The results of comprehensive research on the thermal behavior and molecular and crystalline structures of poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHB-HV) films of different thicknesses, their molecular weights (Mw) and 3-hydroxyvalerate (3-HV) contents are reported. Increasing film thickness from 30 to 100 µm resulted in an isotropic crystal orientation, reducing the crystallite size of the orthorhombic α-phase in the b direction from 22 to 17 nm and increasing the degree of crystallinity of the PHB films without affecting their thermal behavior. Furthermore, despite resulting in the same degree of crystallinity and roughness, an ~8-fold decrease in PHB Mw from 803 kDa to 102 kDa resulted in a decreased number of piezoactive domains. The addition of 5.9% 3-HV resulted in anisotropy in the PHB crystalline structure and increased D(020) from 19 nm to 24 nm. Additionally, a further increase in the 3-HV content to 17.5% in the PHB-HV films led to a decrease in the melting temperature and a decrease in the degree of crystallinity from 57% to 23%, which resulted in the absence of local piezoresponse. Notably, the decrease in the Mw of PHB-HV (~17%) from 1177 kDa to 756 kDa resulted in an increase in the degree of crystallinity from 23% to 32%. Moreover, the PHB-HV films became smoother with increasing 3-HV content. © 2022, The Author(s), under exclusive licence to The Society of Polymer Science, Japan.

The annealing behavior of (1-x)BiMg0.5Ti0.5O3–xBiZn0.5Ti0.5O3 [(1-x)BMT–xBZT] perovskite solid solutions synthesized under high pressure was studied in situ via X-ray diffraction and piezoresponse force microscopy. The as prepared ceramics show a morphotropic phase boundary (MPB) between the non-polar orthorhombic and ferroelectric tetragonal states at 75 mol. % BZT. It is shown that annealing above 573 K results in irreversible changes in the phase diagram. Namely, for compositions with 0.2 < x < 0.6, the initial orthorhombic phase transforms into a ferroelectric rhombohedral phase. The new MPB between the rhombohedral and tetragonal phases lies at a lower BZT content of 60 mol. %. The phase diagram of the BMT–BZT annealed ceramics is formally analogous to that of the commercial piezoelectric material lead zirconate titanate. This makes the BMT–BZT system promising for the development of environmentally friendly piezoelectric ceramics. © 2022 by the authors.

Influence of dopants on structure, microstructure, dielectric and ferroelectric properties of ferroelectric-relaxor (Na0.5Bi0.5)TiO3 ceramics modified by Ba2+ cations and overstoichiometric additives (SiO2 and Na2O) was studied. Changes in structure, microstructure, and dielectric parameters were observed depending on solid solutions compositions. © 2022 Taylor & Francis Group, LLC.

Polymer materials are actively used in dielectric capacitors, in particular for energy storage applications. An enhancement of the stored energy density can be achieved in composites of electroactive polymers and dielectric inorganic fillers with a high dielectric permittivity. In this article, we report on the energy storage characteristics of composites of relaxor terpolymer P(VDF-TrFE-CFE) and BaZr0.2Ti0.8O3 (BZT) nanoparticles. The choice of materials was dictated by their large dielectric permittivity in the vicinity of room temperature. Free-standing composite films, with BZT contents up to 5 vol.%, were prepared by solution casting. The dielectric properties of the composites were investigated over a wide range of frequencies and temperatures. It was shown that the addition of the BZT nanoparticles does not affect the relaxor behavior of the polymer matrix, but significantly increases the dielectric permittivity. The energy storage parameters were estimated from the analysis of the unipolar polarization hysteresis loops. The addition of the BZT filler resulted in the increasing discharge energy density. The best results were achieved for composites with 1.25–2.5 vol.% of BZT. In the range of electric fields to 150 MV/m, the obtained materials demonstrate a superior energy storage density compared to other P(VDF-TFE-CFE) based composites reported in the literature. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.

Piezoelectric poly-L-lactide (PLLA) is a biodegradable polymer used in various biomedical applications. However, tailoring and controlling the structure of PLLA to enhance its piezoelectric response remains a challenge. In this work, extensive characterization was performed to reveal the effect of the reduced graphene oxide (rGO) content (0.2, 0.7, and 1.0 wt%) on the morphology, structure, thermal and piezoelectric behavior of PLLA scaffolds. Randomly oriented homogeneous fibers and a quasi-amorphous structure for pure PLLA and hybrid PLLA-rGO scaffolds were revealed. The addition of rGO affected the molecular structure of the PLLA scaffolds: for example, the number of polar C=O functional groups was increased. Increasing the content of rGO to 1 wt% resulted in decreased glass transition and melting temperatures and increased the degree of crystallinity of the scaffolds. The addition of 0.2 wt% rGO enhanced the effective local vertical and lateral piezoresponses by 2.3 and 15.4 times, respectively, in comparison with pure PLLA fibers. The presence of the shear piezoelectric α-phase (P212121) in uniaxially oriented PLLA fibers and C=O bond rotation in the polymer chains explained the observed piezoresponse. Thus, this study revealed routes to prepare hybrid biodegradable scaffolds with enhanced piezoresponse for tissue engineering applications. © 2022, The Author(s), under exclusive licence to The Society of Polymer Science, Japan.

Domain walls and phase boundaries are fundamental ingredients of ferroelectrics and strongly influence their functional properties. Although both interfaces have been studied for decades, often only a phenomenological macroscopic understanding has been established. The recent developments in experiments and theory allow to address the relevant time and length scales and revisit nucleation, phase propagation and the coupling of domains and phase transitions. This review attempts to specify regularities of domain formation and evolution at ferroelectric transitions and give an overview on unusual polar topological structures that appear as transient states and at the nanoscale. We survey the benefits, validity, and limitations of experimental tools as well as simulation methods to study phase and domain interfaces. We focus on the recent success of these tools in joint scale-bridging studies to solve long lasting puzzles in the field and give an outlook on recent trends in superlattices. © 2021 IOP Publishing Ltd.

Bulk BaTiO3-xCoFe2O4, x = 0.1 – 0.6 magnetoelectric composites were prepared using the phosphate bonded ceramics approach. XRD analysis proved the purity of both phases. The dielectric properties are governed by a series of composition-dependent Maxwell-Wagner relaxations and conductivity at lower frequencies and a phase transition-related anomaly at higher frequencies. A dielectric constant as high as 616 – 9387i is observed at 500 K for BaTiO3-0.6CoFe2O4. The magnetic hysteresis demonstrates a high Ms/Mr ratio of 0.46, which is related to the around 30 nm size of the CoFe2O4 particles. The measured direct magnetoelectric coupling coefficient of 1.1 mV Oe−1 cm−1 is higher than that of the conventionally sintered ceramics and compatible with that of core-shell structures. © 2022 Elsevier B.V.

The ferroelectricity of multivalent codoped Bismuth ferrite (BiFeO3; BFO) nanoparticles (NPs) is revealed and utilized for photocatalysis, exploiting their narrow electronic bandgap. The photocatalytic activity of ferroelectric photocatalysts BiFe0.95Mn0.05O3 (BFM) NPs and mono-, di-, or tri-valent cations (Ag+, Ca2+, Dy3+; MDT) coincorporated BFM NPs are studied under ultrasonication and in acidic conditions. It is found that such doping enhances the photocatalytic activity of the ferroelectric NPs approximately three times. The correlation of the photocatalytic activity with structural, optical, and electrical properties of the doped NPs is established. The increase of spontaneous polarization by the mono- and tri-valent doping is one of the major factors in enhancing the photocatalytic performance along with other factors such as stronger light absorption in the visible range, low recombination rate of charge carriers, and larger surface area of NPs. A-site doping of BFO NPs by divalent elements suppresses the polarization, whereas trivalent (Dy3+) and monovalent (Ag+) cations provide an increase of polarization. The depolarization field in these single domain NPs acts as a driving force to mitigate recombination of the photoinduced charge carriers. © 2022 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH.

Ionic engineering is exploited to substitute Bi cations in BiFe0.95Mn0.05O3 NPs (BFM) with rare-earth (RE) elements (Nd, Gd, and Dy). The sol-gel synthesized RE-NPs are tested for their magnetic hyperthermia potential. RE-dopants alter the morphology of BFM NPs from elliptical to rectangular to irregular hexagonal for Nd, Gd, and Dy doping, respectively. The RE-BFM NPs are ferroelectric and show larger piezoresponse than the pristine BFO NPs. There is an increase of the maximum magnetization at 300 K of BFM up to 550% by introducing Gd. In hyperthermia tests, 3 mg/ml dispersion of NPs in water and agar could increase the temperature of the dispersion up to ∼39°C under an applied AC magnetic field of 80 mT. Although Gd doping generates the highest increment in magnetization of BFM NPs, the Dy-BFM NPs show the best hyperthermia results. These findings show that RE-doped BFO NPs are promising for hyperthermia and other biomedical applications. Copyright © 2022 Dubey, Salamon, Attanayake, Ibrahim, Landers, Castillo, Wende, Srikanth, Shvartsman and Lupascu.

Isolated nonmagnetic substitutional defect ions experience huge coupled electric magnetic interaction in the single-phase multiferroic BiFeO3. In the ferroelectric state above the magnetic Néel temperature TN, the electric environment generates a single symmetric electric field gradient (EFG) parallel to the electric polarization direction. Below TN, a distinct magnetic interaction arises, monitored by the probe nuclei via their magnetic moment. Two magnetic environments arise, given by the relative angle of the local magnetic moment within its easy magnetic plane with respect to the EFG orientation. The angle between field gradient orientation and magnetic field direction is the most stable fitting parameter. The magnetic interaction concomitantly increases the EFG dramatically which reflects an outstandingly large local magnetoelectric coupling. In the set of best fits, two different electric environments form concurrently with two distinctly different local magnetic fields. The magnetic ordering in BiFeO3 thus completely distorts the electric environment of the nonmagnetic probe nucleus. The implications for the local effect of dopants in BiFeO3 are discussed. A third probe environment arising independent of temperature is identified and associated with an iron vacancy. © 2022 American Physical Society.

(1–x)BiMg0.5Ti0.5O3−xBiZn0.5Ti0.5O3 [(1−x)BMT–xBZT] ceramics of perovskite solid solutions, in which BMT and BZT are lead-free structural analogs of PbZrO3 and PbTiO3, respectively, have been synthesized under high pressure. It was found that the as-prepared compositions with a relative BZT content of <75 mol% are orthorhombic (space group Pnnm), while those with a BZT content above this value are tetragonal (P4mm). In the solution with x = 0.75, both phases coexist forming a morphotropic phase boundary (MPB). The compositional dependence of the normalized unit cell volume exhibits a ∼5% jump at x = 0.75. At the same time, the microstructure of the obtained (1–x)BMT−xBZT ceramics shows no particular variation with the chemical composition over MPB. Piezoresponse force microscopy measurements indicate the ferroelectric state of the studied materials and allowed one to estimate their intrinsic piezoelectric coefficients. © 2021 Elsevier Ltd

Ferroelectric materials have gained high interest for photovoltaic applications due to their open-circuit voltage not being limited to the band gap of the material. In the past, different lead-based ferroelectric perovskite thin films such as Pb(Zr,Ti)O3 (Pb,La)(Zr,Ti)O3 and PbTiO3 were investigated with respect to their photovoltaic efficiency. Nevertheless, due to their high band gaps they only absorb photons in the UV spectral range. The well-known ferroelectric PbFe0.5Nb0.5O3 (PFN), which is in a structure similar to the other three, has not been considered as a possible candidate until now. We found that the band gap of PFN is around 2.75 eV and that the conductivity can be increased from 23 S/µm to 35 S/µm during illumination. The relatively low band gap value makes PFN a promising candidate as an absorber material. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

Vats et al. (2016) reported a giant negative electrocaloric effect in multi-ferroic layers. The results were deduced using the polarization from partially switched polarization loops and the Maxwell relation on the electrocaloric effect. First of all, fundamentally, the change of these polarizations with temperature, as presented in their Fig. 5, has no relation with the electrocaloric effect and hence cannot be used to deduce the ECE using the Maxwell relation. Moreover, we are troubled by the data presented in their Fig. 6, which were, as claimed by authors, deduced from the polarization data in their Fig. 5 and the Maxwell relation. We find that the ECE results presented in their Fig. 6 have no direct relation with the polarization-temperature data in their Fig. 5. © 2021 The Royal Society of Chemistry.

We report on temperature-dependent studies of ultrasonic and dielectric properties of (x)0.5(Ba0.7Ca0.3)Ti03–0.5Ba(Ti0.8Zr0.2)O3(BCZT)/(1 − x)NiFe2O4 (BCZT/NFO) composite multiferroics and their relationship to the magnetoelectric (ME) effect in these materials. The most decisive factor in the maximization of the ME effect is the strong elastic softening of the BCZT phase at the phase transition between its ferroelectric phases with orthorhombic and tetragonal symmetry. The proximity of this phase transition to room temperature makes the system promising for practical applications of the ME effect. The magnetostrictive phase does not play any direct role in the determination of the ME temperature dependence because of its weakly temperature-dependent mechanical properties. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.

