Quasi-two-dimensional magnetic correlations in Ni2P2S6 probed by 31P NMR

Detailed P-31 nuclear magnetic resonance (NMR) measurements are presented on well-characterized single-crystals of antiferromagnetic van der Waals Ni2P2S6. An anomalous breakdown is observed in the proportionality of the NMR shift K with the bulk susceptibility chi. This so-called K-chi anomaly occurs in close proximity to the broad peak in chi(T), thereby implying a connection to quasi-two-dimensional (2D) magnetic correlations known to be responsible for this maximum. Quantum chemistry calculations show that crystal field energy level depopulation effects cannot be responsible for the K-chi anomaly. Appreciable transferred hyperfine coupling is observed, which is consistent with the proposed Ni-S-Ni super- and Ni-S-S-Ni super-super-exchange coupling mechanisms. Magnetization and spin-lattice relaxation rate (T-1(-1)) measurements indicate little to no magnetic field dependence of the Neel temperature. Finally, T-1(-1)(T) evidences relaxation driven by three-magnon scattering in the antiferromagnetic state.