量子顺电EuTiO_3材料基态磁性的第一性原理研究

First-principles study of magnetic ground state of quantum paraelectric EuTiO_3 material

EuTiO_3是钙钛矿结构的量子顺电体,实验发现其基态具有平面各向异性G类反铁磁结构,本文运用基于密度泛函理论的第一性原理计算研究了EuTiO_3处于量子顺电相和应力作用下处于铁电四方相时可能的自旋取向和自旋交换耦合作用,分析了自旋耦合作用的路径,探讨了应力对磁性交换路径的作用,结果发现:当体系自由时,EuTiO_3具有自旋沿[110]方向平面内单轴各向异性的G类反铁磁结构,该结构下Eu离子4f电子自旋通过处于面心位置的O 2p实现自旋反铁磁性的超交换耦合,而在外加应力诱导的铁电四方结构下,由于自旋交换路径中Eu—O—Eu键角改变,Eu 4f电子自旋实现了[110]方向的铁磁交换耦合。

Magnetic ground state of perovskite structure quantum paraelectric EuTiO3 has been known to have a planar anisotropic G-type antiferromagnet structure according to the experimental study. In this paper, based on density functional theory, first-principles computations are performed to investigate the magnetic properties and spin-exchange interaction of EuTiO3 in both of the quantum paraelectric phase and ferroelectric tetragonal phase under stress. By analyzing the energies of different magnetic structures and paths of spin exchange coupling as well as the effect of stress on the magnetic exchange paths change, it is found that when the system of EuTiO3 is free, it has a G-type antiferromagnetic structure with uniaxial anisotropic spin along [110] direction. Furthermore, in this structure, Eu 4f electron spin achieves antiferromagnetic super-exchange coupling via O 2p state at face-centered position. However, in the ferroelectric tetragonal phase structure induced by applied stress field, Eu 4f electron spin achieves ferromagnetic exchange coupling in [110] direction due to the variation of Eu-O-Eu bond angle in spin exchange path.