Magnetization Reversal of Almost Perfect Whiskers

Abstract

Single-crystal whiskers of iron and iron-cobalt alloys with diameters from 250 to 4000 Å were prepared by evaporation of the metal in a vacuum. Larger diameter whiskers, from 0.2 to 200 μ, were made by hydrogen reduction of iron bromide. Electron microscopic examination of the vacuum-grown small whiskers showed them to be perfect appearing rods of uniform diameter. Electron diffraction indicated that they were all single crystals with a thin oxide layer. The whisker axes of the small whiskers were all along the 〈100〉 direction, but some of the large whiskers had a 〈111〉 orientation. Magnetic and chemical analysis indicated that the oxide thickness increased with increasing diameter from about 30 Å for 300-Å whiskers to about 100 Å for 1000-Å whiskers. New calculations were carried out for the remanence curves, the intrinsic coercive force and the remanent coercive force of arrays of infinite cylinders with random orientation in either two or three dimensions reversing their magnetization by curling, by fanning, or coherently. The dependence of coercive force, remanence, and rotational hysteresis of the randomly oriented whiskers as a function of diameter correlates well with the curling mode of magnetization reversal for diameters less than about 1000 Å. These results provide the first clear experimental evidence for magnetization curling. This is in sharp contrast to the size-independent behavior of bumpy, irregular, electrodeposited ESD particles, whose properties are described by a fanning mode. The results for alloy whiskers, with up to 60% cobalt and with various crystallographic orientations, were the same as for pure iron. Since the only difference between the alloy and the pure iron whiskers is the crystal anisotropy energy, it is concluded that the crystal anisotropy contribution is negligible. The results suggest a gradual transition from the uniform curling mode below about 1000 Å to some sort of heterogeneous reversal process propagating from one or more nucleation centers in each whisker with increasing diameter.