Coercive Force and Remanence of 25-A to 2000-A Diameter Cobalt, Iron, and Iron-Cobalt Alloy

Abstract

This paper reports an experimental study of the size dependence of the coercive force and remanence of essentially spherical cobalt, iron, and iron-cobalt alloy particles over the diameter range from 25 A to 2000 A. The particles were prepared by low-temperature electrodeposition into a mercury capillary jet, followed by thermal growth; size and shape distributions were determined by electron microscopy. The maximum coercive force at 76°K was 1330 oe for cobalt, 890 oe for iron, and 1380 oe for iron-cobalt alloy; these maxima occurred at a diameter of 30 A for cobalt, 130 A for iron, and 240 A for iron-cobalt alloy. At a sufficiently small diameter superparamagnetism reduced the remanence and coercive force to zero. This occurred at a particle diameter below 25 A for cobalt, at 50 A for iron, and at 40 A for iron-cobalt alloy. The coercive force, remanence, and their temperature coefficients are discussed in terms of the approach to single-domain behavior, the anisotropy constants which determine single-domain rotation, and the transition to superparamagnetic behavior. Crystal anisotropy predominates in the cobalt particles, a combination of shape and crystal in the iron particles, and shape anisotropy in the iron-cobalt alloy particles.