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The Dzyaloshinskii-Moriya interaction, i.e., antisymmetric exchange interaction, combined with a Zeeman magnetic field gives rise to various magnetic states such as chiral, helical, and skyrmionic states. This interaction conventionally originates from spin-orbit coupling and thus needs somewhat heavy elements. In contrast, we here show a Dzyaloshinskii-Moriya interaction which is driven by electric current vorticity. We also find that the vorticity acts on localized spins as a Zeeman field, which may explain a recent experiment on current-driven magnetic skyrmion creation and annihilation without an external magnetic field in a device with a notch structure in FeGe. theory explains the control of skyrmion creation and annihilation by current direction and opens different possibilities for studies of magnetic textures by using structural settings.

Employing unbiased large-scale time-dependent density-matrix renormalization-group simulations, we demonstrate the generation of a charge-current vortex via spin injection in the Rashba system. The spin current is polarized perpendicular to the system plane and injected from an attached antiferromagnetic spin chain. We discuss the conversion between spin and orbital angular momentum in the current vortex that occurs because of the conservation of the total angular momentum and the spin-orbit interaction. This is in contrast to the spin Hall effect, in which the angular-momentum conservation is violated. Finally, we predict the electromagnetic field that accompanies the vortex with regard to possible future experiments., 12 pages, 11 figures

The interaction between spin and nanomechanical degrees of freedom attracts interest from the viewpoint of basic science and device applications. We study the magnon current induced by the torsional oscillation of ferromagnetic nanomechanical cantilever. We find that a finite Dzyaloshinskii-Moriya (DM) interaction emerges by the torsional oscillation, which is described by the spin gauge field, and the DM interaction leads to the detectably-large magnon current with frequency same as that of the torsional oscillation. Our theory paves the way for studying torsional spin-nanomechanical phenomena by using the spin gauge field., Comment: 5 pages, 4 figures

Electron transport in magnetic orders and the magnetic order dynamics have a mutual dependence, which provides the key mechanisms in spin-dependent phenomena. Recently, antiferromagnetic orders have been 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, is doubly degenerate, 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.

We investigate an interfacial spin-transfer torque and $\beta$-term torque with alternating current (AC) parallel to a magnetic interface. We find that both torques are resonantly enhanced as the AC frequency approaches to the exchange splitting energy. We show that this resonance allows us to estimate directly the interfacial exchange interaction strength from the domain wall motion. We also find that the $\beta$-term includes an unconventional contribution which is proportional to the time derivative of the current and exists even in absence of any spin relaxation processes., Comment: 5 pages, 2 figures

We show theoretically that conversion between spin and charge by spin-orbit interaction in metals occurs even in a non-local setup where magnetization and spin-orbit interaction are spatially separated if electron diffusion is taken into account. Calculation is carried out for the Rashba spin-orbit interaction treating the coupling with a ferromagnet perturbatively. The results indicate the validity of the concept of effective spin gauge field (spin motive force) in the non-local configuration. The inverse Rashba-Edelstein effect observed for a trilayer of a ferromagnet, a normal metal and a heavy metal can be explained in terms of the non-local effective spin gauge field.

As a strongly spin-orbit coupled metallic model with ferromagnetism, we have considered an extended Stoner model to the relativistic regime, named Dirac ferromagnet in three dimensions. In the previous paper~[Phys. Rev. B 90, 214418 (2014)], we studied the transport properties giving rise to the anisotropic magnetoresistance~(AMR) and the anomalous Hall effect~(AHE) with the impurity potential being taken into account only as the self-energy. The effects of the vertex corrections~(VCs) to AMR and AHE are reported in this paper. AMR is found not to change quantitatively when the VCs is considered, although the transport lifetime is different from the one-electron lifetime and the charge current includes additional contributions from the correlation with spin currents. The side-jump and the skew-scattering contributions to AHE are also calculated. The skew-scattering contribution is dominant in the clean case as can be seen in the spin Hall effect in the non-magnetic Dirac electron system., Comment: 21 pages, 7 figures

Torque can be provided to magnetization in nanomagnets directly by electric current and/or voltage. This technique enables electric current (voltage)-to-spin conversion without electromagnetic induction, and has been intensively studied for memory device applications. Among the various kinds of torque, torque induced by spin-orbit splitting has recently been found. However, quantitative understanding of bulk-related torque and interface-related torque is still lacking because of their identical symmetry for current-in-plane devices. In this paper, we propose that a pure interface-related torque can be characterized by spin-torque ferromagnetic resonance with a current-perpendicular-to-plane tunnel junction. Epitaxial Fe-MgO-V tunnel junctions are prepared to characterize the interface-related torque at Fe-MgO. We find that the current-driven torque is negligible, and a significant enhancement of the voltage-driven torque is observed when the MgO barrier thickness decreases. The maximum torque obtained is as large as 2.8×10^{−5} J/(Vm^{2}), which is comparable to the voltage-controlled magnetic anisotropy of 180 fJ/Vm. The voltage-driven torque shows strong dc-bias-voltage dependence that cannot be explained by conventional voltage-controlled magnetic anisotropy. Tunnel anisotropic magnetoresistance spectroscopy suggests that the torque is correlated to an interface state at the Fe-MgO. This surface-state-sensitive electric modulation of magnetic properties provides new insight into the field of interface magnetism.

We propose a model ferromagnet based on the Dirac Hamiltonian in three spatial dimensions, and study its transport properties which include anisotropic magnetoresistance (AMR) and anomalous Hall (AH) effect. This relativistic extension allows two kinds of ferromagnetic order parameters, denoted by $\bm{M}$ and $\bm{S}$, which are distinguished by the relative sign between the positive- and negative-energy states (at zero momentum) and become degenerate in the non-relativistic limit. Because of the relativistic coupling between the spin and the orbital motion, both $\bm{M}$ and $\bm{S}$ induce anisotropic deformations of the energy dispersion (and the Fermi surfaces) but in mutually opposite ways. The AMR is determined primarily by the anisotropy of the Fermi surface (group velocity), and secondarily by the anisotropy of the damping; the latter becomes important for ${\bm M}=\pm{\bm S}$, where the Fermi surfaces are isotropic. Even when the chemical potential lies in the gap, the AH conductivity is found to take a finite non-quantized value, $\sigma_{ij} = -(\alpha /3\pi^2 \hbar) \epsilon_{ijk} S_k $, where $\alpha$ is the (effective) fine structure constant. This offers an example of Hall insulator in three spatial dimensions., Comment: 17 pages, 6 figures

Dirac electrons in two-dimensional continuum space subject to $\ensuremath{\delta}$-function impurities, which have inherent ultraviolet divergence, fail to be gauge-invariant if regularized by the standard momentum (Brillouin-zone) cutoff. A regularization scheme which respects gauge invariance is proposed here based on the cutoff in the impurity-potential range. The relation to a lattice model, which has no ultraviolet divergence, is also discussed.