Visualizing ferromagnetism in Fe-doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ topological insulator individual nanowires

Time-reversal symmetry is broken by magnetic doping into the topological insulators, and an energy gap is opened thus generating massive surface carriers. Magnetically-doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ nanowires have been prepared by chemical vapor deposition technique with a high Curie temperature. The magnetic force microscope images still reveal the obvious ferromagnetic contrast signal of the Fe-doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ nanowires even at 40 K, indicating the direct evidence of visulizing ferromagnetism in magnetically-doped topological insulator nanowires. Through systematic magnetoresistance measurements on undoped and Fe-doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ nanowires, we observe the weak anti-localization effect in Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ nanowires indicating a strong spin-orbital coupling effect. Furthermore, we also observed a transition from weak anti-localization to weak localization behavior when magnetically-doping into topological insulator nanowires. Our results pave a potential way for future novel magnetoelectronic device applications.