Abstract—: We have studied the crystal structure and dielectric and local piezoelectric properties of (Na0.5Bi0.5)(Ti1–xMnx)O3 (x = 0–0.1) modified sodium bismuth titanate-based ceramics and observed the formation of a pseudocubic phase with the perovskite structure. Its unit-cell volume first decreases and then, for x ≥ 0.05, increases. The ceramics undergo phase transitions, which show up as anomalies in their dielectric permittivity near ~450 K and peaks at a Curie temperature of ~600 K. As x increases to 0.04, their Curie temperature decreases by 40 K. The phase transitions near 450 K exhibit well-defined relaxor behavior due to the presence of polar regions in the nonpolar matrix. The samples with x < 0.05 have been shown to have an increased room-temperature dielectric permittivity, which correlates with the increased effective piezoelectric coefficient, suggesting that doping with manganese has an advantageous effect on the functional properties of sodium bismuth titanate ceramics. © 2021, Pleiades Publishing, Ltd.

Composites of electroactive polymers and ferroelectric nanoparticles are promising for energy storage and electrocaloric applications. In this paper we report on synthesis and electrocaloric properties of P(VDF-TrFE-CFE)/ BaZr0.20Ti0.80O3 nanocomposites. BaZr0.20Ti0.80O3 (BZT) nanoparticles were synthesized via the hydrothermal route. The P(VDF-TrFE-CFE)/BZT composite films with varying amount (1.25 vol.% to 5 vol.%) of the nanoparticles were prepared by the solution casting method. The nanocomposite films showed a significant increase in the dielectric permittivity with the amount of nanoparticles. An increase in the polarization as well as in hysteresis losses with the amount of nanoparticles was observed. The direct electrocaloric effect was measured using a custom built quasi-Adiabatic calorimeter. The P(VDF-TrFE-CFE)/BZT nanocomposite film with 5 vol.% BZT showed an electrocaloric temperature change of ~ 1.8 K at room temperature and an electric field of 50 MV/m, which is comparable to literature values. © 2021 IEEE.

The temperature dependence of the dielectric permittivity and polarization hysteresis loops of P(VDF-TrFE-CFE) polymer films with different compositions are studied. Among them, the three compositions, 51.3/48.7/6.2, 59.8/40.2/7.3, and 70/30/8.1, are characterized for the first time. Relaxor behavior is confirmed for all studied samples. Increasing the CFE content results in lowering the freezing temperature and stabilizes the ergodic relaxor state. The observed double hysteresis loops are related to the field-induced transition to a ferroelectric state. The critical field corresponding to this transition varies with the composition and temperature; it becomes larger for temperatures far from the freezing temperature. The energy storage performance is evaluated from the analysis of unipolar polarization hysteresis loops. P(VDF-TrFE-CFE) 59.8/40.2/7.3 shows the largest energy density of about 5 J·cm−3 (at the field of 200 MV·m−1) and a charge–discharge efficiency of 63%, which iscomparable with the best literature data for the neat terpolymers. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

Lead iron tungstate ceramics were synthesized by the columbite method. Two compositions were prepared, one with the stoichiometric ratio and the second one with an additional 2 wt% PbO. To investigate the influence of the synthesis conditions on the physical properties, the compositions were calcined and sintered at different temperatures. Due to the excess PbO, the apparent dielectric permittivity and conductivity decreased. Higher calcination and sintering temperatures led to higher dielectric permittivity as well as higher conductivity values. For all samples, weak ferromagnetism was observed caused by local disorder on the B-site sublattice. © 2021 IEEE.

Lead iron niobate, Pb(Fe0.5Nb0.5)O3 (PFN), belongs to the family of multiferroic materials combining ferroelectric and antiferromagnetic ordering. Its properties to a large extent depend on the synthesis conditions. For applications it is important to obtain materials with large polarization and low electric leakage. In this paper we investigate the effect of processing parameters on the structural, electrical and magnetic behaviour of PFN ceramics prepared by the solid state method. The optimal calcination and sintering temperatures are found, which enable us to obtain ceramics with a large polarization Pmax = 28 μC/cm2 and dielectric permittivity εMax ≈ 55 000. We also find that increasing the calcination and sintering temperatures decreases diffuseness of the ferroelectric phase transition and shifts the Néel temperature to lower values, which might be due to a change of the distribution of Fe3+ and Nb5+ across the B-sites of the perovskite towards a more ordered structure. © 2021

Piezoelectric energy harvesters (PEH) hold enormous potential for converting mechanical energy from our surrounding environment into electrical energy that can be used for powering portable electronics. Potassium sodium niobate (KNN) is one of the promising alternatives to replace lead-based piezoelectric materials. This work presents a cutting-edge demonstration of synthesis-function-device integration of piezoelectric nanofibers, where the morphology and the composition are engineered towards achieving high device output. We report a flexible nanogenerator based on electrospun Li and Ta-modified lead-free KNN nanofibers yielding a high voltage output of 5.6 V, which is around 9-fold higher than for the Mn-doped KNN nanofibers reported previously. The influence of Li and Ta-incorporation into the KNN lattice on the electromechanical coupling and the effect of a nanofiber morphology are investigated. The net-shaped KNN and Li and Ta-modified KNN nanofibers, synthesized by electrospinning of appropriate sols, maintain their structural integrity upon calcination and firing steps. The phase analysis (XRD) confirms the formation of single-phase (KNN) material. Li and Ta are found to be incorporated on the A and B-sites of the perovskite lattice, respectively. Piezo force microscopy data show the heat-treated nanofibers to exhibit multi-domain ferroelectric properties. © 2021 Elsevier Ltd

The phase content and sequence, the crystal structure, and the magnetic properties of perovskite solid solutions of the (1−y)BiFeO3–yBiZn0.5Ti0.5O3 series (0.05 ≤ y ≤ 0.90) synthesized under high pressure have been studied. Two perovskite phases, namely the rhombohedral R3c and the tetragonal P4mm, which correspond to the structural types of the end members, BiFeO3 and BiZn0.5Ti0.5O3, respectively, were revealed in the as-synthesized samples. The rhombohedral and the tetragonal phases were found to coexist in the compositional range of 0.30 ≤ y ≤ 0.90. Magnetic properties of the BiFe1−y [Zn0.5Ti0.5 ]yO3 ceramics with y < 0.30 were measured as a function of temperature. The obtained compositional variations of the normalized unit-cell volume and the Néel temperature of the BiFe1−y [Zn0.5Ti0.5 ]yO3 perovskites in the range of their rhombohedral phase were compared with the respective dependences for the BiFe1−yB3+yO3 perovskites (where B3+ = Ga, Co, Mn, Cr, and Sc). The role of the high-pressure synthesis in the formation of the antiferromagnetic states different from the modulated cycloidal one characteristic of the parent BiFeO3 is discussed. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.

The electrocaloric effect (ECE) in dielectrics is characterized by the isothermal entropy change ΔS and adiabatic temperature change ΔT induced by changes of external electric fields. The Maxwell relation, which relates changes of polarization P with temperature T (pyroelectric coefficient) under a fixed electric field E to ΔS for finite intervals in E, provides a convenient way to deduce the ECE from polarization data P(T, E). Hence, this method, known as the indirect method, is widely used in ECE studies in ferroelectrics. Here, we first present the thermodynamic consideration for the Maxwell relation. We then use the indirect method and P(T, E) from bipolar and unipolar polarization curves to deduce the ECE in the normal ferroelectric phase of a P(VDF-TrFE) copolymer. The deduced ECE using the P(T, E) from bipolar polarization curves exhibits a giant negative ECE. In contrast, the directly measured ECE in the same polymer shows the weak and normal ECE. We discuss the constraints of the indirect method and its relation to the polarization-electric field curves measured in practical ferroelectric materials. © 2021 Author(s).

In this reply, we address the concerns that were raised about our paper on CsPbBr3 single crystals. M. Szafrański criticized the dielectric and DSC data in our original paper claiming that they were affected by the experimental artefacts or poor quality of the investigated single crystals, as his DSC and dielectric data did not show any low temperature anomalies in CsPbBr3. We argue in this reply that our main conclusions were not made based on the DSC and dielectric experiments. Here, we emphasize the importance of other experiments like EPR and XRD that were performed to understand if there are any structural transformations of CsPbBr3 at low temperatures. We believe that M. Szafrański did not take into account all the discussion that was presented in our original paper. We hope to clear the doubts in this reply. This journal is © The Royal Society of Chemistry.

The escalated photocatalytic (PC) efficiency of the visible light absorber Ba-doped BiFe0.95Mn0.05O3(BFM) nanoparticles (NPs) as compared to BiFeO3(BFO) NPs is reported for the degradation of the organic pollutants rhodamine B and methyl orange. 1 mol% Ba-doped-BFM NPs degrade both dyes within 60 and 25 minutes under UV + visible illumination, respectively. The Ba and Mn co-doping up to 5 mol% in BFO NPs increases the specific surface area, energy of d-d transitions, and PC efficiency of the BFO NPs. The maximum PC efficiency found in 1 mol% Ba doped BFM NPs is attributed to a cooperative effect of factors like its increased light absorption ability, large surface area, active surface, reduced recombination of charge carriers, and spontaneous polarization to induce charge carrier separation. The 1 mol% Ba and 5 mol% Mn co-incorporation is found to be the optimum dopant concentration for photocatalytic applications. These properties of co-doped BFO NPs can,e.g., be exploited in the field of water splitting. © The Royal Society of Chemistry 2021.

High-pressure synthesis method allows obtaining single-phase perovskite BiFe1−xScxO3 ceramics in the entire concentration range. As-prepared compositions with x from 0.30 to 0.55 have the antipolar orthorhombic Pnma structure but can be irreversible converted into the polar rhombohedral R3c or the polar orthorhombic Ima2 phase via annealing at ambient pressure. Microstructure defects and large conductivity of the high-pressure-synthesized ceramics make it difficult to study and even verify their ferroelectric properties. These obstacles can be overcome using piezoresponse force microscopy (PFM) addressing ferroelectric behavior inside single grains. Herein, the PFM study of the BiFe1−xScxO3 ceramics (0.30 ≤ x ≤ 0.50) is reported. The annealed samples show a strong PFM contrast. Switching of domain polarity by an electric field confirms the ferroelectric nature of these samples. The as-prepared BiFe0.5Sc0.5O3 ceramics demonstrate no piezoresponse in accordance with the antipolar character of the Pnma phase. However, application of a strong enough electric field induces irreversible transition to the ferroelectric state. The as-prepared BiFe0.7Sc0.3O3 ceramics show coexistence of ferroelectric and antiferroelectric grains without poling. It is assumed that mechanical stress caused by the sample polishing can be also a driving force of phase transformation in these materials alongside temperature and external electric field. © 2021 The Authors. physica status solidi (a) applications and materials science published by Wiley-VCH GmbH

Abstract: The effect of modification (in the А and В sublattices) on the crystal structure parameters, microstructure, and dielectric and local piezoelectric properties of perovskite ceramics (1 – x)(K0.5Na0.5)NbO3–xLa(Ag0.5Sb0.5)O3 was studied. At increased concentration of the second component, the lattice volume and average grain diameter decreased, and the temperature of the polymorphic transition and Curie temperature lowered. The dielectric and effective piezoelectric properties of the samples were found to depend nonmonotonically on the composition, average grain diameter, and degree of texturing of the ceramics. © 2021, Pleiades Publishing, Ltd.

The high power conversion efficiency of perovskite solar cells reaching 25.5% today [1], raises the question on the relation between the photovoltaic performance and physical nature of these materials. To get a better understanding of the properties, good quality single crystals are highly demanded. In this paper we report on large single crystals of formamidinium lead halide grown by a combination of inverse temperature crystallization and seed growth. Structure, phase transitions, dielectric and optical properties of the single crystals are shown. © 2021 Taylor & Francis Group, LLC.

The paper proposes a method for forming thin films of polyvinylidene fluoride using a 3D printer. The extrusion parameters of a filament based on PVDF with a diameter of 1.75 were established and the optimal characteristics for printing by 3D printer were selected (nozzle temperature, table temperature, feed rate). Studies have shown that the local polarization of a thin PVDF film is stable for 20 hours. © 2020 IEEE.

Bismuth ferrite (BFO) is the drosophila of research in multiferroic materials due to its simultaneous magnetic and electric ordering at room temperature. The unfortunate detail is its antiferromagnetic ordering, which practically cancels magnetization and magnetoelectric coupling of the crystals. To induce finite coupling, dopants have been introduced with a certain success so far. Nanoparticles (NPs) can additionally constrain the formation of the magnetic cycloid in BFO due to size confinement. Doping nanoparticles can thus potentially provide a sizeable magnetization of BFO, making applications in computer memories and hyperthermia cancer treatment feasible. We show that the codoping of BFO NPs by Ba and Mn balances the electrochemical equilibrium, reduces the particle size, and shifts the magnetic phase transition to lower temperatures. The ferroelectric properties are retained and the remanent magnetization is increased by 1 order of magnitude: Bi0.95Ba0.05Fe0.95Mn0.05O3 possesses a remanent magnetization of 0.277 Am2/kg. Our Mössbauer studies reveal that two effects drive this increase: partial destruction of the spin cycloid due to Mn and increased spin canting due to Ba doping inducing local stress fields. This dopant combination and particular concentration improve the effective magnetization value exceptionally well. © 2020 American Chemical Society.

Below the Néel temperature, TN ∼220 K, at least two nano-scale antiferromagnetic (AFM) phases coexist in the polar polymorph of the BiFe0.5Sc0.5O3 perovskite; one of these phases is a weak ferromagnetic. Non-uniform structure distortions induced by high-pressure synthesis lead to competing AFM orders and a nano-scale spontaneous magnetic phase separated state of the compound. Interface exchange coupling between the AFM domains and the weak ferromagnetic domains causes unidirectional anisotropy of magnetization, resulting in the exchange bias (EB) effect. The EB field, HEB, and the coercive field strongly depend on temperature and the strength of the cooling magnetic field. HEB increases with an increase in the cooling magnetic field and reaches a maximum value of about 1 kOe at 5 K. The exchange field vanishes above TN with the disappearance of long-range magnetic ordering. The effect is promising for applications in electronics as it is large enough and as it is tunable by temperature and the magnetic field applied during cooling. © 2020 Author(s).

