Karsten Rott

Magnetic vortex core reversal by excitation with short bursts of an alternating field

The vortex state, characterized by a curling magnetization, is one of the equilibrium configurations of soft magnetic materials and occurs in thin ferromagnetic square and disk-shaped elements of micrometre size and below. The interplay between the magnetostatic and the exchange energy favours an in-plane, closed flux domain structure. This curling magnetization turns out of the plane at the centre of the vortex structure, in an area with a radius of about 10 nanometres—the vortex core. The vortex state has a specific excitation mode: the in-plane gyration of the vortex structure about its equilibrium position. The sense of gyration is determined by the vortex core polarization. Here we report on the controlled manipulation of the vortex core polarization by excitation with small bursts of an alternating magnetic field. The vortex motion was imaged by time-resolved scanning transmission X-ray microscopy. We demonstrate that the sense of gyration of the vortex structure can be reversed by applying short bursts of the sinusoidal excitation field with amplitude of about 1.5 mT. This reversal unambiguously indicates a switching of the out-of-plane core polarization. The observed switching mechanism, which can be understood in the framework of micromagnetic theory, gives insights into basic magnetization dynamics and their possible application in data storage.

Excitation of microwaveguide modes by a stripe antenna

We have studied experimentally the excitation of propagating spin-wave modes of a microscopic Permalloy-film waveguide by a stripe antenna. We show that due to the strong quantization of the spin-wave spectrum, the excitation of particular modes has essentially different frequency dependencies leading to a nonmonotonous variation of the modulation depth of the resulting spin-wave beam as a function of the excitation frequency. In addition, we address the effect of nonreciprocity of spin-wave excitation and found that for the case of Permalloy microwaveguides this effect is much weaker pronounced than for waveguides made from dielectric magnetic films with low saturation magnetization.

Nano-optics with spin waves at microwave frequencies

With the recent development in nanoscale patterning techniques, the potential of practical applications of nanometer-size structures for signal processing has been growing continuously. Experimental findings on the manipulation of optical signals in nanostructures have recently given rise to a widely addressed scientific area—subwavelength nano-optics. Here, we demonstrate that spin waves in microscopic ferromagnetic film structures represent a superb object for realization of the principles of nano-optics in the microwave frequency range. We show experimentally that by using the unique properties of spin waves, one can easily channelize, split, and manipulate submicrometer-width spin-wave beams propagating in microscopic magnetic-film waveguides.

Tunneling magnetothermopower in magnetic tunnel junction nanopillars.

We study tunneling magnetothermopower (TMTP) in CoFeB/MgO/CoFeB magnetic tunnel junction nanopillars. Thermal gradients across the junctions are generated by an electric heater line. Thermopower voltages up to a few tens of μV between the top and bottom contact of the nanopillars are measured which scale linearly with the applied heating power and hence the thermal gradient. The thermopower signal varies by up to 10  μV upon reversal of the relative magnetic configuration of the two CoFeB layers from parallel to antiparallel. This signal change corresponds to a large spin-dependent Seebeck coefficient of the order of 100  μV/K and a large TMTP change of the tunnel junction of up to 90%.

Spatially resolved ferromagnetic resonance: Imaging of ferromagnetic eigenmodes

Fast magnetization dynamics of ferromagnetic elements on sub-micron length scales is currently attracting substantial scientific interest. Studying the ferromagnetic eigenmodes in such systems provides valuable information in order to trace back the dynamical response to the underlying micromagnetic properties. The inherent time structure of third generation synchrotron sources allows for time-resolved imaging (time resolution: 70–100 ps) of magnetization dynamics at soft x-ray microscopes (lateral resolution down to 20 nm). Stroboscopic pump-and-probe experiments were performed on micron-sized Permalloy samples at a full-field magnetic transmission x-ray microscope (XM-1, beamline 6.1.2) at the ALS at Berkeley, CA. Complementary to these time-domain experiments a frequency-domain “spatially resolved ferromagnetic resonance” (SR-FMR) technique was applied to magnetic x-ray microscopy. In contrast to time-domain measurements which reflect a broadband excitation of the magnetization, the frequency-domain SR...

Excitation of short-wavelength spin waves in magnonic waveguides

By using phase-resolved micro-focus Brillouin light scattering spectroscopy, we demonstrate experimentally a phenomenon of wavelength conversion of spin waves propagating in tapered Permalloy waveguides. We show that this phenomenon enables efficient excitation of spin waves with sub-micrometer wavelengths being much smaller than the width of the microstrip antenna used for the excitation. The proposed excitation mechanism removes restrictions on the spin-wave wavelength imposed by the size of the antenna and enables improvement of performances of integrated magnonic devices.

Tunneling magnetoresistance sensors for high resolutive particle detection

Arrays of tunnel magnetoresistance sensors based on MgO as insulating layer are employed to detect magnetic microbeads. For single bead detection, elliptically shaped sensors of axis lengths of 400 and 100 nm are used. Due to high shape anisotropy a linear response of the sensor signal in a magnetic field range between −500 and 500 Oe can be reported. By performing static detection measurements of magnetic microbeads, a distinct signal shape correlated with the position of beads in respect to the sensor can be observed. The experimental data are compared to micromagnetic simulations carried out on a trilayer model.

Vortex dynamics in coupled ferromagnetic multilayer structures

Magnetization dynamics in ferromagnetic multilayer structures was studied by time-resolved transmission x-ray microscopy. A square-shaped 1×1μm2 trilayer structure consisting of Co(20nm)∕Cu(10nm)∕Permalloy Ni80Fe20(20nm) 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° 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.

Self-focusing of spin waves in Permalloy microstripes

Excitation and propagation of spin waves in Permalloy microstripes magnetized in their plane perpendicularly to the axis have been investigated by means of microfocus Brillouin light scattering spectroscopy with high spatial resolution. We show that the spatial profile of the spin-wave beam demonstrates a focusing at a certain distance from the excitation source depending on the stripe width. A model connecting the observed phenomenon with an interference of different spin-wave modes existing in the stripe due to the finite-size effect is proposed.

Domain wall induced modes of high-frequency response in ferromagnetic elements

The influence of domain wall density on the magnetization dynamics of amorphous CoZrTa thin-film elements was investigated by a combination of microwave magnetometry and quasistatic plus time-resolved wide-field Kerr microscopy. In addition to domain wall motion, permeability rolloff at low frequencies occurs due to rotational processes. The dominating ferromagnetic resonance modes depend on the domain wall density due to the formation of a zone of magnetization curling at the domain walls, which results from a phase lag of domain and domain wall response. Both the amount of permeability reduction and the increase in precessional frequency, can be varied with magnetic history. All effects are avoided by lamination of the ferromagnetic films. The results demonstrate the importance of detailed domain control for the optimization of patterned films for high-frequency applications, beyond the elementary adjustment of material’s high-frequency properties.