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Novel Spintronic Responses of Novel Materials: A Tale of Two Systems
Paul Haney,Fei Xue,Duarte Pereira de Sousa,Jian-Ping Wang,Tony Low 한국자기학회 2021 한국자기학회 학술연구발표회 논문개요집 Vol.31 No.1
The discovery of new materials with unique magnetic ordering, crystal symmetries, and topological properties continues to stimulate the development of new spintronic devices. Spin-orbit coupling underlies many of the spintronic applications in materials, as it couples the electron spin with its real space motion and often plays a key role in determining the topological properties of a material’s electronic structure. In this talk we’ll describe the unique properties of two quite distinct materials systems: antiferromagnetic bilayer CrI3 and magnetic tunnel junctions composed of one or more magnetic Weyl semimetals. Bilayer CrI3 is a two-dimensional Van der Waals material in which two ferromagnetic CrI3 monolayers are coupled antiferromagnetically. We consider electron doped CrI3 and theoretically study the current-induced torques present in this material. In the purely antiferromagnetic state, the two individually inversion symmetry-broken layers of CrI3 form inversion partners, like the well-studied CuMnAs and MnAu. However, the exchange and anisotropy energies are similar in magnitude, unlike previously studied antiferromagnets in which the exchange energy is dominant. This difference leads to qualitatively different behaviors in this material. Using a combination of first-principles calculations of the spin-orbit torque and an analysis of the ensuing spin dynamics, we show that the deterministic electrical switching of the Néel vector is the result of damping like spin-orbit torque, which is staggered on the magnetic sublattices. We then present results on magnetic tunnel junctions composed of one or more magnetic Weyl semimetal layers. For an asymmetric magnetic tunnel junction containing a conventional ferromagnet and a magnetic Weyl semimetal contact, we find unique features of the spin transfer torque. The Weyl semimetal hosts chiral bulk states and topologically protected Fermi arc surface states which we find govern the voltage behavior and efficiency of the spin transfer torque. We discuss the existence of a large field-like torque acting on the magnetic Weyl semimetal, whose efficiency can exceed the theoretical maximum of conventional magnetic tunnel junctions. This large field-like torque is derived from the Fermi arc spin texture and displays a counter-intuitive dependence on the Weyl nodes separation. We finally consider a magnetic tunnel junction composed of two Weyl semimetal contacts. For this system, we show that chirality-magnetization locking leads to a gigantic tunneling magnetoresistance ratio, an effect that does not rely on spin filtering by the tunnel barrier. Our results shed light on the new physics of multilayered spintronic devices comprising of magnetic Weyl semimetals, which might open doors for new energy efficient spintronic devices.
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.
k-asymmetric spin splitting at the interface between transition metal ferromagnets and heavy metals
Grytsyuk, Sergiy,Belabbes, Abderrezak,Haney, Paul M.,Lee, Hyun-Woo,Lee, Kyung-Jin,Stiles, M. D.,Schwingenschlö,gl, Udo,Manchon, Aurelien American Physical Society 2016 Physical Review B Vol.93 No.17
<P>We systematically investigate the spin-orbit coupling-induced band splitting originating from inversion symmetry breaking at the interface between a Co monolayer and 4d (Tc, Ru, Rh, Pd, and Ag) or 5d (Re, Os, Ir, Pt, and Au) transition metals. In spite of the complex band structure of these systems, the odd-in-k spin splitting of the bands displays striking similarities with the much simpler Rashba spin-orbit coupling picture. We establish a clear connection between the overall strength of the odd-in-k spin splitting of the bands and the charge transfer between the d orbitals at the interface. Furthermore, we show that the spin splitting of the Fermi surface scales with the induced orbital moment, weighted by the spin-orbit coupling.</P>