Postprint version. Published in Hyperfine Interactions, Volume 96, Issue 1, December 1, 1995, pages 111-138.
The definitive version is available at http://dx.doi.org/10.1007/BF02066277.
Mössbauer studies of the ubiquitous protein molecule ferritin and its synthetic "biomimic" polysaccharide iron complex (PIC) exhibit an anomaly in the Mössbauer spectrum wherein the recoil free fraction f-factor has a sharp drop with respect to temperature as the temperature rises above 30 K for mammalian ferritin and 60 K for PIC. The anomaly coincides with the disappearance of hyperfine splitting, which is due to superparamagnetic relaxation above the blocking temperature. Different absorbers were used to experimentally investigate the effect of absorber thickness on the Mössbauer spectrum. The anomaly persists for thin absorbers. Also, spectra treated with FFT procedures to eliminate the thickness effect still exhibit this anomaly. Motion of the core with respect to the protein shell was also eliminated as a possible source for this phenomenon, by comparing the Debye temperature obtained from the temperature dependence of f-factor and the isomer shift. A comparison of the magnetic anisotropy constants from magnetization studies with those obtained by relating the hyperfine field H of the Mössbauer spectra to the fluctuations of the magnetization imply that the ferritin and PIC molecules possess magnetic anisotropy energy which may not be strictly uniaxial. This, we believe, may be intimately connected with the mechanism causing the f-factor anomaly.