59 research outputs found
Dynamics and hysteresis in square lattice artificial spin-ice
Dynamical effects under geometrical frustration are considered in a model for
artificial spin ice on a square lattice in two dimensions. Each island of the
spin ice has a three-component Heisenberg-like dipole moment subject to shape
anisotropies that influence its direction. The model has real dynamics,
including rotation of the magnetic degrees of freedom, going beyond the
Ising-type models of spin ice. The dynamics is studied using a Langevin
equation solved via a second order Heun algorithm. Thermodynamic properties
such as the specific heat are presented for different couplings. A peak in
specific heat is related to a type of melting-like phase transition present in
the model. Hysteresis in an applied magnetic field is calculated for model
parameters where the system is able to reach thermodynamic equilibrium.Comment: Revised versio
Domain wall dynamics in expanding spaces
We study the effects on the dynamics of kinks due to expansions and
contractions of the space. We show that the propagation velocity of the kink
can be adiabatically tuned through slow expansions/contractions, while its
width is given as a function of the velocity. We also analyze the case of fast
expansions/contractions, where we are no longer on the adiabatic regime. In
this case the kink moves more slowly after an expansion-contraction cycle as a
consequence of loss of energy through radiation. All these effects are
numerically studied in the nonlinear Klein-Gordon equations (both for the
sine-Gordon and for the phi^4 potential), and they are also studied within the
framework of the collective coordinate evolution equations for the width and
the center of mass of the kink. These collective coordinate evolution equations
are obtained with a procedure that allows us to consider even the case of large
expansions/contractions.Comment: LaTeX, 18 pages, 2 figures, improved version to appear in Phys Rev
Nambu monopoles interacting with lattice defects in two-dimensional artificial square spin ice
The interactions between an excitation (similar to a pair of Nambu monopoles)
and a lattice defect are studied in an artificial two-dimensional square spin
ice. This is done by considering a square array of islands containing only one
island different from all others. This difference is incorporated in the
magnetic moment (spin) of the "imperfect" island and several cases are studied,
including the special situation in which this distinct spin is zero (vacancy).
We have shown that the two extreme points of a malformed island behave like two
opposite magnetic charges. Then, the effective interaction between a pair of
Nambu monopoles with the deformed island is a problem involving four magnetic
charges (two pairs of opposite poles) and a string. We also sketch the
configuration of the field lines of these four charges to confirm this picture.
The influence of the string on this interaction decays rapidly with the string
distance from the defect.Comment: 7 pages, 13 figure
Vortices in the presence of a nonmagnetic atom impurity in 2D XY ferromagnets
Using a model of nonmagnetic impurity potential, we have examined the
behavior of planar vortex solutions in the classical two-dimensional XY
ferromagnets in the presence of a spin vacancy localized out of the vortex
core. Our results show that a spinless atom impurity gives rise to an effective
potential that repels the vortex structure.Comment: 6 pages, 2 figures, RevTex
Magnetic anisotropy of elongated thin ferromagnetic nano-islands for artifical spin ice arrays
The energetics of thin elongated ferromagnetic nano-islands is considered for
some different shapes, aspect ratios, and applied magnetic field directions.
These nano-island particles are important for artificial spin-ice materials.
