Magnetic inhomogeneity in the copper pseudochalcogenide CuNCN

Copper carbodiimide, CuNCN, is a geometrically frustrated nitrogen-based analogue of cupric oxide, whose magnetism remains ambiguous. Here, we employ a combination of local-probe techniques, including ${}^{63,65}$Cu nuclear quadrupole resonance, ${}^{13}$C nuclear magnetic resonance and muon spin rotation to show that the magnetic ground state of the Cu${}^{2+}$ ($S=1\mathrm{/}2$) spins is frozen and disordered. Moreover, these complementary experiments unequivocally establish an onset of intrinsically inhomogeneous magnetic state at $T_h=80$ K. Below $T_h$, the low-temperature frozen component coexist with the remnant high-temperature dynamical component down to $T_l=20$ K, where the latter finally ceases to exist. Based on a scaling of internal magnetic fields of both components we conclude that the two components coexist on a microscopic level.