Large Magnetic Entropy Change in La0.6Ce0.4Fe11.5Si1.5 Alloy Exhibiting First- and Second-Order Phase Transitions

A polycrystalline alloy ingot of La
_0.6
Ce
_0.4
Fe
_11.5
Si
_1.5
was prepared by an arc-melting method. Magnetic measurements of magnetization M as a function of field H and temperature T show a ferromagnetic-paramagnetic phase transition at a Curie temperature TC = 170 K. Interestingly, curves of H/M versus M
²
have a positive slope at low field (H ≤ 10 kOe), which corresponds to a second-order phase transition (SOPT), but a negative slope at high field (H > 10 kOe), which corresponds to a first-order phase transition (FOPT). The magnetic entropy change ΔS
_m
(T) under different ranges of field change ΔH, calculated from M(H) isotherms, has a maximum IΔS
_max
I around TC that increases in magnitude with increasing ΔH. Refrigerant capacity (RC), indicative of magnetic cooling efficiency, also increases with increasing ΔH. Curves of ΔS
_m
(T)/ΔS
_max
versus θ = (T - TC)/(Tr - TC), where Tr is the reference temperature, collapse into a universal curve when ΔH ≤ 10 kOe. In contrast, curves of ΔS
_m
(T)/ΔS
_max
versus θ do not reduce to a universal curve when ΔH > 10 kOe. These suggest a coexistence of FOPT and SOPT properties in the alloy.