Prof. Dr. Anna Grünebohm

Scalebridging Simulation of Functional Composites at ICAMS
Ruhr-Universität Bochum

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  • Multistep stochastic mechanism of polarization reversal in rhombohedral ferroelectrics
    Genenko, Y.A. and Khachaturyan, R. and Vorotiahin, I.S. and Schultheiß, J. and Daniels, J.E. and Grünebohm, A. and Koruza, J.
    Physical Review B 102 (2020)
    A stochastic model for the field-driven polarization reversal in rhombohedral ferroelectrics is developed, providing a description of their temporal electromechanical response. Application of the model to simultaneous measurements of polarization and strain kinetics in a rhombohedral Pb(Zr,Ti)O3 ceramic over a wide time window allows the identification of preferable switching paths, fractions of individual switching processes, and their activation fields. In conjunction, the phenomenological Landau-Ginzburg-Devonshire theory is used to analyze the impact of an external field and stress on switching barriers, showing that residual mechanical stress may promote the fast switching. © 2020 American Physical Society.
    view abstract10.1103/PhysRevB.102.064107
  • Temperature-independent giant dielectric response in transitional BaTiO3 thin films
    Everhardt, A.S. and Denneulin, T. and Grünebohm, A. and Shao, Y.-T. and Ondrejkovic, P. and Zhou, S. and Domingo, N. and Catalan, G. and Hlinka, J. and Zuo, J.-M. and Matzen, S. and Noheda, B.
    Applied Physics Reviews 7 (2020)
    Ferroelectric materials exhibit the largest dielectric permittivities and piezoelectric responses in nature, making them invaluable in applications from supercapacitors or sensors to actuators or electromechanical transducers. The origin of this behavior is their proximity to phase transitions. However, the largest possible responses are most often not utilized due to the impracticality of using temperature as a control parameter and to operate at phase transitions. This has motivated the design of solid solutions with morphotropic phase boundaries between different polar phases that are tuned by composition and that are weakly dependent on temperature. Thus far, the best piezoelectrics have been achieved in materials with intermediate (bridging or adaptive) phases. But so far, complex chemistry or an intricate microstructure has been required to achieve temperature-independent phase-transition boundaries. Here, we report such a temperature-independent bridging state in thin films of chemically simple BaTiO3. A coexistence among tetragonal, orthorhombic, and their bridging low-symmetry phases are shown to induce continuous vertical polarization rotation, which recreates a smear in-transition state and leads to a giant temperature-independent dielectric response. The current material contains a ferroelectric state that is distinct from those at morphotropic phase boundaries and cannot be considered as ferroelectric crystals. We believe that other materials can be engineered in a similar way to contain a ferroelectric state with gradual change of structure, forming a class of transitional ferroelectrics. Similar mechanisms could be utilized in other materials to design low-power ferroelectrics, piezoelectrics, dielectrics, or shape-memory alloys, as well as efficient electro- and magnetocalorics. © 2020 Author(s).
    view abstract10.1063/1.5122954
  • Tuning the magnetic anisotropy of niptmnga by substitution and epitaxial strain
    Herper, H.C. and Grunebohm, A.
    IEEE Transactions on Magnetics 55 (2019)
    Large magnetocrystalline anisotropy (MCA) is of high technical relevance, in particular for magnetic actuators, permanent magnets, and memory devices with high density. Large MCAs have been reported for the low temperature L10 phase of Ni2MnGa. Both, Mn and Pt substitution can stabilize this phase at and above room temperature. Despite the larger spin-orbit coupling in the heavy 5d-element Pt, it has been reported that Pt substitution may result in degeneration of the MCA. In this paper, we study the MCA for a combination of epitaxial strain and Mn and Pt substitution based on density functional theory methods. We show that one can stabilize both large uniaxial and easy-plane anisotropies depending on the value of strain. In particular, small changes of the applied strain may allow to switch between low- and high-anisotropy states or even switch the direction of the easy-axis magnetization direction. © 1965-2012 IEEE.
    view abstract10.1109/TMAG.2018.2856461
  • Origins of the Inverse Electrocaloric Effect
    Grünebohm, A. and Ma, Y.-B. and Marathe, M. and Xu, B.-X. and Albe, K. and Kalcher, C. and Meyer, K.-C. and Shvartsman, V.V. and Lupascu, D.C. and Ederer, C.
