Magnetic anisotropy in ferromagnetic III-Mn-V semiconductors: Issues and observations

Magnetic anisotropy (MA) in III-V-based ferromagnetic (FM) semiconductor alloys (e.g., Ga1-xMnxAs) is of critical importance for the design and implementation of spin-dependent devices based on these materials. This property can be "engineered" and controlled either by imposing appropriate strain conditions on the FM film, or by choosing the hole concentration p. Here it is especially important that the choice of p can be controlled by external mechanisms, such as illumination or voltage. In this paper we present the information on MA in FM semiconductors obtained through a series of complementary methods: ferromagnetic resonance, planar Hall effect, direct imaging of FM domains by magneto-optical methods, and SQUID magnetization data. Special attention is given to the recently-observed reorientation of the easy axis in several of the III1-xMnxV alloys as a function of temperature and/or illumination, and to the somewhat surprising uniaxial anisotropy in the basal (001) plane which some of these FM films exhibit, presumably as a consequence of surface reconstruction during the growth.