Spin-glass-like behavior in SmFeAsO0.8F0.2

The iron-based oxypnictide superconductor SmFeAsO_0.8F_0.2 was synthesized under high pressure and investigated by measuring the dc magnetic susceptibility. The zero-field cooled (ZFC) magnetic susceptibility confirmed the bulk superconductivity of the sample with a critical temperature T_c ≈ 50 K and a significant jump in magnetization at ～4.3 K, usually attributed to the antiferromagnetic ordering of Sm³⁺ ions in this system. Since the occurrence of the jump depends on the cooling history, our data strongly suggest a spin-glass-like behavior.