Keisuke Tomiyasu

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.

Magnetic Structure of NiCr2O4 Studied by Neutron Scattering and Magnetization Measurements

The magnetic ordering of the normal spinel ferrimagnet NiCr 2 O 4 below T C = 74 K was reinvestigated by neutron scattering and magnetization measurements on a powder specimen. We found another magnetic transition at T S = 31 K besides T C in both experiments. The ordering of a ferrimagnetic (longitudinal) component and that of an antiferromagnetic (transverse) component occur at T C and T S , separately. A new magnetic structure model of NiCr 2 O 4 below T S with a spontaneous magnetization of about 0.3 µ B /formula is proposed based on experimental neutron scattering intensity.

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.

Resonance-Like Magnetic Excitations in Spinel Ferrimagnets FeCr2O4 and NiCr2O4 Observed by Neutron Scattering

We performed powder neutron scattering experiments on spinel ferrimagnets A Cr 2 O 4 with magnetic and Jahn–Teller A 2+ ions ( A = Fe, Ni, Cu). In both FeCr 2 O 4 and NiCr 2 O 4 , although geometric frustration had been expected to vanish so far, resonance-like magnetic excitation modes were discovered around Q ≃1.4 A -1 and E ≃5 and 8 to 9 meV in a low-temperature magnetic order phase, similar to the local spin resonance modes early reported in highly frustrated spinel antiferromagnets A Cr 2 O 4 with nonmagnetic and non-Jahn–Teller A 2+ ions ( A = Mg, Zn). In a high-temperature magnetic order phase the resonance-like inelastic scattering transformed into quasielastic scattering. In CuCr 2 O 4 no such diffusive scattering was observed. We interpret the resonance-like magnetic excitations as a dynamical spin-frustration effect, and discuss a necessary condition for the appearance of them.

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.

Emergence of highly degenerate excited states in the frustrated magnet MgCr2O4.

High degeneracy in ground states leads to the generation of exotic zero-energy modes, a representative example of which is the formation of molecular spin-liquid-like fluctuations in a frustrated magnet. Here we present single-crystal inelastic neutron scattering results for the frustrated magnet MgCr(2)O(4), which show that a common set of finite-energy molecular spin excitation modes is sustained in both the liquid-like phase above magnetic ordering temperature T(N) and an ordered phase with an extremely complex magnetic structure below T(N). Based on this finding, we propose the concept of high degeneracy in excited states, which promotes local resonant elementary excitations. This concept is expected to have ramifications on our understanding of excitations in many complex systems, including not only spin but also atomic liquids, complex order systems, and amorphous systems.

Magnetic Excitations in CoO

Magnetic excitations in CoO below T N =289 K were investigated on single crystals and a powder specimen by using inelastic neutron-scattering spectrometers installed at a steady state neutron-source and a pulsed neutron source. In single-crystal data, dispersion relations of the two magnetic excitation modes below 70 meV were determined along the symmetric directions. The magnetic excitations around 120 meV were also observed. Energy spectra on a powder specimen suggest the existence of higher order magnetic excitations spreading up to around 200 meV. The magnetic modes were analyzed in connection with the freedom of an unquenched orbital angular-momentum L of a Co 2+ ion. The magnitude of the spin–orbit interaction and that of the exchange interaction obtained from the analyses are comparable, indicating that those interactions must be simultaneously considered without an approximation of the total-angular-momentum. The lowest two dispersion curves are interpreted as mixing and splitting between a disper...

Spin-state responses to light impurity substitution in low-spin perovskite LaCoO 3

We studied the spin-state responses to light impurity doping in low-spin perovskite LaCoO

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

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Novel Elastic Instabilities within Antiferromagnetically Ordered Phase in the Orbitally-Frustrated Spinel GeCo2O4

(Co^3+: d^6) through magnetization and X-ray fluorescence measurements of single-crystal LaCo