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We present the observation of temperature-induced transition between ferromagnetic (FM) and antiferromagnetic (AFM) interlayer exchange coupling (IEC) in trilayer structures consisting of two (Ga,Mn)(As,P) ferromagnetic layers and a GaAs:Be spacer. Hall resistance measurements were performed on trilayers prepared in as-grown and annealed states. Anomalous Hall resistance (AHR) measurements show two-step hysteresis loops, indicating that coercive fields of the two (Ga,Mn)(As,P) layers in the trilayer are different. Minor loops of AHR show characteristic shifts that correspond to FM IEC at low temperatures and AFM IEC at high temperatures in both as-grown and annealed samples. This temperature-induced transition from FM to AFM IEC occurs at a lower temperature in the annealed sample than in the as-grown sample. We attribute such an enhanced IEC transition effect in the annealed sample to the increase of carrier concentrations obtained by annealing, a characteristic feature of ferromagnetic semiconductors. The observation of AFM IEC in the (Ga,Mn)(As,P)-based trilayer suggests the possibility of realizing synthetic antiferromagnetic systems in such structures, which can be used for antiferromagnetic spintronic device applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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-ofplane 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-ofplane 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. [ABSTRACT FROM AUTHOR]