P. D. Spencer

Direct observation of orbital ordering in La0.5Sr1.5MnO4 using soft X-ray diffraction

We report the first direct resonant soft x-ray scattering observations of orbital ordering. We have studied the low temperature phase of La0.5Sr1.5MnO4, a compound that displays charge and orbital ordering. Previous claims of orbital ordering in such materials have relied on observations at the manganese K edge. These claims have been questioned in several theoretical studies. Instead we have employed resonant soft x-ray scattering at the manganese L(III) and L(II) edges which probes the orbital ordering directly. Energy scans at constant wave vector are compared to theoretical predictions and suggest that at all temperatures there are two separate contributions to the scattering: direct orbital ordering and strong cooperative Jahn-Teller distortions of the Mn3+ ions.

Orbital bi-stripes in highly doped bilayer manganites.

We present high-resolution high-energy and resonant x-ray-diffraction results from La{sub 2-2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} for x=0.55, 0.575, and 0.60. These compounds show superlattice reflections at wave vectors of (h{+-}{delta},k{+-}{delta},l) and (h{+-}2{delta},k{+-}2{delta},l), arising from orbital ordering with associated Jahn-Teller distortions and charge ordering, respectively. We observe a phase transition boundary between the x=0.55 and x=0.575 doping levels. Samples with x=0.55 display structural characteristics similar to those previously reported for x=0.5. Compared to this the long-range order in samples with x=0.55 and x=0.6 have a distinct change in wave-vector and correlation length. We attribute this to a new orbital bi-stripe phase, accompanied by weak, frustrated, charge ordering. The observed azimuthal dependence of the orbital order reflection supports the model proposed for this new phase.

Soft x-ray diffraction study of magnetic ordering in holmium

We have conducted the first single-crystal resonant soft x-ray diffraction study of the magnetic order in holmium at the Ho M V edge. Both the (0, 0, τ) and (0, 0, 2r) satellite reflections can be accessed at the M V edge. Energy scans through these reflections display very different line shapes. The behaviour of the integrated intensity and wavevector modulation was similar to that observed in earlier neutron and resonant x-ray diffraction studies at the L edges, which showed that valid qualitative results can be obtained using the soft x-ray diffraction technique. The sample absorption at the M V edge is considerable, reducing the penetration depth to approximately 100 A.

Novel diffractometer optimized for the study of weak superlattice reflections using crossed parabolic mirrors

We present the development of a novel laboratory-based diffractometer optimized for high intensity and resolution matched to that of flux grown single crystal transition metal–oxide samples studied in solid state physics. This has been implemented using crossed graded d-spacing parabolic multilayer mirrors, a severely off-cut asymmetric analyzer crystal, and a microminiature cryostat. We demonstrate that the wider bandpass of the multilayer mirrors provides a significant increase in intensity compared to the use of silicon and germanium optics, but still provides the necessary resolution to obtain accurate measurements for inverse correlation lengths. The increase in flux allows the observation of features that were previously only visible with third generation synchrotron x-ray sources, shown by the observation of the very weak charge order peak without the use of a synchrotron source. Results on samples previously studied show an increase of a factor of 10 in intensity, coupled together with a factor of...

High energy x-ray scattering studies of strongly correlated oxides

Many transition metal oxides display strongly correlated charge, spin, or orbital ordering resulting in varied phenomena such as colossal magnetoresistance, high temperature superconductivity, metal-insulator transitions etc. X-ray scattering is one of the principle techniques for probing the structural response to such effects. In this paper, we discuss and review the use of synchrotron radiation high energy x-rays (50-200 keV) for the study of transition metal oxides such as nickelates (La 2-x Sr x NiO 4 ) and manganites (La 2-2x Sr 1+2x Mn 2 O 7 ). High energy x-rays have sufficient penetration to allow us to study large flux-grown single crystals. The huge increase in sample scattering volume means that extremely weak peaks can be observed. This allows us to study very weak charge ordering. Measurements of the intensity, width and position of the charge ordering satellites as a function of temperature provide us with quantitative measures of the charge amplitude, inverse correlation length and wavevector of the charge ordering.

Direct Observation of Orbital Ordering inLa0.5Sr1.5MnO4Using Soft X-ray Diffraction

We report the first direct resonant soft x-ray scattering observations of orbital ordering. We have studied the low temperature phase of La0.5Sr1.5MnO4, a compound that displays charge and orbital ordering. Previous claims of orbital ordering in such materials have relied on observations at the manganese K edge. These claims have been questioned in several theoretical studies. Instead we have employed resonant soft x-ray scattering at the manganese L(III) and L(II) edges which probes the orbital ordering directly. Energy scans at constant wave vector are compared to theoretical predictions and suggest that at all temperatures there are two separate contributions to the scattering: direct orbital ordering and strong cooperative Jahn-Teller distortions of the Mn3+ ions.

