Magnetocaloric materials with first-order phase transition: thermal and magnetic hysteresis in LaFe11.8Si1.2 and Ni2.21Mn0.77Ga1.02 (invited)

This is a report on a comparative study of thermal and magnetic hysteresis in LaFe11.8Si1.2 and Ni2.21Mn0.77Ga1.02 polycrystalline samples near their first-order phase transitions by measurements of magnetization and direct measurements of the adiabatic temperature change, ΔTad(H). Based on the obtained M(T) and M(H) dependencies, H-T diagrams were constructed for both alloys and used to explain the ΔTad(H) dependencies. For low thermal hysteresis and a sharp transition in the temperature dependence of magnetization (LaFe11.8Si1.2), the field dependencies of ΔTad(H) are governed by the phase boundary between ferromagnetic and paramagnetic states. For large thermal hysteresis and a broad transition width in temperature (Ni2.21Mn0.77Ga1.02), the phase boundary has little impact on ΔTad(H). It is also shown that, in the phase transition region, ΔTad measured upon the first application of magnetic field (virgin ΔTad(H) leg) can differ from that measured upon subsequent runs of the field. This can lead to an overestimation of the magnetocaloric effect, if only the virgin leg of ΔTad(H) is considered.