M. V. Lubarda

FastMag: Fast micromagnetic simulator for complex magnetic structures (invited)

A fast micromagnetic simulator (FastMag) for general problems is presented. FastMag solves the Landau-Lifshitz-Gilbert equation and can handle problems of a small or very large size with a high speed. The simulator derives its high performance from efficient methods for evaluating the effective field and from implementations on massively parallel Graphics Processing Unit (GPU) architectures. FastMag discretizes the computational domain into tetrahedral elements and therefore is highly flexible for general problems. The magnetostatic field is computed via the superposition principle for both volume and surface parts of the computational domain. This is accomplished by implementing efficient quadrature rules and analytical integration for overlapping elements in which the integral kernel is singular. Thus discretized superposition integrals are computed using a non-uniform grid interpolation method, which evaluates the field from N sources at N collocated observers in ( ) O N operations. This approach allows handling any uniform or non-uniform shapes, allows easily calculating the field outside the magnetized domains, does not require solving linear system of equations, and requires little memory. FastMag is implemented on GPUs with GPU-CPU speed-ups of two orders of magnitude. Simulations are shown of a large array and a recording head fully discretized down to the exchange length, with over a hundred million tetrahedral elements on an inexpensive desktop computer.

Thermal stability of patterned Co/Pd nanodot arrays

We have studied the magnetic reversal and thermal stability of [Co(0.3nm)/Pd(0.7nm)]N multilayers patterned into 35-nm-diameter nanodot arrays. The short-time coercive fields are relatively constant with N while the room-temperature thermal stability parameter increases nearly linearly with N. However the magnetic switching volume extracted from the thermal stability is significantly less than the physical volume of the samples. The experimental results are in quantitative agreement with micromagnetic modeling, which indicates that reversal and thermal stability is controlled by nucleation and propagation of edge domains. V C 2012 American Institute of Physics .[ http://dx.doi.org/10.1063/1.3692574]

Spin-transfer-torque reversal in perpendicular anisotropy spin valves with composite free layers

We describe modeling of spin-transfer-torque (STT) driven reversal in nanopillars with strong out-of-plane magnetic anisotropy where the free layer is a magnetically hard-soft composite structure. By adjusting the exchange coupling between the hard and soft layers, we observed reduced current amplitude and pulse durations required to reverse the magnetization compared to a homogeneous free layer of comparable thermal stability. The reduction in critical current comes from the increased STT efficiency acting on the soft layer. As such, the switching current is relatively insensitive to the damping parameter of the magnetic hard layer. These properties make composite free layers promising candidates for STT-based magnetic memories.

Antiferromagnetically coupled capped bit patterned media for high-density magnetic recording

We report micromagnetic modeling of a bit patterned media where a two-dimensional array of patterned composite islands is antiferromagnetically coupled to a continuous capping layer. This media allows optimization of writability, switching field distributions, and readback response. Lateral and vertical exchange introduced through the coupling with the capping layer compensates the dipolar interactions between islands and antiferromagnetic coupling is employed to modulate the high-density readback response.

Reversal in Bit Patterned Media With Vertical and Lateral Exchange

The extent to which dipolar interactions affect switching field distributions and thermal stability, and subsequently system performance, depends closely on the areal density, materials properties, and the architecture of the media, as well as the recording scheme and field profiles. This paper assesses, via micromagnetic simulations, the potential of heterogeneous capped bit patterned media and other proposed patterned media designs for ultra-high-density magnetic recording with respect to writability, switching field distributions, and thermal stability at different areal densities. Such media is comprised of an array of homogeneous or exchange coupled composite elements with vertical anisotropy that is ferromagnetically or antiferromagnetically coupled to a continuous horizontal layer. It is shown that such systems, characterized by lateral and vertical exchange, enable ultra-high-density recording at low switching fields while ensuring high thermal stability and low switching field distributions. Mechanisms leading to improved performance in capped systems are investigated. Structural and material considerations are provided for all media models.

Advanced Micromagnetic Analysis of Write Head Dynamics Using Fastmag

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.

Micromagnetics on high-performance workstation and mobile computational platforms

The feasibility of using high-performance desktop and embedded mobile computational platforms is presented, including multi-core Intel central processing unit, Nvidia desktop graphics processing units, and Nvidia Jetson TK1 Platform. FastMag finite element method-based micromagnetic simulator is used as a testbed, showing high efficiency on all the platforms. Optimization aspects of improving the performance of the mobile systems are discussed. The high performance, low cost, low power consumption, and rapid performance increase of the embedded mobile systems make them a promising candidate for micromagnetic simulations. Such architectures can be used as standalone systems or can be built as low-power computing clusters.

Large exchange-dominated domain wall velocities in antiferromagnetically coupled nanowires

Magnetic nanowires supporting field- and current-driven domain wall motion are envisioned for methods of information storage and processing. A major obstacle for their practical use is the domain-wall velocity, which is traditionally limited for low fields and currents due to the Walker breakdown occurring when the driving component reaches a critical threshold value. We show through numerical and analytical modeling that the Walker breakdown limit can be extended or completely eliminated in antiferromagnetically coupled magnetic nanowires. These coupled nanowires allow for large domain-wall velocities driven by field and/or current as compared to conventional nanowires.

Characterization of strain and its effects on ferromagnetic nickel nanocubes

We report on the interplay of magnetic properties and intrinsic strain in ferromagnetic nickel nanocubes with cubic anisotropy. Via coherent x-ray diffraction imaging we observed compressive stress at the bottom surface of these cubes. The nanocubes with {100} facets described and imaged in this study were synthesized using a single-step CVD process. Micromagnetic simulations predict the presence of vortices at remanence in the absence of strain. The effects of strain resulting from the compressive stress on the magnetic response of the ferromagnetic cubes is investigated. We observe that measured intrinsic strain is too low to change the magnetic anisotropy of ferromagnetic cubes but topological behavior of magnetic vortices is sensitive to even this low range of strain.

Numerical modeling of heat assisted magnetic recording system

Heat assisted magnetic recording (HAMR) is envisioned as the next technology for boosting the recording densities. In HAMR, a localized optical field is used to locally heat the magnetic recording media layer, thus reducing the thermal stability and allowing recording at sufficiently weak magnetic fields. After cooling down, the recording layer restores the original high thermal stability. From the simulation point of view HAMR requires integrated modeling approaches, including magnetics, optics, heat transport, and electromagnetics.