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A classical ghost imaging experiment performed at the free-electron laser FLASH is presented. The possibility of image formation, a double bar in our case, in the beam that has never interacted with the sample is demonstrated.

Physical review / A covering atomic, molecular, and optical physics and quantum information 101(1), 013820 (2020). doi:10.1103/PhysRevA.101.013820, Here we present the results of a classical ghost imaging experiment accomplished at an XUV free-electronlaser (FEL). To perform such experiment at an FEL source each x-ray pulse was transmitted through a movingdiffuser, which created a noncorrelated speckled beam. This beam was then split in two identical branches byintroducing a beam splitter in the form of a transmission grating. In one of these branches the sample waspositioned. We demonstrate the possibility of image formation, a double bar in our case, in the beam that hasnever interacted with the sample. With this experiment we extend the quantum optics methodology to the FELcommunity., Published by Inst., Woodbury, NY

The intensity correlations in incoherently scattered X-rays from a free-electron laser can be exploited to image 2D objects with a resolution close to or below the diffraction limit. The advent of accelerator-driven free-electron lasers (FEL) has opened new avenues for high-resolution structure determination via diffraction methods that go far beyond conventional X-ray crystallography methods1,2,3,4,5,6,7,8,9,10. These techniques rely on coherent scattering processes that require the maintenance of first-order coherence of the radiation field throughout the imaging procedure. Here we show that higher-order degrees of coherence, displayed in the intensity correlations of incoherently scattered X-rays from an FEL, can be used to image two-dimensional objects with a spatial resolution close to or even below the Abbe limit. This constitutes a new approach towards structure determination based on incoherent processes11,12, including fluorescence emission or wavefront distortions, generally considered detrimental for imaging applications. Our method is an extension of the landmark intensity correlation measurements of Hanbury Brown and Twiss13 to higher than second order, paving the way towards determination of structure and dynamics of matter in regimes where coherent imaging methods have intrinsic limitations14.

The realization of the strong coupling regime between a single cavity mode and an electromagnetic resonance is a centrepiece of quantum optics. In this regime, the reversible exchange of a photon between the two components of the system leads to so-called Rabi oscillations. Strong coupling is used in the optical and infrared regimes, for instance, to produce non-classical states of light, enhance optical nonlinearities and control quantum states. Here, we report the first observation of Rabi oscillations of an X-ray photon between two resonant 57Fe layers embedded in two coupled cavities. The system is described by an effective Hamiltonian, in which the two layers couple strongly. We observe sinusoidal beating as the signature of the Rabi oscillations in the system’s temporal evolution, as well as the splitting of nuclear resonances in the reflected light spectrum. Our results significantly advance the development of the new field of X-ray quantum optics. Rabi oscillations in the regime of hard X-rays are observed between two ensembles of the nuclear resonant isotope 57Fe. The sample system consists of two thin-film cavities stacked on top of each other, coupled through a thin interlayer.

Transient magnetization dynamics are calculated from the Landau-Lifshitz-Gilbert equation. The internal fields are described by partial derivatives of the sample's free energy density. This description allows the calculation of the magnetization trajectory of homogeneously magnetized samples for all kinds of energy contributions. It provides a general solution for transient magnetization dynamics. The model is used to determine the magnetization response to excitation profiles like rectangular or Gaussian pulses, frequency bursts, wavelet excitations, and chirped THz pulses.

Strong coupling between X-rays and matter excitations in arrays of alternating 57Fe and 56Fe layers is demonstrated. The results may open the door to X-ray quantum optics and strong coupling phenomena.

Scientific reports 8(1), 11261 (2018). doi:10.1038/s41598-018-29596-z, We introduce a method to study the spatial profiles of standing spin waves in ferromagnetic microstructures. The method relies on Nuclear Resonant Scattering of 57Fe using a microfocused beam of synchrotron radiation, the transverse coherence length of which is smaller than the length scale of lateral variations in the magnetization dynamics. Using this experimental method, the nuclear resonant scattering signal due to a confined spin wave is determined on the basis of an incoherent superposition model. From the fits of the Nuclear Resonant Scattering time spectra, the precessional amplitude profile across the stripe predicted by an analytical model is reconstructed. Our results pave the way for studying non-homogeneous dynamic spin configurations in microstructured magnetic systems using nuclear resonant scattering of synchrotron light., Published by Nature Publishing Group, London

Ultrafast Phenomena, UP 2018, Hamburg, Germany, 15 Jul 2018 - 20 Jul 2018, The dynamically resolved response of the canted-antiferromagnet FeBO3 excited near amagnon resonance shows fast oscillations after THz-excitation, followed by the magnons’ intrinsicrelaxation, enabling to probe transient magnetic relaxation dynamics over large frequency range.

