We report on the growth, structural as well as magnetic characterization of (Ga,Mn)N epitaxial layers grown directly on 4H–SiC(0001) by reactive molecular-beam epitaxy. We focus on two layers grown under identical conditions except for the Mn/Ga flux ratio. Structural characterization reveals that the sample with the lower Mn content is a uniform alloy, while in the layer with the higher Mn content, Mn-rich clusters are found to be embedded in the (Ga,Mn)N alloy matrix. Although the magnetic behavior of both the samples is similar at low temperatures, showing antiferromagnetic characteristics with a spin-glass transition, the sample with higher Mn content additionally exhibits ferromagnetic properties at and above room temperature. This ferromagnetism most likely originates from the Mn-rich clusters in this sample.

We present experimental evidence of the equilibrium coexistence between crystalline phases in heteroepitaxial films of MnAs on GaAs. The phases, which can coexist in the bulk system only at one temperature point, coexist in the epitaxial film over a wide temperature interval. An apparent contradiction with the Gibbs phase rule is resolved by the presence of strain in the film.

Studies on MnAs, and in particular of its magnetostructural phase transition, have a long history. As a promising material for ferromagnet/semiconductor hybrid structures with new challenges for solid state physics and electronic engineering, MnAs thin films recently came again into the focus of interest. This review summarizes the presently available knowledge about epitaxial growth of MnAs films in a variety of epitaxial orientations on differently oriented GaAs substrates, their interface formation, and the interrelated structural and magnetic properties. In situ growth studies using reflection-high energy electron diffraction and high-resolution x-ray diffraction as well as imaging of the growth morphology by scanning tunnelling microscopy provided a detailed understanding of the growth kinetics. The mismatch accommodation mechanisms elucidated by high-resolution transmission electron microscopy investigations explain how films of high quality can be grown despite a large and anisotropic misfit. Most extensively considered are structural and magnetic properties that are related to the strain evolution in the films during cooling after growth. In clear contrast to bulk MnAs, the structural phase transition in MnAs films exhibits a coexistence of the ferromagnetic α-phase and the paramagnetic β-phase over a wide range of temperatures and a thickness dependent thermal hysteresis. This strain-mediated phase coexistence has been studied in detail by x-ray diffraction and by imaging the magnetic structure using magnetic force microscopy and magnetic circular dichroism photoemission electron microscopy, and also theoretically. It depends in a characteristic manner on the epitaxial orientation. Using high-resolution x-ray diffraction data, it is shown that a unified mechanism explains the shift of the ferromagnetic transition temperature to higher values in as-grown MnAs films of appropriate epitaxial orientation, in MnAs films under external biaxial strain, and in MnAs clusters within a GaAs matrix. The improvement of the structural and magnetic properties of the films by a postgrowth thermal annealing process is demonstrated.The article was invited by Professor K Ploog.

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The temperature-dependent phase coexistence between crystalline phases in heteroepitaxial films of MnAs on GaAs is studied. The epitaxial constraints on the film expansion are analyzed. The x-ray-diffraction data are fitted to a model of periodic elastic domains. The temperature dependencies of phase fractions, the domain sizes, and the misfits are simultaneously obtained. The domain sizes correspond to the minimum of elastic energy, which proves the equilibrium state of the heteroepitaxial system at each temperature.