Magnetic properties and electronic structure of rare earth transition metal intermetallic compounds

Abstract:

A model for the electronic structure of the R,M, intermetallic compounds is proposed in which the s electrons are spread over the crystal and the d states are mostly localized on the transition metal sites, the degree of localization varying from 3d to 4d and 5d. We distinguish between two cases: (i) magnetism driven by the localized f moments (eg GdCo,), and (ii) magnetism sustained by the d band (eg LuFe,). These two situations are discussed in terms of an effective s4 coupling and an s-f exchange: the transition metal magnetic moment in (ii) is calculated using a simple model. The rare earth isomer shifts and the paramagnetic susceptibility of the compounds are also considered; the excess hf fields in the RFe, compounds may be understood within this picture. Finally, it is shown that the s-d hybridization mechanism may account for the observed effective s4 coupling.

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1. Introduction  

The magnetic properties of the R,M, intermetallic compounds (where R is a rare earth and M is a transition metal) have been investigated experimentally using a number of techniques (Taylor 1971). In the present work we will make use of the magnetic hyperfine field data, isomer shifts and information on the presence of magnetic order to suggest a model for the electronic structure of these compounds: some general features of this model were given in an earlier publication (Gomes 1972).

We will use a simple band consisting of s and d electrons, with a sharp f level, the d band being formed of 5d states from the rare earth and nd states (n = 3, 4, 5) from the transition metal. The s electrons are spread out through the entire crystal and have 6s-n’s character (n’ = 4, 5, 6). The degree of localization of the d electrons on the M sites, however. varies from 34 to @ and 5d transition metals. When M is a 3d metal the energy difference between the 3d electrons and the rare earth 5d electrons is so large that the d electrons are more localized on the M site. As we change to 4d or 5d transition elements the density of d electrons on the lanthanide site tends to increase, as suggested by the isomer shift measurements in the rare earth transition metal intermetallic compounds.

Furthermore, one can explain the hyperfine field data obtained in the RFe, compounds by assuming that there are practically no d electrons on the rare earth site (see 8 4). We have therefore taken in our simple model that the d electrons are completely absent on the R site, an assumption which was necessary to explain both isomer shift and hf field measurements in compounds with 3d metals. In these particular systems ow description conflicts with that proposed by Campbell (1972), where the  presence of d electrons is always required to couple rare earth moments to traasition metal moments.

The magnetic moment of the rare earth 4f shell and the d magnetic moment associated to the transition metal are antiparallel when R is a heavy rare earth, and parallel when R is in the first half of the lanthanide series; since J = L + S for the heavy rare earths and J = L - S for the light rare earths it follows that the spins of the f and d electrons are always antiparallel. The d moments attributed to the transition metals in the compounds are generally smaller than the moments in the pure metals, but in some cases may have comparable values.