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THz electron spin resonance (ESR) measurements of the spinel compound GeCo2O4, which consists of the Co2+ pyrochlore structure, were performed. The temperature dependence measurements revealed new antiferromagnetic phases, AF1 and AF2, below TN. Moreover, a wide magnetic field range of ESR study using a single crystal at 1.8 K also revealed various field-induced phases due to the competition between the spin–lattice coupling and the spin frustration. Critical field resonances were observed at 5.0 T, 8.6 T, and 11.0 T for B//[111] and 5.1 T, 7.7 T, 11.2 T, and 13.0 T for B//[110] at 1.8 K. Although it became difficult to observe ESR above 86 K, the g-values of Co2+ ions were estimated to be g[111] = 3.34 and g[110] = 3.27 for [111] and [110], respectively, from the observed ESR mode above the saturation field at 1.8 K. Detailed frequency-field diagrams of the ESR modes at 1.8 K suggested the existence of spin–lattice coupling energy of 250 GHz (= 1.03 meV = 12 K) at the critical field resonances. The spin gap mode in the frequency-field diagram at 1.8 K showed excitation energies of E[111] = 1004 GHz (= 4.15 meV = 48 K) and E[110] = 1044 GHz (= 4.32 meV = 50 K) for [111] and [110], respectively. These results will be discussed in connection with the di-tetramer model suggested previously.

It is shown that the 2p3d resonant inelastic X-ray scattering intensity is distorted by saturation and self-absorption effects, i.e. by incident-energy-dependent saturation and by emission-energy-dependent self-absorption., Angle-dependent 2p3d resonant inelastic X-ray scattering spectra of a LaCoO3 single crystal and a 55 nm LaCoO3 film on a SrTiO3 substrate are presented. Theoretical calculation shows that, with ∼20 meV resolved Co 2p3d resonant inelastic X-ray scattering (RIXS), the excited states of the isotropic 1A1g(Oh) ground state are split by 3d spin–orbit coupling, which can be distinguished via their angular dependence. However, strong self-absorption and saturation effects distort the spectra of the LaCoO3 single crystal and limit the observation of small angular dependence. In contrast, the RIXS on 55 nm LaCoO3 shows less self-absorption effects and preserves the angular dependence of the excited states.

We have measured the electrical resistivity and the thermopower of the electron-doped perovskite cobaltites LaCo$_{1-y}$Te$_y$O$_3$. In contrast to the hole-doped systems such as metallic ferromagnets La$_{1-x}M_x$CoO$_3$ ($M$ = Ca, Sr, Ba), the electron-doped samples show an insulating behavior even in a heavily doped range due to a spin-state blockade mechanism that an electron hopping from high-spin Co$^{2+}$ to low-spin Co$^{3+}$ site is energetically suppressed. We find that, despite the electron doping, the thermopower shows relatively large positive values above $y=0.05$, strikingly distinct from the hole-doped case where it comes close to zero with doping. This prominent electron-hole asymmetry seen in the thermopower originates from a bipolar conduction which consists of a slight amount of mobile holes and the main immobile electrons, demonstrating an impact of spin-state blockade on thermoelectric transport., Comment: 6 pages, 4 figures

We report experimental and theoretical evidence that Rb$_2$Cu$_2$Mo$_3$O$_{12}$ has a nonmagnetic tetramer ground state of a two-leg ladder comprising antiferromagnetically coupled frustrated spin-$1/2$ chains and exhibits a Haldane spin gap of emergent spin-1 pairs. Three spin excitations split from the spin-1 triplet by a Dzyaloshinskii-Moriya interaction are identified in inelastic neutron-scattering and electron spin resonance spectra. A tiny magnetic field generates ferroelectricity without closing the spin gap, indicating a novel class of ferroelectricity induced by a vector spin chirality order., Comment: main text (6 pages, 5 figures) + supplemental material (10 pages, 8 figures), to appear in Phys. Rev. B (R)

