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We demonstrate the possibility of tracking clustering, aggregation, and immobilization processes of magnetic nanoparticles with high sensitivity by measuring their thermal magnetic noise spectra with optically pumped magnetometers.

We describe an extension of the micromagnetic finite difference simulation software MuMax3 to solve elasto-magneto-dynamical problems. The new module allows for numerical simulations of magnetization and displacement dynamics in magnetostrictive materials and structures, including both direct and inverse magnetostriction. The theoretical background is introduced, and the implementation of the extension is discussed. The magnetoelastic extension of MuMax3 is freely available under the GNU General Public License v3.

The development of future spintronic applications requires a thorough and fundamental understanding of the magnetization dynamics. Of particular interest are magnetic nanodisks, in which the vortex state emerges as a stable spin configuration. Here, we focus on how the vortex-core polarization can be reversed periodically by an oscillating magnetic field, applied perpendicularly to the disk's surface. By means of micromagnetic simulations, we demonstrate the presence of several subharmonic switching modes, i.e., the commensurate ratio between the switching frequency of the core and the driving frequency. The underlying mechanism of this periodic behavior depends on the disk thickness. For thin disks, the core switches periodically due to resonant excitation of radial spin wave modes, while it is due to the breathing mode in the case of thick disks. However, overlap of both modes impedes periodic vortex-core switching. For thin disks, the threshold field amplitude required for periodic switching can be substantially lowered by increasing the disk diameter. For thick disks, in contrast, the diameter is less decisive for the minimal field amplitude, as only the average energy density in a central region around the vortex core is relevant to excite the breathing mode. Our results contribute to the understanding of the switching mechanisms in magnetic nanodisks, which are of technological interest due to their potential in nonvolatile memory devices.

Although it is well documented that the vortex core in a micromagnet can switch polarization in response to magnetic field pulses, the temperature dependence of this process has not yet been addressed by experiments. Using scanning transmission x-raymicroscopy, we investigate the magnetic vortex dynamics in a La0.7Sr0.3MnO3 microplatelet at temperatures ranging from 150K up to T-C at 350K. The time-resolved images reveal qualitatively different dynamic regimes as a function of temperature and applied field, as the relative strengths of the micromagnetic energy terms strongly vary over this temperature range. By explicitly accounting for the temperature dependence of the magnetic parameters in our micromagnetic simulations, we found excellent agreement with the experiments over the full measurement range. In line with previous models, the simulations reveal that the vortex core switches polarization when it reaches a critical velocity that mainly depends on the strength of the exchange interaction. It is therefore strongly temperature dependent, thus explaining our observation of the different dynamical regimes.

The end of Moore's law for CMOS technology has prompted the search for low-power computing alternatives, resulting in several promising proposals based on magnetic logic[1-8]. One approach aims at tailoring arrays of nanomagnetic islands in which the magnetostatic interactions constrain the equilibrium orientation of the magnetization to embed logical functionalities[9-12]. Despite the realization of several proofs of concepts of such nanomagnetic logic[13-15], it is still unclear what the advantages are compared to the widespread CMOS designs, due to their need for clocking[16, 17] and/or thermal annealing [18,19] for which fast convergence to the ground state is not guaranteed. In fact, it seems increasingly evident that "beyond CMOS" technology will require a fundamental rethinking of our computing paradigm[20]. In this respect, a type of terminal-agnostic logic was suggested[21], where a given gate is able to "self-organize" into its correct logical states, regardless of whether the signal is applied to the traditional input terminals, or the output terminals. Here, we introduce nanomagnetic self-organizing balanced logic gates, that employ stray-field coupled nanomagnetic islands to perform terminal-agnostic logic. We illustrate their capabilities by implementing reversible Boolean circuitry to solve a two-bit factorization problem via numerical modelling. In view of their design and mode of operation, we expect these systems to improve significantly over those suggested in Ref.[21], thus offering an alternative path to explore memcomputing, whose usefulness has already been demonstrated by solving a variety of hard combinatorial optimization problems[22].