    Energy Technology 6 (2018)
    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.
    view abstract10.1002/ente.201800166
  • Tailoring the Electrocaloric Effect by Internal Bias Fields and Field Protocols
    Ma, Y.-B. and Xu, B.-X. and Albe, K. and Grünebohm, A.
    Physical Review Applied 10 (2018)
    Defect dipoles, strain gradients, and the electric boundary conditions at interfaces and surfaces often impose internal bias fields in acceptor-doped ferroelectrics, ferroelectric films, nanocomposites, and multilayers. In this work, we study the impact of internal bias fields on the electrocaloric effect (ECE), utilizing an analytical model and ab-initio-based molecular-dynamics simulations. We reveal the complex dependency of the ECE on field protocol and the relative strength of internal and external fields. The internal fields may even reverse the sign of the response (inverse or negative ECE). We explore the transition between conventional and inverse ECE and discuss reversible and irreversible contributions to the field-induced specific entropy change. Most importantly, we predict design routes to optimize the cooling and heating response for small external fields by the combination of internal field strengths and the field-loading protocol. © 2018 American Physical Society.
    view abstract10.1103/PhysRevApplied.10.024048
  • The Impact of Hysteresis on the Electrocaloric Effect at First-Order Phase Transitions
    Marathe, M. and Ederer, C. and Grünebohm, A.
    Physica Status Solidi (B) Basic Research 255 (2018)
    We study the impact of thermal hysteresis at the first-order structural/ferroelectric phase transitions on the electrocaloric response in bulk BaTiO3 by performing molecular dynamics simulations for a first-principles-based effective Hamiltonian. We demonstrate that the electrocaloric response can conceptually be separated in two contributions: a discontinuous transitional part, stemming from the jump in entropy at the first order phase transition, and a continuous part, due to the change of polarization and entropy within each phase. This latter part increases with the strength of the applied field, but for small fields it is very small. In contrast, we find a large temperature change of ≈1 K resulting from the transition entropy, which is essentially independent of the field strength. However, due to the coexistence region close to the first order phase transition, this large electrocaloric response depends on the thermal history of the sample and is generally not reversible. We show that this irreversibility can be overcome by using larger fields. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/pssb.201700308
  • Electrocaloric effect in BaTiO3 at all three ferroelectric transitions: Anisotropy and inverse caloric effects
    Marathe, M. and Renggli, D. and Sanlialp, M. and Karabasov, M.O. and Shvartsman, V.V. and Lupascu, D.C. and Grünebohm, A. and Ederer, C.
    Physical Review B 96 (2017)
    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.
    view abstract10.1103/PhysRevB.96.014102
  • First-principles-based calculation of the electrocaloric effect in BaTiO3: A comparison of direct and indirect methods
    Marathe, M. and Grünebohm, A. and Nishimatsu, T. and Entel, P. and Ederer, C.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    We use molecular dynamics simulations for a first-principles-based effective Hamiltonian to calculate two important quantities characterizing the electrocaloric effect in BaTiO3, the adiabatic temperature change ΔT and the isothermal entropy change ΔS, for different electric field strengths. We compare direct and indirect methods to obtain ΔT and ΔS, and we confirm that both methods indeed lead to an identical result provided that the system does not actually undergo a first order phase transition. We also show that a large electrocaloric response is obtained for electric fields beyond the critical field strength for the first order phase transition. Furthermore, our work fills several gaps regarding the application of the first-principles-based effective Hamiltonian approach, which represents a very attractive and powerful method for the quantitative prediction of electrocaloric properties. In particular, we consider the full temperature and field dependence of the calculated specific heat for the indirect calculation of ΔT, and we discuss the importance of maintaining thermal equilibrium during the field ramping when calculating ΔT using the direct method within a molecular dynamics approach. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.054110
