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We report a $^{205}$Tl NMR study of vortex state for an aligned polycrystalline sample of an overdoped high-$T_c$ superconductor Tl$_2$Ba$_2$CuO$_{6+\delta}$ ($T_{c}\sim$85 K) with magnetic field 2 T along the c axis. We observed an imperfect vortex lattice, so-called Bragg glass at $T$=5 K, coexistence of vortex solid with liquid between 10 and 60 K, and vortex melting between 65 and 85 K. No evidence for local antiferromagnetic ordering at vortex cores was found for our sample.

We observed a field-induced magnetic ordering in the quasi one-dimensional antiferromagnetic alternating chain system Pb 2 V 3 O 9 for the first time. The characteristic temperature dependence of t...

Nuclear-magnetic-resonance (NMR) studies were performed in Na0.7CoO2 to investigate its magnetic properties from the microscopic point of view. Na0.7CoO2 is a starting compound of the recently discovered superconductor Na0.35CoO2·1.3H2O, in which Co atoms form a tri- angular structure. Although 23 Na-NMR results show no magnetic anomaly down to 1.5 K, Knight shift ( 23 K) and 23 (1/T1T) of 23 Na continue to increase, suggestive of the vicinity of the magnetic phase. From the relation between 23 K and 23 (1/T1T), it is shown that ferromagnetic fluctuations are dominant below 100 K. In addition to the ferromagnetic correlations, antifer- romagnetic fluctuations are also developed in a low temperature region below 4 K. Magnetic fluctuations suggested by our experiment are discussed. Since the superconductivity in NaxCoO2·yH2O (x � 0.35, y � 1.3) with the transition temperature Tc � 5 K was discovered by Takada et al, 1 understanding of mag- netic properties in the CoO2 plane is one of the most important topics in strongly correlated electron systems. This is because the magnetic properties in the plane might be related to the mechanism of the unconventional superconductivity in the compound. In addition, the Co atoms form a triangular lattice in the plane, which is an- ticipated to possess the geometrical frustrations. Novel types of magnetic transitions and ground states are ex- pected due to a suppression of a long-range magnetic ordering by the geometrical frustrations associated with the triangular structure. In particular, only a few com- pounds have been known to date which possess a metallic property down to the lowest temperature with the geo- metrical frustrations, e.g. (YSc)Mn2 2 and LiV2O4. 3 The NaxCoO2 (0.5 < x < 0.78) compound is considered to be one of the promising metallic compounds with the geometrical frustrations at low temperature. 4 In this paper, we report the magnetic properties in Na0.7CoO2 revealed by our 23 Na-NMR experiments. Powder samples of Na0.7CoO2 were prepared from Na2CO3 (99.99%) and Co3O4(99.9%) by solid-state re- action. Its quality was checked by X-ray powder diffrac- tion. The Na content, which is very sensitive to physical properties, is estimated to be 0.702 by the inductive- coupled plasma atomic emission spectroscopy (ICP- AES) method. Using this well-characterized powder sample, 23 Na- and 59 Co-NMR measurements were per- formed.

We have performed Co-nuclear quadrupole resonance (NQR) studies on Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O compounds with different Na ($x$) and hydrate ($y$) contents. Two samples with different Na contents but nearly the same $T_c$ values ($x$ = 0.348, $T_c$ = 4.7 K ; $x$ = 0.339, $T_c$ = 4.6 K) were investigated. The spin-lattice relaxation rate $1/T_1$ in the superconducting (SC) and normal states is almost the same for the two samples except just above $T_c$. NQR measurements were also performed on different-hydrate-content samples with different $T_c$ values, which were prepared from the same Na-content ($x$ = 0.348) sample. From measurements of $1/T_1$ using the different-hydrate-content samples, it was found that a low-$T_c$ sample with $T_c = 3.9$ K has a larger residual density of states (DOS) in the SC state and a smaller increase of $1/T_1T$ just above $T_c$ than a high-$T_c$ sample with $T_c$ = 4.7 K. The former behavior is consistent with that observed in unconventional superconductors, and the latter suggests the relationship between $T_c$ and the increase in DOS just above $T_c$. This increase, which is seemingly associated with the two-dimensionality of the CoO$_2$ plane, is considered to be one of the most important factors for the occurrence of superconductivity.

