  We investigate the ground state properties of Invar alloys via detailed study
of the electronic structure of Fe$_{1-x}$Ni$_x$ alloys ($x$ = 0.2, 0.3, 0.4,
0.5, 0.6, 0.7, 0.9) employing $x$-ray photoelectron spectroscopy (XPS). While
all the alloys exhibit soft ferromagnetic behavior with Curie temperature much
higher than the room temperature, the results for invar alloy,
Fe$_{0.6}$Ni$_{0.4}$ exhibit anomalous behavior. Moreover, the
magneto-resistance of the Invar alloy becomes highly negative while the end
members possess positive magneto-resistance. The core level spectra of the
Invar alloy exhibit emergence of a distinct new feature below 20~K while all
other Fe-Ni alloys exhibit no temperature dependence down to 10~K.
Interestingly, the shallow core level spectra (3$s$, 3$p$) of Fe and Ni of the
Invar alloy reveal stronger deviation at low temperatures compared to the deep
core levels (2$s$, 2$p$) indicating crystal field effect. It appears that there
is a large precipitation of antiferromagnetic $\gamma^\prime$ phase below 20 K
possessing low magnetic moment (0.5$\mu_B$) on Fe within the $\alpha$ phase.
The discovery of negative magneto-resistance, anomalous magnetization at low
temperature and the emergence of unusual new features in the core levels at low
temperature provide an evidence of mixed phase in the ground state of Invar
alloys.
