NUSNNI-NanoCore, National University of Singapore, Republic of Singapore.
Philos Trans A Math Phys Eng Sci. 2012 Oct 28;370(1977):4927-43. doi: 10.1098/rsta.2012.0198.
We report room-temperature ferromagnetism (FM) in highly conducting, transparent anatase Ti(1-x)Ta(x)O(2) (x∼0.05) thin films grown by pulsed laser deposition on LaAlO(3) substrates. Rutherford backscattering spectrometry (RBS), X-ray diffraction, proton-induced X-ray emission, X-ray absorption spectroscopy (XAS) and time-of-flight secondary-ion mass spectrometry indicated negligible magnetic contaminants in the films. The presence of FM with concomitant large carrier densities was determined by a combination of superconducting quantum interference device magnetometry, electrical transport measurements, soft X-ray magnetic circular dichroism (SXMCD), XAS and optical magnetic circular dichroism, and was supported by first-principles calculations. SXMCD and XAS measurements revealed a 90 per cent contribution to FM from the Ti ions, and a 10 per cent contribution from the O ions. RBS/channelling measurements show complete Ta substitution in the Ti sites, though carrier activation was only 50 per cent at 5 per cent Ta concentration, implying compensation by cationic defects. The role of the Ti vacancy (V(Ti)) and Ti(3+) was studied via XAS and X-ray photoemission spectroscopy, respectively. It was found that, in films with strong FM, the V(Ti) signal was strong while the Ti(3+) signal was absent. We propose (in the absence of any obvious exchange mechanisms) that the localized magnetic moments, V(Ti) sites, are ferromagnetically ordered by itinerant carriers. Cationic-defect-induced magnetism is an alternative route to FM in wide-band-gap semiconducting oxides without any magnetic elements.
我们报告了在室温下具有铁磁性（FM）的高度导电、透明锐钛矿 Ti(1-x)Ta(x)O(2)（x∼0.05）薄膜，该薄膜通过脉冲激光沉积在 LaAlO(3) 衬底上生长。卢瑟福背散射光谱（RBS）、X 射线衍射、质子诱发 X 射线发射、X 射线吸收光谱（XAS）和飞行时间二次离子质谱表明薄膜中几乎没有磁性污染物。通过超导量子干涉仪磁强计、输运测量、软 X 射线磁圆二色性（SXMCD）、XAS 和光学磁圆二色性的组合，确定了具有大载流子密度的 FM 的存在，并且得到了第一性原理计算的支持。SXMCD 和 XAS 测量结果表明，FM 中 90%来自 Ti 离子，10%来自 O 离子。RBS/沟道测量表明 Ta 完全取代了 Ti 位，尽管在 5% Ta 浓度下载流子激活率仅为 50%，这表明存在阳离子缺陷补偿。通过 XAS 和 X 射线光电子能谱分别研究了 Ti 空位（V(Ti)）和 Ti(3+)的作用。结果发现，在具有强 FM 的薄膜中，V(Ti)信号很强，而 Ti(3+)信号不存在。我们提出（在没有任何明显的交换机制的情况下），局域磁矩 V(Ti)位点通过巡游载流子有序化。阳离子缺陷诱导的磁性是在没有任何磁性元素的宽带隙半导体氧化物中实现 FM 的另一种途径。
