Defects and magnetism in cubic GdX 2 Laves phase compounds

Abstract

The relation between the structural and magnetic changes, induced by means of ball milling of GdX 2 compounds that crystallise in the cubic Laves phase structure, is investigated, where X=Pt, Ir, Rh, Al and Mg. These five compounds all exhibit a ferromagnetic transition in the as-prepared, i.e. atomically ordered, state. An explanation for both the similarities and differences in magnetic behaviour as a result of structural changes due to milling is given. Only GdMg 2 was found to disorder in anti-site disorder, whereas the others disordered in quadruple-defect disorder, which is a vacancy type of disorder similar to triple-defect disorder in B2 compounds. GdIr 2 did in fact show both types of disorder. For longer periods of milling the formation of quadruple defects was taken over by the formation of pairs of anti-site defects. In ordered GdPt 2, GdIr 2, and GdRh 2 the Gd-6s-like electrons are the main contributors to the indirect interaction between the Gd moments. This results in an increasing Curie temperature with decreasing lattice parameter and vice versa, a relation which even appears to be linear, probably because of the negligible influence of the non-magnetic element to the Gd–Gd interaction due to their d-electron character. Since the Curie temperature of GdMg 2 was found to behave in a similar way in relation to the lattice parameter as the aforementioned compounds, it is concluded that the conduction electrons in GdMg 2 must also mainly be of Gd-6s-like type. The electronic character of the conduction electrons of ordered GdAl 2 are mainly of Gd-5d-like type, which results in a decreasing Curie temperature with decreasing lattice parameter. This relation was not so perfectly linear, probably because of the p-electron character of the Al atoms. The p electrons do apparently influence the Gd–Gd interaction, when substituted on the Gd sublattice. This even resulted in the introduction of antiferromagnetic interactions, possibly by means of a mechanism similar to superexchange, which finally led to GdAl 2 becoming a spin glass. The freezing temperature of ball milled GdAl 2, turns out to be proportional to the defect concentration. The substitution of Gd atoms on the non-magnetic sublattice in GdIr 2 after long periods of milling and in GdMg 2 (for all milling times), apparently causes these compounds to exhibit, at least, re-entrant spin-glass-like behaviour.