Muon spectroscopy investigation of anomalous dynamic magnetism in NiI₂

We present the results of muon-spin relaxation (μ⁺SR) measurements of the van der Waals magnet NiI₂, which probe magnetic phase transitions at TN1=73K and TN2=60K. Supporting density functional theory (DFT) calculations allow the determination of a single muon stopping site whose magnetic environment is consistent with the proposed ground-state magnetic structure. μ⁺SR measurements of the dynamics reveal behavior consistent with spin-wave excitations below TN2, with an additional contribution from a nonmagnetic component of the material. In the region TN1<T<TN2 the system is magnetically ordered throughout the bulk, but the character of the dynamics changes qualitatively, resulting in an unusual region of temperature-independent fluctuations.