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

TL;DR

This study uses muon-spin relaxation to explore magnetic phase transitions and dynamic magnetism in NiI$_2$, revealing unusual temperature-independent fluctuations between two magnetic transition temperatures.

Contribution

It combines $$SR measurements with DFT calculations to identify muon sites and characterize magnetic dynamics in NiI$_2$, uncovering novel magnetic fluctuation behavior.

Findings

Detection of magnetic phase transitions at 73 K and 60 K.

Observation of spin-wave excitations below 60 K.

Identification of a temperature-independent fluctuation region between 60 K and 73 K.

Abstract

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.