355 research outputs found
Motion of domain walls and the dynamics of kinks in the magnetic Peierls potential
We study the dynamics of magnetic domain walls in the Peierls potential due
to the discreteness of the crystal lattice. The propagation of a narrow domain
wall (comparable to the lattice parameter) under the effect of a magnetic field
proceeds through the formation of kinks in its profile. We predict that,
despite the discreteness of the system, such kinks can behave like sine-Gordon
solitons in thin films of materials such as yttrium iron garnets, and we derive
general conditions for other materials. In our simulations we also observe
long-lived breathers. We provide analytical expressions for the effective mass
and limiting velocity of the kink in excellent agreement with our numerical
results.Comment: 12 pages, 9 figures (incl. supp. mat.
Dirac electrons and domain walls: a realization in junctions of ferromagnets and topological insulators
We study a system of Dirac electrons with finite density of charge carriers
coupled to an external electromagnetic field in two spatial dimensions, with a
domain wall (DW) mass term. The interface between a thin-film ferromagnet and a
three-dimensional topological insulator provides a condensed-matter realization
of this model, when an out-of-plane domain wall magnetization is coupled to the
TI surface states. We show how, for films with very weak intrinsic in-plane
anisotropies, the torque generated by the edge electronic current flowing along
the DW competes with an effective in-plane anisotropy energy, induced by
quantum fluctuations of the chiral electrons bound to the wall, in a mission to
drive the internal angle of the DW from a Bloch configuration towards a N\'eel
configuration. Both the edge current and the induced anisotropy contribute to
stabilize the internal angle, so that for weak intrinsic in-plane anisotropies
DW motion is still possible without suffering from an extremely early Walker
breakdown.Comment: 18 pages, 3 figure
Zero modes in magnetic systems: general theory and an efficient computational scheme
The presence of topological defects in magnetic media often leads to normal
modes with zero frequency (zero modes). Such modes are crucial for long-time
behavior, describing, for example, the motion of a domain wall as a whole.
Conventional numerical methods to calculate the spin-wave spectrum in magnetic
media are either inefficient or they fail for systems with zero modes. We
present a new efficient computational scheme that reduces the magnetic
normal-mode problem to a generalized Hermitian eigenvalue problem also in the
presence of zero modes. We apply our scheme to several examples, including
two-dimensional domain walls and Skyrmions, and show how the effective masses
that determine the dynamics can be calculated directly. These systems highlight
the fundamental distinction between the two types of zero modes that can occur
in spin systems, which we call special and inertial zero modes. Our method is
suitable for both conservative and dissipative systems. For the latter case, we
present a perturbative scheme to take into account damping, which can also be
used to calculate dynamical susceptibilities.Comment: 64 pages, 15 figure
Chirality-dependent transmission of spin waves through domain walls
Spin-wave technology (magnonics) has the potential to further reduce the size
and energy consumption of information processing devices. In the submicrometer
regime (exchange spin waves), topological defects such as domain walls may
constitute active elements to manipulate spin waves and perform logic
operations. We predict that spin waves that pass through a domain wall in an
ultrathin perpendicular-anisotropy film experience a phase shift that depends
on the orientation of the domain wall (chirality). The effect, which is absent
in bulk materials, originates from the interfacial Dzyaloshinskii-Moriya
interaction and can be interpreted as a geometric phase. We demonstrate
analytically and by means of micromagnetic simulations that the phase shift is
strong enough to switch between constructive and destructive interference. The
two chirality states of the domain wall may serve as a memory bit or spin-wave
switch in magnonic devices.Comment: 11 pages, 10 figures (incl. supp. mat.); Phys. Rev. Lett. (accepted
Nonlinear effects in the propagation of optically generated magnetostatic volume mode spin waves
Recent experimental work has demonstrated optical control of spin wave
emission by tuning the shape of the optical pulse (Satoh et al.\ Nature
Photonics, 6, 662 (2012)). We reproduce these results and extend the scope of
the control by investigating nonlinear effects for large amplitude excitations.
We observe an accumulation of spin wave power at the center of the initial
excitation combined with short-wavelength spin waves. These kind of nonlinear
effects have not been observed in earlier work on nonlinearities of spin waves.
Our observations pave the way for the manipulation of magnetic structures at a
smaller scale than the beam focus, for instance in devices with all-optical
control of magnetism.Comment: Added new figures to further illustrate the nonlinear effects to show
time evolution and spectral flow. Added references. Changed perspective on
nonlinear effects w.r.t. applicability of NSE. Added acknowledgemen
Autocatalytic ring opening of N-acylaziridines : Complete control over regioselectivity by orientation at interfaces
Contains fulltext :
14059.pdf (publisher's version ) (Open Access
Universality of cauliflower-like fronts: from nanoscale thin films to macroscopic plants
Chemical vapor deposition (CVD) is a widely used technique to
grow solid materials with accurate control of layer thickness and composition.
Under mass-transport-limited conditions, the surface of thin films thus produced
grows in an unstable fashion, developing a typical motif that resembles the
familiar surface of a cauliflower plant. Through experiments on CVD production
of amorphous hydrogenated carbon films leading to cauliflower-like fronts,
we provide a quantitative assessment of a continuum description of CVD
interface growth. As a result, we identify non-locality, non-conservation and
randomness as the main general mechanisms controlling the formation of these
ubiquitous shapes.We also show that the surfaces of actual cauliflower plants and
combustion fronts obey the same scaling laws, proving the validity of the theory over seven orders of magnitude in length scales. Thus, a theoretical justification
is provided, which had remained elusive so far, for the remarkable similarity
between the textures of surfaces found for systems that differ widely in physical
nature and typical scales.Publicad
Dynamical Renormalization Group Study for a Class of Non-local Interface Equations
We provide a detailed Dynamic Renormalization Group study for a class of
stochastic equations that describe non-conserved interface growth mediated by
non-local interactions. We consider explicitly both the morphologically stable
case, and the less studied case in which pattern formation occurs, for which
flat surfaces are linearly unstable to periodic perturbations. We show that the
latter leads to non-trivial scaling behavior in an appropriate parameter range
when combined with the Kardar-Parisi-Zhang (KPZ) non-linearity, that
nevertheless does not correspond to the KPZ universality class. This novel
asymptotic behavior is characterized by two scaling laws that fix the critical
exponents to dimension-independent values, that agree with previous reports
from numerical simulations and experimental systems. We show that the precise
form of the linear stabilizing terms does not modify the hydrodynamic behavior
of these equations. One of the scaling laws, usually associated with Galilean
invariance, is shown to derive from a vertex cancellation that occurs (at least
to one loop order) for any choice of linear terms in the equation of motion and
is independent on the morphological stability of the surface, hence
generalizing this well-known property of the KPZ equation. Moreover, the
argument carries over to other systems like the Lai-Das Sarma-Villain equation,
in which vertex cancellation is known {\em not to} imply an associated symmetry
of the equation.Comment: 34 pages, 9 figures. Journal of Statistical Mechanics: Theory and
Experiments (in press
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