Magnetization and Mössbauer measurements on maghemite particles with an average particle diameter of 10 nm have been made in the temperature range from 5 K to 353 K spanning the superparamagnetic (SPM) and stable single domain (SD) regimes. The maghemite particles were produced within the iron-storage protein ferritin, resulting in a narrowly-sized, weakly interacting nanocomposite material called magnetoferritin. Experiments combining hysteresis measurements, low temperature remanence, and Mössbauer spectroscopy were used to characterize magnetoferritin and to provide experimental estimates of (1) the pre-exponential frequency factor f 0 in the Néel-Arhennius relaxation equation; (2) the SPM threshold size at room temperature for maghemite; and (3) the SD value of H r IH c at 0 K. The frequency factor was determined from the difference in blocking temperatures measured by dc magnetization and Mössbauer spectroscopy, yielding a value of f 0 =10 9 Hz. This agrees well with the standard value and justifies the usually assumed superparamagnetic blocking condition of KV=25 kT for remanence measurements. The SPM threshold size at room temperature for remanence measurements was estimated to be 20-27 nm and the extrapolated SD value at 0 K for H r IH c is 1.32. The latter value is slightly larger than the theoretical value of 1.09 but may be more appropriate for weakly interacting SD particles commonly found in sediments and soils. However, f 0 for ferrimagnetic magnetoferritin is a factor of 10 3 lower than was determined previously for native ferritin, which contains antiferromagnetic ferrihydrite cores. The difference in f 0 , values between the two varieties of ferritin is probably related to the two different types of magnetic spin ordering of the core minerals and suggests that the higher value of f 0 is more appropriate for antiferromagnetic minerals like hematite and goethite, whereas the lower value is more appropriate for ferrimagnetic minerals like maghemite, magnetite, or greigite.
|Original language||English (US)|
|Number of pages||10|
|Journal||Journal of Geophysical Research B: Solid Earth|
|State||Published - Jan 1 1997|