A mean-field model of ferromagnetism mediated by delocalized or weakly localized holes in zinc-blende and wurzite diluted magnetic semiconductors is presented. The model takes into account strong spin-orbit and k⋅p couplings in the valence band as well as the influence of strain upon the hole density of states. Possible effects of disorder and carrier-carrier interactions, particularly near the metal-to-insulator transition, are discussed. A quantitative comparison between experimental and theoretical results for (Ga,Mn)As demonstrates that the theory describes the values of the Curie temperatures observed in the studied systems as well as explaining the directions of the easy axes and the magnitudes of the corresponding anisotropy fields as a function of biaxial strain. Furthermore, the model reproduces the unusual sign, magnitude, and temperature dependence of the magnetic circular dichroism in the spectral region of the fundamental absorption edge. Chemical trends and various suggestions concerning design of ferromagnetic semiconductor systems are described.

We present a combined theoretical and experimental study of the ferromagnetic semiconductor (Ga,Mn)As which explains the remarkably large changes observed on low-temperature annealing. Careful control of the annealing conditions allows us to obtain samples with ferromagnetic transition temperatures up to 159 K. Ab initio calculations, in situ Auger spectroscopy, and resistivity measurements during annealing show that the observed changes are due to out diffusion of Mn interstitials towards the surface, governed by an energy barrier of 0.7–0.8 eV. Electric fields induced by Mn acceptors have a significant effect on the diffusion.

A new GaAs-based diluted magnetic semiconductor, (Ga,Mn)As, was prepared by molecular beam epitaxy. The lattice constant of (Ga,Mn)As films was determined by x-ray diffraction and shown to increase with the increase of Mn composition, x. Well-aligned in-plane ferromagnetic order was observed by magnetization measurements. Magnetotransport measurements revealed the occurrence of anomalous Hall effect in the (Ga,Mn)As layer.

Magnetotransport properties of p-type (In,Mn)As, a new diluted magnetic semiconductor based on a III-V semiconductor, are studied. The interaction between the holes and the Mn 3d spins is manifested in the anomalous Hall effect, which dominates the Hall resistivity from low temperature (0.4 K) to nearly room temperature, and in the formation of partial ferromagnetic order below 7.5 K, which is a cooperative phenomenon related to carrier localization. The coexistence of remanent magnetization and unsaturated spins as well as the large negative magnetoresistance at low temperatures is explained by the formation of large bound magnetic polarons.

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Thin epitaxial films of the diluted magnetic semiconductor (DMS) GaMnAs have been studied by low energy muon spin rotation and relaxation (LE-µSR) as well as by transport and magnetization measurement techniques. LE-µSR allows measurements of the distribution of magnetic field on the nanometer scale inaccessible to traditional macroscopic techniques. The spatial inhomogeneity of the magnetic field is resolved: although homogeneous above Tc, below Tc the DMS consists of ferromagnetic and paramagnetic regions of comparable volumes. In the ferromagnetic regions the local field inhomogeneity amounts to 0.03 T.

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