Ferromagnetic resonance in Ga1-xMnxAs dilute magnetic semiconductors

We review the phenomenon of ferromagnetic resonance (FMR) in ferromagnetic ( FM) Ga1-xMnxAs semiconductor alloys and their heterostructures in thin film form. We will show that the analysis of FMR in Ga1-xMnxAs films can directly provide values of cubic and uniaxial magnetic anisotropy fields in these materials-i.e. the anisotropy associated with the natural (undistorted) zinc-blende structure and that arising from strain-induced distortion, respectively. In addition to the effects of strain, in this review we will also discuss the use of FMR to determine the effects of annealing, temperature, and doping on magnetic anisotropy. The FMR results attained on the temperature dependence of anisotropy fields ( both cubic and uniaxial) provide a natural explanation of the easy-axis reorientation transition that is observed in these materials as the temperature changes. Using results observed on Ga1-xMnxAs samples where the concentration of holes is controlled either by annealing or by modulation doping, we will show that FMR also provides a convenient tool for studying the correlation between hole concentration and magnetic anisotropy. Additionally, we will show that the FMR studies of Ga1-xMnxAs/Ga1-yAlyAs heterostructures modulation doped by Be reveal that the effective g-factor of Ga1-xMnxAs is also strongly affected by the doping. The measurements of the total g-factor can in turn be used to estimate the contribution which the holes themselves make to the total magnetization of Ga1-xMnxAs. Finally, we will review the results which are currently available on the FMR linewidth, including its dependence on temperature, angle of applied field, and annealing. Although the data on FMR broadening are at this time largely phenomenological, the dependence of the linewidth on hole concentrations suggests that the p-d coupling between the holes and the Mn2+ ions contributes significantly to the damping rate of the magnetization precession in FMR experiments on Ga1-xMnxAs films. Finally, it should be noted that - although in this review we focus on Ga1-xMnxAs, because the overwhelming majority of work on FMR has been carried out on this material - the description of FMR and its analysis presented here can be applied to thin layers of all III1-xMnxV alloys generally.