铋铁石榴石电子性质的从头算研究
Bismuth iron garnet: ab initio study of electronic properties
摘要：
铋铁石榴石（BIG），即Bi3Fe5O12，是一种强亚铁磁体，也具有出色的磁光特性，如已知的最大法拉第旋转。这些性质与磁矩在八面体和四面体位置的分布、态密度中自旋能隙的存在以及强的自旋轨道耦合有关。本文采用密度泛函理论（DFT+U），结合自旋轨道耦合和HSE06杂化泛函，对BIG的结构、电子和磁性进行了第一性原理从头计算.我们发现，在电子结构中的自旋间隙的存在下，从交换和相关效应和晶体场强度的四面体和八面体铁亚晶格之间的相互作用的结果。DFT+U处理倾向于关闭较大U的自旋能隙，这是由于过局域化效应，特别是在八面体位置。另一方面，混合泛函证实了三个自旋能隙的发生在铁态的导带的光学测量所预期的。在价带顶部也得到了一个强的交换分裂与孤对型混合的O p和Bi s，p状态。类似的交换分裂以前没有观察到其他铁基石榴石，如钇铁石榴石。由此可见，空穴掺杂，如通过在Bi位点处的Ca取代所获得的，导致在费米能处的全自旋极化密度。这项工作有助于阐明更多的理论理解BIG的属性，并开辟了使用先进的多体计算预测BIG中的磁光耦合效应与实验测量的直接比较的路线。
英文摘要：
Bismuth iron garnet (BIG), i.e. Bi3Fe5O12, is a strong ferrimagnet that also possess outstanding magneto-optical properties such as the largest known Faraday rotation. These properties are related with the distribution of magnetic moments on octahedral and tetrahedral sites, the presence of spin gaps in the density of state and a strong spin-orbit coupling. In this work, first-principles ab initio calculations are performed to study the structural, electronic and magnetic properties of BIG using Density Functional Theory with Hubbard+U (DFT+U) correction including spin-orbit coupling and HSE06 hybrid functional. We found that the presence of spin gaps in the electronic structure results from the interplay between exchange and correlation effects and the crystal field strengths for tetrahedral and octahedral iron sublattices. The DFT+U treatment tends to close the spin-gaps for larger U due to over-localization effects, notably in the octahedral site. On the other hand, the hybrid functional confirms the occurrences of three spin gaps in the iron states of the conduction band as expected from optical measurements. A strong exchange splitting at the top of the valence bands associated with a lone-pair type mixture of O p and Bi s,p states is also obtained. Similar exchange splitting was not previously observed for other iron based garnets, such as for yttrium iron garnet. It follows that hole doping, as obtained by Ca substitution at Bi sites, results in a full spin polarized density at the Fermi energy. This work helps to shed more light on the theoretical comprehension of the properties of BIG and opens the route towards the use of advanced Many Body calculations to predict the magneto-optical coupling effects in BIG in a direct comparison with the experimental measurements.
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