For low temperature, the magnetic internal energy of an individual particle is
evaluated numerically as a function of the direction of a particle's net
magnetization. This leads to estimations of effective anisotropy constants for
(1) the easy axis along the particle's long direction, and (2) the hard axis
along the particle's thin direction. A spin relaxation algorithm together with
fast Fourier transform for the demagnetization field is used to solve the
micromagnetics problem for a thin system. The magnetic hysteresis is also
found. The results indicate some possibilities for controlling the equilibrium
and dynamics in spin-ice materials by using different island geometries.Comment: 9 pages, two-column, 8 figure
Switching between different vortex states in 2-dimensional easy-plane magnets due to an ac magnetic field
Using a discrete model of 2-dimensional easy-plane classical ferromagnets, we
propose that a rotating magnetic field in the easy plane can switch a vortex
from one polarization to the opposite one if the amplitude exceeds a threshold
value, but the backward process does not occur. Such switches are indeed
observed in computer simulations.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Collective modes for an array of magnetic dots in the vortex state
The dispersion relations for collective magnon modes for square-planar arrays
of vortex-state magnetic dots, having closure magnetic flux are calculated. The
array dots have no direct contact between each other, and the sole source of
their interaction is the magnetic dipolar interaction. The magnon formalism
using Bose operators along with translational symmetry of the lattice, with the
knowledge of mode structure for the isolated dot, allows the diagonalization of
the system Hamiltonian giving the dispersion relation. Arrays of vortex-state
dots show a large variety of collective mode properties, such as positive or
negative dispersion for different modes. For their description, not only
dipolar interaction of effective magnetic dipoles, but non-dipolar terms common
to higher multipole interaction in classical electrodynamics can be important.
The dispersion relation is shown to be non-analytic as the value of the
wavevector approaches zero for all dipolar active modes of the single dot. For
vortex-state dots the interdot interaction is not weak, because, the dynamical
part (in contrast to the static magnetization of the vortex state) dot does not
contain the small parameter, the ratio of vortex core size to the dot radius.
This interaction can lead to qualitative effects like the formation of modes of
angular standing waves instead of modes with definite azimuthal number known
for the insolated vortex state dot
Realization of Rectangular Artificial Spin Ice and Direct Observation of High Energy Topology
In this letter, we have constructed and experimentally investigated
frustrated arrays of dipoles forming two-dimensional artificial spin ices with
different lattice parameters (rectangular arrays with horizontal and vertical
lattice spacings denoted by and respectively). Arrays with three
different ratios , and are
studied. Theoretical calculations of low-energy demagnetized configurations for
these same parameters are also presented. Experimental data for demagnetized
samples confirm most of the theoretical results. However, the highest energy
topology (doubly-charged monopoles) does not emerge in our theoretical model,
while they are seen in experiments for large enough . Our results also
insinuate that magnetic monopoles may be almost free in rectangular lattices
with a critical ratio , supporting previous
theoretical predictions
Noise-induced switching between vortex states with different polarization in classical two-dimensional easy-plane magnets
In the 2-dimensional anisotropic Heisenberg model with XY-symmetry there are
non-planar vortices which exhibit a localized structure of the z-components of
the spins around the vortex center. We study how thermal noise induces a
transition of this structure from one polarization to the opposite one. We
describe the vortex core by a discrete Hamiltonian and consider a stationary
solution of the Fokker-Planck equation. We find a bimodal distribution function
and calculate the transition rate using Langer's instanton theory (1969). The
result is compared with Langevin dynamics simulations for the full many-spin
model.Comment: 15 pages, 4 figures, Phys. Rev. B., in pres
Internal Modes and Magnon Scattering on Topological Solitons in 2d Easy-Axis Ferromagnets
We study the magnon modes in the presence of a topological soliton in a 2d
Heisenberg easy-axis ferromagnet. The problem of magnon scattering on the
soliton with arbitrary relation between the soliton radius R and the "magnetic
length" Delta_0 is investigated for partial modes with different values of the
azimuthal quantum numbers m. Truly local modes are shown to be present for all
values of m, when the soliton radius is enough large. The eigenfrequencies of
such internal modes are calculated analytically on limiting case of a large
soliton radius and numerically for arbitrary soliton radius. It is demonstrated
that the model of an isotropic magnet, which admits an exact analytical
investigation, is not adequate even for the limit of small radius solitons,
R<<Delta_0: there exists a local mode with nonzero frequency. We use the data
about local modes to derive the effective equation of soliton motion; this
equation has the usual Newtonian form in contrast to the case of the easy-plane
ferromagnet. The effective mass of the soliton is found.Comment: 33 pages (REVTeX), 12 figures (EPS
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