THERMODYNAMIC ANALYSIS OF FE72PT28 INVAR

We present measurements of the specific-heat capacity cp(T) and the thermal expansion coefficient (T) on ordered and disordered Fe72Pt28 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 Fe72Pt28 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 Fe3Pt, 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 cp(T) at TC observed in the model calculations is not supported by our experiments. From a comparison of the total measured specific heat with cp(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. © 1995 The American Physical Society.