Intrinsic Anomalous Hall and Anomalous Nernst Effects in Ga1-xMnxAs Ferromagnetic Semiconductors
Jing Shi, Department of Physics and Astronomy, University of California, Riverside, USA

The physical origin of the anomalous Hall effect (AHE) in ferromagnets has been a topic of long-standing theoretical and experimental debate. The Hall current may or may not depend on scattering, which is called extrinsic or intrinsic, respectively, and it has not been settled completely. To help resolve this issue, we carried out a systematic study of both electrical and thermoelectric transport properties in Mn-doped GaMn ferromagnetic semiconductors. We observed a very large anomalous Nernst effect (ANE) - thermoelectric counterpart of AHE, which is directly related to the magnetization instead of the applied magnetic field. Unlike AHE that remains positive below the Curie temperature, ANE can switch its sign at low temperatures. We show that the sign change in ANE is the first indication that the Hall and Nernst currents are not extrinsic.  We further analyzed the temperature dependence of ANE using all measured transport coefficients and obtained excellent agreement which quantitatively yields the exponent of the power-law between Rxx and Rxy.  Out results show the intrinsic nature of the Hall and Nernst currents in GaMnAs.  Meanwhile, we experimentally established the Mott relation for the first time for the intrinsic anomalous transport coefficients recently shown by theorists.