Effects of Composition and Heat Treatment on the Stress Sensitivity and Magnetic Squareness Ratio of Molybdenum Permalloy Tape

Abstract

Some present day memory applications, for example, the Piggyback Twistor,1 require a magnetic material that possesses a coercive force in the vicinity of 0.5 Oe, a low-stress sensitivity and a square hysteresis loop. A study was made of several modified molybdenum permalloys (79%–82% Ni, 12%–17% Fe, 4%–6% Mo) for possible use in these applications. Tapes having dimensions 3.2 by 0.26 mil were made from these alloys by roll flattening 1.1 mil diam wires that had been heat treated (600°–900°C, 3 sec) at a diameter of 2.2 mil. The roll-flattening operation results in a widening of the tape without appreciable elongation. The stress sensitivity of these tapes, as determined by changes in 60 Hz coercive force and squareness ratio with tension, was found to depend on crystallographic texture and on nickel composition. The zero-stress-sensitivity composition increased from about 79% to about 80.5% Ni when the as-flattened tape was annealed (700°–900°C, 15 sec) to reduce the coercive force to the desired value. X-ray examination showed that the tape texture changes from a cold-worked {110} 〈11̄1〉 texture to a recrystallized {001} 〈100〉 texture. The {001} 〈100〉 intensity is influenced by the 2.2 mil wire heat-treatment temperature; a maximum was observed for 650°C. This treatment also corresponds to a maximum in the squareness ratio of the tape. Detailed studies of the strain-insensitive alloys, which contain 80.5% Ni, showed that the squareness ratio increased with molybdenum content for tapes which had received the same processing treatment and, hence, had the same {001} 〈100〉 texture. These observations are consistent with published data, indicating that for the investigated alloys, the first magneto-crystalline anisotropy constant is positive and increases in magnitude with molybdenum additions. In addition, a 6% Mo-80.5% Ni-13.5% Fe alloy was processed to tape of 2.4 by 0.5 mil cross section, by flat-drawing 2.45-mil wire through a diamond die. The flat-drawing process results in an elongation of the tape without appreciable widening. The as-drawn texture consists of {110} 〈11̄2〉+{110} 〈001〉+{112} 〈111̄〉 components, and transforms to a sharp {110} 〈001〉+{210} 〈001〉 texture upon annealing. The large 〈001〉 intensity along the tape axis resulted in an extremely square hysteresis loop (Br/Bs∼1.0).