O. Alejos

Micromagnetic simulations using Graphics Processing Units

The methodology for adapting a standard micromagnetic code to run on graphics processing units (GPUs) and exploit the potential for parallel calculations of this platform is discussed. GPMagnet, a general purpose finite-difference GPU-based micromagnetic tool, is used as an example. Speed-up factors of two orders of magnitude can be achieved with GPMagnet with respect to a serial code. This allows for running extensive simulations, nearly inaccessible with a standard micromagnetic solver, at reasonable computational times.

Micromagnetic dynamic computations including eddy currents

In this paper we present a 3D dynamic micromagnetic model which includes the effect of eddy currents and its application to magnetization reversal processes in permalloy nanocubes.

REMANENT STATES IN PERIODIC ANTIDOT PERMALLOY ARRAYS

It has been recently suggested that periodic antidot arrays in magnetic media could be a suitable system for high density data storage because hard axis magnetic switching has remanence and hence, memory. In this work a numerical micromagnetic study of periodic antidot permalloy arrays is presented. Remanent states are obtained after saturation of the sample along the hard axis direction for different antidot sizes w (0.32 μm⩽w⩽2.16 μm) and different separation distances d between antidots (w/2⩽d⩽3w). Our micromagnetic model allows us to study these systems beyond the experimental results presently available so that we can explore the high density limit. The signal to noise ratio decreases significantly for separation distances d⩾w and increases steadily with the antidot size w in the range analyzed. The remanent configurations are similar to the experimental data obtained by scanning Kerr microscopy.

Thermally activated domain wall depinning in thin strips with high perpendicular magnetocrystalline anisotropy

A theoretical analysis on domain wall dynamics along thin ferromagnetic strips with high perpendicular magnetocrystalline anisotropy driven by both magnetic fields and spin-polarized currents is reported. The domain wall depinning from a constriction is characterized both at zero and at room temperature for different values of the nonadiabatic parameter. The results indicate that engineering of pinning sites in thin strips of high perpendicular anisotropy provides an efficient pathway to achieve both high stability against thermal fluctuations and low current-induced domain wall depinning and, therefore, it can find application on designing memory devices driven by static currents.

Coupled Dzyaloshinskii walls and their current-induced dynamics by the spin Hall effect

The nucleation of domain walls in ultrathin ferromagnetic/heavy-metal bilayers is studied by means of micromagnetic simulations. In the presence of interfacial Dzyaloshinskii-Moriya interaction, the nucleated walls naturally adopt a homochiral configuration with internal magnetization pointing antiparallely. The interaction between these walls was analyzed and described in terms of a classical dipolar force between the magnetic moments of the walls, which couples their dynamics. Additionally, the current-induced motion of two homochiral walls in the presence of longitudinal fields was also studied by means of a simple one-dimensional model and micromagnetic modeling, considering both one free-defect strip and another one with random edge roughness. It is evidenced that in the presence of pinning due to edge roughness, the in-plane longitudinal field introduces an asymmetry in the current-induced depinning, in agreement with recent experimental results.

Micromagnetic analysis of recording processes in periodic antidot arrays: Interaction between adjacent bits

Lithographically defined nonmagnetic structures in thin magnetic films have been proposed as future ultrahigh density storage systems. Previous investigations by our group have shown that these periodic antidot arrays present adequate remanent states for storage. The optimization of material parameters and geometrical dimensions led to maximum areal densities in the order of 750 Gbits/in.2 for materials with exchange lengths in the range of 3 nm. In this work recording processes are computed. Single bit switching is analyzed by micromagnetic simulations. It is observed how the dipolar and exchange interactions between adjacent bits play a key role. If the applied field is not carefully controlled or the magnetocrystalline anisotropy interaction is not strong enough, the switching of one bit can produce undesired switching of neighboring bits.

A Preisach-Stoner–Wohlfarth Vector Model

We recently introduced a Preisach vector model for isotropic media. In this aper, we extend the model to anisotropic media by using the magnetization variation of the outside critical surface of the particle similar to the Stoner-Wohlfarth model. The resulting model can calculate efficiently the vector magnetization variations due to any sequence of vector applied fields

An approach to the magnetic disaccommodation in Ti-substituted Ba–W hexaferrites

The relaxation of the initial permeability has been measured in polycrystalline Ti-doped Ba–W hexaferrites with nominal composition BaTixFe18−xO27, with x ranging among x=0 and 0.3. The samples have been sintered at different temperatures in the 1250 °C<T<1350 °C range in a reducing CO2 atmosphere and rapidly quenched to promote the presence of crystal vacancies. X-ray diffraction spectra reveal the presence of hexaferrite as main phase. In the temperature range between 80 and 500 K, the time decay of the initial permeability after sample demagnetization has been plotted by means of isochronal curves. The isochronal disaccommodation spectra show an increase in the amplitude of the different disaccommodation peaks in substituted samples regarding pure Ba–W hexaferrites. These peaks are centered at 180, 300, and 380 K and they are related to the presence of both ferrous cations and lattice vacancies. The increase of the disaccommodation maxima in Ti-substituted hexaferrites is ascribed to the additional amo...

Proposal for a standard problem for Preisach based models

Preisach based models have been usually considered as very convenient tools to simulate both the static and the transient behavior of the irreversible magnetization. In the present work, a very simple physical system consisting of a square array of aligned magnetic particles has been considered in order to be characterized by means of Preisach parameters. A uniform magnetic field is applied in the direction of the easy axis of the particles so that the system has only one dimension. Results reveal that, despite the simplicity of this system, it cannot be successfully defined by means of Preisach parameters. The distribution of interaction fields in the system does not follow a simple Gaussian function. A distribution composed of three balancing peaks is found instead.

Identification of parameters in multilayer media

Multilayer media are of increasing importance as magneto-optic and perpendicular media. The properties of successive layers evolve as the layers are epitaxially deposited. This complicates both the model for these media and the identification of the model parameters. In this paper, the effect of the variation of the Preisach model parameters is discussed and an identification procedure is outlined. Reentrant-type hysteresis behavior, which is found to be necessary in order to accurately fit both major loop and first-order reversal curves in these multilayer materials, is also considered.