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Research

                                      

Spin wave dynamics

Spin Hall effect

Domain wall motion in nanostructures

Fluctuation properties in domain patterns

Spin injection into semiconductors

Spin wave dynamics

We investigate spin waves - the eigenmodes of ferromagnets - in thin films by time resolved Kerr microscopy. With this technique we are able to directly image spin waves and obtain their characteristics such as wavelength and attenuation length (or simply the complex wave vector). Spin waves are very sensitive tools to probe the magnetic properties of their media – magnetic thin films. As examples, we investigate their response to (i) the application of spin-polarized currents (in so called Spin Wave Doppler experiments), (ii) the modification of interfaces or the composition of the thin films and (iii) the confinement in magnetic nanostructures. Spin waves are therefore a nice probe to study magnetization dynamics in great detail.

Koerner Spin
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Spin Hall effect

The Spin Hall Effect (SHE) describes the finding that in non magnetic metallic materials (NM) with large spin orbit coupling, an electrical current can be converted into a pure spin current which flows perpendicular to the current flow direction. This spin current can diffuse into an adjacent ferromagnetic film (FM) and can exert a torque on the magnetization of FM. For sufficiently large torques (related to efficient charge to spin current conversion) switching of FM has been demonstrated which can be used to e.g. write magnetic bits in future magnetic random access memory devices. We use several techniques to examine the efficiency of charge to spin current conversion. For example, to probe the influence of an injected spin current on the dynamics of the magnetization of FM of a microstructured NM/FM bilayer sample, time resolved magneto optical Kerr microscopy is used.

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In the particular example shown here, FM is brought into ferromagnetic resonance (FMR) in an externally applied magnetic field by microwave excitation. At the same time a dc current is passed through the bilayer to generate the torque on FM. This torque changes the linewidth of the FMR giving access to a quantification of the SHE.

A different approach to quantify the efficiency of spin to charge conversion is to investigate spin-transfer-torque ferromagnetic resonance (STT-FMR) of ferromagnet/heavy metal (e.g. Permalloy/Platin) bilayers to determine the spin Hall angle. This effect has been often used to calculate the spin Hall angle but the reported values differs by more than an order of magnitude. Our goal is to achieve a proper understanding of this method and compare it with other techniques like spin pumping inverse spin Hall effect (SP-ISHE) and modulation of damping (MOD) as described above.

Domain wall motion in nanostructures

Domain walls in nanostructures can be compact micromagnetic objects that can be moved using magnetic field or current pulses. We want to obtain a better understanding of the dynamics of magnetic domain walls and its connection to magnetic damping mechanisms. For that purpose we investigate domain wall motion of vortex walls in nano- and microstripes using a wide field Kerr microscope. Additionally, different experimental methods for sample preparation and characterization are used, such as electron beam lithography, atomic force microscopy and local ferromagnetic resonance investigations using time resolved Kerr microscopy. In order to interpret the experimental results, we compare them to micromagnetic simulations performed using the GPU based program MUMAX.

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Fluctuation properties in domain patterns

In out-of-plane magnetized ultra-thin films the competition between dipolar and exchange energy manifests itself in domain pattern of various shapes and sizes with detailed and well understood phase diagrams. In such systems the fluctuation properties of the individual phases is investigated with high real time temporal and spatial resolution using a PhotoEmission Electron Microscope.

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Spin injection into semiconductors

One of the main goals in spintronics research is the generation of a reasonable spin polarization in semiconductors. We investigate spin injection into GaAs based systems using both electrical characterization of the samples and scanning Kerr microscopy at the cleaved edge, which enables us to examine the spin accumulation directly underneath the injecting contacts. Besides standard electrical spin injection we explore alternative ways of generating a spin imbalance in the semiconductor like thermal spin injection and spin pumping.
The figure shows an experiment for spin injection into a GaAs based 2-dimensional electron gas.

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Spinwavedynamics