Herein, NASICON-type Li1.15Y0.15Zr1.85P3O12 glass ceramics (a-LYZP) were synthesized from amorphous materials. Compared with samples prepared by the solid-state reaction (s-LYZP), a-LYZP showed a higher total ionic conductivity of 7.0 × 10−5 S cm−1 at 298 K and improved inter-particle contact. The structure of this glass ceramic effectively reduced the resistance of the Li1.15Y0.15Zr1.85P3O12. In the temperature range 263–298 K, the activation energy of a-LYZP was 0.43 eV, which was smaller than that for s-LYZP (0.63 eV). Rietveld analysis of XRD patterns suggested that the longer a-axis and larger unit lattice volume of a-LYZP prevented phase change from rhombohedral to triclinic.

We performed terahertz ESR measurements of spin frustrated spinel compound GeCo2O4 using pulsed magnetic fields of up to 40 T. A very broad EPR absorption line was observed at 86 K. The g-values at 86 K were estimated to be g=5.26±0.07, 5.16±0.12 and 4.98±0.07 for B//[111], [100] and [110], respectively. High-field ESR measurements revealed complicated phase structures and a field-induced magnetic phase below 3 K. Critical fields of magnetic phases at 1.8 K for B//[111] were observed at 1.8 T, 5.0 T, 8.6 T, 11.0 T and 12.9 T for B//[111]. An energy gap of 300 GHz (=14.4 K=1.24 meV), which was related to the lattice deformation, was observed for B//[111]. The zero field gap of ω 4 mode, which is considered to be a singlet-triplet excitation of the di-tetramer, was estimated to be 1120 GHz.

Hydrogen release from Al-based complex hydrides composed of metal cation(s) and [AlH4]− was investigated using inelastic neutron scattering viewed from vibrational dynamics. The hydrogen release followed the softening of translational and [AlH4]− librational modes, which was enhanced by vibrational dynamics and the valence(s) of the metal cation(s).

Structural studies were performed on ${R}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ ($R$=La and Ce). X-ray diffraction measurements on single-crystalline samples of both Rh-based compounds, which were synthesized by the molten Sn-flux method, have revealed structural transformations below ${T}_{\mathrm{D}}\ensuremath{\simeq}$ 350 K. The low-temperature superstructures are characterized by the wave vector $\mathbit{q}=(1/2,1/2,0)$. The space group in the low-temperature phases of both compounds is chiral cubic $I{2}_{1}3$, which is the same as that of ${R}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$ ($R$=La and Ce). Neutron diffraction measurement of ${\mathrm{Ce}}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ revealed no magnetic long-range order below the temperature of the specific-heat peaks at 1.2 and 1.8 K. The low-temperature magnetic state in the chiral phase is that of a paramagnetic semimetal, whereas the dynamical spin correlation of Ce is deduced from the specific-heat data, as found previously in ${\mathrm{Ce}}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$.

Fedotovite K_{2}Cu_{3}O(SO_{4})_{3} is a candidate of new quantum spin systems, in which the edge-shared tetrahedral (EST) spin clusters consisting of Cu^{2+} are connected by weak intercluster couplings forming a one-dimensional array. Comprehensive experimental studies by magnetic susceptibility, magnetization, heat capacity, and inelastic neutron scattering measurements reveal the presence of an effective S=1 Haldane state below T≅4 K. Rigorous theoretical studies provide an insight into the magnetic state of K_{2}Cu_{3}O(SO_{4})_{3}: an EST cluster makes a triplet in the ground state and a one-dimensional chain of the EST induces a cluster-based Haldane state. We predict that the cluster-based Haldane state emerges whenever the number of tetrahedra in the EST is even.

