[Show abstract][Hide abstract] ABSTRACT: We have used a combination of resonant magnetic x-ray scattering (RMXS) and
x-ray absorption spectroscopy (XAS) to investigate the properties of the doped
spin-orbital Mott insulator Sr2Ir(1-x)Rh(x)O4 (0.07 < x < 0.70). We show that
Sr2Ir(1-x)Rh(x)O4 represents a unique model system for the study of dilute
magnetism in the presence of strong spin-orbit coupling, and provide evidence
of a doping-induced change in magnetic structure and a suppression of magnetic
order at x_c ~ 0.17. We demonstrate that Rh-doping introduces Rh3+/Ir5+ ions
which effectively hole-dope this material. We propose that the magnetic phase
diagram for this material can be understood in terms of a novel spin-orbital
[Show abstract][Hide abstract] ABSTRACT: We have performed resonant magnetic x-ray scattering (RMXS) measurements on
single crystal samples of the bilayer iridate Sr3Ir2O7. We observe the
development of antiferromagnetic order below T* ~ 260 K, which persists down to
T = 4 K under both field-cooled (fc) and zero-field-cooled (zfc) conditions.
The temperature dependence of the fc and zfc magnetic Bragg peaks suggests the
presence of competition between canted and collinear antiferromagnetic ground
states. Under zfc conditions, we observe a suppression of magnetic peak
intensity at T_D ~ 50 K which can be attributed to a spin reorientation
transition. Under fc conditions, we find that the canted antiferromagnetic
state is stabilized over a much wider range of temperatures, with significantly
enhanced magnetic correlation lengths within the Ir-O layers. The field
dependence of the magnetic Bragg peaks provides no evidence of field-induced
phase transitions for H < 4 T.
[Show abstract][Hide abstract] ABSTRACT: We report x-ray scattering and polarized infrared reflectivity
measurements of the substituted ferroborate
Below TN = 33 K, a new set of commensurate peaks with
resonant enhancements at the rare earth L edges indicates a doubling of
the magnetic structure along the c-axis and simultaneous ordering of the
rare earth and iron ions. Rare earth spin polarizations decrease rapidly
with increasing temperature, in contrast to that of the iron ions.
Shell-specific measurements of the rare earth spin polarizations
indicate similar behaviors of the Sm and Nd 5d states, while the Sm 4f
and 5d states have different temperature dependences. Along the c-axis
we observe negative thermal expansion below ˜75 K and a strong
phonon softening from room temperature down to TN, at which
it freezes in frequency. Also at TN we observe the appearance
of an electromagnon in the ab-plane that gets its spectral weight from
the lowest frequency phonon. These results indicate a lattice
instability linked to magnetism with a strong coupling between magnetic
and elastic properties.
[Show abstract][Hide abstract] ABSTRACT: Interest in RFe3(BO3)4 has been motivated by the magnetoelectric properties of R = Gd and Nd, in which field-induced polarization phases are induced at modest (
[Show abstract][Hide abstract] ABSTRACT: X-ray diffraction with photon energies near the Ru L2-absorption edge was used to detect resonant reflections characteristic of a G-type superstructure in RuSr2GdCu2O8 single crystals. A polarization analysis confirms that these reflections are due to magnetic order of Ru moments, and the azimuthal-angle dependence of the scattering amplitude reveals that the moments lie along a low-symmetry axis with substantial components parallel and perpendicular to the RuO2 layers. Complemented by susceptibility data and a symmetry analysis of the magnetic structure, these results reconcile many of the apparently contradictory findings reported in the literature.
[Show abstract][Hide abstract] ABSTRACT: Resonant x-ray diffraction at the L2- and L3-absorption edges of Ru has been used to investigate the magnetic structure of Ca3Ru2O7, a material with a bilayer perovskite structure that undergoes a transition from a high-temperature metallic to a low-temperature insulating phase at 48 K. In the insulating phase, magnetic Bragg reflections characteristic of A-type antiferromagnetic order (that is, ferromagnetic RuO2 bilayers coupled antiferromagnetically along the c-axis) were identified. The azimuthal-angle dependence of the diffracted intensity implies that the magnetic moments are aligned along the b-axis in the RuO2 planes. In the metallic phase, the A-type magnetic order persists up to the Néel temperature of 56 K, but the sublattice magnetization decreases by a factor of ∼1.7 and rotates by 90° within the planes. Resonant signals characteristic of uniform or staggered orbital order were not found within the experimental sensitivity, probably reflecting a weak orbital polarization in the insulating state.
[Show abstract][Hide abstract] ABSTRACT: We have carried out a detailed investigation of the magnetic field dependence of charge ordering in La2-xBaxCuO4 (x≈(1)/(8)) by utilizing high-resolution x-ray scattering. We find that the charge order correlation length increases as a magnetic field greater than ˜5T is applied in the superconducting phase (T=2K) . The observed unusual field dependence of the charge order correlation length suggests that the static charge stripe order competes with the superconducting ground state in this sample.
