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 t han or equal to x less than or equal to 0.2) are presented far externa l 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 eff ect of the doping on the correlated spin dynamics of the S = 1 Ni2+ in the paramagnetic (PM) and in the antiferromagnetic (AF) phases are ac hieved. In the PM phases the Li-7 relaxation rates W are related to th e critical dynamics of the Ni2+ spins. It is found that the slowing do wn of the spin fluctuations is characterized by critical exponents hav ing little x dependence, while the Neel temperature decreases with x a pproximately in the form T-N(x) = T-N(0)[1-2.2x], faster than theoreti cal predictions for cubic Heisenberg systems or for dilution by locali zed diamagnetic impurities. In the AF phase the hopping of the holes i nduces fluctuating fields at the nuclear site which are responsible fo r a sizable contribution to the Li-7 W. One derives the gaps between i tinerant and localized-charge transfer states. These gaps turn out to be larger than in CuO and decreasing on increasing x, in substantial a greement with electrical conductivity. At a temperature T-m(x), which decreases with increasing x, the hopping frequencies reach the radio-f requency ranee and the holes localize on the oxygen nearest neighbors to the Li+ impurities. Then another relaxation mechanism arises, induc ing marked maxima in W at a x-independent temperature Tsimilar or equ al to 130 K, with a recovery described by a stretched exponential. Thi s low-temperature relaxation mechanism is attributed to progressive fr eezing of the spin fluctuations of effective magnetic moments generate d by the hole localization which interact through the AF matrix and co rrespond to a disordered paramagnet. The activation energy for the fre ezing is found E-A similar or equal to 1300 K, supported also by muon depolarization-rate measurements. [S0163-1829(97)00341-X].