Spin-state transition, also known as spin crossover, plays a key role in diverse systems, including minerals and biological materials. In theory, the boundary range between the low- and high-spin states is expected to enrich the transition and give rise to unusual physical states. However, no compound that realizes a nearly degenerate critical range as the ground state without requiring special external conditions has yet been experimentally identified. This study reports that, by comprehensive measurements of macroscopic physical properties, X-ray diffractometry, and neutron spectroscopy, the Sc substitution in LaCoO$_3$ destabilizes its nonmagnetic low-spin state and generates an anomalous paramagnetic state accompanied by the enhancement of transport gap and magneto-lattice-expansion as well as the contraction of Co--O distance with the increase of electron site-transfer. These phenomena are not well described by the mixture of conventional low- and high-spin states, but by their quantum superposition occurring on the verge of a spin-state transition. The present study enables us to significantly accelerate the design of new advanced materials without requiring special equipment based on the concept of quantum spin-state criticality., Keywords: excitonic insulator, bi-exciton

K3Cu3AlO2(SO4)4 is a highly one-dimensional spin-1/2 inequilateral diamond-chain antiferromagnet. Spinon continuum and spin-singlet dimer excitations are observed in the inelastic neutron scattering spectra, which is in excellent agreement with a theoretical prediction: a dimer-monomer composite structure, where the dimer is caused by strong antiferromagnetic (AFM) coupling and the monomer forms an almost isolated quantum AFM chain controlling low-energy excitations. Moreover, muon spin rotation/relaxation spectroscopy shows no long-range ordering down to 90 mK, which is roughly three orders of magnitude lower than the exchange interaction of the quantum AFM chain. K3Cu3AlO2(SO4)4 is, thus, regarded as a compound that exhibits a Tomonaga-Luttinger spin liquid behavior at low temperatures close to the ground state.

Magnetic properties of a one-dimensional frustrated quantum spin system Rb2Cu2Mo3O12 is investigated by the muon spin relaxation (muSR) technique. Although this system shows an incommensurate spin correlation in neutron scattering, it has not yet been determined whether the ground state reaches to a nonmagnetic spin-singlet state or not. In this study, our zero-field (ZF) muSR result undoubtedly reveals a nonmagnetic ground state, which establishes formation of the nonmagnetic incommensurate spin-singlet state in combination of the previous neutron scattering study. Furthermore, we found that the internal field is slightly enhanced below ~7 K. This temperature dependence resembles that of electric polarization under the magnetic field reported recently, indicating a possibility that a small change occurs even under ZF. In longitudinal-field measurements, a fast fluctuation around a narrowing region is observed. This relaxation is expected to reflect characteristics of formation of the spin-singlet state., 5 pages, 3 figures, to be published in J. Phys. Soc. Jpn.: Conf. Proc

In frustrated spinel oxide $\mathrm{Co}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$, the knowledge of orbital, lattice, and spin structures has been fragmentary thus far. To investigate the structural and magnetic phase transitions in $\mathrm{Co}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$, we performed high-resolution neutron powder diffraction, single crystal synchrotron radiation diffraction, and magnetization and specific heat measurements for high-quality samples. Extremely small crystal distortion from cubic to tetragonal phase ($1\ensuremath{-}c/al0.06%$) was observed using the high-resolution neutron diffraction measurement below ${T}^{*}\ensuremath{\sim}95\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, where a cusp was observed in the temperature dependence of magnetization. The single crystal diffraction experiment revealed that the structural phase transition accompanied by a change in the space group from $I{4}_{1}/amd$ to $I{4}_{1}/a$ occurred at ${T}_{2}=59\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, where the phase transition was observed in the specific heat measurement. Crystal and magnetic structure analysis was carried out with the neutron data, and the results suggest that the magnetic phase transition from collinear to noncollinear magnetic ordering occurs at ${T}^{*}$ and the antiferro-orbital ordering occurs below ${T}_{2}$. We discuss the observed small crystal distortion and orbital characteristics in the light of the boundary between localization and delocalization.

