Magnetic properties of (Ga,Mn)As

The discovery of carrier-mediated ferromagnetism in (III,Mn)V and (II,Mn)VI dilute magnetic semiconductors (DMS) grown by molecular beam epitaxy makes it possible to examine the interplay between physical properties of semiconductor quantum structures and ferromagnetic materials [1]. (Ga,Mn)As serves as a valuable test ground for ferro-DMS, due to the relatively high Curie temperature, TC, and compatibility with the well-characterised GaAs system. The Mn dopants in this III-V host matrix are expected to substitute for the Ga site, and fulfill two roles: to supply a local spin 5/2 magnetic moments, and to act as an acceptor, providing itinerant holes which mediate the ferromagnetic order. The theoretical understanding of this phenomenon [2] is built on Zener's model of ferromagnetism, the Ginzburg-Landau approach to the phase transitions, and the Kohn-Luttinger k⋅p theory of semiconductors. Within this model, the magnitude of TC in Mn-doped arsenides and antimonides, as well as in p-type tellurides and Ge is understood assuming that the long-range ferromagnetic interactions between the localized spins are mediated by delocalised holes in the weakly perturbed valence band [3]. The assumption that the relevant carriers reside in the p-like valence band makes it possible to describe various magnetooptical [4,5] and magnetotransport properties of (Ga,Mn)As, including the anomalous Hall effect and anisotropic magnetoresistance [5,6] as well the negative magnetoresistance caused by the orbital weak-localization effect [7]. From this point of view, (Ga,Mn)As and related compounds emerge as the best understood ferromagnets, providing a basis for the development of novel methods enabling magnetisation manipulation and switching [8,9]. Here, a brief review of micromagnetic properties of (Ga,Mn)As is given. Interestingly, despite much lower spin and carrier concentrations compared to ferromagnetic metals, (III,Mn)V exhibit excellent micromagnetic characteristics, including well defined magnetic anisotropy and large ferromagnetic domains separated by usually straight-line domain walls. It turns out that the above-mentioned p-d Zener model explains the influence of strain on magnetic anisotropy as well as describing the magnitudes of the anisotropy field and domain width. Importantly, the experimentally observed various magnetic easy axis reorientation transitions as a function of the temperature and hole concentration are satisfactory accounted for. This applies also to a weak inplane magnetic anisotropy that has been detected in these systems.