Search

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.

We investigate magnetic properties of mixed crystals of spinel oxides Zn(Cr1−xFex)2O4 and Cd(Cr1−xFex)2O4 to study Cr-site substitution effects on the geometrically-frustrated magnetism of chromite spinels ZnCr2O4 and CdCr2O4. Zn(Cr1−xFex)2O4 and Cd(Cr1−xFex)2O4 with above x ∼ 0.1 exhibit spin-glass-like behavior below ∼10 K which is suppressed by magnetic field H > 1 kOe. Moreover, the Fe concentration x dependence of Weiss temperature ΘW in Zn(Cr1−xFex)2O4 (Cd(Cr1−xFex)2O4) exhibits the sign change of ΘW from negative below x ∼ 0.4 (x ∼ 0.1) to positive above x ∼ 0.4 (x ∼ 0.1). This suggests that, in Zn(Cr1−xFex)2O4 and Cd(Cr1−xFex)2O4, the competition of antiferromagnetic and ferromagnetic interactions (bond frustration) becomes stronger with increasing the Fe concentration x, while the geometrical frustration becomes weaker with increasing x.

Here, the authors perform ultrasound velocity measurements in the frustrated pyrochlore fluoride NaCaCo${}_{2}$F${}_{7}$, which exhibits spin freezing below ${T}_{f}$ $\ensuremath{\sim}$ 2.4 K with the inherent presence of exchange disorder. In this cubic magnet, the bulk modulus and the trigonal shear modulus exhibit anomalies at temperatures below $\ensuremath{\sim}$20 K down to ${T}_{f}$, which reveal an occurrence of magnetic field induced crossover from an isostructural lattice instability toward the spin freezing to a trigonal lattice instability arising from the emergent dynamical spin-cluster state.

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.

We investigate electric and magnetic properties of quasi-one-dimensional transition-metal carbides Sc3TC4 (T = Co, Ru, and Os), and their mixed crystals Sc3(Co1-xRux)C4 and Sc3(Ru1-xOsx)C4. Sc3CoC4 exhibits successive phase transitions of charge-density-wave transition at TCDW ~ 140 K, Peierls-like structural transition at Ts ~ 70 K, and superconducting transition at Tc ~ 5 K. Sc3RuC4 and Sc3OsC4 exhibit a phase transition at T* ~ 220 K and 250 K, respectively, which should occur in the low-dimensional electronic structure. For Sc3CoC4, it is revealed by the investigation of the electric and magnetic properties of Sc3(Co1-xRux)C4 that the phase transitions at TCDW, Ts, and Tc exhibit different robustness against Ru doping. For Sc3RuC4 and Sc3OsC4, it is revealed by the investigation of the electric and magnetic properties of Sc3(Ru1-xOsx)C4 that an identical kind of phase transition occurs at T*. Additionally, the present study reveals that the phase transition at T* in Sc3RuC4 and Sc3OsC4 is inherently different from the phase transitions at TCDW, Ts, and Tc in Sc3CoC4.

We investigated magnetic properties of spinel oxides Zn (Cr1-xFex)2O4 to study element substitution effects on magnetism of spin-frustrated spinel oxides ZnCr2O4 and ZnFe2O4. The present study revealed that an antiferromagnetic order of ZnCr2O4 (TN ~ 13 K) is suppressed by Fe doping. Zn (Cr1-xFex)2O4 with x = 0.1, 0.5, and 0.7 exhibits spin-glass-like behavior below ~ 10 K. On the other hand, Zn (Cr1-xFex)2O4 with x = 0.3 exhibits the absence of magnetic order or spin-glass-like behavior down to low temperature (3 K), which is probably due to the competition of geometrical and bond frustrations.

