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
uctuations. 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 eld, chemical composition). Recently, similar effects (mixed-valence, Kondo
uctuations,
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 Eu2+ state (4f7) has
no orbital component in the usual LS coupling scheme, which can lead to a quite different and
interesting physics. In the EuCu2(SixGe1-x)2 series, where the valence can be tuned by varying
the Si/Ge ratio, it has been reported that a significant valence
uctuation can exist even in the
magnetic order regime. This paper presents a detailed study of the latter material using different
microscopic probes (XANES, Mossbauer 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
uctuations into the antiferromagnetic state over a sizable composition range below the
critical Si concentration xc = 0:65. The sequence of magnetic ground states in the series is shown
to re
ect the evolution of the magnetic spectral response.
