Non-contact characterization of caloric materials: Sub-mK sensitivity even in thin films

Jago Döntgen, Ruhr-Universität Bochum, Bochum, Germany
Jörg Rudolph, Ruhr-Universität Bochum, Bochum, Germany
Tino Gottschall, TU Darmstadt, Darmstadt, Germany
Oliver Gutfleisch, TU Darmstadt, Darmstadt, Germany
Steffen Salomon, Ruhr-Universität Bochum, Bochum, Germany
Alfred Ludwig, Ruhr-Universität Bochum, Bochum, Germany
Daniel Hägele, Ruhr-Universität Bochum, Bochum, Germany

Caloric materials are of high interest for building new energy efficient cooling devices. Solid-state cooling technology can significantly contribute to reducing domestic energy consumption and replacing the currently used fluoro-chloro-carbon coolants that have an adverse impact on the environment. We present measurements of the magnetocaloric and electrocaloric effect obtained by a newly developed measurement scheme utilizing contactless detection of thermal radiation. Modulation of the external field with frequencies beyond 100 Hz and lock-in detection is the key for obtaining ΔT data with a resolution better than 1 mK. By virtue of our Modulation Infrared Thermometry (MIRT) method, the first magneto-caloric peak on a film as thin as 1.4 μm prepared on a thin membrane (Gadolinium on Si3N4/SiO2) could be measured paving the way for a combinatorial materials approach to caloric materials.

Furthermore, a striking quadratic dependence of ΔT on the external magnetic field Hext around Hext = 0 was found at the ΔT peak maximum for all investigated samples including bulk Gadolinium, metamagnetic La-Fe-Co-Si, and amorphous Fe-Mo-Co-B. The quadratic behaviour contradicts an earlier predicted cubic dependence. At higher fields, a transition to the usual Hext2/3 behaviour is found [1].

Measurements in the low-field regime are possible only due to the high sensitivity of our method and help to clarify which parameters are needed for an accurate description of the magnetic field dependence. Measurements on the relaxor dielectric PMN-PT reveal an aging effect of the electrocaloric effect reducing ΔT by as much as 50%. This kind of aging behaviour in relaxor ferroelectrics is known from dielectric measurements and is a fingerprint of the non-ergodic behaviour found in such systems.

[1] J. Döntgen, J. Rudolph, T. Gottschall, O. Gutfleisch, S. Salomon, A. Ludwig and D. Hägele, Appl. Phys. Lett. 106, 032408 (2015)

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