The characteristics of confined magnetoelastic waves in nanoscale ferromagnetic magnetostrictive waveguides have been investigated by a combination of analytical and numerical calculations. The presence of both magnetostriction and inverse magnetostriction leads to the coupling between confined spin waves and elastic Lamb waves. Numerical simulations of the coupled system have been used to extract the dispersion relations of the magnetoelastic waves as well as their mode profiles., 30 pages, 9 figures

Magnetic skyrmions are topologically stabilized nanoscale spin structures that could be of use in the development of future spintronic devices. When a skyrmion is driven by an electric current it propagates at an angle relative to the flow of current-known as the skyrmion Hall angle (SkHA)-that is a function of the drive current. This drive dependence, as well as thermal effects due to Joule heating, could be used to tailor skyrmion trajectories, but are not well understood. Here we report a study of skyrmion dynamics as a function of temperature and drive amplitude. We find that the skyrmion velocity depends strongly on temperature, while the SkHA does not and instead evolves differently in the low- and high-drive regimes. In particular, the maximum skyrmion velocity in ferromagnetic devices is limited by a mechanism based on skyrmion surface tension and deformation (where the skyrmion transitions into a stripe). Our mechanism provides a complete description of the SkHA in ferromagnetic multilayers across the full range of drive strengths, illustrating that skyrmion trajectories can be engineered for device applications. An analysis of skyrmion dynamics at different temperatures and electric drive currents is used to develop a complete description of the skyrmion Hall angle in ferromagnetic multilayers from the creep to the flow regime and illustrates that skyrmion trajectories can be engineered for device applications.

Magnonics research, i.e. the manipulation of spin waves for information processing, is a topic of intense research interest in the past years. FMR, BLS and MOKE measurements lead to tremendous success and advancement of the field. However, these methods are limited in their spatial resolution. X-ray microscopy opens up a way to push to spatial resolutions below 100 nm. Here, we discuss the methodology of STXM for pump-probe data acquisition with single photon counting and arbitrary excitation patterns. Furthermore, we showcase these capabilities using two magnonic crystals as examples: an antidot lattice and a Fibonacci quasicrystal.

Thin ferromagnetic films with an interfacially induced DMI exhibit nontrivial asymmetric dispersion relations that lead to unique and useful magnonic properties. Here we derive an analytical expression for the magnon propagation angle within the micromagnetic framework and show how the dispersion relation can be approximated with a comprehensible geometrical interpretation in the k-space of the propagation of spin waves. We further explore the refraction of spin waves at DMI interfaces in heterochiral magnetic films, after deriving a generalized Snell's law tunable by an in-plane magnetic field, that yields analytical expressions for critical incident angles. The found asymmetric Brewster angles at interfaces of regions with different DMI strengths, adjustable by magnetic field, support the conclusion that heterochiral ferromagnetic structures are an ideal platform for versatile spin-wave guides., 7 pages, 4 figures

In the quest for miniaturizing magnetic devices, the effects of boundaries and surfaces become increasingly important. Here we show how the recently predicted boundary-induced Dzyaloshinskii-Moriya interaction (DMI) affects the magnetization of ferromagnetic films with a $C_{\infty v}$ symmetry and a perpendicular magnetic anisotropy. For an otherwise uniformly magnetized film, we find a surface twist when the magnetization in the bulk is canted by an in-plane external field. This twist at the surfaces caused by the boundary-induced DMI differs from the common canting caused by internal DMI observed at the edges of a chiral magnet. Further, we find that the surface twist due to the boundary-induced DMI strongly affects the width of the domain wall at the surfaces. We also find that the skyrmion radius increases in the depth of the film, with the average size of the skyrmion increasing with boundary-induced DMI. This increase suggests that the boundary-induced DMI contributes to the stability of the skyrmion., 9 pages, 7 figures

