Antiferromagnetic Invar and anti-Invar in Fe-Mn alloys

We present measurements of the thermal-expansion coefficient α(T) on ${\mathrm{Fe}}_{100\mathrm{-}\mathit{x}}$ ${\mathrm{Mn}}_{\mathit{x}}$ alloys with 15<x<60 at. % in the temperature range 4<T<1150 K. The results show that α(T) is anomalously enhanced especially in the high-temperature range. The total volume expansion between 0 K and the melting point ${\mathit{T}}_{\mathit{m}}$ is up to 70% larger as compared to the volume expansion of a normal metal in the range 0<T<${\mathit{T}}_{\mathit{m}}$. A similar enhancement has earlier been found in fcc Fe and is referred to as the anti-Invar effect. An analysis is presented in order to determine the quantitative anti-Invar contribution to α(T) in Fe-Mn. The observed behavior is discussed in terms of moment-volume instabilities, which are known to be responsible for the Invar effect.