Hans Oepen - Profile on Academia.edu (original) (raw)

Papers by Hans Oepen

Ultrafast Demagnetization Excited by Extreme Ultraviolet Light From a Free-Electron Laser

Ultrashort and intense extreme ultraviolet (XUV) and X-ray pulses readily available at free-elect... more Ultrashort and intense extreme ultraviolet (XUV) and X-ray pulses readily available at free-electron lasers (FELs) enable studying non-linear light−matter interactions on femtosecond timescales. Here, we report on the non-linear fluence dependence of magnetic scattering of Co/Pt multilayers, using FERMI FEL’s 70-fs-long single and double XUV pulses, the latter with a temporal separation of 200 fs, with a photon energy slightly detuned to the Co M2,3 absorption edge. We observe a quenching in magnetic scattering that sets-in already in the non-destructive fluence regime of a few mJ/cm² typically used for FEL-probe experiments on magnetic materials. Calculations of the transient electronic structure in tandem with a phenomenological modeling of the experimental data by means of ultrafast demagnetization unambiguously show that XUV-radiation-induced demagnetization is the dominant mechanism for the quenching in the investigated fluence regime of <50 mJ/cm², while light-induced chang...

Optics Express

We present a systematic 2D spatial-coherence analysis of the soft-X-ray beamline P04 at PETRA III... more We present a systematic 2D spatial-coherence analysis of the soft-X-ray beamline P04 at PETRA III for various beamline configurations. The influence of two different beam-defining apertures on the spatial coherence properties of the beam is discussed and optimal conditions for coherence-based experiments are found. A significant degradation of the spatial coherence in the vertical direction has been measured and sources of this degradation are identified and discussed. The Fourier-analysis method, which gives fast and simple access to the 2D spatial coherence function of the X-ray beam, is used for the experiment. Here, we exploit the charge scattering of a disordered nanodot sample allowing the use of arbitrary X-ray photon energies with this method.

Strain relaxation in high-mobility InAs inserted-channel heterostructures with metamorphic buffer

Physica E: Low-dimensional Systems and Nanostructures

... Samples are grown in a solid source Riber 32P MBE system as is described in detail in Ref. ..... more ... Samples are grown in a solid source Riber 32P MBE system as is described in detail in Ref. ... The authors acknowledge Christian Weichsel and Oliver Kronenwerth for help during transport measurements and the Deutsche Forschungsgemeinschaft for financial support via SFB ...

Influence of Growth and Structure on the Magnetism of Epitaxial Cobalt Films on Cu(001)

Magnetism and Structure in Systems of Reduced Dimension

Optics Express, 2016

We present a method to determine the two-dimensional spatial coherence of synchrotron radiation i... more We present a method to determine the two-dimensional spatial coherence of synchrotron radiation in the soft X-ray regime by analyzing the Fourier transform of the magnetic speckle pattern from a ferromagnetic film in a multidomain state. To corroborate the results, a Young's double-pinhole experiment has been performed. The transverse coherence lengths in vertical and horizontal direction of both approaches are in a good agreement. The method presented here is simple and gives a direct access to the coherence properties of synchrotron radiation without nanostructured test objects.

Physical Review B Condensed Matter and Materials Physics, Oct 19, 2009

We have investigated the magnetic properties of single submicron permalloy rectangles with a thic... more We have investigated the magnetic properties of single submicron permalloy rectangles with a thickness of 20 nm and an aspect ratio of 2:1 via anisotropic magnetoresistance ͑AMR͒. Preparation and investigation via magnetotransport are performed in situ in ultrahigh vacuum. The field-dependent magnetization behavior of the two generic cases with the magnetic field applied perpendicular and parallel to the long axis of the rectangles is studied. Due to the high sensitivity of our setup, single field sweeps are sufficient to obtain magnetoresistance curves of structures with dimensions as small as 600ϫ 300 nm 2. To link features of the AMR to changes in the micromagnetic states, the remanent state has been investigated via scanning electron microscopy with polarization analysis. Our main result is that the energy density of micromagnetic states can be obtained from the hard-axis magnetization behavior. It is demonstrated that a C/S state can be distinguished from a Landau state and the energy difference between both states is determined.

Journal of Applied Physics, Apr 14, 1997

Physical Review B Condensed Matter and Materials Physics, Aug 1, 2010

The seeding of vortex domain walls in soft magnetic V-shaped nanowires by magnetic fields has bee... more The seeding of vortex domain walls in soft magnetic V-shaped nanowires by magnetic fields has been investigated via scanning electron microscopy with polarization analysis and micromagnetic simulations. It is found that the orientation of the magnetic seeding field determines the sense of rotation and the position of single vortex domain walls in the state of remanence. The topology of the magnetic microstructure in combination with symmetry considerations gives the key for the explanation of this behavior.

Imaging of magentic microstructures at surfaces. The scanning electron microscope with spin polarization analysis

Scanning Microscopy, 1991

Energy- and spin-analysis of polarized photoelectrons from NEA GaAsP

Appl Phys a Mat Sci Process, 1983

Polarized photoelectrons from NEA GaAs0.6P0.4 are for the first time analysed simultaneously with... more Polarized photoelectrons from NEA GaAs0.6P0.4 are for the first time analysed simultaneously with respect to spin and energy. Parameters of photon energy and temperature were chosen suitable for a polarized electron source cathode. Various contributions to the intensity- and polarization-distribution in connection with several depolarization mechanisms are discussed. Electron-hole exchange scattering together with multiple reflections in the space charge region qualitatively explain the observed distributions. Consequences for a polarized electron source are pointed out.