We show how compensated antiferromagnetic spins can be implemented in the micromagnetic simulation program MuMax3. We demonstrate that we can model spin flop coupling as a uniaxial anisotropy for small canting angles and how we can take into account the exact energy terms for strong coupling between a ferromagnet and compensated antiferromagnet. We also investigate the training effect in biaxial antiferromagnets and reproduce the training effect in a polycrystalline IrMn/CoFe bilayer., 11 pages + Supplementary Material (10 pages)

We describe how a 2.5D approach can be successfully applied to micromagnetic simulations in which the magnetization dynamics is computed on the nanometer scale. In this approach, an infinitely long geometry is treated enabling a 2D spatial discretization. The material properties and field quantities are invariant in the infinite direction but retain their 3D vector character, safeguarding a correct physical description of the magnetization processes. As an example, we apply the 2.5D approach to investigate the equilibrium domain configurations and domain wall types encountered in infinite magnetic strips with varying cross-sectional dimension 4D×D and a varying uniaxial anisotropy strength Ku, imposed along the infinite direction. For each considered point in the parameter space D-Ku, the equilibrium state is computed by relaxing the micromagnetic system starting from four different initial magnetization states. On the basis of these simulations, we can distinguish three different regions, dominated by the anisotropic, magnetostatic, and exchange interactions. Copyright © 2013 John Wiley & Sons, Ltd.

The Dzyaloshinskii-Moriya interaction (DMI) is a chiral interaction that favors formation of domain walls. Recent experiments and ab initio calculations show that there are multiple ways to modify the strength of the interfacially induced DMI in thin ferromagnetic films with perpendicular magnetic anisotropy. In this paper we reveal theoretically the effects of spatially varied DMI on the magnetic state in thin films. In such heterochiral 2D structures we report several emergent phenomena, ranging from the equilibrium spin canting at the interface between regions with different DMI, over particularly strong confinement of domain walls and skyrmions within high-DMI tracks, to advanced applications such as domain tailoring nearly at will, design of magnonic waveguides, and much improved skyrmion racetrack memory.

Magnetic skyrmions are topological spin configurations in materials with chiral Dzyaloshinskii-Moriya interaction (DMI), that are potentially useful for storing or processing information. To date, DMI has been found in few bulk materials, but can also be induced in atomically thin magnetic films in contact with surfaces with large spin-orbit interactions. Recent experiments have reported that isolated magnetic skyrmions can be stabilized even near room temperature in few-atom-thick magnetic layers sandwiched between materials that provide asymmetric spin-orbit coupling. Here we present the minimum-energy path analysis of three distinct mechanisms for the skyrmion collapse, based on ab initio input and the performed atomic-spin simulations. We focus on the stability of a skyrmion in three atomic layers of Co, either epitaxial on the Pt(111) surface or within a hybrid multilayer where DMI nontrivially varies per monolayer due to competition between different symmetry breaking from two sides of the Co film. In laterally finite systems, their constrained geometry causes poor thermal stability of the skyrmion toward collapse at the boundary, which we show to be resolved by designing the high-DMI structure within an extended film with lower or no DMI.

SUMMARY We have developed a micromagnetic simulator for graphical processing units (GPU), using the CUDA framework. In this paper, we discuss the optimization of the effective field calculation, both from a mathematical and from a hardware-specific point of view. By using a finite-difference discretization scheme, the long-range magnetostatic field can be calculated using fast Fourier transforms, an approach well suited for the GPU. We show how the implementation can be tuned to the GPU hardware and how the performance can be further increased by dealing with the large number of zeros that typically occurs in the micromagnetic field computation. Additionally, we show how the ferromagnetic exchange interaction can be readily included in the magnetostatic field calculation without any additional computational cost. The resulting high-performance software can be used to run large-scale simulations that would have been very time-consuming on regular CPU hardware. As an example, we present a case study on the de-pinning of domain walls in racetrack memory devices. Copyright © 2012 John Wiley & Sons, Ltd.