The study of the electrocaloric effect in ensembles of ferroelectric nanoparticles is of significant interest for fundamental research and its practical applications in solidstate coolers. The exploration of electrocaloric cooling is of great importance to finding solutions to environmental and energyefficiency issues in currently available refrigeration technologies. In this work we calculated the polarization, electrocaloric temperature change, and dielectric permittivity of the nanocomposite containing non-interacting ferroelectric nanoparticles with different size distributions, and analyzed the dependences of polarization, dielectric permittivity and electrocaloric temperature change on the distribution function of the particles sizes. As anticipated, the properties of the nanocomposite approach that of the average size nanoparticles with a narrowing in the dispersion of particles sizes within the nanocomposite. A widening in the particles size distribution increases the coercivity (i.e. half-width of the ferroelectric hysteresis loop), but the hysteresis width decreases for the maximum particle radius, which is significantly smaller than the dispersion of the distribution function. © 2020 IEEE.

Laser fragmentation of colloidal submicron-sized bismuth ferrite particles was performed by irradiating a liquid jet to synthesize bismuth ferrite nanoparticles. This treatment achieved a size reduction from 450 nm to below 10 nm. A circular and an elliptical fluid jet were compared to control the energy distribution within the fluid jet and thereby the product size distribution and educt decomposition. The resulting colloids were analysed via UV-VIS, XRD and TEM. All methods were used to gain information on size distribution, material morphology and composition. It was found that using an elliptical liquid jet during the laser fragmentation leads to a slightly smaller and narrower size distribution of the resulting product compared to the circular jet. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

The temperature behavior of the crystal structure as well as dielectric and magnetic properties of the perovskite bismuth chromate ceramics with the 10 mol % Cr3+-to-Sc3+ substitution were studied and compared with those of the unmodified compound. Using a high-pressure synthesis, BiCrO3 and BiCr0.9Sc0.1O3 were obtained as metastable perovskite phases which are monoclinic C2/c with the 6ap × 2ap × 6ap superstructure (where ap is the primitive perovskite unit-cell parameter) under ambient conditions. At room temperature, the unit cell volume of BiCr0.9Sc0.1O3 is ∼1.3% larger than that of BiCrO3. Both perovskites undergo a reversible structural transition into a nonpolar GdFeO3-type phase (orthorhombic Pnma, 2ap × 2ap × 2ap) in the temperature ranges of 410-420 K (BiCrO3) and 470-520 K (BiCr0.9Sc0.1O3) with a relative jump of the primitive perovskite unit cell volume of about -1.6 and -2.0%, respectively. Temperature dependences of the complex dielectric permittivity demonstrate anomalies in the phase transition ranges. The Pnma-to-C2/c crossover in BiCrO3 is accompanied by a decrease in the direct current (dc) conductivity, while in BiCr0.9Sc0.1O3 the conductivity increases. The onset of an antiferromagnetic order in BiCr0.9Sc0.1O3 is observed at the Néel temperature (TN) of about 85 K as compared with TN = 110 K in BiCrO3. In contrast to BiCrO3, which exhibits a spin reorientation at Tsr ∼72 K, no such a transition occurs in BiCr0.9Sc0.1O3. Copyright © 2020 American Chemical Society.

Cesium-lead-bromide (CsPbBr3) is the simplest all inorganic halide perovskite. It serves as a reference material for understanding the exceptional solar cell properties of the organic-inorganic hybrid halide perovskites and is itself discussed as an alternative absorber material. Broadband dielectric spectroscopy has proven to yield an in depth understanding of charge screening mechanisms in the halide solar cell absorbers based on methylammonium and modifications hereof. For a deeper understanding of charge carrier screening, we have investigated CsPbBr3 across wide temperature (120 K-450 K) and frequency ranges. Besides the two known phase transitions at 403 K and 361 K, the dielectric data show another anomaly around 220 K, which can be interpreted as another phase transition. XRD and EPR studies confirm the presence of this anomaly, but Raman scattering spectra do not show any lattice anomalies in the vicinity of 220 K. This additional anomaly is of first order character (different transition temperatures upon cooling and heating) but hardly influences the lattice dynamics. Our broadband dielectric investigations of CsPbBr3 display the same microwave limit permittivity as for MAPbX3 (ϵr ≈ 30, X = Cl, Br, I, MA = CH3NH3+) but do not afford a second permittivity relaxation up to this frequency. Our prior assignment of the second contribution in the methylammonium compounds being due to the relaxation dynamics of the methylammonium ion as a dipole is herewith proven. Nevertheless, CsPbBr3 shows large charge carrier screening up to very high frequencies which can still play a vital role in charge carrier dynamics and exciton behaviour in this material as well. © The Royal Society of Chemistry.

This work reports on the effect of calcination and poling processes on the crystalline phase and piezoresponse of poly(vinylidene fluoride) (PVDF) micropillar arrays. PVDF micropillars were prepared by the imprinting method, heated and treated with high-voltage poling. The effect of the treatment conditions on the crystallization behaviour and the piezoelectric properties of the patterned PVDF films was investigated by piezoresponse force microscopy (PFM), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). DSC data show that poling of the PVDF micropillars increases the crystallinity of the polymer from 12% to 22.7%. FTIR measurements of PVDF films show that the calcination and poling processes affect the γ to β phase transformation. In the imprinted and annealed samples, the γ phase was predominant (58% and 46%, respectively). For the poled samples, up to 42% of the β phase was found. Piezoelectric measurements using PFM showed that the poled PVDF micropillars possess a much higher piezoelectric coefficient (29 pm/V) compared to the annealed sample (10 pm/V). The piezoresponse of the PVDF micropillar arrays is thus substantially enhanced by poling. © 2019 Elsevier B.V.

BaTi1-xZrxO3 nanoparticles (x = 0, 0.05, 0.1, 0.15, 0.2) were successfully produced by the spray-flame synthesis method. The as-synthesized powders are characterized by small (~10 nm) particle sizes as shown by TEM images. The as-synthesized powders were pre-heated at 800 °C to remove organic residuals from the surface. Pellets were then pressed and sintered at 1100 °C for 3 h. XRD measurements of the sintered materials show that the crystallite size decreases with increasing Zr concentration, which was additionally confirmed by TEM. Dielectric measurements show that the Curie temperature shifts towards lower temperatures with increasing Zr concentration accompanied by a decrease in the dielectric permittivity values which is attributed to a decreasing crystallite/particle size. In addition, a frequency dispersion of the permittivity values is discovered. This is mostly ascribed to Maxwell-Wagner polarization effects typical for nanograined ceramics. © 2020 Elsevier Ltd and Techna Group S.r.l.

We consider a model of a nanocomposite based on noninteracting spherical single-domain ferroelectric nanoparticles (NPs) of various sizes embedded in a dielectric matrix. The size distribution function of these NPs is selected as a part of the truncated Gaussian distribution from minimum to maximum radius. For such nanocomposites, we calculate the dependences of the reversible part of the electric polarization, the electrocaloric (EC) temperature change, and the dielectric permittivity on the external electric field, which have the characteristic form of hysteresis loops. We then analyze the change in the shape of the hysteresis loops relative to the particle size distribution parameters. We demonstrate that the remanent polarization, coercive field, dielectric permittivity maximums, and maximums and minimums of the EC temperature change depend most strongly on the most probable radius, moderately on the dispersion, and have the weakest dependence on the maximum radius of the NP. We calculate and analyze the dependences of pyroelectric figures of merit on the average radius of the NPs in the composite. The dependences confirm the presence of a phase transition induced by the size of the NPs, which is characterized by the presence of a maxima near the critical average radius of the particles, the value of which increases with an increasing dispersion of the distribution function. © 1986-2012 IEEE.

Bitumen is the primary binder of asphalt covering most of the roads in the world. The origin of the primary oil, the refinery treatment, the specific chemical structure, and the natural or induced chemical modifications determine the mechanical properties of bitumen. Aging affects the time evolution of its thermo-rheological behaviour. Numerous studies have suggested that the particular local microstructure of bitumen affects its rheological properties including ‘bee’–patterned structures. We have used scanning probe microscopy to distinguish between surface effects and bulk properties. Using scanning probe microscopy we conclude that the ‘bee’ structures exist at the surface only and do not occur in the bulk. In particular, they are not observed on freshly fractured surfaces. A special technique was used to chisel off the bitumen surface. The material relaxes and the ‘bee’ structures disappear from the surface. This proves that the ‘bees’ are formed as a thin surface layer. Improved recycling will thus have to rely on chemical treatment of aged and used bitumen rather than on intentional modifications to the surface microstructure which is merely a surface effect. The bitumen surface microstructure can nevertheless be seen as a fingerprint of the overall bitumen properties to a certain degree. © 2020 Informa UK Limited, trading as Taylor & Francis Group.

Using Landau-Ginzburg-Devonshire theory and effective medium approximation, we analytically calculate typical dependences of the pyroelectric and electrocaloric coefficients on external electric field, temperature, and radius of spherical single-domain ferroelectric nanoparticles. The considered physical model corresponds to the nanocomposite with a small fraction of ferroelectric nanoparticles. Within the framework of the analytical model, we establish how the size changes determine the temperature and field behavior of pyroelectric and electrocaloric coefficients on the example of BaTiO3 nanoparticles covered by a semiconducting shell and placed in a dielectric polymer. We show that by changing the particle size one can induce maxima of the pyroelectric coefficient and electrocaloric temperature variation, control their width and height. Obtained analytical expressions allow selecting the interval of particle sizes, voltage, and/or temperature for which the pyroelectric energy conversion and electrocaloric coefficient are optimal for applications. The observed size effect opens the possibility to control pyroelectric and electrocaloric properties of ferroelectric nanocomposites that can be important for their advanced applications in energy convertors and cooling systems. ©2019 American Physical Society.

Thin film Cu2ZnSnSe4 (CZTSe) solar cells can be grown on flexible and lightweight metal substrates allowing their direct integration on bendable surfaces and where the weight of solar cell is an important criterion. Flexible substrates make it possible to use the roll-to-roll technology of solar cells, which leads to an additional reduction in the cost of production and final cost of solar cells. The CZTSe thin films were fabricated by selenization of electrodeposited metallic precursors onto tantalum (Ta) flexible substrates at different temperature and time. The results of the effect of selenization temperature and time on the morphology, structural, and optical property of the CZTSe films are presented in this work. It was found that the morphology of the CZTSe thin films depend on their elemental composition and time of selenization. Experimental data indicate that composition of the CZTSe films selenized within 10 and 20 min at 560 °C have the CZTSe basic phase and secondary phases (CuSe, SnSe and ZnSe). In contrast, the increase in selenization temperature and/or time leads to disappearing of the secondary phases (CuSe, SnSe) and better crystallization of the CZTSe films. It was found that films selenized at 560 and 580 °C within the same time have similar characteristics. Depending on selenization time and temperature of the CZTSe, thin films exhibited a shift in band gap from 1.16 to 1.19 and to 1.22 eV, respectively. The change of band gap of the CZTSe thin films is associated with changes of elemental and phase compositions, and thickness of the film. These results showed that the received CZTSe films on Ta foil can be used for fabrication of thin film solar cells. © 2018

The electrocaloric effect in P(VDF-TrFE)/ barium zirconium titanate composites was studied by the direct method using a quasi-adiabatic ECE calorimeter. Barium zirconium titanate was synthesised using a hydrothermal method. Analytical results of the prepared material indicate the formation of phase pure particles with particle sizes between 100-200 nm. The polymer composite films with different filler content were prepared using the solution-casting method. The Curie transition temperature of barium zirconium titanate and P(VDF-TrFE) is in the range of 360-380 K. We observe an enhancement of the dielectric constant and the electrocaloric value in this temperature range due to the superposition of the polarization changes in the polymer and the filler. © 2019 IEEE.

We studied magnetostatic response of the Bi0.9La0.1FeO3– KBr composites (BLFO-KBr) consisting of nanosized (≈100 nm) ferrite Bi0.9La0.1FeO3 (BLFO) conjugated with fine grinded ionic conducting KBr. When the fraction of KBr is rather small (less than 15 wt%) the magnetic response of the composite is very weak and similar to that observed for the BLFO (pure KBr matrix without Bi1-xLaxFeO3 has no magnetic response as anticipated). However, when the fraction of KBr increases above 15%, the magnetic response of the composite changes substantially and the field dependence of magnetization reveals ferromagnetic-like hysteresis loop with a remanent magnetization about 0.14 emu/g and coercive field about 1.8 Tesla (at room temperature). Nothing similar to the ferromagnetic-like hysteresis loop can be observed in Bi1-zLazFeO3 ceramics with z ≤ 0.15, which magnetization quasi-linearly increases with magnetic field. Different physical mechanisms were considered to explain the unusual experimental results for BLFO-KBr nanocomposites, but only those among them, which are highly sensitive to the interaction of antiferromagnetic Bi0.9La0.1FeO3 with ionic conductor KBr, can be relevant. © 2019, The Author(s).

