First-principles calculations of the instability leading to the Invar effect.

Abstract

An attempt is made to show the close connection between martensitic transformations, the Invar effect, magnetovolume instabilities, and associated low-moment--high-moment (LM-HM) transitions. For that purpose we study the volume dependence of electron occupation and magnetization of ${\mathit{e}}_{\mathit{g}}$ and ${\mathit{t}}_{2\mathit{g}}$ states in ordered ${\mathrm{Fe}}_{3}$Ni. We show that in this compound the instability against small tetragonal distortions, the Invar behavior, and the LM \ensuremath{\rightleftarrows} HM transitions are a consequence of the delicate balance of charge distribution and magnetic order from d orbitals having different bonding character. Our calculation is a generalization of the cluster calculation of Kaspar and Salahub who have speculated that the Invar effect is a consequence of thermal excitations of electrons from the antibonding majority-spin level to close-lying nonbonding minority-spin orbitals at the Fermi energy ${\mathit{E}}_{\mathit{F}}$. Our fixed-spin-moment (FSM) calculation confirms the presence of these orbitals close to ${\mathit{E}}_{\mathit{F}}$ in ${\mathrm{Fe}}_{3}$Ni. In addition we have extended the FSM calculations to finite temperatures with the help of a semimicroscopic spin-fluctuation theory, which allows the study of the temperature evolution of the HM and LM states of ${\mathrm{Fe}}_{3}$Ni. We find that the states merge with increasing temperature.