Determination of f0 for fine magnetic particles

In this paper we have determined a value for the pre-exponential factor in the Néel-Arhennius equation for the iron oxyhydroxide particles found in the protein ferritin. The data were obtained using a combination of zero field magnetic and Mössbauer spectroscopy studies yielding a value for f0 of (9.5±2.7)X1011 Hz. This value is significantly different to that of 2.8 X 109 Hz commonly used and in closer agreement to that of 1013 Hz obtained for iron particles using an analogous technique. Using our experimental value for f0 gives a revised superparamagnetic criterion for DC magnetic measurements on a 100 second time-scale of KV < 32 kT and a Mössbauer spectroscopy criterion for a measurement time-scale of 10-9 s of KV < 8 kT. Our results together with other published data would suggest that a more appropriate estimate for the value of f0 would lie in the range 1012 to 1013 Hz.