Magnetic state, magnetovolume effects, and atomic order inFe65Ni35Invar alloy: A first principles study

Abstract

We employ the locally self-consistent Green's function technique and exact muffin-tin orbital method to investigate magnetic state and ground state properties of Invar ${\mathrm{Fe}}_{65}{\mathrm{Ni}}_{35}$ alloy. We show that it is in a chemically disordered state, characterized by a relatively small amount of atomic short-range order, above the magnetic ordering temperature. We speculate that it should remain in this state below the Curie temperature upon applying usual heat treatment for the Invar alloys. The magnetic state at the experimental lattice spacing is shown to be sensitive to the type of approximation for the exchange-correlation functional: While the magnetic ground state is purely ferromagnetic in the generalized gradient approximation, there is a small amount of Fe atoms with magnetic moment antiferromagnetically aligned relative to the global magnetization in the local density approximations. The local spin-density approximation, however, fails to yield correctly the equilibrium lattice spacing, whereas the generalized gradient approximation reproduces it reasonably well. The anomalous spontaneous volume magnetostriction leading to the Invar effect is found to be $\ensuremath{\approx}3%$, in fair agreement with the experimental estimate of $\ensuremath{\approx}2.2%$.