Design of thermal hysteresis in nonstoichiometric alloys with giant magnetocaloric effect
The non-stoichiometric Fe$_2$P-type (FeMnP$_{0.5}$Si$_{0.5}$)$_{1-x}$(FeV)$_{x}$ alloys ( $x=0, 0.01$, $0.02$, and $0.03$) have been investigated as potential candidates for magnetic refrigeration near room temperature. The magnetic ordering temperature decreases with increasing FeV concentration, $x$, which can be ascribed to decreased ferromagnetic coupling strength between the magnetic atoms. The strong magnetoelastic coupling in these alloys results in large values of the isothermal entropy change ($ΔS_M$); $15.7$ J/kgK, at $2$ T magnetic field for the $x = 0$ alloy. $ΔS_M$ decreases with increasing $x$. Results from M{ö}ssbauer spectroscopy reveal that the average hyperfine field (in the ferromagnetic state) and average center shift (in the paramagnetic state) have the same decreasing trend as $ΔS_M$. The thermal hysteresis ($ΔT_{hyst}$) of the magnetic phase transition decreases with increasing $x$, while the mechanical stability of the alloys improves due to the reduced lattice volume change across the magnetoelastic phase transition. The adiabatic temperature change $ΔT_{ad}$, which highly depends on $ΔT_{hyst}$, is $1.7$ K at $1.9$ T applied field for the $x = 0.02$ alloy.