No abstract available

[en] A unified approach combining polarized neutron and resonant x-ray magnetic reflectometry has been applied to determine the magnetic structure in an (Fe(35 (angstrom))/Gd(50 (angstrom)))₅ multilayer as a function of temperature and magnetic field. Simultaneous self-consistent refinement of neutron and x-ray data made it possible to resolve the element-specific magnetization profile in the multilayer with unprecedented accuracy. It is shown that the small number of bilayer periods together with the asymmetric termination (Fe-top, Gd-bottom) lead to unique low-temperature magnetic phases characterized by significant twisting of Fe and Gd magnetic moments and nonuniform distribution of vectorial magnetization within Gd layers. A twisted magnetic state was found to be stable at small magnetic fields and at a low temperature of 20 K, which is well below the compensation temperature of this artificial ferrimagnetic system.

[en] Pressure-induced changes in the local structure about Rb atoms in RbCl are investigated by means of x-ray-absorption fine-structure spectroscopy. Rb K-edge data were taken in fine steps across the pressure-induced NaCl-structure (B1)-to-CsCl-structure(B2)transition at 0.55 GPa. Previous diffraction studies of this phase transition indicate several martensiticlike properties, including the coexistence of both phases at 0.49 GPa. Our results are consistent with this coexistence of phases at the transition. In addition, we find no evidence for and intermediate phase

[en] Manganese ferrite (MnFe₂O₄) is a well-known magnetic material widely used in electronics for many years. It is well established that its magnetic behavior is strongly influenced by local structural properties of Mn ions, which are distributed between crystallographically inequivalent tetrahedral and octahedral sites in the unit cell. In order to understand and be able to tune properties of these structures, it is necessary to have detailed site-specific structural information on the system. Here we report on the application of diffraction-anomalous fine structure (DAFS) spectroscopy to resolve site-specific Mn local structures in manganese ferrite films. The DAFS measurements were done at undulator beamline 4-ID-D of the Advanced Photon Source at Argonne National Laboratory. The DAFS spectra (Fig. 1) were measured at several Bragg reflections in the vicinity of the Mn absorption K-edge, having probed separately contributions from tetrahedrally and octahedrally coordinated Mn sites. The DAFS data analysis done with an iterative Kramers-Kroenig algorithm made it possible to solve separately the local structure around different inequivalent Mn sites in the unit cell. The reliability of the data treatment was checked carefully, and it was showed that the site-specific structural parameters obtained with DAFS allow us to describe fluorescence EXAFS spectrum measured independently. Fig. 2 shows individual site contributions to the imaginary part of the resonant scattering amplitude obtained from the treatment of the data of Fig. 1. The analysis of the refined site-specific absorption spectra was done using EXAFS methods based on theoretical standards. We provided direct evidence for the tetrahedral Mn-O bond distance being increased relative to the corresponding Fe-O distance in bulk manganese ferrites. The first coordination shell number was found to be reduced significantly for Mn atoms at these sites. This finding is consistent with the well-known tendency of Mn to be tetrahedrally coordinated in these compounds.

[en] Polarized XAFS measurements on powder of La_2-xSr_xCuO₄ with aligned c-axes find that apical oxygens neighboring only the Sr dopants have a double-site distribution. The occupation of both sites totals one oxygen, is independent of temperature, but changes with x. A theory of the double-site is presented in terms of Jahn-Teller distortions (JT) induced by the extrinsic doped holes paired with the intrinsic holes present in the undoped χ = 0 sample. These pairs reside in singlet and triplet impurity states and satisfy Bose statistics. It is speculated that high T_c occurs because of Bose-condensation of the singlet pairs. Because of the strong electron-lattice interaction of the JT effect, XAFS can determine the corresponding electronic states in this special case. (au)

[en] Images of the magnetic domain structure in a 1600 Aa thick buried SmCo layer of a Fe/SmCo spring magnet were obtained using a newly developed x-ray microprobe. This probe combines circularly polarizing optics with a microfocusing Fresnel zone plate to obtain a highly polarized, small cross-section x-ray beam in the energy range between 5 and 10 keV. X-ray magnetic circular dichroism was used to provide contrast between different magnetic domains as a function of the externally applied magnetic field. The obtained images show domain walls not oriented parallel to the external field but correlated with structural features in the sample. A plausible explanation is pinning of domain walls at stacking faults in the layer. The range of external fields for which the magnetic reorientation of a particular microscopic domain occurred was much smaller than the range measured for a macroscopic hysteresis loop, indicating that the reorientation is due to growth of domains developed at local nucleation points. [copyright] 2001 American Institute of Physics

No abstract available

[en] XAFS and Moessbauer effect measurements were made on impurities that lower the bulk melting temperature, T_m of Pb and Ag. In addition XAFS measurements were made on T1 impurities that raise T_m, of Pb. The spectral intensity of the Moessbauer signal in Pb varies as expected from harmonic vibrations of the 1 at.% Sn Moessbauer atoms below a temperature of about 140K. Above this temperature the spectral intensity drops rapidly which is attributed to a localized hopping associated with a high entropy density. In Ag hosts a similar behavior occurs above 900K. The XAFS signal from Hg impurities in Pb, which like Sn lowers T_m, starts deviating from vibrational behavior above 400K where the apparent coordination number decreases. This is also interpreted as a localized hopping. However, T1, which raises T_m, shows the normal vibrational behavior in XAFS measurements in a Pb host. XAFS measurements on Ga impurities in Ag, which also lower T_m of bulk Ag, show only expected anharmonic behavior up to 900K. All of these results suggest a liquid-like high entropy region within a radius of somewhat more than an atomic spacing around impurities that lower T_m. These high entropy, or 'premelted', regions could act as nucleation sites for three dimensional melting and should dominate over both grain boundaries and free surfaces in nucleating bulk melting in large single crystals. (author)

[en] Recent discoveries in the study of magnetism using synchrotron radiation are helping to elucidate the complex magnetism of the actinides. In this article we discuss the use of resonant X-ray magnetic scattering to investigate multi-k structures and the existence of induced magnetic moments in UAs-USe solid solutions. We present magnetic scattering experiments conducted at the ID20 and XMaS beamlines at the ESRF, Grenoble, France, and the ID-1 SRI-CAT beamline at the APS, Argonne, USA

[en] X-ray magnetic circular dichroism (XMCD) and Small Angle Neutron Scattering (SANS) measurements were performed on thiol capped Au nanoparticles (NPs) embedded into polyethylene. An XMCD signal of 0.8 · 10^-4 was found at the Au L₃ edge of thiol capped Au NPs embedded in a polyethylene matrix for which Superconducting Quantum Interference Device (SQUID) magnetometry yielded a saturation magnetization, M_s, of 0.06 emu/g_Au. SANS measurements showed that the 3.2 nm average-diameter nanoparticles are 28% polydispersed, but no detectable SANS magnetic signal was found with the resolution and sensitivity accessible with the neutron experiment. A comparison with previous experiments carried out on Au NPs and multilayers, yield to different values between XMCD signals and magnetization measured by SQUID magnetometer. We discuss the origin of those differences