Hans Peter Oepen

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.

Experimental method for separating longitudinal and polar Kerr signals

A new procedure is presented which can be easily applied to separate longitudinal and polar Kerr signals. The method is advantageous particularly in systems where in-plane and out-of-plane states of magnetization are involved in the reversal process. The feasibility of the method is demonstrated at the spin-reorientation transition in Co/Au(1 1 1) films.

Magnetic spectromicroscopy from Fe(100)

Magnetic domains on an Fe(100) surface have been imaged by means of energy‐resolved photoemission microscopy. We excited the photoelectrons with circularly polarized synchrotron radiation in the soft x‐ray region, and employed the effect of magnetic circular dichroism in the emitted photoelectrons in order to obtain contrast between differently oriented magnetic domains. This new approach offers a surface sensitive way to combine chemical and magnetic information on a microscopic scale.

Magnetic anisotropy and the cone state in Co/Pt multilayer films

The magnetic anisotropy of Co/Pt multilayers is investigated. The perpendicular uniaxial anisotropy is discussed in second order approximation under a variation in Co and Pt layer thicknesses. The evolution of anisotropy constants is shown in the phase diagram of first and second order anisotropy constants. A thickness driven spin reorientation via the canted phase is observed for a single Co layer as well as for Co/Pt multilayer films.

Soft x-ray holographic microscopy

We present a new x-ray microscopy technique based on Fourier transform holography (FTH), where the sample is separate from the optics part of the setup. The sample can be shifted with respect to the holography optics, thus large-scale or randomly distributed objects become accessible. As this extends FTH into a true microscopy technique, we call it x-ray holographic microscopy (XHM). FTH allows nanoscale imaging without the need for nanometer-size beams. Simple Fourier transform yields an unambiguous image reconstruction. We demonstrate XHM by studying the magnetic domain evolution of a Co/Pt multilayer film as function of locally varied iron overlayer thickness.

The reorientation transition in Co/Au(111)

The spin reorientation transition in as-grown wedge-shaped Co/Au(111) films has been analyzed. Two critical thicknesses have been detected just like in the annealed case. Here, these are shifted to smaller values. The behavior of the system can be explained on the basis of the thickness-driven trajectory in the anisotropy space of the system. Both the first and second anisotropy constants have been determined: K1s=0.66 mJ/m2, K2s=−0.12 mJ/m2. They are both smaller by modulus than their counterparts from the annealed case. The results provide quantitative evidence for the increase of surface anisotropy after annealing.

Magnetic soft x-ray holography study of focused ion beam-patterned Co/Pt multilayers

We report on Fourier transform holography (FTH) experiments on nanostructured Co/Pt multilayer films with 40 nm spatial imaging resolution. The films have been nanostructured by means of focused ion beam (FIB) milling. Applying the ion beam through the supporting membrane with controlled and homogeneous dosing allows for higher resolution magnetic structuring of the ion-sensitive film compared to direct FIB patterning. Nanostructured samples with magnetic stripes exposed to different ion doses and magnetic arrays with 200 nm lattice constant were successfully prepared and imaged by FTH. We present image-processing routines for artifact-free image reconstruction. With this, we could investigate the FIB-induced anisotropy modulation and the perpendicular domain structure in the nanostructured samples, showing how to control the domain size and configuration by applying the appropriate ion dose either homogeneously or concentrated in single spots.

Fabrication of nanomagnet arrays by shadow deposition on self-organized semiconductor substrates

It is demonstrated how large-scale arrays of nanomagnets can be efficiently fabricated by shadow deposition onto faceted surfaces of self-organized Si1−xGex films. By pulsed laser deposition of Co in a grazing incidence geometry, we succeeded to cover just one selected type of facets resulting in isolated Co patches with an areal density of about 0.25×1012/in.2. These uniformly oriented nanomagnets have a parallelogram-shaped base with about 25 nm×35 nm edge lengths. Magneto-optic Kerr effect measurements reveal a clear in-plane anisotropy of the nanomagnets.

Magnetic anisotropy of Co on Cu(1 1 17)

The in‐plane magnetic anisotropy of ultra‐thin Co films, epitaxially grown on Cu(1 1 17), was determined in situ by means of the magneto‐optic Kerr effect down to thicknesses as low as 2 monolayers. Uniaxial and biaxial anisotropy contributions were observed. At room temperature, the uniaxial component is dominant and the easy axis of magnetization is parallel to the step edges. Above 4 monolayers the magnetic anisotropy exhibits a thickness dependence which can be described by volume and interface contributions. For thinner films a pronounced deviation from that behavior is found. The anisotropy drops abruptly by one order of magnitude below 3 monolayers. Thickness dependent relaxations are proposed as driving forces for that behavior.

Multiscale magnetic study of Ni(111) and graphene on Ni(111)

We have investigated the magnetism of the bare and graphene-covered (111) surface of a Ni single crystal employing three different magnetic imaging techniques and ab initio calculations, covering length scales from the nanometer regime up to several millimeters. With low temperature spinpolarized scanning tunneling microscopy (SP-STM) we find domain walls with widths of 60 - 90 nm, which can be moved by small perpendicular magnetic fields. Spin contrast is also achieved on the graphene-covered surface, which means that the electron density in the vacuum above graphene is substantially spin-polarized. In accordance with our ab initio calculations we find an enhanced atomic corrugation with respect to the bare surface, due to the presence of the carbon pz orbitals and as a result of the quenching of Ni surface states. The latter also leads to an inversion of spinpolarization with respect to the pristine surface. Room temperature Kerr microscopy shows a stripe like domain pattern with stripe widths of 3 - 6 {\mu}m. Applying in-plane-fields, domain walls start to move at about 13 mT and a single domain state is achieved at 140 mT. Via scanning electron microscopy with polarization analysis (SEMPA) a second type of modulation within the stripes is found and identified as 330 nm wide V-lines. Qualitatively, the observed surface domain pattern originates from bulk domains and their quasi-domain branching is driven by stray field reduction.