Anders Hahlin

Ultrathin Co films on flat and vicinal Cu(111) surfaces: per atom determination of orbital and spin moments

We have performed x-ray magnetic circular dichroism, x-ray resonant magnetic scattering and scanning tunnelling microscopy measurements on ultrathin Co films deposited on flat and vicinal Cu(111). The range of film thickness varies between one and 25 monolayers. For the vicinal Cu(111), Co deposition below one monolayer leads to the formation of elongated islands preferentially oriented along the step edges. These islands extend over lateral length scales of up to several thousand angstroms. For the thicker films we still observe that the vicinal surface leads to the occurrence of a preferential uniaxial growth direction. No such preferential growth direction can be identified for the flat surface. In correlation to the Co growth we observe an increase of both the orbital and the spin moment per Co atom on vicinal Cu(111) of about 25% versus what was observed for Co on flat Cu(111). This enhancement is observed in both the x-ray absorption and the reflectivity measurements. Similar to what was earlier reported for Co on Cu(100) we also observe for the case Co on Cu(111) an increase in the ratio ml /ms (orbital to spin moments) of 40% for thin Co films. In contrast to what has been reported for Co films on flat and vicinal Cu(100) we do not observe any major variations in the occupancy of the Co 3d states for Co grown on the vicinal Cu(111) surface.

A circularly polarized X-ray study of the temperature-dependent spin-reorientation transition of thin Co films

Utilizing vector magnetometry by means of X-ray circular dichroism, the temperature-driven spin reorientation from an in-plane to out-of-plane orientation in thin Co films is studied in an epitaxially grown Au/Co/Au trilayer with decreasing temperature. Probing the remanent state by changing the photon helicity provides evidence for a smooth transition and the formation of a metastable magnetic multi-domain state at temperatures below the transition temperature.

Observation of excitation-energy-dependent Xe 4d5/2,3/2-1 lifetime widths

The creation and decay of photon-excited Xe 4d5/2,3/2-1 core-hole states has been studied in the photon energy region 70-110 eV, where photon absorption is dominated by the strong shape resonance. Unprecedented experimental quality has enabled the study of nearly inherent Auger electron lineshapes. Analysis on the basis of a post-collision interaction model allows the first ever experimental verification that the Xe 4d5/2,3/2-1 lifetime widths clearly show dynamic, photon-energy-dependent behaviour.

Magnetic X-ray Circular Dichroism on In Situ Grown 3d Magnetic Thin Films on Surfaces

Epitaxic thin and ultrathin films on surfaces allow crystallographic phases that do not occur naturally in the bulk to be stabilized. They also offer new possibilities for an improved understanding of soft X-ray photoabsorption in magnetic systems. Data collected using the Elliptically Polarizing Undulator at BL 5.2 of the Stanford Synchrotron Radiation Laboratory are presented herein. Fe, Co and Ni films were prepared on Cu(100) surfaces. L2,3-edge spectra were recorded with circular and linear light. Fresnel diffractometry was used to quantify the degree of transverse beam coherence. A quantitative analysis of the spectral features indicates a correlation of the spectral intensities and the transverse beam coherence. Resonant reflectivity spectra for Co ultrathin films that exhibit strong dichroism are presented. The reflectivity data indicate that interference effects of the reflected beams at the two interfaces are of importance, even for ultrathin films.

A comparative study of X-ray absorption spectroscopy at various synchrotron facilities and the effect of transverse source coherence

Abstract Core level X-ray Absorption Spectroscopy measurements have been performed at the L 3,2 edges of thin film Fe samples deposited on the (100) surface of Cu. Soft X-rays from five different beamlines have been used to measure spectra from identical samples with very different degrees of transverse coherence in the synchrotron radiation. The degree of source coherence has been characterised by diffraction. The transverse coherence length has been extracted and found to be on the μm length scale at certain beamlines. An increase of the L edge resonance intensities, relative to the continuum states, is observed in spectra measured at these beamlines.

Ultrathin Co films on flat and stepped Cu(111) surfaces: Determination of per atom orbital and spin moments

We have used the x-ray magnetic circular dichroism technique to characterize ultrathin Co films deposited on flat and vicinal Cu(111) single crystal surfaces. All films were prepared and characterized in situ, in ultrahigh vacuum. Similar to other investigators we report a 40% increase of the ratio m(l)/m(s) (orbital to spin moment) of ultrathin Co on both flat and vicinal Cu(111). In addition, an apparently correlated enhancement of both the orbital and spin moments is observed for Co on vicinal Cu(111). This finding is in contrast to what has earlier been reported for a similar system, Co on vicinal Cu(100)

Structure and magnetism on in-situ ultrathin epitaxial films : XMCD and EXAFS on Fe/Ag(100)

We have made in situ investigations of epitaxial ultrathin Fe films on Ag(100). This system is known to exhibit a reorientation of the magnetization, dependent of Fe thickness and temperature. Our aim here is to correlate the local structure with the magnetic properties of this system. The X-ray absorption fine structure has been recorded in situ both in the near edge as well as in the extended X-ray absorption fine structure (EXAFS) range. Using X-ray magnetic circular dichroism we find that, below 300 K, films between 2 and 6 monolayers show an out-of-plane remanent magnetization whereas for thicker films the magnetization lies in-plane. By making use of the Fe L-edge EXAFS we obtain information on the local structure around the Fe atoms, such as the interatomic distances, the coordination number, and the mean square relative displacement of the first neighbor shells

Quantitative analysis of L-edge white line intensities: the influence of saturation and transverse coherence

We have performed x-ray absorption spectroscopy at the Fe, Ni, and Co L2,3 edges of in situ grown thin magnetic films. We compare electron yield measurements performed at SSRL and BESSY-I. Differences in the L2,3 white line intensities are found for all three elements, comparing data from the two facilities. We propose a correlation between spectral intensities and the degree of spatial coherence of the exciting radiation. The electron yield saturation effects are stronger for light with a higher degree of spatial coherence. Therefore the observed, coherence related, intensity variations are due to an increase in the absorption coefficient, and not to secondary channel related effects.