We synthesized polycrystals of cobaltite spinels LiCo2O4, ZnCo2O4, and their mixed crystal (Li1-xZnx)Co2O4, and performed the magnetic susceptibility and specific heat measurements. LiCo2O4 with the Weiss temperature ΘW ~ -114 K exhibits an antiferromagnetic transition at TN ~ 30 K. On the other hand, ZnCo2O4 with the Weiss temperature ΘW ~ -90 K exhibits the absence of magnetic phase transition down to low temperature (2 K), indicating the presence of strong frustration. Taking into account the absence of magnetic phase transition in the orbital-degenerate ZnCo2O4, it is suggested that the antiferromagnetic transition in the charge-orbital-degenerate LiCo2O4 is driven by the charge degree of freedom. Furthermore, the magnetic properties of (Li1-xZnx)Co2O4 suggest that the antiferromagnetic transition in LiCo2O4 is sensitively suppressed by diluting the charge degeneracy.

Calcium vanadate ${\mathrm{CaV}}_{2}{\mathrm{O}}_{4}$ has a crystal structure of quasi-one-dimensional zigzag chains composed of orbital-active ${\mathrm{V}}^{3+}$ ions and undergoes successive structural and antiferromagnetic phase transitions at ${T}_{s}\ensuremath{\sim}140$ K and ${T}_{N}\ensuremath{\sim}70$ K, respectively. We perform ultrasound velocity measurements on a single crystal of ${\mathrm{CaV}}_{2}{\mathrm{O}}_{4}$. The temperature dependence of its shear elastic moduli exhibits huge Curie-type softening upon cooling that emerges above and below ${T}_{s}$ depending on the elastic mode. The softening above ${T}_{s}$ suggests the presence of either onsite Jahn-Teller-type or intersite ferrotype orbital fluctuations in the two inequivalent ${\mathrm{V}}^{3+}$ zigzag chains. The softening below ${T}_{s}$ suggests the occurrence of a dimensional spin-state crossover, from quasi-one to three, that is driven by the spin-lattice coupling along the inter-zigzag-chain orthogonal direction. The successive emergence of the orbital- and spin-driven lattice instabilities above and below ${T}_{s}$, respectively, is unique to the orbital-spin zigzag chain system of ${\mathrm{CaV}}_{2}{\mathrm{O}}_{4}$.

Ultrasound velocity measurements were performed on a single crystal of the orbital-degenerate frustrated spinel CoV2O4 in all the symmetrically-independent elastic moduli of the cubic crystal. The measurements of temperature dependence of the elastic moduli observed discontinuous elastic anomalies due to a ferrimagnetic transition at TC = 165 K and another phase transition at T* = 50 K. Additionally, the measurements observed anomalous temperature dependence of the elastic moduli, specifically, non-monotonic temperature dependence in the magnetically-ordered phase below TC, and magnetic-field-sensitive elastic softening with decreasing temperature in the paramagnetic phase above TC. These anomalous temperature variations below and above TC should be driven by the coupling of lattice to magnetic excitations.

We perform ultrasound velocity measurements on a single crystal of nearly-metallic spinel Co$_{1.21}$V$_{1.79}$O$_4$ which exhibits a ferrimagnetic phase transition at $T_C \sim$ 165 K. The experiments reveal a variety of elastic anomalies in not only the paramagnetic phase above $T_C$ but also the ferrimagnetic phase below $T_C$, which should be driven by the nearly-itinerant character of the orbitally-degenerate V 3$d$ electrons. In the paramagnetic phase above $T_C$, the elastic moduli exhibit elastic-mode-dependent unusual temperature variations, suggesting the existence of a dynamic spin-cluster state. Furthermore, above $T_C$, the sensitive magnetic-field response of the elastic moduli suggests that, with the negative magnetoresistance, the magnetic-field-enhanced nearly-itinerant character of the V 3$d$ electrons emerges from the spin-cluster state. This should be triggered by the inter-V-site interactions acting on the orbitally-degenerate 3$d$ electrons. In the ferrimagnetic phase below $T_C$, the elastic moduli exhibit distinct anomalies at $T_1\sim$ 95 K and $T_2\sim$ 50 K, with a sign change of the magnetoresistance at $T_1$ (positive below $T_1$) and an enhancement of the positive magnetoresistance below $T_2$, respectively. These observations below $T_C$ suggest the successive occurrence of an orbital glassy order at $T_1$ and a structural phase transition at $T_2$, where the rather localized character of the V 3$d$ electrons evolves below $T_1$ and is further enhanced below $T_2$., 9 pages, 4 figures

