Anomalous magnetic relaxation in ferritin

Abstract

We report measurements in natural horse-spleen ferritin that provide a detailed mapping of the blocking temperature, T B , as a function of magnetic field over a broad range up to 20 kOe. Unlike most superparamagnetic materials where it decreases with applied field, T B increases with increasing field at small fields, reaching a maximum at Ϸ3 kOe before exhibiting the expected decrease. The hysteresis loops are anomalously ''pinched'' near zero field. Both observations are consistent with an effective energy barrier that is smaller at zero field than in small finite fields. This may arise from tunneling between pairs of states on opposite sides of the anisotropy barrier that are in resonance in zero magnetic field, regardless of particle size. However, direct measurements of the magnetic viscosity yield ambiguous results, leaving open other possible explanations. ͓S0163-1829͑97͒06441-2͔Quantum tunneling of magnetization has been the focus of renewed interest with the recent discovery 1,2 of resonant tunneling between spin states in the molecular-magnet Mn 12 acetate. The magnetic relaxation rate in Mn 12 was found to increase markedly whenever the applied magnetic field brings spin states on opposite sides of a potential barrier into resonance. These results have now been confirmed, 3,4 and recent experiments have found similar effects in other systems of magnetic molecules. 5,6 The mesoscopic spin-10 magnetic subunits in Mn 12 are each made of 12 strongly superexchange-coupled Mn ions. It would be interesting to observe similar effects in macroscopic systems where the magnetic entities are composed of a larger number ͑thousands͒ of spins. Molecular magnets such as Mn 12 consist of magnetic entities that are nominally identical and, therefore, have resonances at well-defined values of field. This is not the case for most other ensembles of small magnetic particles, which generally have a distribution of moments, anisotropies, and potential barriers. In such systems one expects there to be resonances for some particles at any given field, smearing out the effect and thereby making it unobservable. The only resonance that all particles have in common is the one at zero field, where spin-up and spindown levels are degenerate, and all pairs of states on opposite sides of the anisotropy barrier are in resonance, regardless of particle size. Thus, one expects to observe resonant tunneling effects only near zero applied field in systems containing a distribution of magnetic sizes.Horse-spleen ferritin is in many ways an ideal laboratory for searching for resonant tunneling. It is a superparamagnet that consists of an iron-storage protein with an antiferromagnetic core of diameter Ͻ8 nm such as ͑FeOOH͒ 8 ͑FeOPO 3 H 2 ). Although the magnetic cores are broadly distributed in size in natural ferritin, the maximum cluster size is limited by the volume of the protein cage; moreover, since each is contained within a protein molecule, the interaction between clusters is relatively small. Also, because ferritin is antiferromagn...

Resonant spin tunneling in small antiferromagnetic particles

“…7 we also compare the classical predictions with the blocking temperature T B obtained previously from zerofield cooled dc susceptibility, that presents a minimum at zero field too. [20][21][22] In the calculations, we followed the method described in Refs. 25 and 34 but making ϰV 1/2 that, as we have shown just before, is appropriate for ferritin.…”

Shifted loops and coercivity from field-imprinted high-energy barriers in ferritin and ferrihydrite nanoparticles

“…The magnetic properties of ferritin have been extensively studied in the last decade due to their puzzling features, such as the existence of a maximum in the magnetization derivative at zero field, 1,2 a nonmonotonic field dependence of the magnetic viscosity, [1][2][3] and a decrease of the antiferromagnetic susceptibility with temperature below the Néel temperature when considered at low fields. 4,5 Many of these studies were performed to enlighten the possible existence of quantum tunneling in ferritin in the Kelvin range.…”

Characterization of iron compounds in tumour tissue from temporal lobe epilepsy patients using low temperature magnetic methods

“…Perfect antiferromagnets (AFM) do not show hysteresis, therefore the open loops are probably due to the switching of frustrated surface spins at the ferritin core or to a defect moment (Ne´el 1962). The ordering temperature between 6 and 12 K is also typical for ferritin; horse spleen ferritin has an average blocking temperature from 10 to 20 K (Kilcoyne & Cywinski 1995;Makhlouf et al 1997;Friedman et al 1997;Gilles et al 2000). The only magnetic study on human brain ferritin from globus pallidus by Dubiel et al (1999) reports an average blocking temperature of 8.5 K. The distribution of particle volume, energy barriers and therefore blocking temperatures is known from many superparamagnetic systems.…”

“…The measurement of the induced magnetic moment as a function of temperature in a constant field after ZFC or FC is a useful method in the characterization of horse spleen ferritin (MohieEldin et al 1994;Makhlouf et al 1997;Friedman et al 1997;Gilles et al 2000). Figure 3 shows the DC susceptibility as a function of temperature for ZFC and FC.…”