Magnetism and electronic structure of a local moment ferromagnet

Abstract

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.

Magnetism in (III,Mn)-V diluted magnetic semiconductors: Effective Heisenberg model

“…These results support recent findings obtained by using MC simulations in combination with first-principle methods 15,16 . The presented model should be improved by using a self-consistent method describing the electronic degrees of freedom at finite temperature (such as, e.g., in 32,33 ). In order to obtain a fully quantum mechanical theory, quantum spins should be used instead of classical spins in in the calculation of the effective exchange parameters from the electronic structure.…”

Kondo and anti-Kondo coupling to local moments inEuB6

“…These mechanisms might be of importance in the ∆ < 0 case leading to removal of the first order transition. The mean field study does serve to demonstrate the potential usefulness of the two-band hamiltonian and to stimulate further studies by more advanced techniques, such as the Green's function approach of Nolting et al 30 …”

Low-density approach to the Kondo-lattice model

“…The importance of such effects has been discussed in detail in ref. [10]. Conclusions such as, that at T = 0 the spins of the e g electrons are oriented parallel to the t 2g −spins [30], are correct only for S → ∞.…”

“…Due to some reasons, the above-mentioned theories [10,31] are best justified for weak and intermediate couplings J. In this paper, we propose an approximate scheme which mainly aims at the strong coupling regime (JS >> W : W is the bandwidth) being nevertheless perturbationally correct up to order J 2 .…”