Defects and magnetism in cubic GdX2 Laves phase compounds

The relation between the structural and magnetic changes, induced by means of ball milling of GdX2 compounds that crystallise in the cubic Laves phase structure, is investigated, where X = Pt, Ir, Rh, Al and Mg. These live 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 GdMg2 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. GdIr2 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 CdPt2, GdIr2, and GdRh2 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 GdMg2 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 GdMg2 must also mainly be of Gd-6s-like type. The electronic character of the conduction electrons of ordered GdAl2 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 GdAl2 becoming a spin glass. The freezing temperature of ball milled GdAl2 turns out to be proportional to the defect concentration. The substitution of Gd atoms on the non-magnetic sublattice in GdIr2 after long periods of milling and in GdMg2 (for all milling times), apparently causes these compounds to exhibit, at least, re-entrant spin-glass-like behaviour. (C) 1999 Elsevier Science B.V. All rights reserved.