Magnetic energy-barrier distributions for ferrihydrite nanoparticles formed by reconstituting ferritin

Abstract

The spherical cage-like protein ferritin was reconstituted with varying numbers of iron atoms perprotein shell ranging from approximately 20 to 1100 at temperatures of both 25 and 50 C toproduce ironIII oxyhydroxide ferrihydrite particles with different average particle sizes anddegrees of crystallinity. After characterization of the structural properties of the resultingiron-oxyhydroxide nanoparticles with transmission electron microscopy and M?auerspectroscopy, magnetic viscosity measurements were made in zero applied magnetic field and theresulting data were used to calculate the apparent magnetic-moment-weighted energy barrierdistributions for the samples. The distributions measured were typically comprised of both alognormal distribution and an exponential decay of barrier frequency with increasing barrier height.Evidence that the lognormal component of this distribution arises from the distribution of particlevolumes and moments within the ensemble is strongly supported by the increase in the mode of theenergy barrier distribution with increasing particle size. The exponentially decaying distribution hasa relatively higher contribution to the overall distribution for the more crystalline reconstitutedferritin samples suggesting that it may be associated predominantly with uncompensated spins atparticle surfaces.