Hironori Nishihara

NQR and NMR of 139La in antiferromagnetic La2CuO4−δ

Rather complex Nuclear Quadrupole Resonance (NQR) spectrum of 139 La in La 2 CuO 4-δ , observed at low temperatures is successfully interpreted as a signal from a single La site which suffers a combined effect of a quadrupole coupling (coupling constant ν Q =6.38±0.02 MHz with asymmetry parameter η=0.01±0.01) and a small Zeeman term where the internal field ( H N =0.997±0.010 kOe) is nearly perpendicular to the direction of the maximum field gradient. The spin structure of Cu at low temperatures is suggested to be a simple antiferromagnet with sublattice moments parallel to the a -axis. The temperature dependences of ν Q and H N have been measured in detail. The fractional decrease of H N which is proportional to the Cu sublattice moments is found to be in general accord with the prediction of spin fluctuations theory for weak itinerant antiferromagnets.

Magnetic and Crystallographic Properties of Shape Memory Alloys Ni2Mn1+xSn1-x

X-ray powder diffraction and magnetization measurements were done on the magnetic shape memory alloys Ni2Mn1+xSn1-x. The alloys with 0≤x≾0.4 crystallize in the cubic L21 structure and exhibit the ferromagnetic behavior. X-ray diffraction patterns indicate that the excess Mn atoms occupy the Sn sites. Furthermore, magnetization measurements make clear that the Mn atoms, which substitute for Sn sites, are coupled antiferromagnetically to the ferromagnetic manganese sublattices. The alloys with 0.4≾x≤0.6 undergo a martensitic transition from the high temperature L21 structure to the orthorhombic 4O one. These alloys show a variety of magnetic transitions. A magnetic phase diagram of Ni2Mn1+xSn1-x system is discussed qualitatively on the basis of the interatomic dependence of the exchange interactions.

Synthesis of new ternary tunnel chalcogenides by ion exchange reactions and deintercalation of the ternary chromium selenides

Many ternary tunnel chalcogenides with TlV5S8 and TlV6S8 structures were prepared by ion exchange reactions: TlM5X8 + AI (or ABr) → AM5X8 + TlI (or TlBr), and TlM6X8 + AI (or ABr) → AM6X8 + TlI (or TlBr), where M = Ti, V, and Cr, X=S, Se, and Te, and A=Na, K, Rb, and Cs. The compounds newly obtained are CsxV5S8, ATi5Se8 (A=Na, K, Rb, and Cs), AV5Se8 (A=K, Rb, and Cs), ACr5Se8 (A=Na, Rb, and Cs), ACr5Te8 (A=K, Rb, and Cs), AV6S8 (A=Na and Cs) and Ati6Se8 (A = Na, Rb, and Cs). TlxCr5Se8 (0.33≤x<1.0) was prepared by deintercalation method. The magnetic susceptibility (χ) measurements showed TlxCr5Se8 (x=0.65 and 1.0) to be an antiferromagnet with Neel point at 52 K. Effective magnetic moment Peff (μBCr atom) in TlxCr5Se8 was close to that of Cr3+ and was essentially invariable with x, suggesting the charge of host matrix [Cr5Se8 is constantly −1.

Proton NMR in degraded powder of YBa2Cu3O7−δ

Pulsed NMR of 1H has been applied to characterize aged powders of YBa2Cu3O7-δ and its family. A proton site with an internal field of 150 Oe, which is from antiferromagnetically ordered copper moments, has been observed in antiferromagnetic, tetragonal Yba2Cu3O7-δ. A proton signal inside the superconducting region of orthorhombic YBa2Cu3O7-δ has been observed together with signals from regions of nonmagnetic, nonsuperconducting degraded products for both cases. Overall features are interpreted along with a recently proposed topotactic mechanism of hydration reaction.

Magnetic properties of Co2TiGa compound

The precise magnetization measurement under ambient pressure has been made for Co 2 TiGa using a SQUID magnetometer. The spontaneous magnetization σ s (T) at ambient pressure is expressed empirically as a function of temperature as σ s (T) 2 =σ s (0) 2 1-T 2 /T 2 C ), where σ s (0) and T C are 19.5 emu/g, 128 K, respectively. The pressure derivative of T C has been obtained from the results of temperature dependence of initial permeability under pressure up to 11 kbar. The value of ∂T C /∂P is -1.27 K/kbar, The results are discussed on the basis of the spin fluctuation theory for itinerant electron magnetism.

