7,548 research outputs found
Micromagnetic Simulation of Nanoscale Films with Perpendicular Anisotropy
A model is studied for the theoretical description of nanoscale magnetic
films with high perpendicular anisotropy. In the model the magnetic film is
described in terms of single domain magnetic grains with Ising-like behavior,
interacting via exchange as well as via dipolar forces. Additionally, the model
contains an energy barrier and a coupling to an external magnetic field.
Disorder is taken into account in order to describe realistic domain and domain
wall structures. The influence of a finite temperature as well as the dynamics
can be modeled by a Monte Carlo simulation.
Many of the experimental findings can be investigated and at least partly
understood by the model introduced above. For thin films the magnetisation
reversal is driven by domain wall motion. The results for the field and
temperature dependence of the domain wall velocity suggest that for thin films
hysteresis can be described as a depinning transition of the domain walls
rounded by thermal activation for finite temperatures.Comment: Revtex, Postscript Figures, to be published in J. Appl.Phy
MAGNETISATION REVERSAL AND DOMAIN STRUCTURE IN THIN MAGNETIC FILMS: THEORY AND COMPUTER SIMULATION
A model is introduced for the theoretical description of nanoscale magnetic
films with high perpendicular anisotropy. In the model the magnetic film is
described in terms of single domain magnetic grains, interacting via exchange
as well as via dipolar forces. Additionally, the model contains anisotropy
energy and a coupling to an external magnetic field. Disorder is taken into
account in order to describe realistic domain and domain wall structures.
Within this framework the dependence of the energy on the film thickness can be
discussed. The influence of a finite temperature as well as the dynamics can be
modeled by a Monte Carlo simulation. The results on the hysteresis loops, the
domain configurations, and the dynamics during the reversal process are in good
agreement with experimental findings.Comment: 4 Pages, Postscript, uuencode
Domain State Model for Exchange Bias
Monte Carlo simulations of a system consisting of a ferromagnetic layer
exchange coupled to a diluted antiferromagnetic layer described by a classical
spin model show a strong dependence of the exchange bias on the degree of
dilution in agreement with recent experimental observations on Co/CoO bilayers.
These simulations reveal that diluting the antiferromagnet leads to the
formation of domains in the volume of the antiferromagnet carrying a remanent
surplus magnetization which causes and controls exchange bias. To further
support this domain state model for exchange bias we study in the present paper
the dependence of the bias field on the thickness of the antiferromagnetic
layer. It is shown that the bias field strongly increases with increasing film
thickness and eventually goes over a maximum before it levels out for large
thicknesses. These findings are in full agreement with experiments.Comment: 8 pages latex, 3 postscript figure
Magnetization Switching in Small Ferromagnetic Particles: Nucleation and Coherent Rotation
The mechanisms of thermally activated magnetization switching in small
ferromagnetic particles driven by an external magnetic field are investigated.
For low uniaxial anisotropy the spins rotate coherently while for sufficiently
large uniaxial anisotropy they behave Ising-like, i.e. the switching then is
due to nucleation. The crossover from coherent rotation to nucleation is
studied for the classical three-dimensional Heisenberg model with uniaxial
anisotropy by Monte Carlo simulations. From the temperature dependence of the
metastable lifetime the energy barrier of a switching process can be
determined. For the case of infinite anisotropy we compare numerical results
from simulations of the Ising model with theoretical results for energy
barriers for both, single-droplet and multi-droplet nucleation. The simulated
barriers are in agreement with the theoretical predictions.Comment: 3 pages, Revtex, 4 Figures include
Uniform susceptibility of classical antiferromagnets in one and two dimensions in a magnetic field
We simulated the field-dependent magnetization m(H,T) and the uniform
susceptibility \chi(H,T) of classical Heisenberg antiferromagnets in the chain
and square-lattice geometry using Monte Carlo methods. The results confirm the
singular behavior of \chi(H,T) at small T,H: \lim_{T \to 0}\lim_{H \to 0}
\chi(H,T)=1/(2J_0)(1-1/D) and \lim_{H \to 0}\lim_{T \to 0} \chi(H,T)=1/(2J_0),
where D=3 is the number of spin components, J_0=zJ, and z is the number of
nearest neighbors. A good agreement is achieved in a wide range of temperatures
T and magnetic fields H with the first-order 1/D expansion results [D. A.
Garanin, J. Stat. Phys. 83, 907 (1996)]Comment: 4 PR pages, 4 figures, submitted to PR
Modeling exchange bias microscopically
Exchange bias is a horizontal shift of the hysteresis loop observed for a
ferromagnetic layer in contact with an antiferromagnetic layer. Since exchange
bias is related to the spin structure of the antiferromagnet, for its
fundamental understanding a detailed knowledge of the physics of the
antiferromagnetic layer is inevitable. A model is investigated where domains
are formed in the volume of the AFM stabilized by dilution. These domains
become frozen during the initial cooling procedure carrying a remanent net
magnetization which causes and controls exchange bias. Varying the anisotropy
of the antiferromagnet we find a nontrivial dependence of the exchange bias on
the anisotropy of the antiferromagnet.Comment: 7 pages, 5 figure
Role of temperature-dependent spin model parameters in ultra-fast magnetization dynamics
In the spirit of multi-scale modelling magnetization dynamics at elevated
temperature is often simulated in terms of a spin model where the model
parameters are derived from first principles. While these parameters are mostly
assumed temperature-independent and thermal properties arise from spin
fluctuations only, other scenarios are also possible. Choosing bcc Fe as an
example, we investigate the influence of different kinds of model assumptions
on ultra-fast spin dynamics, where following a femtosecond laser pulse a sample
is demagnetized due to a sudden rise of the electron temperature. While
different model assumptions do not affect the simulational results
qualitatively, their details do depend on the nature of the modelling.Comment: 8 pages, 6 figure
- …