报告人：Dr. Shengqiang Zhou, Helmholtz Young Investigators Group Leader, Helmholtz-Zentrum Dresden-Rossendorf, Germany

报告时间：2011年10月12日上午9：00

地点：固体所三号楼321会议室

报告题目：Ion irradiation as a controllable approach to study the defect-induced ferromagnetism

摘要：Recently, ferromagnetism was observed in nonmagnetically doped, but defective semiconductors or insulators, including TiO2 [1] and ZnO [2-5] (see a review by Coey et al. [6]). This kind of observation challenges the conventional understanding of ferromagnetism, which is rather due to spin-split states or bands. Often the defect-induced ferromagnetism has been observed in samples prepared under non-optimized condition, i.e. by accidence or by mistake. 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 TiO2 [1] and SiC [7]. The saturation magnetization rises and falls with increasing the ion fluence due to the interplay between the amount of defects and the crystalline quality. Using electron spin resonance and positron annihilation spectroscopies, one can determine where the unpaired electrons are located and the concentration of defects. 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.

Reference:

[1] S. Zhou, et al., Phys. Rev. B 79, 113201 (2009).

[2] S. Zhou, et al., J. Phys. D 41, 105011 (2008).

[3] K. Potzger, et al., Appl. Phys. Lett. 92, 182504 (2008).

[4] S. Zhou, et al., Appl. Phys. Lett. 93, 232507 (2008).

[5] Q. Xu, et al., Appl. Phys. Lett. 92, 082508 (2008).

[6] J. M. D. Coey, et al., New J. Phys. 12, 053025 (2010).

[7] L. Li, et al., Appl. Phys. Lett. 98, 222508 (2011).