Determination of the pre-exponential frequency factor for superparamagnetic maghemite particles in magnetoferritin

Magnetization and Mössbauer measurements on maghemite particles with an average particle diameter of 10 nm have been made in the temperature range from 5 K to 353 K spanning the superparamagnetic (SPM) and stable single domain (SD) regimes. The maghemite particles were produced within the iron-storage protein ferritin, resulting in a narrowly-sized, weakly interacting nanocomposite material called magnetoferritin. Experiments combining hysteresis measurements, low temperature remanence, and Mössbauer spectroscopy were used to characterize magnetoferritin and to provide experimental estimates of (1) the pre-exponential frequency factor ƒ0 in the Néel-Arhennius relaxation equation; (2) the SPM threshold size at room temperature for maghemite; and (3) the SD value of Hr/Hc at 0 K. The frequency factor was determined from the difference in blocking temperatures measured by dc magnetization and Mössbauer spectroscopy, yielding a value of f0≈109 Hz. This agrees well with the standard value and justifies the usually assumed superparamagnetic blocking condition of KV = 25 kT for remanence measurements. The SPM threshold size at room temperature for remanence measurements was estimated to be 20–27 nm and the extrapolated SD value at 0 K for Hr/Hc is 1.32. The latter value is slightly larger than the theoretical value of 1.09 but may be more appropriate for weakly interacting SD particles commonly found in sediments and soils. However, ƒ0 for ferrimagnetic magnetoferritin is a factor of 103 lower than was determined previously for native ferritin, which contains antiferromagnetic ferrihydrite cores. The difference in ƒ0 values between the two varieties of ferritin is probably related to the two different types of magnetic spin ordering of the core minerals and suggests that the higher value of ƒ0 is more appropriate for antiferromagnetic minerals like hematite and goethite, whereas the lower value is more appropriate for ferrimagnetic minerals like maghemite, magnetite, or greigite.R.B.F. was supported by NIH grant ROI DK36799-06A4. K.K.W.W. was supported by the BBSRC (U.K.). The Institute for Rock Magnetism (IRM) is supported by grants from the Keck Foundation and the NSF. IRM contribution 9704

Magnetic-field and current-density distributions in thin-film superconducting rings and disks

We show how to calculate the magnetic-field and sheet-current distributions for a thin-film superconducting annular ring (inner radius a, outer radius b, and thickness d<<a) when either the penetration depth obeys lambda < d/2 or, if lambda > d/2, the two-dimensional screening length obeys Lambda = 2 lambda^2/d << a for the following cases: (a) magnetic flux trapped in the hole in the absence of an applied magnetic field, (b) zero magnetic flux in the hole when the ring is subjected to an applied magnetic field, and (c) focusing of magnetic flux into the hole when a magnetic field is applied but no net current flows around the ring. We use a similar method to calculate the magnetic-field and sheet-current distributions and magnetization loops for a thin, bulk-pinning-free superconducting disk (radius b) containing a dome of magnetic flux of radius a when flux entry is impeded by a geometrical barrier.Comment: 10 pages, 13 figure

Excess Spin and the Dynamics of Antiferromagnetic Ferritin

Temperature-dependent magnetization measurements on a series of synthetic ferritin proteins containing from 100 to 3000 Fe(III) ions are used to determine the uncompensated moment of these antiferromagnetic particles. The results are compared with recent theories of macroscopic quantum coherence which explicitly include the effect of this excess moment. The scaling of the excess moment with protein size is consistent with a simple model of finite size effects and sublattice noncompensation.Comment: 4 pages, 3 Postsript figures, 1 table. Submitted to PR

Failing boys and moral panics: perspectives on the underachievement debate

The paper re-examines the underachievement debate from the perspective of the ‘discourse of derision’ that surrounds much writing in this area. It considers the contradictions and inconsistencies which underpin much of the discourse – from a reinterpretation of examination scores, to the conflation of the concepts of ‘under’ and ‘low’ achievement and finally to the lack of consensus on a means of defining and measuring the term underachievement. In doing so, this paper suggests a more innovative approach for understanding, re-evaluating and perhaps rejecting the notion of underachievement