Авторы: Milne G., Dunlop D.

2006 г.

JGR, Solid Earth

We report magnetic hysteresis and back-field demagnetization data measured at 10
intervals of the angle y between the applied field H and the long (magnetically easy) axes of aligned elongated single-domain iron particles electrodeposited in regularly spaced surface pores in an Al substrate. Our three samples have identical particle diameters, d = 17 nm, but varying axial ratios l/d and spacings dc. Measured hysteresis loops resemble ‘‘hysterons’’ predicted by fanning, curling, and coherent rotation models of magnetization reversal, except for two features. For y = 0–60
, particle magnetizations do not reverse simultaneously at a single critical field: distributed particle coercivities produce steep but nonvertical loop segments. Broadened y = 80–90
loops indicate imperfectly aligned particles. Comparing coercive force Hc(y) with theoretical variations for different angular dispersions of axes and Mrs/Ms data with a theoretical cos y variation, we find a variance of 6–8.5
in particle alignment. The Hc(y) results most closely resemble the theoretical variation for fanning rotations when y
50
and coherent rotations when y > 50
. A predicted hump in the Hc(y) curve at intermediate angles marking the changeover from one mechanism to the other is suppressed by particle interactions (packing factors p of 0.127–0.373). Remanent coercive force measurements Hcr(y) show that irreversible changes, unlike reversible rotations, are incoherent at both large and small y. Hcr continues to rise, to a high of 340–430 mT, as y ! 90
, whereas Hc(y) decreases over the same range. Hcr(y) results at large y favor fanning reversals for one sample and curling reversals for the other two. A Day plot of Mrs/Ms versus Hcr/Hc data gives a novel and distinctive trend from which y can determined within ±5
for aligned uniaxial particles.

Файл: 333.pdf