Microscopic magnetic phase evolution in the Weyl semimetal Mn${}_3$Sn revealed by ${\mu }^{+}$SR

Abstract

We report a comprehensive muon spin relaxation (${\mu }^{+}$SR) and bulk magnetization study of the antiferromagnetic (AFM) Weyl semimetal Mn${}_3$Sn (composition Mn$2.99$Sn). Mn$3$Sn is reported to exhibit a commensurate inverse triangular (IT) AFM phase, an incommensurate (IC) helical AFM phase, and a proposed low-temperature spin-glass-like state. In our sample, we identify the characteristic temperatures associated with these regimes as N'eel temperature ($T\mathrm{N}=418$ K), a macroscopic bulk transition temperature between IT-AFM to IC helical phase ($T\mathrm{t}\approx 275$ K), and low-temperature transition $T_{\mathrm{f}}=21$ K. Investigating the low-temperature regime below $T_{\mathrm{f}}$, we find no evidence of a static spin-glass state. Instead, the sample exhibits an increasing ferromagnetic (FM) component accompanied by a localized slowing of spin fluctuations, indicating that these phenomena may be decoupled. In the IC helical AFM phase, the zero-field (ZF) spectra are best described by damped oscillations with an empirical phase offset, consistent with anharmonic and amplitude-modulated order reported by scattering studies. Upon warming above 150 K, a continuous redistribution of muon spectral weight reveals a broad, homogeneous magnetic crossover between the IC helical and IT-AFM phases. In the commensurate IT-AFM phase above $T_{\mathrm{t}}$, a persistent missing fraction in the initial asymmetry indicates that a subset of implanted muons, corresponding to roughly 20% of the sample-related asymmetry, undergoes unresolved ultrafast depolarization. Finally, we observe temperature-driven shifts in muon site populations above 325 K. Ultimately, our results show a highly dynamic magnetic landscape in Mn${}_3$Sn, demonstrating how its complex magnetic orders often coexist and evolve continuously with temperature.

Accepted for publication in Physical Review B. 13 pages total (8 pages main text, 5 pages appendix and references), 7 Figures