Metallurgical Control of Magnetic Properties in Co–Fe and Ni–Fe Alloys for Memory Applications

Abstract

Magnetic memories such as the Twistor require materials which exhibit a square hysteresis loop, controlled value of coercive force, and low stress sensitivity of the magnetic properties. Recent work on optimizing these properties in Co–Fe and Ni–Fe alloys as storage and sensing elements, respectively, will be reviewed. Co–Fe alloys in the face-centered cubic phase (∼90% Co) exhibit low values of magnetostriction and high values of magnetocrystalline anisotropy with 〈111〉 easy axes. Square hysteresis loops are obtained in cold-drawn wires and tapes roll flattened from such wires, as a sharp 〈111〉 axial texture is developed by the cold working. With alloying additions such as Au and Ti, values of coercive force up to 42 Oe can be attained via precipitation aging. For Ni–Fe alloys near the 4% Mo–79% Ni composition, a predominant 〈111〉 axial texture is developed in wire drawing and is converted to 〈100〉 via recrystallization anneal. Since the magnetostriction and magnetocrystalline anisotropy constants are near zero in this vicinity and are sensitive to crystal orientation, optimization of high squareness and low stress sensitivity requires close composition control depending on the particular texture. In cold-worked material, the squareness behavior is also complicated by the dominance of slip-induced anisotropy. Finally, additions such as Be and Zr have been made for precision control of coercive force through precipitation.