In this article, we report on investigation of wave attenuation and longitudinal velocity at ultrasonic frequency in P(VDF-TrFE) composites with fillers of conductive carbon nanotubes (CNT). Large thermal hysteresis was observed in temperature dependences of both the ultrasonic velocity and attenuation. Moreover, the temperature dependences of the ultrasonic velocity and attenuation were shown to be sensitive to thermal cycling over the ferroelectric phase transition. In our knowledge, this is the first time when the effect of annealing on the ferroelectric phase in P(VDF-TrFE) composites is investigated by means of ultrasonic spectroscopy. Obtained results are compared with data of dielectric spectroscopy, differential scanning calorimetry, as well as thermally stimulated discharge current measurements. An analysis of the impact of CNT fillers on the dipolar charge distribution in P(VDF-TrFE)/CNT composites was performed by means of thermally stimulated discharge current measurements. POLYM. COMPOS., 40:1609–1618, 2019. © 2018 Society of Plastics Engineers. © 2018 Society of Plastics Engineers

Pb(Fe1/2Nb1/2)O3 powders have been prepared via different synthesis routes: conventional solid state, molten salt, combustion, hydrothermal and mechanochemical activation assisted syntheses. The homogenized powders were pressed into pellets, sintered at 1000 °C in an oxygen atmosphere and characterized. The ceramic samples prepared by mechanochemical activation assisted synthesis exhibited the best functional properties. These ceramics were highly dense, with no secondary phases exhibiting sharp ferroelectric hysteresis loops with the remanent polarization 28.1 μC/cm2 and coercive field 3.1 kV/cm. In all other samples, the dielectric permittivity, dielectric losses versus temperature and polarization versus electric field measurements reveal enhanced electrical conductivity. © 2019 IEEE.

Abstract: We propose a new method for determining the magnetostriction characteristics of composite multiferroics by measuring the magnetoelectric (ME) response of the material structure. It is established that the integral characteristic of the ME effect coincides to within a constant factor with the magnetostriction curve. The results of an experimental investigation of the physical properties of volume composites based on lead zirconate titanate (PZT) and nickel ferrite spinel are presented. The field dependence of the ME voltage coefficient was used to determine magnetostriction curves of composite structures containing 10–70 wt % ferrospinel. © 2019, Pleiades Publishing, Ltd.

This study was dedicated to the investigation of poly(vinylidene fluoride) (PVDF) micropillar arrays obtained by soft lithography followed by phase inversion at a low temperature. Reduced graphene oxide (rGO) was incorporated into the PVDF as a nucleating filler. The piezoelectric properties of the PVDF-rGO composite micropillars were explored via piezo-response force microscopy (PFM). Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) showed that α,β, and γ phases co-existed in all studied samples, with a predominance of the γ phase. The piezoresponse force microscopy (PFM) data provided the local piezoelectric response of the PVDF micropillars, which exhibited a temperature-induced downward dipole orientation in the pristine PVDF micropillars. The addition of rGO into the PVDF matrix resulted in a change in the preferred polarization direction, and the piezo-response phase angle changed from -120° to 20°-40°. The pristine PVDF and PVDF loaded with 0.1 wt % of rGO after low-temperature quenching were found to possess a piezoelectric response of 86 and 87 pm/V respectively, which are significantly higher than the |d33 eff| in the case of imprinted PVDF 64 pm/V. Thus, the addition of rGO significantly affected the domain orientation (polarization) while quenching increased the piezoelectric response. © 2019 by the authors.

Bismuth Ferrite Nanoparticles (BiFeO3 NPs) are interesting single-phase multiferroic materials due to their notable magnetoelectric properties at room temperature. We enhance the magnetization of BFO NPs via doping 5 % Ba at the Bi site. These NPs were synthesized by a modified and reproducible sol-gel technique and further characterized by XRD (x-ray diffraction), VSM (Vibrating Sample Magnetometer), PFM (Piezoresponse Force Microscopy), DSC (Differential Scanning Calorimetry) and UV-Vis (Absorption spectroscopy) techniques. Rietveld refinement unveils a lattice distortion and shows that the average crystallite size is reduced from 43.7 to 38.4 nm after Ba incorporation. 5 % Ba enhances the ferromagnetism from 0.71 to 0.86 Am2/kg at 300 K. DSC curves confirm no significant variation in magnetic transition temperature (TN) after Ba doping. Moreover, PFM data show the ferroelectric behavior of 5 % Ba doped NPs with reduced piezoresponse. UV-Vis spectra show the presence of crystal field transition and a doubly degenerate d-d transition, whereas the band gap of 5 % Ba doped BFO NPs increases from 2.18 eV for BFO NPs to 2.23 eV. © 2019 IEEE.

In this study we report on the dielectric, ferroelectric, and piezoelectric properties of the conventional polyvinylidene fluoride/trifluoroethylene, P(VDF-TrFE), copolymer of composition 70/30 mol.% with fillers on the basis of lead zirconate titanate (PZT). (Pb0.75Ba0.24Sr0.01)(Zr0.53Ti0.47)O3 (BPZT) fillers with concentrations from 10 to 50 vol.% are additional components in the P(VDF-TrFE) piezoelectric polymer. Dielectric spectroscopy and the characterization of ferroelectricity and piezoelectricity in the P(VDF-TrFE)/BPZT composites are performed over a wide temperature range from 150 to 420 K. The dependence of the effective dielectric permittivity for the composites under investigation on the filler concentration is analyzed by the Lichtenecker mixing rule. The approximation model of effective medium is also applied to explain the impact of BPZT fillers on the ferroelectric and piezoelectric properties of P(VDF-TrFE)/BPZT composites. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In this study we explore the prospect of strain-mediated magnetoelectric coupling in CoFe2O4-BaTiO3 bi-layers as a function of different interfacial boundary conditions. Pulsed laser deposition fabricated thin films on Nb:SrTiO3(100) and Nb:SrTiO3(111) single crystal substrates were characterized in terms of their peculiarities related to the structure-property relationship. Despite the homogeneous phase formation in both films, transmission electron microscopy showed that the bi-layers on Nb:SrTiO3(100) exhibit a higher number of crystallographic defects when compared to the films on Nb:SrTiO3(111). This signifies an intrinsic relationship of the defects and the substrate orientation. To analyze the consequences of these defects on the overall magnetoelectric coupling of the bi-layered films, piezoresponse force microscopy was performed in situ with an applied magnetic field. The local magnetic field dependence of the piezoresponse was obtained using principal component analysis. A detailed analysis of this dependence led to a conclusion that the bi-layers on Nb:SrTiO3(111) exhibit better strain-transfer characteristics between the magnetic and the piezoelectric layer than those which were deposited on Nb:SrTiO3(100). These strain transfer characteristics correlate well with the interface quality and the defect concentration. This study suggests that in terms of overall magnetoelectric coupling, the Nb:SrTiO3(111) grown bi-layers are expected to outperform their Nb:SrTiO3(100) grown counterparts. © 2018 The Royal Society of Chemistry.

Polarized states of polymer/inorganic inclusion P(VDF-TrFE)-(Pb,Ba)(Zr,Ti)O3 composites are studied at the nanoscale using both piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM). It has been shown that inorganic inclusions can be visualized using KPFM due to a discontinuity of the surface potential and polarization at the interface between the inclusions and the polymer matrix. The temperature evolution of the PFM and KPFM signal profiles is investigated. Softening of the polymer matrix on approaching the Curie temperature limits application of the contact PFM method. However non-contact KPFM can be used to probe evolution of the polarization at the phase transition. Mechanisms of the KPFM contrast formation are discussed. © 2017 The Author(s).

For advanced applications in modern industry, it is very important to reduce the volume of ferroelectric nanoparticles without serious deterioration of their polar properties. In many practically important cases, the fixed volume (rather than the fixed size) corresponds to realistic technological conditions of nanoparticles fabrication. The letter is focused on the theoretical study of the behavior of ferroelectric polarization, paramagnetoelectric coefficient and phase diagrams of semi-ellipsoidal nanoparticles with a fixed volume V. Our approach combines the Landau-Ginzburg-Devonshire phenomenology, the classical electrostatics, and the elasticity theory. Our results show that the size effects on the phase diagrams and polarization of semi-ellipsoidal BiFeO3 nanoparticles nontrivially depend on V. These findings provide a path to optimize the polar properties of nanoparticles by controlling their phase diagrams at a fixed volume. © 2018, EDP Sciences, SIF, Springer-Verlag GmbH Germany, part of Springer Nature.

The occurrence of the inverse (or negative) electrocaloric effect, where the isothermal application of an electric field leads to an increase in entropy and the removal of the field decreases the entropy of the system under consideration, is discussed and analyzed. Inverse electrocaloric effects have been reported to occur in several cases, for example, at transitions between ferroelectric phases with different polarization directions, in materials with certain polar defect configurations, and in antiferroelectrics. This counterintuitive relationship between entropy and applied field is intriguing and thus of general scientific interest. The combined application of normal and inverse effects has also been suggested as a means to achieve larger temperature differences between hot and cold reservoirs in future cooling devices. A good general understanding and the possibility to engineer inverse caloric effects in terms of temperature spans, required fields, and operating temperatures are thus of fundamental as well as technological importance. Here, the known cases of inverse electrocaloric effects are reviewed, their physical origins are discussed, and the different cases are compared to identify common aspects as well as potential differences. In all cases the inverse electrocaloric effect is related to the presence of competing phases or states that are close in energy and can easily be transformed with the applied field. © 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

The electrocaloric effect (ECE) in ferroelectric materials is a promising candidate for small, effective, low cost, and environmentally friendly solid state cooling applications. Instead of the commonly used indirect estimates based on Maxwell's relations, direct measurements of the ECE are required to obtain reliable values. In this work, we report on a custom-made quasi-adiabatic calorimeter for direct ECE measurements. The ECE is measured for two promising lead-free materials: Ba(Zr0.12Ti0.88)O3 and Ba(Zr0.2Ti0.8)O3 bulk ceramics. Adiabatic temperature changes of ΔTEC = 0.5 K at 355 K and ΔTEC = 0.3 K at 314 K were achieved under the application of an electric field of 2 kV/mm for the Ba(Zr0.12Ti0.88)O3 and Ba(Zr0.2Ti0.8)O3 samples, respectively. The quasi-adiabatic ECE measurements reliably match other direct EC measurements using a differential scanning calorimeter or an infrared camera. The data are compared to indirect EC estimations based on Maxwell's relations and show that the indirect measurements typically underestimate the effect to a certain degree. © 2018 Author(s).

Using Landau-Ginzburg-Devonshire (LGD) theory for BiFeO3 dense fine-grained ceramics with quasispherical grains and nanosized intergrain spaces enriched by elastic defects, we calculated a surprisingly strong size-induced increase in the antiferromagnetic transition temperature caused by the joint action of rotomagnetic and magnetostrictive coupling. Notably, all parameters included in the LGD functional have been extracted from experiments, not assumed. Complementarily, we performed experiments for dense BiFeO3 ceramics, which revealed that the shift of the antiferromagnetic transition is to TN∼690K instead of TN∼645K for a single crystal. To explain the result theoretically, we consider the possibility of controlling the antiferromagnetic state of multiferroic BiFeO3 via biquadratic antiferrodistortive rotomagnetic, rotoelectric, magnetoelectric, and magnetostrictive couplings. According to our calculations, the highest contribution is the rotostriction contribution, while the magnetostrictive and electrostriction contributions appear smaller. © 2018 American Physical Society.

The term big-data in the context of materials science not only stands for the volume, but also for the heterogeneous nature of the characterization data-sets. This is a common problem in combinatorial searches in materials science, as well as chemistry. However, these data-sets may well be 'small' in terms of limited step-size of the measurement variables. Due to this limitation, application of higher-order statistics is not effective, and the choice of a suitable unsupervised learning method is restricted to those utilizing lower-order statistics. As an interesting case study, we present here variable magnetic-field Piezoresponse Force Microscopy (PFM) study of composite multiferroics, where due to experimental limitations the magnetic field dependence of piezoresponse is registered with a coarse step-size. An efficient extraction of this dependence, which corresponds to the local magnetoelectric effect, forms the central problem of this work. We evaluate the performance of Principal Component Analysis (PCA) as a simple unsupervised learning technique, by pre-labeling possible patterns in the data using Density Based Clustering (DBSCAN). Based on this combinational analysis, we highlight how PCA using non-central second-moment can be useful in such cases for extracting information about the local material response and the corresponding spatial distribution. © 2018 The Author(s).

Local observation of the stress mediated magnetoelectric (ME) effect in composites has gained a great deal of interest over the last decades. However, there is an apparent lack of rigorous methods for a quantitative characterization of the ME effect at the local scale, especially in polycrystalline microstructures. In the present work, we address this issue by locally probing the surface magnetic state of barium titante-hexagonal barium ferrite (BaTiO3-BaFe12O19) ceramic composites using magnetic force microscopy (MFM). The effect of the piezoelectrically induced local stress on the magnetostrictive component (BaFe12O19, BaM) was observed in the form of the evolution of the magnetic domains. The local piezoelectric stress was induced by applying a voltage to the neighboring BaTiO3 grains, using a conductive atomic force microscopy tip. The resulting stochastic evolution of magnetic domains was studied in the context of the induced magnetoelastic anisotropy. In order to overcome the ambiguity in the domain changes observed by MFM, certain generalizations about the observed MFM contrast are put forward, followed by application of an algorithm for extracting the average micromagnetic changes. An average change in domain wall thickness of 50 nm was extracted, giving a lower limit on the corresponding induced magnetoelastic anisotropy energy. Furthermore, we demonstrate that this induced magnetomechanical energy is approximately equal to the K1 magnetocrystalline anisotropy constant of BaM, and compare it with a modeled value of applied elastic energy density. The comparison allowed us to judge the quality of the interfaces in the composite system, by roughly gauging the energy conversion ratio. © 2018 IOP Publishing Ltd.

