J. Stöhr

Lensless imaging of magnetic nanostructures by X-ray spectro-holography

Our knowledge of the structure of matter is largely based on X-ray diffraction studies of periodic structures and the successful transformation (inversion) of the diffraction patterns into real-space atomic maps. But the determination of non-periodic nanoscale structures by X-rays is much more difficult. Inversion of the measured diffuse X-ray intensity patterns suffers from the intrinsic loss of phase information, and direct imaging methods are limited in resolution by the available X-ray optics. Here we demonstrate a versatile technique for imaging nanostructures, based on the use of resonantly tuned soft X-rays for scattering contrast and the direct Fourier inversion of a holographically formed interference pattern. Our implementation places the sample behind a lithographically manufactured mask with a micrometre-sized sample aperture and a nanometre-sized hole that defines a reference beam. As an example, we have used the resonant X-ray magnetic circular dichroism effect to image the random magnetic domain structure in a Co/Pt multilayer film with a spatial resolution of 50 nm. Our technique, which is a form of Fourier transform holography, is transferable to a wide variety of specimens, appears scalable to diffraction-limited resolution, and is well suited for ultrafast single-shot imaging with coherent X-ray free-electron laser sources.

Exploring the microscopic origin of magnetic anisotropies with X-ray magnetic circular dichroism (XMCD) spectroscopy

Symmetry breaking and bonding at interfaces leads to a variety of anisotropy phenomena in transition metal sandwiches and multilayers. The charge density, the spin density and the orbital moment become anisotropic. These effects can be studied by the X-ray magnetic circular dichroism (XMCD) technique which senses the local anisotropy of charge, spin and angular momentum around an atom that is excited by the absorption of polarized X-rays. Here we briefly review the principles of the technique and then apply it to the study of the thickness-dependent electronic and magnetic properties of a Co film sandwiched between Au. The experimental results are compared to those obtained by electronic structure calculations for a free Co monolayer and a Co monolayer sandwiched between Au. In particular, a simple ligand field model is developed which allows one to visualize the origin of the magnetocrystalline anisotropy in terms of the preferred direction of the orbital moment, corresponding to the direction of maximum size. The model supports the intuitive picture that the orbital moment on an atom becomes anisotropic through quenching effects by the anisotropic ligand fields of the neighbors.

Direct observation of the alignment of ferromagnetic spins by antiferromagnetic spins

The arrangement of spins at interfaces in a layered magnetic material often has an important effect on the properties of the material. One example of this is the directional coupling between the spins in an antiferromagnet and those in an adjacent ferromagnet, an effect first discovered in 1956 and referred to as exchange bias. Because of its technological importance for the development of advanced devices such as magnetic read heads and magnetic memory cells, this phenomenon has received much attention. Despite extensive studies, however, exchange bias is still poorly understood, largely due to the lack of techniques capable of providing detailed information about the arrangement of magnetic moments near interfaces. Here we present polarization-dependent X-ray magnetic dichroism spectro-microscopy that reveals the micromagnetic structure on both sides of a ferromagnetic–antiferromagnetic interface. Images of thin ferromagnetic Co films grown on antiferromagnetic LaFeO3 show a direct link between the arrangement of spins in each material. Remanent hysteresis loops, recorded for individual ferromagnetic domains, show a local exchange bias. Our results imply that the alignment of the ferromagnetic spins is determined, domain by domain, by the spin directions in the underlying antiferromagnetic layer.

First experimental results from IBM/TENN/TULANE/LLNL/LBL undulator beamline at the advanced light source

The IBM/TENN/TULANE/LLNL/LBL Beamline 8.0 at the advanced light source combining a 5.0 cm, 89 period undulator with a high‐throughput, high‐resolution spherical grating monochromator, provides a powerful excitation source over a spectral range of 70–1200 eV for surface physics and material science research. The beamline progress and the first experimental results obtained with a fluorescence end station on graphite and titanium oxides are presented here. The dispersive features in K emission spectra of graphite excited near threshold, and found a clear relationship between them and graphite band structure are observed. The monochromator is operated at a resolving power of roughly 2000, while the spectrometer has a resolving power of 400 for these fluorescence experiments.

The adsorption structure of glycine adsorbed on Cu(110); comparison with formate and acetate/Cu(110)

The molecular orientation of an ordered monolayer of glycine adsorbed on Cu(110) has been studied using X-ray Photoelectron Spectroscopy (XPS), Near Edge X-ray Absorption Fine Structure (NEXAFS), X-ray Photoelectron Diffraction (XPD), Low-Energy Electron Diffraction (LEED) and theoretical calculations. In particular, the NEXAFS results are discussed in terms of the spectra of the related molecules ammonia (NH3), formate (HCOO), and acetate (CH3COO) on Cu(110). Whereas the latter two molecules chemisorb in similar geometries, glycine is found to assume a very different chemisorption geometry. Formate and acetate bond through two equivalent oxygen atoms with the molecular plane oriented nearly perpendicular to the surface, aligned along the [110]-azimuth. In the case of adsorbed glycine (NH2CH2COO), the azimuthal orientation is still present, i.e. the bonding oxygen atoms are aligned along the [110]-azimuth, but the molecule is found to bend towards the surface. A second chemisorption bond is formed at the nitrogen end of the molecule, involving copper atoms in the neighboring [110]-row. We therefore have the interesting case of a chemisorption bond involving different functional groups in the same molecule.

