Theory of Invar and Heusler alloys

Abstract

The physical picture of Invar and Heusler alloys provided by parameter-free, self-consistent, spin-polarized energy-band calculations is described. In our view, Invar differs from other itinerant magnets in possessing a nonmagnetic state that can be reached by thermal excitations. Calculations for ordered Fe-Ni compounds containing 0, 25, 50, 75 and 100% Ni agree with zero-temperature measurements, and show the total-energy separation of the magnetic and nonmagnetic states to vanish in the Invar composition range, where both states are stable, but at different volumes. Heusler alloys (X2MnY) possess localized magnetic moments (on the Mn atoms) that are not in nearest-neighbor contact, and therefore provide a testing ground for theories of indirect magnetic coupling. Our calculations indicate that the majority-spin electrons are completely itinerant and that the localized moments result from the exclusion of the minority-spin electrons from the interior of the Mn atoms. Interatomic exchange coupling is studied by comparing calculations for ferromagnetic and antiferromagnetic moment alignments. © 1983.