1,458 research outputs found
Geometric Phases and Topological Effects
Lecture Notes of the 45th IFF Spring School "Computing Solids - Models, ab
initio methods and supercomputing" (Forschungszentrum Juelich, 2014).Comment: 40 pages. January 201
Comparison of conventional and adaptive wall wind tunnel results with regard to Reynolds number effects
A comparison of results from conventional and adaptive wall wind tunnels with regard to Reynolds number effects was carried out. The special objective of this comparison was to confirm or reject earlier conclusions, soley based on conventional wind tunnel results, concerning the influence of viscous effects on the characteristics of partially open wind tunnel walls, hence wall interference. The following postulations could be confirmed: (1) certain classes of supercritical airfoils exhibit a non-linear increase in lift which is, at least in part, related to viscous-inviscid interactions on the airfoil. This non-linear lift characteristic can erroneously be suppressed by sidewall interference effects in addition to being affected by changes in Reynolds number. Adaptive walls seem to relieve the influence of sidewall interference; (2) the degree of (horizontal) wall interference effects can be significantly affected by changes in Reynolds number, thus appearing as true Reynolds number effects; (3) perforated wall characteristics seem much more susceptible to viscous changes than the characteristics of slotted walls; here, blockage interference may be most severely influenced by viscous changes; and (4) real Reynolds number effects are present on the CAST 10-2/DOA 2 airfoil; they were shown to be appreciable also by the adaptive wall wind tunnel tests
The relation of the Dzyaloshinskii-Moriya interaction to spin currents and to the spin-orbit field
Starting from the general Berry phase theory of the Dzyaloshinskii-Moriya
interaction (DMI) we derive an expression for the linear contribution of the
spin-orbit interaction (SOI). Thereby, we show analytically that at the first
order in SOI DMI is given by the ground-state spin current. We verify this
finding numerically by ab-initio calculations in Mn/W(001) and Co/Pt(111)
magnetic bilayers. We show that despite the strong SOI from the 5 heavy
metals DMI is well-approximated by the first order in SOI, while the
ground-state spin current is not. We decompose the SOI-linear contribution to
DMI into two parts. One part has a simple interpretation in terms of the Zeeman
interaction between the spin-orbit field and the spin misalignment that
electrons acquire in magnetically noncollinear textures. This interpretation
provides also an intuitive understanding of the symmetry of DMI on the basis of
the spin-orbit field and it explains in a simple way why DMI and ground-state
spin currents are related. Moreover, we show that energy currents driven by
magnetization dynamics and associated to DMI can be explained by
counter-propagating spin currents that carry energy due to their Zeeman
interaction with the spin-orbit field. Finally, we discuss options to modify
DMI by nonequilibrium spin currents excited by electric fields or light
The inverse thermal spin-orbit torque and the relation of the Dzyaloshinskii-Moriya interaction to ground-state energy currents
Using the Kubo linear-response formalism we derive expressions to calculate
the heat current generated by magnetization dynamics in magnets with broken
inversion symmetry and spin-orbit interaction (SOI). The effect of producing
heat currents by magnetization dynamics constitutes the Onsager reciprocal of
the thermal spin-orbit torque (TSOT), i.e., the generation of torques on the
magnetization due to temperature gradients. We find that the energy current
driven by magnetization dynamics contains a contribution from the
Dzyaloshinskii-Moriya interaction (DMI), which needs to be subtracted from the
Kubo linear response of the energy current in order to extract the heat
current. We show that the expressions of the DMI coefficient can be derived
elegantly from the DMI energy current. Guided by formal analogies between the
Berry phase theory of DMI on the one hand and the modern theory of orbital
magnetization on the other hand we are led to an interpretation of the latter
in terms of energy currents as well. Based on \textit{ab-initio} calculations
we investigate the heat current driven by magnetization dynamics in Mn/W(001)
magnetic bilayers. We predict that fast domain walls drive strong ITSOT heat
currents
Spin-orbit torques and tunable Dzyaloshinskii-Moriya interaction in Co/Cu/Co trilayers
We study the spin-orbit torques (SOTs) in Co/Cu/Co magnetic trilayers based
on first-principles density-functional theory calculations in the case where
the applied electric field lies in-plane, i.e., parallel to the interfaces. We
assume that the bottom Co layer has a fixed in-plane magnetization, while the
top Co layer can be switched. We find that the SOT on the top ferromagnet can
be controlled by the bottom ferromagnet because of the nonlocal character of
the SOT in this system. As a consequence the SOT is anisotropic, i.e., its
magnitude varies with the direction of the applied electric field. We show that
the Dzyaloshinskii-Moriya interaction (DMI) in the top layer is anisotropic as
well, i.e., the spin-spiral wavelength of spin-spirals in the top layer depends
on their in-plane propagation direction. This effect suggests that DMI can be
tuned easily in magnetic trilayers via the magnetization direction of the
bottom layer. In order to understand the influence of the bottom ferromagnet on
the SOTs and the DMI of the top ferromagnet we study these effects in Co/Cu
magnetic bilayers for comparison. We find the SOTs and the DMI to be
surprisingly large despite the small spin-orbit interaction of Cu
Chiral damping, chiral gyromagnetism and current-induced torques in textured one-dimensional Rashba ferromagnets
We investigate Gilbert damping, spectroscopic gyromagnetic ratio and
current-induced torques in the one-dimensional Rashba model with an additional
noncollinear magnetic exchange field. We find that the Gilbert damping differs
between left-handed and right-handed N\'eel-type magnetic domain walls due to
the combination of spatial inversion asymmetry and spin-orbit interaction
(SOI), consistent with recent experimental observations of chiral damping.