Inelastic neutron scattering experiments have been conducted to investigate the spin dynamics and crystal-electric-field level scheme of the Ce $4f$ electrons in ${\mathrm{Ce}}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$. This compound exhibits a large specific heat at low temperatures and anomalous semimetallic transport in the chiral crystallographic phase below 160 K. Distinctly observed magnetic excitations at approximately 6 and 29 meV are asymmetric in spectral shape and are reproduced by two inequivalent crystal-electric-field splitting schemes, which are deduced from the chiral structure. We have also observed the spin dynamics reflecting antiferromagnetic correlations below 1 meV, which is enhanced with an upturn in the electrical resistivity below 15 K and which yields a low-energy density of state relevant to the large specific heat. We discuss the possibility of a three-dimensional Weyl semimetal state, considering the chiral-lattice symmetry, electronic hybridization, and magnetic correlation.

The compounds designated as ${R}_{3}{T}_{4}{\mathrm{Sn}}_{13}$ ($R$ = rare earth and $T$ = transition metal) are hypothesized to be strongly correlated electron systems. Some of these compounds exhibit structural phase transitions with speculated charge-density-wave (CDW) formations. We carried out x-ray diffraction measurements in order to investigate the second-order phase transition of ${R}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$ ($R=\mathrm{La} \text{and} \mathrm{Ce}$) by using single-crystalline samples synthesized by the molten Sn-flux method. Both compounds exhibit the structural superlattice transformations characterized by the wave vector $\mathbit{q}=(1/2,1/2,0)$ below ${T}_{\mathrm{D}}\ensuremath{\simeq}160$ K, which originate from electronic states owing to the common Co and Sn. The space group of the low-temperature phase is evidenced as a chiral cubic $I{2}_{1}3$. The magnetic-susceptibility enhancement and the Co-ion valence shift below ${T}_{\mathrm{D}}$ are signatures of a phase transition not fully attributed to the conventional CDW formation.

We observed all-in type Nb tetrahedral displacement in nonmagnetic pyrochlore niobates ${A}_{2}{\mathrm{Nb}}_{2}{\mathrm{O}}_{7}(A={\mathrm{Nd}}_{0.5}{\mathrm{Ca}}_{0.5}$ and ${\mathrm{Y}}_{0.5}{\mathrm{Ca}}_{0.5})$ through the analysis of the neutron pair distribution function and the extended x-ray absorption function spectroscopy. The all-in type Nb tetrahedral displacement, which has the character of a charge singlet state, is driven by the formation of the bonding orbital. The diffuse scattering in the x-ray diffraction, which has the resonant component in the Nb ${L}_{3}$ edge, indicates that the all-in type Nb tetrahedral displacement has the periodicity with its short-range correlation.

Dynamical properties of protonic conductor K3H(SeO4)2 were examined by using various techniques, Brillouin scattering, quasi elastic, and inelastic neutron scattering. No significant anomaly was observed in longitudinal and transverse acoustic modes. In each experiment, a Lorentzian shaped spectrum was observed at extremely low energy, (less than 0.1 meV). The spectra in Brillouin and inelastic neutron scattering at the zone boundary originate from rotational modes of SeO4 tetrahedra at Г- and L-point, respectively. The peak positions, shapes, and widths of the spectra are consistent with each other, while the integrated intensities show complementary temperature dependence. The lifetimes of those modes, about 10−10 s, agree with the stationary time of conduction protons obtained from Q dependence of width in quasi elastic neutron scattering.

Determination of magnetic structure is an important analytical procedure utilized in various fields ranging from fundamental condensed-matter physics and chemistry to advanced manufacturing. It is typically performed using a neutron diffraction technique; however, finding global solutions of the magnetic structure optimization problem represents a significant challenge. Generally, it is not possible to mathematically prove that the obtained magnetic structure is a truly global solution and that no solution exists when no acceptable structure is found. In this study, the global optimization technique called semidefinite relaxation of quadratic optimization, which has attracted much interest in the field of applied mathematics, is proposed to use as a new analytical method for the determination of magnetic structure, followed by the application of polarized neutron diffraction data. This mathematical approach allows avoiding spurious local solutions, decreasing the amount of time required to find a tentative solution and finding multiple solutions when they exist.

