We use ballistic Hall micromagnetometry to determine depinning fields of domain walls (DWs) in Permalloy nanowires in the temperature range between 5 and 50K. The walls are pinned in constrictions defined by a triangularly shaped notch on one side of the wire. The high sensitivity of the Hall sensor to local stray fields allows the detection of individual DWs and a distinction of different wall types. A strong temperature dependence of the depinning fields is observed that can be described by a model with a single energy barrier. For temperatures above 20K, additional types of DWs occur.

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 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.

Author(s): Kamionka, Thomas; Martens, Michael; Bohlens, Stellan; Bocklage, Lars; Eiselt, Rene; Fischer, Peter; Zielke, Hannah; Matsuyama, Toru; Merkt, Ulrich; Meier, Guido

Current-induced domain-wall dynamics is investigated via high-resolution soft x-ray transmission microscopy by a stroboscopic pump-and-probe measurement scheme at a temporal resolution of 200 ps. A $180\ifmmode^\circ\else\textdegree\fi{}$ domain wall in a restoring potential of a permalloy microstructure is displaced from its equilibrium position by nanosecond current pulses leading to oscillations with velocities up to 325 m/s. The motion of the wall is described with an analytical model of a rigid domain wall in a nonharmonic potential allowing one to determine the mass of the domain wall. We show that Oersted fields dominate the domain-wall dynamics in our geometry.

Magnetic transmission x-ray microscopy is used to directly visualize the influence of a spin-polarized current on domain walls in curved permalloy wires. Pulses of nanosecond duration and of high current density up to 1.0x10(12) A/m(2) are used to move and to deform the domain wall. The current pulse drives the wall either undisturbed, i.e., as composite particle through the wire, or causes structural changes of the magnetization. Repetitive pulse measurements reveal the stochastic nature of current-induced domain-wall motion.

Square lattice arrays of thin microelements of Fe are investigated by magnetic transmission x-ray microscopy. The influence of dipole interaction is analyzed by varying the interelement distance. For comparison isolated elements are prepared on the same sample. The magnetostatic field caused by interelement interaction leads to a substantial stabilization of the elements in the center of the array comparable to the magnetization process previously found by numerical solution of the Landau-Lifshitz equation for magnetic dot arrays. Micromagnetic simulations show that for high field strengths the dipolar interaction is collinear with the external field while in the low-field regime the strayfields have significant perpendicular components leading to a complex reversal mechanism.

Domain imaging techniques are used to analyze the micromagnetic behavior of microelements applied in spin-transport devices. Micromagnetic simulations enable direct comparison of the experimental results and give additional information which is not directly accessible experimentally. As a case study we investigate the stray-field interaction of microelements prepared on thin Si 3 N 4 membranes with magnetic-transmission X-ray microscopy and magnetic-force microscopy. Micromagnetic simulations yield internal parameters such as local stray fields and total magnetic energy. Values for the strength of the stray-field interaction between two microelements of several milli Tesla are deduced. Results also show that pinned magnetizations can explain the magnetization patterns observed in the experiments.

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.

In semiconductor spintronic devices that incorporate ferromagnetic materials the stray-field configuration in the plane of the two-dimensional electron system is of crucial importance. We investigate the stray fields of iron electrodes suitable as injector and detector for spin-polarized transport in hybrid semiconductor/ferromagnet devices. Magnetic-force microscopy images of an electrode pair are derived from simulated magnetization patterns. The calculated averaged stray field is compared to the experimental signal of a ballistic Hall micromagnetometer comprising a two-dimensional electron system in a GaAs/AlGaAs-heterostructure 90 nm below the electrodes. Good agreement between simulation and experiment is obtained.

Iron electrodes suitable as injectors and detectors for spin-polarized transport in ferromagnet/semiconductor hybrid devices are investigated by Hall micromagnetometry. The Hall effect generated by the stray field of the iron structures is measured for single electrodes and electrode pairs with the external magnetic field aligned in plane either parallel or perpendicular to their easy axes. The strength of the stray field of the double structure in the sensor area is comparable for both configurations.