Experiments on THz magnetization dynamics provide insights into a yet unexplored time scale of transient magnetics and spintronics [1-3]. Coherent responses of the magnetization to THz pulses have been demonstrated [1,2]. Further, THz spin currents can be generated by non-equilibrium hot electrons [3]. These THz transients provide new ways to ultrafast spin control and its technical applications. Here, an analytical model is presented that describes transient magnetization dynamics up to the THz regime [4]. The model is used to determine the magnetization response to ultrafast multi and single-cycle THz pulses for a variety of parameters like carrier frequency, width, phase, frequency chirp, and polarization. The THz pulse shape and polarization provide a vectorial control of the magnetization on the sub-picosecond time scale. It is shown that an optimum timing for coherent magnetization control can be achieved. Dynamics of the magnetization induce a spin current in an adjacent non-magnetic material. This effect s known as spin pumping [5]. Here, calculations of THz transient spin current generation by spin pumping are presented. An effective spin current generation is found far above the ferromagnetic resonance up to THz frequencies although dynamic magnetization amplitudes are very small at THz frequencies. At THz frequencies, the coherent reaction of the magnetization also causes a coherent spin current. In contrast to the dc spin current which scales with the susceptibility of the magnetization, the ac spin current does not vanish above the ferromagnetic resonance. Instead the THz ac spin current reaches a value that is comparable to the dc spin current at resonance. The behavior far above resonance can be used to efficiently generate ultrafast spin currents without the need for magnetic systems with very high resonance frequencies. Spin currents on picosecond time scales can be achieved by THz magnetization dynamics. References: [1] C. Vicario et al., Nat. Photonics 7, 720 (2013), [2] T. Kampfrath et al., Nat. Photonics 5, 31 (2011) [3] T. Kampfrath et al., Nat. Nanotechnl. 8, 256 (2013) [4] L. Bocklage, Sci. Rep. 6, 22767 (2016) & ‎J. Magn. Magn. Mater 429, 324 (2017) [5] Y. Tserkovnyak, A. Brataas, and G. E. W. Bauer, Phys Rev. Lett. 88, 117601 (2002) [6] D. Wei et al., Nat. Comm. 5, 3768 (2014) [7] L. Bocklage, (submitted)

Physical review materials 1(2), 023001(2017). doi:10.1103/PhysRevMaterials.1.023001, We present a synchrotron-based x-ray scattering technique which allows disentangling magnetic propertiesof heterogeneous systems with nanopatterned surfaces. This technique combines the nanometer-range spatialresolution of surface morphology features provided by grazing incidence small angle x-ray scattering and thehigh sensitivity of nuclear resonant scattering to magnetic order. A single experiment thus allows attributingmagnetic properties to structural features of the sample; chemical and structural properties may be correlatedanalogously. We demonstrate how this technique shows the correlation between structural growth and evolutionof magnetic properties for the case of a remarkable magnetization reversal in a structurally and magneticallynanopatterned sample system., Published by APS, College Park, MD

Physical review letters 118(25), 257202(2017). doi:10.1103/PhysRevLett.118.257202, The generation of short spin current pulses is the basis for fast spintronic devices. In thin bilayer systems consisting of a nonmagnetic metal and a ferromagnet, a pure spin current is induced by a precessing magnetization into the nonmagnetic layer by spin pumping. This effect has been experimentally demonstrated at ferromagnetic resonance at GHz frequencies. Here, it is theoretically shown that transient magnetization dynamics efficiently generates short spin current pulses that exhibit two transient contributions. An effective coherent spin current generation is found far above the ferromagnetic resonance up to THz frequencies although dynamic magnetization amplitudes are very small in this regime. The results provide a concept for coherent THz spin current generation., Published by APS, College Park, Md.

Physical review letters 118(23), 237204(2017). doi:10.1103/PhysRevLett.118.237204, Nuclear resonant x-ray diffraction in grazing incidence geometry is used to determine the lateral magnetic configuration in a one-dimensional lattice of ferromagnetic nanostripes. During magnetic reversal, strong nuclear superstructure diffraction peaks appear in addition to the electronic ones due to an antiferromagnetic order in the nanostripe lattice. We show that the analysis of the angular distribution together with the time dependence of the resonantly diffracted x rays reveals surface spin structures with very high sensitivity. This scattering technique provides unique access to laterally correlated spin configurations in magnetically ordered nanostructures and, in perspective, also to their dynamics., Published by APS, College Park, Md.

