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 HoB2-xAlx 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 HoB2-xAlx 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 |delta S-M(mu H-0:5 T -> 0 T)| of 32.1 J kg K-1 at T-C1 and a small kink of |delta S-M| at T-C2. Because the |delta S-M| exhibits a maximum at T-C1, stoichiometric HoB2 mainly contributes to the maximum of |delta S-M|.

Automated summary

The Al substitution effect on HoB2's Curie temperature (T-C) was investigated. EDS analysis showed that Al substitution led to inhomogeneous elemental distribution, resulting in segregation into HoB2 and B-substituted HoAl2. The results indicated that Al substitution did not alter T-C1 but varied T-C2. Stoichiometric HoB2 mainly contributed to the maximum value of the magnetic entropy change (|delta S-M|) at T-C1.