Europium mixed-valence, long-range magnetic order, and dynamic magnetic response in EuCu$_{2}$(Si$_{x}$Ge$_{1-x}$)$_{2}$

TL;DR

This study investigates EuCu$_{2}$(Si$_{x}$Ge$_{1-x}$)$_{2}$, revealing how Eu valence fluctuations influence magnetic order and dynamics, with implications for understanding mixed-valence phenomena in Eu-based compounds.

Contribution

It provides a detailed microscopic analysis of how Eu valence states affect magnetic properties across the series, highlighting persistent valence fluctuations within the magnetic regime.

Findings

Valence fluctuations persist into the antiferromagnetic state below x_c ≈ 0.65.

Magnetic ground states evolve with Eu valence and composition.

Magnetic spectral response reflects changes in valence and magnetic order.

Abstract

In mixed-valence or heavy-fermion systems, the hybridization between local $f$ orbitals and conduction band states can cause the suppression of long-range magnetic order, which competes with strong spin fluctuations. Ce- and Yb-based systems have been found to exhibit fascinating physical properties (heavy-fermion superconductivity, non-Fermi-liquid states, etc.) when tuned to the vicinity of magnetic quantum critical points by use of various external control parameters (temperature, magnetic field, chemical composition). Recently, similar effects (mixed-valence, Kondo fluctuations, heavy Fermi liquid) have been reported to exist in some Eu-based compounds. Unlike Ce (Yb), Eu has a multiple electron (hole) occupancy of its $4f$ shell, and the magnetic Eu$^{2+}$ state ($4f^7$) has no orbital component in the usual $LS$ coupling scheme, which can lead to a quite different and interesting physics. In the EuCu$_{2}$(Si$_{x}$Ge$_{1-x}$)$_{2}$ series, where the valence can be tuned by varying the Si/Ge ratio, it has been reported that a significant valence fluctuation can exist even in the magnetic order regime. This paper presents a detailed study of the latter material using different microscopic probes (XANES, M\"ossbauer spectroscopy, elastic and inelastic neutron scattering), in which the composition dependence of the magnetic order and dynamics across the series is traced back to the change in the Eu valence state. In particular, the results support the persistence of valence fluctuations into the antiferromagnetic state over a sizable composition range below the critical Si concentration $x_c \approx 0.65$. The sequence of magnetic ground states in the series is shown to reflect the evolution of the magnetic spectral response.