Magnetic structure of films: Dependence on anisotropy and atomic morphology

A review is given of surface magnetism. In particular the interplay of magnetization and anisotropy at a surface is discussed, after a survey of experimental and theoretical results for magnetic moment and anisotropy at surfaces or interfaces of semi-infinite magnets and thin films. Finally, the importance for surface (or interface) analysis of the random single-ion anisotropy model is stressed.

Surface magnetism; magnetization and anisotropy at a surface

The dynamical mean-field theory (DMFT) is employed to study the correlation-driven metal-insulator transition in the semi-infinite Hubbard model at half-filling and zero temperature. We consider the low-index surfaces of the three-dimensional simple-cubic lattice, and systematically vary the model parameters at the very surface, the intralayer and interlayer surface hopping, and the surface Coulomb interaction. Within the DMFT the self-energy functional is assumed to be local. Therewith, the problem is self-consistently mapped onto a set of coupled effective impurity models corresponding to the inequivalent layers parallel to the surface. Assuming that the influence of the high-energy Hubbard bands on the low-energy quasiparticle resonance can be neglected at the critical point, a simplified ``linearized DMFT'' becomes possible. The linearized theory, however, is formally equivalent to the Weiss molecular-field theory for the semi-infinite Ising model. This implies that qualitatively the rich phenomenology of the Landau description of second-order phase transitions at surfaces has a direct analog for the surface Mott transition. Motivated by this formal analogy, we work out the predictions of the linearized DMFT in detail. It is found that under certain circumstances the surface of a Mott insulator can be metallic, while a Mott-insulating surface of a normal metal is not possible. We derive the corresponding phase diagrams, the (mean-field) critical exponents and the critical profiles of the quasiparticle weight. The results are confirmed by a fully numerical evaluation of the DMFT equations using the exact-diagonalization (ED) method. By means of the ED approach, we especially investigate the noncritical parts of the phase diagrams and discuss the U and layer dependence of the quasi-particle weight. For strong modifications of the surface model parameters, the surface low-energy electronic structure dynamically decouples from the bulk.

Metallic surface of a Mott insulator-Mott insulating surface of a metal

Phase diagrams of the mixed spin Ising ferromagnet with a ferrimagnetic surface

The phase diagram of the three-dimensional transverse Ising system with competing surface and bulk exchange interactions is studied by the use of an effective-field method within the framework of a single-site cluster theory. In addition to this pure case, which shows qualitatively interesting features, we investigated the effects of the bond dilution on the obtained phase diagram. In particular, we studied the influence of the surface dilution parameter on the surface transitions. We find a number of characteristic behaviours of the surface both in the absence and in the presence of the surface transverse field. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Sanchez Jm

Papers

Boundary-condition effects on the surface magnetization of Ising ferromagnets.