Predictive Theory of Anomalous Volume Magnetostriction in Fe-Ni Alloys: Bond Repopulation Mechanism of the Invar Effect

We demonstrate that a quantitative description of the observed magnetovolume anomaly in Fe-Ni Invar alloys can be achieved by taking into account thermally induced longitudinal spin fluctuations on a first-principles basis. The experimental dependence of spontaneous volume magnetostriction on the Ni atomic concentration is readily reproduced. Our calculations predict the Invar anomaly in the Fe65Ni35 INVAR alloy and its gradual vanishing for compositions with more than 50 at. % of Ni. The mechanism of the anomaly is linked to the repopulation of the electronic states in the majority and minority spin bands, accompanied by changes in their bonding character as thermal magnetic disorder increases in the system. These changes can be visualized using first-principles bond population analyses. It provides a simple, chemically intuitive picture of the mechanism of the Invar effect.