The electric conduction mechanism of electrochemically treated anodic porous alumina (APA) resistance switching (RS) memory is investigated. The current-voltage characteristics of the APA were measured for both of high resistance state and low resistant state at room temperature. The conduction mechanism of a high resistance state can be explained by separating into three regions of Ohm's law, space charge limited current (SCLC), and Fowler-Nordheim (F-N) tunneling. It was suggested that the F-N tunneling decreases the reproducibility of switching voltages.

We have studied variation of switching voltage of resistive random access memory (ReRAM). Basically, the switching voltage at every switching has a different value because there are many conduction paths, which are randomly chosen at every switching with different conductivity in an oxide insulator between top and bottom electrodes. Limitation of the number of conductive paths is expected to lead the suppression of the variation of switching voltage. In this study, Ni nanorods buried into the nanoholes of an anodic aluminum oxide (AAO) film have been fabricated to restrict the formation of many filaments and the switching behaviors of the In/NiO/Ni nanorods has been investigated.

We report a detailed study of in-plane lower critical field H c 1 ( T ) and penetration depth λ ab ( T ) of the Lu 2 Fe 3 Si 5 superconductor. Two pronounced changes of the H c 1 curvature are observed on the H c 1 ( T ) curve around 0.4 T c and 0.65 T c , respectively, which are attributed to the two-band nature of superconductivity in Lu 2 Fe 3 Si 5 . The penetration depth of λ ab ( T ) follows a power law (∼ T n ) below 0.4 T c , with n = 2.5 close to the values reported for the FeAs-pnictides, suggesting possible s ± -wave pairing in Lu 2 Fe 3 Si 5 . Temperature dependence of H c 1 can be well fitted by the two-gap BCS model in the full temperature region using two superconducting gaps Δ 1 = 0.19 ± 0.1 meV and Δ 2 = 1.02 ± 0.1 meV.

In the coastal engineering field, particle method is expected for three-dimensional numerical simulations on fluid-structure interaction. Although in realistic three-dimensional numerical simulation, an increase of memory utilization and a computational time are very serious problems due to resolution of distance of particles and increase of number of particles. In this research, the authors develop a hybrid (openMP/MPI) parallel simulation code of MPS method and simulate large-scale simulations for coastal engineering. The simulation of a static water problem evaluates the parallel performance of the proposed simulation code. And the simulation of a collapse of water column and the progressive wave verifies the accuracy. The result of large-scale simulation of ocean wave showed the wave concentration into cavities of structures, and generation of spray as three-dimensional hydraulic phenomenon. This study shows that we can evaluate three-dimensional hydraulic phenomenon more fully with the proposed simulation code.

We have investigated the precise crystal structure and their temperature dependences of oxyarsenides (LaO)TAs; T = Mn, Fe, Co, as the Fe based superconductor’s parent material family using high energy synchrotron radiation x-ray powder diffraction. Lattice constants a and c decrease with decreasing temperature. Focusing the ratios of the changes normalized by room temperature lattice constants, we have found anisotropic shrinks for the superconductor’s parent material of (LaO)FeAs and the ferromagnetic metal (LaO)CoAs. The shrinkage of the lattice constant c along the stacking direction will be discussed by the temperature dependence of the divided three components of the LaO layer, TAs layer, and the interlayer distance.

