Anomalous quantum-critical spin dynamics in YFe2Al10

Abstract

We report results of a muon spin relaxation ($\hat{I}¼$SR) study of YFe${}_2$Al${}_{10}$, a quasi-2D nearly-ferromagnetic metal in which unconventional quantum critical behavior is observed. No static Fe${}^{2+}$ magnetism, with or without long-range order, is found down to 19~mK\@. The dynamic muon spin relaxation rate~$\hat{I}»$ exhibits power-law divergences in temperature and magnetic field, the latter for fields that are too weak to affect the electronic spin dynamics directly. We attribute this to the proportionality of $\hat{I}»(\ddot{I}{‰}_{\hat{I}}¼,T)$ to the dynamic structure factor~$S(\ddot{I}{‰}_{\hat{I}}¼,T)$, where $\ddot{I}{‰}_{\hat{I}}¼\approx 10^5$--$10^7{\mathrm{s}}^{-1}$ is the muon Zeeman frequency. These results suggest critical divergences of $S(\ddot{I}{‰}_{\hat{I}}¼,T)$ in both temperature and frequency. Power-law scaling and a 2D dissipative quantum XY (2D-DQXY) model both yield forms for $S(\ddot{I}‰,T)$ that agree with neutron scattering data ($\ddot{I}‰\approx 10^{12}{\mathrm{s}}^{-1}$). Extrapolation to $\hat{I}¼$SR frequencies agrees semi-quantitatively with the observed temperature dependence of $\hat{I}»(\ddot{I}{‰}_{\hat{I}}¼,T)$, but predicts frequency independence for $\ddot{I}{‰}_{\hat{I}}¼\ll T$ in extreme disagreement with experiment. We conclude that the quantum critical spin dynamics of YFe${}_2$Al${}_{10}$ are not well understood at low frequencies.

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