Influence of hysteresis at magnetostructural transitions on the magnetocaloric properties of Heuslers, Antiperovskites, and Pnictides
A large magnetocaloric effect can be observed in materials with first-order magnetostructural transition. Applying a magnetic field stabilizes the phase with higher magnetization and shifts the transition temperature to higher or lower temperatures. An adiabatically applied field induces a phase transformation and leads to an adiabatic temperature change. This temperature change is used in magnetocaloric refrigerators which is induced by applying a magnetic field cyclically. The material has to have a reversible adiabatic temperature change for each field-cycle to be an effective refrigeration material. The reversibility of the adiabatic temperature change depends on the hysteresis properties of the transition. A minimization of the thermal and magnetic hysteresis is important to improve the efficiency of magnetocaloric refrigerators. The Mn3GaC antiperovskite is a magnetocaloric material with a narrow hysteresis which shows a fully reversible first-order isostructural transformation at about 164 K from the antiferromagnetic state to the ferromagnetic state in an applied field of 2 T. The transformation is accompanied by a volume contraction of about 0.5 %. The small hysteresis can be explained by the low magnetocrystalline-anisotropy-energy which was determined using a Mn3GaC single crystal. The hysteresis properties of polycrystalline and single crystal Mn3GaC are equivalent. However, in pulsed magnetic fields with fast field-sweep rates, the structural response of the transformation cannot follow the field change, which leads to a reduced magnetocaloric effect. This is in contrast to adiabatic magnetization measurements in pulsed fields showing a full transformation. The different time-responses of the structural and magnetic transition can be explained by the large volume change (0.5 %) at the first-order transformation. Mn-Cr-Co-Sb pnictide has a first-order isostructural transition from an antiferromagnetic state to a ferrimagnetic state and has, like Mn3GaC, a narrow hysteresis. The transformation in Mn-Cr-Co-Sb can follow the fast field-sweep rates. This can be explained by the ten times smaller volume change during the transformation in Mn-Cr-Co-Sb compared to Mn3GaC. Another class of magnetocaloric materials are off-stoichiometric Ni-Mn-In Heusler alloys. However, these alloys are not stable and decompose into stoichiometric Ni2MnIn and NiMn when annealed between 650 and 750 K. In Ni-Mn-In alloys with 5 % In, annealing in a magnetic field leads to the formation of shell-ferromagnetic nanoprecipitates. The ferromagnetic hard-shell remains pinned up to fields of 5 T while the rest of the precipitate shows a soft ferromagnetic behavior. A selective formation of precipitates can be a further opportunity to tune the hysteresis in Ni-Mn-In Heusler alloys.
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