Guido Meier

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.

Joule heating in nanowires

We study the effect of Joule heating from electric currents flowing through ferromagnetic nanowires on the temperature of the nanowires and on the temperature of the substrate on which the nanowires are grown. The spatial current density distribution, the associated heat generation, and diffusion of heat is simulated within the nanowire and the substrate. We study several different nanowire and constriction geometries as well as different substrates: (thin) silicon nitride membranes, (thick) silicon wafers, and (thick) diamond wafers. The spatially resolved increase in temperature as a function of time is computed. For effectively three-dimensional substrates (where the substrate thickness greatly exceeds the nanowire length), we identify three different regimes of heat propagation through the substrate: regime (i), where the nanowire temperature increases approximately logarithmically as a function of time. In this regime, the nanowire temperature is well-described analytically by You et al. [APL89, 222513 (2006)]. We provide an analytical expression for the time tc that marks the upper applicability limit of the You model. After tc, the heat flow enters regime (ii), where the nanowire temperature stays constant while a hemispherical heat front carries the heat away from the wire and into the substrate. As the heat front reaches the boundary of the substrate, regime (iii) is entered where the nanowire and substrate temperature start to increase rapidly. For effectively two-dimensional substrates (where the nanowire length greatly exceeds the sub- strate thickness), there is only one regime in which the temperature increases logarithmically with time for large times. We provide an analytical expression, valid for all pulse durations, that allows one to accurately compute this temperature increase in the nanowire on thin substrates

Magnetic reversal of cylindrical nickel nanowires with modulated diameters

Anodic alumina membranes with modulated pore diameters serve as template for the preparation of magnetic nanowires. Filling the pores with Ni by electrodeposition delivers wires replicating the variation in modulation in pore diameter from 80 to 160 nm. Such structures are of interest for the observation and control of magnetic domain wall motion. Single-object characterization utilizing the magneto-optical Kerr effect magnetometry evidences a strong correlation between geometric parameters and magnetic properties. Ensemble magnetization measurements with a superconducting quantum interference device show the effect of dipolar interactions. Analytical models can reproduce the lowering of coercivity due to the presence of enhanced stray fields within the array. Magnetic force microscopy at individual wires indicates the presence of a strong stray field in the vicinity of the diameter change. The preparation technique demonstrates a mass production method of nano-objects with designed geometric irregulariti...

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.

Vertical polarization of quantum magnets in high density arrays of nickel dots with small height-to-diameter ratio

Using interferometric lithography and postexposure processing we have fabricated high density (1.3×1010/in.2) magnetic nickel dot arrays on silicon substrates. The dots have the shape of truncated cones and are 75 nm in height and about 120 nm in diameter. The arrays are characterized using magnetic force microscopy (MFM). We demonstrate that such shallow dots with a small height-to-diameter ratio of only 0.63, show single-domain behavior with vertical, out-of-plane magnetization, i.e., along their short axis. The coercive field of these dots is drastically enlarged due to shape anisotropy and is exceeding the dot interaction strength by about one order of magnitude. Local manipulation of the magnetization state using an additional external field and the stray field of the MFM tip is demonstrated.

Magnetization of small arrays of interacting single-domain particles

We have prepared tailored small arrays of single-domain Ninanomagnets exhibiting an easy-axis perpendicular to the substrate surface. Using ballistic Hall micromagnetometry we probe the magnetization under different tilt angles. For an array of 13 nanomagnets with 700 nm spacing, we observe distinct plateau values in the hysteresis loop indicating the stepwise switching of these particles. The observed hierarchy of reversal comes close to a model on the basis of the magnetostatic interparticle interaction.

Wave modes of collective vortex gyration in dipolar-coupled-dot-array magnonic crystals

Lattice vibration modes are collective excitations in periodic arrays of atoms or molecules. These modes determine novel transport properties in solid crystals. Analogously, in periodical arrangements of magnetic vortex-state disks, collective vortex motions have been predicted. Here, we experimentally observe wave modes of collective vortex gyration in one-dimensional (1D) periodic arrays of magnetic disks using time-resolved scanning transmission x-ray microscopy. The observed modes are interpreted based on micromagnetic simulation and numerical calculation of coupled Thiele equations. Dispersion of the modes is found to be strongly affected by both vortex polarization and chirality ordering, as revealed by the explicit analytical form of 1D infinite arrays. A thorough understanding thereof is fundamental both for lattice vibrations and vortex dynamics, which we demonstrate for 1D magnonic crystals. Such magnetic disk arrays with vortex-state ordering, referred to as magnetic metastructure, offer potential implementation into information processing devices.

Structural, magnetic, and transport properties of Permalloy for spintronic experiments

Permalloy (Ni80Fe20) is broadly used to prepare magnetic nanostructures for high-frequency experiments where the magnetization is either excited by electrical currents or magnetic fields. Detailed knowledge of the material properties is mandatory for thorough understanding its magnetization dynamics. In this work, thin Permalloy films are grown by dc-magnetron sputtering on heated substrates and by thermal evaporation with subsequent annealing. The specific resistance is determined by van der Pauw methods. Point-contact Andreev reflection is employed to determine the spin polarization of the films. The topography is imaged by atomic-force microscopy, and the magnetic microstructure by magnetic-force microscopy. Transmission-electron microscopy and transmission-electron diffraction are performed to determine atomic composition, crystal structure, and morphology. From ferromagnetic resonance absorption spectra the saturation magnetization, the anisotropy, and the Gilbert damping parameter are determined. Co...

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.

Stray fields of domains in permalloy microstructures—Measurements and simulations

We have measured the stray fields of thin permalloy (Ni83Fe17) microstructures with different geometries and several thicknesses by magnetic-force microscopy (MFM). The MFM images are compared to corresponding images calculated from micromagnetic simulations. In particular, the type of 180° domain walls is discussed. We observe a transition from cross-tie to asymmetric Bloch walls between 70 and 100 nm film thickness. Good agreement between measurement and simulation is obtained.