NMR and spin relaxation inLaGa1−xMnxO3:Evidence for thermally activated internal dynamics

Abstract

Single crystals of diamagnetically diluted manganites, ${\mathrm{LaGa}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{O}}_{3}$ $(x=0--0.2)$ have been investigated by means of magnetic-resonance techniques. Detailed study was performed of the NMR line shape, spin-spin and spin-lattice nuclear relaxation of ${}^{69}\mathrm{Ga}$ and ${}^{71}\mathrm{Ga}$ nuclei. The analysis of the temperature and concentration dependencies of the spin-relaxation rates was supported by additional measurements on ${}^{139}\mathrm{La}$ nuclei and electron-spin resonance on Mn electron-spin system. The electrical quadrupole mechanism of nuclear-spin relaxation is shown to be dominant at low Mn concentration $(xl~0.005),$ whereas the magnetic dipolar interaction with the Mn electron spins is prevalent at $xg~0.02.$ The obtained data suggest the presence of thermally activated internal motion with the activation energy of about 50 meV that governs the ${\mathrm{Mn}}^{3+}$ electron spin-lattice relaxation and, via dipolar interaction, the nuclear-spin relaxation of the host nuclei. In the scenario suggested, this motion can be related to the hindered Jahn-Teller dynamics of the ${\mathrm{Mn}}^{3+}$ ions linked to clusters or pinning defects.