Unidirectional switching of the magnetic vortex core can be achieved in micron-sized ferromagnetic platelets by excitation of the gyrotropic mode of the vortex structure with in-plane rotating magnetic fields. Circulating fields with a switchable sense of rotation (clockwise, CW or counter clockwise, CCW) have been generated on a micrometer length scale at frequencies up to 1 GHz by two orthogonal electric RF currents with 90° phase shift flowing through crossed but not isolated striplines. Decoupling of these currents is realized by balanced symmetric RF sources. The amplitudes of the rotating magnetic fields and their spatial distributions are calculated and the stripline geometry is discussed. By taking advantage of this technique, unidirectional vortex core reversal by excitation with CW or CCW rotating magnetic fields has been observed by time resolved scanning transmission X-ray microscopy. An area with reversed magnetization, the “dip,” was observed near the vortex core before vortex core reversal.

When subjected to the interfacially induced Dzyaloshinskii-Moriya interaction, the ground state in thin ferromagnetic films with high perpendicular anisotropy is cycloidal. The period of this cycloidal state depends on the strength of the Dzyaloshinskii-Moriya interaction. In this work, we have studied the effect of confinement on the magnetic ground state and excited states, and we determined the phase diagram of thin strips and thin square platelets by means of micromagnetic calculations. We show that multiple cycloidal states with different periods can be stable in laterally confined films, where the period of the cycloids does not depend solely on the Dzyaloshinskii-Moriya interaction strength but also on the dimensions of the film. The more complex states comprising skyrmions are also found to be stable, though with higher energy., Comment: 8 pages, 10 figures

The motion of domain walls in magnetic materials is a typical example of a creep process, usually characterised by a stretched exponential velocity-force relation. By performing large-scale micromagnetic simulations and analyzing an extended 1D model which takes the effects of finite temperatures and material defects into account, we show that this creep scaling law breaks down in sufficiently narrow ferromagnetic strips. Our analysis of current-driven transverse domain wall motion in disordered Permalloy nanostrips reveals instead a creep regime with a linear dependence of the domain wall velocity on the applied field or current density. This originates from the essentially point-like nature of domain walls moving in narrow, line- like disordered nanostrips. An analogous linear relation is found also by analyzing existing experimental data on field-driven domain wall motion in perpendicularly magnetised media.

We present a combined theoretical and experimental study, investigating the origin of the enhanced non-adiabaticity of magnetic vortex cores. Scanning transmission X-ray microscopy is used to image the vortex core gyration dynamically to measure the non-adiabaticity with high precision, including a high confidence upper bound. Using both numerical computations and analytical derivations, we show that the large non-adiabaticity parameter observed experimentally can be explained by the presence of local spin currents arising from a texture-induced emergent Hall effect. This enhanced non-adiabaticity is only present in two- and three-dimensional magnetic textures such as vortices and skyrmions and absent in one-dimensional domain walls, in agreement with experimental observations., 6 pages, 3 figures

A phenomenological equation called Landau-Lifshitz-Baryakhtar (LLBar) equation, which could be viewed as the combination of Landau-Lifshitz (LL) equation and an extra "exchange damping" term, was derived by Baryakhtar using Onsager's relations. We interpret the origin of this "exchange damping" as nonlocal damping by linking it to the spin current pumping. The LLBar equation is investigated numerically and analytically for the spin wave decay and domain wall motion. Our results show that the lifetime and propagation length of short-wavelength magnons in the presence of nonlocal damping could be much smaller than those given by LL equation. Furthermore, we find that both the domain wall mobility and the Walker breakdown field are strongly influenced by the nonlocal damping., Comment: 10 pages, 6 figures

Magnetic sensing and logic devices based on the motion of magnetic domain walls rely on the precise and deterministic control of the position and the velocity of individual magnetic domain walls. Varying domain wall velocities have been predicted to result from intrinsic effects such as oscillating domain wall spin structure transformations and extrinsic pinning due to imperfections. We use direct dynamic imaging of the nanoscale spin structure to directly check these predictions. We find a new regime of oscillating domain wall motion even below the Walker breakdown correlated with periodic spin structure changes and we show that the extrinsic pinning from defects in the nanowire only affects slow domain walls.

