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A block-inverse preconditioner (BIP) is proposed to accelerate solving implicit time integration in the context of Newton–Krylov approach used in micromagnetic simulations for solving the Landau–Lifshitz–Gilbert equation. A coefficient matrix is generated and stored for the linear system of Newton method. BIP is formulated by subdividing the coefficient matrix into small blocks and directly inverting them. The cost of preconditioning is low since inverting and multiplying small blocks is fast, which can be further minimized by parallel computing on multicore CPUs or GPUs. The effectiveness, speed, robustness, and scalability of BIP is demonstrated by numerical simulation experiments. Comparisons to incomplete LU preconditioning methods are conducted to demonstrate the effectiveness of BIP. [ABSTRACT FROM AUTHOR]

We investigated the effect of interlayer exchange coupling parameter on switching current density and switching time in the [CoPt-ML]/Ta/CoFeB composite free layer. The fundamental parameters for the micromagnetic model were extracted from experimental results and ab-initio calculations of the Fe/MgO and Fe/Ta interfaces. We found that the critical current density and switching current decrease with decreasing interlayer exchange coupling. It was observed experimentally that perpendicular magnetic anisotropy (PMA) increases with increasing thickness of Ta insertion due to enhancement of CoFeB/MgO interfacial anisotropy, whereas the interlayer exchange coupling strength decreases. Therefore, our modeling and experimental results indicate that the optimized Ta insertion in the composite layer leads to improved thermal stability via combined interface and bulk anisotropies, lower critical current density, and reduced switching time as compared to the composite layer without Ta insertion. [ABSTRACT FROM AUTHOR]

A new software framework is presented for fast optimal designs of micromagnetic devices. The developed framework adopts a fast micromagnetic (FastMag) simulator as a micromagnetic solver, and utilizes distributed evolutionary algorithm in Python (DEAP). The framework makes use of a simple Linux utility for resource management (SLURM) in order to run and manage multiple micromagnetic simulations concurrently on a Linux cluster. Various design variables can be designated including material and geometry parameters. The whole design cycle is automated including the mesh generation, and the status of the evolution and multiple simulations can be monitored and managed easily. By combining high computing speed of FastMag and distributed optimization capability of DEAP and SLURM, the optimal designs of micromagnetic devices can be performed rapidly and efficiently. As numerical examples, the optimal designs of a spin-torque oscillator and a magnetic random access memory device are performed. [ABSTRACT FROM AUTHOR]

A micromagnetic solver using the finite-difference method on a graphics processing unit (GPU) and its integration with the object-oriented micromagnetic framework (OOMMF) are presented. Two approaches for computing the magnetostatic field accelerated by the fast Fourier transform are implemented. The first approach, referred to as the tensor approach, is based on the tensor spatial convolution to directly compute the magnetostatic field from magnetic moments. The second approach, referred to as the scalar potential approach, uses differential operator evaluation through finite differences (divergence for magnetic charge and gradient for magnetostatic field) and spatial convolution for magnetic scalar potential. Comparisons of implementation details, speed, memory consumption, and accuracy are provided. The GPU implementation of OOMMF shows up to $32\times $ GPU–CPU speedup. [ABSTRACT FROM AUTHOR]

A discretized magnetic media model is developed to improve our ability to describe the high-frequency recording performance of perpendicular magnetic media. The new model differs from the traditional macrospin model in that the magnetic grains are subdivided into cells, which have varying magnetic properties and independent magnetic moments. This discretized media model has two realizations: one with a discretized magnetic boundary and one without the magnetic boundary. We focus on the resolution and the signal-to-noise ratio obtained from a high-density bit pattern as the bit length approaches the grain pitch. Simulation results demonstrating the impact of a discretized cap in this recording regime will be discussed. The increased computational effort needed to perform a simulation using this discretized media model, and has been addressed by porting this model to a cluster of graphics processing units. [ABSTRACT FROM AUTHOR]

Micromagnetic simulations are used to evaluate the effect of thermal fluctuations on the performance of perpendicular BaFe media. The write simulations are performed using a highly efficient particle micromagnetic solver running on graphics processing units. Thermal effects are found to reduce the field required to write optimally the medium, in agreement with the decay of the medium mean switching field with temperature. Thermal fluctuations also affect the system write performance: a broadband signal-to-noise ratio (BB-SNR) loss of several decibels is observed for a recording temperature of 400 K compared to the 0 K case. The SNR correlates with the increase of the written transition width and relates to the broadening of the medium switching field distribution at increasing recording temperatures. The presented results are also relevant for understanding thermal effects in granular media used in magnetic recording. [ABSTRACT FROM AUTHOR]