Direct observation of orbital ordering in La

We report the first direct resonant soft x-ray scattering observations of orbital ordering. We have studied the low temperature phase of La0.5Sr1.5MnO4, a compound that displays charge and orbital ordering. Previous claims of orbital ordering in such materials have relied on observations at the manganese K edge. These claims have been questioned in several theoretical studies. Instead we have employed resonant soft x-ray scattering at the manganese L(III) and L(II) edges which probes the orbital ordering directly. Energy scans at constant wave vector are compared to theoretical predictions and suggest that at all temperatures there are two separate contributions to the scattering: direct orbital ordering and strong cooperative Jahn-Teller distortions of the Mn3+ ions.

_{0.5}

Abstract The interplay between charge, spin and orbital degrees of freedom in transition metal oxides has been a matter of much interest in recent years. In this paper we present X-ray scattering results of charge and orbital ordering in the electron doped manganite Bi 0.24 Ca 0.76 MnO 3 . Using High Energy X-ray Scattering (HEXS) the charge ordering, structural modulation due to the Jahn–Teller ordering and the harmonic of the structural modulation have been measured. The superlattice reflections due to the Jahn–Teller ordering are found to maximise in intensity at the Neel point. We have confirmed using resonant X-ray scattering at the K-edge of manganese and the anisotropic tensor of susceptibility technique (ATS) that superlattice reflections occurring at a modulation wave vector of (0.5, 0, 0) are due to charge ordering in a Mn 3+ /Mn 4+ ordering pattern. In addition superlattice reflections corresponding to the orbital order were measured at a modulation wave vector of (0.25, 0, 0). From the temperature dependence of the orbital and charge order it appears that the orbital order is driven by the charge order.

Sr

The interplay between charge, spin and orbital degrees of freedom in transition metal oxides has been a matter of much interest in recent years. In this paper we present X-ray scattering results of charge and orbital ordering in the electron doped manganite Bi0.24Ca0.76MnO3. Using High Energy X-ray Scattering (HEXS) the charge ordering, structural modulation due to the Jahn–Teller ordering and the harmonic of the structural modulation have been measured. The superlattice reflections due to the Jahn–Teller ordering are found to maximise in intensity at the Neel point. We have confirmed using resonant X-ray scattering at the K-edge of manganese and the Anisotropic Tensor of Susceptibility technique (ATS) that superlattice reflections occurring at a modulation wavevector of (0.5, 0, 0) are due to charge ordering in a Mn3+/Mn4+ ordering pattern. In addition superlattice reflections corresponding to the orbital order were measured at a modulation wavevector of (0.25, 0, 0). The temperature dependence of both the Jahn–Teller structural distortion peak (1.75, 2, 0) and the charge ordering peak (3 0 0) were observed to behave identically, gradually increasing with decreasing temperature.

_{1.5}

The La2-xSrxNiO4 system is isostructural with the high TC superconducting cuprate La2-xSrxCuO4 and is a prototypical system for the understanding of strongly correlated electron-phonon coupling, and the resultant effects on material properties. X-ray scattering studies have been performed on La5/3Sr1/3NiO4 that demonstrate the two-dimensional nature of these charge stripes. Such studies, demonstrate the very high correlation length of the stripes (~ 2000 A) at low temperatures. We have undertaken a series of experiments measuring the wavevector and charge stripe correlation length on a variety of crystals with the compositions La2-xSrxNiO4 (x=0.20, 0.25, 0.275, 0.30 and 0.33) using ~10 keV X-rays. The results demonstrate that for x=0.275, and above, the charge stripes are highly correlated in a well-ordered crystalline lattice. Measurements of the incommensurability, e, as a function of temperature for the series revealed that it is commensurate and temperature independent for the x=0.33 sample. For other compositions it is incommensurate and also temperature dependent. However for the x=0.20 and 0.25 crystals a much reduced correlation length was observed suggestive of a charge stripe glass. However, such experiments are sensitive to such charge ordering only in the near (top few micron) surface region. High energy X-rays however can probe the charge stripe ordering within the bulk of the single crystal by utilising the dramatic increase in penetration depth. We have used 130 keV X-rays and demonstrate that in La5/3Sr1/3NiO4 the charge stripes are far less correlated in the bulk than in the near surface region. This reduced correlation length (~300 A), consistent with neutron scattering measurements, is indicative of a charge stripe glass, reminiscent of that observed below x=0.25, in the near surface region.