We probe the spin dynamics in a thin magnetic film at ferromagnetic resonance by nuclear resonant scattering of synchrotron radiation at the 14.4 keV resonance of ^{57}Fe. The precession of the magnetization leads to an apparent reduction of the magnetic hyperfine field acting at the ^{57}Fe nuclei. The spin dynamics is described in a stochastic relaxation model adapted to the ferromagnetic resonance theory by Smit and Beljers to model the decay of the excited nuclear state. From the fits of the measured data, the shape of the precession cone of the spins is determined. Our results open a new perspective to determine magnetization dynamics in layered structures with very high depth resolution by employing ultrathin isotopic probe layers.

Group velocity control is demonstrated for x-ray photons of 14.4 keV energy via a direct measurement of the temporal delay imposed on spectrally narrow x-ray pulses. Sub-luminal light propagation is achieved by inducing a steep positive linear dispersion in the optical response of ${}^{57}$Fe M\"ossbauer nuclei embedded in a thin film planar x-ray cavity. The direct detection of the temporal pulse delay is enabled by generating frequency-tunable spectrally narrow x-ray pulses from broadband pulsed synchrotron radiation. Our theoretical model is in good agreement with the experimental data., Comment: 8 pages, 4 figures

The confinement of spin waves in inhomogeneous fields and spin wave interaction with domain walls has attracted interest due to possible applications in magnonics. We investigate spin waves in curved ferromagnetic nanowires. The field dispersion and localization of spin waves is revealed by comparison to known modes in stripes and taking into account the specific field reversal of the curved wire. In small wires we find a strongly altered mode spectrum in a certain field regime. Micromagnetic simulations show an extended domain wall within the wire in this field region. The domain wall shows several dynamic modes and changes the remaining spin wave modes. We find mode suppression as well as newly arising modes due to the strong inhomogenous internal field of the wall.

The motion of domain walls is influenced by their internal structure and their structural changes during motion. The macroscopic motion is well understood but the internal dynamics during motion have not been experimentally studied in detail. We study vortex domain walls excited by nanosecond long magnetic field pulses in a pinning potential and above the pinning threshold. The dynamic response is imaged by transmission x-ray microscopy and the structural changes are analyzed. From the directly observed inertial behavior of the wall the domain-wall mass equivalent is estimated. During motion, oscillations of both the vortex core around the center of the domain wall and the domain-wall width are observed. The wall shows a fully elastic behavior by compression and overexpansion when being pushed by the field.

A wide variety of coupled harmonic oscillators exist in nature. Coupling between different oscillators allows for the possibility of mutual energy transfer between them and the information-signal propagation. Low-energy input signals and their transport with negligible energy loss are the key technological factors in the design of information-signal processing devices. Here, utilizing the concept of coupled oscillators, we experimentally demonstrated a robust new mechanism for energy transfer between spatially separated dipolar-coupled magnetic disks - stimulated vortex gyration. Direct experimental evidence was obtained by a state-of-the-art experimental time-resolved soft X-ray microscopy probe. The rate of energy transfer from one disk to the other was deduced from the two normal modes' frequency splitting caused by dipolar interaction. This mechanism provides the advantages of tunable energy transfer rates, low-power input signals and negligible energy loss in the case of negligible intrinsic damping. Coupled vortex-state disks might be implemented in applications for information-signal processing.

Heusler films with L 2 1 and B2 structure are deposited simultaneously on amorphous carbon films, Si(1 0 0) surfaces, and in situ cleaved InAs(1 1 0) surfaces by coevaporation of Ni and the alloy MnIn. Morphology, structure, and stoichiometry are investigated with transmission-electron microscopy, electron diffraction, and X-ray spectroscopy. The almost perfect lattice match supports highly oriented growth of Ni 2 MnIn on InAs, which is proven by electron diffraction under grazing incidence. The electrical resistivity of thin films on Si show metallic behavior. At temperatures of liquid helium point-contact Andreev reflection spectroscopy is performed on films grown on Si(1 0 0) and in situ cleaved InAs(1 1 0) surfaces yielding spin polarizations comparable to the ones of Fe, Ni, Co, and permalloy ( Ni 80 Fe 20 ) .

