大濱 哲夫

We performed zero-field nuclear magnetic resonance (ZF NMR) measurements of the antiferromagnetic and pressure-induced magnetically ordered phases of α-Mn at high pressures using a piston cylinder cell and an opposed-anvil high-pressure cell. We found that 55Mn internal magnetic fields disappeared discontinuously at 4.2–4.3 GPa. This suggests a first-order phase transition.

Abstract Comprehensive analyses of the ¹¹⁵In-NQR spectra employing a magnetic dipolar model and a transferred hyperfine model are performed to determine the magnetic structures of Ce₃PtIn₁₁. Based on this analysis, we confirmed that the magnetic moment at the Ce(1) site is very small (∼ 4% of that at the Ce(2) site) but finite. Further, for T_N2 < T < T_N1 and T < T_N2, the magnetic moments at the Ce(1) and Ce(2) sites are parallel to the c-axis, and the in-plane propagation vectors in the two Ce atoms sublattices are (q_a, q_b) = (1/2, 1/2). In the temperature interval T_N2 <T < T_N1, for example, both of them have a form of q = (1/2, 1/2, 0). For T < T_N2, the magnetic structure in the Ce(2) sublattice is defined by the propagation vector q = (1/2, 1/2, 1/2 or 0), whereas the Ce(1) sublattice forms a six-fold period structure given by q = (1/2, 1/2, 1/6).

We have clarified that the internal magnetic field in α-Mn is suppressed by pressure and that a first-order phase transition occurs at around 1.4-1.5 GPa by zero-field nuclear magnetic resonance. In the pressure-induced ordered state above about 1.4 GPa, the scenario of the canted-antiferromagnetic order or the ferrimagnetic order seems to be preferable to account for the weak ferromagnetism.

The low-temperature (LT) superstructure of NaV2O5 was determined by synchrotron radiation X-ray diffraction. Below the phase transition temperature at 34 K, the LT structure is monoclinic. It was determined that the LT structure is (a - b) × 2b × 4c with the space group A112, where a, b and c represent the high-temperature orthorhombic unit cell. The valence estimation of V ions shows that the V sites are clearly separated into two groups of V4+ and V5+ with a zigzag charge-ordering pattern. © 2003 Elsevier Science B.V. All rights reserved.

NaTiSi2O6 is a spin-1/2 member of the quasi-one-dimensional magnets; pyroxene family. This compound exhibits the phase transition at 210 K, which is accompanied by the structural change and the spin-singlet formation. The alternation of Ti-Ti distance within the chain made up of edge-shared TiO6 octahedra in the low temperature phase was observed in X-ray and neutron diffraction. This is the direct evidence for the formation of Ti3+-Ti3+ singlet pairs. © 2003 Elsevier Science B.V. All rights reserved.

We measured the 51V nuclear spin relaxation rate in the high-temperature phase of the quarter-filled ladder compound α′-NaV2O5. We compare the results with theories for the spin-1/2 Heisenberg chain and and no serious discrepancies between them. © 2003 Elsevier Science B.V. All rights reserved.

The low-temperature (LT) superstructure of α′-NaV 2O5 was determined by synchrotron radiation X-ray diffraction. Below the phase transition temperature associated with atomic displacement and charge ordering at 34 K, we observed Bragg peak splittings, which provide evidence that the LT structure is monoclinic. It was determined that the LT structure is (a - b) x 2b x 4c with the space group A112, where a, b and c represent the high-temperature orthorhombic unit cell. The valence estimation of V ions according to the bond valence sum method shows that the V sites are clearly separated into two groups of V4+ and V 5+ with a zigzag charge-ordering pattern. This LT structure is consistent with resonant X-ray and NMR measurements, and is in striking contrast to the LT structure previously reported, which includes V 4.5+ sites.

The nuclear spin-lattice relaxation rate (1/T1) at the V sites in CaV2O5 was measured. The relaxation rate in the two-leg ladder was calculated with a strong-coupling expansion (J⊥ ≫ J∥), and the result was compared with the experimental data. The comparison of the temperature dependence shows a good agreement and provides an estimate of both J⊥ and J∥ as well as the energy gap Δ. The magnetic field dependence of 1/T1 is also reproduced with this approximation in the temperature range T ≲ Δ/4. It is found that the magnon-magnon collision, which leads to a diffusive behavior, is negligible in this temperature range. The effect of inter-ladder coupling is investigated and found to be weak in CaV2O5. Thus the strong-coupling expansion calculation a grees satisfactorily with the experimental data, but a discrepancy between the exchange parameters deduced from the magnetic susceptibility and 1/T1 is found.

23Na NMR spectrum measurements in α′-NaV2O5 with a single-crystalline sample are reported. In the charge-ordered phase, the number of inequivalent Na sites observed is more than that expected from the low-temperature structures of space group Fmm2 reported so far. This disagreement indicates that the real structure, including both atomic displacement and charge disproportionation, is of lower symmetry. It is suggested that zigzag ordering is the most probable. It is also found that the temperature variation of the NMR spectra near the transition temperature is incompatible with that of second-order transitions.

We report 51V NMR study of the Na-deficient Na1 -XV2O5 (x = 0, 0.02, 0.04) with single-crystal samples. Above the charge-ordering temperature (Tc), noticeable line broadening was observed at low temperatures in all the samples investigated. This line broadening is of magnetic origin and significantly decreases below Tc. The origin of this observation is discussed concerning one-dimensional character of this system. © 2000 Elsevier Science B.V. All rights reserved.

