NMR relaxation mechanisms and line widths in insulators below 1 K

Spin-lattice relaxation in insulating diamagnets is extremely slow at millikelvin temperatures, though far less slow than conventional theory would suggest. However spins at the surface of particles immersed in liquid 3He relax efficiently by the Friedman-Richardson mechanism, which is shown to be dipolar in nature and possibly informative about surface structure. Relaxation of interior spins is shown to occur by spin diffusion from the surface. This process can be impeded by a barrier arising from interaction with foreign spins. Resonances in finely divided solids suffer unexpectedly large inhomogeneous broadening, though the natural widths are in accordance with expectations.