[en] The structural and magnetic properties of phase-separated (Ga, Mn)As granular material, obtained by annealing diluted (Ga, Mn)As grown by molecular-beam epitaxy at low temperature, are investigated using transmission electron microscopy, high-resolution X-ray diffraction, and superconducting quantum interference device magnetometry. Synthesis strategies are defined to achieve specific magnetic properties through the control of the material structure

[en] The magnetic scattering in the paramagnetic phases of MnAs has been investigated in a spin polarised neutron scattering experiment. The magnetic correlations are found to indicate ferromagnetic short-range order which persists in both the orthorhombic as well as the high temperature hexagonal modification of MnAs. In particular, the unusual magnetic properties of the orthorhombic phase, as revealed by magnetisation measurements, are not mirrored in the magnetic scattering results which have been obtained in an almost zero external field experiment. The magnitude and the scattering vector dependence of the magnetic moment do not change as a function of temperature. It is argued that the unusual temperature dependence of the magnetic susceptibility is a phenomenon induced by the application of an external magnetic field and arises due to a coupling of magnetic and lattice degrees of freedom

[en] 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 post-growth thermal annealing process is demonstrated

[en] The discontinuous change in the lattice constant that occurs at the first-order phase transition between α- and β-MnAs gives rise to a coexistence of the two phases in MnAs layers grown on GaAs substrates. When the GaAs substrates are oriented in the (111)B direction, the c axis of MnAs is aligned normal to the growth plane. We identify the domain structure of α- and β-MnAs for this crystal orientation by utilizing the different reactivities of the two phases against wet chemical etching. Submicrometer-size islands of α-MnAs are found to be interwoven in a honeycomblike network of β-MnAs. We also show that this domain structure combined with strain effects results in a formation of MnAs lumps by etching

[en] By using scanning tunneling microscopy and reflection high-energy electron diffraction it is demonstrated that self-organized ordered Si structures develop during submonolayer Si deposition on vicinal GaAs(001) surfaces, provided the preferred adsorption sites in the trenches of the (2x4) reconstruction are filled with Ga. The evolution of different reconstructions with increasing Si coverages is accompanied by step bunching and de-bunching processes. This unexpected behavior is explained by the interaction between reconstructions and steps from a thermodynamic equilibrium view. For particular coverages the complex process leads to a separation of different surface phases and Si coverages on terraces and in step regions

[en] We report on an overgrowth of shallowly buried quantum wells (QW) of diluted magnetic semiconductor (DMS) (with non-magnetic CdMgTe or ZnTe barriers) on ferromagnetic MnAs and GaMnAs films. Atomic force microscopy (AFM) of the structures grown on MnAs revealed protruded micrometer-size islands, surrounded by areas with small-scale roughness. Magnetic force microscopy (MFM) imaging at room temperature showed clear magnetic domains within these islands. The structures overgrown on GaMnAs exhibited smooth surfaces, but no magnetic signal was detectable. The luminescence (PL) emission attributable to CdMnTe quantum wells was observed from all overgrown structures. Spatially resolved μ-PL showed that the peak energy of emission depended on the position of the excitation and PL collection spot for the samples overgrown on MnAs and was slightly blue-shifted when recorded from the magnetized islands area

[en] We have investigated epitaxial MnAs/GaAs(0 0 1) films in the phase transition region between 10 deg. C and 40 deg. C where hexagonal, ferromagnetic α-MnAs and paramagnetic, orthorhomic β-MnAs coexist. Quantitative stress measurements with a cantilever beam magnetometer reveal that during the phase transition the atomic distances change only in the vertical direction providing insight into the nature of the exchange coupling mechanism

[en] We present an analysis of thin MnAs films on GaAs(001) by the x-ray grazing incidence diffraction during molecular-beam epitaxy and immediately after deposition near the growth temperature. Separate MnAs peaks are detected for average thicknesses starting from ≅1 monolayer, indicating the formation of a relaxed MnAs lattice. The variation of the position and shape of the MnAs peaks during growth yields the time dependence of relaxation and island sizes. The MnAs domains of different orientations are detected and their amount is analyzed quantitatively. A line broadening due to the size and strain effects is observed. Both the effects are separated for each of the main directions along the interface. The lateral domain sizes of 10-40 nm and strain values of 0.2%-0.6% are found in the MnAs films. We find that the positions of the misfit dislocations at the interface are correlated

[en] MnAs films were grown by molecular beam epitaxy (MBE) on GaAs(110) substrates, since this orientation was recently identified as promising for the increase of spin lifetimes in semiconductor heterojunctions, which is of interest in spin injection experiments. A single epitaxial orientation was revealed for the MnAs films which consist of both the ferromagnetic, hexagonal α-MnAs and the paramagnetic, orthorhombic β-MnAs phase at room temperature. This phase coexistence could be imaged as a well ordered stripe pattern, whose periodicity depends on the film thickness. The study of the ferromagnetic properties shows a strong influence of the film thickness on the measured coercive fields and saturation magnetizations

[en] We study the nonconserved coarsening kinetics of a reconstructed semiconductor surface. The domain size evolution is obtained in situ by time-resolved surface x-ray diffraction. The system exhibits four equivalent domain types with two nonequivalent types of domain boundaries. Small domains are prepared by molecular beam epitaxy deposition of one GaAs layer. We find the correlation lengths of the domain size distribution to depend on time as l∝t^0.42±0.05 in the half-order reflections and l∝t^0.22±0.05 in the quarter-order reflections. The fraction of the higher energy domain boundaries increases as lnt