Interfacial spin-orbit torques

Spin-orbit torques offer a promising mechanism for electrically controlling magnetization dynamics in nanoscale heterostructures. While spin-orbit torques occur predominately at interfaces, the physical mechanisms underlying these torques can originate in both the bulk layers and at interfaces. Classifying spin-orbit torques based on the region that they originate in provides clues as to how to optimize the effect. While most bulk spin-orbit torque contributions are well studied, many of the interfacial contributions allowed by symmetry have yet to be fully explored theoretically and experimentally. To facilitate progress, we review interfacial spin-orbit torques from a semiclassical viewpoint and relate these contributions to recent experimental results. Within the same model, we show the relationship between different interface transport parameters. For charges and spins flowing perpendicular to the interface, interfacial spin-orbit coupling both modifies the mixing conductance of magnetoelectronic circuit theory and gives rise to spin memory loss. For in-plane electric fields, interfacial spin-orbit coupling gives rise to torques described by spin-orbit filtering, spin swapping and precession. In addition, these same interfacial processes generate spin currents that flow into the non-magnetic layer. For in-plane electric fields in trilayer structures, the spin currents generated at the interface between one ferromagnetic layer and the non-magnetic spacer layer can propagate through the non-magnetic layer to produce novel torques on the other ferromagnetic layer.

Interface-Generated Spin Currents

Transport calculations based on ab initio band structures reveal large interface-generated spin currents at Co/Pt, Co/Cu, and Pt/Cu interfaces. These spin currents are driven by in-plane electric fields but flow out of plane and can have similar strengths to spin currents generated by the spin Hall effect in bulk Pt. Each interface generates spin currents with polarization along z[over ^]×E, where z[over ^] is the interface normal and E denotes the electric field. The Co/Cu and Co/Pt interfaces additionally generate spin currents with polarization along m[over ^]×(z[over ^]×E), where m[over ^] gives the magnetization direction of Co. The latter spin polarization is controlled by-but not aligned with-the magnetization, providing a novel mechanism for generating spin torques in magnetic trilayers.

Experimental demonstration of scanned spin-precession microscopy

We present a new tool for imaging spin properties. We show that a spatially averaged spin signal, measured as a function of a scanned magnetic probe's position, contains information about the local spin properties. In this first demonstration we map the injected spin density in GaAs by measuring spin photoluminescence with a resolution of 1.2  μm. The ultimate limit of the technique is set by the gradient of the probe's field, allowing for a resolution beyond the optical diffraction limit. Such probes can also be integrated with other detection methods. This generality allows the technique to be extended to buried interfaces and optically inactive materials.

Detection of electrically modulated inverse spin hall effect in an Fe/GaAs microdevice

We report the detection of the inverse spin Hall effect (ISHE) in n-gallium arsenide (n-GaAs) combined with electrical injection and modulation of the spin current. We use epitaxial ultrathin-Fe/GaAs injection contacts with strong in-plane magnetic anisotropy. This allows us to simultaneously perform Hanle spin-precession measurements on an Fe detection electrode and ISHE measurements in an applied in-plane hard-axis magnetic field. In this geometry, we can experimentally separate the ordinary from the spin-Hall signals. Electrical spin injection and detection are combined in our microdevice with an applied electrical drift current to modulate the spin distribution and spin current in the channel. The magnitudes and external field dependencies of the signals are quantitatively modeled by solving drift-diffusion and Hall-cross response equations.