  The rare-earth based ternary intermetallic compounds $R_2TX_3$ ($R$ =
rare-earth, $T$ = transition-metal, $X$ = Si, Ge, Ga, In) have attracted
considerable interest due to a wide range of interesting low temperature
properties. Here we investigate the magnetic state of Nd$_{2}$PdSi$_{3}$ using
neutron diffraction, muon spin relaxation ($\mu$SR) and inelastic neutron
scattering (INS). This compound appears anomalous among the $R_{2}$PdSi$_{3}$
series, since it was proposed to order ferromagnetically, whereas others in
this series are antiferromagnets. Although some members of the $R_2TX_3$ series
have been reported to form ordered superstructures, our data are well described
by Nd$_{2}$PdSi$_{3}$ adopting the AlB$_2$-type structure with a single Nd
site, and we do not find evidence for superlattice peaks in neutron
diffraction. Our results confirm the onset of long range magnetic order below
$T_0=17$~K, where the whole sample enters the ordered state. Neutron
diffraction measurements establish the presence of a ferromagnetic component in
this compound, as well as an antiferromagnetic one which has a propagation
vector $\mathbf{k_2}=(1/2,1/2,1/4-\delta)$ with a temperature dependent
$\delta\approx0.02-0.04$, and moments orientated exclusively along the
$c$-axis. $\mu$SR measurements suggest that these components coexist on a
microscopic level, and therefore the magnetic structure of Nd$_{2}$PdSi$_{3}$
is predominantly ferromagnetic, with a sinusoidally modulated antiferromagnetic
contribution which reaches a maximum amplitude at 11~K, and becomes smaller
upon further decreasing the temperature. INS results show the presence of
crystalline-electric field (CEF) excitations above $T_0$, and from our analysis
we propose a CEF level scheme.
