G. Krill

Sequence and symmetry of hole injection in YBa2Cu3O6+x in situ and ex situ experiments on powders and single crystals using X-ray absorption spectroscopy

Abstract Very accurate XAS in situ and ex situ measurements at the Cu K-edge on powders and single crystals of YBa2Cu3O6+x have been carried out using the time-resolved dispersive spectrometer at LURE-DCI. The whole range of oxygen content has been investigated and the x-dependence of the amount of Cu(I) has been evaluated at room and at high temperatures (from 400 to 750°C). Owing to the balance of charges, we derived the amount of the oxygen 2p holes. The sequence of injection was separated into steps, associated with three oxygen bands of distinct symmetries and with the well-known steps in the superconducting behavior.

Use of an asymmetric wiggler to study the magnetic circular x‐ray dichroism of Ni, Ho, and Tb

The asymmetric wiggler, inserted since the end of 1990 on the Super‐Aco storage ring in Orsay, has been built to produce a high rate and high flux of circularly polarized soft x rays in the 100–3000 eV range. This insertion device is particularly well suited to measure x‐ray absorption spectra corresponding to dipole transitions towards electronic orbitals that are directly responsible for magnetism (3d states of first row transition metals, 4f states of rare earths). In this article we report the first magnetic circular x‐ray dichroism spectra obtained with this insertion device. They exemplify the wide prospects of this experimental setup.

X-ray magnetic circular dichroism of vanadium thin films grown on Ag(100) and Fe(100)

Abstract In the last few years, magnetism at the surfaces and interfaces of transition metals has attracted considerable experimental and theoretical attention. The reduced symmetry and the lower coordination number offer the possibility of inducing new and exotic phenomena. In this context, vanadium has played a fundamental role since the prediction of a sizable magnetic moment in one or two vanadium monolayers on top of Ag(100) or Fe(100). We have investigated the magnetic properties of V deposited onto Fe(100) and Ag(100) by X-ray magnetic circular dichroism at the VL 2,3 absorption edges. No evidence of ferromagnetic ordering of the V layer (1–2 atomic layers) on silver was found for temperatures as low as 100 K. On the other hand, we observed room temperature antiferromagnetic alignment between V (1–25 atomic layers) and the iron substrate. The intensity of the dichroic signal decreases as the thickness of the V film increases. This dependence is discussed.

Hard X-rays magnetic EXAFS

Abstract The results of a systematic study of Magnetic EXAFS at K edges of transition metals (TM) and L edges of rare earth (RE) compounds are presented. At the K edges of TM the magnetic EXAFS is in phase with the normal EXAFS in all studied cases and its intensity is related to the variation of the mean magnetization. At L edges, magnetic EXAFS is not correlated with the complex magnetic dichroism observed at the edge. By studying the phase difference between the magnetic and normal EXAFS, we observe that there exists an interplay between the contribution of the central atom (giving a contribution to the phase in quadrature with normal EXAFS) and the magnetic neighbours (giving a contibution in phase). Multiple excitations are frequently observed in the spectra with a greater sensitivity than in normal EXAFS.

Investigation of uranium edges in by x-ray magnetic circular dichroism

We report a high statistic x-ray magnetic circular dichroism study of carried out at the uranium edges. Using the sum rules, the detailed analysis of our data yields orbital and spin moments of the 5f uranium electrons in reasonable agreement with values extracted from a localized moment approach.

1 ML Mn/Fe(001): Is Mn coupling with Fe antiferromagnetic or ferromagnetic?

The magnetic properties of Mn thin films deposited on (001) bcc iron were studied by soft X-ray magnetic circular dichroism (XMCD). The coupling between Mn and Fe at the interface is found to be ferromagnetic. This result is in contradiction with a similar recent study (Roth et al., Phys. Rev. B 52 (1995) 15691), where an antiferromagnetic coupling was observed. These contradictory results are explained to come from the different sample preparation used by both groups. This leads us to conclude that 1 ML Mn/bcc Fe(001) is ferromagnetic with a ferromagnetic with a ferromagnetic interfacial Mn/Fe coupling.

Palladium magnetism in Pd/Fe multilayers studied by XMCD at the PdL2,3 edges

Abstract We have performed direct measurements of the Pd4d orbital and spin moments in Pd/Fe multilayers with different Pd interlayer thicknesses using X-ray magnetic circular dichroism (XMCD) at the L2,3 absorption edges of Pd. The atoms directly at the interface are strongly polarized with a total moment of 0.4μB/atom. The orbital moments are found to be small (0.0–0.04μB). The thickness dependence of the XMCD shows that Pd atoms are magnetically polarized up to four layers from the interface.

Magnetic characteristics of Er(Mn12-xFex) compounds (x=7, 9) determined by X-ray magnetic circular dichroism

Abstract The RMn 12− x Fe x series of compounds of ThMn 12 structure type exhibit particularly complicated and composition sensitive magnetic properties. Previous neutron diffraction and magnetisation experiments have permitted to relate the main features to the strength of the local magnetic moments (3d polarisation) and the nature exchange forces between the Mn, Fe and R sublattices. X-ray magnetic circular dichroism analyses have been undertaken on iron rich compounds with R=Er. They allow to better understand the thermal and field behaviour of the local magnetic moments as well as the nature of the magnetic couplings between the 3d metals.

X-ray Magnetic Circular Dichroism study of vanadium in thin films (V/Ag(100) and V/Fe(100)) and alloys (Fe0.82V0.18/Au(111))

Abstract We have investigated the magnetic properties of V deposited onto Fe(100) and Ag(100) by X-ray magnetic circular dichroism (XMCD) at the V–L 2,3 absorption edges. No evidence of a ferromagnetic ordering of the V layer (1–2 ml) on silver was found for temperatures as low as 80 K. On the other hand, we have observed room-temperature antiparallel coupling between V (1–25 ml) and the iron substrate. The intensity of the dichroic signal decreases as the thickness of the V film is increased. The 3d magnetic moment in Fe 0.82 V 0.18 /Au(111) alloys was measured by the same method as a function of the thickness (3–6 ml). The magnitude of the V–L 2,3 dichroic signal normalised to the isotropic intensity is independent on the alloy layer thickness, indicating that the 3d magnetic moment per V atom remains constant. On the contrary, the number of 3d holes per V atom decreases considerably as the number of atomic layers of alloy film is reduced.

X-ray circular dichroism of thin Fe0.82V0.18 interlayers in Au(111)

Abstract We present a magnetic circular X-ray dichroism study performed at room temperature at the V- and Fe-L 2,3 absorption edges of the Au/Fe 0.82 V 0.18 /Au(111) system as a function of the thickness of the Fe V alloy layer (3 to 6 atomic layers). We find that the 3d magnetic moment localized on the iron atoms does not change significantly as a function of the thickness of the alloy film. Although this moment is strongly reduced with respect to the one of bulk iron, its decomposition in spin and orbital contribution remains the same. The dichroic signals obtained at the V-L 2,3 edges prove that the 3d electronic states of vanadium carry a magnetic moment aligned antiparallel to the one of iron. The magnitude of the V-L 2,3 dichroic signal normalized to the intensity of the edge is independent on the alloy layer thickness, indicating that the 3d magnetic moment per V atom remains constant.