Multiferroic crossover in perovskite oxides
Multiferroic crossover in perovskite oxides
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DOI：
10.1103/physrevb.93.165210
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发表时间：
2016-04-29
影响因子：
3.7
通讯作者：
Stampfl, C.
中科院分区：
文献类型：
--
作者：
Weston, L.;Cui, X. Y.;Stampfl, C.
在ABO（3）钙钛矿氧化物中，铁电性和磁性的共存是罕见的，这种现象被称为铁电“d（0）规则”。“最近，钙钛矿BiCoO 3已被实验证明与PbTiO 3同构，同时d（6）Co 3+离子具有C型反铁磁有序的高自旋基态。有人认为Bi 6s态与O 2 p价带的杂化稳定了极性相，然而，我们最近证明了钙钛矿结构中的Co 3+离子可以通过Co 3d-O 2 p共价相互作用促进铁电畸变[L.韦斯顿等人，物理修订信函114，247601（2015）]。在本文中，使用精确的混合密度泛函计算，我们调查的原子，电子和磁性结构的BiCoO 3阐明起源的多铁性状态。开始，我们进行了更一般的第一性原理调查的作用，d电子在影响钙钛矿材料的倾向，表现出铁电畸变，这是通过一个定性的趋势研究，在人工立方和tetraxylLaBO 3钙钛矿。我们选择La作为A阳离子，以消除Bi 6s杂化的影响。晶格的不稳定性被确定为在立方相的声子模式的软化，以及由与铁电畸变相关的能量降低。对于LaBO 3系列，其中B是来自3d块的d（0）-d（8）阳离子，趋势研究表明，增加d轨道占据最初消除了极性畸变的趋势，正如预期的那样。然而，对于高自旋d（5）-d（7）和d（8）阳离子，恢复了强铁电不稳定性。这种效应可以解释为增加的伪Jahn-Teller（PJT）p-d振动耦合。PJT效应由有利于立方相的稳定力（K-0）和驱动铁电态（K-v）的电子振动项之间的竞争来描述。高自旋d（5）-d（7）和d（8）阳离子的晶格不稳定性的恢复是由于（i）由于σ键合的轴向d e（g）轨道的布居引起的显著体积增加而导致的K-0的减少，以及（ii）由于增加的p-d杂化引起的K-v贡献的增加;我们的计算表明前一种机制是主导的。令人惊讶的是，我们能够证明，在某些情况下，未成对电子自旋实际上驱动铁电性，而不是抑制它，这代表了对铁电性和磁性如何在钙钛矿氧化物中相互作用的理解的转变。因此，对于BiCoO 3的情况下，Co 3+离子在铁电晶格不稳定性中起主要作用。重要的是，当Co 3+离子处于高自旋态时，与处于低自旋态的Co 3+离子相比，铁电极化大大增强，并且存在大的电和磁极化耦合。通常，对于ABO（3）钙钛矿中的d（5）-d（7）B阳离子，通过多铁交叉效应存在固有的且显著强的磁电耦合，由此，自旋状态的转换强烈地影响铁电极化，并且潜在地，利用外部施加的电场对极化的操纵可以引起自旋状态转变。这种新的效果证明BiCoO 3，其中的地面自旋状态切换通过减少内部铁电极化。这些结果为钙钛矿铁电体和多铁性提供了更深入的了解。
The coexistence of ferroelectricity and magnetism in ABO(3) perovskite oxides is rare, a phenomenon that has become known as the ferroelectric "d(0) rule." Recently, the perovskite BiCoO3 has been shown experimentally to be isostructural with PbTiO3, while simultaneously the d(6) Co3+ ion has a high-spin ground state with C-type antiferromagnetic ordering. It has been suggested that the hybridization of Bi 6s states with the O 2p valence band stabilizes the polar phase, however, we have recently demonstrated that Co3+ ions in the perovskite structure can facilitate a ferroelectric distortion via the Co 3d-O 2p covalent interaction [L. Weston, et al., Phys. Rev. Lett. 114, 247601 (2015)]. In this paper, using accurate hybrid density functional calculations, we investigate the atomic, electronic, and magnetic structure of BiCoO3 to elucidate the origin of the multiferroic state. To begin with, we perform a more general first-principles investigation of the role of d electrons in affecting the tendency for perovskite materials to exhibit a ferroelectric distortion; this is achieved via a qualitative trend study in artificial cubic and tetragonal LaBO3 perovskites. We choose La as the A cation so as to remove the effects of Bi 6s hybridization. The lattice instability is identified by the softening of phonon modes in the cubic phase, as well as by the energy lowering associated with a ferroelectric distortion. For the LaBO3 series, where B is a d(0)-d(8) cation from the 3d block, the trend study reveals that increasing the d orbital occupation initially removes the tendency for a polar distortion, as expected. However, for high-spin d(5)-d(7) and d(8) cations a strong ferroelectric instability is recovered. This effect is explained in terms of increased pseudo-Jahn-Teller (PJT) p-d vibronic coupling. The PJT effect is described by the competition between a stabilizing force (K-0) that favors the cubic phase, and a vibronic term that drives the ferroelectric state (K-v). The recovery of the lattice instability for high-spin d(5)-d(7) and d(8) cations is due to (i) a reduction in K-0 due to a significant volume increase arising from population of the sigma-bonded axial d e(g) orbitals, and (ii) an increase in the K-v contribution arising from increased p-d hybridization; our calculations suggest that the former mechanism is dominant. Surprisingly, we are able to show that, in some cases unpaired electron spins actually drive ferroelectricity, rather than inhibit it, which represents a shift in the understanding of how ferroelectricity and magnetism interact in perovskite oxides. It follows, that for the case of BiCoO3, the Co3+ ion plays a major role in the ferroelectric lattice instability. Importantly, the ferroelectric polarization is greatly enhanced when the Co3+ ion is in the high-spin state, when compared to the nonmagnetic, low-spin state, and a large coupling of the electric and magnetic polarization is present.Generally, for d(5)-d(7) B cations in ABO(3) perovskites, an inherent and remarkably strong magnetoelectric coupling exists via the multiferroic crossover effect, whereby switching the spin state strongly affects the ferroelectric polarization and, potentially, manipulation of the polarization with an externally applied electric field could induce a spin-state transition. This novel effect is demonstrated for BiCoO3, for which the ground spin state is switched by reducing the internal ferroelectric polarization. These results provide a deeper insight into perovskite ferroelectrics and multiferroics.