  • Influence of defects on ferroelectric and electrocaloric properties of BaTiO3
    Grünebohm, A. and Nishimatsu, T.
    Physical Review B - Condensed Matter and Materials Physics 93 (2016)
    We report modifications of the ferroelectric and electrocaloric properties of BaTiO3 by defects. For this purpose, we have combined ab initio based molecular dynamics simulations with a simple model for defects. We find that different kinds of defects modify the ferroelectric transition temperatures and polarization, reduce the thermal hysteresis of the transition, and are no obstacle for a large caloric response. For a locally reduced polarization, the ferroelectric transition temperature and the adiabatic response are slightly reduced. For polar defects, an intriguing picture emerges. The transition temperature is increased by polar defects and the temperature range of the large caloric response is broadened. Even more remarkable, we find an inverse caloric effect in a broad temperature range. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.93.134101
  • Molecular dynamics simulations of chemically disordered ferroelectric (Ba,Sr)TiO3 with a semi-empirical effective Hamiltonian
    Nishimatsu, T. and Grünebohm, A. and Waghmare, U.V. and Kubo, M.
    Journal of the Physical Society of Japan 85 (2016)
    We present a semi-empirical effective Hamiltonian to capture effects of disorder associated with Ba and Sr cations occupying A sites in (BaxSr1-x)TiO3 on its ferroelectric phase transition. Averaging between the parameters of firstprinciples effective Hamiltonians of end members BaTiO3 and SrTiO3, we include a term with an empirical parameter to capture the local polarization and strains arising from the difference between ionic radii of Ba and Sr. Using mixed-space molecular dynamics of the effective Hamiltonian, we determine T-dependent ferroelectric phase transitions in (BaxSr1-x)TiO3 which are in good agreement with experiment. Our scheme of determination of semi-empirical parameters in effective Hamiltonian should be applicable to other perovskite-type ferroelectric solid solutions. ©2016 The Physical Society of Japan.
    view abstract10.7566/JPSJ.85.114714
  • On the rich magnetic phase diagram of (Ni, Co)-Mn-Sn Heusler alloys
    Grünebohm, A. and Herper, H.C. and Entel, P.
    Journal of Physics D: Applied Physics 49 (2016)
    We put a spotlight on the exceptional magnetic properties of the metamagnetic Heusler alloy (Ni, Co)-Mn-Sn by means of first principles simulations. In the energy landscape we find a multitude of local minima, which belong to different ferrimagnetic states and are close in total magnetization and energy. All these magnetic states correspond to the local high spin state of the Mn atoms with different spin alignments and are related to the magnetic properties of Mn. Compared to pure Mn, the magneto-volume coupling is reduced by Ni, Co and Sn atoms in the lattice and no local low-spin Mn states appear. For the cubic phase we find a ferromagnetic ground state whereas the global energy minimum is a tetragonal state with a complicated spin structure and vanishing magnetization which so far has been overlooked in simulations. © 2016 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/49/39/395001
  • Positive and negative electrocaloric effect in BaTiO3 in the presence of defect dipoles
    Ma, Y.-B. and Grünebohm, A. and Meyer, K.-C. and Albe, K. and Xu, B.-X.
    Physical Review B - Condensed Matter and Materials Physics 94 (2016)
    The influence of defect dipoles on the electrocaloric effect (ECE) in acceptor doped BaTiO3 is studied by means of lattice-based Monte-Carlo simulations using a Ginzburg-Landau type effective Hamiltonian. Oxygen vacancy-acceptor associates are described by fixed local dipoles with orientation parallel or antiparallel to the external field. By a combination of canonical and microcanonical simulations the ECE is directly evaluated. Our results reveal that in the case of antiparallel defect dipoles the ECE can be positive or negative depending on the dipole density. Moreover, a transition from a negative to positive ECE can be observed when the external field increases. These transitions are due to the delicate interplay of internal and external fields and are explained by the domain structure evolution and related field-induced entropy changes. The results are in good qualitative agreement to those obtained by molecular dynamics simulations employing an ab initio based effective Hamiltonian. Finally, a modified electrocaloric cycle, which makes use of the negative ECE in the presence of defect dipoles, is proposed to enhance the cooling effect. © 2016 American Physical Society.
