Rare earth sesquisulfides \({\alpha }\) - \({R_2}\) S \(_3\) (R = Dy, Sm) possess an orthorhombic crystal structure having two crystallographically inequivalent rare earth sites, R1 and R2. The compound \(\alpha \) - \(\textrm{Dy}_2\textrm{S}_3\) exhibits successive antiferromagnetic transitions at \(T_\text {N1} = 11.4\) K and \(T_\text {N2} = 6.4\) K, while \({\alpha }\) -Sm \(_2\) S \(_3\) shows successive weak-ferromagnetic transitions at \(T_\text {C1} = 3.6\) K and \(T_\text {C2} = 1.8\) K. Furthermore, they are fascinating because they exhibit a very large increase and recovery in electrical resistivity within a narrow temperature range just above \(T_\text {N2}\) and \(T_\text {C1}\) , respectively. In this study, the AC magnetic susceptibility was measured near \(T_\text {N1}\) and \(T_\text {N2}\) using \({\alpha }\) -Dy \(_2\) S \(_3\) single crystals, and near \(T_\text {C1}\) using \({\alpha }\) -Sm \(_2\) S \(_3\) single crystals. The imaginary part of AC susceptibility for each compound exhibited sharp peak near \(T_\text {N2}\) or \(T_\text {C1}\) , respectively, clearly indicating a phase delay in the magnetic moment motion within each temperature range, where it could not follow the changes in the AC magnetic field. Furthermore, it was found that in \({\alpha }\) -Dy \(_2\) S \(_3\) , the peak height increases with increasing AC magnetic field frequency, whereas in \({\alpha }\) -Sm \(_2\) S \(_3\) , it decreases, reflecting the difference in magnetic ordering between the two compounds.
