Electron spin-lattice relaxation of low-symmetry Ni2+ centers in LiF

The spin-lattice relaxation times of charge-compensated Ni2+ centers in LiF single crystals were measured using the electron spin echo technique. An analysis of the results revealed a very high relaxation rate with a linear dependence on temperature within the intermediate temperature range of 5–15 K. This acceleration of the relaxation rate was found to be due to the inhomogeneous distribution of Ni2+ ions in the LiF lattice. It seems that the effective cross-relaxation mechanism through the exchange-coupled clusters of Ni2+ ions can play a dominant role. Two-phonon Raman type relaxation dominates at higher temperatures involving the Debye phonon spectrum of the LiF lattice.