We performed experimentally the electron-density mapping on the crystal structure of the iron-based superconductor Lu2Fe3Si5 which revealed the Fe-Si covalent-bonding network with the fully ionized Lu-sites. And, for Lu2Fe3Si5, we investigated the effect of Fe- and Si-site substitutions on the superconductivity. The Fe-site-substitution study revealed that, in addition to the disorder-sensitive superconductivity, the additionally-provided 3d electrons or holes tend to be itinerant rather than localized. Taking into account that the localized magnetic moments introduced on the Lu-sites are inactive as magnetic pair breakers, the present results suggest that, in Lu2Fe3Si5, the Fe-Si bonding network should be the playground for the exotic superconductivity which is characterized by the sign-reversed superconducting gap function.

We have successfully prepared the new family of fluoride-arsenide Sr1-xNdxFeAsF (0 ≤ x ≤ 0.5), and investigated the electrical and magnetic properties. Sr1-xNdxFeAsF with x = 0.4 and 0.5 show superconductivity, and their superconducting transition temperatures are 32 and 49 K, respectively. The superconducting volume fractions of Sr0.6Nd0.4FeAsF and Sr0.5Nd0.5FeAsF are ~11 and ~ 17 %, respectively. On the other hand, the sample of Sr1-xNdxFeAsF (0 ≤ x ≤ 0.35) is not superconducting but shows a metallic conductivity. All compounds show a weak ferromagnetism with the Curie temperature of above room temperature. The origin of the ferromagnetism may be due to the tiny Fe impurity.

We have prepared single crystal of layered Mott-insulator FePS3 and investigated the pressure dependence of the electrical resistivity. FePS3 shows a metal-insulator transition between 10 and 11 GPa. In the insulating region, the activation energy increases continuously with decreasing the pressure and it is well extrapolated to the ambient pressure value. In the metallic region, the electrical resistivity is propotional to the square of the temperature within 50 to 100 K.

We report electromigration techniques for the fabrication of planar-type tunnel junctions with ferromagnetic nanogap system. In these techniques, by monitoring the current passing through the devices, we are easily able to obtain the planar-type Ni-Vacuum-Ni tunnel junctions. In this paper, magnetoresistance (MR) properties of the planar-type Ni-based tunnel junctions formed by stepwise feedback-controlled electromigration (SFCE) and field-emission-induced electromigration (activation) are studied. We performed the SFCE method for Ni nanoconstrictions connecting asymmetrical butterfly-shape electrodes. Furthermore, the activation technique was applied to Ni nanogaps with separations of 15-45 nm. MR ratio of the devices formed by the SFCE exhibited approximately 4% at 16 K . On the other hand, the devices fabricated by the activation showed MR ratio of above 300% at 16 K. These results suggest that it is possible to fabricate planar-type ferromagnetic tunnel junctions with vacuum barriers by electromigration techniques.

We have investigated the magnetism and the specific heat of transition metal phosphorous trisulfide Mn1−xFexPS3 and determined the magnetic phase diagram. A spin glass state appears in the range 0.5 ≤ x ≤ 0.6 . The remarkable feature of this spin glass phase is that the spin freezing is not suppressed up to a rather high magnetic field. Reentrant spin glass states appear in the ranges 0.3 ≤ x ≤ 0.45 and 0.6 x ≤ 0.7; antiferrmomagnetic ordered states appear in the ranges 0.0 ≤ x ≤ 0.2 and 0.8 ≤ x ≤ 1.0 . The temperature dependence of the magnetization of the antiferromagnets except x = 0.0 and 1.0 does not approach zero at 0 K. The molar specific heat of Mn1−xFexPS3 below 4 K is larger than that of x = 0.0 and 1.0. These results strongly suggest that the spin fluctuation exists in Mn1−xFexPS3 even in the antiferromagnetic ordered state.

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.

