We present a method for performing atomistic spin dynamic simulations. A comprehensive summary of all pertinent details for performing the simulations such as equations of motions, models for including temperature, methods of extracting data and numerical schemes for performing the simulations is given. The method can be applied in a first principles mode, where all interatomic exchange is calculated self-consistently, or it can be applied with frozen parameters estimated from experiments or calculated for a fixed spin-configuration. Areas of potential applications to different magnetic questions are also discussed. The method is finally applied to one situation where the macrospin model breaks down; magnetic switching in ultra strong fields.

We show that the three most relevant magnetic properties (magnetic moment, critical temperature, and interlayer exchange coupling) of metallic multilayers can be reproduced with good accuracy by first principles theory, provided that the picture of atomically sharp interfaces is abandoned and one allows instead for both interface alloying and interface roughness. The interface of a metallic multilayer (exemplified by the Fe͞V system) is demonstrated to, at best, have interdiffusion essentially over two to three atomic layers on each side of the interface. Our conclusions are the result of combining experimental work with theoretical modeling, and we argue that this approach is the best avenue to obtain accurate information about the interface quality of metallic multilayers.T he study of physical properties of multilayers (structural, elastic, magnetic, and transport) is one of the most prosperous and rich branches of materials science today (1-3). By growing different atomic layers on top of each other, new phenomena have been observed, e.g., a reduced critical temperature for magnetic ordering, an oscillating interlayer exchange coupling (4), and giant magneto resistance (5). There are several ways in which these properties are used in nanotechnology, using for instance the giant magneto resistance effect (4, 5) in data storage and computer memories. Single-crystal multilayers represent the highest obtainable degree of perfection, with respect to interface and crystalline quality. This type of multilayer is often referred to as a superlattice.Typically, superlattices are grown by molecular beam epitaxy or sputtering. The growth temperature has to be sufficiently high to ensure large surface mobility and thereby enable layer-bylayer growth (6). At the same time, the temperature must be sufficiently low to prevent the metals from interdiffusing and thereby forming alloys. This delicate balance between surface and bulk mobility is of uttermost importance because, as we shall see, all properties that are special for multilayers (interlayer exchange coupling, magnetic moments, and critical temperatures) depend on the interface quality. Although it has been difficult to assess the interface quality in multilayers, it is safe to conclude that an ideal layer-by-layer growth is never obtained.In magnetic superlattices, interface roughness and interface mixing (7) are the most important types of imperfections. These types are schematically illustrated in Fig. 1. In the figure we specify the geometrical extent of the roughness and mixing via the parameters ⌫ T and ⌫ C , respectively. The mixing is assumed to follow a normal distribution with a width specified by ⌫ C . For the roughness we assume that it is composed of terraces with large lateral extent, such that a multilayer of formal composition, e.g., Fe 3 V 7 , may be thought to be composed of multilayers Fe 2 V 8 , Fe 3 V 7 , and Fe 4 V 6 , with a distribution specified by ⌫ T . For details of how the structure of the interface was constructed mathematically, please...

The dynamical behavior of the magnetism of diluted magnetic semiconductors ͑DMSs͒ has been investigated by means of atomistic spin-dynamics simulations. The conclusions drawn from the study are argued to be general for DMS systems in the low-concentration limit, although all simulations are done for 5% Mndoped GaAs with various concentrations of As antisite defects. The magnetization curve M͑T͒ and the Curie temperature T C have been calculated and are found to be in good correspondence to the results from Monte Carlo simulations and experiments. Furthermore, equilibrium and nonequilibrium behaviors of the magnetic pair-correlation function have been extracted. The dynamics of DMS systems reveals a substantial short-ranged magnetic order even at temperatures at or above the ordering temperature, with a nonvanishing pair-correlation function extending up to several atomic shells. For the high As antisite concentrations the simulations show a short-ranged antiferromagnetic coupling and a weakened long-ranged ferromagnetic coupling. For sufficiently large concentrations we do not observe any long-ranged ferromagnetic correlation. A typical dynamical response shows that starting from a random orientation of moments, the spin correlation develops very fast ͑ϳ1 ps͒ extending up to 15 atomic shells. Above ϳ10 ps in the simulations, the pair correlation is observed to extend over some 40 atomic shells. The autocorrelation function has been calculated and compared with ferromagnets such as bcc Fe and spin-glass materials. We find no evidence in our simulations for a spin-glass behavior for any concentration of As antisites. Instead the magnetic response is better described as slow dynamics, at least when compared to that of a regular ferromagnet such as bcc Fe.

We have studied alloying of the nonmagnetic spacer layer with a magnetic material as a method of tuning the interlayer coupling in magnetic multilayers. We have specifically studied the Fe/V(100) system by alloying the spacer V with various amounts of Fe. For some Fe concentrations in the spacer, it is possible to create a competition between antiferromagnetic Ruderman-Kittel-Kasuya-Yoshida exchange and direct ferromagnetic exchange coupling. The exchange coupling and transport properties for a large span of systems with different spacer concentrations and thicknesses were calculated and measured experimentally and good agreement between observations and theory was observed. A reduction in magnetoresistance of about 50% was observed close to the switchover from antiferromagnetic to ferromagnetic coupling.

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