Evidence in rare-earth (R)–transition metal (M) intermetallics for a systematic dependence of R-M exchange interactions on the nature of the R atom

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Abstract
In rare earth (R)–transition metal (M) compounds, large R-M magnetic interactions can occur, which give rise to higher values of the ordering temperature (TC,TN) for compounds with magnetic R elements than for compounds with R nonmagnetic, i.e., La, Lu, Y. Due to the localized character of the 4f shell, these R-M interactions are indirect, mediated by the 5d, 6s conduction electrons. The highest value of the ordering temperature is obtained for Gd compounds, and as a first approximation it is reasonable to write the interaction energy as ER-M=−nRMMSRMSM, where MSR and MSM are the rare-earth and transition metal spin moments, respectively. The molecular field coefficient nRM is generally assumed to be a constant throughout a given series, owing to the similarities of band structure for all R elements. In this paper, the molecular field coefficient nRM has been obtained for a number of series of rare earth-transition metal compounds. The analysis reveals that nRM is not a constant going across a given series but decreases by a factor of 2. The observed variation in nRM is shown to be related to the variation of the exchange interactions between 4f and 5d electrons which are larger for light rare-earth elements since the difference between the spatial extent of the 4f and 5d electrons is reduced. As a general rule the observed enhancement of magnetic interactions for lightest rare earths in all rare-earth metallic systems can be attributed to the same phenomenon.