We propose the eta-carbide-type correlated-electron metal Fe3Mo3N as a new candidate for testing geometric frustration in itinerant-electron magnets. In this compound, ferromagnetism is abruptly induced from a non-magnetic non-Fermi-liquid ground state either when a magnetic field (similar to 14T) applied to it or when it is doped with a slight amount of impurity (similar to 5%Co). We observed a peak in the paramagnetic neutron scattering intensity at finite wave vectors, revealing the presence of the antiferromagnetic correlation hidden in the magnetic measurements. It causes a new type of geometric frustration in the stella quadrangula (SQ) lattice of the Fe sublattice, which is a corner-shared lattice of two nested regular tetrahedrons. Although the SQ lattice is one of pyrochlore-derived lattices, we anticipate the unique nature of the frustration due to the competition between first-and second-neighbour interactions, J(1) and J(2), possibly of direct exchange type. We propose that the frustrated antiferromagnetic correlation suppresses the ferromagnetic correlation to its marginal point and is therefore responsible for the origin of the ferromagnetic quantum critical behaviour in pure Fe3Mo3N Copyright (C) EPLA, 2011