Friday, April 18, 2008

Phys. Rev. B

57Fe NMR and spin structure of manganese ferriteH. Štěpánková, B. Sedlák, and V. Chlan, P. Novák and Z. Šimša


NMR of 57Fe in five MnFe2O4 single crystals with different degrees of inversion was measured in liquid He temperature. At the zero external field, two lines originating from Fe3+ ions on the octahedral sites are observed at 68.7 and 71.1 MHz, while the line at 72.0 MHz, the amplitude of which increases with increasing inversion, is ascribed to Fe3+ ions on the tetrahedral sites. Measurement in the external field shows that the spin structure is in accord with the Goodenough-Kanamori rules. This contradicts to an abnormal spin structure which Shim et al. [Phys. Rev. B 75, 134406 (2007)] proposed recently on the basis of 57Fe NMR measured in polycrystalline manganese ferrite. Reinterpretation of the NMR in polycrystalline compounds is given.

Phys. Rev. B 77, 104301 (2008)

6Li and 7Li NMR line-shape and stimulated-echo studies of lithium ionic hopping in LiPO3 glass
Sandra Faske, Hellmut Eckert, and Michael Vogel


6Li and 7Li NMRs are used to investigate the lithium ion dynamics in LiPO3 glass. In particular, 6Li NMR stimulated-echo experiments are used to provide straightforward access to two-time correlation functions characterizing the lithium ionic hopping motion in the millisecond regime in a glassy ion conductor. Temperature-dependent measurements serve to separate the spin diffusion contribution and the dynamic contribution to the stimulated-echo decays. The 6Li NMR correlation functions of LiPO3 glass describing the lithium ionic motion show pronounced nonexponential decays, which can be well described by a stretched exponential function with a temperature-independent small stretching parameter =0.27, indicating the complex nature of the lithium dynamics. The temperature dependence of the mean correlation times resulting from these stimulated-echo experiments is described by an activation energy Ea=0.66 eV. The values of are in good agreement with time constants from previous electrical and mechanical relaxation studies. At appropriate temperatures, the 6Li and 7Li NMR spectra are superpositions of a broad and a narrow spectral component, which result from slow and fast lithium ions, respectively, on the NMR time scale. A detailed analysis of the temperature dependence of these line shapes provides information about the distribution of correlation times.

Phys. Rev. B 77, 115130 (2008)

Electronic properties of Sc5M4Si10 (M=Co,Rh,Ir) probed by NMR and first-principles calculations
C. S. Lue, R. F. Liu, Y. F. Fu, and C. Cheng

We report a systematic study of 45Sc NMR measurements on the Sc5Co4Si10-type silicides Sc5M4Si10 (M=Co,Rh,Ir). From the central transition line shapes, three nonequivalent Sc sites have been identified. We thus measured the Knight shift and spin-lattice relaxation time (T1) for each of the three crystallographic sites. Results of experimental Knight shift and T1 together with theoretical band structure calculations provide evidence that orbital electrons are responsible for the observed shifts as well as the relaxation rates. In addition, we found no correlation between the Fermi-level density of states and the superconducting transition temperature of the studied materials. Further analyses clearly indicate that the effect of electron-phonon coupling plays a significant role for the superconductivity of Sc5M4Si10, and these materials should be classified as moderate-coupling superconductors.

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