Studying multiferroic magnetoelectrics has been a focus field for the last decade and a half, and the exploration of new materials is one of the several aspects of this quest. Here we report on the synthesis and characterization of NiFe2O4-based multiferroic composites which employ (Ba,Ca)(Zr,Ti)O3 as the ferroelectric/piezoelectric component and NiFe2O4 as the magnetostrictive phase. We find that these composites show excellent magnetoelectric properties. Especially the composite with 30 vol% of NiFe2O4 has a converse ME coefficient approximately two times larger than the previously reported one for BaTiO3-CoFe2O4 composites. A relationship between the phase connectivity within these composites and the ME properties was explored by the time of flight secondary ion mass microscopy. We believe that our investigation will be helpful for the design of magnetoelectric materials as components of sensors and memory devices. © 2017 Acta Materialia Inc.

In this work we address the peculiarities of the macroscopic responses in ternary 0.1Na0·5Bi0·5TiO3-0.6SrTiO3-0.3PbTiO3 (0.1NBT-0.6ST-0.3PT) solid solutions. These solid solutions exhibit a spontaneous first order relaxor to normal ferroelectric phase transition. The phase transition is accompanied by a broad dielectric relaxation which expands over 10 orders of magnitude in frequency just above the phase transition temperature. The temperature dependence of polarization shows that non-zero net polarization persists above the phase transition temperature. Below the phase transition temperature, it is not possible to describe the temperature dependence of polarization with a power law function which is valid in normal ferroelectrics. The piezoresponse force microscopy studies reveal that 0.1NBT-0.6ST-0.3PT solids solutions display several local polarization patterns which arise due to the bimodal distribution of grains in the ceramics. We associate the peculiar macroscopic responses to these complex polar structures and their different temperature behaviours. © 2018 Acta Materialia Inc.

Polar structures of CaxBa1-xNb2O6 (CBN100x) single crystals were investigated using piezoresponse force microscopy. Increasing Ca content results in decreasing domain size and enhancement of the polar disorder. For the composition with x = 0.32 the characteristic domain size is similar to that reported for relaxor Sr0.61Ba0.39Nb2O6 (SBN61). However, decay of an artificial macroscopic domain in CBN32 takes place below the macroscopic transition temperature, contrary to SBN61, where random fields stabilize it above the transition temperature. We can conclude that CBN with 0.26 ≤ x ≤ 0.32 does not display classical relaxor behavior and might be considered as a disordered ferroelectric. © 2017 by the authors.

A study of dielectric properties of composite films on the base of poly(vinylidene fluoride-trifluoroethylene) copolymer P(VDFTrFE) and ferroelectric ceramics of barium lead zirconate titanate (BPZT) solid solution is presented in this work. The composite films containing up to 50 vol.% of BPZT grains with size ~1 μm were prepared by the solvent cast method. Frequency dependences of real and imaginary components of the complex permittivity were determined. The concentration dependence of the dielectric constant was discussed. © The Author(s).

Due to the unprecedented rapid increase of their power conversion efficiency, hybrid organic-inorganic perovskites CH3NH3PbX3 (X = I, Br, Cl) can potentially revolutionize the world of solar cells. However, despite tremendous research activity, the origin of the exceptionally large diffusion length of their photogenerated charge carriers, that is, their low recombination rate, remains elusive. Using frequency and temperature-dependent dielectric measurements across the entire frequency spectrum, it is shown that the dielectric constant conserves very high values (>27) for frequencies below 1 THz in all three halides. This efficiently prevents photocarrier trapping and their recombination owing to the strong screening of charged entities. By combining ultrasonic and Raman spectroscopy with dielectric analysis, similarly large contributions to the dielectric constant are attributed to the dipolar disorder of the CH3NH3 + cations as well as lattice dynamics in the gigahertz range yielding dielectric constants of εstat = 62 for the iodide, 58 for the bromide, and about 45 for the chloride below 1 GHz at room temperature. Disorder continuously reduces for decreasing temperature. Dipole dynamics prevail in the intermediate tetragonal phase. The low-temperature orthorhombic state is antipolar. No indications of ferroelectricity are found. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this study, we report on investigation of the electrocaloric (EC) effect in lead-free Ba(SnxTi1-x)O3 (BSnT) ceramics with compositions in the range of 0.08 ≤ x ≤ 0.15 by the direct measurement method using a differential scanning calorimeter. The maximum EC temperature change, ΔTEC-max = 0.63 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.11 at ∼44 °C around the multiphase coexistence region. We observed that the EC effect also peaks at transitions between ferroelectric phases of different symmetries. Comparison with the results of indirect EC measurements from our previous work shows that the indirect approach provides reasonable estimations of the magnitude of the largest EC temperature changes and EC strength. However, it fails to describe correctly temperature dependences of the EC effect for the compositions showing relaxor-like behaviour (x = 0.14 and 0.15) because of their non-ergodic nature. Our study provides strong evidence supporting that looking for multiphase ferroelectric materials can be very useful to optimize EC performance. © 2017 Author(s).

One of the methods to enhance the functional properties of two-phase multiferroic magnetoelectrics is to increase magnetostriction of the ferrite phase. Al3+-modified cobalt ferrite Co(Al0.5Fe1.5)O4 shows better magnetostriction than unmodified cobalt ferrite. It is used in combination with (Ba,Ca)(Zr,Ti)O3 which has very good piezoelectric properties, to form a multiferroic composite. The composite shows good magnetoelectric characteristics, both macroscopically (converse magnetoelectric coefficient of 11 ps/m) and microscopically. Al3+ proves to be the best non-magnetic dopant to enhance magnetostriction in CoFe2O4 and thus the magnetoelectric coefficient. © 2017 Springer Science+Business Media, LLC

We study the electrocaloric (EC) effect in bulk BaTiO3 (BTO) using molecular dynamics simulations of a first principles-based effective Hamiltonian, combined with direct measurements of the adiabatic EC temperature change in BTO single crystals. We examine in particular the dependence of the EC effect on the direction of the applied electric field at all three ferroelectric transitions, and we show that the EC response is strongly anisotropic. Most strikingly, an inverse caloric effect, i.e., a temperature increase under field removal, can be observed at both ferroelectric-ferroelectric transitions for certain orientations of the applied field. Using the generalized Clausius-Clapeyron equation, we show that the inverse effect occurs exactly for those cases where the field orientation favors the higher temperature/higher entropy phase. Our simulations show that temperature changes of around 1 K can, in principle, be obtained at the tetragonal-orthorhombic transition close to room temperature, even for small applied fields, provided that the applied field is strong enough to drive the system across the first-order transition line. Our direct EC measurements for BTO single crystals at the cubic-tetragonal and at the tetragonal-orthorhombic transitions are in good qualitative agreement with our theoretical predictions, and in particular confirm the occurrence of an inverse EC effect at the tetragonal-orthorhombic transition for electric fields applied along the [001] pseudocubic direction. © 2017 American Physical Society.

Magnetic properties of polycrystalline multiferroic Bi0.65La0.35Fe0.5Sc0.5O3 synthesized under high-pressure (6 GPa) and high-temperature (1500 K) conditions were studied using a SQUID magnetometer technique. The temperature dependent static magnetic moment M was measured in both zero-field-cooled and field-cooled modes over the temperature range of 5-300 K in low magnetic field H = 0.02 kOe. The field dependent magnetization M(H) was measured in magnetic fields up to 50 kOe at different temperatures up to 230 K after zero-field cooling procedure. A long-range magnetic ordering of the antiferromagnetic type with a weak ferromagnetic contribution takes place below TN ≈ 220 K. Magnetic hysteresis loops taken below TN show a huge coercive field up to Hc ≈ 10 kOe, while the magnetic moment does not saturate up to 50 kOe. A strong effect of magnetic field on the magnetic properties of the compound has been found. Below TN ≈ 220 K the derivatives of the initial magnetization curves demonstrate the existence of a temperature-dependent anomaly in fields of H = 15÷25 kOe. The nature of the anomaly is unknown and requires additional study.

We present a systematic study on the relation of the electrocaloric effect (ECE) and the relaxor state transition of BaZrxTi1- xO3 (BZT) using a combination of computer simulation and experiment. The results of canonical and microcanonical lattice-based Monte Carlo simulations with a Ginzburg-Landau-type Hamiltonian are compared with measurements of BaZrxTi1- xO3 (x = 0.12 and 0.2) samples. In particular, we study the ECE at various temperatures, domain patterns by piezoresponse force microscopy at room temperature, and the P-E loops at various temperatures. We find three distinct regimes depending on the Zr-concentration. In the compositional range 0≤x≤0.2, ferroelectric domains are visible, but the ECE peak drops considerably with increasing Zr-concentration. In the range 0.3≤x≤0.7, relaxor features become prominent, and the decrease in the ECE with Zr-concentration is moderate. In the range of high concentrations, x≥0.8, the material is almost nonpolar, and there is no ECE peak visible. Our results reveal that BZT with a Zr-concentration around x=0.12∼0.3 exhibits a relatively large ECE in a wide temperature range at rather low temperature. © 2017 Author(s).

The stability of ferroelectric domain patterns at the nanoscale has been a topic of much interest for many years. We investigated the relaxation of the polarized state created by application of a local electric field using a conductive tip of a scanning probe microscope for the model uniaxial relaxor system SrxBa(1-x)Nb(2)O(6) (SBN) in its pure and Ce-doped form. The temporal relaxation of the induced PFM contrast was measured at various temperatures. The average value of the induced contrast decreases during heating for all investigated crystals. Below the freezing temperature the induced state remains stable after an initial relaxation. Above the freezing temperature the induced state is unstable and gradually decays with time. The stability of the induced state is strongly affected by the measuring conditions, so continuous scanning results in a faster decay of the poled domain. The obtained effects are attributed to a decrease of the induced polarization and backswitching of the polarized area under the action of the depolarization field.

The orthorhombically distorted perovskite NaMnF3 has been predicted to become ferroelectric if an a = c distortion of the bulk Pnma structure is imposed. In order to test this prediction, NaMnF3 thin films were grown on SrTiO3 (001) single crystal substrates via molecular beam epitaxy. The best films were smooth and single phase with four different twin domains. In-plane magnetization measurements revealed the presence of antiferromagnetic ordering with weak ferromagnetism below the Néel temperature TN = 66 K. For the dielectric studies, NaMnF3 films were grown on a 30 nm SrRuO3 (001) layer used as a bottom electrode grown via pulsed laser deposition. The complex permittivity as a function of frequency indicated a strong Debye-like relaxation contribution characterized by a distribution of relaxation times. A power-law divergence of the characteristic relaxation time revealed an order-disorder phase transition at 8 K. The slow relaxation dynamics indicated the formation of super-dipoles (superparaelectric moments) that extend over several unit cells, similar to polar nanoregions of relaxor ferroelectrics. © 2017 Author(s).

The lead-free ferroelectric 0.5Ba(Zr 0.2 Ti 0.8)O 3 - 0.5(Ba 0.7 Ca 0.3)TiO 3 (BCZT) is a promising component for multifunctional multiferroics due to its excellent room temperature piezoelectric properties. Having a composition close to the polymorphic phase boundary between the orthorhombic and tetragonal phases, it deserves a case study for analysis of its potential for modern electronics applications. To obtain magnetoelectric coupling, the piezoelectric phase needs to be combined with a suitable magnetostrictive phase. In the current article, we report on the synthesis, dielectric, magnetic, and magnetoelectric characterization of a new magnetoelectric multiferroic composite consisting of BCZT as a piezoelectric phase and CoFe 2 O 4 (CFO) as the magnetostrictive phase. We found that this material is multiferroic at room temperature and manifests a magnetoelectric effect larger than that of BaTiO 3 -CoFe 2 O 4 bulk composites with similar content of the ferrite phase.

In this paper, we report on studies of the electrocaloric (EC) effect in lead-free (1−x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 ceramics with compositions range between 0.32 ≤ x ≤ 0.45. The EC effect was measured directly using a modified differential scanning calorimeter. The maximum EC temperature change, ΔTdirect = 0.33 K under an electric field of 2 kV/mm, was observed for the composition with x = 0.32 at ~63°C. We found that the EC effect peaks not only around the Curie temperature but also at the transition between the ferroelectric phases with different symmetries. A strong discrepancy observed between the results of the direct measurements and indirect estimations points out that using Maxwell's equations is invalid for the thermodynamic nonequilibrium conditions that accompany only partial (incomplete) poling of ceramics. We also observe a nonlinearity of the EC effect above the Curie temperature and in the temperature range corresponding to the tetragonal ferroelectric phase. © 2016 The American Ceramic Society

Solid-state refrigeration using the electrocaloric effect (ECE) in ferroelectric materials is a promising alternative to the conventional vapor-compression technology. In spite of growing interest to the investigation of the ECE, direct measurements of the effect are still rare. In this paper, we report on a modification of a differential scanning calorimeter for direct ECE measurements. The importance of proper estimation of the thermal correction factor and use of proper values of the heat capacitance for correct ECE measurements is discussed. The ECE measurements were performed for Ba(Zr0.2Ti0.8)O3 and Ba(Zr0.12Ti0.88)O3 bulk ceramics. Large electrocaloric temperature changes of 0.54 and 0.34 K are achieved under the application of an electric field of 2 kV/mm for the Ba(Zr0.12Ti0.88)O3 and Ba(Zr0.2Ti0.8)O3 samples, respectively. The relation between the directly measured ECE values and frequently used indirect estimation based on Maxwell's relations is discussed. © 2016 IEEE.

