Spin-lattice order in frustrated ZnCr2O4

Using synchrotron X-rays and neutron diffraction we disentangle spin-lattice order in highly frustrated ZnCr$_2$O$_4$ where magnetic chromium ions occupy the vertices of regular tetrahedra. Upon cooling below 12.5 K the quandary of anti-aligning spins surrounding the triangular faces of tetrahedra is resolved by establishing weak interactions on each triangle through an intricate lattice distortion. The resulting spin order is however, not simply a N\'{e}el state on strong bonds. A complex co-planar spin structure indicates that antisymmetric and/or further neighbor exchange interactions also play a role as ZnCr$_2$O$_4$ resolves conflicting magnetic interactions

Orbital and spin chains in ZnV2O4

Our powder inelastic neutron scattering data indicate that \zvo is a system of spin chains that are three dimensionally tangled in the cubic phase above 50 K due to randomly occupied $t_{2g}$ orbitals of V$^{3+}$ ($3d^2$) ions. Below 50 K in the tetragonal phase, the chains become straight due to antiferro-orbital ordering. This is evidenced by the characteristic wave vector dependence of the magnetic structure factor that changes from symmetric to asymmetric at the cubic-to-tetragonal transition

Electron-hole and plasmon excitations in 3d transition metals: Ab initio calculations and inelastic x-ray scattering measurements

We report extensive all-electron time-dependent density-functional calculations and nonresonant inelastic x-ray scattering measurements of the dynamical structure factor of 3d transition metals. For small wave vectors, a plasmon peak is observed which is well described by our calculations. At large wave vectors, both theory and experiment exhibit characteristic low-energy electron-hole excitations of d character which correlate with the presence of d bands below and above the Fermi level. Our calculations, which have been carried out in the random-phase and adiabatic local-density approximations, are found to be in remarkable agreement with the measured dynamical structure factor of Sc and Cr at energies below the semicore onset energy (M-edge) of these materials.Comment: To appear in Phys. Rev.

X-Ray Studies of Phonon Softening in TiSe 2

The charge-density-wave transition in TiSe 2 , which results in a commensurate ͑2 3 2 3 2͒ superlattice at temperatures below ϳ200 K, presumably involves softening of a zone-boundary phonon mode. For the first time, this phonon-softening behavior has been examined over a wide temperature range by synchrotron x-ray thermal diffuse scattering

Antiferromagnetism and Its Relation to the Superconducting Phases of UPt3

Using magnetic x-ray and neutron diffraction in UPt3, we find that a suppression of the antiferromagnetic scattering intensity in the superconducting phase is due to a reduction in the magnitude of the staggered moment with no change in symmetry. The existence of the suppression as well as the magnetic correlation lengths are not affected by the presence or absence of a visible splitting in the superconducting transition. The simplest models wherein antiferromagnetic order provides the symmetry-breaking field for the splitting do not provide a compete explanation of our results

Charge transport and magnetization profile at the interface between a correlated metal and an antiferromagnetic insulator

A combination of spectroscopic probes was used to develop a detailed experimental description of the transport and magnetic properties of superlattices composed of the paramagnetic metal CaRuO$_3$ and the antiferromagnetic insulator CaMnO$_3$. The charge carrier density and Ru valence state in the superlattices are not significantly different from those of bulk CaRuO$_3$. The small charge transfer across the interface implied by these observations confirms predictions derived from density functional calculations. However, a ferromagnetic polarization due to canted Mn spins penetrates 3-4 unit cells into CaMnO$_3$, far exceeding the corresponding predictions. The discrepancy may indicate the formation of magnetic polarons at the interface.Comment: 4 pages, 3 figure

Direct measurement of antiferromagnetic domain fluctuations

Measurements of magnetic noise emanating from ferromagnets due to domain motion were first carried out nearly 100 years ago and have underpinned much science and technology. Antiferromagnets, which carry no net external magnetic dipole moment, yet have a periodic arrangement of the electron spins extending over macroscopic distances, should also display magnetic noise, but this must be sampled at spatial wavelengths of order several interatomic spacings, rather than the macroscopic scales characteristic of ferromagnets. Here we present the first direct measurement of the fluctuations in the nanometre-scale spin- (charge-) density wave superstructure associated with antiferromagnetism in elemental Chromium. The technique used is X-ray Photon Correlation Spectroscopy, where coherent x-ray diffraction produces a speckle pattern that serves as a "fingerprint" of a particular magnetic domain configuration. The temporal evolution of the patterns corresponds to domain walls advancing and retreating over micron distances. While the domain wall motion is thermally activated at temperatures above 100K, it is not so at lower temperatures, and indeed has a rate which saturates at a finite value - consistent with quantum fluctuations - on cooling below 40K. Our work is important because it provides an important new measurement tool for antiferromagnetic domain engineering as well as revealing a fundamental new fact about spin dynamics in the simplest antiferromagnet.Comment: 19 pages, 4 figure

Static displacements of atoms in crystalline metallic solid solutions

Measurement of diffuse x-ray scattering from binary crystalline solid solutions at more than one x-ray energy is a sensitive probe of the average interatomic distance between neighboring pairs of atoms. Deviations of these bond distances by as little as 0.001 {angstrom} are easily detected in the diffuse scattering from binary alloys consisting of atoms nearby in the periodic table and/or where x-ray energies chosen near their absorption edges can change the scattering contrast. Individual bond distances (AA, AB and BB) are recovered out to several near neighbor shells by combining three scattering measurements at x-ray energies chosen for maximum and minimum contrast. Average bond distances recovered from four metal alloys Fe{sub 77.5}Ni{sub 22.5}, Fe{sub 46.5}Ni{sub 53.5}, Cr{sub 47}Fe{sub 53} and Cr{sub 20}Ni{sub 80} are discussed. The authors find that the interatomic distances measured for these alloys give interesting insights to atomic displacements in solid solutions. Nearest neighbor bond distances are not well represented by phenomenological models based on pure element atomic size and concentration dependence of the lattice constants. Radial and nonradial displacements are recovered from these measurements of diffuse scattering

Large crystal local-field effects in the dynamical structure factor of rutile TiO2

We present ab initio time-dependent density-functional calculations and nonresonant inelastic x-ray scattering measurements of the dynamical structure factor of rutile TiO2. Our calculations are in good agreement with experiment and prove the presence of large crystal local-field effects below the Ti M edge, which yield a sharp loss peak at 14 eV whose intensity features a remarkable nonmonotonic dependence on the wave vector. These effects, which impact the excitation spectra in the oxide more dramatically than in transition metals, provide a signature of the underlying electronic structure.I.G.G. and J.M.P. acknowledge partial support by the Basque Unibertsitate, Hezkuntza eta Ikerketa Saila, the UPV/EHU, and the MCyT. W.K. acknowledges support from the U.S. DOE under Contract No. DE-AC02-98CH10886. A.G.E. acknowledges support from NSF ITR DMR0219332. ORNL research sponsored by the DOE, Office of Science, DMS under contract with UT-Battelle, LLC; the UNICAT beamline supported by the FS-MRL, ORNL, NIST, and UOP Res.; the Advanced Photon Source (APS) supported by the DOE.Peer reviewe