Tunneling spectroscopy of GaMnAs anisotropies

M. GRANADA; R. GIRAUD; E. BRIONES; A. LEMAÎTRE; U. GENNSER; G. FAINI

Ordizia

Workshop; Recent Trends in Nanomagnetism, Spintronics and Their Applications; 2011

We present a study of the valence band anisotropy of GaMnAs by means of tunnelling transport measurements in p+-GaMnAs/n+-GaAs diodes. The samples studied in this work were epitaxially grown under in-plane (IP) compressive strain on (001) GaAs, then the plane of the junction is a plane of easy magnetization. We performed tunnelling transport measurements with an IP applied magnetic field H; varying the bias voltage Vbias on micrometric diodes allowed us to study the energy dependence of GaMnAs anisotropies. In the forward regime, i.e. Vbias>0, the electrons tunnel from occupied states in the n+-GaAs conduction band into unoccupied states in the p+-GaMnAs valence band. A variation of the measured resistance R with the angle between the magnetization M and [110] crystal axis is observed, due to the anisotropy of the valence band structure. These anisotropies are strongly dependent on Vbias, i.e. on the energy of the tunnelling carriers, revealing their valence band nature. The IP magneto-resistance MR(phi) = [R(phi)-R(0)]/ R(0), can be described by the combination of a cubic and two uniaxial terms, MR(phi) = MR0 +MRc cos(4phi) +MRu[110] cos(2phi) + MRu[100] cos[2(phi-pi/4)]. For positive MRi constants, the axes indicated in Fig.1b are those that minimize MR(phi). We fitted the measured data using this formula and extracted the relative weights of the different components. The cubic term, arising from the crystalline anisotropy, presents the same behaviour as the tunnelling anisotropic magnetoresistance measured in similar samples with a perpendicular magnetic field. The behaviour of MRc and MRu[110] in the Vbias>0 regime can be associated with the magnetic anisotropy energy calculated by a k.p model including exchange interaction and strain.