CLASSICAL AND QUANTUM MAGNETIC PHENOMENA IN NATURAL AND ARTIFICIAL FERRITIN PROTEINS

Artificial ferritin has been synthesized with control of both the magn etic state (antiferromagnetic or ferrimagnetic) and the particle size over an order of magnitude in the number of iron atoms. The magnetic p roperties of the artificial ferritin were compared with those of natur al horse spleen ferritin in a range of temperatures (20 millikelvin to 300 kelvin) and fields (1 nanotesla to 27 tesla). In the classical re gime, the blocking temperature was found to correlate with the average particle size. A correlation was also observed in the quantum regime between the resonance frequency of macroscopic quantum tunneling of th e Neel vector and the particle size. At high magnetic fields (to 27 te sla), a spin flop transition with a strong dependence on orientation w as seen in the natural ferritin, providing evidence of antiferromagnet ism in this system.