Magnetic order and spin dynamics in the proximity of a ferromagnetic quantum critical point: AμSR study of YbNi4P2

Abstract

The local 4$f$-electronic spin dynamics and magnetic order in YbNi${}_{4}$P${}_{2}$ were studied by means of muon-spin relaxation measurements. Zero-field muon-spin relaxation proves static magnetic order with a strongly reduced ordered Yb${}^{3+}$ moment of $(2.5--4.6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}{\ensuremath{\mu}}_{B}$, below ${T}_{C}=140$ mK. Above ${T}_{C}$, the muon-spin polarization $P(t,B)$ is dominated by quasihomogeneous spin fluctuations and exhibits a time-field scaling relation $P(t,B)=P(t/{B}^{\ensuremath{\gamma}})$, indicating cooperative critical spin dynamics in the system. At $T=190$ mK, slightly above ${T}_{C}$, $\ensuremath{\gamma}=0.81(5)$, suggesting time-scale invariant power-law behavior for the dynamic electronic spin-spin autocorrelation function.