K. Yamada

Emergence of Magnetic Long-range Order in Frustrated Pyrochlore Nd2Ir2O7 with Metal–Insulator Transition

In this study, we performed powder neutron diffraction and inelastic scattering measurements of frustrated pyrochlore Nd 2 Ir 2 O 7 , which exhibits a metal–insulator transition at a temperature T MI of 33 K. The diffraction measurements revealed that the pyrochlore has an antiferromagnetic long-range structure with propagation vector q 0 of (0,0,0) and that it grows with decreasing temperature below 15 K. This structure was analyzed to be of the all-in all-out type, consisting of highly anisotropic Nd 3+ magnetic moments of magnitude 2.3±0.4µ B , where µ B is the Bohr magneton. The inelastic scattering measurements revealed that the Kramers ground doublet of Nd 3+ splits below T MI . This suggests the appearance of a static internal magnetic field at the Nd sites, which probably originates from a magnetic order consisting of Ir 4+ magnetic moments. Here, we discuss a magnetic structure model for the Ir order and the relation of the order to the metal–insulator transition in terms of frustration.

High-energy spin and charge excitations in electron-doped copper oxide superconductors

The evolution of electronic (spin and charge) excitations upon carrier doping is an extremely important issue in superconducting layered cuprates and the knowledge of its asymmetry between electron- and hole-dopings is still fragmentary. Here we combine X-ray and neutron inelastic scattering measurements to track the doping dependence of both spin and charge excitations in electron-doped materials. Copper L3 resonant inelastic X-ray scattering spectra show that magnetic excitations shift to higher energy upon doping. Their dispersion becomes steeper near the magnetic zone centre and they deeply mix with charge excitations, indicating that electrons acquire a highly itinerant character in the doped metallic state. Moreover, above the magnetic excitations, an additional dispersing feature is observed near the Γ-point, and we ascribe it to particle-hole charge excitations. These properties are in stark contrast with the more localized spin excitations (paramagnons) recently observed in hole-doped compounds even at high doping levels.

Molecular spin resonance in the geometrically frustrated magnet MgCr2O4 by inelastic neutron scattering.

We measured two magnetic modes with finite and discrete energies in an antiferromagnetic ordered phase of a geometrically frustrated magnet MgCr2O4 by single-crystal inelastic neutron scattering, and clarified the spatial spin correlations of the two levels: one is an antiferromagnetic hexamer and the other is an antiferromagnetic heptamer. Since these correlation types are emblematic of quasielastic scattering with geometric frustration, our results indicate instantaneous suppression of lattice distortion in an ordered phase by spin-lattice coupling, probably also supported by orbital and charge. The common features in the two levels, intermolecular independence and discreteness of energy, suggest that the spin molecules are interpreted as quasiparticles (elementary excitations with energy quantum) of highly frustrated spins, in analogy with the Fermi liquid approximation.

Ni-Impurity Effects on Incommensurate Spin Correlations in the Superconducting Phase of La2-xSrxCuO4 near the Spin-Glass Phase Boundary

Neutron scattering experiments were carried out to explore Ni-impurity effects on static spin correlations in La 2- x Sr x CuO 4 in the vicinity of the superconductor-insulator boundary where both parallel and diagonal spin-density modulations (SDMs) coexist at low temperature. Dilute Ni substitution completely destroys the bulk superconductivity and alters well-defined incommensurate peaks of Ni-free SDMs into a single broad peak at (π,π). As the Ni concentration ( y ) reaches ∼ x , the magnetic diffuse scattering is transformed to an intense sharp Bragg peak, corresponding to a three-dimensional antiferromagnetic (AF) order in bulk. T N of such Ni-induced AF order depends on x and appears to disappear at x ∼0.1. A simplified cross section for the Q spectra taking into account the orthorhombic domain structure reproduces the Ni-impurity effect on the static spin correlations well, which results in a decreasing incommensurability of both types of SDMs. These effects are approximately ascribed by a reducti...

Magnetic Excitations in Infinite-Layer Antiferromagnetic Insulator

The magnetic excitations of an infinite-layer antiferromagnetic insulator SrFeO 2 were examined by powder inelastic neutron scattering to find dispersive magnetic excitations from ∼15 up to 63 meV. The scattering intensity distribution is well described by a spin wave model, confirming that the out-of-plane direct Fe…Fe exchange is comparable in strength to the in-plane Fe–O–Fe superexchange. SrFeO 2 shows an additional magnetic excitation mode around 30 meV, which is suggestive of an orbital magnon arising from small orbital moment on Fe 2+ brought about by spin–orbit coupling.

Charge Excitations in the Stripe-Ordered La5/3Sr1/3NiO4 and La2-x(Ba, Sr)xCuO4 Superconducting Compounds

S. Wakimoto, H. Kimura, K. Ishii, K. Ikeuchi, T. Adachi, M. Fujita, K. Kakurai, Y. Koike, J. Mizuki, Y. Noda, A. H. Said, Y. Shvyd’ko, and K. Yamada 7 1 Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan 2 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan 3 Synchrotron Radiation Research Unit, Japan Atomic Energy Agency, Hyogo 679-5148, Japan 4 Department of Applied Physics, Tohoku University, Sendai 980-8579, Japan 5 Institute for Materials Research, Tohoku University, Katahira, Sendai 980-8577, Japan 6 Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA 7 Advanced Institute for Materials Research, Katahira, Sendai 980-8577, Japan (Dated: June 20, 2008)

Incommensurate Spin Correlations Induced by Magnetic Fe ions Substituted into Overdoped Bi1.75Pb0.35Sr1.90CuO6+z

Spin correlations in the overdoped region of

Energy-dependent crossover from anisotropic to isotropic magnetic dispersion in lightly doped La1.96Sr0.04CuO4

{text{Bi}}_{1.75}{text{Pb}}_{0.35}{text{Sr}}_{1.90}{text{CuO}}_{6+z}

Hydrogen release from Li alanates originates in molecular lattice instability emerging at ∼100 K

have been explored with Fe-doped single crystals characterized by neutron scattering, muon-spin-rotation spectroscopy, and magnetic-susceptibility measurements. Static incommensurate spin correlations induced by the Fe spins are revealed by elastic neutron scattering. The resultant incommensurability

A New Growth Mechanism of Polar Nanoregions by Phonon-Relaxation Mode-Coupling in a Relaxor Ferroelectric

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