Nuclear magnetic and quadrupole resonance studies of local environments in magnetic and superconducting systems

Authors

Zhaohui Han, University of Connecticut

Date of Completion

January 2006

Keywords

Physics, Condensed Matter

Degree

Ph.D.

Abstract

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 RuSr₂RECu ₂O₈ (RE 1212, 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 (AFM) 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 (FM) 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 = 33 K, T₂ = 17, and T₃ < 2 K which are attributed to the Ru-Ru, Ru-Gd, and Gd-Gd AFM interactions, respectively. Weak FM is associated with the transition at 17 K. 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 FM phase in the AFM 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. ^

Recommended Citation

Han, Zhaohui, "Nuclear magnetic and quadrupole resonance studies of local environments in magnetic and superconducting systems" (2006). Doctoral Dissertations. AAI3234308.
https://digitalcommons.lib.uconn.edu/dissertations/AAI3234308