Relaxor/ferroelectric ceramic/ceramic composites have shown to be promising in generating large electromechanical strain at moderate electric fields. Nonetheless, the mechanisms of polarization and strain coupling between grains of different nature in the composites remain unclear. To rationalize the coupling mechanisms we performed advanced piezoresponse force microscopy (PFM) studies of 0.92BNT-0.06BT-0.02KNN/0.93BNT-0.07BT (ergodic/non-ergodic relaxor) composites. PFM is able to distinguish grains of different phases by characteristic domain patterns. Polarization switching has been probed locally, on a sub-grain scale. k-Means clustering analysis applied to arrays of local hysteresis loops reveals variations of polarization switching characteristics between the ergodic and non-ergodic relaxor grains. We report a different set of switching parameters for grains in the composites as opposed to the pure phase samples. Our results confirm ceramic/ceramic composites to be a viable approach to tailor the piezoelectric properties and optimize the macroscopic electromechanical characteristics. © The Royal Society of Chemistry 2016.

We report on a systematic study of the magnetoelectric effect in cobalt ferrite (CoFe2O4)—barium titanate (BaTiO3) ceramic composites with (0-3) connectivity. Both the converse magnetoelectric coefficient, αC, and the direct voltage magnetoelectric coefficient, αE, were measured in dependence on composition and electric and magnetic bias fields. The strongest ME effect was observed in the composition (1−x) CoFe2O4–xBaTiO3 with x = 0.5 yielding values αC = 25 psm−1 and αE = 3.2 mV/(cm·Oe). We show that the proper conversion between these two coefficients demands knowledge about the dielectric permittivity of the sample. For low BaTiO3 content the dielectric coefficient of the composite yields a better correspondence, whereas for high BaTiO3 content the sample's average dielectric coefficient yields a better match. The influence of mutual orientation of polarization and magnetization on the ME effect is addressed. We found that for measurements performed parallel to the polarization direction (longitudinal effect), the ME coefficient is approximately twice as large and of opposite sign in comparison to the measurements perpendicular to the polarization direction (transverse effect). This difference has been rationalized in terms of the different contributions of the material coefficient tensor components to the ME effect, the demagnetizing factor, and losses. The obtained results provide a better understanding of peculiarities of the ME effect in bulk ceramic composites. © 2016 The American Ceramic Society

The domain structure of lithium-niobate thin films grown on Si(111) and Si(100) substrates coated with a native oxide layer with a thickness of no less than 2 nm is investigated by X-ray diffraction, scanning electron microscopy and piezoresponse force microscopy. The films are synthesized by the rf magnetron sputtering of a single-crystal lithium-niobate target. A high degree of grain orientation in the polycrystalline films is demonstrated. The piezoelectric coefficients dzz of the lithium-niobate films on Si(111) and Si(100) substrates are calculated from the measured dependences of the amplitude of the piezoresponse signal on the ac voltage applied between the cantilever tip and the substrate. Piezoelectric hysteresis loops are obtained in the remanent piezoelectric response regime © 2016, Pleiades Publishing, Ltd.

Polycrystalline lanthanum lead zirconate titanate (PLZT) thin films were deposited on Pt/TiO2/SiO2/Si substrates to study the effects of the thickness and grain size on their structural and piezoresponse properties at nanoscale. Thinner PLZT films show a slight (100)-orientation tendency that tends to random orientation for the thicker film, while microstrain and crystallite size increases almost linearly with increasing thickness. Piezoresponse force microscopy and autocorrelation function technique were used to demonstrate the existence of local self-polarization effect and to study the thickness dependence of correlation length. The obtained results ruled out the bulk mechanisms and suggest that Schottky barriers near the film-substrate are likely responsible for a build-in electric field in the films. Larger correlation length evidence that this build-in field increases the number of coexisting polarization directions in larger grains leading to an alignment of macrodomains in thinner films. © 2016 Author(s).

In this work we report on the ultrasonic and dielectric properties of composites consisting of conventional piezoelectric polymer, polyvinylidene fluoride/trifluoroethylene (PVDF-TrFE) copolymer of composition 70/30 mol. %, and (Pb0.75Ba0.24Sr0.01)(Zr0.53Ti0.47)O3 (BPZT) particles. BPZT fillers concentration was varied from 10 to 50 vol.%. Investigation of ultrasonic wave attenuation, velocity and piezoelectric voltage in these composites was performed over a temperature range from 300 K to 420 K using an automatic ultrasonic pulse-echo method. The temperature dependences of ultrasonic velocity and attenuation showed anomalies attributed to the glass transition temperature Tg and Curie temperature Tc of the polymer matrix. © 2016 Elsevier Ltd. All rights reserved.

Theelectrocaloric effect (ECE) in ferroelectric materials is a promising mechanism for the development of small, effective, low cost, and environmentally friendly solid state refrigerators. During the last decade, an increased interest has been paid to studies of this effect. Getting reliable values requires direct measurements of the ECE instead of the frequently used indirect estimates based on Maxwell's relation. In this paper, we report on the modification of a differential scanning calorimeter for direct ECE measurements. The importance of proper estimation of the heat capacitance and thermal losses for the correct ECE measurements is discussed. The ECE was measured for bulk ceramics Ba(Zr0.2Ti0.8)O3. The temperature change reaches a value of ΔTEC= 0.44 K at 305 K under an electric field of 2 kV/mm. The obtained data are compared with results of the evaluation of the indirect ECE. © 2015 IEEE.

In this work, pyroelectric properties of composite films on the basis of poly(vinylidene fluoride-trifluoroethylene) copolymer with a various level of ferroelectric ceramics inclusions of barium lead zirconate titanate solid solution were investigated by the dynamic method. The composite films were prepared by the solvent cast method. The unusual spike-like dynamic response with a quasi-stationary component was observed. It is supposed that composite films may be effectively used for pyroelectric applications. © 2015, Springer-Verlag Berlin Heidelberg.

This paper presents an extract of modifications of lead magnesium niobate-lead titanate in order to optimize its electrocaloric properties. Electrocaloric entropy (δS<inf>EC</inf>) and temperature changes (δT<inf>EC</inf>) are measured in a temperature range from -5°C up to 100°C using differential scanning calorimetry. The influence of dopants (Li+, Mn2+, Sr2+, Ta5+) on the electrocaloric effect as well as on microstructure, dielectric and ferroelectric behavior is investigated. The maximum of relative permittivity can be shifted to lower or higher temperatures depending on the added dopant and decreased ferroelectric hysteresis losses were observed for all altered compositions. For the undoped PMN-8PT ceramics maximum δT<inf>EC</inf> of 0.58K and δS<inf>EC</inf> of 0.51Jkg-1K-1 were measured, when applying an electric field of 2kVmm-1 at 30°C. The electrocaloric temperature change showed lower values for all doped PMN-8PT. Remarkably, this is accompanied with an increase of the entropy change for the Li-doped sample. © 2015 Elsevier Ltd.

Exchange bias phenomenon, evident of antiferromagnetic-ferromagnetic phase segregation state, has been observed in (Nd1-xYx)2/3Ca1/3MnO3 (x = 0, 0.1) compounds at low temperatures. A contribution to the total magnetization of the compounds due to the ferromagnetic phase has been evaluated. It has been found that yttrium doping leads to the growth of the ferromagnetic phase fraction. The ferromagnetic phase in the doped compound has a lower coercivity Hc and more rectangular form of the hysteresis loop. The values of the exchange bias field HEB and coercivity are found to be strongly dependent on the cooling magnetic field Hcool. In sufficiently high magnetic fields, Hcool &gt; 5 kOe, HEB in the doped compound is about twice as low as in the parent compound. This difference is attributed to a lower exchange interaction and higher saturation magnetization of the ferromagnetic phase in (Nd0.9Y0.1)2/3Ca1/3MnO3. © 2015 AIP Publishing LLC.

A study on magnetoelectric phenomena in the barium titanate-barium hexaferrite (BaTiO3-BaFe12O19) composite system, using high resolution techniques including switching spectroscopy piezoresponse force microscopy (SSPFM) and spatially resolved confocal Raman microscopy (CRM), is presented. It is found that both the local piezoelectric coefficient and polarization switching parameters change on the application of an external magnetic field. The latter effect is rationalized by the influence of magnetostrictive stress on the domain dynamics. Processing of the Raman spectral data using principal component analysis (PCA) and self-modelling curve resolution (SMCR) allowed us to achieve high resolution phase distribution maps along with separation of average and localized spectral components. A significant effect of the magnetic field on the Raman spectra of the BaTiO3 phase has been revealed. The observed changes are comparable with the classical pressure dependent studies on BaTiO3, confirming the strain mediated character of the magnetoelectric coupling in the studied composites. This journal is © The Royal Society of Chemistry.

Abstract We report on the effect of an applied static magnetic field on the dielectric properties of ferroelectric relaxor/ferrimagnetic composites consisting of [Ba(Sn<inf>0.3</inf>Ti<inf>0.7</inf>)O<inf>3</inf>]<inf>0.8</inf>-[CoFe<inf>2</inf>O<inf>4</inf>]<inf>0.2</inf> (BST<inf>0.8</inf>-CFO<inf>0.2</inf>). The pure Ba(Sn<inf>0.3</inf>Ti<inf>0.7</inf>)O<inf>3</inf> (BST30) as well as the composites, were synthesized by solid state reaction method. The X-ray diffraction analysis confirmed that BST30 and CFO coexist in the composite without any secondary phase. The real and the imaginary part of the dielectric permittivity were studied as a function of temperature, with and without static magnetic field, respectively. Relaxor characteristics such as dielectric permittivity and its peak temperature are observed to vary with the magnetic field. This is explained in the context that the applied magnetic field creates magnetostriction in the ferrite phase which is transferred to the relaxor phase via the interface coupling. The strain in the relaxor phase results in the reorientation of local polarization entities, polar nano regions (PNRs), which alters the dielectric characteristics of the sample. This effect is explained in relation to local order parameter q(T) which is found to increase in a certain temperature range above the typical ferroelectric temperature regime with the application of magnetic field. © 2015 Elsevier B.V. All rights reserved.

In this article we report on the synthesis and multiferroic properties of cobalt ferrite (CoFe2O4)-barium titanate (BaTiO3) biphasic composites. The initial composite nanopowder was synthesized by a combination of co-precipitation and organosol methods. A ceramic sample with (3-0) connectivity, i.e. BaTiO3 grains in a CoFe2O4 matrix was obtained by a combination of spark plasma sintering and annealing. In order to understand the correlations between morphology, electric properties, and magnetization, we present a detailed study at different preparation steps and compare it to the properties of a conventionally sintered sample with the traditional (0-3) connectivity, i.e. CoFe2O4 grains in a BaTiO3 matrix. We observe that the (3-0) sample shows improved magnetic properties in comparison to the conventionally sintered composite of the same composition. In spite of relatively large leakage current for the (3-0) sample compared to the traditional (0-3) one, it exhibits a converse magnetoelectric effect that follows the Hdc dependence of the piezomagnetic coefficient. The magnetic field-dependence of electric polarization at the surface was investigated utilizing X-ray absorption spectroscopy and its associated linear and circular dichroisms. © 2015 Acta Materialia Inc.

Magnetoelectric coupling is the material based coupling between electric and magnetic fields without recurrence to electrodynamics. It can arise in intrinsic multiferroics as well as in composites. Intrinsic multiferroics rely on atomistic coupling mechanisms, or coupled crystallographic order parameters, and even more complex mechanisms. They typically require operating temperatures much below T = 0C in order to exhibit their coupling effects. Room temperature applications are thus excluded. Consequently, composites have been designed to circumvent this limitation. They rely on field coupling between magnetostrictive and piezoelectric materials or in more advanced scenarios on quantum coupling in between both phases. This overview will describe experimental techniques and their particular limitations in accessing these coupling phenomena at different scales. Strain coupling is the dominant coupling mechanism at the macroscale as well as down to the micrometer. At the nanoscale more subtle effects can arise and some care has to be taken when investigating local coupling at interfaces using scanning probe techniques, e. g. due to semiconductor effects, field screening, or gradient and surface effects. At the smallest length scale atomic or molecular coupling can be tested using X-ray dichroism or probe atoms like 57Fe in Mössbauer spectroscopy. We display a selection of measuring techniques at the different scales and outline possible pitfalls for experimentalists as well as theoreticians when using material parameters extracted from such experimental work. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Tailoring of ferroelectric properties of copolymer P(VDF-TrFE) by incorporation of ceramic inclusions in the polymer matrix is promising for advanced applications in sensorics. We have observed experimentally that in composites of P(VDF-TrFE) with barium-doped lead zirconate titanate (BPZT), the remanent polarization increases, while the coercive field substantially decreases in comparison with the pure polymer samples. Results of simulation in framework of the modified Weiss model have shown that the changes of the hysteresis loops characteristics are due to increase of the dielectric susceptibility of the composite as compared to pure PVDF-TrFE. This originates from the strong dispersion of the mean field constant α, which describes the feedback of the polarization on the electric field at the location of the dipoles and is closely related with the local increase of composite susceptibility in the vicinity the BPZT inclusions. This phenomenon effectively becomes macro-scale due to the long-range nature of the inhomogeneous elastic and electric fields occurring at the interfaces between the matrix and inclusions. © 2015 AIP Publishing LLC.

