Magnetoelectric Decoupling in Bismuth Ferrite

It is still an open question if magnetoelectric coupling occurs at the atomic scale in multiferroic BiFeO3. Nuclear solid-state techniques monitor local fields at the atomic scale. Using such an approach, we show that, contrary to our own expectation, ferroelectric and magnetic ordering in bismuth ferrite (BiFeO3 or BFO) decouple at the unit-cell level. Time differential perturbed angular correlation (TDPAC) data at temperatures below, close, and above the magnetic N´ eel temperature show that the coupling of the ferroelectric order to magnetization is completely absent at the bismuth site. It is common understanding that the antiferromagnetic order and the cycloidal ordering due to the Dzyaloshinskii-Moriya interaction generate a net zero magnetization of the sample canceling out any magnetoelectric effect at the macroscopic level. Our previous data show that a very large coupling of magnetic moment and electrical distortions arises on the magnetic sublattice (Fe site). The oxygen octahedra around the iron site experience a large tilt due to the onset of magnetic ordering. Nevertheless, the Bi-containing complementary sublattice carrying the largest part of ferroelectric order is practically unaffected by this large structural change in its direct vicinity. The magnetoelectric coupling thus vanishes already at the unit cell level. These experimental results agree well with an ab initio density functional theory (DFT) calculation.多铁性铋铁氧体（multiferroic BiFeO3）中是否存在原子尺度的磁电耦合，仍是一个悬而未决的科学问题。核固态技术可用于探测原子尺度的局域场。依托该研究手段，我们的实验结果与预期相悖：铋铁氧体（BiFeO3，简称BFO）中的铁电序与磁序在单胞层面发生了解耦。在低于、接近及高于磁奈尔温度的温度条件下获取的时间微分扰动角关联（Time differential perturbed angular correlation, TDPAC）数据表明：铋位点处铁电序与磁化强度的耦合完全消失。学界普遍认为，由德热纳-森重相互作用（Dzyaloshinskii-Moriya interaction）引发的反铁磁序与螺旋有序，会使样品的净磁化强度为零，进而在宏观尺度上抵消所有磁电效应。我们此前的研究数据显示，磁亚晶格（铁位点）上存在磁矩与电畸变的极强耦合。铁位点周围的氧八面体因磁序的出现发生了大幅倾斜。尽管如此，承载绝大部分铁电序的含铋互补亚晶格，在其直接邻近区域几乎不受该大幅结构变化的影响。因此，磁电耦合早在单胞层面就已消失。上述实验结果与从头算密度泛函理论（ab initio density functional theory, DFT）的计算结果吻合度极高。