Evidence for reversible control of magnetization in a ferromagnetic material by means of spin–orbit magnetic field

Abstract

Conventional computer electronics creates a dichotomy between how information is processed and how it is stored. Silicon chips process information by controlling the flow of charge through a network of logic gates. This information is then stored, most commonly, by encoding it in the orientation of magnetic domains of a computer hard disk. The key obstacle to a more intimate integration of magnetic materials into devices and circuit processing information is a lack of efficient means to control their magnetization. This is usually achieved with an external magnetic field or by the injection of spin-polarized currents [1,2,3]. The latter can be significantly enhanced in materials whose ferromagnetic properties are mediated by charge carriers [4]. Among these materials, conductors lacking spatial inversion symmetry couple charge currents to spin by intrinsic spin-orbit (SO) interactions, inducing nonequilibrium spin polarization [5,6,7,8,9,10,11] tunable by local electric fields. Here we show that magnetization of a ferromagnet can be reversibly manipulated by the SO-induced polarization of carrier spins generated by unpolarized currents. Specifically, we demonstrate domain rotation and hysteretic switching of magnetization between two orthogonal easy axes in a model ferromagnetic semiconductor.In crystalline materials with inversion asymmetry, intrinsic spin-orbit interactions (SO) couple the electron spin with its momentumhk. The coupling is given by the Hamiltonian H so =h 2σ · Ω(k), whereh is the Planck's constant andσ is the electron spin operator (for holesσ should be replaced by the total angular momentum J). Electron states with different sign of the spin projection on Ω(k) are split in energy, analogous to the Zeeman splitting in an external magnetic field. In zinc-blende crystals such as GaAs there is a cubic Dresselhaus term[12] Ω D ∝ k 3 , while strain introduces a term Ω ε = C∆ε(k x , −k y , 0) that is linear in k, where ∆ε is the difference between strain in theẑ andx,ŷ directions [13]. In wurzite crystals or in multilayered materials with structural inversion asymmetry there also exists the Rashba term[14] Ω R which has a different symmetry with respect to the direction of k,, whereẑ is along the axis of reduced symmetry. In the presence of an electric field the electrons acquire an average momentumh∆k(E), which leads to the generation of an electric current j =ρ −1 E in the conductor, whereρ is the resistivity tensor. This current defines the preferential axis for spin precession Ω(j) . As a result, a nonequilibrium current-induced spin polarization J E Ω(j) is generated, whose magnitude J E depends on the strength of various mechanisms of momentum scattering and spin relaxation [5,15]. This spin polarization has been measured in non-magnetic semiconductors using optical [7,8,9,11,16] and electron spin resonance [17] techniques. It is convenient to parameterize J E in terms of an effective magnetic field H so . Different contributions to H so have different current dependencies (∝ j or j 3 ), as we...

Current-induced spin–orbit torques

“…The experimental detection of a Dresselhaus type of SO torque in an FM was first reported by Chernyshov et al [18] for dilute magnetic semiconductor Ga 1−x Mn x As films epitaxially grown on the (001) surface of GaAs. The films, with Mn concentration x = 6-7%, present FM behaviour up to a Curie temperature Figure 4c reports the change in sign of R xy owing to the rotation of f M by 90 • as M switches from the easy direction parallel to [010] to the easy direction parallel to [100] under an in-plane applied field of constant amplitude and variable direction f H .…”

“…Equations (2.1)-(2.3) are relevant not only for quantum well structures in zinc blende semiconductors [21,29,30], but also for (Ga,Mn)As films [18] and metal/semiconductor layers such as Fe/GaAs [31]. In the case of metal surfaces [32,33] and metal layers deposited between asymmetric cubic and amorphous interfaces [19,34], g = b = l = 0, i.e.…”

Emergent phenomena at oxide interfaces

“…92 A representative heterostructure effect is the magneto-electric coupling in the spin transistor proposed by DattaDas 93 where the spin precession is manipulated by the electric field, as well as the spin-galvanic effect, i.e., the current-induced spin polarization or the current-control of the magnetization. 94,95 i S  When the time-dependence of the SOI is considered, more rich phenomena can be designed, including adiabatic spin pumping and the manipulation of quantum qubits by electric field. Even without mobile carriers, the coupling between the spin and electric field is induced by the SOI.…”

Tetragonal phase of epitaxial room-temperature antiferromagnet CuMnAs

“…This is manifested in the realization of large magnetoresistances because of anisotropies in the density of states and chemical potential and in the realization of carrier-induced spin reorientation phenomena. Many of these relativistic spintronic effects were first observed in the ferromagnetic Mn-doped zincblende GaAs 12,13,34,35 . CuMnAs shares the spin orbit, broken space-inversion symmetry character of the electronic structure with these magnetic zincblende compounds, and the tetragonal distortion of its lattice further enhances the magnetic anisotropy.…”