Abstract
Magnetization switching at the interface between ferromagnetic and paramagnetic metals, controlled by current-induced torques, could be exploited in magnetic memory technologies. Compelling questions arise regarding the role played in the switching by the spin Hall effect in the paramagnet and by the spin-orbit torque originating from the broken inversion symmetry at the interface. Of particular importance are the antidamping components of these current-induced torques acting against the equilibrium-restoring Gilbert damping of the magnetization dynamics. Here, we report the observation of an antidamping spin-orbit torque that stems from the Berry curvature, in analogy to the origin of the intrinsic spin Hall effect. We chose the ferromagnetic semiconductor (Ga,Mn)As as a material system because its crystal inversion asymmetry allows us to measure bare ferromagnetic films, rather than ferromagnetic-paramagnetic heterostructures, eliminating by design any spin Hall effect contribution. We provide an intuitive picture of the Berry curvature origin of this antidamping spin-orbit torque as well as its microscopic modelling. We expect the Berry curvature spin-orbit torque to be of comparable strength to the spin-Hall-effect-driven antidamping torque in ferromagnets interfaced with paramagnets with strong intrinsic spin Hall effect.
Highlights
We start by deriving the intuitive picture of our Berry phase anti-damping spin-orbit torque (SOT) based on the Bloch equation description of the carrier spin dynamics
The spin Hall effect (SHE) in these experiments originates from the Berry phase effect in the band structure of a clean crystal[2,24,25,27,33] and the anti-damping spin-transfer torque (STT) is based on a disorderindependent transfer of spin from carriers to magnetization
The SOT alone can induce magnetization dynamics based on a scattering-independent principle
Summary
Materials: The 18 nm thick (Ga0.95,Mn0.05)As epilayer was grown on a GaAs [001] substrate by molecular beam epitaxy, performed at a substrate temperature of 230 C It was subsequently annealed for 8 hours at 200 C. Devices: Two terminal microbars are patterned in different crystal directions by electron beam lithography to have dimensions of 4×40 μm These bars have a typical low temperature resistance of 10 kΩ (data-table in supplementary information). The resistance change of the micro-bar due to Joule heating of a direct current is measured. We assume the same Joule heating (and resistance change of the micro-bar) for the same direct and rms microwave currents, enabling us to calibrate the unknown microwave current against the known direct current
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