Spin Dynamics Studied by 77Se Nuclear Spin-Echo Decay Measurements in FeSe

We carried out 77Se nuclear magnetic resonance measurement in the iron-chalcogenide superconductor FeSe, which has only orbital order without magnetic order. From the measured 77Se nuclear transverse relaxation rate 1/T2, we evaluate the longitudinal fluctuations near zero frequency uzz(0), which develop on cooling below the structural transition temperature of Ts,nem = 90 K. The contrasting behavior between uzz(0) and the transverse fluctuations at an NMR angular frequency as a nuclear spin-lattice relaxation rate 1/T1(co0) indicates the sensitive change of spin fluctuations against frequency and the manifestation of ultra-slow spin dynamics in FeSe, the latter of which will be discussed in terms of the fluctuating domain walls and magnetic frustration effect.