BiFeO<inf>3</inf> (BFO) is a classical multiferroic material with both ferroelectric and magnetic ordering at room temperature. Doping of this material with rare-earth oxides was found to be an efficient way to enhance the otherwise low piezoelectric response of unmodified BFO ceramics. In this work, we studied two types of bulk Sm-modified BFO ceramics with compositions close to the morphotropic phase boundary (MPB) prepared by different solid-state processing methods. In both samples, coexistence of polar R3c and antipolar P<inf>bam</inf> phases was detected by conventional X-ray diffraction (XRD); the non-polar P<inf>nma</inf> or P<inf>bnm</inf> phase also has potential to be present due to the compositional proximity to the polar-to-non-polar phase boundary. Two approaches to separate the phases based on the piezoresponse force microscopy measurements have been proposed. The obtained fractions of the polar and non-polar/anti-polar phases were close to those determined by quantitative XRD analysis. The results thus reveal a useful method for quantitative determination of the phase composition in multi-phase ceramic systems, including the technologically most important MPB systems. © 2015 AIP Publishing LLC.

Solid solutions of (1-x)Ba(Zr0.2Ti0.8)O3-x(Ba0.7Ca0.3)TiO3 promise to exhibit a large electrocaloric effect (ECE), because their Curie temperature and a multiphase coexistence region lie near room temperature. We report on direct measurements of the electrocaloric effect in bulk ceramics 0.65Ba(Zr0.2Ti0.8)O3-0.35(Ba0.7Ca0.3)TiO3 using a modified differential scanning calorimeter. The adiabatic temperature change reaches a value of ΔTEC = 0.33 K at ∼65 °C under an electric field of 20 kV/cm. It remains sizeable in a broad temperature interval above this temperature. Direct measurements of the ECE proved that the temperature change exceeds the indirect estimates derived from Maxwell relations by about ∼50%. The discrepancy is attributed to the relaxor character of this material. © 2015 AIP Publishing LLC.

High piezoelectric activity of many ferroelectrics has been the focus of numerous recent studies. The structural origin of this activity remains poorly understood due to a lack of appropriate experimental techniques and mixing of different mechanisms related to ferroelectricity and ferroelasticity. Our work reports on the study of a uniaxial Sr0.5Ba0.5Nb2O6 ferroelectric where the formation of regions with different spontaneous strains is ruled out by the symmetry and where the interrelation between piezoelectricity and ferroelectricity can be inspected in an isolated fashion. We performed x-ray diffraction experiments on a single crystalline sample under alternating electric field and observed an unknown hidden-in-the-bulk mechanism, which suggests that the highest piezoelectric activity is realized in the volumes where nucleation of small ferroelectric domains takes place. This new mechanism creates a novel roadmap for designing materials with enhanced piezoelectric properties. © 2015 American Physical Society.

The effect of heterovalent B-site doping on ergodicity of relaxor ferroelectrics is studied using (1 - y)(0.81Bi1/2Na 1/2TiO3-0.19Bi1/2K1/2TiO 3)-yBiZn1/2Ti1/2O3 (BNT-BKT-BZT) with y = {0.02;0.03;0.04} as a model system. Both the large- and small-signal parameters are studied as a function of electric field. The crystal structure is assessed by means of neutron diffraction in the initial state and after exposure to a high electric field. In order to measure ferroelastic domain textures, diffraction patterns of the poled samples are collected as a function of sample rotation angle. Piezoresponse force microscopy (PFM) is employed to probe the microstructure for polar regions at a nanoscopic scale. For low electric fields E < 2 kV·mm-1, large- and small-signal constitutive behavior do not change with composition. At high electric fields, however, drastic differences are observed due to a field-induced phase transition into a long-range ordered state. It is hypothesized that increasing BZT content decreases the degree of non-ergodicity; thus, the formation of long-range order is impeded. It is suggested that frozen and dynamic polar nano regions exist to a different degree, depending on the BZT content. This image is supported by PFM measurements. Moreover, PFM measurements suggest that the relaxation mechanism after removal of the bias field is influenced by surface charges. © 2014 AIP Publishing LLC.

The stability of the field-induced ferroelectric (FE) state was studied in relaxor lead-free ceramics (1 - y)[0.81Bi1/2Na1/2TiO3-0.19Bi1/2K1/2TiO3]-yBiZn1/2Ti1/2O3 both macroscopically and microscopically. A strong dc electric field results in the formation of a stable FE state with a large piezoelectric coefficient for compositions with a small amount of Bi(Zn1/2Ti1/2)O3, which are in the non-ergodic relaxor state at room temperature. Increasing temperature promotes ergodic relaxor behavior, which is accompanied by the rapid destabilization of the induced state, that is, small relaxation times. Based on the obtained data, it is proposed that the depolarization is a two-step process consisting of an initial realignment of the FE domains and their subsequent breakup into polar nanoregions. The ergodic relaxor behavior is also promoted by increasing the Bi(Zn1/2Ti1/2)O3 content. The related charge disorder results in an enhancement of random electric fields and consequently a stable FE state cannot be induced even at room temperature. © 2014 The American Ceramic Society.

Dense nanocrystalline barium strontium titanate Ba0.6Sr 0.4TiO3 (BST) ceramics with an average grain size around 40 nm and very small dispersion were obtained by spark plasma sintering at 950°C and 1050°C starting from nonagglomerated nanopowders (~20 nm). The powders were synthesized by a modified "Organosol" process. X-ray diffraction (XRD) and dielectric measurements in the temperature range 173-313 K were used to investigate the evolution of crystal structure and the ferroelectric to paraelectric phase transformation behavior for the sintered BST ceramics with different grain sizes. The Curie temperature TC decreases, whereas the phase transition becomes diffuse for the particle size decreasing from about 190 to 40 nm with matching XRD and permittivity data. Even the ceramics with an average grain size as small as 40 nm show the transition into the ferroelectric state. The dielectric permittivity ε shows relatively good thermal stability over a wide temperature range. The dielectric losses are smaller than 2%-4% in the frequency range of 100 Hz-1 MHz and temperature interval 160-320 K. A decrease in the dielectric permittivity in nanocrystalline ceramics was observed compared to submicrometer-sized ceramics. © 2014 The American Ceramic Society.

We report on temperature dependence of local electromechanical properties of lead-free (K,Na)NbO3-based ceramics that macroscopically manifests a large temperature-insensitive strain. Piezoresponse force microscopy reveals the particular role of the orthorhombic-tetragonal phase transition, where a reconstruction of the domain structure occurs and local piezoelectric response shows a peak value. A good quantitative agreement between temperature dependences of the local and previously reported macroscopic small-signal piezoelectric coefficients is observed. An influence of the polymorphic phase transition on polarization switching kinetics was revealed. © 2014 AIP Publishing LLC.

The ferroelectric and magnetic behaviour of multiferroic BiFeO3 nanoparticles has been studied using piezoresponse force microscopy (PFM), Mössbauer spectroscopy and SQUID magnetometry. The results of the PFM studies indicate a decay of the spontaneous polarization with decreasing particle size. Nevertheless, particles with diameter ∼50 nm still manifest ferroelectric behaviour. At the same time these particles are weakly ferromagnetic. The Mössbauer spectroscopy studies prove that the weak ferromagnetic state is due to non-compensated surface spins rather than distortions of the cycloidal spin structure characteristic for bulk BiFeO 3. © 2013 IOP Publishing Ltd.

The local piezoelectric properties of ferroelectric composites consisting of P(VDF-TrFE) copolymer matrix with barium lead zirconate titanate ceramic inclusions were addressed both experimentally using piezoresponse force microscopy technique and theoretically applying the Landau-Ginzburg-Devonshire formalism. A transient region with a width of approximately 40 nm has been found at the interface between the two constituents. It is shown that the piezoresponse in the vicinity of the interface is strongly affected by inhomogeneous stresses originating from an incompatibility of thermal expansion coefficients of PVDF and lead zirconate titanate. © 2013 AIP Publishing LLC.

The temperature and bias field dependences of macroscopic, measured by pulsating load method, and local, measured by piezoresponse force microscopy, longitudinal piezoelectric responses have been studied in (001)-oriented flux-grown (1 - x)Pb(Mg1/3Nb2/3)O3-xPbTiO 3 (0.0 x 0.29) single crystals. Both types of responses exhibit a dramatic enhancement with increasing bias fields. At the same time, their temperature maxima shift from the Vogel-Fulcher temperature to the vicinity of the dielectric permittivity maximum, where the critical point in the E-T phase diagram is located. Both datasets confirm a quasicritical nature of the giant field-induced piezoelectric response in relaxor single crystals. © 2013 AIP Publishing LLC.

In this paper we report on a new approach to synthesize core/shell cobalt iron oxide/barium titanate composite nanoparticles combining the co-precipitation and organosol crystallization techniques. The weight fraction of CoFe2O4 and BaTiO3 was 20% and 80% respectively. The obtained core/shell powder was used to sinter (0-3) composite multiferroic ceramics. Ferroelectric, magnetic, and magnetoelectric properties of the ceramics were studied. It was found that the value of the converse magnetoelectric coefficient, c, reaches 4.4·10-12 s·m-1 at the magnetic field μ 0 H dc = 0.15 T and T = 285 K. © 2013 Copyright Taylor and Francis Group, LLC.

Ferroelectric relaxors continue to be one of the most mysterious solid-state materials. Since their discovery by Smolenskii and coworkers, there have been many attempts to understand the properties of these materials, which are exotic, yet useful for applications. On the basis of the numerous experimental data, several theories have been established, but none of them can explain all the properties of relaxors. The recent advent of piezoresponse force microscopy (PFM) has allowed for polarization mapping on the surface of relaxors with subnanometer resolution. This development thus leads to the question of whether the polar nanoregions that contribute to diffuse X-ray scattering and a range of macroscopic properties can be visualized. This review summarizes recent advancements in the application of PFM to a number of ferroelectric relaxors and provides a tentative explanation of the peculiar polarization distributions related to the intriguing physical phenomena in these materials. © Copyright © 2013 by Annual Reviews. All rights reserved.

The development of lead-free piezoceramics has attracted great interest because of growing environmental concerns. A polymorphic phase transition (PPT) has been utilized in the past to tailor piezoelectric properties in lead-free (K,Na)NbO3 (KNN)-based materials accepting the drawback of large temperature sensitivity. Here a material concept is reported, which yields an average piezoelectric coefficientd33 of about 300 pC/N and a high level of unipolar strain up to 0.16% at room temperature. Most intriguingly, field-induced strain varies less than 10% from room temperature to 175 °C. The temperature insensitivity of field-induced strain is rationalized using an electrostrictive coupling to polarization amplitude while the temperature-dependent piezoelectric coefficient is discussed using localized piezoresponse probed by piezoforce microscopy. This discovery opens a new development window for temperature-insensitive piezoelectric actuators despite the presence of a polymorphic phase transition around room temperature. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In this work, we report on local ferroelectric and piezoelectric properties of nanostructured polymer composites P(VDF-TrFE)+x(Ba,Pb)(Zr,Ti)O3 (x = 0-50%). High-resolution imaging of ferroelectric domains, local polarization switching, and polarization relaxation dynamics were studied by piezoresponse force microscopy. In particular, we found that (Ba,Pb)(Zr,Ti)O3 inclusions usually show a strong unipolar piezoresponse signal, as compared to the polymer matrix. By scanning under high dc voltage the films can be polarized uniformly under both positive and negative electric fields. Stability of the polarized state is discussed. © 2013 Materials Research Society.

Cobalt ferrite/barium titanate nanoparticles with a core/shell structure were synthesized by combining co-precipitation and organosol methods. The average particle size was about 110 nm with an average shell thickness of about 40 nm. Dielectric and magnetic properties of the particles were studied using impedance and Mössbauer spectroscopy, respectively. The particles are promising for fabrication of multiferroic ceramics with the core-shell structure. Copyright © Taylor & Francis Group, LLC.

In the relaxor ferroelectric Sr 0.8Ba 0.2Nb 2O 6 (SBN80) the relaxation of polar nanoregions (PNRs) reveals an anisotropic breathing mode when approaching the transition temperature, T C ≈ 289 K, on cooling. Two distinct dispersion steps and peaks, respectively, of the complex susceptibility components in the milli- and megahertz frequency range reflect their large length-to-width aspect ratio. Below T C power-law frequency dispersion characterizes the creep motion of the emerging ferroelectric domain walls and the fractal dimension D ≈ 2 of their contour lines. Piezoforce micrographs reveal nanometric jagged domains in accordance with the large disorder and random field magnitude in the extreme relaxor SBN80. © 2012 American Institute of Physics.

Feature size is a natural determinant of material properties. Its design offers the technological perspectives for material improvement. Grain size, crystallite size, domain width, and structural defects of different nature constitute the classical design elements. Common ferroelectric ceramics contain micrometer grain sizes and submicrometer domain widths. Domain wall mobility is a major contribution to their macroscopic material properties providing approximately half of the macroscopic output in optimized materials. The extension into the dynamic nanoworld is provided by relaxor ferroelectrics. Ionic and nanoscale field disorders form the base to a state with natural nanometer-size polar structures even in bulk materials. These polar structures are highly mobile and can dynamically change over several orders of magnitude in time and space being extremely sensitive to external stimuli. This yields among the largest dielectric and piezoelectric constants known. In this feature article, we want to outline how lead-free relaxors will offer a route to an environmentally safer option in this outstanding material class. Properties of uniaxial, planar, and volumetric relaxor compositions will be discussed. They provide a broader and more interesting scope of physical properties and features than the classical lead-containing relaxor compositions. © 2011 The American Ceramic Society.