Magnetization distribution of 180� domain walls at Fe(100) single-crystal surfaces

Phys Rev Lett, 1989

... In thin films, on the other hand, 1800 Bloch walls are known to yield notoriously low Bitter ... more ... In thin films, on the other hand, 1800 Bloch walls are known to yield notoriously low Bitter contrast. This was explained by La Bonte14 and Hubert,15 who came to the same conclusion on different theoretical grounds: On both surfaces of the film the wall is of Neel-type charac ...

Magnetization distribution of 180o domain walls at Fe(100) single-crystal surfaces

Physical Review Letters, 1989

Applied Physics a Materials Science Processing, Jan 3, 2003

The basics of the scanning electron microscope with polarization analysis are briefly reviewed, e... more The basics of the scanning electron microscope with polarization analysis are briefly reviewed, emphasizing the achievable magnetic resolution and image contrast. The design of an optimized spin-polarization detector based on the wellestablished LEED scattering principle is presented. Results of first tests are reported.

Applied Physics Letters, 2016

The effect of tilted edges on the shape anisotropy and stray field coupling of uniformly magnetized rectangular elements

Journal of Applied Physics, Aug 15, 2009

ABSTRACT The influence of tilted edges on the magnetostatic properties of uniformly magnetized th... more ABSTRACT The influence of tilted edges on the magnetostatic properties of uniformly magnetized thin rectangular elements is studied. To calculate the magnetostatic energy, the Poisson equation is solved. The shape of the magnetic element is approximated by horizontally assembled thin cuboids and the solutions of Rhodes and Rowlands [Proc. Leeds Phil. Soc. 6, 191 (1954)] are utilized. A second approach is the straightforward integration of the Poisson equation taking into account the trapezoidal shape of the side faces due to the tilted edges. For an adequate number of cuboids, both methods agree very well. It is found that the shape anisotropy of a single magnetic element with tilted edges is reduced compared to that of an ideal cuboid. For a two element system the shape anisotropy competes with the magnetostatic interaction favoring a magnetization orientation parallel to the connecting line of the elements. If the elements are oriented in-line with their short axes, the easy magnetization axis switches at a critical distance between the elements. This distance increases when the elements have tilted edges.

J Magn Magn Mater, 2003

We demonstrate that the discrete dipolar sums can be separated into two contributions: thickness-... more We demonstrate that the discrete dipolar sums can be separated into two contributions: thickness-and geometrydependent parts. The geometry-dependent part is analogous to the shape dependence of the continuum approach. The correct normalization of the dipolar energy eliminates the apparent discrepancies of the discrete summation with the experimental results and continuum Maxwell theory. The superposition of the two contributions explains a new phenomenon, i.e. the size-dependent spin reorientation transition and/or enhancement of the effective perpendicular anisotropy.

Journal of Magnetism and Magnetic Materials, 2000

Besides the work mentioned in this thesis, I also spent 9 months working in developing a new tech... more Besides the work mentioned in this thesis, I also spent 9 months working in developing a new technique, i.e., the component resolved Kerr effect. In the spin-reorientation transition (SRT) when the magnetization changes between perpendicular and in-plane orientations at least two components of magnetization can be involved in the reversal process due to a field sweep. This causes a mixing of polar and longitudinal Kerr signal. This kind of mixing is also commonly found when the external field is not strictly aligned. As the longitudinal and polar Kerr signals are mixed with a field-dependent strength, quantitative data analysis is difficult. To overcome this problem, we developed a new experimental method to deconvolute the polar and longitudinal signals by two measurements of the Kerr signal in reversed geometries. By including the transverse component, the method is expanded to 3-D studies of magnetization by simple Kerr measurements, i.e., the component resolved Kerr effect. By using four different geometries, related to each other by mirror symmetry and a 90 • rotation, all the polar, longitudinal and transverse components of the magnetization can be obtained. The technique has been used to study the spin-reorientation transition in Co/Au(111). With thickness within the SRT region we find hysteresis loops with non-vanishing remanence in all three components when a field is applied within the film plane. A vertical field, however, drives the same film into a single domain state exhibiting full remanence. The fact that remanence is found in all magnetization components, and full remanence is obtained in a vertical field, rules out that the transition proceeds via a state of canting of magnetization and indicates that it proceeds via a state of coexisting phases. As the details of the technique has been published, I attached the original publication in this thesis as reference.

J Magn Magn Mater, 2005

We demonstrate theoretically that a large class of ultrathin ferromagnetic films shows anisotropy... more We demonstrate theoretically that a large class of ultrathin ferromagnetic films shows anisotropy in the domain wall orientation in spite of the isotropy in the exchange interactions. The reason is an orientation-dependent density of nearest-neighbor atomic bonds due to the symmetry of an underlying atomic lattice.

The magnetic domain structure of ultrathin Co-microstructures on Cu(0 0 1) has been investigated ... more The magnetic domain structure of ultrathin Co-microstructures on Cu(0 0 1) has been investigated by means of a scanning electron microscope with polarization analysis. In particular, the influence of a magnetic field on the domain structure has been studied. Here, we concentrate on squares of different size (3-30mm) and variable thickness, although triangles and bars with aspect ratios up to