Spin reorientations and crystal field modification in Ho1−y GdyAl2 compounds

We have performed an experimental and theoretical investigation on the spin reorientation transitions Ho1−y GdyAl2 compounds. Crystallographic, magnetic and calorimetric measurements have been performed for five selected samples in the Ho1−y GdyAl2 series (y = 0, 0.25, 0.5, 0.75 and 1.0). From the results, a linear increase in the lattice parameter as a function of Gd-content (y) is observed. Magnetization measurements show that all samples present collinear ferromagnetism. Specific heat measurements were performed in order to detail the first-order spin reorientation transition (FOSRT) that appears for HoAl2 around 20 K. Furthermore, we investigated how the increase in Gd content affects the character of such phase transition. Our results show a slightly modification in the spin reorientation transition temperature, which moves to lower temperatures as Gd is increased in HoAl2, whereas for the selected concentrations up to x = 0.75 the first-order character of the transition is maintained. The theoretical analysis by means of a model Hamiltonian that includes crystal field, exchange and Zeeman interactions reveals a modification in the crystal-field acting on Ho ions as Gd-content is increased.