Theoretical consideration is given to the behavior of the anisotropy field of iron—nickel films ranging in composition from 65% Ni—35% Fe to 90% Ni—10% Fe as a function of applied external stresses and magnetic fields in various directions in the plane of the film. It is postulated that internal stresses in the substrate film system may account for observed discrepancies. An expression is derived which relates the magnetoelastic parameter (&eegr;) to the magnetostriction (&lgr;s) of the film. This parameter is measured as a function of composition and the predicted value of &lgr;sagrees within the limits of precision to that observed for bulk material. Assuming the films to be isotropic in the plane of the substrate, a relation between applied stress and change in the anisotropy field is developed and within limits experimentally verified. It is postulated that observed discrepancies are due to nonlinear anisotropic internal stresses and dispersion in the direction of the easy axis of the film.