High-temperature moment-volume instability and anti-Invar of γ-Fe

Just as Invar describes a system with a smaller than ‘‘normal’’ thermal expansion in a magnetically ordered state, a system with a larger than ‘‘normal’’ thermal expansion in a magnetically disordered state can be described as anti-Invar. Anti-Invar had been already observed in a number of fcc binary and ternary alloys of the 3d series well above magnetic ordering temperatures. While Invar is characterized by low-temperature moment-volume instabilities, anti-Invar signifies the importance of high-temperature moment-volume instabilities. Both are closely related to the ground-state properties. We have reinvestigated the ${\mathrm{Fe}}_{50}$ ${\mathrm{Ni}}_{\mathit{x}}$ ${\mathrm{Mn}}_{50\mathrm{-}\mathit{x}}$ alloy system and confirmed the existence of enhanced thermal expansion at high temperatures, i.e., the anti-Invar effect. With the aid of the thermal properties of these alloys we deduce the temperature dependence of the thermal expansion of γ-Fe and show its anti-Invar nature. The enhanced volume expansion in γ-Fe amounts to 2.8%, which is larger than the Invar effect (2%) in ${\mathrm{Fe}}_{65}$ ${\mathrm{Ni}}_{35}$.