Nuclear-spin relaxation of59Cocorrelated with the spin-state transitions inLaCoO3

Abstract

The ${}^{59}\mathrm{C}\mathrm{o}\ensuremath{-}\mathrm{N}\mathrm{M}\mathrm{R}$ on single crystals of ${\mathrm{LaCoO}}_{3}$ has been studied in order to investigate the spin-state transition through the nuclear-spin relaxation in this compound. Below $T\ensuremath{\sim}35\mathrm{K},$ the spin-lattice and spin-spin relaxation rates show a thermal-activation-type temperature dependence with an energy gap of \ensuremath{\sim}180 K. This value is close to the energy difference between the low spin state $(S=0)$ and the intermediate spin state $(S=1)$ predicted in the two-stage spin-state transition model with the intermediate spin state. The apparent spin-spin relaxation rates depend on the resonance peaks split due to the nuclear-electric-quadrupole interaction, which is explained by an indirect nuclear spin-spin interaction through the interacting paramagnetic electron spins. This observation is consistent with the ferromagnetic short-range correlation among the electron spins on ${\mathrm{Co}}^{3+}$ ions confirmed by neutron-scattering experiments.