Lucjan E. Misiak

EPR study of high-Tc superconductor Y0.9Ho0.1Ba2Cu3O7−δ

Abstract X-band EPR study of polycrystalline Y0.9Ho0.1Ba2Cu3O7−δ high-Tc superconductor has been made in the temperature range 4.2-300 K. As the temperature is lowered, the dependence of the intensity of the Cu2+ line on temperature exhibits a jump at 80 K, while the low-field microwave absorption, indicating onset of superconducting properties, begins to occur at the superconducting transition temperature Tc = 60 K. The estimated sizes of Josephson junctions are in the range 0.2–0.4 μm below 60 K. The amplitude of the low-field microwave absorption line fitted to (Tc−T)n, yields the critical exponent to be n = 1.45. The values of the lower critical field, the thermodynamic critical field, and the upper critical field for the Y0.9Ho0.1Ba2Cu3O7−δ sample, extrapolated to 0 K, have been determined to be 75 mT, 1.75 T, and 173 T, respectively. The electronic specific heat coefficient is estimated to be γ = 3.4 mJ/(mole Cu)K2. Significant covalency due to Cu2+−O2- bonds is observed. The Cu2+ EPR spectrum is due to the phase with increased oxygen vacancies.

Electron paramagnetic resonance of the high-Tc superconductor Y0.9Er0.1Ba1.9Sr0.1Cu3O7−δ

Abstract The Y0.9Er0.1Ba1.9Sr0.1Cu3O7−δ superconductor has been investigated by X-band EPR in the temperature range 4.2–300 K. The superconducting transition temperature of the sample has been determined to be Tc = 88 K. No copper EPR line was observed at any temperature, contrary to that observed in the undoped high-Tc superconductor YBa2Cu3O7−δ. The sizes of the Josephson junctions are determined to be in the range 0.4–0.7 μm. The dependence of the lower critical field Hcl on temperature exhibits an upward inflection at ∼ 15 K, possibly due to the dimensional effect, which is observed in multilayered and granular weakly coupled superconductors. The lower critical field and the upper critical field, as extrapolated to 0 K, have been estimated to be Hc1(0) = 19 mT and Hc2(0) = 83 T, respectively, while the Sommerfeld constant γ = 4.8 mJ mol−1 K−2. The spin-glass nature of the sample, below Tc, has also been confirmed by the observed shift in the base line. The values of the critical exponent defined by I ≃ (T c − T) n , where I is the intensity of the low-field microwave absorption, is determined to be n = 1.32.

Variable temperature EPR study of a Ni2CdCl6.12H2O single crystal

Abstract X-band (9.45 GHz) EPR spectra have been recorded on a single crystal of Ni2CdCl6·12H2O for several orientations of the external magnetic field at many temperatures in the range 3.8-295 K. All the EPR line positions observed at any temperature were simultaneously fitted in a least-squares procedure employing numerical diagonalization of the spin Hamiltonian matrix to evaluate the spin-Hamiltonian parameters g∥, g⊥ and D. A detailed examination of the EPR line positions reveals that there exist two magnetically inequivalent sites for Ni2+ in the unit cell, characterized by considerably different values of the zero-field splitting parameter (D). The intensities of the EPR lines at room temperature indicate that three Ni2+ positions in the unit cell are occupied by one set of magnetically equivalent ions, while the other set of the inequivalent ions occupy the fourth position. The temperature dependence of D for both sets of ions is found to contain linear, quadratic and cubic terms in temperature. The relative intensities of lines at liquid helium temperature indicate the absolute sign of the parameter D to be negative.

Gd3+ EPR study of phase transitions in A NH4Nd(SO4)2·4H2O single crystal and superposition-model calculation of the zero-field splitting parameters

Abstract The phase transitions undergone by a NH 4 Nd(SO 4 ) 2 ·4H 2 O (ANST) single crystal have been studied via the EPR of the Gd 3+ ion between 3.8 and 304 K. Three first-order phase transitions, at 280±0.5 K, at 176±0.5 K and at 19±1 K have been found to occur, as indicated by the behaviour of the overall splittings of EPR lines and that of the EPR linewidths. The spin-Hamiltonian parameters for Gd 3+ at room temperature have been evaluated from the positions of the allowed EPR lines by the use of a least-squares fitting procedure. The local symmetry of the Gd 3+ ion in an ANST host lattice has been deduced to be monoclinic, with the two-fold symmetry axis parallel to the magnetic z axis. The absolute sign of the zero-field splitting parameter b 0 2 for the Gd 3+ ion in ANST has been determined to be positive by using the superposition model of Newman; the intrinsic parameters, b 2 and b 4 , for Gd 3+ in the ANST crystal have been calculated to be -5.263 and 0.021 GHz, respectively, with t 2 = 8.5, t 4 = 14.0, associated with an angular distortion of Δθ = 1.5° of the ligand ion positions.

EPR of polycrystalline high-Tc superconductor YBa1.9K0.1Cu3O7−δ

Abstract The high-Tc superconductor YBa1.9K0.1Cu3O7−δ has been studied by X-band EPR in the temperature range 4.2–303 K. The temperature dependence of the EPR spectra indicates that the low-field microwave absorption begins to occur below 140 K, and the intensity of the Cu2+ line due to the phase with increased oxygen vacancies exhibits two peaks around 94 K within an interval of 2 K, indicating the superconducting transition temeprature Tc = 94 K. The values of the lowercritical field (Hc1), the thermodynamic-critical field (Hc), and the upper-critical field (Hc2), as extrapolated to 0 K, are estimated to be 31.5 mT, 0.73 T, and 73 T, respectively. The estimated values of the spin-orbit reduction parameters, using the measured principal values of the Cu2+g tensor in the phase of the sample with increased vacancies, indicate significant degree of Cu-O bond covalency.

Electron paramagnetic resonance of a Gd3+ -doped Pr(BrO3)3 · 9H2O single crystal

Abstract X-band (∼9.24 GHz) EPR measurements have been made on a single crystal of Gd 3+ -doped Pr(BrO 3 ) 3 · 9H 2 O in the temperature range 4–295 K. The values of the spin-Hamiltonian parameters (S.H.P.) have been determined at 295, 230 and 4 K, using a least-squares fitting procedure. The absolute signs of S.H.P. are determined from the relative intensities of lines at 4 K. The absolute value of the zero-field splitting parameter, b 2 0 , is found to increase as the temperature is decreased.