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Register a new userCurrent induced torques between ferromagnets and compensated antiferromagnets: symmetry and phase coherence effects
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It is shown that the current-induced torques between a ferromagnetic layer and an antiferromagnetic layer with a compensated interface vanish when the ferromagnet is aligned with an axis of spin-rotation symmetry of the antiferromagnet. For properly chosen geometries this implies that the current induced torque can stabilize the out-of-plane (or hard axis) orientation of the ferromagnetic layer. This current-induced torque relies on phase coherent transport, and we calculate the robustness of this torque to phase breaking scattering. From this it is shown that the torque is not linearly dependent on applied current, but has an absolute maximum.
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We analyse the influence of current induced torques on the magnetization configuration of a ferromagnet in a circuit containing a compensated antiferromagnet. We argue that these torques are generically non-zero and support this conclusion with a microscopic NEGF calculation for a circuit containing antiferromagnetic NiMn and ferromagnetic Co layers. Because of symmetry dictated differences in the form of the current-induced torque, the phase diagram which expresses the dependence of ferromagnet configuration on current and external magnetic field differs qualitatively from its ferromagnet-only counterpart.
In bilayer systems consisting of an ultrathin ferromagnetic layer adjacent to a metal with strong spin-orbit coupling, an applied in-plane current induces torques on the magnetization. The torques that arise from spin-orbit coupling are of particular interest. Here, we calculate the current-induced torque in a Pt-Co bilayer to help determine the underlying mechanism using first principles methods. We focus exclusively on the analogue to the Rashba torque, and do not consider the spin Hall effect. The details of the torque depend strongly on the layer thicknesses and the interface structure, providing an explanation for the wide variation in results found by different groups. The torque depends on the magnetization direction in a way similar to that found for a simple Rashba model. Artificially turning off the exchange spin splitting and separately the spin-orbit coupling potential in the Pt shows that the primary source of the field-like torque is a proximate spin-orbit effect on the Co layer induced by the strong spin-orbit coupling in the Pt.
We report measurements of current-induced torques in heterostructures of Permalloy (Py) with TaTe$_2$, a transition-metal dichalcogenide (TMD) material possessing low crystal symmetry, and observe a torque component with Dresselhaus symmetry. We suggest that the dominant mechanism for this Dresselhaus component is not a spin-orbit torque, but rather the Oersted field arising from a component of current that flows perpendicular to the applied voltage due to resistance anisotropy within the TaTe$_2$. This type of transverse current is not present in wires made from a single uniform layer of a material with resistance anisotropy, but will result whenever a material with resistance anisotropy is integrated into a heterostructure with materials having different resistivities, thereby producing a spatially non-uniform pattern of current flow. This effect will therefore influence measurements in a wide variety of heterostructures incorporating 2D TMD materials and other materials with low crystal symmetries.
Electron transport in magnetic orders and the magnetic orders dynamics have a mutual dependence, which provides the key mechanisms in spin-dependent phenomena. Recently, antiferromagnetic orders are focused on as the magnetic order, where current-induced spin-transfer torques, a typical effect of electron transport on the magnetic order, have been debatable mainly because of the lack of an analytic derivation based on quantum field theory. Here, we construct the microscopic theory of spin-transfer torques on the slowly-varying staggered magnetization in antiferromagnets with weak canting. In our theory, the electron is captured by bonding/antibonding states, each of which is the eigenstate of the system, doubly degenerates, and spatially spreads to sublattices because of electron hopping. The spin of the eigenstates depends on the momentum in general, and a nontrivial spin-momentum locking arises for the case with no site inversion symmetry, without considering any spin-orbit couplings. The spin current of the eigenstates includes an anomalous component proportional to a kind of gauge field defined by derivatives in momentum space and induces the adiabatic spin-transfer torques on the magnetization. Unexpectedly, we find that one of the nonadiabatic torques has the same form as the adiabatic spin-transfer torque, while the obtained forms for the adiabatic and nonadiabatic spin-transfer torques agree with the phenomenological derivation based on the symmetry consideration. This finding suggests that the conventional explanation for the spin-transfer torques in antiferromagnets should be changed. Our microscopic theory provides a fundamental understanding of spin-related physics in antiferromagnets.
One of the main obstacles that prevents practical applications of antiferromagnets is the difficulty of manipulating the magnetic order parameter. Recently, following the theoretical prediction [J. v{Z}elezny et al., PRL 113, 157201 (2014)], the electrical switching of magnetic moments in an antiferromagnet has been demonstrated [P. Wadley et al., Science 351, 587 (2016)]. The switching is due to the so-called spin-orbit torque, which has been extensively studied in ferromagnets. In this phenomena a non-equilibrium spin-polarization exchange coupled to the ordered local moments is induced by current, hence exerting a torque on the order parameter. Here we give a general systematic analysis of the symmetry of the spin-orbit torque in locally and globally non-centrosymmetric crystals. We study when the symmetry allows for a nonzero torque, when is the torque effective, and its dependence on the applied current direction and orientation of magnetic moments. For comparison, we consider both antiferromagnetic and ferromagnetic orders. In two representative model crystals we perform microscopic calculations of the spin-orbit torque to illustrate its symmetry properties and to highlight conditions under which the spin-orbit torque can be efficient for manipulating antiferromagnetic moments.
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