Muon spectroscopy investigation of anomalous dynamic magnetism in NiI$_2$

We present the results of muon-spin relaxation ($\mu^{+}$SR) measurements of the van der Waals magnet NiI$_2$, which probe magnetic phase transitions at $T_{\mathrm{N1}}=73$K and $T_{\mathrm{N2}}=60$K. 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. $\mu^{+}$SR measurements of the dynamics reveal behavior consistent with spin-wave excitations below $T_{\mathrm{N2}}$, with an additional contribution from a nonmagnetic component of the material. In the region $T_{\mathrm{N1}}<T<T_{\mathrm{N2}}$ 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.