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 $T_{\mathrm{N}1}=73$~K and $T_{\mathrm{N}2}=60$~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 $T_{\mathrm{N}2}$. In the region $T_{\mathrm{N}2}<T<T_{\mathrm{N}1}$ the character of the dynamics changes qualitatively, resulting in an unusual region of temperature-independent fluctuations.