[14a-C41-7~12] 10.4 半導体・有機・光・量子スピントロニクス

▼ [14a-C41-9] Tunneling magnetoresistance induced by spin-splitting of the valence band in GaMnAs

〇Hiroshi Terada¹、Shinobu Ohya^1,2、Masaaki Tanaka^1,2 (1.The Univ. of Tokyo、2.CSRN)

キーワード：GaMnAs, TMR, Magnetic anisotropy

Both tunneling magnetoresistance (TMR) and tunneling anisotropic magnetoresistance (TAMR) are associated with the density of states of the (ferromagnetic) electrodes of tunneling junctions. The density of states in a ferromagnetic electrode depends on its magnetization direction. In the previous study, it was reported that the in-plane magnetic-field direction dependence of TAMR is strongly dependent on the bias voltage V applied to the tunneling junctions composed of GaMnAs/ AlAs/ p⁺GaAs. This results can be understood by considering that the valence band (VB) and the impurity band (IB) in GaMnAs have different symmetries of the in-plane magnetization-direction dependence of the density of states; the VB has a four-fold symmetry, and IB has a two-fold symmetry. Meanwhile, previous studies of the magneto-optical spectroscopy on GaMnAs has indicated that the VB has a small spin splitting. Thus, it is expected that TMR effects can be observed even when holes are injected to the valence band of GaMnAs, and the above mentioned difference between the IB and VB is expected to be observed also in the magnetic-field direction dependence of TMR as a function of V in GaMnAs-based magnetic tunnel junctions (MTJs). In this study, we have systematically measured TMR and TAMR as a function of the magnetic-field direction, magnetic-field strength, and V in a GaMnAs-based MTJ. We grew a heterostructure composed of Ga_0.95Mn_0.05As (10 nm)/ GaAs (11 nm)/ Ga_0.95Mn_0.05As (3 nm)/ GaAs: Be on a p⁺GaAs (001) substrate by low-temperature molecular beam epitaxy. After the growth, we prepared a cylindrical mesa diode with 200 μm in diameter. We measured the magnetoresistance (MR) varying the in-plane magnetic-field direction φ at V = -100 mV, -5 mV, and 140 mV at 3.8 K. The measurements were performed at every 10° step of φ. When V = -5 mV, clear spin-valve characteristics were obtained, where the uniaxial magnetic anisotropy along [-110] is dominant. The MR characteristics are well explained by the relative magnetization direction between two GaMnAs layers. Meanwhile, by increasing the magnitude of V, additional anisotropic MR characteristics appear. When V = -100 mV, positive MR with a biaxial anisotropy along <110> was observed, and when V = 140 mV, negative MR with a biaxial anisotropy along <110> was observed. In the presentation, we discuss the origin of the V dependence of the anisotropy of the MR characteristics.