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Electrical Spin Current Generation in Ferromagnets and Antiferromagnets
Vivek Amin,Fei Xue,Paul Haney,Mark Stiles 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.1
Electrical control of magnetic order has widespread applications for information and communications technology. One way to manipulate magnetic order in layered structures is to generate a spin current in a source layer that is absorbed by a nearby magnetic layer, causing a transfer of spin angular momentum or spin torque. Under an applied electric field, nonmagnetic, ferromagnetic, and antiferromagnetic materials all generate such spin currents. However, it is typically assumed that the spin torque occurs in a different layer than where the spin current was generated. For ferromagnetic and antiferromagnetic metals with appreciable spin-orbit coupling, conduction electrons can carry a substantial spin current flowing perpendicularly to the electric field with spin directions misaligned with the magnetic order parameter. In some cases, these symmetry-allowed spin currents can flow into the layer boundaries and exert substantial torques that can be measured through optical techniques such as MOKE. Thus, magnetic materials can be simultaneously the source and receiver of spin torques, suggesting a promising avenue to optimize electrical control of magnetic order. <br.>In this talk, I discuss several mechanisms to electrically generate spin currents in ferromagnets, antiferromagnets, and magnetic interfaces. Each mechanism can have a different dependence on magnetization direction, crystal structure, and/or disorder. While measurements of spin torques at layer boundaries provide evidence of spin current generation, disentangling contributions from spin currents and from other sources remains an open challenge. We present both first principles and semiclassical transport calculations giving the strength and magnetization dependence of electrically generated spin currents in magnetic systems via intrinsic and/or extrinsic mechanisms. Shedding light on these mechanisms could help optimize electrical control of magnetic order with potential applications for information processing.
Spin currents and spin–orbit torques in ferromagnetic trilayers
Baek, Seung-heon C.,Amin, Vivek P.,Oh, Young-Wan,Go, Gyungchoon,Lee, Seung-Jae,Lee, Geun-Hee,Kim, Kab-Jin,Stiles, M. D.,Park, Byong-Guk,Lee, Kyung-Jin Nature Publishing Group 2018 NATURE MATERIALS Vol.17 No.6
<P>Magnetic torques generated through spin-orbit coupling(1-8) promise energy-efficient spintronic devices. For applications, it is important that these torques switch films with perpendicular magnetizations without an external magnetic field(9-14). One suggested approach(15) to enable such switching uses magnetic trilayers in which the torque on the top magnetic layer can be manipulated by changing the magnetization of the bottom layer. Spin currents generated in the bottom magnetic layer or its interfaces transit the spacer layer and exert a torque on the top magnetization. Here we demonstrate field-free switching in such structures and show that its dependence on the bottom-layer magnetization is not consistent with the anticipated bulk effects(15). We describe a mechanism for spin-current generation(16,17) at the interface between the bottom layer and the spacer layer, which gives torques that are consistent with the measured magnetization dependence. This other-layer-generated spin-orbit torque is relevant to energy-efficient control of spintronic devices.</P>