Al substitution effect on magnetic properties of magnetocaloric material HoB2

Abstract

We investigate the Al substitution effect for HoB2 to tune the Curie temperature (TC). The HoB2−xAlx samples with nominal x (xnom) of 0, 0.1, 0.3, and 0.5 are characterized by powder X-ray diffraction measurements, EDS analysis and magnetization measurements. The HoB2−xAlx samples with xnom of 0.1, 0.3, and 0.5 show two ferromagnetic transitions at TC1 of ∼15 K and TC2 of >32 K. The TC1 is identical to the TC of HoB2 and does not change with Al substitution. The TC2 is similar to the TC 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 xnom = 0.5 sample exhibits the maximum |ΔSM(μ0H:5 T→0 T)| of 32.1 J kg K−1 at TC1 and a small kink of |ΔSM| at TC2. Because the |ΔSM| exhibits a maximum at TC1, stoichiometric HoB2 mainly contributes to the maximum of |ΔSM|.