In this work, a self-consistent thermodynamic approach to micromagnetism is presented. The magnetic degrees of freedom are modeled using the Landau-Lifshitz-Baryakhtar theory, that separates the different contributions to the magnetic damping, and thereby allows them to be coupled to the electron and phonon systems in a self-consistent way. We show that this model can quantitatively reproduce ultrafast magnetization dynamics in Nickel., 5 pages, 3 figures

We present Vinamax, a simulation tool for nanoparticles that aims at simulating magnetization dynamics on very large timescales. To this end, each in dividual nanoparticle is approximated by a macrospin. Vinamax numerically solves the Landau-Lifshitz equation by adopting a dipole approximation method, while temperature effects can be taken into account with two stochastic methods. It describes the influence of demagnetizing and anisotropy fields on magnetic nanoparticles at finite temperatures in a space and time-dependent externally applied field. Vinamax can be used in biomedical research where nanoparticle imaging techniques are under developement., Comment: 7 pages, 7 figures

We report a numerical implementation of the Landau-Lifshitz-Baryakhtar theory that dictates that the micromagnetic relaxation term obeys the symmetry of the magnetic crystal, i.e., replacing the single intrinsic damping constant with a tensor of corresponding symmetry. The effect of anisotropic relaxation is studied in a thin saturated ferromagnetic disk and an ellipse with and without uniaxial magnetocrystalline anisotropy. We investigate the angular dependence of the linewidth of magnonic resonances with respect to the given structure of the relaxation tensor. The simulations suggest that the anisotropy of the magnonic linewidth is determined by two factors: the projection of the relaxation tensor onto the plane of precession and the ellipticity of the latter.

By combining magnetic transmission x-ray microscopy with a stroboscopic pump and probe technique using synchrotron radiation we are able to image the magnetization dynamics in micron sized magnetic particles on a sub-100 ps time scale with a lateral spatial resolution down to 21 nm. We report first observations in squared elements indicating locally varying precessional frequencies which are in agreement with micromagnetic simulations. The experiment opens a route towards a high spatiotemporal resolution of spin patterns which is needed to understand the microscopic origin of magnetization reversal of micron sized and nano-sized magnetic particles.

Magnetic sensing and logic devices based on the motion of magnetic domain walls rely on the precise and deterministic control of the position and the velocity of individual magnetic domain walls in curved nanowires. Varying domain wall velocities have been predicted to result from intrinsic effects such as oscillating domain wall spin structure transformations and extrinsic pinning due to imperfections. Here we use direct dynamic imaging of the nanoscale spin structure that allows us for the first time to directly check these predictions. We find a new regime of oscillating domain wall motion even below the Walker breakdown correlated with periodic spin structure changes. We show that the extrinsic pinning from imperfections in the nanowire only affects slow domain walls and we identify the magnetostatic energy, which scales with the domain wall velocity, as the energy reservoir for the domain wall to overcome the local pinning potential landscape., Nature Communications, 4, ISSN:2041-1723

The unidirectional shift of the hysteresis loop and the athermal training effect are two key features of the exchange bias phenomenon in most polycrystalline FM/AFM bilayers. We show that, by using MuMax3 which is an open source micromagnetic simulation program, we are able to reproduce experimental data (bias field and coercivity) for a polycrystalline Co/CoO bilayer. We also demonstrate that rotatable uncompensated spins can be responsible for the athermal training effect and that the reversal mechanism between the ascending and descending branch in the first hysteresis loop can be different, as has frequently been noticed in experiments.

