Anomalous second ferromagnetic phase transition in Co_0.08Bi_1.92Se₃ topological insulator

We report the observation of ferromagnetism in topological insulator Co_0.08Bi_1.92Se₃ single crystal. The structural, magnetic, and microstructure properties of Co_0.08Bi_1.92Se₃ are investigated. The existence of complicated ferromagnetic ordering, indicates the anomalous second ferromagnetic phase transition below 30 K. Well-defined ferromagnetic hysteresis in the magnetization was found in the sample. The origin of bulk ferromagnetism in Co_0.08Bi_1.92Se₃ is concerned with three aspects: Co cluster, RKKY interactions, and the spin texture of Co impurities. - Highlights: • The bulk ferromagnetism have been found in the C_o0.08Bi_1.92Se₃ single crystal. • The anomalous second ferromagnetic phase transition is found below 30 K. • The origin of bulk ferromagnetism in Co_0.08Bi_1.92Se₃ is concerned with three aspects.