Catalina Salazar Mejia, HLD.
Magnetic-refrigeration and hydrogen-liquefaction technology based on the magnetocaloric effect are emerging as climate-friendly alternatives to conventional methods. To make these applications a reality, the proper characterization of the respective materials is now more crucial than ever. It is necessary to determine the magnetocaloric and other physical properties under various stimuli such as magnetic fields and mechanical loads. We demonstrated that pulsed magnetic fields are a powerful tool to study and characterize magnetocaloric materials. The high-field regime allows determining, for instance, the saturation value of the magnetocaloric effect and its maximum temperature span and we can induce the transition of the material over a wide temperature range. We recently summarized an overview of the characterization techniques established at the Dresden High Magnetic Field Laboratory to measure the temperature change of a sample directly under applied fields that reach even beyond 50 T. The short pulse duration of a about 100 ms provides excellent adiabatic conditions during the experiment allowing the direct determination of the adiabatic temperature change of a material, ΔTad, without any heat loss. ΔTad, together with the isothermal entropy change, ΔST, are the most important parameters to characterize a magnetocaloric material. In Figure 1, we present ΔTad measured up to 50 T for HoAl2 as function of the initial temperature Ti. Laves phases are promising materials for gas liquefaction based on the magnetocaloric effect. Therefore, measurements down to low temperatures are necessary to study these materials. The inset shows the temperature change as function of applied field for 2, 10, 20, and 50 T pulses starting at Ti = 30 K. We found no saturation of ΔTad even up to 50 T. We also show a good agreement between the direct measurements and the values extracted from specific-heat data. To understand fully the magnetocaloric behavior of materials in pulsed magnetic fields, we need to measure different physical properties simultaneously. As we show in Figure 2 for a Fe-Rh sample, we can measure simultaneously magnetostriction, magnetization and temperature changes. This gives a comprehensive picture of the material´s behavior.
Figure 1: ΔTad measured up to 50 T for HoAl2 as function of initial temperature Ti. Values calculated from specific-heat data (lines) are shown for 10 T. The inset shows the temperature change as function of applied field for 2, 10, 20, and 50 T pulses starting at Ti = 30 K.
Figure 2: Magnetization M (green), relative length change Δl/l0 (blue), and ΔTad (orange) as a function of field, measured simultaneously for a Fe–Rh sample at Ti = 200 K for a 26 T pulse.
On the high-field characterization of magnetocaloric materials using pulsed magnetic fields, C. Salazar Mejía, T. Niehoff, M. Straßheim, E. Bykov, Y. Skourski, J. Wosnitza, and T. Gottschall, J. Phys. Energy 5, 034006 (2023).
https://iopscience.iop.org/article/10.1088/2515-7655/acd47d/meta
Contact: c.salazar-mejia@hzdr.de
