First-principles investigations of atomic disorder effects on magnetic and structural instabilities in transition-metal alloys

In this paper we use the coherent-potential approximation within the Korringa-Kohn-Rostocker band-structure scheme to investigate the influence of atomic disorder on magnetism and crystal structure of transition-metal alloys like iron-nickel. This method allows an investigation of disordered alloys on an equally well-defined basis as an investigation of corresponding stoichiometrically ordered phases. In particular we have calculated the magnetic and structural binding surfaces of fcc ${\mathrm{Fe}}_{\mathit{x}}$ ${\mathrm{Ni}}_{1\mathrm{-}\mathit{x}}$ for concentrations close to the critical concentration x=0.65 which corresponds to the Invar alloy ${\mathrm{Fe}}_{65}$ ${\mathrm{Ni}}_{35}$, with the help of the fixed-spin-moment method. We find that magnetism in the ground state gradually vanishes as we go from ${\mathrm{Fe}}_{60}$ ${\mathrm{Ni}}_{40}$, which has a well-defined magnetic ground state being separated from the nonmagnetic state by 1.0 mRy/atom, to ${\mathrm{Fe}}_{75}$ ${\mathrm{Ni}}_{25}$ which is nonmagnetic. The criical concentration for which this disorder driven magnetic-nonmagnetic transition occurs is x≊0.65–0.70 in accordance with the magnetic phase diagram of ${\mathrm{Fe}}_{\mathit{x}}$ ${\mathrm{Ni}}_{1\mathrm{-}\mathit{x}}$. These calculations have to be compared with ab initio calculations for ordered fcc ${\mathrm{Fe}}_3$Ni; here the magnetic ground state is by 1.25 mRy more stable than the nonmagnetic state. This different magnetic behavior of disordered and ordered phases can be explained on statistical grounds. Furthermore, the magnetic disordered ground state is unstable with respect to a martensitic fcc→bcc transition on the Fe-rich side in accordance with the structural phase diagram of ${\mathrm{Fe}}_{\mathit{x}}$ ${\mathrm{Ni}}_{1\mathrm{-}\mathit{x}}$. We have furthermore calculated the temperature evolution of the binding surfaces with the help of a finite-temperature fluctuation theory. We find interesting reentrant ferromagnetic phase transitions in the fcc phase close to the Invar concentration x=0.65.