  We have investigated the magnetic properties of CuNCN, the first
nitrogen-based analogue of cupric oxide, CuO. Our muon spin relaxation, nuclear
magnetic resonance and electron spin resonance studies reveal that classical
magnetic ordering is absent down to lowest temperatures. However, large
enhancement of spin correlations and unexpected inhomogeneous magnetism have
been observed below 80 K. We attribute this to a peculiar fragility of the
electronic state against weak perturbations due to geometrical frustration,
which selects between competing spin-liquid and more conventional frozen
states.
