A. Remhof

Invited Article : Vector and Bragg Magneto-optical Kerr effect for the analysis of nanostructured magnetic arrays

Experimental and theoretical aspects of obtaining the magnetic information carried by laser beams diffracted from an array of micro- or nanosized magnetic objects are reviewed. We report on the fundamentals of vector magneto-optic Kerr effect (MOKE), Bragg-MOKE, and second-order effects in the Kerr signal in longitudinal Kerr geometry as well as on an experimental setup used for vector and Bragg-MOKE experiments. The vector and Bragg-MOKE technique in combination with micromagnetic simulation is a reliable tool for measuring the complete magnetization vector and for characterizing the reversal mechanism of lateral magnetic nanostructures. We discuss the Bragg-MOKE effect for three standard domain configurations during the magnetization reversal process and present the expected behavior of the magnetic hysteresis loops.

Ultrasound-Induced Gradient Crystals Observed by High-Energy X-rays

High-energy X-ray scattering (90u2005keV) has been performed at the high-energy beamline ID15 at the European Synchrotron Radiation Facility, Grenoble, on perfect Si crystals excited by compressional ultrasound waves in the [111] direction. Two-dimensional intensity maps of the {bar3}51 Bragg peak reveal a purely longitudinal distortion of the lattice with respect to the applied distortion field, which is, for the present scattering geometry, inclined by 72° to the diffraction vector. A maximum gain factor of 50 for the diffracted intensity is observed for the strongest sound excitation. A quantitative analysis of the line shape by a transfer-matrix method shows the transition from the diffraction behavior of an ideal crystal towards a crystal slightly deformed by internal stresses.

Remote control of the Fe magnetic moment in magnetic heterostructures

In magnetic superlattices with interlayer exchange coupling, not only the coupling strength but also the magnetic moment of the ferromagnetic layer can be altered non-locally by modifying the electronic structure of the non-magnetic spacer layer. Specifically, changes of the electronic structure of the V spacer layers in Fe/V (001) superlattices are seen to affect the adjacent Fe layers. By reversibly loading the V layer with hydrogen, the magnetic moment of Fe increases, whereas the induced magnetic moment in V remains unchanged. The nature of this remote and non-local control of the magnetic moment is connected with a d-electron charge transfer and effective shift of the Fermi level relative to the d-bands of Fe and V, as elucidated on the basis of self-consistent electronic structure calculations.

Structural and magnetic properties of stoichiometric epitaxial CoO ∕ Fe exchange-bias bilayers

We present a detailed study of the magnetic and structural properties of the CoO/Fe bilayers by using a combination of x-ray diffraction (crystalline structure), Rutherford backscattering (chemical composition), and SQUID magnetometry (magnetic characterization) measurements. We prepared stoichiometric and single crystalline CoO thin films by a post-deposition annealing process in ultrahigh-vacuum conditions from sputter deposited hyperstoichiometric, polycrystalline CoO films. A simultaneous increase of the CoO crystalline quality and chemical phase purity has been achieved. Subsequently, the annealed CoO layers served as a template for the growth of epitaxial Fe films. The epitaxial relation between CoO and Fe was found to be of the Nishiyama-Wassermann type, where the Fe[001] direction is parallel aligned to the CoO[1-10] direction. The magnetic properties of CoO/Fe bilayers are strongly influenced by the stoichiometry of the antiferromagnetic CoO layer. Especially the blocking temperature is sensitive to the oxygen concentration within the CoO layer. While the blocking temperature of stoichiometric, epitaxial CoO/Fe exchange-bias bilayers almost equals the Neel temperature of bulk CoO, it is strongly reduced for superstoichiometric, polycrystalline CoO.

Hydrogen assisted growth of Fe/V superlattices

The functionality of magnetic heterostructures is strongly affected by the interface structure and morphology as well as by the crystalline quality. Using Fe/V superlattices, serving as model systems for exchange coupling and for superconducting spin valves, we show that the excellent interfacial properties can be further improved by hydrogen assisted growth. The role of the hydrogen thereby is twofold. It reduces thickness fluctuations and improves the crystalline quality. Sputter deposited Fe/V superlattices grown at a partial pressure of 2 × 10−6 mbar are atomically flat and possess a residual resistivity ratio which is significantly improved as compared to those of conventionally grown samples.

Investigation of ultrasonic fields by time-resolved x-ray diffraction

Time resolved detection in both detector and synchrotron resolution mode are presented. Using this technique the response of the Bragg profile in perfect Silicon to a MHz ultrasonic wave field has been studied on the high resolution triple crystal diffractometer at the ESRF high energy beamline ID15A. High photon energies up to 500 keV have been used for the analysis of a longitudinal acoustic wave in the 10 mm bulk. First, conventional time averaged rocking curves show intensity gains by up to a factor of 50, and a reciprocal space mapping reveals full information on the acoustic wave vector. Secondly, time and space resolved reflection curves have been taken. They give detailed insights into the properties of the acoustic wave field in space and time. In particular, they allow us to identify uniquely true standing waves without parasitic strain components or higher harmonic excitation. Time averaged and snapshots of the time resolved rocking curves represent different aspects of the density of states for the lattice parameter distribution. They are understood analytically by the elliptic integral K and the inverse circle function, respectively.