    view abstract10.1103/PhysRevB.94.094113
  • Tuning the caloric response of BaTiO3 by tensile epitaxial strain
    Grünebohm, A. and Marathe, M. and Ederer, C.
    EPL 115 (2016)
    We investigate the effect of epitaxial strain on the electrocaloric effect (ECE) in BaTiO3 by means of ab-initio-based molecular-dynamics simulations. We show that tensile strain can be used to optimize the operation range for ferroic cooling. Strain in the range of ≤ 1% can be used to shift the operation temperature by several hundreds of kelvin both to higher and lower temperatures, depending on the direction of the external field. In addition, the transformation between multi-domain and mono-domain states, induced by an in-plane electric field, results in an additional peak of the adiabatic temperature change at lower temperatures, and a broad temperature interval where the caloric response scales linearly with the applied field strength, even up to very high fields. © Copyright 2016 EPLA.
    view abstract10.1209/0295-5075/115/47002
  • Ab initio phase diagram of BaTiO3 under epitaxial strain revisited
    Grünebohm, A. and Marathe, M. and Ederer, C.
    Applied Physics Letters 107 (2015)
    We revisit the phase diagram of BaTiO<inf>3</inf> under biaxial strain using a first principles-based effective Hamiltonian approach. We show that, in addition to the tetragonal (c), quasi-rhombohedral (r), and quasi-orthorhombic (aa) ferroelectric phases that have been discussed previously, there are temperature and strain regions, in particular, under tensile strain, where the system decomposes into multi-domain structures. In such cases, the strained system, at least on a local level, recovers the same phase sequence as the unclamped bulk material. Furthermore, we extend these results from the case of "uniform" biaxial strain to the situation where the two in-plane lattice constants are strained differently and show that similar considerations apply in this case. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4930306
  • Large magnetocaloric effects in magnetic intermetallics: First-principles and Monte Carlo studies
    Entel, P. and Gruner, M.E. and Ogura, M. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Grünebohm, A. and Arróyave, R. and Uebayashi, K. and Singh, N. and Talapatra, A. and Duong, T. and Acet, M. and Çakir, A.
    MATEC Web of Conferences 33 (2015)
    We have performed ab initio electronic structure calculations and Monte Carlo simulations of frustrated ferroic materials where complex magnetic configurations and chemical disorder lead to rich phase diagrams. With lowering of temperature, we find a ferromagnetic phase which transforms to an antiferromagnetic phase at the magnetostructural (martensitic) phase transition and to a cluster spin glass at still lower temperatures. The Heusler alloys Ni-(Co)-Mn-(Cr)-(Ga, Al, In, Sn, Sb) are of particular interest because of their large inverse magnetocaloric effect associated with the magnetostructural transition and the influence of Co/Cr doping. Besides spin glass features, strain glass behavior has been observed in Ni-Co-Mn-In. The numerical simulations allow a complete characterization of the frustrated ferroic materials including the Fe-Rh-Pd alloys. © Owned by the authors, published by EDP Sciences, 2015.
    view abstract10.1051/matecconf/20153302001
  • Predictions of a Large Magnetocaloric Effect in Co- and Cr-Substituted Heusler Alloys Using First-Principles and Monte Carlo Approaches
    Sokolovskiy, V.V. and Buchelnikov, V.D. and Zagrebin, M.A. and Grünebohm, A. and Entel, P.
    Physics Procedia 75 (2015)
    The effect of Co- and Cr-doping on magnetic and magnetocaloric poperties of Ni-Mn-(In, Ga, Sn, and Al) Heusler alloys has been theoretically studied by combining first principles with Monte Carlo approaches. The magnetic and magnetocaloric properties are obtained as a function of temperature and magnetic field using a mixed type of Potts and Blume-Emery-Griffiths model where the model parameters are obtained from ab initio calculations. The Monte Carlo calculations allowed to make predictions of a giant inverse magnetocaloric effect in partially new hypothetical magnetic Heusler alloys across the martensitic transformation. © 2015 The Authors. Published by Elsevier B.V.
    view abstract10.1016/j.phpro.2015.12.155
  • The metamagnetic behavior and giant inverse magnetocaloric effect in Ni-Co-Mn-(Ga, In, Sn) Heusler alloys
    Entel, P. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Ogura, M. and Gruner, M.E. and Grünebohm, A. and Comtesse, D. and Akai, H.
    Journal of Magnetism and Magnetic Materials 385 (2015)
