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 co
mpounds all exhibit a ferromagnetic transition in the as-prepared, i.e. ato
mically ordered, state. An explanation for both the similarities and differ
ences in magnetic behaviour as a result of structural changes due to millin
g is given. Only GdMg2 was found to disorder in anti-site disorder, whereas
the others disordered in quadruple-defect disorder, which is a vacancy typ
e 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 form
ation 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 co
ntributors to the indirect interaction between the Gd moments. This results
in an increasing Curie temperature with decreasing lattice parameter and v
ice versa, a relation which even appears to be linear, probably because of
the negligible influence of the non-magnetic element to the Gd-Gd interacti
on 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 a
s the aforementioned compounds, it is concluded that the conduction electro
ns in GdMg2 must also mainly be of Gd-6s-like type.
The electronic character of the conduction electrons of ordered GdAl2 are m
ainly of Gd-5d-like type, which results in a decreasing Curie temperature w
ith decreasing lattice parameter. This relation was not so perfectly linear
, probably because of the p-electron character of the Al atoms. The p elect
rons 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, w
hich finally led to GdAl2 becoming a spin glass. The freezing temperature o
f ball milled GdAl2 turns out to be proportional to the defect concentratio
n.
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 ca
uses these compounds to exhibit, at least, re-entrant spin-glass-like behav
iour. (C) 1999 Elsevier Science B.V. All rights reserved.