Oxide Electronics - Materials and Applications

The most intriguing open question in magnetic materials is the understanding of the mechanism for magnetism in the magnetic semiconductors. Although the evidence for magnetic semiconductors (not simply semiconductors which are ferromagnetic) is compelling, there is much uncertainty in the mechanism for the carrier polarization, suggesting that it must be quite novel. In this presentation, I will briefly describe the excitement that has been generated by magnetic semiconductors and describe the experimental evidence for their existence. To do this, I will describe the basics of what it means for a material to be classified a semiconductor, what it means for a material to be classified a ferromagnet, and what a hybrid term like �magnetic semiconductor� means. Along the way, I will also describe what a half-metallic ferromagnet is and how each might be used in real applications. At the end of my presentation, I will then focus on recent work that addresses whether the underlying mechanism describing magnetic semiconductors is created by cation vacancies (similar to the polaron percolation theory proposed by Kaminski and Das Sarma [1], which was recently applied specifically to doped oxides by Coey et al. [2]). Included in this discussion will be our work where we have measured the X-ray absorption spectra for low concentrations (0-4%) of Co doped into a variety of magnetic oxide semiconductors including TiO2, La1-xSrxO3, HfO2, In2O3, and others. We have also measured the X-ray Magnetic Circular Dichroism spectra for a large class of magnetic semiconductors. Although these materials show strong bulk magnetization, we are unable to detect a robust dichroism feature associated with magnetic elements in the host semiconductor, suggesting that the defect states not only mediate the ferromagnetism, but generate it as well. 1. Kaminski and S. Das Sarma, Physical Review Letters 88, 247202 (2002). 2. J. M. D. Coey, M. Venkatesan, and C. B. Fitzgerald, Nature Materials 4, 173 (2005).