Biology offers one of the most convenient model systems for the study of small particle magnetism - ferritin. Ferritin is an approximately spherical protein cage with an external diameter of 12nm and an internal diameter of 8nm. Iron is stored in the cavity of the protein in the form of a particle of iron(III) oxyhydroxide. The particle size is limited by the internal cavity dimensions of the protein. Thus a sample of ferritin contains single domain superparamagnetic particles with a narrow size distribution, each particle being separated from the others by a protein coat. Since the protein is soluble, the particle-particle distances can be adjusted by simply varying the concentration of a solution of ferritin. In our laboratories we have developed methods of removing the iron oxyhydroxide particle and replacing it under controlled conditions so that the degree of crystallographic order can be controlled. We have also developed methods of incorporating varying quantities of phosphate within the iron oxyhydroxide structure. Thus a range of particle structures and compositions is available. A question of major practical importance is: how does the structure and composition of a superparamagnetic particle affect its magnetic properties? We have several projects aimed at addressing this question.
Macroscopic Quantum Tunnelling of Magnetization?
Over the past decade the iron storage protein ferritin has featured prominently in a debate on whether or not resonant quantum tunneling of magnetization can be observed in bulk measurements on polydisperse mesoscopic antiferromagnetic particles [1-13 ]. Ferritin proteins each consist of a hollow approximately spherical shell of polypeptide with external diameter 12 nm and internal diameter 8 nm. Ferritin proteins isolated from biological systems are usually found to contain up to 4500 iron atoms within the central cavity in the form of a hydrated iron(III) oxyhydroxide particle (up to 8 nm in size) with trace amounts of phosphate. Horse spleen ferritin has been the ferritin of choice for study since it is commercially available. Horse spleen ferritin typically contains an average of 3000 Fe atoms per protein shell when highly loaded with iron.
Initial experiments designed to observe resonant quantum tunneling of
the magnetization of horse spleen ferritin particles were made at temperatures
of approximately 30 mK by measuring the frequency-dependent magnetic noise
and magnetic susceptibility of the particles using an integrated dc SQUID
microsusceptometer [1, 2 ]. A sharply defined resonance near 1 MHz was
observed at temperatures up to 0.2 K. The data were interpreted in terms
of macroscopic quantum tunneling of the weak magnetic moment of ferritin
that is assumed to be coupled to the Néel vector of the generally
antiferromagnetic iron(III) oxyhydroxide particles. The adjective "macroscopic"
refers to the fact that the system being described is significantly larger
than atomic scale systems which are well described by quantum mechanics.
The tunneling refers to the tunneling of the Néel vector through
the magnetic anisotropy energy barrier that separates easy directions of
magnetization for each particle. Although there were some objections to
the interpretation of the microsusceptometer measurements in terms of superpositions
of macroscopic quantum states [4, 14, 15], these discoveries prompted other
researchers to look for additional phenomena that may be related to quantum
tunneling of the ferritin moments. 3, 7, 8, 12, 13 Four basic phenomena
have been identified for ferritin at temperatures above 1 K and have been
interpreted in terms of quantum tunneling of magnetization in very small
or zero applied fields. These phenomena are (a) temperature independent
magnetic viscosity S below a critical temperature (approx 2 K), 3
(b) a non-monotonic dependence of the superparamagnetic blocking temperature
TB on applied magnetic field H (temperatures of approximately
15 K) [7, 8, 12, 13], (c) a maximum in the rate of change of magnetization
M with H at zero applied field during constant rate sweeps
of H (at temperatures between 5 and 13 K) [7, 12] and (d) a non-monotonic
dependence of S on H at temperatures between 4 and 6.5 K with
S increasing as H approaches zero [7, 12, 13]. Quantum tunneling
is expected to be suppressed above a certain temperature owing to the interaction
of the environment with the system [16 ]. Thus the observations of these
phenomena have important implications for the upper temperature limit at
which quantum tunneling of magnetization can be observed. However, it has
been pointed out by several workers that phenomena (a), (b), and (c) can
also be explained in terms of classical models of magnetic particles [9,
11]. We are currently embarking on studies of the magnetic viscosity of
ferritin that shed further light on its magnetic relaxation processes at
temperatures above 1 K.
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