Multicaloric Effect Studied in High Magnetic Fields—Dissipation Losses Limiting First-Order Phase Transition Materials for Cryogenic Caloric Cooling

With the world's increasingly affluent population demanding more comfortable living and working conditions and rising global temperatures, leading to an ever growing demand for cooling [1], which is predicted to exceed the energy consumption for heating in this century [2], it is vital that we address the development of much more efficient cooling technologies as an urgent priority. A promising alternative approach is based on solid-state refrigeration by one of the caloric effects – electrocaloric, magnetocaloric, barocaloric or elastocaloric - where the material's temperature is changing under the application of an electrical, magnetic, or mechanical field [3]. However, there is also the possibility to combine these different effects in a beneficial way, in the so-called multicaloric cooling cycle. Magnetic Ni-Mn-based Heusler alloys are ideally suited for multicaloric applications due to their coupled magnetostructural transformation between martensite and austenite [4].<br/><br/>Ni-Mn-based Heusler alloys, are highly promising materials for solid-state cooling, because large multicaloric effects can be achieved across their magnetostructural martensitic transformation over a broad temperature range, which is possibly also relevant for cryogenic applications such as the liquefaction of hydrogen. In this work, we simultaneously measured the responses of the magnetic structural and electronic subsystems to the temperature- and field-induced martensitic transformation of Ni(-Co)-Mn-Ti at low temperatures, showing an abnormal increase of hysteresis and consequently dissipation energy at cryogenic temperatures [5]. Simultaneous measurements of magnetization and adiabatic temperature change in high pulsed magnetic fields up to 50 T [6] reveal a positive and irreversible adiabatic temperature change as a consequence of increased dissipation losses and decreased heat capacity. This is in contradiction to the expected negative adiabatic temperature change, which was determined indirectly by heat capacity measurements. Most importantly, this irreversible phenomenon was also observed in other material systems and is universal to any first-order material with non-negligible hysteresis, effectively limiting the utilization of their caloric effects for gas liquefaction at cryogenic temperatures.<br/><br/>[1] DUPONT J. L. Et al., 38^th Note on Refrigeration Technologies (2019).<br/>[2] M. Isaac er al., Energy Policy 37 (2) (2009).<br/>[3] T. Gottschall et al., Nat. Mater. 17, 929 (2018).<br/>[4] T. Gottschall et al., J. Appl. Phys. 127, 185107 (2020).<br/>[5] B. Beckmann et al. submitted for publication.<br/>[6] T. Gottschall et al., Phys. Rev. B <b>99</b>, 134429 (2019).