Physical review / B 87, 064420(2013). doi:10.1103/PhysRevB.87.064420, Vortex cores in Landau-domain patterns are resonantly excitable by alternating magnetic fields in the sub-gigahertz regime. We present a highly sensitive method to detect the vortex gyration in single micrometer-sized elements spectroscopically by measuring spectra of induction voltages caused by the stray fields of a single ferromagnetic square exposed to an alternating Oersted field. A distinct change of the induction voltage is observed around the resonance frequency of the vortex core. The shape of the measured spectra deviates from Lorentzian profiles due to voltages induced by magnetic fringing fields of the exciting currents. An analytical description of the measured spectra is given. A characteristic frequency shift in external magnetic fields proves that signals detected by the induction sensor originate from the dynamically excited Landau-domain pattern. The measurements on a single square are compared with measurements on an ensemble of uniform squares., Published by APS, College Park, Md.

Applied physics letters 103(9), 092406 (2013). doi:10.1063/1.4819729, We have investigated the generation of magnetic domain walls by nanosecond magnetic field pulses of a strip line. Domain wall creation is sensitive to an externally applied field concerning wall type and threshold amplitudes. The domain wall creation is stochastic similar to domain wall depinning. In the experiment reliable domain wall generation require up to 8 ns long pulses at the threshold field amplitude. The required pulse length can be reduced by higher field amplitudes. Time-resolved measurements and micromagnetic simulations show that the domain wall is generated within three nanoseconds. The creation proceeds via vortex core formation under the strip line., Published by American Institute of Physics, Melville, NY

We demonstrate the feasibility of investigating periodically driven magnetization dynamics in a scanning electron microscope with polarization analysis based on spin-polarized low-energy electron diffraction. With the present setup, analyzing the time structure of the scattering events, we obtain a temporal resolution of 700 ps, which is demonstrated by means of imaging the field-driven 100 MHz gyration of the vortex in a soft-magnetic FeCoSiB square. Owing to the efficient intrinsic timing scheme, high-quality movies, giving two components of the magnetization simultaneously, can be recorded on the time scale of hours.

Full-field magnetic transmission x-ray microscopy at high spatial resolution down to 20 nm is used to directly observe field-driven domain wall motion in notch-patterned permalloy nanowires. The depinning process of a domain wall around a notch exhibits a stochastic nature in most nanowires. The stochasticity of the domain wall depinning sensitively depends on the geometry of the nanowire such as the wire thickness, the wire width, and the notch depth. We propose an optimized design of the nanowire for deterministic domain wall depinning field at a notch.

A wide variety of coupled harmonic oscillators exist in nature. Coupling between different oscillators allows for the possibility of mutual energy transfer between them and the information-signal propagation. Low-energy input signals and their transport with negligible energy loss are the key technological factors in the design of information-signal processing devices. Here, utilizing the concept of coupled oscillators, we experimentally demonstrated a robust new mechanism for energy transfer between spatially separated dipolar-coupled magnetic disks - stimulated vortex gyration. Direct experimental evidence was obtained by a state-of-the-art experimental time-resolved soft X-ray microscopy probe. The rate of energy transfer from one disk to the other was deduced from the two normal modes' frequency splitting caused by dipolar interaction. This mechanism provides the advantages of tunable energy transfer rates, low-power input signals and negligible energy loss in the case of negligible intrinsic damping. Coupled vortex-state disks might be implemented in applications for information-signal processing.

