Glass-like and Verwey transitions in magnetite in details

We have measured low field DC and low frequency AC magnetic properties of single crystals of magnetite with continuously reading high-resolution SQUID magnetometer in temperature range from 5 to 150 K. The measurements show hysteresis of Verwey transition in temperature and linear opening of energy gap of about 40meV above the transition. Below the transition an AC response may be well explained by a thermally activated charge hopping. For frequencies of the driving AC field close to the hopping frequency a spin glasslike behavior is observed. Extracted values of the energy gap, activation energy and attempt period agree with those found from infrared complex conductivity measurements. Characteristic temperature of the glass-like transition coincides with minimum of the spin-spin relaxation time observed by NMR with this sample. Both NMR and our data may be explained consistently on the basis of the fluctuation-dissipation theorem. (c) 2006 Elsevier B.V. All rights reserved.