The high values of initial and maximum permeabilities found with quenched 78.5% nickel-iron suggest that the effective anisotropy energy is a minimum at this composition. The magnetocrystalline anisotropy coefficient K₁ is known to go through zero at 75% nickel. At 78.5% nickel K₁ is small and negative, giving lowest energy with magnetization along [111] axes. The magnetostriction is anisotropic, with λ₁₀₀ > λ₁₁₁, so that the magnetoelastic energy, for randomly oriented internal stress, is also anisotropic with deepest energy minima along [100] axes. The combination of these two anisotropy terms can account for the minimum value of effective anisotropy at 78.5% nickel.

A magnetic anneal induces uniaxial anisotropy and assists alignment of easy axes throughout the material, giving very high values of maximum permeability, by 180° boundary movement. The optimum composition is that with the lowest effective anisotropy energy and, again, is slightly displaced from the composition for which K₁ = 0 by magnetoelastic energy associated with anisotropic magnetostriction.