Longitudinal magnetic fluctuations observed by the ⁷⁷Se nuclear spin-spin relaxation measurement in FeSe

A relaxation rate of the transverse nuclear magnetization, so-called 1/T₂, can be induced by longitudinal magnetic fluctuations along the applied fields in addition to transverse one as the nuclear spin-lattice relaxation rate 1/T₁. The former can manifest itself when there is an ultra-slow spin dynamics[1-3]. Here, we performed the ⁷⁷Se nuclear magnetic resonance measurement on one of the typical iron-chalcogenide superconductors FeSe, which exhibits only orbital ordering at T_s,nem ≃ 90 K whereas iron-pnictide materials exhibit both spin and orbital ordering[4-7]. We measured 1/T₂ to study the spin dynamics of FeSe in magnetic fields of B||c. As a result, 1/T₂ increases on cooling below T_s,nem while 1/T₁ decreases gradually. This contrasting behavior between 1/T₂ and 1/T₁ indicates that the contribution of longitudinal fluctuations along the c-direction is dominant over that of 1/T₁ and thus ultra-slow spin dynamics appear in FeSe. The existence of longitudinal fluctuations along the c-direction at the Se sites can be understood by considering the striped correlations between iron electron-spin, which is consistent with the inelastic neutron scattering result[8]. The ultra-slow spin dynamics in FeSe may be ascribed to the magnetic frustration coming from local moments. The frustrated magnetism with the nearest and next-nearest exchange couplings between iron electron-spin may play a role in unconventional superconductivity in FeSe.