Unconventional Magnetism in a Nitrogen-Containing Analog of Cupric Oxide

Abstract

We have investigated the magnetic properties of CuNCN, the first nitrogen-based analog 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 the lowest temperatures. However, a large enhancement of spin correlations and an 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.

High-resolution neutron diffraction study of CuNCN: New evidence of structure anomalies at low temperature

“…6,7 One observes 7 anomalies in the temperature dependence of the a and c lattice parameters around 100 K and an additional anomaly in c at ∼35 K as detected from synchrotron radiation experiments. This behavior mirrors the transition from a 1D-RVB state at temperatures around 80-100 K to the 2D-RVB state at lower temperatures, and it is also responsible for the switch from the temperature-independent to the activation-like behavior of the magnetic susceptibility.…”

“…Based on neutron-diffraction data, electrical resistivity, specific heat, and magnetic susceptibility measurements as well as first-principles calculations, CuNCN was first assumed to be a two-dimensional S = 1 2 frustrated triangular Heisenberg quantum antiferromagnet. 2 In subsequent publications, 3-7 a plethora of different magnetic models were proposed including a nonmagnetic ground state, 3 an "unconventional" spin glass, 6 the onset of a magnetic long-range order (LRO) at around 70 K 4, 5 and a resonating valence bond (RVB) state 6,7 with the latter two being under extensive discussion. Puzzlingly, CuNCN was described 4, 5 as a uniform spin-1 2 chain system with the predominant coupling along c and a very strong antiferromagnetic (AFM) coupling constant (J ≈ 2300 K).…”

Magnetic inhomogeneity in the copper pseudochalcogenide CuNCN

“…Furthermore, the assessment of the intrinsic nature of such intricate magnetism has not been provided either. In principle, static magnetism and the coexistence of two magnetic components could be triggered by external perturbations, e.g., by muons perturbing the local environment in the µSR experiments and by strong applied magnetic fields in the NMR experiments, 27 if the dynamical disordered state was unstable.…”

“…The 13 C NMR thus complements the previous 14 N NMR measurements that could detect only the dynamical magnetic component below T h , while the static component was inaccessible. 27 Finally, simultaneous detection of the two components is achieved also by a complementary µSR experiment in a strong transverse magnetic field (TF) that, in contrast to previous µSR experiments, 21,27 by far exceeds the internal static fields. The new results reveal intrinsic coexistence of the two fundamentally different magnetic components, i.e., a dynamical and a static one, which in CuNCN compete on a microscopic scale in the broad temperature range between T h = 80 K and T l = 20 K.…”

Mean-field RVB ground states of lattice models of CuNCN

“…It also does not manifest any magnetic neutron scattering [2] so there is no long-range magnetically ordered state to which one could ascribe the susceptibility decay. These findings brought us to the idea that in CuNCN the antiferromagnetically interacting Cu 2+ local spins 1/2 which are unequivocally observed in the Pauli paramagnetic phase with use of EPR may form resonating valence bond (RVB) phases [3,4]. This incidentally makes CuNCN an RVB material at highest temperature observed so far.…”