Jonathan Lang

We combine x-ray magnetic circular dichroism spectroscopy at Fe L{sub 2,3} edges, at Eu M{sub 4,5} edges, x-ray absorption spectroscopy (XAS) investigation of Eu valence, and local spin density calculations, to show that the filled skutterudite Eu{sub 0.95}FeSb is a ferrimagnet in which the Fe 3d moment and the Eu{sup 2+} 4f moment are magnetically ordered with dominant antiferromagnetic coupling. From Eu L edge XAS, we find that about 13% of the Eu have a formal valence of 3{sup +}. We ascribe the origin of ferrimagnetism at a relatively high transition temperature T{sub c} of 85 K in Eu{sub 0.95}FeSb to f-electron interaction with the nearly ferromagnetic [FeSb]{sup 2.2-} host lattice.

We present a portable pulsed-magnet system for x-ray studies of materials in high magnetic fields (up to 30 T). The apparatus consists of a split-pair of minicoils cooled on a closed-cycle cryostat, which is used for x-ray diffraction studies with applied field normal to the scattering plane. A second independent closed-cycle cryostat is used for cooling the sample to near liquid helium temperatures. Pulsed magnetic fields (∼1 ms in total duration) are generated by discharging a configurable capacitor bank into the magnet coils. Time-resolved scattering data are collected using a combination of a fast single-photon counting detector, a multichannel scaler, and a high-resolution digital storage oscilloscope. The capabilities of this instrument are used to study a geometrically frustrated system revealing strong magnetostrictive effects in the spin-liquid state.

SrCu2(BO3)2 is a quasi-2D quantum spin system known to possess a collective singlet ground state. It serves as an experimental realization of the Shastry-Sutherland model for interacting S=1/2 dimers. The ratio of the intra and inter-dimer exchange in this compound is close to a quantum critical point, where the ground state transforms from a gapped, non-magnetic state to a gapless long-range ordered antiferromagnetic state as a function of the ratio of the strength of the magnetic interactions. We use synchrotron x-ray diffraction in a diamond anvil cell to investigate the pressure-driven quantum phase transition in high-quality single crystals of SrCu2(BO3). We will present the evolution of both the magnetic and structural properties up to pressures of 5 GPa.

The temperature dependence (T=10-300K) of circular magnetic x-ray dichroism at the rare earth L3 edge of REFe2 compounds (RE=Ho-Lu) has been studied. The magnetization of the rare earth 4f and 5d states has been separately determined by measuring the magnitude of the quadrupolar and dipolar features in the dichroic spectra. The 5d moment is found to scale with the RE 4f moment in all compounds except Lu, in which it scales with the Fe moment. An explanation of this and other observed features will be discussed. fˇill footnoterule ^*Work at the Advanced Photon Source is supported by USDOE-BES under contract No. W-31-109-ENG-38. Ames Laboratory is operated for USDOE by Iowa State Univ. under contract No. W-7405-ENG-82.

We have observed a Fermi-surface (FS) induced lattice modulation in a YBa2Cu3O7-x superconductor with a wave vector along CuO chains; i.e., q1=(0,δ,0) . The value of δ˜0.21 is twice the Fermi wave vector (2kF) along b∗ connecting nearly nested FS “ridges.” The q1 modulation exists only within O-vacancy-ordered islands [characterized by q0=((1)/(4),0,0) ] and persists well above and below Tc . Our results are consistent with the presence of a FS-induced charge-density wave.

The magnetic phase diagram of ferrimagnetic Gd/Fe multilayers has long been known to exhibit collinear and twisted magnetic configurations that result from a delicate balance between inter/intra-layer exchange and Zeeman energies. At low applied fields exchange dominates and collinear structures are favored, the majority component (Gd) aligning with the field while the minority component (Fe) is constrained anti-parallel by interlayer exchange. At high applied fields a twisted phase arises when it becomes advantageous to reduce Zeeman energy losses in the anti-parallel minority component by a twist which reduces its projected moment along the field direction. It has also been proposed(LePage and Camley, Phys. Rev. Lett. 65), 1152 (1990) that when the multilayer is terminated with the minority component a transition into a ``surface'' twisted phase occurs at a lower field than that of its ``bulk'' twist counterpart. Using x-ray magnetic circular dichroism (XMCD) we have, for the first time, unambiguously determined the existence of this surface twisted phase.

