Superparamagnetic properties of γ-Fe2O3 particles:: Mossbauer spectroscopy and d.c. magnetic measurements

Particles of Fe oxide were prepared by chemical coprecipitation. and their sizes were shown by TEM and confirmed by XRD to be in the range of 5 nm. The Mossbauer spectra at 120 K clearly indicated the absence of magnetite and presence of the maghemite (gamma-Fe2O3) phase. We studied the transition of the system to superparamagnetic behaviour, which strongly depends on the relevant time window amounting to similar to 10(-7) s for Mossbauer spectroscopy of Fe-57 and units of seconds for d.c. magnetic measurements. From the temperature dependences of magnetic moments of zero-field-cooled (ZFC)) and field-cooled (FC) samples, the distributions of blocking temperatures were determined. The comparison of the transition temperatures derived from these two types of measurements gave an independent estimate for the pre-exponential factor and the energy barrier and thus magnetocrystalline anisotropy in an order-of-magnitude agreement with the published data for bulk gamma-Fe2O3.