    The magnetic and magnetocaloric properties of Ni-Co-Mn-(Ga, In, Sn) Heusler intermetallics are discussed on the basis of ab initio and Monte Carlo calculations. The main emphasis is on the different reference spin states and magnetic exchange coupling constants of high-temperature austenite and low-temperature martensite which are very important for the calculation of magnetocaloric effect. The origin of metamagnetic behavior is considered in the framework of orbital resolved magnetic exchange parameters of austenite and martensite. The decomposition of exchange constants on orbital contributions has shown that a strong ferromagnetic interaction of magnetic moments in austenite is caused by the more itinerant d-electrons with t2g states while a strong antiferromagnetic interaction in martensite is associated with the more localized eg states. In addition, the appearance of a paramagnetic gap between magnetically weak martensite and ferromagnetically ordered austenite can be realized because of strong competition of magnetic exchange interactions. As a result, large magnetization drop and giant inverse magnetocaloric effect can be achieved across the magnetostructural phase transition. ©2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jmmm.2015.03.003
  • Ab initio and Monte Carlo approaches for the magnetocaloric effect in Co- and in-doped Ni-Mn-Ga Heusler alloys
    Sokolovskiy, V. and Grünebohm, A. and Buchelnikov, V. and Entel, P.
    Entropy 16 (2014)
    The complex magnetic and structural properties of Co-doped Ni-Mn-Ga Heusler alloys have been investigated by using a combination of first-principles calculations and classical Monte Carlo simulations. We have restricted the investigations to systems with 0, 5 and 9 at% Co. Ab initio calculations show the presence of the ferrimagnetic order of austenite and martensite depending on the composition, where the excess Mn atoms on Ga sites show reversed spin configurations. Stable ferrimagnetic martensite is found for systems with 0 (5) at% Co and a c/a ratio of 1.31 (1.28), respectively, leading to a strong competition of ferro- and antiferro-magnetic exchange interactions between nearest neighbor Mn atoms. The Monte Carlo simulations with ab initio exchange coupling constants as input parameters allow one to discuss the behavior at finite temperatures and to determine magnetic transition temperatures. The Curie temperature of austenite is found to increase with Co, while the Curie temperature of martensite decreases with increasing Co content. This behavior can be attributed to the stronger Co-Mn, Mn-Mn and Mn-Ni exchange coupling constants in austenite compared to the corresponding ones in martensite. The crossover from a direct to inverse magnetocaloric effect in Ni-Mn-Ga due to the substitution of Ni by Co leads to the appearance of a “paramagnetic gap“ in the martensitic phase. Doping with In increases the magnetic jump at the martensitic transition temperature. The simulated magnetic and magnetocaloric properties of Co- and In-doped Ni-Mn-Ga alloys are in good qualitative agreement with the available experimental data. © 2014 by the authors; licensee MDPI, Basel, Switzerland.
    view abstract10.3390/e16094992
  • First-principles calculation of the instability leading to giant inverse magnetocaloric effects
    Comtesse, D. and Gruner, M.E. and Ogura, M. and Sokolovskiy, V.V. and Buchelnikov, V.D. and Grünebohm, A. and Arróyave, R. and Singh, N. and Gottschall, T. and Gutfleisch, O. and Chernenko, V.A. and Albertini, F. and Fähler, S. and Entel, P.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    The structural and magnetic properties of functional Ni-Mn-Z (Z=Ga, In, Sn) Heusler alloys are studied by first-principles and Monte Carlo methods. The ab initio calculations give a basic understanding of the underlying physics which is associated with the strong competition of ferro- and antiferromagnetic interactions with increasing chemical disorder. The resulting d-electron orbital dependent magnetic ordering is the driving mechanism of magnetostructural instability which is accompanied by a drop of magnetization governing the size of the magnetocaloric effect. The thermodynamic properties are calculated by using the ab initio magnetic exchange coupling constants in finite-temperature Monte Carlo simulations, which are used to accurately reproduce the experimental entropy and adiabatic temperature changes across the magnetostructural transition. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.184403
  • Optimizing the magnetocaloric effect in Ni-Mn-Sn by substitution: A first-principles study
    Grünebohm, A. and Comtesse, D. and Hucht, A. and Gruner, M.E. and Maslovskaya, A. and Entel, P.
