Complex magnetism in a transition metal nitride

High-symmetry lattices of transition metal ions such as the body-centred tetragonal (bct) arrangement are model systems for correlated electron physics and frustrated magnetism. Transition metal oxides or halides with the bct K₂NiF₄-type structure are extensively studied and have ground states dominated by strong interactions within the square layers, but materials exemplifying other bct ground states are much rarer. Here we show that Ca₂CoN₂, from the underexplored class of transition metal nitrides, has one of the complex magnetic ground states predicted for the bct lattice lying close to a spin-liquid instability. Ca₂CoN₂ has been synthesised at high pressure in a large-volume press, and has a Na₂HgO₂-type structure where weakening of intralayer superexchange interactions leads to frustration. Our magnetisation and neutron diffraction experiments reveal an incommensurate spin order below 54 K, consistent with a double-k spin density wave with propagation vectors (k00) and (0k0) where k = 0.57. This phase lies close to the predicted crossover (at k = 0.64-0.69) to a modulated spin liquid, which could be explored through further chemical tuning. This work demonstrates the potential of transition metal nitrides as magnetic model systems, alongside the much-studied oxides and halides, for exploring complex spin orders and associated quantum phases.