Spin reorientations and crystal field modification in Ho$_{1-y}$Gd$_y$Al$_2$ compounds

TL;DR

This study combines experimental and theoretical methods to analyze how Gd substitution in HoAl2 affects its spin reorientation transition and crystal field, with implications for magnetocaloric applications.

Contribution

It provides new insights into the impact of Gd content on spin reorientation and crystal field modifications in Ho$_{1-y}$Gd$_y$Al$_2$ compounds, supported by comprehensive measurements and modeling.

Findings

Gd content linearly increases lattice parameter.

Spin reorientation temperature decreases with Gd addition.

First-order transition persists up to y=0.75.

Abstract

We have performed an experimental and theoretical investigation on the spin reorientation transitions Ho$_{1-y}$Gd$_y$Al$_2$ compounds. Crystallographic, magnetic and calorimetric measurements have been performed for five selected samples in the Ho$_{1-y}$Gd$_y$Al$_2$ 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 ($x$) 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 HoAl$_2$ 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 HoAl$_2$, whereas for the selected concentrations up to x = 0.75 the first-order character of the transition is maintained. Such behavior may lead to a large refrigerant capacity (RC) over a wide temperature range, which is an important parameter to evaluate the magnetocaloric effect (MCE) of such compounds. 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.