Magnetization and magnetic field dynamics arising when switching a realistic recording head model is studied. The write head design comprises a return pole, yoke, main pole, tapered trapezoidal pole tip, tapered wrap around shield (WAS), and soft underlayer. The analysis was performed using the high-performance micromagnetic simulator FastMag, which is well suited for the write head dynamic problems due to its ability to handle complex magnetic devices discretized into many millions of elements. The head dynamics is considered for different mesh densities, switching data rates, and current waveforms. It is demonstrated that improper discretization may result in a very different magnetization behavior. This is especially pronounced for cases of high switching rates, for which meshes of insufficient density resulted in a completely incorrect behavior, e.g. absence of switching. On the other hand, sufficiently dense meshes resulted in reliable dynamics and switching behavior. Furthermore, magnetization dynamics effects in WAS and their effects on the magnetostatic fields in the media layer were studied. WAS significantly improves the head field gradients in both down- and off-track directions, which is important for high areal recording densities. However, the presence of WAS leads to reduced write fields below the pole tip and to significant undesired magnetostatic fields below the side shields in the media layer. Such undesired fields can be obtained close to the pole tip as well as far from the tip. These phenomena result from the domain wall creation, propagation, and annihilation in WAS due to the switching. The field close to the pole tip can result in adjacent track erasure, while fields far from the tip can lead to far track erasure. The existence of these fields should be accounted for when performing recording system design optimization and analysis. [ABSTRACT FROM AUTHOR]

An efficient micromagnetic solver running on graphics processing units (GPU) is demonstrated. The solver implements a nonuniform grid interpolation method (NGIM) to compute the superposition integral for the magnetostatic field with O (N) operations and memory requirements. The NGIM divides the computational domain into a hierarchy of boxes containing sources and observers, and it uses spatial interpolation from sparse nonuniform grids to achieve computational savings. Efficiency of the GPU solver is achieved by using coalesced memory accessing requiring arranging data in contiguous addresses, one-block-per-box computations with a block of threads handling an observation box to achieve the best utilization of the GPU threads, and on-fly computation of all grids and interpolation coefficients leading to reduced memory and increased speed. The GPU-CPU speed-ups are shown to be in the range 40-100 depending on the problem size and accuracy. A simple and inexpensive GPU is shown to handle efficiently problems comprising discretizations of more than 16 million of spins. [ABSTRACT FROM AUTHOR]

Magnetization reversal mechanisms for single patterned media elements have been examined. Sizes and magnetic properties have been examined for parallelepipeds of fixed aspect ratio. For element sizes for 1 Tbit/in² recording magnetization reversal is nearly uniform for a uniform applied field. For fields covering a small percentage of the element volume nonuniform reversal occurs and the reversal field increases at a much smaller rate than inversely as the volume covered. [ABSTRACT FROM AUTHOR]

We present an approach for minimizing the critical current for the magnetization switching in magnetic tunnel junctions by optimizing the spatial distribution of the current density. We show that such a minimization is possible because critical current is determined by the condition of making one of the magnetization eigenstates grow in time. The excitation of the eigenstates is enhanced when the spatial distributions of the eigenstates and current density overlap. Critical current can be viewed as a functional of the current density spatial distribution and it can be minimized by optimizing this distribution. Such an optimization results in a major reduction of the critical current and increase of the switching efficiency, viz. the ratio between the energy barrier and critical current. The minimized critical current increases approximately linearly with the magnetic tunnel junction size, which is much slower than critical current for the case of a uniform current density. The optimized efficiency can be approximately a constant with respect to the magnetic tunnel junction size, which is much higher than the efficiency for the uniform current density. Additional optimization can be achieved by spatially modulating the material parameters, e.g., the saturation magnetization. The presented approach and obtained scaling of the critical current and efficiency offers opportunities for the magnetic tunnel junction optimization. [ABSTRACT FROM AUTHOR]

• Micromagnetic code based on Voronoi tessellation. • Magnetostatic field computed using a non-uniform fast Fourier method augmented with exact near field computation. • Exchange field computed on Voronoi geometry. • GPU accelerated computation. • Used to simulate SNR and Domain Wall Motion in granular recording media. A micromagnetic code based on Voronoi tessellation and the non-uniform fast Fourier transform (NUFFT) method is presented. The code is capable of efficiently and accurately simulating magnetization dynamics in large and structurally complex granular systems, such as multilayer granular media used for perpendicular magnetic recording, bit patterned media, granular nanowires, and read heads. In these systems the granular microstructure and distributions in grain and interface properties play an important role in device performance. The presented Voronoi simulator allows comprehensive studies to be performed as it accounts for the detailed granular microstructure and distributions that characterize true systems. Simulation time is greatly reduced by a non-uniform fast Fourier transform algorithm and implementation on graphics processing units (GPUs). Simulations of conventional magnetic recording, heat-assisted magnetization reversal, domain wall dynamics in granular nanowires, and particulate tape recording are presented. [ABSTRACT FROM AUTHOR]