New concepts of high-density and ultrafast nonvolatile data storage devices involve the controlled motion of magnetic domain walls (DWs) in nanowires [1]. To realize such a device, reproducible and reliable pinning sites for individual DWs are required. Geometric constrictions are widely used to create local confining potentials acting as pinning sites [2]. As an alternative, pinning sites can be induced via a local modification of magnetic properties by ion irradiation [3]. In this case, a variation in the wire geometry on the nanoscale is not required. Implantation of chromium ions into permalloy is known to cause alloying and structural defects which lead to a reduction in the saturation magnetization MS, and the magnetic anisotropy as well as to a change in the exchange constant and the damping parameter [4]. The strength of the pinning potential can be tuned by the chromium ion fluence applied to induce the so-called magnetic soft spots [3]. Micromagnetic simulations, high resolution magnetic transmission soft X-ray microscopy at beamline 6.1.2 of the Advanced Light Source in Berkeley, CA, USA, and electrical measurements of the anisotropic magnetoresistance are employed to characterize the pinning potential which significantly differs for transverse and vortex walls. We demonstrate field-induced DW pinning and depinning as well as reliable DW depinning by single current pulses in a permalloy nanowire containing a square-shaped magnetic soft spot [5]. Lower requirements on the lithography in comparison to geometric constrictions on the nanoscale, a smaller distribution of properties due to parallel processing during implantation, and fine tunability of the pinning potential via the chromium ion fluence make the magnetic soft spots a promising candidate for applications. Financial support by the Deutsche Forschungsgemeinschaft via Grant Nos. FA314/3-2 and MC9/7-2, the SFB 668 and the GrK 1286 as well as the Forschungs- und Wissenschaftsstiftung Hamburg via the Exzellenzcluster “Nano- Spintronik” is gratefully acknowledged. Operation of the X-ray microscope is supported by the US Department of Energy under Contract No. DE-AC02-05-CH11231. References: [1] D. A. Allwood, Gang Xiong, M. D. Cooke, C. C. Faulkner, D. Atkinson, N. Vernier, R. P. Cowburn, Science 296, 2003 (2002); S. S. P. Parkin, U. S. Patent No. US 683 400 5 (2004). [2] M.-Y. Im, L. Bocklage, P. Fischer, and G. Meier, Phys. Rev. Lett. 102, 147204 (2009). [3] A. Vogel, S. Wintz, J. Kimling, M. Bolte, T. Strache, M. Fritzsche, M.-Y. Im, P. Fischer, G. Meier, and J. Fassbender, IEEE Trans. Mag. 46, 1708 (2010). [4] J. Fassbender and J. McCord, J. Magn. Magn. Mater. 320, 579 (2008). [5] A. Vogel, S. Wintz, T. Gerhardt, L. Bocklage, T. Strache, M.-Y. Im, P. Fischer, J. Fassbender, J. McCord, and G. Meier, Appl. Phys. Lett. 98, 202501 (2011).

Domain-wall oscillation in a pinnig potential is described analytically in a one dimensional model for the feld-driven case. For a proper description the pinning potential has to be extended by nonharmonic contributions. Oscillations of a domain wall are observed on its genuine time scale by magnetic X-ray microscopy. It is shown that the nonharmonic terms are present in real samples with a strong restoring potential. In the framework of our model we gain deep insight into the domain-wall motion by looking at different phase spaces. The corrections of the harmonic potential can change the motion of the domain wall significantly. The damping parameter of permalloy is determined via the direct imaging technique.

A dependence of current-induced domain-wall motion in nanowires on the temporal shape of current pulses is observed. The results show that the motion of the wall is amplified for alterations of the current on a time scale smaller than the intrinsic time scale of the domain wall which is a few nanoseconds in permalloy. This effect arises from an additional force on the wall by the spin-transfer torque due to a fast changing current and improves the efficiency of domain-wall motion. The observations provide an understanding for the efficient domain-wall motion with nanosecond current pulses.

The stochastic field-driven depinning of a domain wall pinned at a notch in a magnetic nanowire is directly observed using magnetic X-ray microscopy with high lateral resolution down to 15 nm. The depinning-field distribution in Ni{sub 80}Fe{sub 20} nanowires considerably depends on the wire width and the notch depth. The difference in the multiplicity of domain-wall types generated in the vicinity of a notch is responsible for the observed dependence of the stochastic nature of the domain wall depinning field on the wire width and the notch depth. Thus the random nature of the domain wall depinning process is controllable by an appropriate design of the nanowire.

Current-induced domain-wall dynamics is investigated via high-resolution soft x-ray transmission microscopy by a stroboscopic pump-and-probe measurement scheme at a temporal resolution of 200 ps. A $180\ifmmode^\circ\else\textdegree\fi{}$ domain wall in a restoring potential of a permalloy microstructure is displaced from its equilibrium position by nanosecond current pulses leading to oscillations with velocities up to 325 m/s. The motion of the wall is described with an analytical model of a rigid domain wall in a nonharmonic potential allowing one to determine the mass of the domain wall. We show that Oersted fields dominate the domain-wall dynamics in our geometry.

