Hidden magnetic order in CuNCN

TL;DR

This study combines experimental and theoretical methods to reveal hidden antiferromagnetic order in CuNCN, a quasi-one-dimensional quantum magnet, highlighting strong quantum fluctuations and complex microstructural effects.

Contribution

It provides a comprehensive analysis confirming magnetic order in CuNCN and introduces a detailed microscopic magnetic model supported by multiple experimental techniques.

Findings

Magnetic transition occurs around 70 K.

Strong quantum fluctuations reduce the ordered moment to about 0.4 μ_B.

Neutron data are inconclusive on magnetic order, but spectroscopic techniques show signatures of long-range order.

Abstract

We report a comprehensive experimental and theoretical study of the quasi-one-dimensional quantum magnet CuNCN. Based on magnetization measurements above room temperature as well as muon spin rotation and electron spin resonance measurements, we unequivocally establish the localized Cu+2-based magnetism and the magnetic transition around 70 K, both controversially discussed in the previous literature. Thermodynamic data conform to the uniform-spin-chain model with a nearest-neighbor intrachain coupling of about 2300 K, in remarkable agreement with the microscopic magnetic model based on density functional theory band-structure calculations. Using exact diagonalization and the coupled-cluster method, we derive a collinear antiferromagnetic order with a strongly reduced ordered moment of about 0.4 mu_B, indicating strong quantum fluctuations inherent to this quasi-one-dimensional spin system. We re-analyze the available neutron-scattering data, and conclude that they are not sufficient to resolve or disprove the magnetic order in CuNCN. By contrast, spectroscopic techniques indeed show signatures of long-range magnetic order below 70 K, yet with a rather broad distribution of internal field probed by implanted muons. We contemplate the possible structural origin of this effect and emphasize peculiar features of the microstructure studied with synchrotron powder x-ray diffraction.