Circular magnetic x-ray dichroism (CMXD) has been used to probe the magnetic properties of rare earth compounds. This study has been undertaken in two distinct parts. First, the angular dependence of the CMXD signal has been examined in order to determine the multiple nature of the transitions involved. Second, the nature of the dichroic spectra of amorphous and crystalline GdFe_2 has been probed. CMXD offers a unique probe of the magnetic properties of amorphous materials providing information that is not available from any other technique. Previous CMXD measurements at the L edges of rare earth materials have demonstrated two distinct features, one above and one below the Fermi energy. Theoretical calculations have ascribed the two features to dipolar and quadrupolar transitions to the empty 5d and 4f states respectively. This study sought to determine the multipole nature of the transitions involved by measuring the angular dependence of the CMXD signal. Two experiments attempting to observe this angular dependence were undertaken, one at the L_3 edge of Er in Er _2Fe_{14}B, and another at the L_2 and L _3 edges of grain oriented REFe_2 compounds. All features in the CMXD spectra, within experimental error, were found to be consistent with dipolar transitions. We discuss some possible reasons for the apparent absence of the predicted quadrupolar angular dependence of the features below E_{rm f }.. The spin dependent absorption of circularly polarized x-rays at the K edge of Fe and the L_2 and L_3 edges Gd in amorphous and crystalline GdFe_2 has also been studied. Large differences in the magnitude of the dichroic signal are observed between the two samples. The application of recently derived sum rules indicated substantial quenching of the orbital moment in the amorphous sample. The results are compared to a theoretical spectrum for crystalline GdFe_2.

Using high-energy synchrotron x-ray scattering experiments, we present clear evidence of lattice modulations in twinned and detwinned single crystals of optimally doped YBa_2Cu_3O_6.92 superconductor. Diffuse superlattice peaks associated with a real-space priodicity of four unit cells (4a) along the shorter Cu-O-Cu bond direction are found at positions q_0=(1/4,0,0) (in units of frac2πa where a is the lattice parameter) away from the Bragg points in the momentum space. We observe the q0 peaks at room temperature, which grow significantly stronger well into the superconducting state. Furthermore, our observations of interplanar correlations unambiguously show that modulations occurring on the CuO2 planes are coupled to those on the CuO chains and BaO planes, respectively. Intriguingly, q0 is the same as the 4a periodicity observed previously in the local density of states near vortex cores and is the wave vector where discontinuity in phonon dispersions was observed in other high-temperature superconductors, respectively. In addition, the appearance of anisotropic diffuse scattering around Bragg points is indicative of the coexistence of lattice strain effects in this material.

The occurrence of a remarkably high Curie temperature ( ~ 400 K) in ferrimagnetic vanadium-tetracyanoethylene (V[TCNE]_x, x ~ 2) molecular magnet is of great interest. Understanding the chemical structure of this material is the first step towards solving the more difficult problem of magnetic ordering. Since samples grown as thin films are amorphous, the x-ray absorption fine structure (XAFS) technique is ideal for deriving the local structure around V-ions. Temperature-dependent XAFS measurements at the vanadium K-edge indicate a coordination of 4.1(4) nitrogen atoms at an average distance < R >=2.08(1) ÅThe V-N bonding is very strong with a characteristic Einstein temperature above 1000 K. This leads to a narrow distribution of V-N nearest neighbor distances with an rms variation of sigma ~ 0.03 ÅThe absorption data also indicates lack of centro-symmetry at the V-ion site, and preliminary evidence suggests a valence state for V-ion higher than 2.