Magnetic vortices and antivortices are usually observed coexisting in cross-tie walls within ferromagnetic thin films. An antivortex can be isolated by utilizing the shape anisotropy. The isolation offers the possibility to investigate its fundamental dynamics as a two-dimensional oscillator in a confining potential. Hitherto, the gyration mode of vortices and its excitation has been studied intensely. Here, a detailed investigation of the coupling of an antivortex to in-plane rotating magnetic fields is presented. The resonant response is imaged by time-resolved scanning transmission x-ray microscopy to determine the amplitude as well as the phase of the gyration. The experimental results are compared with the analytical model of a two-dimensional harmonic oscillator derived from the Thiele equation. As a cause for deviations from the model, a buckled energy landscape due to local defects is discussed.

We experimentally study the magnetization dynamics of pairs of micron-sized permalloy squares coupled via their stray fields. The trajectories of the vortex cores in the Landau-domain patterns of the squares are mapped in real space using time-resolved scanning transmission x-ray microscopy. After excitation of one of the vortex cores with a short magnetic-field pulse, the system behaves like coupled harmonic oscillators. The coupling strength depends on the separation between the squares and the configuration of the vortex-core polarizations. Considering the excitation via a rotating in-plane magnetic field, it can be understood that only a weak response of the second vortex core is observed for equal core polarizations.

Micron-sized magnetic platelets in the flux-closed vortex state are characterized by an in-plane curling magnetization and a nanometer-sized perpendicularly magnetized vortex core. Having the simplest non-trivial configuration, these objects are of general interest to micromagnetics and may offer new routes for spintronics applications. Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field core toggling by excitation of the gyrotropic eigenmode at sub-GHz frequencies was established. At frequencies more than an order of magnitude higher vortex state structures possess spin wave eigenmodes arising from the magneto-static interaction. Here we demonstrate experimentally that the unidirectional vortex core reversal process also occurs when such azimuthal modes are excited. These results are confirmed by micromagnetic simulations, which clearly show the selection rules for this novel reversal mechanism. Our analysis reveals that for spin-wave excitation the concept of a critical velocity as the switching condition has to be modified., Micron and submicron-sized magnetic platelets in a vortex configuration may be useful in micromagnetics and spintronics applications. Kammerer et al. show that a fast unidirectional vortex core reversal process occurs when azimuthal spin wave modes are excited at GHz frequency.

Topological singularities occur as antivortices in ferromagnetic thin-film microstructures. Antivortices behave as two-dimensional oscillators with a gyrotropic eigenmode which can be excited resonantly by spin currents and magnetic fields. We show that the two excitation types couple in an opposing sense of rotation in the case of resonant antivortex excitation with circular-rotational currents. If the sense of rotation of the current coincides with the intrinsic sense of gyration of the antivortex, the coupling to the Oersted fields is suppressed and only the spin-torque contribution locks into the gyrotropic eigenmode. We report on the experimental observation of purely spin-torque induced antivortex-core reversal. The dynamic response of an isolated antivortex is imaged by time-resolved scanning transmission x-ray microscopy on its genuine time and length scale.

Time-resolved X-ray microscopy is used to image the influence of alternating high-density currents on the magnetization dynamics of ferromagnetic vortices. Spin-torque induced vortex gyration is observed in micrometer-sized permalloy squares. The phases of the gyration in structures with different chirality are compared to an analytical model and micromagnetic simulations, considering both alternating spinpolarized currents and the current's Oersted field. In our case the driving force due to spin-transfer torque is about 70% of the total excitation while the remainder originates from the current's Oersted field. This finding has implications to magnetic storage devices using spin-torque driven magnetization switching and domain-wall motion., Comment: 10 pages, 3 figures

A magnetic vortex occurs as an equilibrium configuration in thin ferromagnetic platelets of micron and sub-micron size and is characterised by an in-plane curling magnetisation. At the centre, a magnetic singularity is avoided by an out-of-plane magnetisation core. This core has a gyrotropic excitation mode, which corresponds to a circular motion of the vortex around its equilibrium position, where the rotation sense is determined by the direction of the vortex core magnetisation, its polarisation. Unlike linear fields or spin polarised currents, which excite both polarisation states, an in-plane rotating field can selectively excite one of the polarisation states. Here we report the observation of vortex dynamics in response to rotating magnetic fields, imaged with time-resolved scanning X-ray microscopy. We demonstrate that the rotating field only excites the gyrotropic mode if the rotation sense of the field coincides with the vortex gyration sense and that such a field can selectively reverse the vortex polarisation.

