Passivated, particulate high curie temperature magnetic alloys

Abstract

A process, and the compositions produced thereby, wherein an element characterized as aluminum, silicon or chromium, notably aluminum, is alloyed in relatively small concentrations with iron, cobalt or nickel, notably iron or cobalt, or both, which have relatively high Curie temperatures. Small amounts of yttrium, hafnium, zirconium or lanthanium can also be added. Alloys of specified composition are formed into particles of selected size, and the particles then contacted in an atmosphere of controlled oxygen activity at elevated temperature over selected time periods. By treatment with a low oxygen-containing, or low moisture-containing gas an oxide of the alloying element is formed which diffuses to the surface of the particles to form a film. The addition of these elements to the base metal necessarily, and unavoidably reduces the Curie temperature of the metal. However, by limiting the particle size and the amount of oxidation of the alloys, a relatively thin impervious oxide film can be formed on the surface of the alloy particle, and much of the lost Curie temperature can be recovered by the increased magnetization which occurs at the particle core when the oxidized form of the metal migrates, or diffuses, to the surface of the particle.