Microstructural evolution and magnetocaloric properties of off-stoichiometric La_1.2Fe_11.6Si_1.4 alloys with interstitial C atoms

This study investigated the effects of interstitial carbon doping on the microstructural and magnetocaloric properties of off-stoichiometric La_1.2Fe_11.6Si_1.4C_x (x = 0, 0.25, 0.5, 0.75, 1) alloys. The alloys were prepared by melt-spinning following vacuum arc-melting. For the as-prepared and annealed samples, the carbon existed in the La₂Fe₂Si₂C and NaZn₁₃-type La(Fe, Si)₁₃ (denoted by 1:13) phases, respectively. During the annealing process, the C atoms inhibited the diffusion reaction and depressed the generation of 1:13 phase, reducing mass fraction of the 1:13 phase in annealed La_1.2Fe_11.6Si_1.4C_x compounds. The introduction of carbon resulted in lattice expansion and increased the Curie temperature (T_C) from 192 K to 273 K with x = 0.5. The first-order magnetic transition was gradually transformed into the second-order magnetic transition with increasing carbon content, which induced the significant reduction of thermal and magnetic hysteresis, as well as the maximum magnetic entropy change and adiabatic temperature change vary from 18.92 J/(kg⋅K) to 4.60 J/(kg⋅K) and from 4.9 K to 2.2 K under an applied field change of 0–2 T. The results demonstrate that interstitial carbon doping is an effective strategy to improve the magnetocaloric performance of La(Fe,Si)₁₃ alloys.