Dynamics of dilute spin systems in TiO2:Cr3+

The NMR line widths of 17O with natural abundance in doped rutile TiO2: Cr3+ is mainly determined by the magnetic ions. At a Cr3+ concentration of approximately 0.05% the shape of the unsaturated 10O spectrum can be calculated from a representative ensemble of 17O - Cr3+ spin pairs. Experimentally the NMR of 17O is investigated at the temperature of liquid He under dynamic nuclear polarisation conditions. When sweeping a marginal oscillator of frequency ν the spectrum shows marked transients which stem from the partial saturation of the swept NMR transitions. It has been found that the saturated lines recover to their equilibrium value with a characteristic time constant tausub(R) such that tausub(1S)<tausub(R)<tausub(1I) when tausub(1S) and tausub(1I) are the spin-lattice relaxation times of the electronic and nuclear spins. The dynamical behaviour of the coupled spin systems has been simulated on a computer using a set of rate equations which needs the experimental parameters tausub(1S), tausub(1I), tausub(R) and Tsub(I), Tsub(S), sigma where Tsub(I), Tsub(S) are the spin temperatures and where sigma is a saturation parameter for NMR transitions. The agreement between the calculated line from F(ν,sigma,7) and the observed curves is gratifying for different experimental conditions. (author)