We present laser-excited ultrahigh-resolution photoemission spectroscopy of YNi 2 B 2 C and ErNi 2 B 2 C. The extremely high energy resolution enables us to discuss the superconducting (SC) gap function of YNi 2 B 2 C and to measure the density of states which is coexisted between superconductivity and antiferromagnetism in ErNi 2 B 2 C. For YNi 2 B 2 C, s + g-wave SC gap symmetry cannot reproduce the experimental data, implying importance of taking the complicated electronic structures into consideration. For ErNi 2 B 2 C, the observed coherent peak is very broad and finite density of state exists at Fermi level.

We review our angle-resolved photoemission spectroscopy (ARPES) results across superconducting transitions in 2 H –NbSe 2 and MgB 2 , and also show recent results on YNi 2 B 2 C. For 2 H –NbSe 2 and MgB 2 , we observe that the superconducting gap exhibits clear Fermi surface (FS) sheet dependence for two types of FS sheets with different characters, thus providing evidence that the FS sheet dependence is a general feature for a superconductor having multiple FS sheets of different character. In YNi 2 B 2 C, we show experimental FSs, and momentum-dependent superconducting gap showing large anisotropy observed on a single FS. These ARPES results provide direct evidence for and importance of momentum-dependent superconducting gaps of phonon-mediated superconductors.

We have measured the ultrasound velocity of quasi-two-dimensional superconductor CeCoIn 5 with extremely large Pauli paramagnetic susceptibility. The results indicate that the new high field superconducting phase, which is revealed by the recent heat capacity measurements, is characterized by the unusual softening of flux line lattice. The softening is most likely due to the collapse of the flux line lattice tilt modulus and transition to quasi-two-dimensional vortex state. These results provide a strong evidence that the high field phase is the Fulde–Ferrell–Larkin–Ovchinnikov phase, in which the order parameter is spatially modulated and has planar nodes aligned perpendicularly to the vortices.

In order to determine the superconducting gap structure of the borocarbide superconductor YNi 2 B 2 C, we have measured the angular variation of the c-axis thermal conductivity κ zz in magnetic field rotated within the ab-planes. A clear fourfold symmetry with narrow cusps was observed in the angular variation of κ zz . These results provide a strong evidence that the gap function has point nodes which are located along the a- and b-axes of the crystal.

We have investigated the thermal expansion of the ferrimagnet FeSe of the hexagonal system by synchrotron radiation powder X-ray diffraction from room temperature down to 90 K. The lattice constants show anisotropic change against temperature. Especially, the c-axis expands down to 170 K, exhibiting negative linear thermal expansion. Almost the same behavior is found in the temperature dependence of the magnetization. This implies that the anomalous change of the c-axis has a strong correlation with magnetism. Magnetic fields up to 0.4 T do not affect the lattice constants at room temperature. Electrical resistivity indicates metallic behavior, i.e., there is a finite density of the state at the Fermi energy, consisting of the Fe3d and the Se4p states.

Ultrasound velocity measurements of the orbitally degenerate frustrated spinel ${\mathrm{MgV}}_{2}{\mathrm{O}}_{4}$ are performed in a high-purity single crystal which exhibits successive structural and antiferromagnetic phase transitions, and in the disorder-introduced single crystal which exhibits spin-glass-like behavior. The measurements reveal that two types of unusual temperature dependence of the elastic moduli coexist in the cubic paramagnetic phase, which are resolved by magnetic-field and disorder sensitivities: huge Curie-type softening with decreasing temperature (convex temperature dependence), and concave temperature dependence with a characteristic minimum. These elastic anomalies suggest the coupling of the lattice to coexisting orbital fluctuations and spin-cluster excitations.

