L. Severin

Ab initio calculation of molecular field interactions in rare‐earth transition‐metal intermetallics (invited)

The interaction, KRM, between the rare‐earth 4f moment and the transition‐metal 3d moments in rare‐earth transition‐metal intermetallics is shown to depend upon the R‐5d moment, which is due to 3d–5d hybridization, and local 4f–5d exchange integrals. Both the R‐5d moment and KRM may be calculated ab initio from the local spin‐density approximation to density functional theory in self‐consistent energy‐band calculations with the localized 4f‐moments fixed at their Russel–Saunders values. Detailed examples are given for the RFe2 (R=Gd−Yb) series. The exchange integrals are similar to those entering into the density functional version of Stoner theory and their energy dependence must be treated carefully. The calculated local exchange integrals are shown to be related to the molecular fields derived from spin Hamiltonians, hence to the spin‐wave spectra. Reasonable agreement with values of the molecular fields extracted from inelastic neutron scattering and high field susceptibility measurements is obtained.

Orbital magnetism in energy bands

Abstract Recent work on orbital magnetism in itinerant magnets is reviewed. It is shown that such an orbital contribution to the magnetic moment is especially important for magnets involving actinide systems and that it is of a size comparable to the spin contribution. In particular, the results for the AnFe 2 (An = U, Np and Pu) compounds are discussed and their anomalous properties, as compared to corresponding standard rare-earth systems, are explained in terms of 3d–5f hybridization and orbital polarization within the 5f band manifold. As an introduction to the AnFe 2 systems we discuss CeFe 2 for which we predict 4f band magnetism in addition to the normal iron magnetism. It is most significant that, in agreement with experiment, the size of the iron moment is found to be reduced in comparison to its value in rare-earth magnets like YFe 2 . The α-γ transition in Ce metal is also treated and the calculated volume collapse agrees with experiment. Related calculations for Pr and Nd are also briefly discussed. Finally, the orbital contribution to the magnetism in Fe, Co and Ni is considered, and its maximum for Co is explained as the result of the HCP crystal structure.

Spin and orbital magnetization densities in itinerant magnets

Abstract The theoretical analysis of the contributions to the spin and orbital magnetization densities of transition metals, rare earths and actinides is described. Emphasis is placed upon the relative signs of both the local and diffuse spin moments and the orbital contributions to the moments. Examples include Fe, Co, Ni, rare earth and actinide intermetallic compounds and uranium NaCl-type monochalcogenides.

Density functional theory of molecular fields in R-M systems

Abstract In the local spin density approximation to the density functional theory the interaction, K RM , between the rare-earth 4f moment and local 4f-5d exchange integrals. Detailed examples are given for the RFe 2 (R=Gd-Yb) series. The calculated local exchange integrals are shown to be energy dependent and are then related to the molecular fields.

Theory of the Curie temperatures of the rare earth metals

Abstract The curie temperatures of the rare earth metals have been calculated ab initio using the local pin density approximation. The exchange splitting of the 5d-states depends upon local 4f-5d exchange integrals, κ 415d , which are calculated ab initio in the local spin density approximation. The Curie temperatures may then be obtained without use of adjustable parameters but are calculated to be too high by a factors of two to three if normal itinerant electron mean field theory is used. We have found that it is necessary allow for disordered local 5d moments above T c and to calculate the local susceptibility. When the local fluctuation contribution to the Landau Parameter, A , is included the Curie temperatures are actually reduced to below those measured.

Magnetism and crystal structure in orthorhombic Fe2P: a theoretical and experimental study

A theoretical and experimental study of the electronic and magnetic structure of Fe2P1-xSix is performed. A hexagonal-orthorhombic crystal structure transition occurs with increasing Si substitution. The latter crystal structure is extremely complex with six inequivalent Fe sites. The magnetic moments, as obtained by our electronic structure calculations, vary substantially between the different sites, from 0.7 mu B to 2.6 mu B. The latter value is one of the largest Fe moments ever found. The estimated hyperfine fields are in fair agreement with Mossbauer data. For this compound it is found that the hyperfine field cannot be assumed to be proportional to the spin magnetic moment when the majority spin band is saturated. This is explained in terms of a hybridization mechanism between the Fe 3d and 4s states, which results in a 4s valence electron contribution that may be large and of either sign.

Mechanisms for itinerant magnetism in intermetallic compounds

Abstract We consider how magnetism in itinerant intermetallic systems arises within the local spin density approximation (LSDA) through the concepts of local exchange and hybridization. The delicate interplay between these two mechanisms explains trends in ground state magnetic properties. This is exemplified with the AFe 2 (A = 5d transition metal atom) and AnFe 2 (An = actinide) series of intermetallic compounds.

Density functional theory of spin and orbital magnetization densities in actinide magnets

Abstract We summarize recent attempts to calculate the contributions to the spin and orbital magnetization densities of transition metals and actinide compounds. Emphasis is placed on the relative signs of both the local and the diffuse spin moments and the orbital contributions to the moments. Examples include Fe, Co, Ni, actinide transition metal intermetallic compounds and uranium NaCl-type monochalcogenides.

EXPERIMENTAL AND THEORETICAL-STUDIES OF SI/P SUBSTITUTED ORTHORHOMBIC FE2P

A Mössbauer spectroscopic study of orthorhombic Fe2P1−xSix withx ≤ 0.35 was performed. A large spread in magnetic hyperfine fields was found at the six Fe positions ranging from 10–26 T at 4.2 K. Small rearrangements in the crystal positions as compared to the hexagonal phase cause large changes in the magnetic field. Large changes in Fe magnetic moments have also been obtained in a spin-polarized LMTO band calculation performed on orthorhombic Fe2P as compared to a similar calculation of hexagonal Fe2P.

THEORY OF ORBITAL SPLITTING IN METALS

A theory for orbital splitting is derived from a statistical Hartree-Fock (HF) treatment of open shell interactions. In a scaling procedure, where the HF spinpolarization matrix is replaced by the corresponding local spin density matrix, an expression for the orbital splitting is derived which is well suited for implementation in the standard ab initio calculational scheme. Results for Co metal as well as for the itinerant 5f ferromagnet US is presented, which are in good agreement with experiment.