Masakazu Nishi

New Inorganic Spin-Peierls Compound NaV 2O 5 Evidenced by X-Ray and Neutron Scattering

In this letter we report lattice dimerization and spin-gap formation of NaV 2 O 5 below its spin-Peierls transition temperature T sp =35.3 K, in which there are V 4+ ( S =1/2) linear chains along the b axis of its orthorhombic structure. A single-crystal X-ray scattering experiment shows superlattice reflections with a lattice modulation vector q =(1/2, 1/2, 1/4) . Inelastic neutron scattering from a powder sample at T =7 K clearly demonstrates a gap energy of 9.8 meV at a scattering vector | Q |=1.0 A -1 corresponding to the reciprocal lattice point (1, 1/2, 0).

Direct evidence for the localized single-triplet excitations and the dispersive multitriplet excitations in SrCu2(BO3)2.

We performed inelastic neutron scattering on the 2D Shastry-Sutherland system SrCu2(11BO3)2 with an exact dimer ground state. Three energy levels at around 3, 5, and 9 meV were observed at 1.7 K. The lowest excitation at 3.0 meV is almost dispersionless with a bandwidth of 0.2 meV at most, showing a significant constraint on a single-triplet hopping owing to the orthogonality of the neighboring dimers. In contrast, the correlated two-triplet excitations at 5 meV exhibit a more dispersive behavior.

BaFeO3: A Ferromagnetic Iron Oxide

Magnetic attraction: The cubic perovskite BaFeO(3) (see picture, Ba blue, Fe brown, O white), which is obtained by a low-temperature reaction using ozone as an oxidant, exhibits ferromagnetism with a fairly large moment of 3.5 μ(B) per Fe ion above a small critical field of approximately 0.3 T. This specific ferromagnetism is attributed to the enhancement of O→Fe charge transfer that arises from deepening of the Fe(4+) d levels.

CaFeO2: a new type of layered structure with iron in a distorted square planar coordination.

CaFeO(2), a material exhibiting an unprecedented layered structure containing 3d(6) iron in a high-spin distorted square-planar coordination, is reported. The new phase, obtained through a low-temperature reduction procedure using calcium hydride, has been characterized through powder neutron diffraction, synchrotron X-ray diffraction, Mossbauer spectroscopy, XAS experiments as well as first-principles DFT calculations. The XAS spectra near the Fe-K edge for the whole solid solution (Sr(1-x)Ca(x))FeO(2) supports that iron is in a square-planar coordination for 0 </= x </= 0.8 but clearly suggests a change of coordination for x = 1. The new structure contains infinite FeO(2) layers in which the FeO(4) units unprecedentedly distort from square-planar toward tetrahedra and rotate along the c-axis, in marked contrast to the well-studied and accepted concept that octahedral rotation in perovskite oxides occurs but the octahedral shape is kept almost regular. The new phase exhibits high-spin configuration and G-type antiferromagnetic ordering as in SrFeO(2). However, the distortion of the FeO(2) layers leads to only a slight decrease of the Neel temperature with respect to SrFeO(2). First-principles DFT calculations provide a clear rationalization of the structural and physical observations for CaFeO(2) and highlight how the nature of the cation influences the structural details of the AFeO(2) family of compounds (A = Ca, Sr, Ba). On the basis of these calculations the driving force for the distortion of the FeO(2) layers in CaFeO(2) is discussed.

Strong antiferromagnetic spin fluctuations and the quantum spin-liquid state in geometrically frustrated -Mn, and the transition to a spin-glass state caused by non-magnetic impurity

The magnetic properties of -Mn and the effect of the substitution of non-magnetic Al were systematically investigated by measuring thermodynamic and transport properties, and the NMR and polarized neutron scattering for alloys with -Mn is paramagnetic down to the lowest temperature (1.4 K) but with strong spin fluctuations. Substitution of Al atoms which preferentially occupy one of two crystallographic sites (site II) results in the formation of local moments on Mn atoms, and makes the ground state into a spin-glass-like state. NMR measurements revealed almost independent magnetic behaviour of the two different sites, and static magnetic ordering of Mn at site II for , although the possibility of very weak magnetism of the site-I atoms cannot be excluded. The temperature dependence of the spin - lattice relaxation rate indicates antiferromagnetic correlations of paramagnetic spin fluctuations, and its saturation at higher temperatures for and a critical slowing down of the fluctuations at low temperatures for . Polarized neutron scattering experiments showed directly strong spin fluctuations with antiferromagnetic correlations for both -Mn and . The characteristic energy of fluctuations for -Mn is fairly large even at 7 K , implying a quantum origin of the fluctuations, while the energy spectrum of becomes very sharp at low temperatures, within a resolution-limited energy width, indicating damping of spin fluctuations into a spin-glass-like state. These results can be interpreted in terms of the transition from a spin liquid to a spin glass caused by the substitution of Al that gives rise to the release of the antiferromagnetic frustration of the characteristic crystal lattice. Considerable similarity of the characteristic features to those of the highly frustrated Laves phase compounds is argued. The possible frustration on site II in the -Mn structure, a three-dimensional network of corner-sharing regular triangles, which is similar to the two-dimensional Kagome lattice, is pointed out.

