Key researcher: S. Zhou (s(dot)zhou(at)hzdr.de)
Making semiconductor ferromagnetic is a long dream. One approach is to dope semiconductors with transition metals (TM). TM ions act as local moments and they couple with free carriers to develop collective magnetism. However, there are no fundamental reasons against the possibility of local moment formation from localized sp states. Recently, ferromagnetism was observed in nonmagnetically doped, but defective semiconductors or insulators, including ZnO and TiO2. This kind of observation challenges the conventional understanding of ferromagnetism. Often the defect-induced ferromagnetism has been observed in samples prepared under non-optimized condition, i.e. by accidence or by mistake. Therefore, in this field theory goes much ahead of experimental investigation. To understand the mechanism of the defect-induced ferromagnetism, one needs a better controlled method to create defects in the crystalline materials. As a nonequilibrium and reproducible approach of inducing defects, ion irradiation provides such a possibility. Energetic ions displace atoms from their equilibrium lattice sites, thus creating mainly vacancies, interstitials or antisites. The amount and the distribution of defects can be controlled by the ion fluence and energy. By ion irradiation, we have generated defect-induced ferromagnetism in ZnO, TiO2 and SiC. Ion irradiation combined with proper characterizations of defects could allow us to clarify the local magnetic moments and the coupling mechanism in defective semiconductors. Otherwise we may have to build a new paradigm to understand the defect-induced ferromagnetism. We take SiC as the test bed since the crystalline quality and material purity can be much better controlled compared with oxides and graphites.
Ferromagnetism in defective SiC
6H-SiC (silicon carbide) single crystals containing VSi-VC divacancies are investigated with respect to magnetic and structural properties. We found that an initial increase of structural disorder leads to pronounced ferromagnetic properties at room temperature. Further introduction of disorder lowers the saturation magnetization and is accompanied with the onset of lattice amorphization. Close to the threshold of full amorphization, also divacancy clusters are formed and the saturation magnetization nearly drops to zero.
|Fig. (above) (a). Ferromagnetic hysteresis loops recorded at 5 K or 300 K for sample 1X14. The magnetization M was related to a thin layer of 460 nm thickness. The 5 K as-measured loops without subtracting the diamagnetic background are shown in the inset. The magnetization M was related to the whole sample weight. (b) Evolution of the saturation magnetization with the displacements per atom (dpa). (c) Raman spectra for virgin and Ne+ implanted 6H-SiC single crystals. (d) shows the relative intensity variation of the folded longitudinal optical (FLO) mode marked in (c) along with the displacements per atom (dpa). (e) S-parameter depending on positron implantation energy and Ne+ fluence implanted. Since the positron implantation profile corresponds to a broad Makhov-type distribution, it probes defects also with the distribution tails, i.e. the mean positron implantation depth does not correspond directly to the geometric depth of the sample. (f) Dependence of the S-parameter (with respect to the bulk value Sbulk) as well as the estimated number N (agglomerated) of on the VSi-VC divacancies in the cluster on the displacements per atom (dpa). Figure is from ref. Appl. Phys. Lett. 98, 222508 (2011).|
(1) Y. Wang, L. Li, S. Prucnal, X. Chen, W. Tong, Z. Yang, F. Munnik, K. Potzger, W. Skorupa, S. Gemming, M. Helm, and S. Zhou, Disentangling defect-induced ferromagnetism in SiC, Phys. Rev. B 89, 014417 (2014).
(2) Y. Wang, X. Chen, L. Li, A. Shalimov, W. Tong, S. Prucnal, F. Munnik, Z. Yang, W. Skorupa, M. Helm, S. Zhou, Structural and magnetic properties of irradiated SiC, J. Appl. Phys. 115, 17C104, (2014).
(3) Lin Li, S. Prucnal, S. D. Yao, K. Potzger, W. Anwand, A. Wagner, and Shengqiang Zhou, Rise and fall of defect induced ferromagnetism in SiC single crystals, Appl. Phys. Lett. 98, 222508 (2011). Full version: arXiv:1106.0966v1, DOI:10.1063/1.3597629.
(4) Shengqiang Zhou, E. Čižmár, K. Potzger, M. Krause, G. Talut, M. Helm, J. Fassbender, S. A. Zvyagin, J. Wosnitza, and H. Schmidt, Origin of magnetic moments in defective TiO2 single crystals, Phys. Rev. B 79, 113201 (2009). Citation: 72.
(5) K. Potzger, and Shengqiang Zhou, Non-DMS related ferromagnetism in transition metal doped zinc oxide, phys. stat. sol. (b) 246, 1147 invited feature article (2009).
(6) Qingyu Xu, H. Schmidt, Shengqiang Zhou, K. Potzger, M. Helm, H. Hochmuth, M. Lorenz, A. Setzer, P. Esquinazi, C. Meinecke, and M. Grundmann, Room temperature ferromagnetism in ZnO films due to defects, Appl. Phys. Lett. 92, 082508 (2008). Citation: 195.
(7) Shengqiang Zhou, Q. Xu, K. Potzger, G. Talut, R. Grötzsche, J. Fassbender, M. Vinnichenko, J. Grenzer, M. Helm, H. Hochmuth, M. Lorenz, M. Grundmann, and H. Schmidt, Room temperature ferromagnetism in carbon-implanted ZnO, Appl. Phys. Lett. 93, 232507 (2008). Citation: 104.
(8) Shengqiang Zhou, K Potzger, G Talut, H Reuther, K Kuepper, J Grenzer, Qingyu Xu, A Mücklich, M Helm, J Fassbender and E Arenholz, Ferromagnetism of Fe implanted ZnO – a phenomenon related to defects? J. Phys. D: Appl. Phys. 41, 105011(2008).