(Ge,Mn)Te is a IV-VI diluted magnetic semiconductor exhibiting ferromagnetic transition induced by a very high conducting hole concentration. The Curie temperature in crystalline (Ge,Mn)Te layers can be increased up to TC = 190 K by independent optimization of Mn2+ ions content and carrier concentration [1,2]. GeTe and (Ge,Mn)Te crystals are known to undergo structural (ferroelectric) transition in which high-temperature cubic (rock-salt) structure becomes distorted along cube’s diagonal to form low-temperature rhombohedral structure. Recent magnetization and ferromagnetic resonance (FMR) studies of (Ge,Mn)Te epitaxial layers on BaF2 (111) substrates revealed that in monocrystalline layers with 10 at. % of Mn content the easy axis of magnetization is perpendicular to the layer (perpendicular magnetic anisotropy - PMA) while the usual easy-plane anisotropy is observed in layers with 20 at. % of Mn content. Additional annealing of layers results in the change of the easy axis of magnetization to the in-plane direction in samples with Mn content 10 – 20 at. %. These experimental findings indicate the key role of lattice distortions (the change of rhombohedral angle) in magnetic anisotropy of (Ge,Mn)Te layers.
In this work, we quantitatively examine this effect by FMR and SQUID superconducting magnetometry techniques in monocrystalline (Ge,Mn)Te layers (thickness of 0.7 µm) grown on BaF2 (111) substrates by molecular beam epitaxy with Mn concentration varying from 10 to 20 at. %. Post-grown annealing was performed to compare magnetic properties of as-grown and annealed samples. Our experimental results on the angular dependence of the FMR resonant field are analyzed in model calculations taking into account the contribution of demagnetization fields and the magnetocrystalline anisotropy terms in magnetic free energy. We propose a microscopic model of the PMA effect in (Ge,Mn)Te/BaF2(111) layers with the key role of the hybridization of 3d5 electronic orbitals of Mn2+ ions and the valence band states of rhombohedrally  - distorted crystal lattice of p-(Ge,Mn)Te.
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This work was supported by the research project UMO 2011/01/B/ST3/02486 of Polish National Science Center (NCN).