| As a newly discovered quantum material, topological insulator is characterized by a bulk insulating gap and a spin-momentum locked surface state protected by time reversal symmetry. Topological insulator has drawn intensive interests due to its potential application in spintronics and new electronic devices. It has been predicted that bulk doped or surface absorbed magnetic atoms will form ferromagnetic order with out-of-plane easy axis which will further lead to gap opening at Dirac point and quantum anomalous Hall effect. In this thesis, we studied the growth and magnetism of magnetically doped topological insulator by various means. Both the adsorbing of magnetic molecules Fe Pc and the bulk doping of magnetic elements(including rare earth Gd and transition metal Fe, Cr, V) were investigated.The details show here:Firstly, surface absorbed magnetic molecules. Fe Pc was thermally evaporated on Bi2Te3 and Bi2Se3 surface. In both cases, Fe Pc forms a well ordered single molecular layer. However, Fe Pc has a packing density of 0.64 molecule/nm2 on Bi2Te3, higher than on Bi2Se3 and most metallic substrates. Therefore, Bi2Te3 was chosen as the substrate for further identifying the structure and magnetism of Fe Pc. Scanning tunneling microscopy observation shows that the absorption of Fe Pc is site dependent leading to a large unit cell with 20 Fe Pc molecules which is further confirmed by the first principle calculation. However, even with such a high packing density, spin polarized scanning tunneling microscopy measurement found that Fe Pc is paramagnetic with an in-plane easy axis. No ferromagnetic ordering was observed which is possibly due to weak Van der Waals interactions between molecules and substrate as calculated by the density functional theory. The calculation also shows that the magnetic moment and magnetic anisotropy energy are apparently influenced by the molecule-substrate interaction, although the change is not enough to switch the magnetic easy axis. Our results show the importance of choosing proper molecule and substrate in obtaining the ferromagnetism mediated by the topological surface states.Secondly, rare earth metal Gd doped topological insulators. The modified Bridgman method was applied for the growth of Bi2-xGdxSe3. The samples show very high quality as confirmed by X-ray diffraction. However, the maximum doping density was only 1% measured by inductively coupled plasma. No gap opening at the Dirac point was observed in scanning tunneling spectroscopy and angle resolved photoemission spectroscopy measurements. Vibrating sample magnetometer was used to identify the magnetism which shows paramagnetism with a large magnetic moment of 7μB per Gd atom. Although the large magnetic moment of Gd ion is favored as comparing with the reduced magnetic moments of transition metal doping, the high localization of the 4f electron hinders the formation of ferromagnetism. Our results show that one must balance between localization and unlocalization in order to obtain strong magnetic topological insulator.Thirdly, bulk doping of transition metals. Topological insulator with ferromagnetism can be obtained by doping transition magnetic metals. By doping Cr in Sb2Te3, ferromagnetic order forms and the magnetism is associated with carrier density. Comparatively, ferromagnetism with high cure temperature and large coercivity forms by doping V in Sb2Te3 instead of Cr. In order to tune the EF, BixSb2-x-yVyTe3 was grown by the coevaperation of Bi, Sb, V and Te by adjusting the ratio of Bi, Sb and V. Together with the back gate, it was proven that the carrier type can be tuned from hole to electron. Finally, in 5 quintuple layer BixSb2-x-yVyTe3, a Hall resistivity close to quantum Hall resistivity was observed. However, unlike the quantum anomalous Hall effect, the system shows a large dissipative longitudinal resistivity which is possibly related to the high disorder of the sample. |
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