Physical Review B 05/2008; 77(18). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report X-ray scattering studies of the c-axis lattice parameter in Ca3Ru2O7 as a function of temperature and magnetic field, for B || the ground state easy (b) axis. The step-like, ∼0.1% change coincides in zero field with a spin reorientation and a metal–insulator transition, at Tm−i≈48 K, and shifts to a lower temperature upon application of magnetic field. At temperatures well below Tm−i, negligible change in the c-axis lattice parameter is observed at the metamagnetic transition to the spin-polarized phase, at Bc≈6 T, which indicates that this transition does not couple to the lattice degree of freedom.
Physica B Condensed Matter 04/2008; 403:1577-1578. · 1.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The rare earth iron borates have interesting magnetic properties due to the subtle interactions between the rare earth and the iron moments. Among these materials, GdFe3(BO3)4 has the most complex phase diagram as suggested by previous studies. [1,2] These studies suggest that iron moments order antiferromagnetically below TN˜36 K and that there are several additional magnetic phase transitions below TN. Yet whether and at what temperature the Gd moments order and the nature of the additional transitions, remain largely unknown. Using x-ray magnetic scattering, we have verified that the moments order antiferromagnetically with a propagation vector (0 0 3/2). Large resonant scattering enhancements at the Gd LII and LIII edges show unambiguously that Gd moments order at TN. Both resonant and nonresonant scattering data exhibit a splitting of the magnetic peak along c* above ˜ 10 K which indicates an incommensurate phase transition, with the incommensurability delta increasing continuously as a function of temperature (delta˜.0038 near TN). Use of the NSLS/BNL is supported by the U. S. DOE under Contract no. DE-AC02-98CH10886.  F. Yen et. al, PRB 73, 54435 (2006)  A. I. Pankrats et. al, JETP 99, 766 (2004)
[Show abstract][Hide abstract] ABSTRACT: The question of how bulk electronic order is terminated at a surface is an intriguing one, and one with possible practical implications--for example in nanoscaled systems that may be characterized by their surface behaviour. One example of such order is orbital order, and in principle it should be possible to probe the termination of this order with surface X-ray scattering. Here, we report the first observation of the scattering arising from the termination of bulk orbital order at the surface of a crystal--so-called 'orbital truncation rods'. The measurements, carried out on a cleaved perovskite, La(0.5)Sr(1.5)MnO(4), reveal that whereas the crystallographic surface is atomically smooth, the orbital 'surface', which is observed through the atomic displacements caused by the orbital order, is much rougher, with a typical scale of the surface roughness of approximately 7 degrees A. Interestingly, the temperature dependence of this scattering shows evidence of a surface-induced second-order transition.
Nature Material 01/2008; 6(12):972-6. · 36.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have carried out a detailed investigation of the magnetic field dependence of charge ordering in La2−xBaxCuO4 (x≈1/8) by utilizing high-resolution x-ray scattering. We find that the charge order correlation length increases as a magnetic field greater than ∼5 T is applied in the superconducting phase (T=2 K). The observed unusual field dependence of the charge order correlation length suggests that the static charge stripe order competes with the superconducting ground state in this sample.
[Show abstract][Hide abstract] ABSTRACT: We report x-ray scattering studies of the c-axis lattice parameter in Ca3Ru2O7 as a function of temperature and magnetic field. These structural studies complement published transport and magnetization data, and therefore elucidate the spin-charge-lattice coupling near the metal-insulator transition. Strong anisotropy of the structural change for field applied along orthogonal in-plane directions is observed. Competition between a spin-polarized phase that does not couple to the lattice and an antiferromagnetic metallic phase that does gives rise to a rich behavior for B‖b.
[Show abstract][Hide abstract] ABSTRACT: The top layer of a cleaved surface of the single layered manganite La0.5Sr1.5MnO4 is measured with crystal truncation rod scattering. Knowledge of the surface structure of strongly correlated electron systems is needed for nano-science and device application of such systems. The result shows that the cleaved surface is terminated by La/Sr layer and has little surface roughness.
Thin Solid Films 05/2007; 515(14):5741-5743. · 1.87 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Understanding the role of stripe physics in cuprate superconductors is believed to be essential in elucidating the superconducting mechanism of the cuprates. Despite the fundamental importance of charge ordering in the cuprates, a comprehensive examination of the relationship between charge stripes and superconductivity is still lacking. We have carried out a detailed investigation of temperature and magnetic field dependence of charge order in La7/8Ba1/8CuO4 utilizing high-resolution x-ray scattering. We find that the correlation length of the charge order exhibits unusual temperature and magnetic field dependence. Specifically, at zero field the correlation length decreases as the sample is cooled below ˜12K, while it increases as magnetic field is applied in the superconducting phase. These observations suggest that the size of the charge ordered region seems to be inversely correlated with superconductivity. This finding clearly shows that static charge order competes with the superconducting ground state, and supports the microscopic phase separation picture discussed by the recent muSR work.
