J. C. Lang

Electronic structure and magnetism in compressed 3d transition metals

The authors present a systematic study of high-pressure effects on electronic structure and magnetism in 3d transition metals (Fe, Co, and Ni) based on x-ray magnetic circular dichroism measurements. The data show that the net magnetic moment in Fe vanishes above 18GPa upon the transition to hcp Fe, while both cobalt and nickel remain ferromagnetic to well over 100GPa. The authors estimate the total disappearance of moment in hcp Co at around 150GPa and predict a nonmagnetic Ni phase above 250GPa. The present data suggest that the suppression of ferromagnetism in Fe, Co, and Ni is due to pressure-induced broadening of the 3d valence bands.

New high temperature furnace for structure refinement by powder diffraction in controlled atmospheres using synchrotron radiation

A low thermal gradient furnace design is described which utilizes Debye–Scherrer geometry for performing high temperature x-ray powder diffraction with synchrotron radiation at medium and high energies (35–100 keV). The furnace has a maximum operating temperature of 1800 K with a variety of atmospheres including oxidizing, inert, and reducing. The capability for sample rotation, to ensure powder averaging, has been built into the design without compromising thermal stability or atmosphere control. The ability to perform high-resolution Rietveld refinement on data obtained at high temperatures has been demonstrated, and data collected on standard Al2O3 powder is presented. Time-resolved data on the orthorhombic to rhombohedral solid state phase transformation of SrCO3 is demonstrated using image plates. Rietveld refinable spectra, collected in as little as 8 s, opens the possibility of performing time-resolved structural refinements of phase transformations.

A unique polarized x-ray facility at the advanced photon source

To use the unique element-specific nature of polarized x-ray techniques to study a wide variety of problems related to magnetic materials, we have developed a dual-branch sector that simultaneously provides both hard and soft x-ray capabilities. This facility, which is located in sector 4, is equipped with two different insertion devices providing photons in both the intermediate (0.5–3 keV) and hard x-ray regions (3–100 keV). This facility is designed to allow the simultaneous branching of two undulator beams generated in the same straight section of the ring.

Bragg transmission phase plates for the production of circularly polarized x rays

The x‐ray optics for a thin‐crystal Si (400) Bragg transmission phase plate have been constructed for the production of 5 to 12 keV circularly polarized x rays. Using multiple beam diffraction from a GaAs crystal, a direct measurement of the degree of circular polarization as a function of off‐Bragg position was made. These measurements indicated nearly complete circular polarization (‖Pc‖≥0.95) and full helicity reversal on opposite sides of the rocking curve.

Magnetically Driven Metal-Insulator Transition in NaOsO3

The metal-insulator transition (MIT) is one of the most dramatic manifestations of electron correlations in materials. Various mechanisms producing MITs have been extensively considered, including the Mott (electron localization via Coulomb repulsion), Anderson (localization via disorder), and Peierls (localization via distortion of a periodic one-dimensional lattice) mechanisms. One additional route to a MIT proposed by Slater, in which long-range magnetic order in a three dimensional system drives the MIT, has received relatively little attention. Using neutron and x-ray scattering we show that the MIT in NaOsO(3) is coincident with the onset of long-range commensurate three dimensional magnetic order. While candidate materials have been suggested, our experimental methodology allows the first definitive demonstration of the long predicted Slater MIT.

Evidence for Ligand-Induced Paramagnetism in CdSe Quantum Dots

We report evidence that paramagnetism in CdSe QDs can be induced via manipulation of the surface chemistry. Using SQUID magnetometry and X-ray absorption spectroscopy, we demonstrate that the paramagnetic behavior of the CdSe QDs can be varied by changing the ligand end-group functionality of the passivating layer. Contrary to previous reports, no evidence for ferromagnetism was observed. The results suggest that the paramagnetism is induced via pi back-bonding between Cd 4d orbtials and ligands with empty pi* orbitals.

Performance of the advanced photon source 1-BM beamline optics

Bending magnet beamlines at third-generation synchrotron sources combined with well-designed optics offer unique capabilities for providing high x-ray fluxes into relatively small focal spots. This article provides a description of the x-ray optics used in the Advanced Photon Source 1-BM beamline. The performance of these optics in terms of the delivered flux ( 9×1011 ph/s/100 mA at 10 keV), energy resolution [ΔE/E≈1.5×10−4 with Si(111)], and focusing properties (spot size ≈0.25×0.60 mm) is compared with that expected from ideally reflecting and shaped optics.

Orbital ordering transition in Ca2RuO4 observed with resonant x-ray diffraction

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.

Nature of Ho magnetism in multiferroic HoMnO3.

Using x-ray resonant magnetic scattering and x-ray magnetic circular dichroism, techniques that are element specific, we have elucidated the role of Ho3+ in multiferroic HoMnO3. In zero field, Ho3+ orders antiferromagnetically with moments aligned along the hexagonal c direction below 40 K, and undergoes a transition to another magnetic structure below 4.5 K. In applied electric fields of up to 1 x 10(7) V/m, the magnetic structure of Ho3+ remains unchanged.

Imaging spiral magnetic domains in Ho metal using circularly polarized Bragg diffraction

We have used circularly polarized x rays to image the spiral magnetic domain structure in a single crystal of Ho metal. In these structures, the magnetization direction rotates between successive atomic layers forming a helix. At magnetic Bragg diffraction peaks, circularly polarized x rays are sensitive to the handedness of such a helix (i.e., either right or left handed). By reversing the helicity of the incident beam with phase-retarding optics and measuring the difference in the Bragg scattering, contrast between domains of opposing handedness can be obtained.