  Half-metallic ferromagnets (HMF) are on one of the most promising materials
in the field of spintronics due to their unique band structure consisting of
one spin sub-band having metallic characteristics along with another sub-band
with semiconductor-like behavior. In this work, we report the synthesis of a
novel quaternary Heusler alloy FeMnVAl and have studied the structural,
magnetic, transport, and electronic properties complemented with
first-principles calculations. Among different possible structurally ordered
arrangements, the optimal structure is identified by theoretical energy
minimization. The corresponding spin-polarized band structure calculations
indicates the presence of a half-metallic ferromagnetic ground state. A
detailed and careful investigation of the x-ray diffraction data, M\"{o}ssbauer
and nuclear magnetic resonance spectra suggest the presence of site-disorder
between the Fe and Mn atoms in the stable ordered structure of the system. The
magnetic susceptibility measurement clearly establishes a ferromagnetic-like
transition below $\sim$213 K. The ${^{57}}$Fe M\"{o}ssbauer spectrometry
measurements suggest only the Mn-spins could be responsible for the magnetic
order, which is consistent with our theoretical calculation. Surprisingly, the
density-functional-theory calculations reveal that the spin-polarization value
is almost immunized (92.4\% ${\rightarrow}$ 90.4\%) from the Mn-Fe structural
disorder, even when nonmagnetic Fe and moment carrying Mn sites are entangled
inseparably. Robustness of spin polarization and half metallicity in the
studied FeMnVAl compound comprising structural disorder is thus quite
interesting and could provide a new direction to investigate and understand the
exact role of disorders on spin polarization in these class of materials, over
the available knowledge.