Superstructures characterized by q=(qx, 0, 0) are observed throughout the phase diagram of yttrium-barium cuprates (YBa2Cu3O6+x, YBCO); q decreases with doping from 12 (2-unit-cell) in the heavily un-derdoped compound to 15 in the overdoped material. A 4-unit-cell superstucture is stable in the vicinity of optimal doping. The superstructures in YBCO correspond to short-range ordered regions of coupled atomic displacements on neighboring CuO, BaO, and CuO2 planes, respectively. T-dependent measurements suggest that these ``nanodomains'' experience anharmonic thermal motion. These regions induce a long-range strain in the host, which manifests as ``bow-tie''-shape Huang diffuse scattering below ˜200 K. X- ray diffuse scattering results will be presented within the context of the oxygen ordering and the phase diagram.

The electronic structure and magnetism of Ir 5d5 states in nonmetallic, weakly ferromagnetic BaIrO3 are probed with x-ray absorption techniques. Contrary to expectation, the Ir 5d orbital moment is found to be ˜1.5 times larger than the spin moment. This unusual, atomiclike nature of the 5d moment is driven by a strong spin-orbit interaction in heavy Ir ions, as confirmed by the nonstatistical large branching ratio at Ir L2,3 absorption edges. As a consequence, orbital interactions cannot be neglected when addressing the nature of magnetic ordering in BaIrO3. The local moment behavior persists even as the metallic-paramagnetic phase boundary is approached with Sr doping or applied pressure.

We report a temperature-dependent increase below ~220 K of diffuse superlattice peaks corresponding to q0=(~2/5,0,0) in an underdoped YBa2Cu3O6+x superconductor (x~0.63). These peaks reveal strong c-axis correlations involving the CuO2 bilayers, show a nonuniform increase below ~220 K with a plateau for ~100-160 K, and appear to saturate in the superconducting phase. We propose that this temperature dependence of the superlattice peaks is a possible manifestation of charge stripes in the CuO2 planes which are coupled to the oxygen-ordered superstructure.

We investigate the electronic and magnetic properties of high pressure cobalt using X-ray emission spectroscopy (XES) and magnetic circular dichroism (MCD) measurements in a diamond anvil cell. We ascribe the changes in the line-shape of the Kbeta emission as signature of an electronic (spin) transition, similar to those reported in iron and manganese compounds. We further interpret the observed pressure-induced decrease in the MCD signal as evidence of a gradual loss of magnetic order in high-density cobalt. Experiments were conducted at the Advanced Photon Source in Argonne, IL. The work at LLNL was performed under the auspices of the U.S. Department of Energy by University of California, under Contract W-7405-Eng-48.

We used magnetic diffraction anomalous near-edge structure (MDANES) to study element-specific and site-specific magnetism in materials; MDANES involves measuring the asymmetry in diffracted intensity for opposite helicities of circularly polarized x-rays. We have applied this technique to magnetite (Fe_3O_4). Without polarization analysis of the scattered beam, this technique measures the projection of site-specific and element-specific magnetic moments in the scattering

The multiferroic material HoMnO3 displays electrical polarization Pc = 5.6 muC cm-2 along the hexagonal c axis below the Curie temperature TC = 875 K and antiferromagnetic Mn^3+ ordering at the N&#39;eel temperature, TN = 75 K. The recently reported ferromagnetic response of Ho^3+ by an applied electric field opens up the possibility of electric field controlled magnetic data storage.

Dramatic changes in the Er L 2 and L 3 XMCD spectra in Er 2 Fe 17 as a function of temperature have been investigated with detailed experiments and first principles calculations. This study seeks to understand the fundamental mechanisms governing the spectral shape and ...

