ABSTRACT
Co doped ZnO (Zn(1-x)Co(x)O) is studied as a prototype material for transition metal doped II-VI diluted magnetic semiconductors (DMSs) from first-principles and Monte Carlo simulations. The exchange interactions are calculated using the Korringa-Kohn-Rostoker (KKR) Green's function method. The exchange coupling constants thus obtained are treated in the classical Heisenberg model and the magnetic phase transitions are studied by the Monte Carlo technique. Our results show that the defect free substitutional DMSs of Zn(1-x)Co(x)O do not sustain magnetization at low concentration. At high concentration, we find layered magnetic structures. Ferromagnetism, with Curie temperature below room temperature, is stable at intermediate Co concentrations. First-principles studies with the generalized gradient approximation (GGA) and the GGA together with the Hubbard U are discussed with respect to structural and electronic properties of ZnO.
ABSTRACT
We analyze the depinning transition of a driven interface in the three-dimensional (3D) random field Ising model (RFIM) with quenched disorder by means of Monte Carlo simulations. The interface initially built into the system is perpendicular to the [111] direction of a simple cubic lattice. We introduce an algorithm which is capable of simulating such an interface independent of the considered dimension and time scale. This algorithm is applied to the 3D RFIM to study both the depinning transition and the influence of thermal fluctuations on this transition. It turns out that in the RFIM characteristics of the depinning transition depend crucially on the existence of overhangs. Our analysis yields critical exponents of the interface velocity, the correlation length, and the thermal rounding of the transition. We find numerical evidence for a scaling relation for these exponents and the dimension d of the system.
