We have investigated the magnetic properties of the Ni films deposited on a GaAs and a Bi2Se3 buffer grown by molecular beam epitaxy on a GaAs (001) substrate. The magnetization measurements at 4 K revealed that the coercivity of the Ni films decreases monotonically with increasing thickness up to 25 nm in both cases. However, the coercivity measured at 4 K was always larger in the Ni film deposited on the surface of Bi2Se3 than in the film deposited on the GaAs. Such enhancement of the coercivity decreases with increasing temperature and film thickness. This suggests that the Bi2Se3 surface alters the magnetic properties of the Ni film. The increase of the coercivity was more serious in an un-capped Ni/Bi2Se3 sample, which showed an exchange bias effect due to the oxidation of the top surface of the Ni film. These observations are important for the investigation of spin dependent phenomena in magnetic systems involving a ferromagnet/topological insulator interface.

We report a systemeatic investigation of magnetic anisotropy of quaternary GaMnAsP ferromagnetic semiconductor films by magneto-transport. Hall measurements showed a transition of the easy magnetization direction from in-plane to out-of plane with incorporation of the P into the GaMnAs films. Quantitative information on magnetic anisotropy of the films is obtained by fitting the angular dependence of Hall resistance data to magnetic free energy using the coherent rotation model. Values of magnetic anisotropy parameters show that in-plane anisotropy decreases and out-of-plane anisotropy increases with increasing P content in these films. The out-of-plane magnetic anisotropy in GaMnAsP layers is further enhanced by low temperature annealing. By optimizing the growth and annealing conditions, we were able to obtain a Curie temperature of 125 K in such quaternary films, with strong out-of-plane anisotropy. This study showed that the magnetic anisotropy of the GaMnAsP films can be controlled by adjusting the concentration of the P, and by appropriate post-growth annealing.

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We construct the Feynman integral for the Schrödinger propagator with combinations of exponentially growing and harmonic oscillator potentials as well-defined white noise functionals.