Microstructure, phase evolution and magnetocaloric properties of LaFe_11.6Si_1.4/La₇₀Co₃₀ composite

LaFe_11.6Si_1.4/16 wt%La₇₀Co₃₀ composites were fabricated by hot-pressing (HP) followed by thermal diffusion treatment in the temperature range of 923–1273 K. Significant microstructural and phase evolution occurred during diffusion heat treatment. Thermal decomposition (TD) occurred between 1023 and 1223 K, which increased the Curie temperature (T_C) but smeared out the magnetic transition, decreasing magnetic entropy change (–ΔS_M). Co atoms first diffused into the α-Fe(Si) and LaFeSi (1:1:1) phases. And then, the α-Fe(Co,Si) and La(Fe,Co)Si phases, attached to large LaFe_11.6Si_1.4 particles, transformed to a NaZn₁₃-type La(Fe,Co,Si)₁₃ (1:13) phase at 1273 K. Hence, a core-shell La(Fe,Co,Si)₁₃ phase formed, resulting in a table-like magnetic entropy change over a wide temperature range (∼202–292 K). Greater Co diffusion and more homogeneity of the composition of the 1:13 phase could be achieved by increasing the annealing time. The maximum magnetic entropy change (–ΔS_M)^max of the composites at 0–2 T could be increased from 5.62 J/(kg·K) at ∼196 K to 8.81 J/(kg·K) at ∼244 K.