DETERMINATION OF F(0) FOR FINE MAGNETIC PARTICLES

In this paper we have determined a value for the pre-exponential facto r in the Neel-Arhennius equation for the iron oxyhydroxide particles f ound in the protein ferritin. The data were obtained using a combinati on of zero field magnetic and Mossbauer spectroscopy studies yielding a value for f0 of (9.5 +/- 2.7) x 10(11) Hz. This value is significant ly different to that of 2.8 x 10(9) Hz commonly used and in closer agr eement to that of 10(13) Hz obtained for iron particles using an analo gous technique. Using our experimental value for f0 gives a revised su perparamagnetic criterion for DC magnetic measurements on a 100 second time-scale of KV < 32 kT and a Mossbauer 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 estim ate for the value of f0 would lie in the range 10(12) to 10(13) HZ.