Spin-spin relaxation:

This mechanism of relaxation involves mutual exchange of spins by two precessing nuclei in close proximity of each other. For example, if there are two nuclei then one of these can flip down and the other may flip up by mutual exchange of spins. This is also known as transverse relaxation. It shortens the life time of an individual nucleus in the higher state but does not contribute to the maintenance of the required excess in the lower energy state. Because of the relaxation mechanisms, the excited states have small life times.

The lifetime of excited state is inversely related to the width of absorption line. As a result of inverse proportionality between the line width and lifetime of the excited state sharp resonance lines are observed for nuclei having long lived excited states and broad lines for short lived states. Therefore, spin-spin and spin-lattice both relaxation processes contribute to the line width.