Nuclear Magnetic Relaxation in Magnetite

Abstract

The nuclear magnetic relaxation of 57Fe on tetrahedral sites in polycrystalline magnetite has been studied by spin-echo method with special interest in the relation to the low temperature transition and the electron diffusion among octahedral sites. Spin-lattice relaxation time T1 increases from about 0.7 msec at room temperature to 9 msec at 130°K. At the transition point (118°K) T1 decreases abruptly by the factor of about seven. It makes a minimum of 0.6 msec at about 88°K, then increases with decreasing temperature up to about 500 msec at 4.2°K. The minimum of T1 may be interpreted by the electron diffusion among B sites. This effect is considered to be dominant from the transition temperature to liquid-nitrogen temperature. The discontinuity of the relaxation time at the transition temperature suggests that the hopping frequency increases very rapidly as the order collapses and this mechanism becomes no more effective to the nuclear magnetic relaxation. Spin-spin relaxation time T2 is equal to T1 above about 60°K and becomes smaller than T1 below that temperature. T2 (=10 msec) at 4.2°K may be attributed to Suhl's interaction.