Al substitution effect on magnetic properties of magnetocaloric material HoB₂

We investigate the Al substitution effect for HoB₂ to tune the Curie temperature (T_C). The HoB_2−xAl_x samples with nominal x (x_nom) of 0, 0.1, 0.3, and 0.5 are characterized by powder X-ray diffraction measurements, EDS analysis and magnetization measurements. The HoB_2−xAl_x samples with x_nom of 0.1, 0.3, and 0.5 show two ferromagnetic transitions at T_C1 of ∼15 K and T_C2 of >32 K. The T_C1 is identical to the T_C of HoB₂ and does not change with Al substitution. The T_C2 is similar to the T_C of HoAl₂ and varies with Al content. In an EDS analysis, the inhomogeneous elemental distribution is observed in Al-substituted HoB₂, where the Al-rich regions exist. Our results suggest that the Al added to HoB₂ does not form a solid solution, but rather leads to the phase segregation into stoichiometric HoB₂ and B-substituted HoAl₂. The x_nom = 0.5 sample exhibits the maximum |ΔS_M(μ₀H:5 T→0 T)| of 32.1 J kg K⁻¹ at T_C1 and a small kink of |ΔS_M| at T_C2. Because the |ΔS_M| exhibits a maximum at T_C1, stoichiometric HoB₂ mainly contributes to the maximum of |ΔS_M|.