We report a method for accurate determination of the strength of the current-induced spin-orbit (SO) field in ferromagnetic GaMnAs films. The SO-field manifests itself in the form of a hysteresis between planar Hall resistances (PHR) measured with positive and negative currents as an applied magnetic field is rotated in the sample plane at constant field strength. The width of the hysteresis, which is related to the strength of the SO-field, is observed to change significantly for different values of the rotating external field strength. Since the SO field occurring at a given current is an intrinsic property of the crystal, such a field dependence of the hysteresis indicates that the width of the hysteresis measured with a single field strength is insufficient for determining the SO field. However, using a model based on magnetic free energy that includes the effects of magnetic anisotropy and the SO-field as developed in the present paper, we show that the SO field for a given current density can be accurately established by fitting to the experimentally observed dependence of transition angles of PHR measured with different applied field strengths. Using the known dependence of magnetic anisotropy of GaMnAs on temperature, we also show that this method applies reliably as the temperature varies.

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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Spin–orbit-induced (SOI) effective magnetic field in GaMnAs film with in-plane magnetic anisotropy has been investigated by planar Hall effect measurements. The presence of SOI field was identified by a shift between planar Hall resistance (PHR) hystereses observed with positive and negative currents. The difference of switching fields occurring between the two current polarities, which is determined by the strength of the SOI field, is shown to depend on the external field direction. In this paper we have developed a method for obtaining the magnitude of the SOI fields based on magnetic free energy that includes the effects of magnetic anisotropy and the SOI field. Using this approach, the SOI field for a given current density was accurately obtained by fitting to the observed dependence of the switching fields on the applied field directions. Values of the SOI field obtained with field scan PHR measurements give results that are consistent with those obtained by analyzing the angular dependence of PHR, indicating the reliability of the field scan PHR method for quantifying the SOI-field in GaMnAs films. The magnitude of the SOI field systematically increases with increasing current density, demonstrating the usefulness of SOI fields for manipulation of magnetization by current in GaMnAs films.