Evidence in rarehyphen;earth (R)ndash;transition metal (M) intermetallics for a systematic dependence of Rhyphen;M exchange interactions on the nature of the R atom

摘要

In rare earth (R)ndash;transition metal (M) compounds, large Rhyphen;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 4fshell, these Rhyphen;M interactions are indirect, mediated by the 5d, 6sconduction 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 asERhyphen;M=minus;nRMMSRMSM, whereMSRandMSMare the rarehyphen;earth and transition metal spin moments, respectively. The molecular field coefficientnRMis 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 coefficientnRMhas been obtained for a number of series of rare earthhyphen;transition metal compounds. The analysis reveals thatnRMis not a constant going across a given series but decreases by a factor of 2. The observed variation innRMis shown to be related to the variation of the exchange interactions between 4fand 5delectrons which are larger for light rarehyphen;earth elements since the difference between the spatial extent of the 4fand 5delectrons is reduced. As a general rule the observed enhancement of magnetic interactions for lightest rare earths in all rarehyphen;earth metallic systems can be attributed to the same phenomenon.