18 research outputs found
Mobility of Bloch Walls via the Collective Coordinate Method
We have studied the problem of the dissipative motion of Bloch walls
considering a totally anisotropic one dimensional spin chain in the presence of
a magnetic field. Using the so-called "collective coordinate method" we
construct an effective Hamiltonian for the Bloch wall coupled to the magnetic
excitations of the system. It allows us to analyze the Brownian motion of the
wall in terms of the reflection coefficient of the effective potential felt by
the excitations due to the existence of the wall. We find that for finite
values of the external field the wall mobility is also finite. The spectrum of
the potential at large fields is investigated and the dependence of the damping
constant on temperature is evaluated. As a result we find the temperature and
magnetic field dependence of the wall mobility.Comment: 20 pages, 5 figure
One-dimensional Gapless Magnons In A Single Anisotropic Ferromagnetic Nanolayer.
Gapless magnons in a plane ferromagnet with normal axis anisotropy are shown to exist besides the usual gapped modes that affect spin dependent transport properties only above a finite temperature. These magnons are one-dimensional objects, in the sense that they are localized inside the domain walls that form in the film. They may play an essential role in the spin dependent scattering processes even down to very low temperatures.9122680
NMR linewidth and Skyrmion localization in quantum Hall ferromagnets
The non-monotonic behavior of the NMR signal linewidth in the 2D quantum Hall
system is explained in terms of the interplay between skyrmions localization,
due to the influence of disorder, and the non-trivial temperature dependent
skyrmion dynamics.Comment: 5 pages, 2 figure
Optical conductivity of charge carriers interacting with a two-level systems reservoir
Using the functional-integral method we investigate the effective dynamics of
a charged particle coupled to a set of two-level systems as a function of
temperature and external electric field. The optical conductivity and the
direct current (dc) resistivity induced by the reservoir are computed. Three
different regimes are found depending on the two-level system spectral
function, which may lead to a non-Drude optical conductivity in a certain range
of parameters. Our results contrast to the behavior found when considering the
usual bath of harmonic oscillators which we are able to recover in the limit of
very low temperatures.Comment: 13 pages, 7 figure
Domain-wall profile in the presence of anisotropic exchange interactions: Effective on-site anisotropy
Starting from a D-dimensional XXZ ferromagnetic Heisenberg model in an
hypercubic lattice, it is demonstrated that the anisotropy in the exchange
coupling constant leads to a D-dependent effective on-site anisotropy
interaction often ignored for D>1. As a result the effective width of the wall
depends on the dimensionality of the system. It is shown that the effective
one-dimensional Hamiltonian is not the one-dimensional XXZ version as assumed
in previous theoretical work. We derive a new expression for the wall profile
that generalizes the standard Landau-Lifshitz form. Our results are found to be
in very good agreement with earlier numerical work using the Monte Carlo
method. Preceding theories concerning the domain wall contribution to
magnetoresistance have considered the role of D only through the modification
of the density of states in the electronic band structure. This Brief Report
reveals that the wall profile itself contains an additional D dependence for
the case of anisotropic exchange interactions.Comment: 4 pages; new title and abstract; 1 figure comparing our results with
earlier numerical work; a more general model containing the usual on-site
anisotropy; new remarks and references on the following two topics: (a)
experimental evidence for the existence of spin exchange anisotropy, and (b)
preceding theories concerning the domain wall contribution to
magnetoresistance; to appear in Phys. Rev.
Dissipative transport in quantum Hall ferromagnets by spin-wave scattering
We report on a study of the effect upon electrical transport of spin-wave scattering from charged quasiparticles in nu = 1 quantum Hall ferromagnets, including both Heisenberg (single layer) and easy-plane (bilayer) cases. We derive a quantum Langevin equation to describe the resulting diffusive motion of the charged particle and use this to calculate the contribution to low-temperature conductivity from a density of charged particles. This conductivity has a power-law dependence upon temperature. The contribution is small at low temperatures increasing to a large value at relatively modest temperatures. We comment upon high-temperature transport and upon the contribution of scattering to the width of the zero bias peak in tunneling conductivity