435 research outputs found
Atomistic-continuum multiscale modelling of magnetisation dynamics at non-zero temperature
In this article, a few problems related to multiscale modelling of magnetic
materials at finite temperatures and possible ways of solving these problems
are discussed. The discussion is mainly centred around two established
multiscale concepts: the partitioned domain and the upscaling-based
methodologies. The major challenge for both multiscale methods is to capture
the correct value of magnetisation length accurately, which is affected by a
random temperature-dependent force. Moreover, general limitations of these
multiscale techniques in application to spin systems are discussed.Comment: 30 page
Moving toward an atomistic reader model
With the move to recording densities up to and beyond 1 Tb/in/sup 2/, the size of read elements is continually reducing as a requirement of the scaling process. The expectation is for read elements containing magnetic films as thin as 1.5 nm, in which finite size effects, and factors such as interface mixing might be expected to become of increasing importance. Here, we review the limitations of the current (micromagnetic) approach to the theoretical modeling of thin films and develop an atomistic multiscale model capable of investigating the magnetic properties at the atomic level. Finite-size effects are found to be significant, suggesting the need for models beyond the micromagnetic approach to support the development of future read sensors
Magnetic free energy at elevated temperatures and hysteresis of magnetic particles
We derive a free energy for weakly anisotropic ferromagnets which is valid in
the whole temperature range and interpolates between the micromagnetic energy
at zero temperature and the Landau free energy near the Curie point T_c. This
free energy takes into account the change of the magnetization length due to
thermal effects, in particular, in the inhomogeneous states. As an
illustration, we study the thermal effect on the Stoner-Wohlfarth curve and
hysteresis loop of a ferromagnetic nanoparticle assuming that it is in a
single-domain state. Within this model, the saddle point of the particle's free
energy, as well as the metastability boundary, are due to the change in the
magnetization length sufficiently close to T_c, as opposed to the usual
homogeneous rotation process at lower temperatures.Comment: 16 pages, 4 figure
Homogenization in magnetic-shape-memory polymer composites
Magnetic-shape-memory materials (e.g. specific NiMnGa alloys) react with a
large change of shape to the presence of an external magnetic field. As an
alternative for the difficult to manifacture single crystal of these alloys we
study composite materials in which small magnetic-shape-memory particles are
embedded in a polymer matrix. The macroscopic properties of the composite
depend strongly on the geometry of the microstructure and on the
characteristics of the particles and the polymer.
We present a variational model based on micromagnetism and elasticity, and
derive via homogenization an effective macroscopic model under the assumption
that the microstructure is periodic. We then study numerically the resulting
cell problem, and discuss the effect of the microstructure on the macroscopic
material behavior. Our results may be used to optimize the shape of the
particles and the microstructure.Comment: 17 pages, 4 figure
Stabilization methods in relaxed micromagnetism
The magnetization of a ferromagnetic sample solves a
non-convex variational problem, where its relaxation by convexifying
the energy density resolves relevant
macroscopic information.
The numerical analysis of the relaxed model
has to deal with a constrained convex
but degenerated, nonlocal energy functional in mixed formulation for
magnetic potential u and magnetization m.
In [C. Carstensen and A. Prohl, Numer. Math. 90
(2001) 65–99], the conforming P1 - (P0)d-element in d=2,3 spatial
dimensions is shown to lead to
an ill-posed discrete problem in relaxed micromagnetism, and suboptimal
convergence.
This observation motivated a
non-conforming finite element method which leads to
a well-posed discrete problem, with solutions converging at
optimal rate.
In this work, we provide both an a priori and a posteriori error analysis for two
stabilized conforming methods which account for inter-element jumps of the
piecewise constant magnetization.
Both methods converge at optimal rate;
the new approach is applied to a macroscopic nonstationary
ferromagnetic model [M. Kružík and A. Prohl, Adv. Math. Sci. Appl. 14 (2004) 665–681 – M. Kružík and T. Roubíček, Z. Angew. Math. Phys. 55 (2004) 159–182 ]
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