F. Yakhou

ID20: a beamline for magnetic and resonant X-ray scattering investigations under extreme conditions.

A new experimental station at ESRF beamline ID20 is presented which allows magnetic and resonant X-ray scattering experiments in the energy range 3-25 keV to be performed under extreme conditions. High magnetic field up to 10 T, high pressure up to 30 kbar combined with low temperatures down to 1.5 K are available and experiments can be performed at the M-edges of actinide elements, L-edges of lanthanides and K-edges of transition metals.

Nature of the magnetic order and origin of induced ferroelectricity in TbMnO3

The magnetic structures which endow TbMnO(3) with its multiferroic properties have been reassessed on the basis of a comprehensive soft x-ray resonant scattering (XRS) study. The selectivity of XRS facilitated separation of the various contributions (Mn L(2) edge, Mn 3d moments; Tb M(4) edge, Tb 4f moments), while its variation with azimuth provided information on the moment direction of distinct Fourier components. When the data are combined with a detailed group theory analysis, a new picture emerges of the ferroelectric transition at 28 K. Instead of being driven by the transition from a collinear to a noncollinear magnetic structure, as has previously been supposed, it is shown to occur between two noncollinear structures.

Dynamics of long-wavelength phason fluctuations in the i-Al-Pd-Mn quasicrystal.

We report on the dynamics of phason modes in the i-Al–Pd–Mn icosahedral quasicrystal, measured between room temperature and 650°C, using the X-ray intensity fluctuation spectroscopy (XIFS) technique. Up to 500°C, the autocorrelation function, ℑ(q, t), displays almost no time evolution as expected for frozen-in phason fluctuations at low temperature. At higher temperatures, ℑ(q, t) follows a single exponential time decay from which the characteristic time τ c(q) is extracted. These results are compared to the expected shape of ℑ(q, t) as derived from the expressions of the eigenvalues and eigenvectors of the C ⊥ ⊥(q) phason dynamical matrix. In agreement with the hydrodynamic theory of quasicrystals, which predicts phasons with diffusive character, we find that τ c(q) varies linearly with q −2 at 650°C. The corresponding diffusion coefficient is 2.2(±0.5) × 10−18 m2 s−1 and the activation energy is estimated around 2.3(±1) eV.

Antiferromagnetically spin polarized oxygen observed in magnetoelectric TbMn2O5.

We report the direct measurement of antiferromagnetic spin polarization at the oxygen sites in the multiferroic TbMn2O5, through resonant soft x-ray magnetic scattering. This supports recent theoretical models suggesting that the oxygen spin polarization is key to the magnetoelectric coupling mechanism. The spin polarization is observed through a resonantly enhanced diffraction signal at the oxygen K edge at the commensurate antiferromagnetic wave vector. Using the fdmnes code we have accurately reproduced the experimental data. We have established that the resonance arises through the spin polarization on the oxygen sites hybridized with the square based pyramid Mn3+ ions. Furthermore we have discovered that the position of the Mn3+ ion directly influences the oxygen spin polarization.

Magnetic domain fluctuations observed by coherent X-ray scattering

Abstract Coherent X-ray-scattering measurements were performed in the type-I antiferromagnetic phase of UAs at the M IV uranium absorption edge. Speckle patterns from a magnetic reflection were followed up through the magnetic transition. The origin of the speckles is discussed in terms of antiferromagnetic domain configuration and fluctuations.

Holographic imaging of interlayer coupling in Co/Pt/NiFe

We present a method to determine the magnetic configuration of an in-plane magnetized permalloy layer using Fourier transform holography with extended references in an off-normal geometry. We use a narrow slit as an extended holographic reference to record holograms with the sample surface orthogonal to the incident x-ray beam, as well as rotated by 30° and 45° with respect to the beam. To demonstrate the sensitivity of the technique to in-plane magnetization, we present images of flux closed ground states in thin (∼50 nm) permalloy elements, less than 1 μm in lateral size. Images of the in-plane domain pattern which is magnetostatically imprinted into a permalloy film by the stray fields generated by an adjacent Co/Pt multilayer were obtained. It is found that, whilst the domain patterns within the two magnetic layers show a strong resemblance at remanence within a pristine sample, the similarities disappear after the sample is exposed to a saturating magnetic field.

Critical Reexamination of Resonant Soft X-Ray Bragg Forbidden Reflections in Magnetite

Magnetite, Fe{sub 3}O{sub 4}, displays a highly complex low-temperature crystal structure that may be charge and orbitally ordered. Many of the recent experimental claims of such ordering rely on resonant soft x-ray diffraction at the oxygen K and iron L edges. We have reexamined this system and undertaken soft x-ray diffraction experiments on a high-quality single crystal. Contrary to previous claims in the literature, we show that the intensity observed at the Bragg forbidden (001/2){sub c} reflection can be explained purely in terms of the low-temperature structural displacements around the resonant atoms. This does not necessarily mean that magnetite is not charge or orbitally ordered but rather that the present sensitivity of resonant soft x-ray experiments does not allow conclusive demonstration of such ordering.

Interplay between orbital and magnetic long range order by resonant X-ray scattering in (V1−xCrx)2O3☆

Abstract Resonant X-ray scattering experiments have been performed in 2.8% Cr-doped V 2 O 3 single crystal at the Vanadium K-photoabsorption edge. Using linear polarization analysis and comparing the angular dependence of scattered photons with structure factor calculations we can discriminate the nature of the different resonant X-ray processes involved in forbidden lattice reflections enhanced by resonances. We present an experimental method to extract information on local properties of edge-atom such as the anisotropy of the local atomic environment, the atomic magnetic moment orientation and orbital ordering.

Ordering of localized electronic states in multiferroic TbMnO3: a soft x-ray resonant scattering study

Soft x-ray resonant scattering (XRS) has been used to observe directly, for the first time, the ordering of localized electronic states on both the Mn and the Tb sites in multiferroic TbMnO3. Large resonant enhancements of the x-ray scattering cross-section were observed when the incident photon energy was tuned to either the Mn L or Tb M edges which provide information on the Mn 3d and Tb 4f electronic states, respectively. The temperature dependence of the XRS signal establishes, in a model independent way, that in the high-temperature phase (28 K≤T≤42 K) the Mn 3d sublattice displays long-range order. The Tb 4f sublattices are found to order only on entering the combined ferroelectric/magnetic state below 28 K. Our results are discussed with respect to recent hard XRS experiments (sensitive to spatially extended orbitals) and neutron scattering.

Magnetism in lanthanide superlattices

Abstract Neutron diffraction studies of heavy rare-earth superlattices have revealed the stabilization of novel magnetic phases that are not present in bulk materials. The most striking result is the propagation of the magnetic ordering through nonmagnetic spacer materials. Here we describe some recent X-ray magnetic resonant scattering studies of light rare-earth superlattices, which illuminate the mechanism of interlayer coupling, and provide access to different areas of Physics, such as the interplay between superconductivity and magnetism. Magnetic X-ray diffraction is found to be particularly well suited to the study of the modulated magnetic structures in superlattices, and provides unique information on the conduction-electron spin–density wave responsible for the propagation of magnetic order.