Abstract:
The Invar anomaly, a phenomenon observed in the canonical INVAR alloys Fe-Ni, has been investigated on a first-principles basis, with the inclusion of longitudinal spin fluctuations in the paramagnetic region. We obtained an almost perfect quantitative description of the spontaneous volume magnetostrictions and their dependence on the alloy's chemical composition. With an increase in the Ni concentration, the Invar anomaly vanishes in Fe-Ni, and spontaneous magneto-volume magnetostrictions even reach slightly negative values in compositions with over 50% of Ni (anti-Invar). This feature is readily reproduced in our calculations, being a consequence of the longitudinal spin fluctuations (LSF) that increase the local atomic moment as temperature increases. We will provide an overview of the history of the methodology that allows for an effective handling of LSF on a first-principles basis and its impact on the theory of magnetism in transition metal alloys. We demonstrate that the mechanism of the Invar anomaly is rooted in the repopulation of anti-bonding states in the majority spin band and bonding states in the minority spin band at the Fermi level. This repopulation occurs due to thermal magnetic disorder effects that reduce the local atomic moments compare to their values in the ferromagnetic ground state due to itinerant character of the magnetism. We provide evidence that such repopulation of majority and minority spin states occurs with change of their bonding character in Fe-Ni alloys due to special position of the Fermi level in the metallic d-band. We visualize the bonding/anti-bonding character of electronic states in Fe-Ni alloy at the Fermi level using first-principle
based Crystal Orbital Hamiltonian Population (COHP) analyses