    IEEE Transactions on Magnetics 50 (2014)
    We optimize the magnetic and structural properties of Ni(Co,Cu)MnSn Heusler alloys for the magnetocaloric effect (MCE) by means of density functional theory combined with Monte Carlo simulations of a classical Heisenberg model. NiMnSn alloys show a drop of magnetization at the martensitic phase transition, which leads to the inverse MCE. We find either disordered or frustrated magnetic configurations directly below the martensitic transition temperature. However, the jump of magnetization at the magnetostructural transition is small as the austenite is in a ferrimagnetic state and not fully magnetized. For Co and Cu substitution, the structural phase transition temperature shifts to lower temperatures. In particular, Co substitution is promising, as the magnetization of the austenite increases by additional ferromagnetic interactions, which enhances the jump of magnetization. © 2014 IEEE.
    view abstract10.1109/TMAG.2014.2330845
  • Ab initio study of the electronic and magnetic structure of the TiO 2 rutile (110)/Fe interface
    Grünebohm, A. and Entel, P. and Herper, H.C.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    Adsorption of Fe on the rutile (110) surface is investigated by means of ab initio density functional theory calculations. We discuss the deposition of single Fe atoms and increasing Fe coverage, as well as the adsorption of small Fe clusters. It is shown that the different interface structures found in experiment can be understood in terms of the adsorption of the Fe atoms landing first on the rutile surface. Strong interface bonds form if single Fe atoms are deposited. The Fe-Fe bonds in deposited Fe clusters lead to a three-dimensional growth mode. Mainly ionic Fe oxide bonds are formed in both cases and the electronic band gap of the surface is reduced due to interface states. In addition to the structural and electronic properties, we discuss the influence of the interface on the magnetic properties, finding stable Fe moments and induced moments within the interface which lead to a large spin polarization of the Fe atoms at the rutile (110)/Fe interface. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.155401
  • First-principles investigation of incipient ferroelectric trends of rutile TiO2 in bulk and at the (110) surface
    Grünebohm, A. and Entel, P. and Ederer, C.
    Physical Review B - Condensed Matter and Materials Physics 87 (2013)
    The ferroelectric trends of rutile (TiO2) in bulk and at the (110) surface are investigated by means of ab initio density functional theory. We discuss the underlying mechanism of the incipient ferroelectric behavior of rutile in terms of Born effective charges, which we decompose in individual contributions by means of maximally localized Wannier functions. We show that a ferroelectric phase can be stabilized for a variety of different lattice distortions, which all enlarge the shortest Ti-O distance, even if the longer apical Ti-O bond is simultaneously shortened. At the (110) surface, the ferroelectric trends are modified compared to the bulk, but nevertheless, ferroelectric phases with large polarization even in the topmost surface layer can be stabilized by uniaxial strain. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.87.054110
  • Reply to "comment on 'First-principles study of the influence of (110)-oriented strain on the ferroelectric properties of rutile TiO2' "
    Grünebohm, A. and Ederer, C. and Entel, P.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    In their Comment, Refson criticize the use of the "frozen-core" approximation for the Ti 3s and 3p semicore states in our Brief Report, and they dispute whether the methods employed by us are capable of supporting our conclusions. In this Reply we summarize the main features of our work and we show that indeed none of the conclusions of our Brief Report are affected by the frozen-core treatment of the Ti semicore states. The criticism of Refson seems to be based mostly on a misinterpretation what the main goal of our study is and what the main conclusions of our work are. While the detailed comparison between different pseudopotentials containing different numbers of valence electrons presented in the comment by Refson is very interesting and instructive in its own right, it has no consequences regarding the validity of our work. ©2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.136102
  • Domain structure in the tetragonal phase of BaTiO 3-From bulk to nanoparticles
    Grnebohm, A. and Gruner, M.E. and Entel, P.
    Ferroelectrics 426 (2012)
