Influence of Gd and Er Substitution on the Structural, Magnetic, and Cryogenic Magnetocaloric Properties of HoB 2

Abstract

This work reports the effect of partially substituting 20 % of the Ho atoms with Gd or Er on the structural, magnetic, and magnetocaloric properties of the binary boride HoB 2 . Bulk arc-melted Ho 0.8 R 0.2 B 2 alloys with R = Ho, Gd, and Er crystallize in the hexagonal AlB 2 -type structure, with a small amount of the tetragonal UB 4 -type secondary phase. Thermomagnetic measurements reveal two magnetic transitions in the three alloys: a spin- reorientation transition at ~10 K and a ferromagnetic–to–paramagnetic phase transition at higher temperatures. The HoB 2 and Ho 0.8 Er 0.2 B 2 alloys exhibit similar magnetic and structural properties, with Curie temperatures T C = 16 K and magnetic entropy change curves similar in shape and maximum values. In contrast, Gd substitution significantly modifies the magnetic interactions, shifting T C to about 25 K and broadening the temperature range of second-order magnetic phase transition. Large magnetocaloric responses are reported for a magnetic field change of μ 0 ΔH = 5 T. The maximum magnetic entropy change reaches 40.6 J kg –1 K –1 , 27.1 J kg –1 K –1 , and 37.0 J kg –1 K –1 , while the maximum adiabatic temperature change is 12 K, 10 K, and 11 K for HoB 2 , Ho 0.8 Gd 0.2 B 2 , and Ho 0.8 Er 0.2 B 2 , respectively. Although the magnetocaloric effect of Ho 0.8 Gd 0.2 B 2 is reduced compared with the parent alloy, its broader transition results in a higher refrigerant capacity and an extended working temperature range. These results highlight the potential of compositional tuning in rare- earth diborides to optimize magnetocaloric performance in the 10–30 K temperature range, which is relevant for cryogenic cooling and hydrogen liquefaction using magnetic refrigeration.