Determination of the preexponential frequency factor for superparamagnetic maghemite particles in magnetoferritin

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 f₀ in the Néel-Arhennius relaxation equation; (2) the SPM threshold size at room temperature for maghemite; and (3) the SD value of H_rIH_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 f₀=10⁹ 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_rIH_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, f₀ for ferrimagnetic magnetoferritin is a factor of 10³ lower than was determined previously for native ferritin, which contains antiferromagnetic ferrihydrite cores. The difference in f₀, 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 f₀ 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.