The "organosol" precipitation method is proposed to produce nanosized particles of barium titanate (BaTiO 3) at temperatures as low as room temperature. The advantages of this method are a high yield, a simple but precise control of the size of the particles, low process temperature, short reaction time, as well as low cost of reagents. The particles were systematically characterized by powder X-ray diffraction (XRD), Raman scattering, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), thermogravimetric thermal analysis (TGA/DSC), infrared spectroscopy (IR), and impedance analysis. The as-prepared BaTiO 3 nanocrystals exhibit a granular shape of around 15 nm in diameter. Oleic acid retards crystallization and thus allows generation of a uniformly small grain size and excellent dispersibility in organic solvents. The surface energy of the particles is modified and crystallization in cubes also arises. The mechanism of powder formation is discussed. The method offers an alternate low-cost route to perovskite nanopowders easily dispersed in organic media. © 2012 The Royal Society of Chemistry.

Polycrystalline Sr 0.98Mn 0.02TiO 3 thin films are prepared by sol-gel spin-coating method on Si/SiO 2/TiO 2/Pt substrates. Their dielectric permittivity, polarization, and magnetization are investigated as a function of temperature (from 10 to 300 K), electric and magnetic fields. Temperature dependences of the magnetization M ZFC-FH(T), real part of the dielectric permittivity (T), and dissipation factor tan(T) of Sr 0.98Mn 0.02TiO 3 films show anomalies around 41-45 K, implying an interrelation between polar and magnetic order. Butterfly-like dc bias dependences of the real part of the dielectric permittivity (E dc), as well as slim polarization and magnetization hysteresis loops are observed, suggesting that Sr 0.98Mn 0.02TiO 3 thin films belong to multiglass systems. © Taylor & Francis Group, LLC.

We have successfully synthesized CoFe 2O 4/BaTiO 3 composite nanoparticles with core-shell structure using a new organosol crystallization method. The weight fraction of the ferroelectric and the ferrimagnetic phase was 80% and 20%, respectively. The nanopowders were sintered to form a ceramic composite. The ceramics exhibits the magnetoelectric effect. The value of the converse magnetoelectric coefficient α c measured by a modified SQUID susceptometer reaches 4.4·10 -12 s·m -1 at the magnetic field μ 0H dc = 0.15 T and T = 285 K. © 2012 IEEE.

Multiferroic (MF) materials with simultaneous magnetic and electric long range order and occasionally, mutual magnetoelectric (ME) coupling, have recently attracted considerable interest. The small linear ME effect has been shown to control spintronic devices very efficiently, e.g. via the classic ME antiferromagnet Cr2O3 using exchange bias. Similar nano-engineering concepts exist also for type-I MF single phase materials, whose magnetic and polar orders have distinct origins like BiFeO3. Strong ME coupling occurs in type-II multiferroics, where ferroelectricity is due to spiral spin order as in TbMnO3. Record high ME response coming close to applicability arises in stress-strain coupled multiphase magnetoelectrics such as PZT/FeBSiC composites. Higher order ME response in disordered systems ("type-III multiferroics") extends the conventional MF scenario toward ME quantum paraelectric and multiglass materials with polarization-induced control of magnetic exchange, as e.g. in EuTiO3, Sr 0.98Mn0.02TiO3, and PbFe0.5Nb 0.5O38. © Owned by the authors, published by EDP Sciences, 2012.

Mixed Cu(In xCr 1-x)P 2S 6 crystals of ferrielectric CuInP 2S 6 and antiferroelectric CuCrP 2S 6 were investigated by means of broadband dielectric spectroscopy (10 mHz-3 GHz), for several values of x. The phase boundary between the ferrielectric phase and the dipolar glass state was found to lie between 0.5 and 0.7, and a similar boundary between the dipolar glass state and the antiferroelectric phase is observed to be located between 0.4 and 0.2. The dipole freezing results in a broad distribution of the relaxation times. The parameters of the double-well potentials, the local polarization distribution function, and glass order parameter have been extracted from the dielectric measurements. From these results the complete phase diagram has been constructed. © 2012 American Physical Society.

Multiferroic composites were prepared by covering CoFe2O 4 nanoparticles with a shell of BaTiO3 using a sol-gel technique. Scanning probe microscopy confirmed the formation of a core-shell structure with a magnetic core and a piezoelectric shell. The converse magnetoelectric effect was studied at different temperatures and bias fields. The magnetoelectric coefficient peaks at approximately 270K and reaches the value αC≈(2.2 0.1)10- 11sm- 1, which surpasses those reported previously for similar structures. A change of the sign of the magnetoelectric coefficient observed for an increasing magnetic bias field is related to the non-monotonic field dependence of magnetostriction in polycrystalline CoFe2O4. © 2011 IOP Publishing Ltd.

Dielectric and noise characteristics of CoFe 2O 4-BaTiO 3 ceramic composites were investigated. The complex dielectric permittivity displays a broad maximum around 300 K, where a phase transition occurs. The dielectric spectra of the complex dielectric permittivity are caused by the conductivity above 300 K. Intensive almost periodic telegraphic noise with relaxation type spectra is observed in the same temperature range (around 300 K). An investigation of the noise characteristic shows that several noise intensity peaks occur during this phase transition. Copyright © Taylor & Francis Group, LLC.

The lamellar antiferromagnet CuCr1-xInxP 2S6 (TN 32 K) rapidly loses magnetic percolation upon diamagnetically diluting the triangular Cu-Cr-P2 planar network with In3+ ions. For 0 ≤ x < 0.3 anti-ferroelectricity (Tc ≤ 150 K) and antiferromagnetic spin order (TN ≤ 32 K) coexist. Both orders are superposed by ferroic fluctuations. Dynamic polar clustering with glasslike polydispersive dielectric susceptibility emerges for x &gt; 0 at T ≤ Tc, while for x < 0.3 pseudocritical planar 2D ferromagnetic fluctuations are encountered. They give rise to Langevin-type magnetization saturation and large quasimolecular magnetic anisotropy. Third-order magnetoelectric coupling is observed in collinear field configuration. © 2011 American Physical Society.

The polarization state of relaxor PbMg1/3Nb2/3O 3 single crystals was studied using piezoresponse force microscopy. Areas of correlated polarization with a size of ∼50 nm were observed in the nominally ergodic relaxor state. These areas were attributed to ensembles of frozen polar nanoregions, which may exist in vicinity of the sample surface. Relatively strong piezoresponse could be induced by applying a dc electric field. After removing the field, a relaxation of the piezoelectric signal obeying power time dependence was observed. This induced piezoresponse was attributed to a contribution from dynamic polar nanoregions. © by Oldenbourg Wissenschaftsverlag, München.

Room temperature crystal structure, ferroelectric and magnetic properties of polycrystalline Bi0.85Sm0.15FeO3 samples were investigated. X-ray diffraction study shows that the compound possesses a dominant PbZrO3-like orthorhombic structure with √2a× 2√2a × √2a superlattice (a is the parameter of the cubic perovskite subcell). In contrast to piezoresponse force microscopy data demonstrating some features characteristic of ferroelectrics, polarization vs. electric field measurements reveal the behavior expected for nonpolar materials. Investigation of magnetic properties confirms that 15% samarium substitution suppresses the spin modulation typical of BiFeO3 and induces the appearance of spontaneous magnetization. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Nanopowders of cobalt iron oxide (CoFe2O4) were successfully fabricated by the co-precipitation method followed by a technique to prevent particle agglomeration. Particle sizes were in the range of 24 to 44 nm. The size of cobalt iron oxide particles decreases with increasing the concentration of the precipitation agent. The crystal structure was confirmed by X-ray diffraction (XRD), the chemical composition by energy dispersive spectroscopy (EDS), and phase changes by thermogravimetric differential thermal analysis (TGA-TDA). The particle morphology was analyzed by scanning electron microscopy (SEM). Magnetic properties were investigated by SQUID magnetometry and Mössbauer spectroscopy. Being nearly monodisperse and non-agglomerated the prepared cobalt iron oxide powders are the base for synthesizing magnetoelectric composites embedded in a ferroelectric BaTiO3 matrix. © 2012 Materials Research Society.

The coexistence of cluster glass with long-range antiferromagnetic order in the relaxor ferroelectric PbFe0.5Nb0.5O3 is elucidated. While the transition at TN=153K on the infinite antiferromagnetic cluster induces 3m symmetry with large EH2 magnetoelectric response, the disconnected subspace of isolated Fe3+ ions and finite clusters accommodates the cluster glass below Tg=10.6K with field-induced m′ symmetry and EH-type magnetoelectric response. Critical slowing-down, memory and rejuvenation after aging, occurrence of a de Almeida-Thouless phase line, and stretched exponential relaxation of remanence corroborate the glass nature. © 2010 The American Physical Society.

Investigation of crystal structure and multiferroic properties of polycrystalline Bi1-xSmxFeO3 (0.1 ≤ x ≤ 0.2) samples was performed by X-ray diffraction, piezoresponse force microscopy and SQUID-magnetometry techniques. It was shown that increasing samarium content induced a polar-to-nonpolar phase transition near x = 0.2. Within the polar region, a rhombohedral and two orthorhombic modifications of Bi1-xSmxFeO3 were found. It was shown that samarium substitution resulted in the appearance of spontaneous magnetization, which was significantly enhanced upon the composition-driven transition from a rhombohedral to an orthorhombic phase. © 2009 Acta Materialia Inc.

The spontaneous transition between the ferroelectric and relaxor states was investigated in 0.86PbMg1/3Nb2/3O3 -0.14 PbTiO3 ceramics using piezoresponse force microscopy (PFM). Macroscopically, the transition from the ferroelectric to relaxor phases manifests itself by an anomaly in the temperature dependences of the dielectric permittivity and by a sharp decline of the remanent polarization. Alternatively, PFM reveals a decay of the ferroelectric micron-size domains at the macroscopic Curie temperature, TC. Simultaneously, smaller domains of submicron sizes are observed at temperatures appreciably above TC, being concentrated near grain boundaries. It is argued that the particular mechanical and electrical conditions at the grain boundaries promote nucleation of the ferroelectric phase. © 2010 American Institute of Physics.

The third-order E2 H2 -type magnetoelectric (ME) response of polycrystalline EuTiO3 changes sign under magnetic bias and shows a large anomaly at the antiferromagnetic (AF)-paramagnetic phase boundary below TN ≈5.3 K. It is attributed to critical fluctuations of the AF order parameter reinforced by quantum paraelectric polar correlations. The underlying biquadratic spin-lattice coupling involves electric field induced Dzyaloshinskii-Moriya interaction as described within mean-field approximation. Single domaining by ME annealing (or cooling) significantly enhances the response by additional EH and E H2 effects. © 2010 The American Physical Society.

Magnetoelectric (ME) materials are of utmost interest in view of both fundamental understanding and novel desirable applications. Despite its smallness, the linear ME effect has been shown to control spintronic devices very efficiently, e.g., by using the classic ME antiferromagnet Cr 2O3. Similar nano-engineering concepts exist also for type-I multiferroic single phase materials like BiFeO3 and BiMnO 3. Record high ME response has been realized in stress-strain coupled multiphase magnetoelectrics like PZT/FeBSiC composites, enabling applications in sensors. In type-II multiferroics, whose ferroelectricity is due to modulated magnetic ordering, the ME coupling is of fundamental interest. Higher-order ME response characterizes disordered systems, which extend the conventional multiferroic scenario toward ME multiglass (e.g., Sr1-xMn xTiO3). © 2010 IEEE.

In ceramics of KTaO3 doped with 3 at. % of Mn the dielectric response is dominated by the polydispersive behavior of Mn2+ centered polar regions, whereas the magnetic and magnetoelectric (ME) behaviors reflect an intimate coupling between A -site substituted Mn2+ ions and minute amounts of Mn3 O4 precipitates mediated by the polar host material. This becomes apparent by the common onset at Tc ≈42 K of the ordering of ferrimagnetic Mn3 O4 and of a spin cluster glass, which is characterized by memory and rejuvenation effects. The composite magnetic system exposed to external magnetic and electric dc fields shows large third order ME susceptibility with a sharp anomaly at Tc and 1/ T2 dependence as T→0. © 2010 American Institute of Physics.

Film deposition of Ni2 MnGa phaselike alloy by radio frequency (rf) magnetron sputtering was performed onto bare Si(100) substrates and LaNi O3 /Pb (Ti,Zr) O3 (LNO/PZT) ferroelectric buffer layer near room temperature. The prepared samples were characterized using conventional x-ray diffraction (XRD), superconducting quantum interference device, and electron dispersive x-ray spectroscopy from scanning electron microscope observations. The optimized films deposited under high rf power and low argon pressure present good surface quality and highly textured phase crystallization. The positioning distance between the substrate and the target-holder axis has some limited effect on the film's composition due to the specific diffusion behavior of each element in the sputtering plasma. Extended four pole high resolution XRD analysis allowed one to discriminate the intended Ni-Mn-Ga tetragonal martensitic phase induced by the (100) LNO/PZT oriented buffer. This low temperature process appears to be very promising, allowing separate control of the functional layer's properties, while trying to achieve high electromagnetoelastic coupling. © 2010 American Vacuum Society.