We have investigated the rare earth elements dependence on the magnetism to understand the contribution to physical properties of the 4f electrons of (LnO)MnAs (Pn = La, Ce, Pr, Nd). (CeO)MnAs, (PrO)MnAs and (NdO)MnAs shows the antiferromagnetic behaviors at low temperature. (CeO)MnAs and (NdO)MnAs have the magnetic anomalies around 34 K and 24 K, respectively. So, it is speculated that the anomalies depend on the Mn -Mn distance directly

We have focused on the layered antiferromagnetic semiconductor (LaO)MnAs and investigated carrier dependences of the physical properties to understand the semiconducting origin. The absolute values of electrical resistivity of (LaO1-xFx)MnAs decrease with F substitution, but the Zn substitution samples don’t show apparent dependences in the resistivity. All substituted samples indicate paramagnetic behaviors, where (LaO)ZnAs without magnetic moments presents very small temperature independent magnetization. The magnetism of (LaO)MnAs changes from antiferromagnetism to paramagnetism by the carrier doping.

We have performed ultrahigh-resolution photoemission spectroscopy of Y(Ni1-xPtx)2B2C ( x = 0.0 and 0.2) in order to study the changes in the density of states across the superconducting transition. Because of a drastic increase in energy resolution, we clearly observe the opening of superconducting gaps across T(c) in both compounds. Furthermore, we find a small but significant difference in the superconducting-state spectral shape. This can be explained in terms of reduction in gap anisotropy by introducing impurities and provides spectroscopic evidence for an anisotropic s-wave gap in YNi2B2C.

Microscopic needle-like structures with submicron sizes were observed in the films prepared by electrolysis. Macroscopic needle-like structures with few millimeters length were also assembled by an extraction from nonelectrolysis supersaturated toluene solution. The anisotropic activation energy for two kinds of growing surfaces was investigated from the observation of their characteristic structure. Preparation conditions with thermal-equilibrium were thought to be effective to obtain a very long and/or fine assembled structure of C60. Conclusively a huge needle-like C60 was obtained by a very slow extraction. In the sample, ultrafine fibers with a diameter of about 50 nm were observed.

We have succeeded to prepare La 1− x OFeP ( 0 ≤ x ≤ 0.2 ) by sintering La, FeO and P powders in a vacuum at 1373 K for 192 h. This is a first report of hole doped superconductivity without introducing fifth elements in the LnOFePn (Ln: rare earths; Pn: pnictogen elements). The lattice constants a and c are 0.3961 and 0.8506 nm, respectively for x = 0 , and they are almost independent of x . However, the superconducting transition temperature T c increases with x and has a maximum value of 7 K at x = 0.10 and then decreases. This indicates that the hole concentration is optimum for x = 0.10 and becomes over doping above x = 0.10 . The lower and upper critical magnetic fields H c 1 and H c 2 are 2 × 1 0 2 and 1.7 × 1 0 4 Oe, respectively, for x = 0.10 , which are larger than those of LaOFeP and LaO 1− x F x FeP.

We have investigated the photoluminescence (PL) and the magnetic properties of CdPS3 intercalated with Ce and Eu ions. Two kinds of the intercalated phases are obtained. In the first phases (PCe and PEu), the hydrated rare earth ions exist in the van der Waals gap. The second phases (DCe and DEu) are obtained by the dehydration of PCe and PEu. Rare earth ions are inserted into the lamellar layers in them. All phases are paramagnetic. While the magnetic properties of PCe, PEu and DCe are explained by Ce3+ and Eu3+ ions, respectively, those of DEu suggest the coexistence of Eu2+ and Eu3+ ions. In the PL spectra at 4.2 K, while 4f → 5d transition of Ce3+ ions is observed in DCe, 4f → 4f transition of Eu3+ ions and 4f → 5d transition of Eu2+ ions are observed in DEu. These are consistent with the magnetic properties.

