Markus Bolte

Time-resolved X-ray microscopy of spin-torque-induced magnetic vortex gyration

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 spin-polarized currents and the currents 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 currents Oersted field. This finding has implications to magnetic storage devices using spin-torque driven magnetization switching and domain-wall motion.

Observation of coupled vortex gyrations by 70-ps-time- and 20-nm-space-resolved full-field magnetic transmission soft x-ray microscopy

Observation of coupled vortex gyrations by 70-ps-time- and 20-nm-space- resolved full-field magnetic transmission soft x-ray microscopy Hyunsung Jung 3 , Young-Sang Yu, Ki-Suk Lee, Mi-Young Im, Peter Fischer, 1,a) Lars Bocklage, Andreas Vogel, Markus Bolte, Guido Meier, and Sang-Koog Kim Research Center for Spin Dynamics and Spin-Wave Devices, and Nanospinics Laboratory, Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea Center for X-ray Optics, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Institut fur Angewandte Physik und Zentrum fur Mikrostrukturforschung, Universitat Hamburg, Hamburg 20355, Germany We employed time-and space-resolved full-field magnetic transmission soft x-ray microscopy to observe vortex-core gyrations in a pair of dipolar-coupled vortex-state Permalloy (Ni80Fe20) disks. The 70 ps temporal and 20 nm spatial resolution of the microscope enabled us to simultaneously measure vortex gyrations in both disks and to resolve the phases and amplitudes of both vortex-core positions. We observed their correlation for a specific vortex-state configuration. This work provides a robust and direct method of studying vortex gyrations in dipolar-coupled vortex oscillators.

Harmonic oscillator model for current-and field-driven magnetic vortices

Institut fu¨r Angewandte Physik und Zentrum fu¨r Mikrostrukturforschung,Universita¨t Hamburg, Jungiusstr. 11, 20355 Hamburg, Germany(Dated: February 2, 2008)In experiments the distinction between spin-torque and Oersted-field driven magnetization dynamics is stillan open problem. Here, the gyroscopic motion of current- andfield-driven magnetic vortices in small thin-film elements is investigated by analytical calculations an d by numerical simulations. It is found that for smallharmonic excitations the vortex core performs an elliptical rotation around its equilibrium position. The globalphase of the rotation and the ratio between the semi-axes aredetermined by the frequency and the amplitude ofthe Oersted field and the spin torque.

Proposal for a Standard Problem for Micromagnetic Simulations Including Spin-Transfer Torque

of micromagnetic simulation tools. The work is based on the micromagnetic model extended by the spin-transfer torque in continuously varying magnetizations as proposed by Zhang and Li. The standard problem geometry is a permalloy cuboid of 100 nm edge length and 10 nm thickness, which contains a Landau pattern with a vortex in the center of the structure. A spin-polarized dc current density of 10 12 A/m 2 ows laterally through the cuboid and moves the vortex core to a new steady-state position. We show that the new vortex-core position is a sensitive measure for the correctness of micromagnetic simulators that include the spin-transfer torque. The suitability of the proposed problem as a standard problem is tested by numerical results from four dierent nite-dierence and nite-element-based simulation tools.

Current-and field-driven magnetic antivortices for nonvolatile data storage

We demonstrate by micromagnetic simulations that magnetic antivortices are potential candidates for fast nonvolatile data-storage elements. These storage elements are excited simultaneously by alternating spin-polarized currents and their accompanying Oersted fields. Depending on the antivortex-core polarization p and the orientation of the in-plane magnetization c around the core, the superposition of current and field leads to either a suppression of gyration (logical “zero”) or an increased gyration amplitude (logical “one”). Above an excitation threshold the gyration culminates in the switching of the antivortex core. The switching can be seen as a cp-dependent writing of binary data, allowing to bring the antivortex into a distinct state. Furthermore a read-out scheme using an inductive loop situated on top of the element is investigated.

Current-driven domain-wall dynamics in curved ferromagnetic nanowires

The current-induced motion of a domain wall in a semicircle nanowire with applied Zeeman field is investigated. Starting from a micromagnetic model we derive an analytical solution which characterizes the domain-wall motion as a harmonic oscillation. This solution relates the micromagnetic material parameters with the dynamical characteristics of a harmonic oscillator: i.e., domain-wall mass, resonance frequency, damping constant, and force acting on the wall. The time derivative of the current density greatly contributes to the force on the domain wall. For wires with strong curvature the dipole moment of the wall as well as its geometry influence the eigenmodes of the oscillator. Based on these results we suggest experiments for the determination of material parameters which otherwise are difficult to access. Numerical calculations confirm our analytical solution and show its limitations.

Current- and field-driven magnetic antivortices

Antivortices in ferromagnetic thin-film elements are in-plane magnetization configurations with a core pointing perpendicular to the plane. By using micromagnetic simulations, we find that magnetic antivortices gyrate on elliptical orbits similar to magnetic vortices when they are excited by alternating magnetic fields or by spin-polarized currents. The phase between high-frequency excitation and antivortex gyration is investigated. In case of excitation by spin-polarized currents the phase is determined by the polarization of the antivortex, while for excitation by magnetic fields the phase depends on the polarization as well as on the in-plane magnetization. Simultaneous excitation by a current and a magnetic field can lead to a maximum enhancement or to an entire suppression of the amplitude of the core gyration, depending on the angle between excitation and in-plane magnetization. This variation of the amplitude can be used to experimentally distinguish between spin-torque and Oersted-field driven motion of an antivortex core.

Vortices and antivortices as harmonic oscillators

It is shown that the current- and field-induced gyration of magnetic vortices and antivortices follows the analytical model of a two-dimensional harmonic oscillator. Quantities of the harmonic oscillator, i.e., resonance frequency, damping constant, gyration amplitude, and phase, can be linked to material parameters and sample dimensions. This description is useful for the investigation of vortex-switching and vortex-antivortex annihilation processes.

Domain-Wall Pinning and Depinning at Soft Spots in Magnetic Nanowires

The local modification of magnetic properties by ion irradiation opens the possibility to create pinning sites for domain walls in magnetic nanowires without geometric constrictions. Implantation of chromium ions into Ni80Fe20 nanowires is used to cause a local reduction of the saturation magnetization Ms and thus a decrease of the energy associated with the domain wall. Field-driven pinning and depinning of a domain wall at the here so-called magnetic soft spots is directly observed using magnetic transmission soft X-ray microscopy. The pinning rate and the depinning field considerably depend on the wire width and the chromium fluence.

Comparative study of magnetization reversal in isolated and strayfield coupled microcontacts

Ferromagnetic microcontacts are key components for future spintronic devices in full metal as well as in hybrid ferromagnet/semiconductor systems. Control of the micromagnetic behavior and especially the reversal process is crucial for the functionality of such devices. We have prepared isolated and strayfield coupled micron sized rectangular Ni∕Fe double layer contacts on silicon nitride membranes. High-resolution magnetic microscopy studies in external fields are performed on identical samples comparing full field magnetic transmission x-ray microscopy and magnetic-force microscopy. The results of both techniques are in good agreement. We find evidence for a strayfield-induced coupling of the domain structure in adjacent contacts in accordance with micromagnetic simulations.