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

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 eg and t2g states in ordered Fe₃Ni. We show that in this compound the instability against small tetragonal distortions, the Invar behavior, and the LM {} 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 EF. Our fixed-spin-moment (FSM) calculation confirms the presence of these orbitals close to EF in Fe₃Ni. 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 Fe₃Ni. We find that the states merge with increasing temperature.