Electrical and magnetic properties of oxypnictides LnOZnPn (Ln = rare earths, Pn = pnictogen elements) with a ZrCuSiAs structure are investigated. The electrical resistivity of LnOZnPn decreases with an atomic number of Pn. The electrical resistivity has a maximum at Ln = Ce and then decreases with an atomic number of Ln in LnOZnAs, and it decreases with an atomic number of Ln in LnOZnSb. While LnOZnPn (Ln = La, Ce, Pr, Pn = P, As) are semiconducting, LnOZnSb (Ln = La, Ce) show a metal–semiconductor transition and NdOZnPn (Pn = As, Sb) become metallic. While LaOZnPn shows a diamagnetism, other LnOZnPn are paramagnetic. The effective magnetic moment μ eff deviates from the trivalent free ion values with an atomic number of Pn and Ln. The correlation between the deviation of μ eff and the decrease of the electrical resistivity suggests that 4f electrons contribute to the electrical conduction.

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.

Hydrogen anneal effects on reproducibility of the switching voltages for a resistive switching memory using anodic porous alumina have been investigated. The hydrogen anneal made the reproducibility worse.

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.

Thermal transport measurements have been made on the Fe-based superconductor ${\mathrm{Lu}}_{2}{\mathrm{Fe}}_{3}{\mathrm{Si}}_{5}$ (${T}_{c}\ensuremath{\sim}6\text{ }\text{ }\mathrm{K}$) down to a very low temperature ${T}_{c}/120$. The field and temperature dependences of the thermal conductivity confirm the multigap superconductivity with fully opened gaps on the whole Fermi surfaces. In comparison to ${\mathrm{MgB}}_{2}$, ${\mathrm{Lu}}_{2}{\mathrm{Fe}}_{3}{\mathrm{Si}}_{5}$ reveals a remarkably enhanced quasiparticle heat conduction in the mixed state. The results can be interpreted as a consequence of the unequal weight of the Fe $3d$-electron character among the distinct bands.

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)

We have succeeded to prepare La 1− x OFeP ( 0 ≤ x ≤ 0.2 ) by sintering La, FeO and P powders in a vacuum at 1373 K for 192 h. This is a first report of hole doped superconductivity without introducing fifth elements in the LnOFePn (Ln: rare earths; Pn: pnictogen elements). The lattice constants a and c are 0.3961 and 0.8506 nm, respectively for x = 0 , and they are almost independent of x . However, the superconducting transition temperature T c increases with x and has a maximum value of 7 K at x = 0.10 and then decreases. This indicates that the hole concentration is optimum for x = 0.10 and becomes over doping above x = 0.10 . The lower and upper critical magnetic fields H c 1 and H c 2 are 2 × 1 0 2 and 1.7 × 1 0 4 Oe, respectively, for x = 0.10 , which are larger than those of LaOFeP and LaO 1− x F x FeP.

We studied the effect of nonmagnetic and magnetic impurities on superconductivity in ${\text{Lu}}_{2}{\text{Fe}}_{3}{\text{Si}}_{5}$ by small-amount substitution of the Lu site and investigated structural, magnetic, and electrical properties of nonmagnetic ${({\text{Lu}}_{1\ensuremath{-}x}{\text{Sc}}_{x})}_{2}{\text{Fe}}_{3}{\text{Si}}_{5}$, ${({\text{Lu}}_{1\ensuremath{-}x}{\text{Y}}_{x})}_{2}{\text{Fe}}_{3}{\text{Si}}_{5}$, and magnetic ${({\text{Lu}}_{1\ensuremath{-}x}{\text{Dy}}_{x})}_{2}{\text{Fe}}_{3}{\text{Si}}_{5}$. The rapid depression of ${T}_{c}$ by nonmagnetic impurities in accordance with the increase in the residual resistivity reveals the strong pair breaking dominated by disorder.

