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Register a new userResonant soft x-ray scattering from stepped surfaces of SrTiO3
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We studied the resonant diffraction signal from stepped surfaces of SrTiO3 at the Ti 2p -> 3d (L2,3) resonance in comparison with x-ray absorption (XAS) and specular reflectivity data. The steps on the surface form an artificial superstructure suited as a model system for resonant soft x-ray diffraction. A small step density on the surface is sufficient to produce a well defined diffraction peak, showing the high sensitivity of the method. At larger incidence angles, the resonant diffraction spectrum from the steps on the surface resembles the spectrum for specular reflectivity. Both deviate from the XAS data in the relative peak intensities and positions of the peak maxima. We determined the optical parameters of the sample across the resonance and found that the differences between the XAS and scattering spectra reflect the different quantities probed in the different signals. When recorded at low incidence or detection angles, XAS and specular reflectivity spectra are distorted by the changes of the angle of total reflection with energy. Also the step peak spectra, though less affected, show an energy shift of the peak maxima in grazing incidence geometry.
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We describe a strategy for using resonant soft x-ray scattering (RSXS) to study the electronic structure of transition metal oxide quantum wires. Using electron beam lithography and ion milling, we have produced periodic, patterned arrays of colossal magnetoresistance (CMR) phase La(1-x)Sr(x)MnO(3) consisting of ~ 5000 wires, each of which is 80 nm in width. The scattered intensity exhibits a series of peaks that can be interpreted as Bragg reflections from the periodic structure or, equivalently, diffraction orders from the grating-like structure. RSXS measurements at the Mn L(2,3) edge, which has a large magnetic cross section, show clear evidence for a magnetic superstructure with a commensurate period of five wires, which we interpret as commensurately modulated antiferromagnetism. This superstructure, which is accompanied by non-trivial reorganization of the magnetization within each wire, likely results from classical dipole interactions among the wires. We introduce a simple, exactly soluble, analytic model of the scattering that captures, semi-quantitatively, the primary features in the RSXS data; this model will act as a foundation for forthcoming, detailed studies of the magnetic structure in these systems.
A general formalism of X-ray scattering from different kinds of surface morphologies is described. Based on a description of the surface morphology at the atomic scale through the use of the paracrystal model and discrete distributions of distances, the scattered intensity by non-periodic surfaces is calculated over the whole reciprocal space. In one dimension, the scattered intensity by a vicinal surface, the two-level model, the N-level model, the faceted surface and the rough surface are addressed. In two dimensions, the previous results are generalized to the kinked vicinal surface, the two-level vicinal surface and the step meandering on a vicinal surface. The concept of crystal truncation rod is generalized considering also the truncation of a terrace by a step (yielding a terrace truncation rod) and a step by a kink (yielding a step truncation rod).
Resonant soft X-ray scattering was used to determine the presence of more subtle orderings not detected in standard structural analyses. By tuning to specific Co absorption edges, arrangements particular to the electronic states of those elements are enhanced. We have discovered an ordering commensurate to the lattice at the ($frac{1}{4}~frac{1}{4}~frac{1}{4}$) position. Incommensurate peaks near this position were also observed. The intensity of these peaks depends on the oxygen concentration of the sample, and can be suppressed at temperatures above 320 K. Regular orderings of charge density which closely match the underlying lattice may help to explain the observed propensity for SrCoO$_{3-y}$ (0 $leq$ y $leqfrac{1}{2}$) to stabilize at particular phases.
Soft x-ray resonant scattering has been used to examine the charge and magnetic interactions in the cycloidal antiferromagnetic compound dyfeal. By tuning to the Dy $M_4$ and $M_5$ absorption edges and the Fe L(2) and L(3) absorption edges we can directly observe the behavior of the Dy 4f and Fe 3d electron shells. Magnetic satellites surrounding the (110) Bragg peak were observed below 60 K. The diffraction peaks display a complex spectra at the Dy M(5) edge, indicative of a split 4f electron band. This is in contrast to a simple resonance observed at the Fe L(3) absorption edge, which probes the Fe 3d electron shell. Temperature dependant measurements detail the ordering of the magnetic moments on both the iron and the dysprosium antiferromagnetic cycloids. The ratio between the intensities of the Dy M(4) and M(5) remained constant throughout the temperature range, in contrast to a previous study conducted at the Dy L(2,3) edges. Our results demonstrate the ability of soft x-ray diffraction to separate the individual magnetic components in complicated multi-element magnetic structures.
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 Tb sites in multiferroic TbMnO$_3$. Large resonant enhancement 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 $leq$ T $leq$ 42 K) the Mn 3d sublattices 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.
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