Modeling the magnetic behavior of horse spleen ferritin with a two-phase core structure

The growth of the nanotechnology industry has led to an increased interest in characterizing magnetic nanoparticles. A natural material with well-defined grain size in the nanoparticle range is commercially available-horse spleen ferritin, an iron storage protein. Modeling of the magnetic properties of commercial horse spleen ferritin is often based on the assumption of a single-phase core of ferrihydrite (5Fe(2)O(3)center dot 9H(2)O). Low temperature hysteresis measurements indicate, however, that the ferritin cores contain at least two magnetic phases. Initial magnetization curves measured at temperatures between 50 and 300 K have been modeled using four methods. A model that used a sum of two Langevin functions fitted the data 70% better on average than a model that used a single Langevin function. It was also superior to both a random mean orientation model and a model that takes account of crystalline anisotropy. The two-phase model consists of a phase with a high coercivity that does not undergo saturation and a second phase with a low coercivity and a saturation field of 300 mT. The high-coercivity phase is compatible with antiferromagnetic ferrihydrite, while the low-coercivity phase could be magnetite, maghemite, or a mixture of both. The results from this study are consistent with earlier microscopic studies that characterize horse spleen ferritin as a multiphase system with up to 30% of magnetite-maghemitelike cores. (c) 2006 American Institute of Physics.