Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications

Magnetic refrigeration relies on a substantial entropy change in a magnetocaloric material when a magnetic field is applied. Such entropy changes are present at first‐order magnetostructural transitions around a specific temperature at which the applied magnetic field induces a magnetostructural phase transition and causes a conventional or inverse magnetocaloric effect (MCE). First‐order magnetostructural transitions show large effects, but involve transitional hysteresis, which is a loss source that hinders the reversibility of the adiabatic temperature change ΔTad. However, reversibility is required for the efficient operation of the heat pump. Thus, it is the mastering of that hysteresis that is the key challenge to advance magnetocaloric materials. We review the origin of the large MCE and of the hysteresis in the most promising first‐order magnetocaloric materials such as Ni–Mn‐based Heusler alloys, FeRh, La(FeSi)13‐based compounds, Mn3GaC antiperovskites, and Fe2P compounds. We discuss the microscopic contributions of the entropy change, the magnetic interactions, the effect of hysteresis on the reversible MCE, and the size‐ and time‐dependence of the MCE at magnetostructural transitions. Understanding hysteresis: Materials with magnetostructural phase transitions (MSPT) show a large magnetocaloric effect (MCE). The understanding of MSPT and its thermal hysteresis requires the knowledge about the electronic, magnetic, and lattice entropy contributions. In this Review, we provide an overview of the properties of MSPT in La–Fe–Si, Heusler alloys, Mn3GaC, and Fe2P‐type materials with respect to the MCE and its reversibility based on studies under static/dynamic conditions at micro‐ and mesoscopic scales.