We carried out cold neutron scattering experiments on a single crystal of CoCr 2 O 4 . We compare the quasielastic scattering, measured at the top and on the side of peak of the magnetic satellite reflection, and we discuss the distribution of the clusters of the spiral component.

Crystal structures of protonic conductor K 3 H(SeO 4 ) 2 in the phase I (conducting phase) and phase II (room-temperature phase) have been determined using a time-of-flight (TOF) neutron diffractometer VEGA. SeO 4 tetrahedra in the phase I were rotationally displaced with the occupancy 1/3 and the crystal structure in the phase II was remain locally. Proton distribution maps were obtained from the maximum entropy method (MEM) describe that the amount of the proton in a hydrogen bond were decline from 87.3% to 21.4% with increasing the temperature and 13.4% of protons exist at inter-layer space. Incoherent quasi-elastic neutron scattering (QENS) experiments performed by LAM80-ET spectrometer yield the QENS spectra of conducting protons. Proton diffusion constants are estimated as 1.9(1) × 10 − 10 m 2 /s at 393 K and 2.1(3) × 10 − 10 m 2 /s at 413 K, respectively.

Ultrasound velocity measurements were performed on a single crystal of spin-frustrated ferrite spinel ${\mathrm{ZnFe}}_{2}{\mathrm{O}}_{4}$ from 300 K down to 2 K. In this cubic crystal, all the symmetrically independent elastic moduli exhibit softening with a characteristic minimum with decreasing temperature below $\ensuremath{\sim}100$ K. This elastic anomaly suggests a coupling between dynamical lattice deformations and molecular-spin excitations. In contrast, the elastic anomalies, normally driven by the magnetostructural phase transition and its precursor, are absent in ${\mathrm{ZnFe}}_{2}{\mathrm{O}}_{4}$, suggesting that the spin-lattice coupling cannot play a role in relieving frustration within this compound. The present study infers that, for ${\mathrm{ZnFe}}_{2}{\mathrm{O}}_{4}$, the dynamical molecular-spin state evolves at low temperatures without undergoing precursor spin-lattice fluctuations and spin-lattice ordering. It is expected that ${\mathrm{ZnFe}}_{2}{\mathrm{O}}_{4}$ provides the unique dynamical spin-lattice liquidlike system, where not only the spin molecules but also the cubic lattice fluctuate spatially and temporally.

A chemical bond includes both covalent and ionic characteristics. We develop an experimental method to estimate the degree of each contribution based on magnetic susceptibility measurements, in which Pascal's scheme for Larmor diamagnetism is combined with electronegativity. The applicability to metal hydrides is also shown.

We studied the Co valences and spin states in electron-doped LaCo$_{1-y}$Te$_{y}$O$_3$ by measuring x-ray absorption spectra and electron spin resonance. The low-temperature insulating state involves the low-spin Co$^{3+}$ ($S=0$) and the high-spin Co$^{2+}$ state, which is described by $g=3.8$ and $j_{\rm eff}=1/2$. The results, in concurrence with the electron-hole asymmetry confirmed in electrical resistivity, coincide with a spin-blockade phenomenon in this system. Further, we discuss the $g$ factor in terms of the strong covalent-bonding nature and consider multiple origins of this phenomenon., 7 pages, 5 figures, and 1 table

