A solution to the permalloy problem-A micromagnetic analysis with magnetostriction

A long-standing puzzle in the understanding of magnetic materials is the Permalloy problem, i.e., why the particular composition of Permalloy, Fe21.5Ni78.5, achieves a dramatic drop in hysteresis and concomitant increase in initial permeability, while its material constants show no obvious signal of this behavior. In fact, the anisotropy constant kappa(1) and the magnetostriction constants lambda(100), lambda(111) all vanish at various nearby, but distinctly different, compositions than Fe21.5Ni78.5. These compositions are in fact outside the compositional region where the main drop in hysteresis occurs. We use our newly developed coercivity tool [A. Renuka Balakrishna and R. D. James, Acta Mater. 208, 116697 (2021)] to identify a delicate balance between local instabilities and magnetic material constants that lead to a dramatic decrease in coercivity at the Permalloy composition Fe21.5Ni78.5. Our results demonstrate that specific values of magnetostriction constants and anisotropy constants are necessary for the dramatic drop of hysteresis at 78.5% Ni. Our findings are in agreement with the Permalloy experiments and provide theoretical guidance for the development of other low hysteresis magnetic alloys.

Automated summary

The study reveals that the reduction in hysteresis of Permalloy is related to specific values of magnetostriction and anisotropy constants, which are balanced at the particular composition Fe21.5Ni78.5, resulting in a significant decrease in hysteresis.