A. Kobs

ultrafast optical demagnetization manipulates nanoscale spin structure in domain walls

During ultrafast demagnetization of a magnetically ordered solid, angular momentum has to be transferred between the spins, electrons, and phonons in the system on femto- and picosecond timescales. Although the intrinsic spin-transfer mechanisms are intensely debated, additional extrinsic mechanisms arising due to nanoscale heterogeneity have only recently entered the discussion. Here we use femtosecond X-ray pulses from a free-electron laser to study thin film samples with magnetic domain patterns. We observe an infrared-pump-induced change of the spin structure within the domain walls on the sub-picosecond timescale. This domain-topography-dependent contribution connects the intrinsic demagnetization process in each domain with spin-transport processes across the domain walls, demonstrating the importance of spin-dependent electron transport between differently magnetized regions as an ultrafast demagnetization channel. This pathway exists independent from structural inhomogeneities such as chemical interfaces, and gives rise to an ultrafast spatially varying response to optical pump pulses.

Tuning of the nucleation field in nanowires with perpendicular magnetic anisotropy

We report on domain nucleation in nanowires consisting of Co/Pt multilayers with perpendicular magnetic anisotropy that are patterned by electron-beam lithography, sputter deposition, and lift-off processing. It is found that the nucleation field can be tuned by changing the geometry of the wire ends. A reduction of the nucleation field by up to 60% is achieved when the wire ends are designed as tips. This contrasts with the behavior of wires with in-plane anisotropy where the nucleation field increases when triangular-pointed ends are used. In order to clarify the origin of the reduction of the nucleation field, micromagnetic simulations are employed. The effect cannot be explained by the lateral geometrical variation but is attributable to a local reduction of the perpendicular anisotropy caused by shadowing effects due to the resist mask during sputter deposition of the multilayer.

Imaging the in-plane magnetization in a Co microstructure by Fourier transform holography

We report on experiments using Fourier transform holography to image the in-plane magnetization of a magnetic microstructure. Magnetic sensitivity is achieved via the x-ray magnetic circular dichroism effect by recording holograms in transmission at off-normal incidence. The reference beam is defined by a narrow hole milled at an inclined angle into the opaque mask. We present magnetic domain images of an in-plane magnetized cobalt element with a size of 2 μm × 2 μm× 20 nm. The domain pattern shows a multi-vortex state that deviates from the simple Landau ground state.

Integrated setup for the fabrication and measurement of magnetoresistive nanoconstrictions in ultrahigh vacuum

A UHV instrument is presented for in situ fabrication of nanostructures and in situ investigation of their magnetoresistance. Nanostructures of diverse shape and size are created from thin films utilizing a focused ion beam. The magnetic nanostructures are contacted via a micromanipulator, which makes it possible to address the individual structures. The system is additionally equipped with a scanning electron microscope column, which is used for damage-free navigation and control of the structuring and contacting. First magnetoresistance measurements of structures carved into a Permalloy film demonstrate the high sensitivity and the flexibility of the new setup.

On the variation of magnetic anisotropy in Co/Pt(111) on silicon oxide

The structural properties and magnetic anisotropy of Pt/Co/Pt trilayers grown on thermally oxidized (Si/SiO2) and naturally oxidized silicon (Si/Siox) are presented. Although similar substrates and identical preparation conditions are used distinct differences in the structural composition are found which stem from the Pt seed layer created via ion assisted sputtering. While for thermal oxidized Si a Pt/Co/Pt trilayer is formed, for systems grown on naturally oxidized Si a complex PtSi alloy formation within the seed layer is observed as a consequence of the high ion energies of ion assisted sputtering. The composition of the PtSi alloy varies along the growth direction with a low Si content at the interface to Co and the lattice constant is similar to bulk Pt. The latter provides a much higher magnetic interface anisotropy constant compared to Pt/Co/Pt on thermal oxidized Si of about 0.9 mJ/m2 which is comparable to the highest values found for MBE grown Co on single crystalline Pt(111).

Femtosecond-laser–induced modifications in Co/Pt multilayers studied with tabletop resonant magnetic scattering

We characterize the magnetic domain structure of Co/Pt multilayer films on length scales below one hundred nanometers using resonant magnetic scattering and magnetic force microscopy. The extreme ultraviolet light for the scattering experiment is created by a laser-based high-order harmonic generation source. After illumination with intense ultrashort infrared laser pulses, we observe pronounced changes in the magnetic structure and morphology. This study points out the importance of a detailed analysis of the different laser-induced modifications of a magnetic thin film that influence the scattering patterns.

