Growth and magnetic properties of Ni-doped Bi2Se3 topological insulator crystals

• NixBi2-xSe3 single crystals have been grown by the modified Bridgeman method.
• The magnetic changes as the Ni dopant concentrations increased.
• The ferromagnetism of NixBi2-xSe3 is the combined effect of 3 aspects.

Transition metal doped topological insulators NixBi2−xSe3 were grown by the modified Bridgeman method. Their phase structures, electrical and magnetic transport properties were studied. The lattice constant c decreased with the increasing Ni concentration. All samples are highly c-axis oriented and exhibit weak metallic resistivity. The resistivity increased with both the increasing applied magnetic field and Ni concentration. The resistivity data could be fitted by different formulas below and above 30 K, respectively. The magnetic changed as the Ni dopant concentrations increased, which implied the nickel entering the matrix structure. For the sample with small amount of Ni (x=0.03), a behavior in the curves of temperature dependent of magnetism closely resembled a paramagnet. Bulk ferromagnetism was observed in highly doped samples (x≥0.05) from M(T) data. The samples with (x≥0.05) showed clear hysteresis loops, which suggested the existence of ferromagnetism ordering. All Ni-doped samples are observed with similar weak diamagnetic signals. It was considered that there were three possible origins of ferromagnetism: Ni-Se compound, the interaction of the doped Ni atoms and magnetic contamination.