Thermodynamic analysis ofFe72Pt28Invar

Abstract

We present measurements of the specific-heat capacity ${\mathit{c}}_{\mathit{p}}$(T) and the thermal expansion coefficient α(T) on ordered and disordered ${\mathrm{Fe}}_{72}$ ${\mathrm{Pt}}_{28}$ Invar in the temperature range 4<T<1200 K. In a detailed discussion of the various contributions to the total specific heat we show that after properly correcting for the thermal expansion of the alloy, the specific-heat anomaly around the Curie temperature in ordered ${\mathrm{Fe}}_{72}$ ${\mathrm{Pt}}_{28}$ is sharper than in the disordered alloy. The results for the magnetic specific heat are compared with theoretical finite-temperature expansions of the T=0 band structure for ${\mathrm{Fe}}_3$Pt, exhibiting pronounced moment-volume instabilities. In accordance with the theoretical predictions we find that the magnetic contribution to the specific heat in the paramagnetic range decreases with increasing temperature, and thus seems to be dominated by strong spin fluctuations. However, the jump in ${\mathit{c}}_{\mathit{p}}$(T) at ${\mathit{T}}_{\mathit{C}}$ observed in the model calculations is not supported by our experiments. From a comparison of the total measured specific heat with ${\mathit{c}}_{\mathit{p}}$(T) of a fictitious, nonmagnetic reference material we estimate the energy, which stabilizes the magnetic Invar phase, to be about 1 mRy/atom. This is comparable to the energetic stability of the fcc austenitic phase as compared to the bcc martensitic phase.