G. Brown

Monte Carlo simulations of interacting magnetic nanoparticles

Motivated by recent advances in synthesis techniques of nanometer size magnetic particles, we have performed Monte Carlo simulations of the magnetic properties of such assemblies of particles. The particles are assumed to be point dipoles, which interact magnetostatically and have uniaxial anisotropy. Spatial distributions of the particles are either generated numerically or are taken from experimental data sets. The properties, such as the remnant magnetization, are studied as a function of temperature for a wide range of parameters. The role of magnetostatic interaction is found to be important in many cases where it has typically been neglected. For example, we find that for a thin film of iron particles with 3.5 nm average diameter and only 13% area coverage, the magnetostatic interactions raise the blocking temperature by 20%, and the particles do not have Stoner–Wolfarth character.

Kinetic Monte Carlo simulations of a model for heat-assisted magnetization reversal in ultrathin films

To develop practically useful systems for ultra-high-density information recording with densities above terabits/cm

First principles calculation of finite temperature magnetism in Fe and Fe3C

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First principles approach to the magneto caloric effect: Application toNi2MnGa

, it is necessary to simultaneously achieve high thermal stability at room temperature and high recording rates. One method that has been proposed to reach this goal is heat-assisted magnetization reversal (HAMR). In this method, one applies a high-coercivity material, whose coercivity is temporarily lowered during the writing process through localized heating. Here we present kinetic Monte Carlo simulations of a model of HAMR for ultrathin films, in which the temperature in the central part of the film is momentarily increased above the critical temperature, for example by a laser pulse. We observe that the speed-up achieved by this method, relative to the switching time at a constant, subcritical temperature, is optimal for an intermediate strength of the writing field. This effect is explained using the theory of nucleation-induced magnetization switching in finite systems. Our results should be particularly relevant to recording media with strong perpendicular anisotropy, such as ultrathin Co/Pt or Co/Pd multilayers.

Transition state in magnetization reversal

Density functional calculations have proven to be a useful tool in the study of ground state properties of many materials. The investigation of finite temperature magnetism, on the other hand, has to rely usually on the usage of empirical models that allow the large number of evaluations of the systems Hamiltonian that are required to obtain the phase space sampling needed to obtain the free energy, specific heat, magnetization, susceptibility, and other quantities as function of temperature. We have demonstrated a solution to this problem that harnesses the computational power of today’s large massively parallel computers by combining a classical Wang–Landau Monte-Carlo calculation [F. Wang and D. P. Landau, Phys. Rev. Lett. 86, 2050 (2001)] with our first principles multiple scattering electronic structure code [Y. Wang et al., Phys. Rev. Lett. 75, 2867 (1995)] that allows the energy calculation of constrained magnetic states [M. Eisenbach et al., Proceedings of the Conference on High Performance Comput...

Angular dependence of switching properties in single Fe nanopillars

The magneto-caloric effect (MCE) is a possible route to more efficient heating and cooling of residential and commercial buildings. The search for improved materials is important to the development of a viable MCE based heat pump technology. We have calculated the magnetic structure of a candidate MCE material: Ni2MnGa. The density of magnetic states was calculated with the Wang Landau statistical method utilizing energies fit to those of the locally self-consistent multiple scattering method. The relationships between the density of magnetic states and the field induced adiabatic temperature change and the isothermal entropy change are discussed.

Reversal modes of simulated iron nanopillars in an obliquely oriented field

We consider a magnet with uniaxial anisotropy in an external magnetic field along the anisotropy direction, but with a field magnitude smaller than the coercive field. There are three representative magnetization configurations corresponding to three extrema of the free energy. The equilibrium and metastable configurations, which are magnetized approximately parallel and antiparallel to the applied field, respectively, both correspond to local free-energy minima. The third extremum configuration is the saddle point separating these minima. It is also called the transition state for magnetization reversal. The free-energy difference between the metastable and transition-state configurations determines the thermal stability of the magnet. However, it is difficult to determine the location of the transition state in both experiments and numerical simulations. Here it is shown that the computational Projective Dynamics method, applied to the time dependence of the total magnetization, can be used to determine the transition state. From large-scale micromagnetic simulations of a simple model of magnetic nanowires with no crystalline anisotropy, the magnetization associated with the transition state is found to be linearly dependent on temperature, and the free-energy barrier is found to be dominated by the entropic contribution at reasonable temperatures and external fields. The effect of including crystalline anisotropy is also discussed. Finally, the influence of the spin precession on the transition state is determined by comparison of the micromagnetic simulations to kinetic Monte Carlo simulations of precession-free (overdamped) dynamics.We consider a magnet with uniaxial anisotropy in an external magnetic field along the anisotropy direction, but with a field magnitude smaller than the coercive field. There are three representative magnetization configurations corresponding to three extrema of the free energy. The equilibrium and metastable configurations, which are magnetized approximately parallel and antiparallel to the applied field, respectively, both correspond to local free-energy minima. The third extremum configuration is the saddle point separating these minima. It is also called the transition state for magnetization reversal. The free-energy difference between the metastable and transition-state configurations determines the thermal stability of the magnet. However, it is difficult to determine the location of the transition state in both experiments and numerical simulations. Here it is shown that the computational Projective Dynamics method, applied to the time dependence of the total magnetization, can be used to determine...

Improved methods for calculating thermodynamic properties of magnetic systems using Wang-Landau density of states

The continued increase in areal densities in magnetic recording makes it crucial to understand magnetization reversal in nanoparticles. We present finite-temperature micromagnetic simulations of hysteresis in Fe nanopillars with the long axis tilted at angles from 0° to 90° to the applied sinusoidal field. The field period is 15 ns, and the particle size is 9×9×150u2009nm. The system is discretized into a rectangular pillar of 7×7×101 spins each with uniform magnetization. At low angles, reversal begins at the endcaps and proceeds toward the center of the particle. At 90° reversal proceeds along the entire length of the particle (save at the ends). The switching field was observed to increase over the entire range of angles, consistent with recent experimental observations. A second, lower-resolution micromagnetic simulation with 1×1×17 spins, does not agree with experiment, but shows behavior very similar to that of the Stoner–Wohlfarth model of coherent rotation.

Novel nanophysics in antiferromagnetic Heisenberg chains

Stochastic micromagnetic simulations are employed to study switching in three-dimensional magnetic nanopillars exposed to highly misaligned fields. The switching appears to proceed through two different decay modes, characterized by very different average lifetimes and different average values of the transverse magnetization components.

Convergence for the Wang-Landau density of states.

The Wang-Landau method [F. Wang and D. P. Landau, Phys. Rev. E 64, 056101 (2001)] is an efficient way to calculate the density of states (DOS) for magnetic systems, and the DOS can then be used to rapidly calculate the thermodynamic properties of the system. A technique is presented that uses the DOS for a simple Hamiltonian to create a stratified sample of configurations which are then used calculate a “warped’’ DOS for more realistic Hamiltonians. This technique is validated for classical models of bcc Fe with exchange interactions of increasing range, but its real value is using the DOS for a model Hamiltonian calculated on a workstation to select the stratified set of configurations whose energies can then be calculated for a density-functional Hamiltonian. The result is an efficient first-principles calculation of thermodynamic properties such as the specific heat and magnetic susceptibility. Another technique uses the sample configurations to calculate the parameters of a model exchange interaction ...