Activation volume of alpha-Fe particles in alumite films

The time-dependent properties of arrays of elongated and mutually aligned iron particles in alumite (anodized aluminum oxide film) have been studied. At a constant particle diameter of 11 nm, the magnetic viscosity is nearly independent of the particle length (varied from 140 to 440 nra). The activation volume (1.1 to 1.2 X 10(-18) cm(3)) is almost independent of the particle length and fields: this nearly constant value is much smaller than previously reported values, but very close to the theoretical superparamagnetic limit 1.0 X 10(-18) cm(3) at room temperature. (C) 1997 American Institute of Physics.