Time-resolved X-ray microscopy is used to image the influence of alternating high-density currents on the magnetization dynamics of ferromagnetic vortices. Spin-torque induced vortex gyration is observed in micrometer-sized permalloy squares. The phases of the gyration in structures with different chirality are compared to an analytical model and micromagnetic simulations, considering both alternating spinpolarized currents and the current's Oersted field. In our case the driving force due to spin-transfer torque is about 70% of the total excitation while the remainder originates from the current's Oersted field. This finding has implications to magnetic storage devices using spin-torque driven magnetization switching and domain-wall motion., Comment: 10 pages, 3 figures

We demonstrate the fast generation of domain walls with defined chiralities in nanowires. Nanosecond long current pulses are passed through a stripline to generate Oersted fields which create vortex domain walls in the nanowire. The direction of the Oersted field with respect to the nanowire is varied by the polarity of the voltage pulses as well by the alignment between the stripline and the nanowire. The results show that by using a tilted stripline and short current pulses domain walls with defined chirality and type can be generated.

We study the creation and annihilation of domain walls in a permalloy nanowire by local Oersted fields of current pulses passing through a perpendicularly aligned stripline. The occurrence of both processes is investigated for current pulses of different polarities and for various external magnetic fields. Reliable creation and annihilation are achieved for small and zero external fields, while higher externally applied fields lead to the suppression of both processes as well as to the creation of multiple domain walls in the wire.

Physical properties of Permalloy (Ni80Fe20) can be considerably enhanced by sputtering on substrates heated to temperatures around 300 °C. To enable the use of sputtered Permalloy in micro- and nanostructures for spintronic experiments a subtractive preparation process has to be established, as common lift-off processing is incompatible with high temperatures. Two subtractive fabrication processes for curved Permalloy nanowires are executed: Ion-milling and rf-sputter-etching. The results of current-induced domain-wall depinning experiments with these nanowires are compared.

The depinning probability of magnetic domain walls at constrictions by current pulses depends on the external magnetic field and on the temporal shape of current pulses. We investigate the depinning of domain walls in permalloy nanowires in dependence of the rise time of a current pulse. We observe a change in the depinning probability when varying the pulse rise time and the pulse amplitude at a constant magnetic field. The experiments reveal that the current density for domain-wall depinning can be strongly reduced by current pulses with short rise times.

We employed time-and space-resolved full-field magnetic transmission soft x-ray microscopy to observe vortex-core gyrations in a pair of dipolar-coupled vortex-state Permalloy (Ni{sub 80}Fe{sub 20}) disks. The 70 ps temporal and 20 nm spatial resolution of the microscope enabled us to simultaneously measure vortex gyrations in both disks and to resolve the phases and amplitudes of both vortex-core positions. We observed their correlation for a specific vortex-state configuration. This work provides a robust and direct method of studying vortex gyrations in dipolar-coupled vortex oscillators.

Domain walls in curved Permalloy (Ni80Fe20) nanowires are depinned by nanosecond current pulses and magnetic fields. A significant dependence of the depinning probability on the pulse length is observed. We measure an oscillatory dependence with a characteristic frequency of 270 MHz. The quality of the Permalloy used for the nanowires is crucial for efficient domain-wall depinning. By sputtering Permalloy on heated substrates we are able to considerably decrease the specific resistance that is found to directly correspond to the depinning field of the domain wall.

We demonstrate by micromagnetic simulations that magnetic antivortices are potential candidates for fast nonvolatile data-storage elements. These storage elements are excited simultaneously by alternating spin-polarized currents and their accompanying Oersted fields. Depending on the antivortex-core polarization p and the orientation of the in-plane magnetization c around the core, the superposition of current and field leads to either a suppression of gyration (logical “zero”) or an increased gyration amplitude (logical “one”). Above an excitation threshold the gyration culminates in the switching of the antivortex core. The switching can be seen as a cp-dependent writing of binary data, allowing to bring the antivortex into a distinct state. Furthermore a read-out scheme using an inductive loop situated on top of the element is investigated.

We present point-contact Andreev reflection (PCAR) spectroscopy on Permalloy (Ni80Fe20) and on the half-metallic Heusler alloy Ni2MnIn. The thin Permalloy films are deposited on Si, the Ni2MnIn films on Si as well as on in situ cleaved (110) surfaces of InAs. A highly conductive layer under the ferromagnetic film almost eliminates a series resistance and thus facilitates the determination of the spin polarization from the differential conductance curves. We obtain a spin polarization of Permalloy of 35%. The spin polarization of Ni2MnIn depends on the substrate, presumably due to the growth of different crystal structures. It is shown that the surface sensitive PCAR spectroscopy cannot determine the spin polarization of the bulk material of half-metals where the degree of spin polarization strongly depends on the crystal structure.