99 research outputs found
Microscopic theory of spin-orbit torques and skyrmion dynamics
We formulate a general microscopic approach to spin-orbit torques in thin
ferromagnet/heavy-metal bilayers in linear response to electric current or
electric field. The microscopic theory we develop avoids the notion of spin
currents and spin-Hall effect. Instead, the torques are directly related to a
local spin polarization of conduction electrons, which is computed from
generalized Kubo-St\v{r}eda formulas. A symmetry analysis provides a one-to-one
correspondence between polarization susceptibility tensor components and
different torque terms in the Landau-Lifshitz-Gilbert equation for
magnetization dynamics. The spin-orbit torques arising from Rashba or
Dresselhaus type of spin-orbit interaction are shown to have different
symmetries. We analyze these spin-orbit torques microscopically for a generic
electron model in the presence of an arbitrary smooth magnetic texture. For a
model with spin-independent disorder we find a major cancelation of the
torques. In this case the only remaining torque corresponds to the
magnetization-independent Edelstein effect. Furthermore, our results are
applied to analyze the dynamics of a Skyrmion under the action of electric
current.Comment: 13 pages, 4 figure
Magnetoelectric domain wall dynamics and its implications for magnetoelectric memory
Domain wall dynamics in a magnetoelectric antiferromagnet is analyzed, and
its implications for magnetoelectric memory applications are discussed.
CrO is used in the estimates of the materials parameters. It is found
that the domain wall mobility has a maximum as a function of the electric field
due to the gyrotropic coupling induced by it. In CrO the maximal
mobility of 0.1 m/(sOe) is reached at V/nm. Fields of
this order may be too weak to overcome the intrinsic depinning field, which is
estimated for B-doped CrO. These major drawbacks for device
implementation can be overcome by applying a small in-plane shear strain, which
blocks the domain wall precession. Domain wall mobility of about 0.7
m/(sOe) can then be achieved at V/nm. A split-gate scheme is
proposed for the domain-wall controlled bit element; its extension to
multiple-gate linear arrays can offer advantages in memory density,
programmability, and logic functionality.Comment: 5 pages, 2 figures, revised and corrected version, accepted in
Applied Physics Letter
Thermoelectric efficiency of topological insulators in a magnetic field
We study the thermoelectric properties of three-dimensional topological
insulators in magnetic fields with many holes (or pores) in the bulk. We find
that at high density of these holes in the transport direction the
thermoelectric figure of merit, ZT, can be large due to the contribution of the
topologically protected conducting surfaces and the suppressed phonon thermal
conductivity. By applying an external magnetic field a subgap can be induced in
the surface states spectrum. We show that the thermoelectric efficiency can be
controlled by this tunable subgap leading to the values of ZT much greater than
1. Such high values of ZT for reasonable system parameters and its tunability
by magnetic field make this system a strong candidate for applications in heat
management of nanodevices, especially at low temperatures.Comment: 9 pages, 4 figures, Proceedings of MMM 201
Staggered Dynamics in Antiferromagnets by Collective Coordinates
Antiferromagnets can be used to store and manipulate spin information, but
the coupled dynamics of the staggered field and the magnetization are very
complex. We present a theory which is conceptually much simpler and which uses
collective coordinates to describe staggered field dynamics in
antiferromagnetic textures. The theory includes effects from dissipation,
external magnetic fields, as well as reactive and dissipative current-induced
torques. We conclude that, at low frequencies and amplitudes, currents induce
collective motion by means of dissipative rather than reactive torques. The
dynamics of a one-dimensional domain wall, pinned at 90 at its ends,
are described as a driven harmonic oscillator with a natural frequency
inversely proportional to the length of the texture.Comment: 4 pages, 2 figure
Stripes in thin ferromagnetic films with out-of-plane anisotropy
We examine the T=0 phase diagram of a thin ferromagnetic film with a strong
out-of-plane anisotropy in the vicinity of the reorientation phase transition
(with Co on Pt as an example). The phase diagram in the anisotropy-applied
field plane is universal in the limit where the film thickness is the shortest
length scale. It contains uniform fully magnetized and canted phases, as well
as periodically nonuniform states: a weakly modulated spin-density wave and
strongly modulated stripes. We determine the boundaries of metastability of
these phases and point out the existence of a critical point at which the
difference between the SDW and stripes vanishes. Out-of-plane magnetization
curves exhibit a variety of hysteresis loops caused by the coexistence of one
or more phases. Additionally, we study the effect of a system edge on the
orientation of stripes. We compare our results with recent experiments.Comment: added references and clarified derivations in response to referee
comment
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