The behaviour of nuclear spins near highly polarised paramagnetic centres

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Abstract
In two dilute paramagnets (insulating solids containing both dipolar nuclei and paramagnetic centres) the nuclear magnetisation is found to decay surprisingly rapidly in the presence of a weak radiofrequency field distant from the nuclear resonance by many linewidths. The decay is shown to be the result of a direct saturation by the RF field of those nuclei through which the majority communicate with the electronic reservoirs; its study confirms that nuclear spin diffusion is possibly well into the field gradient surrounding each paramagnetic centre, providing the dipolar energy generated can be relaxed sufficiently quickly. The extra Zeeman splitting of nuclei within these gradients, up to a certain limit, contributes to the nuclear dipolar reservoir, greatly enhancing its heat capacity in these materials at low temperature. Direct measurements of the nuclear dipolar relaxation rate are also presented.