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The addition of 1%–5% Mn to Ni–Fe films reduces Hk by ∼3× and increases magnetostriction by ∼4×, but also raises dispersion by ∼3×, so that the product α50Ku stays within ±10% over the range of interest of from 1% to 5% Mn. Since high dispersion (α50>5°) was found to cause disturbs in memory applications, it is not advisable to use more than ∼3% Mn for a memory with 20-mil-wide film strips.

For the sonic film memory highly strain-sensitive ferromagnetic thin films are required with magnetic properties comparable to those of good Permalloy memory films. Research has shown that films prepared by evaporation from 60% Ni, 25% Fe, 15% Co melts meet the requirements. These films have Hc and Hk values below 15 Oe and are usually inverted. They are well oriented with good squareness in the easy axis. X-ray diffraction measurements show them to have a single phase (fcc). The strain-sensitivity is about an order of magnitude higher than that of the most sensitive Ni–Fe alloys with comparable values of Hc and Hk. Typically, a strain of about 5×10−5 causes: (1) a rotation of the easy axis by ∼25°, (2) a significant change in the values of Hc, Hk, and (3) a decrease in the switching threshold by at least a factor of two. The change in Hk and the rotation of the easy axis agree in general with theoretically predicted behavior. Dynamic strain measurements show these films to be suitable for memory use with coincidence writing (strain pulse—magnetic field pulse) and nondestructive read.