Toshinobu Tsuda

Observation of Nuclear Resonance of Cu in Antiferromagnetic La2CuO4-δ and CuO

The zero-field nuclear magnetic resonance (NMR) of Cu in the high- T c related oxide, La 2 CuO 4-δ , has been observed at 93.85±0.1 MHz for 63 Cu and 100.1±0.1 MHz for 65 Cu with well-articulated quadrupole splittings at 1.3 K. The resonance pattern is successfully analyzed as a result of the combined effect of a quadrupole coupling (coupling constant ν Q =31.9 MHz for 63 Cu and 29.5 MHz for 65 Cu with asymmetry parameter η=0.03) and a Zeeman term where the internal field ( H N =78.78 kOe) is nearly perpendicular to the direction of the maximum electric field gradient (EFG). The quadrupole split zero-field NMR signals have also been observed in CuO. The observed frequencies yield parameter values of 137.1±0.1 MHz, 146.9±0.1 MHz, 20.07 MHz, 18.57 MHz, 0.20, 121.5 kOe, and the direction of H N is found to be nearly parallel to the direction of the maximum EFG.

Nuclear Spin-Lattice Relaxation of 63,65Cu at the Cu(2) Sites of the High Tc Superconductor YBa2Cu3O7-δ

Nuclear spin-lattice relaxation times T 1 of 63 Cu and 65 Cu in the Cu(2) sites of the high T c superconductor YBa 2 Cu 3 O 7-δ (δ≃0.1, T c =91 K) have been measured for the wide temperature range between 2.2 K and 300 K. The temperature dependence of 1/ T 1 in the normal state has been found to be the one unexpected for simple metals. The influences of antiferromagnetic spin fluctuations in the d–p bands in the Cu(2)–O layers are discussed. The temperature dependence of 1/ T 1 below T c has also been found to be the one which is not expected for isotropic BCS superconductors. The power-law like temperature dependence of 1/ T 1 without any enhancement just below T c has been observed between 40 K and 91 K.

Site assignment for Cu NQR lines in YBa2Cu3O7−δ superconductor

A clear evidence is presented on the site assignment for two distinct 63 Cu NQR lines observed at around 22 MHz and 31 MHz in YBa 2 Cu 3 O 7-δ superconductor ( T c =91 K). Using the highly oriented powder sample under the applied magnetic field in the superconducting state where the c -axis being perpendicular to the field, the 63 Cu NMR spectra have carefully been examined. From analysis of the spectra, the directions of the principal axes of the electric field gradient (EFG) tensors at the two 63 Cu sites have been determined. Comparing these experimental results with the calculated EFG tensors at the crystallographically inequivalent two Cu sites [the chain Cu(1) and the planar Cu(2) sites] in YBa 2 Cu 3 O 7 , the site associated with the lower and the higher NQR frequencies are uniquely assigned to the Cu(1) and the Cu(2) sites, respectively.

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.

Orbital and Spin Susceptibilities of the Plane Copper Sites in YBa2Cu3Ox System

The tensor components of the magnetic susceptibility (χ) and the Knight shift ( K ) at the plane Cu(2) sites have been precisely measured in YBa 2 Cu 3 O x with x =6.98, 6.48 and 6.0. Using the temperature dependent χ and K observed in YBa 2 Cu 3 O 6.48 , we have carried out the analysis of K -χ diagram to make a separation of the observed susceptibility into orbital and spin contributions. Assuming the orbital contribution from the Cu(2) ions is independent of x for 6.0≤ x ≤7.0, we have obtained the x dependent spin susceptibility of the Cu(2) ions. The obtained spin susceptibility is discussed in comparison with the theoretical predictions of the Heisenberg model, the Stoner model and a band structure calculation. A complete set of the parameter values used in the crystal field model of a Cu 2+ ion (the crystal field splittings, the orbital reduction factor, the spin-orbit coupling and the g -factors) has been determined from the K -χ analysis.

