Spin dynamics in hole-doped NiO from Li-7 NMR in Ni1-xLixO

Li-7 NMR spectra and relaxation measurements in Ni1-xLixO (0.01 less than or equal to x less than or equal to 0.2) are presented far external fields ranging from 1 T up to 9.4 T in the temperature range 10 less than or equal to T less than or equal to 700 K. Insights into the effect of the doping on the correlated spin dynamics of the S = 1 Ni2+ in the paramagnetic (PM) and in the antiferromagnetic (AF) phases are achieved. In the PM phases the Li-7 relaxation rates W are related to the critical dynamics of the Ni2+ spins. It is found that the slowing down of the spin fluctuations is characterized by critical exponents having little x dependence, while the Neel temperature decreases with x approximately in the form T-N(x) = T-N(0)[1-2.2x], faster than theoretical predictions for cubic Heisenberg systems or for dilution by localized diamagnetic impurities. In the AF phase the hopping of the holes induces fluctuating fields at the nuclear site which are responsible for a sizable contribution to the Li-7 W. One derives the gaps between itinerant and localized-charge transfer states. These gaps turn out to be larger than in CuO and decreasing on increasing x, in substantial agreement with electrical conductivity. At a temperature T-m(x), which decreases with increasing x, the hopping frequencies reach the radio-frequency ranee and the holes localize on the oxygen nearest neighbors to the Li+ impurities. Then another relaxation mechanism arises, inducing marked maxima in W at a x-independent temperature T*similar or equal to 130 K, with a recovery described by a stretched exponential. This low-temperature relaxation mechanism is attributed to progressive freezing of the spin fluctuations of effective magnetic moments generated by the hole localization which interact through the AF matrix and correspond to a disordered paramagnet. The activation energy for the freezing is found E-A similar or equal to 1300 K, supported also by muon depolarization-rate measurements. [S0163-1829(97)00341-X].