Kazumi Horai

EPR and 19F-ENDOR of Cr3+ Impurity Centres in K2ZnF4 and K2MgF4

The EPR spectra of three kinds of Cr 3+ centres in K 2 ZnF 4 and K 2 MgF 4 are reported. The Cr 3+ ion in each centre replaces the divalent cation substitutionally. The centre I (tetragonal symmetry) is ascribed to a vacancy-free Cr 3+ ion. In centre II (nearly-trigonal) a vacancy is present at the nearest K + site and in centre III (nearly-tetragonal) at divalent cation site. For each centre the values of the parameters ( g // - g ⊥ ) and D in the spin Hamiltonian are discussed by considering the effective negative charge distribution surrounding the Cr 3+ ion. In the vacancy-associated centres the values of D are interpreted by superposition of two uniaxial spin-Hamiltonian terms. Results of 19 F-ENDOR of K 2 MgF 4 : Cr 3+ are also presented.

Irreversibility behavior investigated with microwave absorption in the high-Tc Bi(Pb)-Sr-Ca-Cu-O superconducting ceramics

Abstract Reversible and irreversible microwave absorption is observed as a function of temperature and field in the high-Tc phase of a Pb doped Bi-Sr-Ca-Cu-O superconductor. The difference between zero-field-cooled and field-cooled absorption behaviors is related to the flux-pinning property. The irreversibility temperature Tirr scales with the applied field as H 1 3 , in accord with an exponent for a Tl-based superconductor, but in contrast to H 2 3 power law in the Y-based superconductor or spin glasses.

Electron Paramagnetic Resonance of Gd3+ Centres in Several Perovskite Fluorides

EPR measurements for substituted Gd 3+ centres in CsCaF 3 , CsCdF 3 , RbCdF 3 and KCdF 3 have been made at several temperatures in a range 77-487 K. In all the crystals the spectrum with tetragonal symmetry for Gd 3+ centre associated with the nearest-neighbour cation vacancy has been observed together with that for cubic centre. The observed fourth-order parameters in the spin Hamiltonian of both cubic and tetragonal centres are discussed in the series of perovskite fluorides. The result suggests that the parameter b 4 for Gd 3+ in 6-fold cubic coordination changes its sign as the lattice constant decreases. The superposition model for the fourth-order parameter is examined.

EPR of Several Iron-Group Impurities in LiBaF3 Having Inverse Perovskite Structure

Using 34 GHz band, the spin-Hamiltonian parameters of Cr 3+ , Mn 2+ , Fe 3+ and Co 2+ centres with cubic symmetry and Ti 3+ centre with tetragonal symmetry in LiBaF 3 have been determined. Co 2+ ion with g =4.316 indicates that it substitutes for the Li + ion which is surrounded by a fluorine octahedron.

An Investigation of V2+ Impurity Center in KMgF3 and KZnF3 by 51V-ENDOR Measurements

51 V-ENDOR measurements for a substitutional V 2+ impurity center in KMgF 3 and KZnF 3 have been made using a microwave frequency of 35 GHz. The spin-Hamiltonian parameters including higher order hyperfine interaction term U ( S 3 I ) have been determined as following values: A =-258.337±0.005 MHz, g I β N / h =(11.255±0.005)×10 -4 MHz/G, U =0.110±0.005 MHz for KMgF 3 , and A =-259.157±0.005 MHz, g I β N / h =(11.260±0.005)×10 -4 MHz/G, U =0.118±0.005 MHz for KZnF 3 . To interprete the parameter U , calculation for V 2+ in a cubic octahedral crystalline field including the contribution from excited 2 G term is carried out. Other parameters are also discussed. A new value for the nuclear magnetic moment of 51 V nucleus is proposed.

Experimental Studies of Rhombohedral Cr2S3 Single Crystal. I. Magnetic Properties

The magnetic properties of rhombohedral Cr 2 S 3 single crystals were investigated. The remarkable temperature dependences of the small spontaneous magnetization, the magnetic susceptibilities and ...

Study of Fluorine Hyperfine Interactions in KZnF3: V2+ and Cr3+ by ENDOR

Fluorine ENDOR measurements have been made on first and second fluorine neighbours to V 2+ and Cr 3+ in KZnF 3 in order to clarify the role of the intervening Zn 2+ ions. The isotropic hf interaction parameters for the second fluorine neighbours have been obtained as +79 kHz and -22 kHz for v 2+ and Cr 3+ respectively. The negative sign of the result for the Cr 3+ strongly suggests that the cation-cation charge transfer interaction between the 3d 3 impurity and Zn 2+ ions can play important roles in these systems. Differences between the KMgF 3 and KZnF 3 for both V 2+ and Cr 3+ are discussed from the same standpoint.

Transmission Electron Spin Resonance Investigations in Aluminium

Conduction electron spin resonance transmission in aluminium has been observed at a frequency of 22.6 GHz under the completely damped spin-wave condition. Line shape fittings based on two TESR theories have been performed. The theory of Walker has turned out to explain fairly well the data in the anomalous skin effect condition at 18 K. The probability e of a spin flip at the surface has been found to be about 1 ×10 -3 , which is in good agreement with that obtained from the reflection measurement. DC resistivity in the same samples has also been measured. The applicability of several TESR theories has been considered, referring to the skin effect condition, g anisotropy and electron-electron interaction.

Examinations of Superconducting Transition Width, Oxygen Deficiency and Crystallographic Quality in YBa2Cu3O7-δ

A series of high-T c YBa2Cu3O7-δ ceramics has been investigated through electrical resistivity measurements, least squares analyses and Rietveld refinements of X-ray powder diffractions; with emphasis on crystallographic and superconducting homogeneities. We have applied a simple method for estimating oxygen contents in the specimen through determination of unit cell parameters by X-ray diffraction peaks. Structural and stoichiometric examinations have also been performed through the Rietveld refinement. By the good agreement of our X-ray Rietveld analyses with the neutron diffraction results, we have confirmed the reliability of our X-ray studies. Nonlinear correlation between full widths at half-maximum of several diffraction peaks and superconducting transition widths has been observed, and the importance of local structural variation has been discussed.

Electron Paramagnetic Resonance Studies of Gd3+ in KZnF3 and KCdF3

EPR investigations of Gd 3+ impurity ions in KZnF 3 and KCdF 3 have been made at the temperatures 487, 300, 77 and 4.2 K for KZnF 3 and at 487 K for KCdF 3 . The spectra for cubic symmetry have been observed and the spin Hamiltonian parameters in cubic crystalline field have been determined. At the temperature of 487 K, the fourth-order zero-field splitting parameters in the spin Hamiltonian show remarkable difference for both crystals as following values: b 4 =-6.97×10 -4 cm -1 for KZnF 3 and | b 4 |=1.6×10 -4 cm -1 for KCdF 3 . This difference implies that Gd 3+ impurities substitute for different cubic sites. From the size consideration of the ionic radii among K + , Zn 2+ , Cd 2+ and Gd 3+ , it is concluded that Gd 3+ substitute for 12-fold K + site in KZnF 3 and 6-fold Cd 2+ site in KCdF 3 .