IR SPECTROSCOPY ON FENI AND FEPT INVAR-ALLOYS

UHV IR emission spectroscopy has been carried out at energies 9-33 mRy (wavelength 2.8-9.8 mum) in the temperature range 430-800 K on single crystals of FeNi and FePt Invar, with Curie temperatures in the respective range. The results show a drastic (up to 30%) decrease of the IR absorptivity around T(c) in the energy range 23-33 mRy (2.8-4 mum), while at small energies around 9 mRy (10 mum) the absorptivity is temperature independent, as expected for 3d metals, and found on pure Ni. The decrease in absorptivity on the Invar alloys cannot be understood by the common temperature dependence of interband transitions; rather, it calls for the existence of moment-volume instabilities, characterized by transitions between a high-spin ground state and a temperature-induced low-spin state. Qualitative agreement of the absorptivity with respective total density-of-states calculations on Fe3Ni is demonstrated.