Magnetism-induced ordering in two and three dimensions

Effective Interatomic Interactions VIA The TB-LMTO Method

The energetics of metallic alloys, their surfaces or interfaces, and magnetic multilayers is studied in terms of effective interatomic (or interlayer) interactions that are determined from ab initio electronic structure calculations using the TB-LMTO method combined with the coherent potential approximation and the method of surface Green functions. First the theoretical background (force theorem, Lloyd formula, generalized perturbation method for bulk and surfaces, vertex cancellation theorem, method of infinitesimal rotations) is discussed, and then the applications to the phase stability of bulk alloys, surface segregation in disordered alloys, magnetism-induced ordering in two- and three-dimensional systems, phase diagram of two-dimensional alloys, interlayer exchange coupling in metallic multilayers, and the construction of Heisenberg-like Hamiltonians for magnetic systems are presented.

Ordering in random overlayers: the correlated cluster mean-field method

We have extended a recently proposed correlated cluster mean-field (CCMF) theory to two-dimensional binary alloys with interactions up to the next-nearest neighbors and investigated their critical behavior. A very good agreement with results from the renormalization group theory was obtained for the critical temperatures of a simple model. We finally successfully compared our results obtained using the CCMF with the results of Monte Carlo simulations for the case of ordering in a random Fe(0.5)Co(0.5) overlayer on the Cu(001) substrate. The proposed theory represents a numerically efficient alternative to existing approaches like the cluster variation method and Monte Carlo simulations.