The ultimate speed of magnetic switching in granular recording media

In magnetic memory devices, logical bits are recorded by selectively setting the magnetization vector of individual magnetic domains either ‘up’ or ‘down’. In such devices, the fastest and most efficient recording method involves precessional switching: when a magnetic field Bp is applied as a write pulse over a period τ, the magnetization vector precesses about the field until Bpτ reaches the threshold value at which switching occurs. Increasing the amplitude of the write pulse Bp might therefore substantially shorten the required switching time τ and allow for faster magnetic recording. Here we use very short pulses of a very high magnetic field to show that under these extreme conditions, precessional switching in magnetic media supporting high bit densities no longer takes place at well-defined field strengths; instead, switching occurs randomly within a wide range of magnetic fields. We attribute this behaviour to a momentary collapse of the ferromagnetic order of the spins under the load of the short and high-field pulse, thus establishing an ultimate limit to the speed of deterministic switching and magnetic recording.

Photoemission electron microscope for the study of magnetic materials

The design of a high resolution photoemission electron microscope (PEEM) for the study of magnetic materials is described. PEEM is based on imaging the photoemitted (secondary) electrons from a sample irradiated by x rays. This microscope is permanently installed at the Advanced Light Source at a bending magnet that delivers linearly polarized, and left and right circularly polarized radiation in the soft x-ray range. The microscope can utilize several contrast mechanisms to study the surface and subsurface properties of materials. A wide range of contrast mechanisms can be utilized with this instrument to form topographical, elemental, chemical, magnetic circular and linear dichroism, and polarization contrast high resolution images. The electron optical properties of the microscope are described, and some first results are presented.

Orientation and absolute coverage of benzene, aniline, and phenol on Ag(110) determined by NEXAFS and XPS

Abstract The orientation, absolute coverage, and core-electron binding energies of benzene, aniline, and phenol have been measured with near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). At 0.13 ± 0.02 monolayer (ML) coverage, the plane of the benzene ring orients with a maximum tilt angle with respect to the plane of the surface of 18°. The aromatic ring of aniline tilts at 39 ± 5 ° at 0.27 ± 0.07 ML coverage, and the phenol aromatic ring tilts 40 ± 5 ° from the plane of the surface at a coverage of 0.35 ± 0.06 ML. The saturation coverage of benzene on the Ag(110) surface is 0.31 ± 0.05 ML. After annealing a multilayer of phenol to 225 K the coverage of phenol remaining on the surface is 0.35 ± 0.06 ML. Aniline is present at 0.75 ± 0.15 ML at saturation coverage, suggesting the presence of a second layer; temperature-programmed desorption shows states indicative of a second layer also. The C(1s) XPS binding energy of benzene monolayer is 284.7 ± 0.3 eV. As expected, two C(1s) XPS binding energies were measured both for aniline at 284.4 ± 0.3 eV and 285.5 ± 0.3 eV and for phenol at 284.4 ± 0.3 eV and 285.9 ± 0.3 eV. The splitting due to different carbons of 1.1 eV observed for aniline agrees well with the gas phase splitting of 1.2 eV, and the 1.6 eV splitting observed for phenol is identical to the splitting measured for gas phase phenol. A clear splitting of the π∗ resonances is also observed in the NEXAFS spectra of aniline and phenol and has been assigned to transitions from the two C(1s) levels into the three π∗ molecular Orbitals.

The orientation of Langmuir–Blodgett monolayers using NEXAFS

Carbon K‐shell NEXAFS (near edge x‐ray absorption fine structure) spectra of oriented hydrocarbon chains in Langmuir–Blodgett (LB) monolayers were measured and used to study the orientation of these molecules. The LB monolayers were assembled from arachidic acid or cadmium or calcium arachidate on the oxidized Si(111) surface. The observed NEXAFS resonances are assigned to transitions to excited states which are localized on individual CH2 groups or C–C bonds. From a detailed analysis using curve‐fitting techniques of the angular dependence of the various spectral peaks, the hydrocarbon chains of the cadmium arachidate monolayer is estimated to lie within 15° of the surface normal, the hydrocarbon chains of the calcium arachidate monolayer is estimated to be tilted by 33±5° from the surface normal, while the arachidic acid monolayer is not ordered at all. The determined chain orientations are discussed in terms of a microscopic model involving lateral interactions between the zig–zag hydrocarbon chains.

X-RAY MAGNETIC CIRCULAR DICHROISM SPECTROSCOPY OF TRANSITION METAL THIN FILMS

Abstract A review is given of the principles underlying X-ray magnetic circular dichroism (XMCD) spectroscopy. The main strengths of the technique are highlighted. They are the quantitative determination of spin and orbital magnetic moments and their anisotropies through sum rule analysis of experimental spectra. A discussion is given why and how the spin and orbital magnetic moments in transition metal thin films are modified relative to the bulk. Selected XMCD results for thin films are discussed. “Non-magnetic” spacer layers sandwiched between magnetic layers are shown to become magnetic. The orbital moment is shown to become anisotropic in thin films, revealing that it is the microscopic origin of the magnetocrystalline anisotropy.