The pseudo two-dimensional (2D) magnetic systems (LaS) 1.14 [ M x (NbS 2 ) 2 ] ( M = Mn, Fe, Co) were synthesized for the first time, and their magnetic and transport properties were investigated. (LaS) 1.14 [ M x (NbS 2 ) 2 ] show three-dimensional (3D) magnetic transitions at 4.4, 6.2 and 15 K for M = Mn, Co and Fe, respectively. The 3D transition temperatures are lowered by the quasi-2D magnetic property brought about the insertion of the extra layer LaS to the layered magnets M x NbS 2 , and the 2D short-range order effects are observed in the heat capacity at much higher than the transition temperatures. The minimum behavior of ρ and the maximum behavior of R H appear in the low temperatures. We consider that the weak localization in the quasi-2D transport system causes the anomalous behaviors of the transport properties.

We applied 57 Fe Mossbauer spectroscopy for investigating the Ising spin triangular lattice antiferromagnet (TLA) (CeS) 1.16 [Fe 0.33 (NbS 2 ) 2 ] between 2 and 300 K. The spectra revealed that the relaxation time of the hyperfine field markedly changes in the intermediate phase between T N1 =22 K and T N2 =15 K due to strong spin fluctuation. The relaxation of the hyperfine field is not sufficiently fast as a paramagnet even at 77 K, which is much higher than T N1 , and the inverse susceptibility of (LaS) 1.14 [Fe 0.33 (NbS 2 ) 2 ] deviates from the Curie–Weiss law below 100 K. These results indicate that an unusual short-range order exists above T N1 . The temperature dependence of the Mossbauer spectra can be explained by phase transition of the three-dimensional TLA model with weak interlayer exchange interactions.

We report on the anion substitution and pressure effects on the layered compound 1T-CrSe2 with unusual-valent chromium. Owing to the competition between the orbital degrees of freedom and the valence instability of Cr4+, the systems of Cr(Se1−xSx)2 and Cr(Se1−yTey)2 exhibit several types of phase transition. As well as the parent compound CrSe2, Te-substituted samples and slightly S-substituted samples of x ≤ 0.025 show two phase transitions, which are likely to be related to both the valence change and orbital ordering of Cr4+. The highly S-substituted samples of x > 0.04 show one phase transition, which can be attributed to orbital ordering without a valence change. The intermediate S-substituted samples of \(0.025 < x \lesssim 0.04\) exhibit the above three types of phase transition and, in addition, another type of transition. In contrast, the pressure effect on CrSe2 is qualitatively different from these anion substitution effects, which indicates that these anion substitution effects are not simply ...

We have successfully synthesized a single-crystal sample of the layered structure ferromagnet Fe3GeTe2 and present the results of its complete characterization using powder X-ray diffraction (XRD), magnetic susceptibility χ(T), isotherm magnetization \(M(H)\), specific heat \(C(T)\), and electrical resistivity ρ(T) measurements. The rietveld refinement of the powder XRD data confirms that Fe3GeTe2 crystallizes in the space group \(p6_{3}/mmc\). The χ(T), \(C(T)\), and ρ(T) data indicate bulk ferromagnetic ordering at \(T_{\text{C}} = 220\) K with a saturation moment of 1.625 µB/Fe at zero temperature. The magnetic measurements of the external magnetic field \(H\) applied along the \(c\)-axis and \(ab\)-plane show a large anisotropy in this material. We found that the modified Arrott plot (\(M^{4}\)–\(H/M\)) rather than the \(M^{2}\)–\(H/M\) plot is obeyed in this material. The generalized Rhodes–Wohlfarth ratio indicates an itinerant magnetism in this material. The spin fluctuation parameters are estimate...

We investigated five \(S = 1/2\) (Ti3+) double perovskite fluorides of the form \(A_{2}B\)TiF6 (\(A\) = \(K\), Rb, Cs; \(B\) = Na, K, Rb; \(A \neq B\)). All five compounds are cubic at room temperature and show various phase transitions at low temperatures, depending on lattice instability. Rb2NaTiF6 shows a Jahn–Teller distortion at 24 K. For Cs2RbTiF6 and Rb2KTiF6, structural transitions with octahedral rotations are observed at 115 and 230 K, respectively. In contrast, K2NaTiF6 shows successive structural transitions at 170 and 110 K. Moreover, Cs2KTiF6 exhibits a unique phase transition at 12 K, which is a result of the competition among the Jahn–Teller effect, lattice instability, and spin frustration. At high temperatures, all have the negative Curie–Weiss temperature \(\theta_{\text{CW}}\simeq -40\) K, indicating dominant antiferromagnetic interactions. At low temperatures, \(g\)-factors decrease and \(\theta_{\text{CW}}\) reaches about 0 K, although all five compounds have different low-temperatur...

