Invar and Anti-Invar: Magnetovolume Effects in Fe-Based Alloys Revisited

In search for a cheaper length standard than Pt–Ir, Guillaume found in 1897 “Invar,” a ferromagnetic (FM) fcc Fe65Ni35 alloy showing small, almost temperature independent thermal expansion below the Curie temperature. After discovering “Elinvar,” an FM fcc Fe–Ni–Cr alloy with constant elastic behavior, “revolutionizing” the watch industry with newly designed balance springs, Guillaume was honored with the Nobel Price in physics in 1920. In spite of worldwide research, understanding of Invar and Elinvar remained a centennial problem until the beginning of the 1990s, when ab initio calculations gave basic insights into the relations between lattice structure, atomic volume and magnetic properties. At that time, “anti-Invar” also was recognized, the effect being the enlargement of the atomic volume of fcc Fe and fcc Fe-rich alloys at high temperatures in the paramagnetic range by FM spin fluctuations. In this chapter we briefly review the basic experimental properties of Invar and anti-Invar and discuss the properties of Fe, the element inspiring Weiss to his historical 2γ-states model. We then revisit the theoretical situation and specifically discuss the models stressing the importance of charge transfer at the Fermi energy between electronic levels with different symmetry (eg and t2g) and nonbonding or antibonding character.