Magnetic properties of epitaxial MnAs thin films on GaAs (001)

The magnetic properties of two types of epitaxial MnAs films on GaAs (001) substrates in the thickness range of 20 similar to 200 nm were studied. Using longitudinal a magneto-optical Kerr-effect (MOKE) apparatus at lambda =632.8 nm, we determined the Curie temperatures of the 100-nm thick films to be 54.0+/-0.5 degreesC and 63.7+/-0.5 degreesC for type A films and type B films, respectively. The observed Curie temperatures corresponded to increases of 36.8 degreesC and 33.9 degreesC per one percent increase in the unit cell volume for type A and B, respectively. The normalized maximum MOKE signal from the type A film exhibited a first-order-like magnetic transition while that of type B underwent a second-order-like transition. These different behaviors between types A and B stem from different residual stresses being exerted on the hexagonal phase. Utilizing a Foner-type vibrating sample magnetometer at room temperature, we examined the thickness dependence of the coercive force and the saturation magnetization of the film. The coercive force of films thicker than 10 nm decreased as the thickness increased, which was interpreted in terms of a decreasing magneto-static energy contribution to the Bloch domain wall energy with increasing film thickness. The saturation magnetization had a maximum at a thickness of 50 nm. This observation reflects the dependence of the saturation magnetization on the a-constant of the hexagonal phase.