Structural and magnetic investigations of epitaxial ferromagnetic τ MnAl films grown on GaAs/AlAs by molecular-beam epitaxy

The crystalline quality and magnetic properties of epitaxial ferromagnetic τ MnxAl100−x (x=50–70) films with perpendicular anisotropy grown on AlAs/GaAs by molecular-beam epitaxy are improved by ex situ rapid thermal annealing compared to as-grown thin films. An increase in magnetization of up to 230% is observed for moderate annealing temperatures (≊450 °C). This is strongly related to an improved ordering in the occupation of the two sublattices, which are antiferromagnetically coupled. At the same time a strong reduction in coercive field (up to a factor of 4) upon annealing is attributed to a decreased density of antiphase boundaries in the metal film. Annealing at higher temperatures (≊550 °C and above) results in the partial relaxation of the τ phase, and eventually in the transformation of the entire film to the nonmagnetic ε phase. The composition has a strong influence since the presence of excess Mn (x≳50) reduces the magnetization. The remanent magnetization Mr=465 kA/m for τ Mn50Al50 is close to the bulk value of 490 kA/m. The thickness of the film has little influence on the annealing behavior.