A. Fontaine

EXAFS studies of the Ni66Y33 and Cu60Zr40 amorphous alloys: A modelling of the pair distribution function with two subshells

Abstract Disorder effects in amorphous Ni66Y33 and Cu60Zr40 alloys have been investigated by the EXAFS technique. EXAFS spectra have been fitted using a double pair distribution function for the TM-TM (transition metal) pairs and for the unlike atom pairs. Both alloys have been found to be chemically ordered. For Cu60Zr40 the amorphous structure is very similar to the crystalline one.

Magnetic properties of REFe2 (RE: Ce, Gd, Lu) compounds studied by magnetic X-ray dichroism

Magnetic X-ray dichroism (MXD) experiments have been performed on several REFe2 Laves phases compounds (RE: Ce, Gd, Lu), both at the K edge of iron and at the LIII edges of the rare-earth elements. In all compounds large spin-dependent absorption effects are observed, below the Curie temperatures, at both the K edges of iron and the LIII of the rare earth. In the case of CeFe2 the MXD signal observed at the Ce edges follows the same temperature dependence as the bulk magnetization and confirms that an ordered magnetic moment develops at the Ce sites in this mixed-valence compound. A special attention is given to the problem of final-state effects in the description of MXD experiments on cerium, particularly we show that the spin-dependent absorption cross-section reproduces the well-known double-bump structure, typical of cerium mixed-valence compounds.

Study of 5d magnetism in rare-earth-transition-metal (Fe, Co) intermetallic compounds by magnetic circular X-ray dichroism

Abstract Magnetic circular X-ray dichroism (MCXD) experiments have been performed at the L 2,3 of rare-earth (RE) elements in several Re-transition-metal (Fe, Co) compounds. Experiments at the L 2,3 edges give, in principle, direct information on the 5d magnetic properties of the RE atom which results from the hybridization between the 3d and the 5d electrons. We show in this paper that the use of MCXD experiments at the L 2,3 absorption edges seems to be really complicated when localized 4f electrons are present in the ground state. This unexpected situation is clearly demonstrated by comparing the results obtained on a cerium mixed-valent compound (CeFe 2 ), in which the 4f electrons are highly hybridized with the conduction electrons, and those obtained on a normal trivalent cerium (4f 1 ) compound (CeRu 2 Ge 2 ). Our results suggest that the 4f electrons play an essential role in the description of MCXD at the edges of rare earths.

Dispersive XAS at third-generation sources: strengths and limitations.

The experience of two years of operation of the ESRF Dispersive XAS beamline is reviewed. The examples of experiments performed show that the advent of third generation sources has given a new perspective to the field of X-ray absorption in the dispersive mode. The achievement of an aberration-free horizontal focal spot of- 20 ktm FWHM has opened up unique possibilities for performing X-ray absorption studies in extreme conditions of temperature and pressure. The implementation of quarter wave plates has allowed extremely accurate XMCD and nanosecond-resolved XMCD using the pump-probe scheme to be recorded. However, the reduction of source size combined to the lengthening of beamlines have introduced new problems related to the much higher sensitivity to beam instabilities amplified by phase contrast phenomena. Possibilities for the future have been investigated and shall be discussed.

Experimental evidence of pressure-induced magnetic phase transition in Fe72Pt28 Invar alloy

X-ray magnetic circular dichroism (XMCD) measurements have been performed under high pressure and used to study the disordered Fe72Pt28 Invar alloy in conjunction with x-ray diffraction. The high spin to low spin state transition is observed by both techniques and is compared with previous work. Furthermore, XMCD gives the first evidence of the complete disappearance of the magnetic moment at high pressure in this alloy. It also proves the existence of an hysteresis cycle due to the pressure-dependent magnetic moment.

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.

Structural and magnetic properties of Cu/Co and Au/Co multilayers

The structural and magnetic properties of Cu//Co and Au//Co multilayers are studied by X-ray absorption spectroscopy. The results of EXAFS measurements show that thin cobalt layers in Cu/Co and Au/Co multilayers have a different structure imposed by the mismatch with Cu (2%) and Au (14%). The most interesting results for Cu/Co multilayers are provided by magnetic circular X-ray dichroism (MCXD) measurements. The presence of an MCXD signal at the Cu K edge indicates the presence of a p-band moment on copper.

Redox Mechanisms and Density of Holes by XAS in the Compensated Series Bi2-xPbxSr2Ca1-xYxCu2O8+δ (0 ≤ x ≤ 1)

Bi[sub 2[minus]x]Pb[sub x]Sr[sub 2]Ca[sub 1[minus]x]Y[sub x]Cu[sub 2]O[sub 8+[delta]] (0 [le] x [le] 1) of structural type Bi(2212) is a very interesting solid solution since one is able to adjust the critical temperature over a large range of x by means of a mere formal exchange of charge between cations which do not belong to the essential [CuO[sub 2]][sub [infinity]] planes. X-ray absorption spectroscopy was performed at bismuth, lead, and copper L[sub 3]-edges on the Bi[sub 2[minus]x]Pb[sub x]Sr[sub 2]Ca[sub 1[minus]x]Y[sub x]Cu[sub 2]O[sub 8+[delta]] (0 [le] x [le] 1) solid solution in order to investigate the formal valence states of the elements and the correlation between hole density on the [CuO[sub 2]][sub [infinity]] planes and superconducting properties. It is shown that lead substitution does affect the role of the [BiO][sub [infinity]] planes which exchange electrons (holes) with the [CuO[sub 2]][sub [infinity]] planes. The decrease of the hole density in the [CuO[sub 2]][sub [infinity]] planes with increasing x has been correlated to the decrease of [Tc]s, whereas the rapid decrease of the diamagnetic volume for 0.3 [le] x [le] 0.7 has been understood on the basis of a segregation between bismuth- and lead-rich phases. 37 refs., 11 figs., 1 tab.

X ray absorption spectroscopy investigation of phase transition in Ge, GaAs and GaP

Abstract The phase transitions of GaAs, GaP and Ge under pressure have been investigated by x-ray absorption spectroscopy (XAS). At the onset of the transition the Debye-Waller factor increases and the x-ray absorption near edge structure (XANES) is progressively modified. A mixing of the low and high pressure phase can be determined by XAS as well as amorphization of the sample on pressure release.

Two recent developments in XMCD.

This paper reports on two new technical developments concerning sample environments for X-ray magnetic circular dichroism (XMCD). The first measurements under high pressures of up to 30 GPa are described. The difficulties of combining the techniques of high pressure and XMCD are commented on. The second development involves the use of a fast-switching magnetic field. A new superconducting device is used to perform XMCD measurements on paramagnetic compounds in magnetic fields of up to 6 T. The small amplitude of the XMCD signal imposes, for a given signal-to-noise ratio, a noise less than a few 10(-5). The signal-to-noise ratio is improved by the use of a series of acquisitions, switching the magnetic field between each acquisition. A very fast switching mechanism has been built based on mechanical rotation of a superconducting coil, with the sample kept in place inside the coil. The XMCD signals at the L(II,III)-edges of paramagnetic rare-earth compounds have been measured at 4.5 K in fields of up to 6 T with a switching time of 11 s.