1. Adjustable magnetization in codoped topological insulator Bi2Se3
F. H. Ji, B. Li , Z. Liu, F. C. Xi and S. Qiao
Phys. Dept., Fudan Univ., Shanghai, People’s Republic of China
The ferromagnetism could be achieved by introducing magnetic dopants into the three-dimensional TI dibismuth triselenide(Bi2Se3). XAFS experiments show the ferromagnetism is strongly related to the occupation sites of Fe atoms in Bi2Se3 crystal. We developed a codoping method to adjust the ferromagnetic properties of TI by changing the density of electronic state near Fermi level which can be achieved by the adjustment of the Fermi level.
Breaking the time reversal symmetry
The exchange field breaks the time reversal symmetry of the Hamiltonian in Bi2Se3 bulk, changing the chern number. As a result, electron in Dirac point become massive,creating a gap in the surface state of Bi2Se3. If the 4-band system is originally in the topologically trivial phase, the exchange field will induce a band inversion. The system will be in the quantum spin hall(QSH) phase, which is exactly the situation in HgTe/CdTe quantum well system.
Spontaneously spin-split bands
Experiments show that the magnetic moment per Fe atom in doped Bi2Se3 is about 1% uB, which can not be explained by the models based on localized moments on Fe atoms. Here we employ the Stoner model to solve this problem. A small number of electrons near the Fermi surface could be excited from the spin-down band to spin-up band by exchange interaction. The total change of energy is
Magnetization is determined by the DOS at Fermi level, given by
Fermi level
Codoped Bi2Se3
Loaction of Fe atoms in Bi2Se3
Fe,Mn doped Bi2Se3. With the same Fe atom concentration, we could adjust the magnetization by controlling the Mn concentration in Bi2Se3 sample, which changed the Fermi level. We observed the fluctuation of magnetiztion when Mn doping is increased, which means the exchange interaction varies with Fermi level as well.
Fe,Mg doped Bi2Se3. The result is almost the same as Fe,Mn doped sample. The carriar concentration was decreased by Mg doping, lowering the Fermi level, then the magnetization was changed.
XAFS data show the Fe atoms are located on either substitutional or interstitial sites of Bi2Se3. Different growing methods could make different Fe location in the crystal, resulting in different magnetic property. Evidences show that ferromagnetism could be obtained when Fe atoms occupy interstitial sites.
Conclusion
Fe doped Bi2Se3 has a high Curie temprature of 350K. By doping Mg and Mn, we can adjust its magnetization by a magnitude of 90%.
The relation between magnetization and DOS at Fermi level could be verified by future ARPES experiments
Magnetic susceptbility will increase after Fermi level passes the Dirac point if Stoner model holds. Because DOS equals zero at the Dirac point, and it increases when the energy shifts.
For thin films with several quintuple layers, Fermi level could be shifted by employing a bias gate voltage. In this way, the magnetization could be controlled more directly.
References
(1)S. Blundell, Magnetism in Condensed Matter
(2)R. Yu, W. Zhang, H. J. Zhang, S. C. Zhang and Z. Fang,Science 2 July
2010: Vol. 329 no pp
(3)Y. L. Chen, J. H. Chu, J. G. Anlytis, Z.K. Liu, K. Igarashi, H. H. Kuo,
Z. Hussain and Z. X. Shen, Science 6 August 2010:Vol.329 no. 5992 pp