Anisotropic Thermopower and Planar Nernst Effect in Ga_1-xMn_xAs Ferromagnetic Semiconductors

We present the first experimental study of the thermopower in Mn-doped GaAs ferromagnetic semiconductors. Large magnetothermopower effects in both longitudinal and transverse directions have been observed below the ferromagnetic transition temperature. Unlike magnetoresistance, neither the transverse thermopower (planar Nernst effect) nor the longitudinal thermopower explicitly depend on the strength of the in-plane magnetic field, but rather are intimately related to each other through the magnetization. These newly discovered effects can be satisfactorily explained by an extension of anisotropic magnetotransport model and place important constraints on potential microscopic descriptions of the scattering mechanisms in these materials.