Nuclear Quadrupole Resonance of Cu in the Paramagnetic State of CuO

The 63 Cu nuclear quadrupole resonance (NQR) technique has been utilized to investigate the dynamical properties of CuO in paramagnetic states. The NQR frequency increases linearly with increasing temperature. The spin-lattice relaxation rate 1/ T 1 asymptotically approaches a constant value 1/ T 1∞ of 1.16±0.02×10 3 sec -1 at high temperatures. Below about 350 K, the rate begins to increase, and near the Neel temperature of 229∼230 K, it diverges due to the critical slowing down. The critical region is found to be not very wide (( T - T N )/T_N\lesssim0.5). A n e s t i m a t e d v a l u e f o r 1/T_1∞ i n t e r m s o f e x c h a n g e c o u p l e d l o c a l m o m e n t s i s i n g o o d a g r e e m e n t w i t h t h e e x p e r i m e n t a l v a l u e . F r o m t h e v i e w p o i n t o f t h e s p i n - l a t t i c e r e l a x a t i o n e x p e r i m e n t s , C u O a p p e a r s t o b e a w e l l - l o c a l i z e d m a g n e t i c i n s u l a t o r .

Spin-Lattice Relaxation of 63Cu in High-T c Related Antiferromagnetic Cu Oxides

We have made the measurements of the temperature dependence of the nuclear spin-lattice relaxation rate 1/ T 1 of 63 Cu in La 2 CuO 4 , YBa 2 Cu 3 O 6 and CuO. At 4.2 K, 1/T 1 of La 2 CuO 4 , YBa 2 Cu 3 O 6 and CuO are about 5×10 4 sec -1 , 5×10 3 sec -1 and 3×10 -3 sec -1 respectively. The sharp temperature dependence of 1/ T 1 has been observed in CuO, while in La 2 CuO 4 1/ T 1 has exhibited a T 2∼3 dependence below 4.2 K and has become nearly temperature independent above 4.2 K. In YBa 2 Cu 3 O 6 1/ T 1 has exhibited a weak temperature dependence with nearly T 2/3 dependence. The observed behaviors in CuO are in general accord with the theoretical prediction based on the spin wave picture if we take its low dimensional nature into account while those in other two compounds are not consistent with the theoretical prediction.

Hyperfine Field and 51V Nuclear Relaxation in the Paramagnetic State of the Triangular Lattice Antiferromagnet VX2 (X=Cl, Br and I)

The microscopic magnetic properties of the paramagnetic state in the triangular Heisenberg antiferromagnet VX 2 (X=Cl, Br and I) have been investigated by using the NMR technique. From the 51 V shift vs the magnetic susceptibility plot the hyperfine coupling constant due to the d-spin core polarization effect is determined to be -95 ±2, -79 ±2 and -69 ±2 kOe/ µ B for VCl 2 , VBr 2 and VI 2 , respectively. Also it has been revealed that the high temperature value of the 51 V nuclear spin-lattice relaxation rate is explained from the Guassian model for the auto-correlation function of the spin fluctuations.

Magnetic Susceptibility of an Antiferromagnetic KCoF3 Single Crystal–19F NMR and Static Measurements

The temperature dependence of the parallel and perpendicular susceptibilities of an antiferromagnetic KCoF 3 single crystal has been obtained from the shift of 19 F NMR and the static magnetic measurements in both the paramagnetic and antiferromagnetic states. A large residual value of the parallel susceptibility, (4.6±0.1)×10 3 emu/mole, is observed even at 4.2 K, which is ascribed to the residual orbital momentum of Co 2+ ions. The absolute value and the temperature dependence of the susceptibility are in general accord with the theoretical prediction by N. Suzuki, T. Isu and K. Motizuki based on the correlated effective field approximation.

207Pb NMR Study of BaPbO3

The Knight shift K and spin-lattice relaxation time T 1 for the 207 Pb nuclei in BaPbO 3 have been measured in the temperature range between 1.4 K and 4.2 K. The observed shift of the 207 Pb NMR signal was found to be temperature-independent, and the Knight shift K s associated with the conduction electron paramagnetism is estimated to be 0.98%. From the measurement of the temperature dependence of T 1 , the Korringa product T 1 T was also found to be temperature-independent with a value of 0.036 s·K.