Tailoring of the structural and magnetic properties of MnAs films grown on GaAs-strain and annealing effects

MnAs films were deposited by molecular-beam epitaxy on GaAs(001) and GaAs(111)B surfaces. Imaging of the temperature-dependent magnetic structure by x-ray magnetic circular dichroism photoemission electron microscopy, and the comparison with magnetization measurements by superconducting quantum interference device (SQUID) magnetometry, is used to study the impact of the different strain state of MnAs/GaAs(001) and of MnAs/GaAs(111)B films on the phase transition between ferromagnetic alpha-MnAs and paramagnetic beta-MnAs, the spatial distribution of the two structural and magnetic phases,. and the transition temperature. For the isotropically strained MnAs/GaAs(111)B films, the phase coexistence range is much wider than for the anisotropically strained MnAs/GaAs(001) films. The characteristic change of the saturation magnetization with film thickness is found to be a universal property of films with different epitaxial orientation, if at least one MnAs (1120) direction is in the film plane. For MnAs/GaAs(001) films this variation is related to the striped coexistence of a and 8 MnAs and the changing intra- and inter-stripe magnetic interaction with film thickness and temperature. The magnetic structure of MnAs/GaAs(111)B is more complex due to the existence of three symmetry-equivalent a-phase domains superimposed by a honeycomb-like network of the coexisting 8 phase. The magnetic properties (saturation magnetization, domain size) of thin MnAs/GaAs(001) films can be improved by postgrowth annealing. Above a certain film thickness this is inhibited by the complex magnetic structure of the film.