Interplay between quantum criticality and geometric frustration in Fe ₃Mo₃N with stella quadrangula lattice

We propose the η-carbide-type correlated-electron metal Fe ₃Mo₃N 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 (∼14 T) applied to it or when it is doped with a slight amount of impurity (∼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, J₁ and J₂, 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.