Multiscale and multimodel simulation of Bloch point dynamics

We present simulation results on the structure and dynamics of micromagnetic point singularities with atomistic resolution. This is achieved by embedding an atomistic computational region into a standard micromagnetic algorithm. Several length scales are bridged by means of an adaptive mesh refinement and a seamless coupling between the continuum theory and a Heisenberg formulation for the atomistic region. The code operates on graphical processing units and is able to detect and track the position of strongly inhomogeneous magnetic regions. This enables us to reliably simulate the dynamics of Bloch points, which means that a fundamental class of micromagnetic switching processes can be analyzed with unprecedented accuracy. We test the code by comparing it with established results and present its functionality with the example of a simulated field-driven Bloch point motion in a soft-magnetic cylinder

Numerical micromagnetism of strong inhomogeneities

The size of micromagnetic structures, such as domain walls or vortices, is comparable to the exchange length of the ferromagnet. Both, the exchange length of the stray field $l_s$ and the magnetocrystalline exchange length $l_k$ are material-dependent quantities that usually lie in the nanometer range. This emphasizes the theoretical challenges associated with the mesoscopic nature of micromagnetism: the magnetic structures are much larger than the atomic lattice constant, but at the same time much smaller than the sample size. In computer simulations, the smallest exchange length serves as an estimate for the largest cell size admissible to prevent appreciable discretization errors. This general rule is not valid in special situations where the magnetization becomes particularly inhomogeneous. When such strongly inhomogeneous structures develop, micromagnetic simulations inevitably contain systematic and numerical errors. It is suggested to combine micromagnetic theory with a Heisenberg model to resolve such problems. We analyze cases where strongly inhomogeneous structures pose limits to standard micromagnetic simulations, arising from fundamental aspects as well as from numerical drawbacks

Magnetization dynamics in a three-dimensional interconnected nanowire array

Three-dimensional magnetic nanostructures have recently emerged as artificial magnetic material types with unique properties bearing potential for applications, including magnonic devices. Interconnected magnetic nanowires are a sub-category within this class of materials that is attracting particular interest. We investigate the high-frequency magnetization dynamics in a cubic array of cylindrical magnetic nanowires through micromagnetic simulations based on a frequency-domain formulation of the linearized Landau-Lifshitz-Gilbert equation. The small-angle high-frequency magnetization dynamics excited by an external oscillatory field displays clear resonances at distinct frequencies. These resonances are identified as oscillations connected to specific geometric features and micromagnetic configurations. The geometry- and configuration-dependence of the nanowire array's absorption spectrum demonstrates the potential of such magnetic systems for tuneable and reprogrammable magnonic applications.Comment: 7 pages, 5 figure

Spin-Transfer Torque Induced Vortex Dynamics in Fe/Ag/Fe Nanopillars

We report experimental and analytical work on spin-transfer torque induced vortex dynamics in metallic nanopillars with in-plane magnetized layers. We study nanopillars with a diameter of 150 nm, containing two Fe layers with a thickness of 15 nm and 30 nm respectively, separated by a 6 nm Ag spacer. The sample geometry is such that it allows for the formation of magnetic vortices in the Fe disks. As confirmed by micromagnetic simulations, we are able to prepare states where one magnetic layer is homogeneously magnetized while the other contains a vortex. We experimentally show that in this configuration spin-transfer torque can excite vortex dynamics and analyze their dependence on a magnetic field applied in the sample plane. The center of gyration is continuously dislocated from the disk center, and the potential changes its shape with field strength. The latter is reflected in the field dependence of the excitation frequency. In the second part we propose a novel mechanism for the excitation of the gyrotropic mode in nanopillars with a perfectly homogeneously magnetized in-plane polarizing layer. We analytically show that in this configuration the vortex can absorb energy from the spin-polarized electric current if the angular spin-transfer efficiency function is asymmetric. This effect is supported by micromagnetic simulations.Comment: The article has been sent to J. Phys. D. Submitted on August 9, 2010. (7 pages and 4 figures.

Ultrafast dynamics of a magnetic antivortex - Micromagnetic simulations

The antivortex is a fundamental magnetization structure which is the topological counterpart of the well-known magnetic vortex. We study here the ultrafast dynamic behavior of an isolated antivortex in a patterned Permalloy thin-film element. Using micromagnetic simulations we predict that the antivortex response to an ultrashort external field pulse is characterized by the production of a new antivortex as well as of a temporary vortex, followed by an annihilation process. These processes are complementary to the recently reported response of a vortex and, like for the vortex, lead to the reversal of the orientation of the antivortex core region. In addition to its fundamental interest, this dynamic magnetization process could be used for the generation and propagation of spin waves for novel logical circuits.Comment: 4 pages, 4 figures. To be published in Physical Review B (R

Finite element calculations on the single-domain limit of a ferromagnetic cube – a solution to µmag standard problem

Abstract Zero field states of the magnetization in a uniaxial ferromagnetic sample of cubic shape are calculated by means of micromagnetic finite element modeling. With increasing size the minimum energy arrangement changes from a singledomain configuration (flower state) to a vortex configuration. An intermediate arrangement (twisted flower state) between the flower state and the vortex state is observed. A further magnetization state resulting in the calculation is a vortex state with a singularity of the directional field of the magnetization. This work provides our solution to the micromagnetic Standard Problem No. 3 posed by the mMAG micromagnetic modeling activity group at the National Institute of Standards and Technology (NIST).