Fe substitution in URu$_2$Si$_2$: singlet magnetism in an extended Doniach phase diagram

TL;DR

This study investigates how Fe substitution in URu$_2$Si$_2$ affects its electronic and magnetic properties, revealing non-monotonic spectral changes and proposing an extended Doniach phase diagram to explain singlet magnetism in the material.

Contribution

It introduces an extended Doniach phase diagram incorporating density of states dependence and demonstrates singlet magnetism as the ground state in Fe-substituted URu$_2$Si$_2$.

Findings

Non-monotonic changes in 4f core-level spectra with Fe substitution.

Proposed an extended Doniach phase diagram including density of states.

Ground state remains a singlet or quasi-doublet state despite magnetic order.

Abstract

The application of pressure as well as the successive substitution of Ru with Fe in the hidden order (HO) compound URu$_2$Si$_2$ leads to the formation of the large moment antiferromagnetic phase (LMAFM). Here we have investigated the substitution series URu$_{2-x}$Fe$_x$Si$_2$ from $x$\,=\,0.0 to 2.0 by U\,4$f$ core-level photoelectron spectroscopy and have observed non-monotonic changes in the spectra. The initial increase and subsequent decrease of the spectral weight of the 4$f$ core level satellite with increasing $x$ stands for a non-monotonic 5$f$ filling across the substitution series. The competition of chemical pressure and increase of the density of states at the Fermi energy, both due to substitution of Ru with Fe, can explain such a behavior. An extended Doniach phase diagram including the $x$ dependence of the density of states is proposed. Also in URu$_{2-x}$Fe$_x$Si$_2$ the ground state is a singlet or quasi-doublet state consisting of two singlets. Hence, the formation of magnetic order in the URu$_{2-x}$Fe$_x$Si$_2$ substitution series must be explained within a singlet magnetism model.