Abstract
Anisotropic magnetoresistance effect has been studied systematically in (311)A-oriented as-grown and annealed GaMnAs films at different temperatures. Besides a uniaxial term, the measured angular dependence of longitudinal and transverse magnetoresistivities contains higher order terms arising from the crystal symmetry of GaMnAs. A third order crystalline term accounts for the observed antisymmetric planar Hall effect which is intrinsic to (311)A-oriented GaMnAs films. The uniaxial term persists above the Curie temperature, while the higher order terms can only be observed below the Curie temperature, with their magnitudes increasing with decreasing temperatures. It is revealed that the higher order crystalline terms originate from the long range ferromagnetic (FM) phase and the uniaxial one from isolated magnetic polarons (MPs) as well as the FM phase. With decreasing temperatures, the dominant contribution to the uniaxial term changes from isolated MPs to the FM phase. In addition, low temperature annealing of GaMnAs films is found to increase the magnitudes of all these terms.
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