We present detailed ab-initio lattice dynamical analysis of the Fe-O infinite-layer compounds CaFeO2 and SrFeO2 in various magnetic configurations. These indicate strong spin-phonon coupling in SrFeO2 in contrast to that in case of CaFeO2. Powder neutron inelastic scattering experiments on SrFeO2 have also been performed at temperatures from 5 K to 353 K in the antiferromagnetic phase and analyzed using the ab-initio calculations. These suggest distortion of the ideal infinite planer structure above 300 K. From our ab-initio calculations in SrFeO2 as a function of volume, we suggest that the distortion in SrFeO2 above 300 K is similar to that known in CaFeO2 at ambient conditions. The distortion of the planer structure of CaFeO2 involves doubling of the planer unit cell that may be usually expected to be due to a soft phonon mode at the M-point (1/2 1/2 0). However, our ab-initio calculations show quite unusually that all the M-point (1/2 1/2 0) phonons are stable, but two stable M3+ and M2-modes anharmonically couple with an unstable Bu mode at the zone centre and lead to the cell doubling and the distorted structure. Magnetic exchange interactions in both the compounds have been computed on the basis of the ideal planar structure (P4/mmm space group) and with increasing amplitude of the Bu phonon mode. These reveal that the magnetic exchange interactions reduce significantly with increasing distortion. We have extended the ab-initio phonon calculation to high pressures, which reveal that, above 20 GPa of pressure, the undistorted planer CaFeO2 becomes dynamically stable. We also report computed phonon spectra in SrFeO3 that has a cubic structure, which is useful to understand the role of the difference in geometry of oxygen atoms around the Fe atom with respect to planer SrFeO2., Comment: 28 Pages, 13 Figures

We derived analytical expressions of energy (E) and momentum (Q) resolutions on a chopper spectrometer, including timing and geometrical parameters, such as a pulse width at a moderator, an opening time of a chopper, spectrometer lengths, sizes of a moderator, of a sample and of a position-sensitive detector (PSD) and pixel sizes in a PSD. For a long linear PSD, resolutions change for pixels dependently on their positions inside the PSD, because of the change in the flight path lengths. Controlling the pixel sizes can reduce such nonuniformity in resolutions. However, in order to diminish the nonuniformity in both of the E and Q resolutions simultaneously, the PSD length was found to be limited.

Spin fluctuations were studied over a wide momentum ($\ensuremath{\hbar}Q$) and energy ($E$) space in the frustrated $d$-electron heavy-fermion metal ${\mathrm{LiV}}_{2}{\mathrm{O}}_{4}$ by time-of-flight inelastic neutron scattering. We observed the overall $Q\ensuremath{-}E$ evolutions near the characteristic $Q=0.6\text{ }\text{ }{\AA{}}^{\ensuremath{-}1}\text{ }$ peak and found another weak broad magnetic peak around $2.4\text{ }\text{ }{\AA{}}^{\ensuremath{-}1}$. The data are described by a simple response function, a partially delocalized magnetic form factor, and antiferromagnetic short-range spatial correlations, indicating that heavy-fermion formation is attributable to spin-orbit fluctuations with orbital hybridization.

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.

We studied electron-doped \(\text{LaCo}_{1 - y}\text{Te}_{y}^{6 + }\text{O}_{3}\) by magnetization measurements and neutron scattering. The effective Bohr magneton, estimated by Curie–Weiss fitting around room temperature, is independent of y. This suggests that magnetic Co3+(HS), not nonmagnetic Co3+(LS), is mainly replaced by doped magnetic Co2+(HS). At the lowest temperatures, a Brillouin-function-like saturating behavior persists in the magnetization curves even in the high-y samples, and neither a clear magnetic reflection nor magnetic dispersion is observed by neutron scattering. These findings indicate that the magnetic correlation is very weak, in contrast to the well-known hole-doped LaCoO3 accompanied by a drastic transition to a ferromagnetic metal. However, we also found that the low-y samples exhibit nonnegligible enhancement of the saturated magnetization by ∼2μB per a doped electron. All these characteristics are discussed in the light of the activation and inactivation of a spin-state bloc...