Ultrasound velocity measurements of cubic spinel ${\text{GeCo}}_{2}{\text{O}}_{4}$ in single crystal were performed for the investigation of shear and compression moduli. The shear moduli in the paramagnetic state reveal the absence of Jahn-Teller activity despite the presence of orbital degeneracy in the ${\text{Co}}^{2+}$ ions. Such a Jahn-Teller inactivity indicates that the intersite orbital-orbital interaction is much stronger than the Jahn-Teller coupling. The compression moduli in the paramagnetic state near the N\'eel temperature ${T}_{N}$ reveal that the most relevant exchange path for the antiferromagnetic transition lies in the [111] direction. This exchange-path anisotropy is consistent with the antiferromagnetic structure with the wave vector $q\ensuremath{\parallel}[111]$, suggesting the presence of bond frustration due to competition among a direct ferromagnetic interaction and several distant-neighbor antiferromagnetic interactions. In the Jahn-Teller-inactive condition, the bond frustration can be induced by geometrical orbital frustration of ${t}_{2g}\text{\ensuremath{-}}{t}_{2g}$ interaction between the ${\text{Co}}^{2+}$ ions, which can be realized in the pyrochlore lattice of the high-spin ${\text{Co}}^{2+}$ with ${t}_{2g}$-orbital degeneracy. In ${\text{GeCo}}_{2}{\text{O}}_{4}$, the tetragonal elongation below ${T}_{N}$ releases the orbital frustration by quenching the orbital degeneracy.

Electrical and magnetic properties of oxypnictides LnOZnPn (Ln = rare earths, Pn = pnictogen elements) with a ZrCuSiAs structure are investigated. The electrical resistivity of LnOZnPn decreases with an atomic number of Pn. The electrical resistivity has a maximum at Ln = Ce and then decreases with an atomic number of Ln in LnOZnAs, and it decreases with an atomic number of Ln in LnOZnSb. While LnOZnPn (Ln = La, Ce, Pr, Pn = P, As) are semiconducting, LnOZnSb (Ln = La, Ce) show a metal–semiconductor transition and NdOZnPn (Pn = As, Sb) become metallic. While LaOZnPn shows a diamagnetism, other LnOZnPn are paramagnetic. The effective magnetic moment μ eff deviates from the trivalent free ion values with an atomic number of Pn and Ln. The correlation between the deviation of μ eff and the decrease of the electrical resistivity suggests that 4f electrons contribute to the electrical conduction.

We have performed temperature- (T-)dependent laser-photoemission spectroscopy of the antiferromagnetic (AF) superconductor ErNi 2 B 2 C to study the electronic-structure evolution reflecting the interplay between antiferromagnetism and superconductivity. The spectra at the superconducting (SC) phase show a very broad spectral shape. A T-dependent SC gap shows a sudden deviation from the BCS prediction just below T N . This observation can be explained well by the theoretical model and thus represents the characteristic bulk electronic structure of the AF SC phase for the first time.

We investigated magnetic properties of spinel oxides Ge(Co1-xMgx)2O4 with x = 0 ~ 0.5 to study the spin-orbit dilution effects on the magnetism of spin-orbit frustrated spinel GeCo2O4. We discovered that the magnetic moment per single Co2+ ion is decreased with increasing nonmagnetic Mg2+ concentration, which indicates the spin-orbit decoupling caused by the spin-orbit dilution. Additionally, small-amount substitution of Mg2+ for Co2+ causes the rapid increase of the positive Weiss temperature indicating the enhancement of ferromagnetic interactions, while the Mg2+ substitution suppresses the antiferromagnetic ordering resulting in the appearance of spin glass behavior. The present results suggest that the spin-orbit dilution causes the spin-orbit decoupling and the reinforcement of ferromagnetic frustration in GeCo2O4.

Ultrasound velocity measurements of the orbital-degenerate frustrated spinel MgV2O4 are performed in the disorder-free high-purity single crystal which exhibits successive structural and antiferromagnetic phase transitions, and in the disorder-introduced single crystal which exhibits spin-glass-like behavior. The measurements reveal coexisting two types of anomalous temperature dependence of the elastic moduli in the cubic paramagnetic phase: Curie-type softening with decreasing temperature, and softening with a characteristic minimum with decreasing temperature. These elastic anomalies should respectively originate from the coexisting orbital fluctuations and spin-cluster excitations.