We study the normal state properties of a noncentrosymmetric superconductor Mo 3 Al 2 C by means of the nuclear magnetic resonance (NMR) experiment. The change of NMR line shape and the peak of the nuclear spin-echo decay rate at ≃130 K indicate that the Mo 3 Al 2 C exhibits the phase transition prior to the emergence of superconductivity. The reduction in the conduction electron density is inferred from the decrease in the nuclear spin–lattice relaxation rate. By considering the result of the energy band calculation, we discussed the nature of the newly found phase transition.

Temperature dependences of the electrical resistivity, magnetic susceptibility and specific heat have been measured on the single crystal Ca 3 Ir 4 Sn 13 . This compound shows a bulk superconducting transition at ≈7.0 K. We observe peak-like anomalies, which are attributed to the ferromagnetic spin-fluctuations, in susceptibility and resistivity. The resistivity shows a non Fermi-liquid behavior as ρ=ρ 0 + A * T n ( n =1.2 for H =0) in normal state at low temperatures, and develops into a Fermi-liquid behavior by the suppression of the ferromagnetic spin fluctuation on increasing magnetic fields ( n =2 for H =14 T). The superconductivity disappears for magnetic field H ≥7 T as well as the diminution of the ferromagnetic spin fluctuations. We find that the superconductivity coexists with the ferromagnetic spin fluctuations and seems to cooperate in this material. The large values of Δ C /γ n T c =2.78, Kadowaki–Woods ratio A /γ n 2 =1.14×10 -5 µΩ·cm (K·mol/mJ) 2 and Wilson ratio R w =π 2 k B 2 χ p /3µ B 2 ...

The spin-fluctuation effect in the Se-substituted single crystalline Fe 1+δ Te 1- x Se x ( x = 0, 0.05, 0.12, 0.20, 0.30, 0.33, 0.45, and 0.48; 0≤δ≤0.12) and the polycrystalline Fe 1.11 Se has been studied by the measurements of the X-ray diffraction, the magnetic susceptibility under high magnetic fields and the electrical resistivity under magnetic fields up to 14 T. The samples with x = 0.05, 0.12, 0.20, 0.30, 0.33, 0.45, and 0.48 show superconducting transition temperatures in the ranger of 10–14 K. We obtained their intrinsic susceptibilities by the Honda–Owen method. A nearly linear-in- T behavior in magnetic susceptibility of Se-rich superconducting samples was observed, indicating the antiferromagnetic spin fluctuations have a strong link with the superconductivity in this series. The upper critical field µ 0 H c2 orb for T →0 was estimated to exceed the Pauli paramagnetic limit. The Kadowaki–Woods and Wilson ratios indicate that electrons are strongly correlated in this system. Furthermore, the s...

The structural and magnetic properties of Li 7 RuO 6 were investigated in single crystals. This ruthenate is composed of quasi-two-dimensional distorted triangular lattices of RuO 6 octahedra isolated from each other. Because of the ( t 2 g ) 3 electron configuration of Ru 5+ , a Heisenberg-type magnetic behavior of S =3/2 spins is expected. The magnetic susceptibility reveals an antiferromagnetic transition at T N =6.5 K, much lower than the absolute value of the Weiss temperature Θ=-75 K, indicating a strong geometric frustration. The small and almost isotropic drop of the susceptibility below T N suggests a helical magnetic structure. Specific heat revealed an additional phase transition at T ' N =12 K, suggesting an unconventional magnetic phase diagram.

The Si-intercalated compounds Ni 3 AlSi x ( x = 0, 0.01, 0.02, 0.05, 0.07, and 0.1) and Si substituted compounds Ni 3 Al 1- y Si y ( y = 0, 0.01, 0.015, 0.02, 0.04, 0.06, 0.1, and 0.2) were synthesized and investigated by macroscopic and microscopic magnetic measurements. We found that the common features to these two systems are: (i) drastic disappearance of ferromagnetism in Ni 3 Al with only a small Si concentration x c ≃0.01 or y c ≃0.02, (ii) T 4/3 -temperature dependences in both square spontaneous magnetization M (0, T ) 2 and inverse initial susceptibility χ 0 -1 near the Curie temperature T C for ferromagnetic metal. T 4/3 -temperature dependence of χ 0 -1 are found to hold even for the paramagnetic metals that are classified as nearly itinerant ferromagnets very close to the ferromagnetic instability. On the other hand, our 27 Al NMR results show that the Si intercalation leads to the changes of the hyperfine coupling constant A hf , the characteristic parameters of spin fluctuations, T 0 and T ...