Extended longitudinal vector and Bragg magneto-optic Kerr effect for the determination of the chirality distribution in magnetic vortices

We have measured longitudinal magneto-optic Kerr effect (MOKE) for off-specular beams diffracted by a square array of Permalloy nanodots outside the plane of incidence and found that only the measurements performed outside the plane of incidence are sensitive to the distribution of rotational sense of vortices (vortex chirality) in the dot array. An asymmetry was introduced into the dot shape to ensure a uniform vortex chirality distribution in the dot array and to obtain a clear, well-defined chirality contrast in the MOKE signal. We also demonstrate an alternative method to measure the vortex chirality in dots of asymmetric shape without switching the chirality during magnetization reversal. In addition to the experiment, we have developed a general formalism that can be used to describe MOKE measured for off-specular beams inside as well as outside of the plane of incidence in all three Kerr geometries (polar, longitudinal, and transversal) with an arbitrary polarization state of incident light. Combin...

Neutron storage in a longitudinally vibrating silicon crystal

The time structure and integrated diffraction profile of cold neutrons of wavelength )~ = 6.27 Jk transmitted through a longitudinally vibrating silicon crystal were calculated by Monte Carlo simulations and measured on the backscattering spectrometer IN10 at the Institut -1 Laue-Langevin. Neutrons of velocity Vx = 630 ms require t-r = 158.5 Its for direct transit through the 10(Imm-long silicon resonator. This time is long compared with the vibration period Tt, = 22.3 !us and most of the neutrons experience multiple Bragg reflections in the oscillating Doppler-strain field. Monte Carlo calculations predict that neutrons will be stored in the crystal and released with a time structure determined by the vibration period and by the energy width of the incident beam. For a continuous beam, the usual time modulation of the diffracted neutrons with twice the vibration frequency is expected. If a neutron pulsc much shorter than the vibration period impinges on the crystal, the transmitted signal should be a decaying sequence of pulses separated by the vibration period. For pulses which are long compared with the vibration period, the effect of neutron storage should be manifest as a delayed staircase-like intensity variation on both pulse edges. A silicon crystal was set with the (111) lattice planes in backreflection and excited in a ~,/2 resonance at 44.78 kHz. A typical deformation amplitude was u0 = 1 Itm corresponding to a scattering range of AE = +1.9 IteV. The response of the ~k/2 resonator to a quasicontinuous beam and to neutron pulses of lengths Att, = 3 ms and 33 jas FWHM was measured. Experiments were performed with neutron beams of two energy widths AE = ±0.35 and ±1.23 laeV. In agreement with the Monte Carlo simulations, neutron storage in the silicon crystal was observed. The storage time was 250 Its for both long and short incident pulses. This time equals about 11 vibration periods of the crystal resonator and is in good agreement with the calculations for the Si 111 reflection and chosen vibration parameters. A first indication of the dependence of the time structure on the energy width of the neutron beam was seen in the experiments. The predicted pulsed structure of the transmitted signal

Hydrogen in thin epitaxial Nb films

Abstract The solubility isotherms of hydrogen in epitaxial Nb(100) films have been determined via in situ X-ray lattice-parameter measurements. The Nb films were grown by molecular-beam epitaxial techniques on R-Al 2 O 3 substrates with different thicknesses varying from 34 to 204 nm. Similar to Nb(110) films, the maximum out-of-plane lattice expansion scales inversely with Nb(100) film thickness. Furthermore, the critical temperature T c for the α-α′ phase transition as derived from the solubility isotherms depends strongly on the film thickness. For both orientations we determine similar T c ’s but for film thicknesses below 30 nm, T c in Nb(100) films are higher by about 80 K as compared to Nb(110) films. The influence of finite size scaling and adhesion of the films to the substrates on the solubility isotherms will be discussed.

Magnetization reversal in nanowires with a spiral shape

Magnetic nanowires have been shaped in the form of spirals and arranged in different patterns. A two-dimensional periodic array of Fe spiral structures was fabricated by electron-beam lithography. The spirals had a radius of 2.8u2002μm, a linewidth of 100 nm, and a thickness of 20 nm. The magnetization reversal was studied by longitudinal vector magneto-optic Kerr effect (MOKE) in specular geometry as well as in Bragg MOKE geometry, using the diffraction spots from the grating for hysteresis measurements. The measurements are compared with the results of micromagnetic simulation, which allows a detailed interpretation of the experimental data. The magnetization reversal is characterized by an onion state in remanence and a coercivity which is different for the inner and outer parts of the spiral structures. In general the inner parts of the spiral are more stable and switch later than the outer ones. The switching of the outer parts depends on the boundary condition.