Neutron scattering studies were carried out for investigating heavy-electron-like behaviors of PrCu4Au. Excitation spectra are composed of crystal-field (CF) excitations in the Pr3+4f2 electron configuration and a broad component. The CF splitting ground state is a triplet 5. The type-II antiferromagnetic ordering characterized by the propagation vector (1/2, 1/2, 1/2) appears below approximately 4 K. The powder diffraction pattern is reproduced by the ordered magnetic moments aligning along the direction perpendicular to the propagation vector. The magnitude of the ordered magnetic moment is evaluated as 1.36uB/Pr, which is less than 2.0uB/Pr expected from the 5 ground state. Dual electronic states of both localized and itinerant character are inferred.

We studied the spin-state responses to light impurity doping in low-spin perovskite LaCoO$_{3}$ (Co^3+: d^6) through magnetization and X-ray fluorescence measurements of single-crystal LaCo$_{0.99}$$M_{0.01}$O$_{3}$ ($M$ = Cr, Mn, Fe, Ni). In the magnetization curves measured at 1.8 K, a change in the spin-state was not observed for Cr, Mn, or Fe doping but was observed for Ni doping. Strong magnetic anisotropy along the [100] easy axis was also found in the Ni-doped sample. The fluorescence measurements revealed that the valences were roughly estimated to be Cr^3+, Mn^4+, Fe^(3+delta)+, and Ni^3+. From the observed chemical trends, we propose that the chemical potential is a key factor in inducing the change of the low-spin state. By expanding a model of the ferromagnetic spin-state heptamer generated by hole doping, we discuss the emergence of highly anisotropic ferromagnetic spin-state clusters induced by low-spin Ni^3+ with Jahn-Teller activity. We also discuss applicability of the present results to mantle materials and impurity-doped pyrites with Fe (d^6).

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 ${\mathrm{MgCr}}_{2}{\mathrm{O}}_{4}$, 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 ${T}_{\mathrm{N}}$ and an ordered phase with an extremely complex magnetic structure below ${T}_{\mathrm{N}}$. 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.

Ultrasound velocity measurements of the magnesium chromite spinel MgCr${}_{2}$O${}_{4}$ reveal elastic anomalies in the paramagnetic phase that are characterized as due to geometrical frustration. The temperature dependence of the tetragonal shear modulus $({C}_{11}\ensuremath{-}{C}_{12})/2$ exhibits huge Curie-type softening, which should be the precursor to spin Jahn-Teller distortion in the antiferromagnetic phase. The trigonal shear modulus ${C}_{44}$ exhibits nonmonotonic temperature dependence with a characteristic minimum at $\ensuremath{\sim}$50 K, indicating a coupling of the lattice to a dynamical molecular-spin state. These results strongly suggest the coexistence of the dynamical spin Jahn-Teller effect and a dynamical molecular-spin state in the paramagnetic phase, which is compatible with the coexistence of magnetostructural order and a dynamical molecular-spin state in the antiferromagnetic phase.

We describe powder and single-crystal inelastic neutron scattering experiments on a spinel-type antiferromagnet GeCo${}_{2}$O${}_{4}$, represented by an effective total angular momentum ${J}_{\mathrm{eff}}=1/2$. Several types of nondispersive short-range magnetic excitations were discovered. The scattering intensity maps in $\mathbit{Q}$ space are well reproduced by dynamical structure factor analyses using molecular model Hamiltonians. The results of analyses suggest that the molecular excitations below ${T}_{N}$ arise from molecular ground states, one of which consists of antiferromagnetically coupled ferromagnetic subunits. The quasielastic excitations above ${T}_{N}$ are interpreted as its precursor. A combination of frustration and ${J}_{\mathrm{eff}}=1/2$ might induce these quantum phenomena.

Ultrasound velocity measurements of the orbitally-frustrated GeCo$_2$O$_4$ reveal unusual elastic instabilities due to the phonon-spin coupling within the antiferromagnetic phase. Shear moduli exhibit anomalies arising from the coupling to short-range ferromagnetic excitations. Diplike anomalies in the magnetic-field dependence of elastic moduli reveal magnetic-field-induced orbital order-order transitions. These results strongly suggest the presence of geometrical orbital frustration which causes novel orbital phenomena within the antiferromagnetic phase.

