Probing the energy / momentum dependence of GaMnAs anisotropies by inter-band tunneling transport of a Zener-Esaki diode

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

Troyes

Jornada; 12e Journées de la Matière Condensée; 2010

A key issue in semiconductors-based spintronics is the integration of ferromagnetic materials with conventional semiconductors. The diluted ferromagnetic semiconductor GaMnAs can be epitaxially grown on InGaAs, and has strong anisotropic properties which make it a very interesting material to develop new devices. Although GaMnAs has been widely studied in the last years, there is no agreement in the community on the exact nature of charge carriers in the metallic-like regime, and much work is still devoted to this subject. There are two different models: one is based on hole transport (Valence-band model) and the other one involves only delocalized carriers in an impurity band (“Impurity-band model”) [1]. Here, we will present an experimental study of GaMnAs anisotropic carriers by means of energy- or momentum-resolved tunnel spectroscopy using a p-GaMnAs/n-GaAs Esaki diode. The results give a strong insight on the valence-band nature of carriers close to the Fermi level, and the possible influence of an impurity band close to the top of the valence band. Since a tunnel junction acts as a filter in k-space, the tunnel current gives a direct access to the local density of states (LDOS) in the tunnelling direction. Due to a strong spin-orbit coupling, the GaMnAs band structure is highly anisotropic. Besides, due to exchange interaction, the LDOS is modified under the rotation of the magnetization vector, leading to a tunnel anisotropic magnetoresistance (TAMR) [2]. Contrary to ohmic AMR, TAMR depends on the magnetization direction with respect to the crystal axes. This leads to make the distinction between out-of-plane (OP) and in-plane (IP) TAMR, the latter being only due to the crystalline anisotropy. We previously evidenced the large OP-TAMR signal, up to 40%, of a ferromagnetic p++GaMnAs / n+GaAs Zener-Esaki tunnel diode, and demonstrated that interband tunneling allows us to separate different valence subbands contributions [3]. The IP-TAMR signal is much smaller (~1%) but it clearly reveals cubic and in-plane uniaxial anisotropies whose contributions strongly depend on the applied voltage. We performed a study of the IP-TAMR on a 10mm x 10mm tunnel diode as a function of voltage (“energy spectroscopy”) and magnetic field (“momentum spectroscopy”) which reveals qualitative features supporting the “Valence band model” mentioned above. Further, we extract the different contributions to the anisotropy, cubic and uniaxial, as a function of the applied voltage and attempt to use these results to go deeper into a semi-quantitative analysis of the valence-band/impurity-band interplay.   [1] T. Jungwirth et al.,  Phys. Rev. B 76, 125206 (2007). [2] C. Gould, C. Rüster, T. Jungwirth, E. Girgis, G.M. Schott, R. Giraud, K. Brunner, G. Schmidt, and L.W. Molenkamp,  Phys. Rev. Lett. 93, 117203 (2004). [3] R. Giraud, M. Gryglas, L. Thevenard, A. Lemaître, G. Faini, Appl. Phys. Lett. 87, 242505 (2005).