NMR Study of the Metal-Insulator-Magnetic Transitions in BaVS3

Microscopic magnetic properties of BaVS 3 have been investigated by means of NMR of 51 V in the high-temperature metallic phase, in the low-temperature insulating phase which exhibits magnetic order and in the intermediate temperature insulating phase. The hyperfine coupling constant was obtained to be -113 ±3 (kOe/µ B ) in the high temperature phase. The temperature dependences of the line shift, the line width and the spin-echo decay time in the intermediate phase suggest that the mechanism of the metal-to-insulator transition around 70 K involves a gradual pairing of the magnetic vanadium atoms in linear chains to form a non-magnetic state with decreasing temperature. The coexistence of both magnetic and nonmagnetic sites of vanadium atoms and the spin structure in the low temperature phase are discussed.

NMR of Co59 in an Approximate Ising System: CoCl2·2H2O

Nuclear Magnetic Resonance (NMR) of Co 59 has been investigated in the three metamagnetic phases of CoCl 2 ·2H 2 O at liquid helium temperatures. The hyperfine field in zero external field at 0 K was found to be +421.1±0.4 kOe, which agrees well with the calculated value where the orbital contribution is dominant. When the external field H 0 was applied parallel to b -axis, the interaction of the nucleus with the electronic moment induced by H 0 was observed to be about 0.29 times the direct coupling with the external field. The jumps of the hyperfine field were also observed as the manifestation of the changes of the exchange interactions of Co 2+ spin with adjacent linear chains. The most striking feature of the data is the effects of the spin reduction of 0.1∼1% depending on H 0 caused by the magnon bound states in an approximate Ising system to CoCl 2 ·2H 2 O.

Magnetic properties of Heusler compounds Ru2CrGe and Ru2CrSn

We have succeeded in synthesizing the Heusler compounds Ru2CrGe and Ru2CrSn and performed magnetization measurements in high magnetic fields up to 18T in order to investigate the magnetic properties of Ru2CrGe and Ru2CrSn. We confirmed that the compounds have an ordered Heusler L21 structure. It was found that Ru2CrGe is an antiferromagnet with the Neel temperature TN=13K and Ru2CrSn shows a spin-glass-like behavior below 7K. The Heisenberg exchange constants determined on the basis of the experimental results for Ru2CrGe are different from the values of Mn-based Heusler compounds.

NMR of 109Ag and 19F in Ag2F–Modified Korringa Relation in a Two-Dimensional Metal

The spin-lattice relaxation time T 1 and the Knight shift K for 109 Ag in Ag 2 F have been measured between 4.2 and 77 K at 10.7 MHz. The results T 1 T =12.9±1.3 (secK) and K =+0.38±0.01 (%) are discussed in terms of the modified Korringa relation in the case of a two-dimensional metal. The temperature and frequency dependence of T 1 of 19 F was reexamined. The contribution of the Knight shift to the total shift of 19 F in Ag 2 F is estimated to be 0.025±0.002 (%).

Phase relations and some electrical properties of compounds in TiS2-NbS2 system

The phase relations of the TiS{sub 2}-NbS{sub 2} system have been studied in the temperature range from 500 to 1,100 C. Three polytypes, 1T, 2H{sub a}, and 3R, are observed. The 1T-polytype is a high temperature phase, the 2H{sub a}-polytype is a medium temperature phase, and the 3R-polytype is a low temperature phase. The 1T-polytype spreads over almost the whole composition range of the system. The 2H{sub a}-polytype has a very narrow homogeneity range around 100% NbS{sub 2}. The 3R-polytype is stable in the composition range between 100% and 96% NbS{sub 2}. The electrical resistivity has been measured from 30 to 300 K. The magnetic susceptibility has been measured from 5 to 300 K. The samples of the 1T-Ti{sub 1{minus}x}Nb{sub x}S{sub 2} (X {le} 0.90) show the semiconductive behavior caused by an impurity scattering mechanism.