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Unique properties of magnetotransport in GaMnAs films grown on vicinal and high-index planes

Ferromagnetic III–Mn–V semiconductors such as GaMnAs represent systems in which electronic and magnetic properties are closely intertwined, which results in entirely new effects in electrical transport. We illustrate this by the unique and somewhat unexpected magnetotransport phenomena observed in GaMnAs films grown on vicinal substrates (i.e., on surfaces tilted by several degrees relative to the (100) plane) as well as on substrates with high-index-plane surfaces. In particular, it will be shown that such vicinal or high-index-plane GaMnAs layers manifest a striking asymmetry in the dependence of the planar Hall resistance Rxy on magnetic field, caused by the superposition of the planar Hall effect (PHE) and the anomalous Hall effect (AHE). This asymmetry is a direct manifestation of the effect of magnetocrystalline anisotropy in GaMnAs that confines the magnetization M to a preferred crystal plane rather than to the plane of the film, resulting in turn in a finite component of M normal to the sample plane. The ability to investigate PHE and AHE occurring simultaneously in the same sample revealed a clear relationship between the two effects, suggesting that PHE and AHE are fundamentally connected. The asymmetry of the resistance Rxy occurring in the PHE geometry in these GaMnAs layers also allows one to obtain four distinct zero-field resistance states that depend on the history of the experiment, making this effect of potential interest for building a unique four-state magnetic memory device. Moreover, measurements with the magnetic field normal to the growth plane in such tilted samples have revealed a new highly complex hysteresis behaviour of Rxy. These results, together with measurements of anisotropic magnetoresistance (AMR) measured on samples grown on high-index planes, point to the intimate relationship between magnetotransport and magnetocrystalline anisotropy in GaMnAs, providing new insights into this complex interdependence, as well as opening new opportunities for exploiting magnetic anisotropy for use in spintronic devices.