Heat Capacity Studies of Nanocrystalline Magnetite (Fe3O4)

The Verwey transition in magnetite (Fe3O4) has been studied extensively with a wide assortment of experimental techniques to investigate the effects of impurities, oxygen stoichiometry, and crystal quality; however, no studies have tested the effects of crystal size on the Verwey transition. In this study, the heat capacity of a magnetite powder with an average crystallite size of 13 nm was measured in the temperature range of 0.5-350 K. No obvious anomaly was observed in the heat capacity in the vicinity of 120 K, yet the measurements exhibited unusual thermal behaviors between 50 and 90 K with relaxation times increasing from an average of 25 min to as much as 5 h in this temperature interval. The behavior is typical of a phase transition, so the lack of a distinct anomaly suggests either the phenomenon is spread out over a large temperature interval with little enthalpy or an insulating phase with different thermal conductivities appears. The heat capacity below 90 K was dependent on cooling rate with an inconsistent anomaly found below 4 K. The results suggest that the Verwey transition has a particle-size dependence. Additionally, theoretical fits below 15 K suggest that magnetite nanoparticles display anisotropic ferrimagnetic behavior and a superparamagnetic contribution to the heat capacity which are properties not observed in bulk magnetite.