  We have carried out muon spin relaxation (muSR), neutron diffraction and
inelastic neutron scattering (INS) investigations on polycrystalline samples of
Ce(Ru1-xFex)2Al10 (x=0, 0.3, 0.5, 0.8 and 1) to investigate the nature of the
ground state (magnetic ordered versus paramagnetic) and the origin of the spin
gap formation as evident from the bulk measurements in the end members. Our
zero-field muSR spectra clearly reveal coherent two-frequency oscillations at
low temperature in x=0, 0.3 and 0.5 samples, which confirms the long-range
magnetic ordering of the Ce-moment with TN=27, 26 and 21 K respectively. On the
other hand the muSR spectra of x=0.8 and x=1 down to 1.4 K and 0.045 K,
respectively exhibit a temperature independent Kubo-Toyabe term confirming a
paramagnetic ground state. The long-range magnetic ordering in x=0.5 below 21 K
has been confirmed through the neutron diffraction study. INS measurements of
x=0 clearly reveal the presence of a sharp inelastic excitation near 8 meV
between 5 K and 26 K, due to an opening of a gap in the spin excitation
spectrum, which transforms into a broad response at and above 30 K.
Interestingly, at 4.5 K the spin gap excitation broadens in x=0.3 and exhibits
two clear peaks at 8.4(3) and 12.0(5) meV in x=0.5. In the x=0.8 sample, which
remains paramagnetic down to 1.2 K, there is a clear signature of a spin gap of
10-12 meV at 7 K, with a strong Q-dependent intensity. Evidence of a spin gap
of 12.5(5) meV has also been found in x=1. The observation of a spin gap in the
paramagnetic samples (x=0.8 and 1) is an interesting finding in this study and
it challenges our understanding of the origin of the semiconducting gap in
CeT2Al10 (T=Ru and Os) compounds in terms of hybridization gap opening only a
small part of the Fermi surface, gapped spin waves, or a spin-dimer gap.

Strongly Correlated Electrons