The magnetic phase diagram of ferrimagnetic Gd/Fe multilayers has long been known to exhibit collinear and twisted magnetic configurations that result from a delicate balance between inter/intra-layer exchange and Zeeman energies. At low applied fields exchange dominates and collinear structures are favored, the majority component (Gd) aligning with the field while the minority component (Fe) is constrained anti-parallel by interlayer

We have studied the inerplay of electronic instabilities and oxygen-vacancy ordering in underdoped YBCO superconductor using high-energy x-ray-diffraction techniques. We present a temperature-dependent increase below 300 K of diffuse superlattice peaks corresponding to q_0=( ~2/5,0,0), hitherto associated with oxygen-vacancy ordering, in an under-doped YBa_2Cu_3O_6+x superconductor (x&amp;ap;0.63). These peaks reveal strong c-axis correlations involving the CuO2 bilayers, show a non-uniform increase below ~220 K with a plateau for ~100-160 K, and appear to saturate in the superconducting phase. We interpret this unconventional T-dependence of the ``oxygen-ordering&#x27;&#x27; peaks as a manifestation of a charge density wave in the CuO2 planes coupled to the oxygen-vacancy ordering.

We report on comprehensive quantitative analyses of x-ray diffuse scattering studies of nanoscale inhomogeneities in the optimally doped YBCO superconductor. In addition to previously studied ${\bf q}_0=\left(\frac{1}{4},0,0\right)$ superstructure due to oxygen vacancy ordered ...

Using xray diffraction we studied in detail the crystal structure of a 100 nm thick La$_{0.7}$ Sr$_{0.3 }$MnO$_{3}$ film grown on a SrTiO$_{3}$ substrate. Satellite peaks are observed at ($H \quad \pm \quad \delta h$, $K$, $L)$ for nonzero $K$ and ($H$, $K \quad \pm \quad \ ...

Bulk measurements reveal disorder-induced unconventional quantum critical behaviors in (Sr1-xCax)3Ru2O7 (SCRO) compounds, in particular, near x = 0.3. Here we report X-ray scattering studies on SCRO with x =0.3, as well as those for the end members. We find that at x = 0.3 robust 2-unit-cell periodic lattice modulations exist that are characterized by (12,0,0) and (0,12,0), respectively, even at room temperature. These modulations are transversely polarized and quasi-3D ordered in that they are fully coherent in the basal plane with c axis correlations at least one unit cell in extent. These modulations are due to correlated displacements of the O atoms. The displacement pattern is consistent with t2g-modes of distortion of RuO6 octahedra, signifying the presence of lattice and orbital correlations, although no long-range magnetic or orbital order is present. These modulations are absent in the end members.

Pulsed magnets have emerged as a viable approach at synchrotron x-ray facilities for studying materials in high magnetic fields. We are developing a new high-field (30 Tesla) pulsed magnet system for single-crystal x-ray diffraction studies. It consists of a single 18mm-bore solenoid, designed and built at Tohoku University using high-tensile-strength and high conductivity CuAg wires. A dual-cryostat scheme has been developed at Advanced Photon Source in order to cool the coil using liquid nitrogen and the sample using a closed-cycle cryostat independently. Liquid nitrogen cooling allows repetition rate of a few minutes for peak fields near 30 Tesla. This scheme is unique in that it allows the applied magnetic field to be parallel to the scattering plane. Time-resolved scattering data are typically collected using a fast one-dimensional strip detector. Opportunities and challenges for experiments and instrumentation will be discussed.

We address the question of possible coexistence between weak ferromagnetism (W-FM) and superconductivity (SC) in the Ru-1222 rutheno-cuprate layered structure using element-specific x-ray magnetic circular dichroism (XMCD) and x-ray absorption fine structure (XAFS) measurements. XMCD probes Ru magnetization independently from the paramagnetic contributions of rare-earth ions and XAFS is ideally suited for detection of nano-sized impurities that may go undetected in diffraction measurements. We report the presence of a significant zero-field FM component (0.21 muB/Ru) associated with Ru ions in the Ru-1222 lattice. The results, together with bulk susceptibility and resistivity measurements, imply by necessity coexistence of W-FM and SC at the atomic level in this rutheno-cuprate structure [see N. M. Souza-Neto et al., Phys. Rev. B 80, R140414 (2009).]