Direct method for measuring the canting angle of magnetization

We present a method to accurately determine the canting angle of magnetization in Co/Pt multilayers by utilizing magnetoresistance effects. In a current-in-plane geometry, the longitudinal voltage drop is determined as a function of the direction of an externally applied magnetic field. The field strength is sufficient to prevent domain decay. Measuring the change of resistance for two slightly differing field strengths allows the determination of the canting angle with high accuracy.

Endstation for ultrafast magnetic scattering experiments at the free-electron laser in Hamburg

An endstation for pump-probe small-angle X-ray scattering (SAXS) experiments at the free-electron laser in Hamburg (FLASH) is presented. The endstation houses a solid-state absorber, optical incoupling for pump-probe experiments, time zero measurement, sample chamber, and detection unit. It can be used at all FLASH beamlines in the whole photon energy range offered by FLASH. The capabilities of the setup are demonstrated by showing the results of resonant magnetic SAXS measurements on cobalt-platinum multilayer samples grown on freestanding Si(3)N(4) membranes and pump-laser-induced grid structures in multilayer samples.

Polarization control in magnonic vortex crystals

Summary form only given. Wave transmission media created by a periodically modulated magnetic material are referred to as magnonic crystals [1]. The dynamics of magnonic crystals are described by common concepts of solid state physics, i.e., group velocity, density of states, and band structure. We investigate so-called magnonic vortex crystals created via rectangular arrangements of magnetic vortices. Here we aim at the control of vortex-core polarizations by perpendicularly aligned bias fields and use magnetic force microscopy and broadband-ferromagnetic transmission spectroscopy. In the first step, arrays of CoPt-multilayer disks arranged in a checkerboard pattern are prepared by electron-beam lithography, sputter deposition, and lift-off processing, compare figure 1(a). Two layer types A (Pt/0.7 nm Co/Pt) and B (Pt/2x(0.8 Co/1.1 nm Pt)/Pt) of the pattern differ in the magnetic anisotropy and thus yield different switching fields of the perpendicularly magnetized disks [2]. The switching fields are investigated by Kerr microscopy. After saturation in negative field direction, disks of type B start to switch at a field strength of μ0 H = +29 mT visible in the hysteresis loop shown in figure 1(b). The switching is completed at +37 mT where the greyscale intensity stays constant. A stable state of antiparallel magnetization of both types of disks persists up to +56 mT, where disks of type A start to switch. At +68 mT the magnetizations of disks of type A and B are aligned parallel with the positive field direction, compare figure 1(c). The switching of the multilayer disks follows a normal distribution and shows no dependence on the interdisk distance. In the next step, permalloy disks are prepared on top of the CoPt disks by electron-beam lithography, thermal evaporation, and lift-off processing. A thin Si interlayer is used to avoid direct contact of the perpendicularly and in-plane magnetized ferromagnets. The magnetization of the CoPt disks is adjusted using a perpendicularly aligned magnetic field. Subsequently an in-plane field is used to nucleate vortices in the permalloy disks. It is expected that due to stray field coupling the polarization of the vortex core in each permalloy disk is determined by the subjacent CoPt disk and coincides with the direction of the magnetization of the CoPt disk. The vortex-core polarizations are investigated using magnetic force microscopy (not shown). Analytical calculations [3] show that the dispersion relations and the density of states of magnonic vortex crystals depend crucially on the polarization pattern of the vortices. It has also been shown experimentally [4] that the polarization patterns in magnonic vortex crystals are determined by the frequency of excitation and the strength of the dipolar interaction between the vortices. Here we aim at the definition of the properties of the magnonic vortex crystal, i.e., its band structure via the polarization control of its vortex-core polarizations. We have realized three different polarization patterns of the vortex cores: uniform polarization of all disks, a checkerboard pattern and a stripe pattern. The density of states of these magnonic vortex crystals with its enforced polarization pattern is determined by broadband-ferromagnetic transmission spectroscopy. Financial support of the Deutsche Forschungsgemeinschaft via the Sonderforschungsbereich 668, the Graduiertenkolleg 1286, and the excellence cluster “The Hamburg Centre for Ultrafast Imaging - Structure, Dynamics and Control of Matter on the Atomic Scale” is gratefully acknowledged.