Dzyaloshinski-Moriya Interaction in the 2D Spin Gap System SrCu2(BO3)2

The Dzyaloshinski-Moriya interaction partially lifts the magnetic frustration of the spin-1/2 oxide SrCu(2)(BO(3))(2). It explains the fine structure of the excited triplet state and its unusual magnetic field dependence, as observed in previous ESR and new neutron inelastic scattering experiments. We claim that it is mainly responsible for the dispersion. We propose also a new mechanism for the observed ESR transitions forbidden by standard selection rules, which relies on an instantaneous Dzyaloshinski-Moriya interaction induced by spin-phonon couplings.

Electron paramagnetic resonance governed by the Dzyaloshinsky - Moriya antisymmetric exchange interaction in

Several reports of electron paramagnetic resonance experiments performed on the spin - Peierls compound have appeared so far, but none of them have precisely explained the origin of the line broadening and the temperature dependence of both the linewidth and the resonance field, all of which differ from those of conventional one-dimensional Heisenberg antiferromagnets in which the dipolar or anisotropic exchange interaction brings about line broadening. In the present report, it is clarified that the antisymmetric exchange interaction, with -axis (magnetic chain), between nearest-neighbour Cu spins on the c-axis governs all factors which characterize the EPR line of , i.e., the value and the angular dependence of the linewidth at high temperatures where the short-range order is completely absent, the temperature dependence of the linewidth, and the resonance field, as well as the high-temperature lineshape. The present conclusion indicates that the crystal symmetry of this compound is lower than that given by the space group Pbmm which was reported by Vollenkle et al in 1967, because the symmetry Pbmm does not allow the antisymmetric exchange interaction mentioned above.

Study of Spin-Gap Formation in Quasi-Two-Dimensional S= 1/2 System CaV 4O 9: Neutron Scattering and NMR

Neutron inelastic scattering study has been performed on aligned single crystals of quasi-two-dimensional spin-gap system CaV 4 O 9 . By analyzing the dispersion curves and the structure factor of the triplet excitation, a physical picture of the present spin system has been constructed: The plaquette singlet units of four S =1/2 V-atoms within four “corner-linked” VO 5 pyramids form the two-dimensional linkage of the spins by the interactions between the V-spins within the “edge-linked” VO 5 pyramids. We have remeasured the NMR relaxation rates and obtained the spin-gap behavior consistent with the neutron results.

Direct Observation of Orbital Ordering in YTiO3 by Means of the Polarized Neutron Diffraction Technique

The orbital ordering in the ferromagnetic YTiO 3 ( T C ≃30 K) has been observed by means of the polarized neutron diffraction technique. The observed magnetic form factors obtained from the aspheri...

Spin dynamics in NaV2O5 : Inelastic neutron scattering

Inelastic neutron scattering measurement of NaV 2 O 5 single crystal samples was carried out to measure their magnetic excitations in the spin-Peierls dimerized state. We found a steep dispersion along the b * -axis (chain direction), having a spin-Peierls gap of almost 10 meV at (3, 0.5, 0). Along the a * -axis (interchain direction), two excitation modes with a weak dispersion have been observed. In addition, a novel intensity modulation has been discovered along this direction, which has not been expected from the crystal structure reported so far.Inelastic neutron scattering measurement of NaV 2 O 5 single crystal samples was carried out to measure their magnetic excitations in the spin-Peierls dimerized state. We found a steep dispersion along the b*-axis (chain direction), having a spin-Peierls gap of almost 10 meV at (3, 0.5, 0). Along the a*-axis (interchain direction), two excitation modes with a weak dispersion have been observed. In addition, a novel intensity modulation has been discovered along this direction, which has not been expected from the crystal structure reported so far.