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 $T_{ \mathrm{ N2}}$. In the region $T_{ \mathrm{ N2}}<T<T_{ \mathrm{ N1}}$ the character of the dynamics changes qualitatively, resulting in an unusual region of temperature-independent fluctuations.
Categories: cond-mat.str-el