Exchange interactions in R-M intermetallics

Some unsolved questions in the understanding of the magnetism of R-M interm etallics (essentially R-Fe) are addressed. The variation of the R-M interac tions at the R-sites from one compound to another and the concomitant varia tion of the 5d polarisation are discussed in terms of 3d-5d hybridisation. Discrepancies in the strength of the R-M interactions derived from differen t experimental methods are interpreted in terms of temperature-dependent ba nd-splitting at the Fe sites in R-rich compounds (e.g. GdFe2). The weakness of the R-R interactions is ascribed to the d-electrons distribution being highly aspherical, because d-d hybridisation favours R-M bonding instead of R-R bonding. The value of E-FeFe in many different compounds is discussed in terms of d-band filling. This allows the 3d interactions in Fe and Co co mpounds to be interpreted consistently. The observed 4f moment reduction, i n some Pr-M and Nd-M compounds, is related to the occurrence of 4f-d hybrid isation, itself favoured by 3d-5d hybridisation. (C) 1999 Elsevier Science B.V. All rights reserved.