Mixed ground state in Fe-Ni Invar alloys

TL;DR

This study reveals a mixed phase ground state in Fe-Ni Invar alloys, characterized by anomalous magnetic and electronic behaviors at low temperatures, indicating complex phase coexistence and crystal field effects.

Contribution

It provides the first detailed spectroscopic evidence of a mixed magnetic phase in Invar alloys, highlighting temperature-dependent electronic structure changes.

Findings

Emergence of a new spectral feature below 20 K in Invar alloys.

Negative magneto-resistance observed in Invar alloys.

Strong deviation in shallow core levels indicating crystal field effects.

Abstract

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.