We propose a self-consistent approximate solution of the s - f model for describing the exchange coupling of a local moment system with a partially filled energy band. Induced electronic correlations account for the characteristic quasiparticle band effects which become manifest via striking temperature dependencies, band deformations and splittings. For weak s - f exchange interactions a `Stoner-like' spin splitting of the conduction band proportional to the f magnetization occurs. As soon as the coupling exceeds a critical value an additional spin splitting of the quasiparticle dispersion sets in, which is due to different elementary excitations. One of these appears as a repeated emission and reabsorption of a magnon by the conduction electron, resulting in an effective electron - magnon attraction. This gives rise to a polaron-like quasiparticle (a `magnetic polaron'). Other elementary processes are connected to magnon emission or absorption by the conduction electron (`scattering states'). The polarization of the conduction band due to the s - f exchange interaction J feeds back to the localized spin system leading to an indirect coupling between the spins. For weak s - f coupling the RKKY mechanism dominates , but with remarkable deviations for intermediate and strong couplings. The Curie temperature saturates with increasing J, where the saturation value is strongly dependent on the band occupation n. The oscillating behaviour of the effective exchange integral connecting the localized spins restricts ferromagnetism to special regions for n. The magnetization curve, the spin polarization of the itinerant electrons, and f - f as well as s - f spin correlation functions are worked out for a simple cubic lattice and discussed in terms of the band occupation n and the s - f exchange coupling J.

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%'e report here the magnetic and nonmagnetic electronic-structure calculations of the perovskite oxides LaMO3 (M=Ti to Ni) and the orthorhombic LaSc03 performed using the tight-binding linearmufin-tin-orbital method. Total-energy calculations for these systems reproduce the magnetic phases observed at low temperature. The calculations correctly account for the insulating properties of the systems for M= Sc, Cr, Fe, and Co. However, for M= Ti, V, and Mn, the calculations fail, which is basically due to the failure of spin-density-functional theory in providing the required splitting of the 3d t2g and eg bands of M atoms. But the calculated magnetic moments, p-d band separations, and insulating gaps agree well with the experimental values. Moreover, the calculated total densities of states show a good agreement overall with photoemission data. Furthermore, high-pressure studies of these systems for M=Sc, Cr, Fe, and Co are also reported.

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The electronic structure calculations of the perovskite oxides SrCrO 3 and PbCrO 3 performed both in the paramagnetic and antiferromagnetic phases are reported here. The calculations were carried out using the Linear Muffin Tin Orbital method within the Atomic Sphere Approximation. The quantitative results obtained are found to give a good description of the electronic states of SrCrO 3 and are in agreement with the Goodenough’s qualitative chemical picture. However, it is not able to predict the semiconducting gap in PbCrO 3 which is an antiferromagnetic semiconductor. But the value of the theoretically calculated magnetic moment at the Cr site in PbCrO 3 is found to be in good agreement with the experimentally observed value. The calculations show strong hybridisation between the Cr -3d and O -2p orbitals and the density of states at the Fermi energy has major contributions from these hybridised orbitals.

Li 2 Pd 3 B is known to be superconducting, while the isotypical Li 2 Pt 3 B compound is not. Electronic structures of Li 2 Pd 3 B and Li 2 Pt 3 B have been calculated in order to obtain an insight into this surprising difference, through an analysis of the differences in the band structures. The electronic structures of these systems were obtained using the Full Potential Linear Augmented Plane Wave plus local orbitals (FP-LAPW+lo) method and it was found that four bands cross the Fermi level (E F ). Out of these four bands, only two bands contribute significantly to the density of states at the E F . One of these bands is a hole band and the other an electron band. Thus at least a two-band model is required for studying the electronic properties of the Pd and Pt compounds. These two bands are rather narrow and hence the coulombic correlations effects can be significant.