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 $T_{\rm MI}$ of 33 K. The diffraction measurements revealed that the pyrochlore has an antiferromagnetic long-range structure with propagation vector $\vec{q}_{0}$ 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\pm0.4$$\mu_{\rm B}$, where $\mu_{\rm B}$ is the Bohr magneton. The inelastic scattering measurements revealed that the Kramers ground doublet of Nd$^{3+}$ splits below $T_{\rm MI}$. 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., Comment: 6 pages, 1 table, 3 figures

We describe a method of white-beam inelastic neutron scattering for improved measurement efficiency. The method consists of matrix inversion and selective extraction. The former is to resolve each incident energy component from the white-beam data, and the latter eliminates contamination by elastic components, which produce strong backgrounds that otherwise obfuscate the inelastic scattering components. In this method, the optimal experimental condition to obtain high efficiency will strongly depend on the specific aim of the individual experiments., Comment: 6 pages, 3 figures

We report single-crystal neutron diffraction studies on a spinel antiferromagnet GeCo$_2$O$_4$, which exhibits magnetic order with a trigonal propagation vector and tetragonal lattice expansion ($c/a\simeq1.001$) below $T_{\rm N}=21$ K. For this inconsistency between spin and lattice in symmetry, magnetic Bragg reflections with a tetragonal propagation vector were discovered below $T_{\rm N}$. We discuss spin and orbital states of Co$^{2+}$ ion underlying the new magnetic component., Comment: 3 pages 2 figures, submitted to ICFCM proceeding (Journal of Physics: Conference Series, 2011)

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.

Neutron powder diffraction and inelastic neutron scattering experiments were performed to clarify the mechanism of phase transition and protonic conductivity of ${\text{K}}_{3}\text{H}{({\text{SeO}}_{4})}_{2}$. A rotational ${\text{SeO}}_{4}$ tetrahedra mode at the zone boundary $L$-point accounts for the symmetrical change in space group from $R\overline{3}m$ to $C2/c$. No significantly anomaly was observed in the transversal acoustic and under-damped optical modes, while a broad spectrum was observed around 0 meV at the $L$-point above ${T}_{C}$. The spectrum depends on the temperature and the dependence is represented by the soft mode due to an over-damped phonon both $Q$ and energy space. The rotational mode of tetrahedra drives the improper ferroelastic phase transition.

Using elastic neutron scattering on single crystals of ${\text{La}}_{1\ensuremath{-}x}{A}_{x}{\text{CoO}}_{3}$ ($A={\text{Ca}}^{2+}$, ${\text{Sr}}^{2+}$, and ${\text{Ba}}^{2+}$), we found the development of magnetic superstructures below the global magnetic transition to be strongly dependent on the size of the $A$-site dopant, $⟨{r}_{A}⟩$, in an unusual way. Upon reducing the $⟨{r}_{A}⟩$ (i.e., as with Ca doping), only a commensurate ferromagnetic cluster phase is evident. On expanding the $⟨{r}_{A}⟩$, the tendency toward coexistence of competing ferromagnetic and antiferromagnetic orders increases giving rise to an inhomogeneous ground state. The antiferromagnetic ordered state, initially incommensurate, continuously strengthens and becomes commensurate with long-range order and a characteristic cubic wave vector of ${\stackrel{P\vec}{Q}}_{c}=(0.25,0.25,0.25)$ with $x$. The two competing order parameters become comparable in magnitude indicative of the phase-separated nature of the cobalt perovskite system.

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.

We performed ultrasound velocity measurements in the geometrically-frustrated ferrite spinel ZnFe2O4. In the low-temperature paramagnetic phase, the elastic moduli exhibit softening with a characteristic minimum with decreasing temperature. This elastic anomaly suggests the coupling of lattice to molecular-spin excitations.