^{99,101}Ru NMR and ^{63,65}Cu NQR Study of the Magnetic Superconductors   RuSr_{2}EuCu_{2}O_{8} and RuSr_{2}GdCu_{2}O_{8}
  In order to investigate the coexistence of magnetism and superconductivity on a microscopic scale, local studies using zero-field spin-echo ^{99,101}Ru NMR and ^{63,65}Cu NQR have been carried out on both superconducting and non-superconducting samples of magnetically ordered RuSr_{2}EuCu_{2}O_{8} and RuSr_{2}GdCu_{2}O_{8}. ^{99,101}Ru NMR signals were observed at 1.3 K over two distinct frequency ranges. The high frequency spectra (110 to 150 MHz), which are essentially the same for all samples, are characterized by five sharp peaks for each isotope corresponding to a hyperfine field of approximately 590 kOe with the appropriate quadrupole splitting. The low frequency spectra (40 to 90 MHz), which show significant variation from sample to sample, correspond to a much lower hyperfine field with considerable broadening and quadrupole features that are barely resolvable. As reported previously, the low frequency spectra are attributed to the Ru^{4+} valence state while the high frequency spectra are attributed to the Ru^{5+} valence state. ^{63,65}Cu NQR features were observed (26 to 34 MHz), although with considerable broadening. The results are discussed in terms of the microscopic magnetic structure, mixed valence state for Ru, occupancy of the Ru sites, and the existence and role of impurity phases. 
Z.H. Han, J.I. Budnick, and W.A. Hines, P.W. Klamut, M.Maxwell, and   B.Dabrowski