We report (Formula presented) NMR measurements in (Formula presented)-(Formula presented), which has been considered as a spin-Peierls (SP) system. Above the transition temperature (Formula presented) only one set of V sites was observed in the NMR spectra, indicating that all the V sites are in a uniform electronic state with the average oxidation of (Formula presented) Below (Formula presented) there appear two inequivalent sets of V sites assigned to (Formula presented) and (Formula presented) states. This result indicates that the phase transition in (Formula presented)-(Formula presented) is not a conventional SP transition but a charge ordering. Below (Formula presented) 1/(Formula presented) at the (Formula presented) sites has an activated T dependence, confirming a spin-singlet ground state with the excitation gap (Formula presented). © 1999 The American Physical Society.

51V NMR measurements in the uniform state of NaV2O5 are presented. The NMR shift and the electrical field gradient (EFG) were measured at the tetravalent V sites. Observed anisotropy of the NMR shift and the EFG indicates that the dxy-like orbital is occupied. It has been found that the orbital susceptibility in the uniform state is significantly smaller than the residual susceptibility in the spin-Peierls state. The possible origins of this disagreement are discussed.

23Na NMR spectrum measurementsin a spin-Peierls system, NaV 2O 5, are presented.Below the spin-Peierls transition temperature (T sp),two distinct sites of Na appear, confirming the structural phase transitionat T sp.The orbital susceptibility was evaluatedusing the temperature dependence of the 23Na NMR shift;it is suggested that the orbital susceptibility changes near T sp.The temperature dependence of the spin susceptibility obeysan activation law below ∼20 K with a gap of 98 K.

The NMR shift and the nuclear spin-lattice relaxation rate (1/T1) were measured at the V sites in the two-dimensional compound CaV4O9. The NMR shift becomes a positive constant below ∼15 K, indicating that the intrinsic spin susceptibility vanishes at low temperatures. The activated T dependence of 1/T1 has revealed the dynamical spin gap ΔR ∼ 104 K, which is rather smaller than the static spin gap determined from the susceptibility ΔS ∼ 135 K.

The anisotropies of the Knight shift and the nuclear spin-lattice relaxation time (T1) at the Sn sites have been measured up to 300 K with a magnetically aligned powder sample of CeNiSn. In contrast to the results of previous neutron scattering measurements, the anisotropy of 1/T1 is weak and almost temperature (T)-independent. This indicates that the substantial spectral weight exists in the higher energy region than the quasielastic magnetic excitation at high T. Above about 60 K, 1/T1 increases linearly with T, suggesting that the T dependence of the quasielastic line width is weak. © 1995, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

The magnetic susceptibility and the Cu and Si Knight shifts have been measured in CeCu2 Si2. The transferred hyperfine couplings at both the sites are strongly temperature-dependent and noticeably anisotropic at low temperatures. The negative hyperfine couplings observed for the magnetic field along the c-axis at the low temperatures are anomalous as in Ce intermetallic compounds. It is pointed out that the transferred hyperfine coupling due to the hybridization of the f electrons with the ligand s electrons can be anisotropic and also can reverse its sign at the lower temperatures than the crystal-field splitting energy. It is concluded that this hybridization is the dominant mechanism of the transferred hyperfine coupling and responsible for the anomaly. © 1995, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

The temperature dependence and the anisotropy of the magnetic susceptibility and the Knight shifts have been measured in PrCu2 Si2 and GdCu2 Si2 with magnetically aligned powder samples. For GdCu2Si2, the isotropic s-f exchange model provides a good description of its magnetic properties, but in PrCu2Si2 an anisotropic two-ion coupling has been found. It has been revealed that the hyperfine coupling at the Si sites is enhanced in PrCu2Si2, thereby suggesting preferential hybridization of the f-electrons with the Si valence electrons. This effect is responsible for the unusually high Neel temperature of PrCu2Si2. © 1995, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

In the metallic compound U3Co3Sb4, the nuclear spin-lattice relaxation time T1 and the NQR frequency vQ at the Sb sites have been measured. The temperature dependence of T1 and vQ can be understood as the properties of a nearly ferromagnetic metal. © 1993.

NMR and NQR measurements have been made for PauH paramagnetic UNi2Ga. The temperature dependence of the Knight shift and the nuclear relaxation rate 1/T1 at the Ga sites has been measured between 4.2 K and 290 K. Fermi Hquid behavior, that is, temperature-independent Knight shift and 1/T1 proportional to temperature, has been observed over the measured temperature range. A quantitative analysis suggests that the enhancement of the wave-vector-dependent susceptibility X(q) is fairly localized near q~inq space. © 1993, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.

The 121Sb Knight shift and the 121,123Sb nuclear spin-lattice relaxation time T1 in U3Ni3 Sb4 have been measured. In the temperature range between 25 and 250 K, 1/T1, which has been measured by nuclear quadrupole resonance (NQR), follows an activation law with an activation energy of 21.1 meV. The relaxation process is dominated by the magnetic contribution. The Knight shift is represented as the sum of a strongly temperature-dependent anisotropic term and a temperature-independent term. The transferred hyperfine field from the U 5f electrons through the U 5f-Sb 5p hybridization makes an important contribution to the Knight shift. © 1992, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.