Hamed Ben Mohamed Saidaoui

Spin-Swapping Transport and Torques in Ultrathin Magnetic Bilayers.

Planar spin transport in disordered ultrathin magnetic bilayers comprising a ferromagnet and a normal metal (typically used for spin pumping, spin Seebeck and spin-orbit torque experiments) is investigated theoretically. Using a tight-binding model that puts the extrinsic spin Hall effect and spin swapping on equal footing, we show that the nature of spin-orbit coupled transport dramatically depends on the ratio between the layer thickness d and the mean free path λ. While the spin Hall effect dominates in the diffusive limit (d≫λ), spin swapping dominates in the Knudsen regime (d≲λ). A remarkable consequence is that spin swapping induces a substantial fieldlike torque in the Knudsen regime.

Spin transfer torque in antiferromagnetic spin valves: From clean to disordered regimes

Current-driven spin torques in metallic spin valves composed of antiferromagnets are theoretically studied using the nonequilibrium Greens function method implemented on a tight-binding model. We focus our attention on G-type and L-type antiferromagnets in both clean and disordered regimes. In such structures, spin torques can either rotate the magnetic order parameter coherently (coherent torque) or compete with the internal antiferromagnetic exchange (exchange torque). We show that, depending on the symmetry of the spin valve, the coherent and exchange torques can either be in the plane, proportional to n x (q x n) or out of the plane proportional to n x q, where q and n are the directions of the order parameter of the polarizer and the free antiferromagnetic layers, respectively. Although disorder conserves the symmetry of the torques, it strongly reduces the torque magnitude, pointing out the need for momentum conservation to ensure strong spin torque in antiferromagnetic spin valves.

Robust spin transfer torque in antiferromagnetic tunnel junctions

A.M. acknowledges the financial support of the King Abdullah University of Science and Technology (KAUST) through the Office of Sponsored Research (OSR; Grant No. OSR-2015-CRG4-2626).

Theory of spin transport in antiferromagnets(Conference Presentation)

Antiferromagnets (AF) have long remained an intriguing and exotic state of matter, their application being restricted to enabling interfacial exchange bias in spin-valves. Their role in the expanding field of applied spintronics has been mostly passive and the in-depth investigation of their basic properties considered as fundamental condensed matter physics. A conceptual breakthrough was achieved ten years ago with the proposal that spin transfer torque could be used to electrically control the direction of the order parameter of AF spin valves, henceforth making these materials potential candidates for low energy spin devices. In spite of substantial theoretical efforts and experimental attempts to observe such a torque, the difficulty to independently detect the direction of the AF order parameter has remained a major obstacle. In this talk, I will first introduce the original concept of spin transfer torque in AF spin-valves, demonstrating that it is strongly limited by the spin decoherence and dramatically vanishes in the presence of disorder, leaving little hope to observe this effect experimentally. Then, I will present the newly proposed concept of spin-orbit torque that utilizes bulk or interfacial the spin-orbit coupling in non-centrosymmetric magnets to directly generate a torque on the AF order parameter. This torque, being local, is much more robust against impurities, as will be demonstrated for the specific case of interfacial Rashba spin-orbit coupling. Finally, I will discuss about spin motive force and torques in antiferromagnetic textures, intriguing effects that remained to be experimentally observed.

Antiferromagnetic spin-orbitronics

Antiferromagnets have long remained an intriguing and exotic state of matter, whose application has been restricted to enabling interfacial exchange bias in metallic and tunneling spin-valves [1]. Their role in the expanding field of applied spintronics has been mostly passive and the in-depth investigation of their basic properties mostly considered from a fundamental perspective.