We report on the observation of magnetic vortex domain wall chirality reversal in ferromagnetic rings that is controlled by the sense of rotation of a magnetic field. We use time-resolved X-ray microscopy to dynamically image the chirality-switching process and perform micromagnetic simulations to deduce the switching details from time-resolved snapshots. We find experimentally that the switching occurs within less than 4 ns and is observed in all samples with ring widths ranging from 0.5 μm to 2 μm, ring diameters between 2 μm and 5 μm, and a thickness of 30 nm, where a vortex domain wall is present in the magnetic onion state of the ring. From the magnetic contrast in the time-resolved images, we can identify effects of thermal activation, which plays a role for the switching process. Moreover, we find that the process is highly reproducible so that the domain wall chirality can be set with high fidelity.

Employing time-resolved x-ray microscopy, we investigate the dynamics of a pinned magnetic vortex domain wall in a magnetic nanowire. The gyrotropic motion of the vortex core is imaged in response to an exciting ac current. The elliptical vortex core trajectory at resonance reveals asymmetries in the local potential well that are correlated with the pinning geometry. Using the analytical model of a two-dimensional harmonic oscillator, we determine the resonance frequency of the vortex core gyration and, from the eccentricity of the vortex core trajectory at resonance, we can deduce the stiffness of the local potential well.

Magnetization dynamics in ferromagnetic multilayer structures was studied by time-resolved transmission x-ray microscopy. A square-shaped 1x1 mu m(2) trilayer structure consisting of Co(20 nm)/Cu(10 nm)/Permalloy Ni80Fe20(20 nm) was investigated. Each ferromagnetic layer showed a Landau-like domain configuration with a single vortex. A gyrotropic vortex motion was excited by an in-plane magnetic field alternating at a frequency of 250 MHz. The movement of the magnetic vortex in each individual magnetic layer was imaged by taking advantage of the element specificity of the x-ray magnetic circular dichroism. A 180 degrees phase shift between the gyrotropic vortex motion in the Permalloy and the Co layer was observed. This phase shift can be ascribed to the magnetic coupling between the layers. (C) 2006 American Institute of Physics.

A large amount of future spintronic devices is based on the control of the static and dynamic properties of magnetic domain walls in magnetic nanowires. For these applications, understanding the domain wall mobility under the action of spin polarized currents is of paramount importance. Numerous studies describe the spin-current driven domain wall motion in nanowires with ideal material properties, while only some authors take into account the influence of the nanowire edge roughness [1]. In this contribution we numerically investigate the influence of distributed disorder on the vortex domain wall mobility in Permalloy nanowires. To this aim, we use the GPU based micromagnetic software package MuMax[2] to simulate the propagation of vortex domain walls in nanowires with cross sectional dimensions of 400x10 nm². We apply spin polarized currents acting on the domain wall by means of the Spin Transfer Torque (STT) mechanism, considering a system with perfect adiabaticity (β=0) and with non-adiabatic STT contributions (β=α and β=2α, α is the Gilbert damping). As in [3], the disorder is simulated as a random distribution of 3.125x3.125nm² sized voids. For each current value, average domain wall velocities are computed considering 25 different realisations of the disorder. We find that even very small disorder concentrations have a huge impact on the domain wall mobility. In the non-adiabatic case (β=2α), the domain wall velocity is largely suppressed below the Walker breakdown since the disorder is able to pin the vortex structure hindering the formation of the transverse domain wall, characteristic to the movement in this current region. In the adiabatic case (β=0), the intrinsic depinning threshold is largely reduced. Even very small disorder densities disable the domain wall to internally balance the Landau-Lifshitz-Gilbert torques with the STT torques, resulting in a non-zero domain wall speed. At low currents, the disorder pins the domain wall structure.