Electronic, magnetic and thermal behavior near the Invar compositions of Fe-Ni alloys

Abstract

The structural, magnetic and electronic properties of Fe${}_{1-x}$Ni${}_x$ ($x$ = 0.32, 0.36, 0.40, 0.50) alloys have been investigated using synchrotron based x-ray diffraction (XRD), superconducting quantum interference device (SQUID) magnetometer and the high resolution x-ray photoelectron spectroscopy (XPS) measurements. The XRD measurement was done down to 50 K temperature. The XRD studies suggest a single phase with fcc structure for $x$ = 0.36, 0.40, and 0.50 alloys and a mixed phase for $x$ = 0.32 alloy containing both bcc and fcc structures. The lattice parameter of the alloys exhibits a linear dependence on temperature giving rise to a temperature independent coefficient of thermal expansion (CTE). The lowest CTE is observed for $x$ = 0.36 Invar alloy as expected while $x$ = 0.50 alloy exhibits the highest CTE among the alloys studied. The CTE of the fcc component of mixed phase alloy is close to that of Invar alloy. The temperature dependence of magnetization of the alloys down to 2 K reveals an overall antiferromagnetic interactions within the ferromagnetic phase causing the magnetization decreasing with lowering temperature. The field cooled and zero field cooled data show larger differences for the Invar compositions; this is also manifested in the magnetic hysteresis data at 2 K and 300 K. The Fe 2$p$ and Ni 2$p$ core level spectra exhibit spin–orbit split features along with a satellite feature in the Ni 2$p$ spectra. The spectral line shapes are almost similar for all the compositions studied. Interestingly, the spin–orbit splitting for Fe 3$p$ spectra is larger than that observed for Ni 3$p$ suggesting additional contributions due to the exchange interaction between the Fe 3$p$ core hole with the Fe 3$d$ moment. This suggests large magnetic moment contribution from Fe as expected. The core level and valence band spectra, and the magnetization data suggest significant role of disorder for the Invar compositions.