J. P. Whitehead

Micromagnetic simulations of interacting dipoles on an fcc lattice: application to nanoparticle assemblies.

Micromagnetic simulations are used to examine the effects of cubic and axial anisotropy, magnetostatic interactions and temperature on M-H loops for a collection of magnetic dipoles on fcc and sc lattices. We employ a simple model of interacting dipoles that represent single-domain particles in an attempt to explain recent experimental data on ordered arrays of magnetoferritin nanoparticles that demonstrate the crucial role of interactions between particles in an fcc lattice. Significant agreement between the simulation and experimental results is achieved, and the impact of intra-particle degrees of freedom and surface effects on thermal fluctuations is investigated.

Simulations of magnetic hysteresis loops for dual layer recording media

A Kinetic Monte-Carlo algorithm is applied to examine MH loops of dual-layer magnetic recording media at finite temperature and long time scales associated with typical experimental measurements. In contrast with standard micromagnetic simulations, which are limited to the ns-μs time regime, our approach allows for the direct calculation of magnetic configurations over periods from minutes to years. The model is used to fit anisotropy and coupling parameters to experimental data on exchange-coupled composite media which are shown to deviate significantly from standard micromagnetic results. Sensitivities of the loops to anisotropy, inter-layer exchange coupling, temperature, and sweep rate are examined.

Micromagnetic simulations of sweep-rate dependent coercivity in perpendicular recording media

The results of micromagnetic simulations are presented, which examine the impact of thermal fluctuations on sweep rate dependent coercivities of both single-layer and exchange-coupled-composite (ECC) perpendicular magnetic recording media. M-H loops are calculated at four temperatures and sweep rates spanning five decades with fields applied normal to the plane and at 45°. The impact of interactions between grains is evaluated. The results indicate a significantly weaker sweep-rate dependence for ECC media suggesting more robustness to long-term thermal effects. Fitting the modeled results to Sharrock-like scaling proposed by Feng and Visscher [J. Appl. Phys. 95, 7043 (2004)] is successful only in the case of single-layer media with the field normal to the plane.

Effects of dilution on the magnetic ordering of a two-dimensional lattice of dipolar magnets

Monte Carlo simulations are used to study the effects of dilution by random vacancies on the phenomenon of order arising from disorder in an ultrathin magnetic film. At very low concentrations of vacancies, both the collinear ordered phase observed in the undiluted system and the microvortex state are observed, and the boundary on which the reorientation transition between these states occurs is found to be consistent with the predictions of earlier work. However, even at vacancy densities as low as 0.5% there is evidence that the vacancies result in a energy landscape with a number of very nearly degenerate minima.

Memory effects and slow dynamics in ultra thin magnetic films

Monte Carlo studies of a uniaxial spin system on a square lattice with both ferromagnetic exchange interactions and dipolar interactions reveal three distinct dynamical regimes at low temperatures. In the first regime the magnetization decays through nucleation and growth of spin islands, in the second regime, despite the magnetization being effectively zero, there is a persistent memory effect most clearly revealed by the asymmetry between the numbers of up and down islands. The third regime is characterized by slow crystallization of the equilibrium smectic phase from the quenched polycrystalline state.

Monte Carlo simulations of intragrain spin effects in a quasi-2D Heisenberg model with uniaxial anisotropy

A combination of Metropolis and modified Wolff cluster algorithms is used to examine the impact of uniaxial single-ion anisotropy on the phase transition to ferromagnetic order of Heisenberg macrospins on a 2D square lattice. This forms the basis of a model for granular perpendicular recording media where macrospins represent the magnetic moment of grains. The focus of this work is on the interplay between anisotropy D, intragrain exchange J and intergrain exchange J on the ordering temperature T(C) and extends our previous reported analysis of the granular Ising model. The role of intragrain degrees of freedom in heat assisted magnetic recording is discussed.

