Zero Magnetostriction Composition of NiFe Films

Measurements of the magnetoelastic strain sensitivity of evaporated NiFe films in the range 64.1–84.1 wt % nickel show the average zero magnetostriction composition (ZMC) to be 81.5±0.2% nickel for films in the vicinity of 1000 Å. This result agrees with the ZMC reported by Wolf and Crowther for electroplated and evaporated 1000- Å films, but disagrees with the value of 80.3% nickel reported by Long for 10 000- Å electroplated films, even though the same analyst and analytic techniques were used. In an attempt to resolve this difference, the effect on the ZMC of a uniform composition gradient is examined. Strong exchange coupling is assumed to make the magnetization direction uniform throughout the film. A quadratic expression is fitted to the nonlinear magnetoelastic strain coefficient vs composition data. It can then be shown that the apparent ZMC, as a function of film thickness and composition gradient, may be represented by ZMC=55.86+[− (G2T2/12)+657.4]½, where G is the gradient in nickel concentration in weight percent Ni per 1000 Å and T is the film thickness in thousands of angstroms. The apparent discrepancy with Long's result would be resolved if a composition gradient in the order of 2.7% 1000 Å existed in the electroplated films. Since a gradient of this magnitude is not unreasonable in an electroplating process even with agitation, the ZMC of 80.3% Ni reported by Long is consistent with the model proposed here. It is concluded that the ZMC of NiFe films is 81.5% Ni in the absence of a gradient, although whatever mechanisms are responsible for Metzdorf's recent findings may also be significant here.