    We present a first-principles density functional theory study of domain wall structures in tetragonal BaTiO 3 and its nanoparticles. For the bulk material the domain wall profiles, their width and their formation energy are computed and preliminary investigations on thin BaTiO 3 films up to 4 monolayers and small nanoparticles of 15.8 have been performed. While the 180 wall is atomically sharp, we find a lower bond for the 90 wall width of 16.5 . Although, no ferroelectric state can be stabilized neither in films nor in the nanoparticles of this small size, a large local polarization exits in both cases. © Taylor & Francis Group, LLC.
    view abstract10.1080/00150193.2012.671090
  • First-principles study of the influence of (110) strain on the ferroelectric trends of TiO 2
    GrüNebohm, A. and Siewert, M. and Ederer, C. and Entel, P.
    Ferroelectrics 429 (2012)
    We investigate the impact of uniaxial strain on atomic shifts, dipolar interactions, polarization and electric permittivity in TiO 2 (rutile) by using two different implementations of density functional theory. It is shown that calculations using the Vienna ab inito simulation package (VASP) and the plane-wave self-consistent field method (PWscf) yield qualitatively the same atomic relaxations and ferroelectric trends under strain. The phonon dispersion curves of unstrained and strained TiO 2 (rutile) obtained by employing the linear response method confirm previous calculations of the giant LO-TO splitting and the appearance of soft polar modes. A second order phase transition into a ferroelectric phase with polarization along (110) appears under expansive strain in (110) direction.
    view abstract10.1080/00150193.2012.676945
  • Simulation of cluster sintering, dipolar chain formation, and ferroelectric nanoparticulate systems
    Grünebohm, A. and Hucht, A. and Meyer, R. and Comtesse, D. and Entel, P.
    NanoScience and Technology 79 (2012)
    Magnetic and ferroelectric nanoparticles are subjects of increasing basic research for future technologies. In this work Fe and Ni clusters and near-ferroelectric TiO 2 clusters have been chosen as representatives in order to discuss fundamental issues such as sintering of magnetic and near-ferroelectric clusters as well as configurations resulting from cluster agglomeration due to magnetic dipolar interactions. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/978-3-642-28546-2_6
  • First-principles study of the influence of (110)-oriented strain on the ferroelectric properties of rutile TiO2
    Grünebohm, A. and Ederer, C. and Entel, P.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    We use first-principles density-functional theory to investigate the softening of polar phonon modes in rutile TiO2 under tensile (110)-oriented strain. We show that the system becomes unstable against a ferroelectric distortion with polarization along (110) for experimentally accessible strain values. The resulting polarization, estimated from the Born effective charges, even exceeds the bulk polarization of BaTiO3. Our calculations demonstrate the different strain dependence of polar modes polarized along (110) and (001) directions, and we discuss the possibility of strain engineering the polarization direction, and the resulting dielectric and piezoelectric response, in thin films of TiO2 grown on suitable substrates. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.132105
  • A comparative study of (Fe, Fe3Si)/GaAs and Heusler/MgO for spintronics applications
    Grünebohm, A. and Siewert, M. and Herper, H.C. and Gruner, M.E. and Entel, P.
    Journal of Physics: Conference Series 200 (2010)
    Many Heusler-like compounds as well as elemental Fe show large spin polarization in their bulk phase, which makes them possible candidates for spintronic applications. However, it has turned out that the magnetic properties may change significantly if the ferromagnet is grown on a semiconductor. Here, we investigate from first principles the magnetic properties of thin Fe, Fe 3Si, and Heusler films on GaAs(110) and MgO(001) substrates, which show rather flat interfaces due to the absence of reconstruction. We observe high spin polarization for the Fe containing systems and both Heusler alloys considered here, even for ultra thin layers. These systems are mainly unaffected by the substrate. © 2010 IOP Publishing Ltd.
    view abstract10.1088/1742-6596/200/7/072038
  • ab initio calculations

  • density functional theory

  • ferroelectrics

  • ferroic cooling

  • magnetic properties

  • magnetism

  • martensite

  • modelling and simulation

  • molecular dynamics

  • monte carlo methods

  • multiferroics

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