[Show abstract][Hide abstract] ABSTRACT: Surface x-ray diffraction measurements were made on a (001) cleaved surface of the orbitally ordered layered manganite, La0.5Sr1.5MnO4 at X22C and X21 at the NSLS and at 6ID at the APS. The termination of crystal by a surface gives rise to rods of scattered intensity normal to that surface; so-called crystal truncation rods. The intensity distribution along such rods is sensitive to details of the surface order. In principle, similar rods should arise from the surface termination of orbital order. We have successfully observed such orbital truncation rods for the first time. We find that the intensity distribution of the orbital rod has a slightly steeper L dependence than that for the allowed Bragg rods, indicating that the surface roughness of the orbital ordering is larger than the crystallographic roughness. As the transition temperature is approached, this orbital roughness is seen to increase.
[Show abstract][Hide abstract] ABSTRACT: Resonant x-ray diffraction performed at the L(II) and L(III) absorption edges of Ru has been used to investigate the magnetic and orbital ordering in Ca2RuO4 single crystals. A large resonant enhancement due to electric dipole 2p-->4d transitions is observed at the wave-vector characteristic of antiferromagnetic ordering. Besides the previously known antiferromagnetic phase transition at T(N)=110 K, an additional phase transition, between two paramagnetic phases, is observed around 260 K. Based on the polarization and azimuthal angle dependence of the diffraction signal, this transition can be attributed to orbital ordering of the Ru t(2g) electrons. The propagation vector of the orbital order is inconsistent with some theoretical predictions for the orbital state of Ca2RuO4.
[Show abstract][Hide abstract] ABSTRACT: Orbital excitations have been sought in three systems, LaMnO3, KCuF3 and YTiO3, using high-resolution inelastic x-ray scattering, with an energy resolution of 6 meV. Motivated by the recent Raman scattering result of Saitoh et al (2001 Nature 410 180), we measured the energy transfer spectra of LaMnO3 in the energy range up to 200 meV. We did not find any signal above background that could be associated with orbiton excitations. Since significant interaction between the spin and orbital degrees of freedom is expected in KCuF3 and YTiO3, energy spectra were measured above and below the respective magnetic ordering temperature. We were not able to detect any change in the excitation spectra due to the magnetic ordering temperature of these materials. We discuss the implications of the experimental findings and estimate an upper bound on the orbiton x-ray cross-section.
New Journal of Physics 11/2004; 6(1):161. · 3.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report the characterization of the crystal structure, low-temperature charge and orbital ordering, transport and magnetization of Pr0.6Ca0.4MnO3 films grown on LaAlO3, NdGaO3 and SrTiO3 substrates, which provide compressive (LaAlO3) and tensile (NdGaO3 and SrTiO3) strain. The films are observed to exhibit different crystallographic symmetries from the bulk material and the low-temperature ordering is found to be more robust under compressive—as opposed to tensile—strain. In fact, bulk-like charge and orbital ordering is not observed in the film grown on NdGaO3, which is the substrate that provides the least amount of measured, but tensile, strain. This result suggests the importance of the role played by the Mn–O–Mn bond angles in the formation of charge and orbital ordering at low temperatures. Finally, in the film grown on LaAlO3, a connection between the lattice distortion associated with orbital ordering and the magnetization is reported.
[Show abstract][Hide abstract] ABSTRACT: We present the results of a systematic x-ray scattering study of the charge and orbital ordering in the manganite series Pr1-xCaxMnO3 with x=0.25, 0.4, and 0.5. The temperature dependence of the scattering at the charge and orbital wavevectors, and of the lattice constants, was characterized throughout the ordered phase of each sample. It was found that the charge- and orbital-order wavevectors are commensurate with the lattice, in striking contrast to the results of electron and neutron diffraction studies of samples with x=0.5. High-momentum-transfer resolution studies of the x=0.4 and 0.5 samples further revealed that while long-range charge order is present, long-range orbital order is never established. Above the charge and orbital ordering temperature T0, the charge-order fluctuations are more highly correlated than the orbital fluctuations. This suggests that charge order drives orbital order in these samples. In addition, a longitudinal modulation of the lattice with the same periodicity as the charge and orbital ordering was discovered in the x=0.4 and 0.5 samples. For x=0.25, only long-range orbital order was observed with no indication of charge ordering, nor of an additional lattice modulation. We also report the results of a preliminary investigation of the loss of charge and orbital ordering in the x=0.4 sample by application of a magnetic field. Finally, the polarization and azimuthal dependence of the charge and orbital ordering in these compounds are characterized both in the resonant and nonresonant limits, and compared with the predictions of current theories. The results are qualitatively consistent with both cluster and local density approximation +U calculations of the electronic structure.