Spin-echo nuclear magnetic and quadrupole resonance (NMR/NQR) studies, along with complementary x-ray diffraction and bulk magnetization measurements, have been carried out on the magnetic superconducting system RuSr₂RECu₂O₈ (RE1212, RE = Gd or Eu) and systems based on or derived from SrRuO₃.

In RE1212, two sets of Ru NMR signals are found, indicating the existence of a mixed valence for Ru. This result suggests a hole doping mechanism to account for the superconductivity that originates in the CuO₂ planes. The evolution of the Ru NMR spectra in external magnetic fields provides information concerning the magnetic structure of the Ru sublattice which is consistent with the previously proposed type-I antiferromagnetic structure. Based on this model, the interplane magnetic coupling and the in-plane spin-flop critical field are determined.

While substituting for the Ru-site in SrRuO₃, Mn and vacancies suppress the ferromagnetic ordering temperature drastically. Cr, on the other hand, increases the ordering temperature. The valence states for Mn, Cr, and Ru are determined from the NMR spectra, which, combined with dc magnetization, also provide information with regard to the various magnetic interactions. The hybridization of the Ru 4d and Mn or Cr 3d orbitals is believed to play a key role in the variation of the ordering temperature. Upon cooling, three consecutive and distinct magnetic transitions are observed for Sr₂RuGdO₆ at T₁ = 33K, T₂ = 17K, and T₃ < 2K which are attributed to the Ru-Ru, Ru-Gd, and Gd-Gd antiferromagnetic interactions, respectively. Weak ferromagnetism is associated with the transition at 17K. These features point to the unique role played by Gd in this system. Ru NMR spectra, in the presence of an external magnetic field, demonstrate the existence of a ferromagnetic phase in the antiferromagnetic matrix which behaves superparamagnetically. These results strongly suggest that, at low temperature, the magnetic ground state of Sr₂RuGdO₆ involves complex coexistence and competition of different interactions.

In summary, the research in this dissertation explores two types of ruthenates: (1) RE1212 which shows complex superconductivity and magnetism, and (2) systems based on SrRuO₃ which show either itinerant ferromagnetism or insulating antiferromagnetism.