Critical slowing-down and field-dependent paramagnetic fluctuations in the skyrmion host EuPtSi: μ SR and NMR studies

We report the results of muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) and $^{29}\mathrm{Si}$ NMR measurements carried out on the cubic chiral magnet EuPtSi. This compound exhibits a helimagnetic transition at ${T}_{\mathrm{N}}=4$ K, and hosts a unique skyrmion phase characterized by a short modulation period and strong anisotropy under magnetic fields. Our zero-field $\ensuremath{\mu}\mathrm{SR}$ experiments revealed the development of critical slowing down of Eu spin fluctuations over a relatively wide critical region $(T\ensuremath{-}{T}_{\mathrm{N}})/{T}_{\mathrm{N}}l5$ above ${T}_{\mathrm{N}}$. We also found from $^{29}\mathrm{Si}$ NMR that the spin fluctuations are strongly suppressed by magnetic field in the paramagnetic state above 20 K. These characteristic spin dynamics observed over a wide region of temperature and magnetic field suggest the presence of magnetic frustration in the spin system. Such frustration would underlie the mechanism stabilizing the short-period skyrmion lattice observed in this compound.