Ferromagnetic resonance in Ga1−xMnxAs dilute magnetic semiconductors

Abstract

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 FMRin Ga1−xMnxAs films can directly provide values of cubic and uniaxial magnetic anisotropy fields in thesematerials—i.e. the anisotropy associated with the natural (undistorted) zinc-blendestructure and that arising from strain-induced distortion, respectively. In addition to theeffects of strain, in this review we will also discuss the use of FMR to determine the effectsof annealing, temperature, and doping on magnetic anisotropy. The FMR results attainedon the temperature dependence of anisotropy fields (both cubic and uniaxial)provide a natural explanation of the easy-axis reorientation transition that isobserved in these materials as the temperature changes. Using results observed onGa1−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 holeconcentration and magnetic anisotropy. Additionally, we will show that the FMR studies ofGa1−xMnxAs /Ga1−yAlyAs heterostructures modulation doped by Be reveal that the effectiveg-factorof Ga1−xMnxAs is also strongly affected by the doping. The measurements of the totalg-factorcan in turn be used to estimate the contribution which the holes themselves make to the total magnetizationof 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 thedata on FMR broadening are at this time largely phenomenological, the dependence of thelinewidth on hole concentrations suggests that the p–d coupling between the holes and theMn2+ ionscontributes significantly to the damping rate of the magnetization precession in FMR experimentson Ga1−xMnxAs films. Finally, it should be noted that—although in this review we focus onGa1−xMnxAs, because the overwhelming majority of work on FMR has been carried out on this material—thedescription of FMR and its analysis presented here can be applied to thin layers of allIII1−xMnxV alloys generally.