Monte Carlo simulations of magnetic ordering in the fcc Kagome lattice

Monte Carlo simulation results are reported on magnetic ordering in ABC stacked Kagome layers with fcc symmetry for both XY and Heisenberg models which include exchange interactions with the eight near-neighbors. Well known degeneracies of the 2D system persist in the 3D case and analysis of the numerical data provides strong evidence for a fluctuation-driven first-order transition to well- defined long-range order characterized as the layered q = 0 (120-degree) spin structure. Effects of varying the inter-layer coupling are also examined. The results are relevant to understanding the role of frustration in IrMn3 alloys widely used by the magnetic storage industry as thin-films in the antiferromagnetic pinning layer in GMR and TMR spin valves. Despite the technological importance of this structure, it has not previously been noted that the magnetic Mn-ions of fcc IrMn3 form Kagome layers.

Phase behaviour of the antiferromagnetic plane rotator model

The phase diagram of an ultrathin film of magnetic rotors confined to the plane of the film has been determined from Monte Carlo simulations. In this square lattice system, the classical spins interact through a nearest neighbour antiferromagnetic exchange and the dipolar interactions. The phase diagram shows a dipolar antiferromagnetic phase for low values of |J|/g, and a simple antiferromagnetic phase for large values of |J|/g. The Monte Carlo data also indicate that the dipolar phase separates into two distinct phases for different values of |J|/g. In the first phase the spins are aligned along the x-or y-axis, while in the second phase the sublattice magnetization is oriented at π/4 to the x-and y-axis. The results for the plane rotator model are compared with previous results obtained for the Heisenberg model, which showed an analogous phase behaviour. This comparison clarifies the role played by out-of-plane degree of freedom and provides some further insight into this intriguing transition.

Monte Carlo simulations of ABC stacked kagome lattice films.

Properties of films of geometrically frustrated ABC stacked antiferromagnetic kagome layers are examined using Metropolis Monte Carlo simulations. The impact of having an easy-axis anisotropy on the surface layers and cubic anisotropy in the interior layers is explored. The spin structure at the surface is shown to be different from that of the bulk 3D fcc system, where surface axial anisotropy tends to align spins along the surface [1 1 1] normal axis. This alignment then propagates only weakly to the interior layers through exchange coupling. Results are shown for the specific heat, magnetization and sub-lattice order parameters for both surface and interior spins in three and six layer films as a function of increasing axial surface anisotropy. Relevance to the exchange bias phenomenon in IrMn3 films is discussed.

Thermally activated switching at long time scales in exchange-coupled magnetic grains

Rate coefficients of the Arrhenius-Neel form are calculated for thermally activated magnetic moment reversal for dual layer exchange-coupled composite (ECC) media based on the Langer formalism and are applied to study the sweep rate dependence of MH hysteresis loops as a function of the exchange coupling I between the layers. The individual grains are modelled as two exchange coupled Stoner-Wohlfarth particles from which the minimum energy paths connecting the minimum energy states are calculated using a variant of the string method and the energy barriers and attempt frequencies calculated as a function of the applied field. The resultant rate equations describing the evolution of an ensemble of non-interacting ECC grains are then integrated numerically in an applied field with constant sweep rate and the magnetization calculated as a function of the applied field H. MH hysteresis loops are presented for a range of values I and a figure of merit (FOM) that quantifies the advantages of ECC media is proposed. The results are also used to examine the accuracy of certain approximate models that reduce the complexity associated with the Langer based formalism and which provide some useful insight into the reversal. Of particular interest is the clustering of minimum energy states that are separated by relatively low energy barriers into metastates. It is shown that while approximating the reversal process in terms of metastates results in little loss of accuracy, it can reduce the run time of a Kinetic Monte Carlo (KMC) simulation of the magnetic decay of an ensemble of dual layer ECC media by 2~3 orders of magnitude. The essentially exact results presented in this work for two coupled grains are analogous to the Stoner-Wohlfarth model of a single grain and serve as an important precursor to KMC based simulation studies on systems of interacting dual layer ECC media.