Additionally, we find that also the spectroscopic factor differs between
left-handed and right-handed N\'eel-type domain walls, which we call chiral
gyromagnetism. We also investigate the gyromagnetic ratio in the Rashba model
with collinear magnetization, where we find that scattering corrections to the
factor vanish for zero SOI, become important for finite spin-orbit
coupling, and tend to stabilize the gyromagnetic ratio close to its
nonrelativistic value
Direct and inverse spin-orbit torques
In collinear magnets lacking inversion symmetry application of electric
currents induces torques on the magnetization and conversely magnetization
dynamics induces electric currents. The two effects, which both rely on
spin-orbit interaction (SOI), are reciprocal to each other and denoted direct
spin-orbit torque (SOT) and inverse spin-orbit torque (ISOT), respectively. We
derive expressions for SOT and ISOT within the Kubo linear response formalism.
We show that expressions suitable for density-functional theory calculations
can be derived either starting from a Kohn-Sham Hamiltonian with time-dependent
exchange field or by expressing general susceptibilities in terms of the
Kohn-Sham susceptibilities. For the case of magnetic bilayer systems we derive
the general form of the ISOT current induced under ferromagnetic resonance.
Using \textit{ab initio} calculations within density-functional theory we
investigate SOT and ISOT in Co/Pt(111) magnetic bilayers. We determine the
spatial distribution of spin and charge currents as well as torques in order to
expose the mechanisms underlying SOT and ISOT and to highlight their
reciprocity on the microscopic level. We find that the spin Hall effect is
position-dependent close to interfaces
Phase-Space Berry Phases in Chiral Magnets: Dzyaloshinskii-Moriya Interaction and the Charge of Skyrmions
The semiclassical motion of electrons in phase space, x=(R, k), is influenced
by Berry phases described by a 6-component vector potential, A=(A^R, A^k). In
chiral magnets Dzyaloshinskii-Moriya (DM) interactions induce slowly varying
magnetic textures (helices and skyrmion lattices) for which all components of A
are important inducing effectively a curvature in mixed position and momentum
space. We show that for smooth textures and weak spin-orbit coupling phase
space Berry curvatures determine the DM interactions and give important
contributions to the charge. Using ab initio methods we calculate the strength
of DM interactions in MnSi in good agreement with experiment and estimate the
charge of skyrmions.Comment: 5 pages, 1 figure; 5 pages of supplemental material with 1 figure;
substantial changes: Berry phase theory of DM interactions + extra Fermi
surface contribution
Scattering-Independent Anomalous Nernst Effect in Ferromagnets
Using the full-potential linearized augmented plane-wave method within the
density functional theory, we compute all contributions to the scattering
independent part of the thermoelectric conductivity tensor, namely the
intrinsic contribution and the side-jump contribution. For the ferromagnetic
materials bcc Fe, hcp Co, fcc Ni and L1_0 ordered alloys FePd and FePt, our
investigations of the energy and temperature dependence of the intrinsic and
side-jump contributions show that they are both of equal importance. Overall,
our calculations are able to correctly reproduce the order of magnitude and
sign of the experimentally measured signal, suggesting that the scattering
independent part plays an important role in the anomalous Nernst effect of
ferromagnets.Comment: 5 pages, 2 figures plus supplement, accepted for publication as a
Rapid Communication in Physical Review
Spin-orbit torques in L1-FePt/Pt thin films driven by electrical and thermal currents
Using the linear response formalism for the spin-orbit torque (SOT) we
compute from first principles the SOT in a system of two layers of
L1-FePt(001) deposited on an fcc Pt(001) substrate of varying thickness. We
find that at room temperature the values of the SOTs that are even and odd with
respect to magnetization generally lie in the range of values measured and
computed for Co/Pt bilayers. We also observe that the even SOT is much more
robust with respect to changing the number of layers in the substrate, and as a
function of energy it follows the general trend of the even SOT exerted by the
spin Hall current in fcc Pt. The odd torque, on the other hand, is strongly
affected by modification of the electronic structure for a specific energy
window in the limit of very thin films. Moreover, taking the system at hand as
an example, we compute the values of the thermal spin-orbit torque (T-SOT). We
predict that the gradients of temperature which can be experimentally created
in this type of systems will cause a detectable torque on the magnetization. We
also underline the correlation between the even T-SOT and the spin Nernst
effect, thus motivating a more intensive experimental effort aimed at
observation of both phenomena.Comment: 8 pages, 4 figure
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