Volume dependence of the Curie temperature of rare-earth-3d transition metal compounds

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Abstract
The pressure variation of the Curie temperature TC has been measured up to 3.6 kbar for GdNi, RNi3 (R=Gd, Tb, Dy), R2Ni7 (R=Gd, Tb, Dy, Nd, Y), GdNi5, R2Ni17 (R=Dy, Ho, Er, Y) and Gd4Co3 ordering at temperatures between 32 and 225K. The values of dTC/dP are small ranging from -0.48 to 0.08K kbar-1. Thermal expansion measurements on GdNi3, Tb2Ni7, Ho2Ni17 and Y2Ni17 show no marked spontaneous volume magnetostriction. The quantity Gamma =d 1n TC/d 1n V is observed to decrease with decreasing TC. This contradicts the TC-2 behaviour which can be derived from the Stoner-Wohlfarth model of weak itinerant ferromagnetism and has been found to be valid for all R-Co and R-Fe compounds studied earlier but for those with TC lower than about 400K. Since small or even negative values of gamma are observed for compounds where R=Y or La the reduction of Gamma cannot be attributed to R-3d and R-R interactions. The phenomenon is discussed also in terms of a compression-induced transfer of 3d electrons to other bands and of alloy heterogeneities, but a more plausible explanation is given by a combined model of localised and itinerant d electrons.