$^{75}$As and $^{139}$La field-swept NMR spectra were obtained for the novel weakly itinerant ferromagnet LaCoAsO with 2D layered structure above the Curie temperature of 55 K. By analyzing NMR spectra, temperature dependences of Knight shift $K$ and nuclear quadrupole resonance frequency $\nu_Q$ were obtained successfully for each nucleus. We confirmed from the so-called $K$-$\chi$ plots that the macroscopic magnetization of our {LaCoAsO} powder sample is intrinsic and does not contain the contribution from impurity phases. We estimated hyperfine coupling constants from the slope of $K$-$\chi$ plots and compared to that of iron-arsenide superconductor., Comment: 5 pages, 5 figures, published on J. Phys. Soc. Jpn. at Vol.79, pp.054703 (2010).

The itinerant-electron ferromagnet Ni 3 Al and its carbon-intercalated compounds Ni 3 AlC x ( x = 0.01, 0.02, 0.04, 0.05, 0.08, and 0.1) were synthesized and investigated by magnetization measurements between 2 and 300 K with the magnetic field H up to 5 T and 27 Al NMR measurements at H = 7.4847 T. The Curie temperature is suppressed and disappears only by the carbon intercalation with x = 0.02. For the samples with x ≤0.02, the T 4/3 -temperature dependence of M s 2 ( M s : spontaneous magnetization) and χ 0 -1 (χ 0 : initial susceptibility) are newly observed, being consistent with the predication of the self-consistent renormalization theory of spin fluctuations for weak itinerant ferromagnets. The coupling constant between the 27 Al Knight shift and the bulk susceptibility of Ni 3 AlC 0.1 is three times larger than that of Ni 3 Al, suggesting the drastic change of the electronic state by the carbon intercalation.

We report 125 Te and 139 La NMR studies for single crystals of LaTe 3 between 10 and 160 K under an applied field of H = 7.4841 T. We observed the broad 125 Te(1) NMR signals of metallic Te(1) sheets with a superlattice modulation and the sharp 125 Te(2) and 139 La NMR signals of LaTe(2) b i -layers. Temperature dependence of 125 Te(1) nuclear spin–lattice relaxation times of the modulated Te(1) sheets obeys a modified Korringa relation. The results indicate that the electronic state on the Te(1) sheets is a Landau–Fermi liquid on a misfit superlattice or a Tomonaga–Luttinger liquid in a two-dimensional charge-density wave ordering state.

Co nuclear-quadrupole-resonance (NQR) measurements were performed on various bilayered hydrate cobaltate Na x (H 3 O) z CoO 2 · y H 2 O with different values of the superconducting and magnetic-ordering temperatures, T c and T M , respectively. From measurements of the temperature and sample dependence of the NQR frequency, it was revealed that the NQR frequency is changed by the change of the electric field gradient (EFG) along the c axis ν z z rather than the asymmetry of EFG within the a b -plane. In addition, it is considered that the change of ν z z is gaverned mainly by the trigonal distortion of the CoO 2 block layers along the c axis, from the relationships between ν z z and the various physical parameters. We found the tendency that samples with ν z z larger than 4.2 MHz show magnetic ordering, whereas samples with lower ν z z show superconductivity. We measured the nuclear spin–lattice relaxation rate 1/ T 1 in these samples, and found that magnetic fluctuations depend on samples. The higher-ν z...

We report 59Co nuclear quadrupole resonance (NQR) studies of bilayer water intercalated sodium cobalt oxides NaxCoO2.yH2O (BLH) with the superconducting transition temperatures, 2 K < T_c +-7/2 and found a dome shape superconducting phase. The 59Co NQR spectrum of the non-superconducting BLH shows a broadening below T_M without the critical divergence of 1/T_1 and 1/T_2, suggesting an unconventional magnetic ordering. The degree of the enhancement of 1/T_1T at low temperatures increases with the increase of nu_3 though the optimal nu_3~12.30 MHz. In the NaxCoO2.yH2O system, the optimal-T_c superconductivity emerges close to the magnetic instability. T_c is suppressed near the phase boundary at nu_3~12.50 MHz, which is not a conventional magnetic quantum critical point., 4 pages, 5 figures

We report a $^{63}$Cu nuclear quadrupole resonance (NQR) study of the vortex state for an aligned polycrystalline sample of a slightly overdoped high-$T_c$ superconductor YBa$_2$Cu$_3$O$_{7-\delta}$ ($T_{c}\sim$92 K) at a low magnetic field of 96 mT along the c axis, near a lower critical field $H_{c1}$. We observed the frequency distribution of the nuclear spin-lattice relaxation time $^{63}T_1$ in the Zeeman-perturbed $^{63}$Cu NQR spectrum below $T_c$. The characteristic behavior of 1/$^{63}T_1$, taking the minimum values with respect to temperature and frequency, indicates the significant role of antiferromagnetic spin fluctuations in the Doppler-shifted quasiparticle energy spectrum inside and outside vortex cores., Comment: 4 pages, 4 figures