The structure, magnetic and magnetocaloric effect of misch-metal (MM)₂Fe_17−xAl_x (x = 0–2) compounds

In this study, the structural, magnetic and magnetocaloric effects (MCE) of misch-metal MM₂Fe_17−xAl_x (0 ≤ x ≤ 2) compounds are investigated. The XRD patterns indicate that all the samples crystallize primarily in a rhombohedral Th₂Zn₁₇ type crystal structure coexisting with a small amount of precipitation phase α-Fe. The lattice parameters and the Curie temperature (T_C) increase linearly with the increase of Al content. This allows the MM₂Fe_17−xAl_x compounds to achieve magnetic refrigeration over the entire room temperature range. The maximum values of entropy changes (ΔS_M) for MM₂Fe_17−xAl_x (x = 0, 0.5, 1 and 1.5) compounds with a magnetic field variation of 0–2 T and 0–5 T are determined to be −2.1 J Kg^-1 K⁻¹, −1.3 J Kg^-1 K⁻¹, −1.9 J Kg^-1 K⁻¹, 1.9 J Kg^-1 K⁻¹ and −4.0 J Kg^-1 K⁻¹, −2.7 J Kg^-1 K⁻¹, −3.5 J Kg^-1 K⁻¹, −3.5 J Kg^-1 K⁻¹, respectively. The relative cooling power (RCP) for all the studied compounds is larger than 90 and 270 Jkg⁻¹ while the full-width half maxima (δT_FWHM) is larger than 60 K and 100 K with the field changes of 0–2 T and 0–5 T, respectively. These results indicate that MM₂Fe_17−xAl_x (0 ≤ x ≤ 2) compounds could have higher magnetocaloric cost-performance comparable to the room-temperature magnetic refrigeration materials R₂Fe₁₇ (R = Pr, Nd) and Gd with the second-order phase transition (SOPT).