Nuclear 57Fe relaxation in Fe2+-containing NiFe2O4 and CoFe2O4 single crystals

Spin-lattice (T-1) and spin-spin (T-2) relaxation times of Fe-57 nuclei in the single-crystal NiFe2O4 and CoFe2O4 ferrites containing Fe2+ ions have been studied in the temperature range of 4.2-100 K by a spin-echo technique. The peaks of relaxation rates T-1(-1) and T-2(-1) caused by the presence of Fe2+ ions were observed for both ferrites in the ranges 38-42 and 28-32 K, respectively. The analysis of the results obtained with invocation of the data on ferromagnetic resonance and the measurements of the temperature dependence of resistivity shows that the mechanism of nuclear relaxation responsible for "impurity" peaks T-1(-1) and T-2(-1) is a slow relaxation process caused by electron exchange Fe2+ <-> Fe3+, characterized by a low activation energy. (C) 2000 MAIK "Nauka/Interperiodica".