  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.
