Magnetic exchange coupling and electrical switching in multiferroic BiFeO3/double perovskite hetero structures

同时表现出磁性和铁电有序的多铁性材料提供了一种独特的可能性，可以通过电场改变磁化方向。多铁性BiFeO_3(BFO)既是铁电材料，又是反铁磁体，它将与铁磁覆盖层相结合，实现G型反铁磁体中有序的Fe自旋与双钙钛矿中Fe原子的空间有序的直接交换耦合。在多铁体中旋转铁电极化方向会强制其反铁磁方向旋转。由于界面处的交换耦合，铁磁体的磁化方向遵循。这使得铁磁体的开关具有极低的损耗，因为只需要电容器结构的小充电电流流动。到目前为止，这一设想不能在室温下以可重复的方式实现，因为最常见的是尝试实现与诸如金属钴或混合价态锰氧化物的“传统的、简单的”铁磁体的交换耦合。然而，在理想的界面上，反铁磁体的自旋之间的净交换耦合消失了，反铁磁体的自旋显示为左右交替，铁磁的自旋显示为全部左(右)。在双钙钛矿的有序结构中，如Sr2FeMoO6，Fe和Mo原子交替占据原子位置。因此，在一个界面上，双钙钛矿的Fe原子总是可以耦合到反铁磁体的左指向Fe自旋，而反铁磁体的右指向Fe自旋是非磁性Mo原子的邻居。这保证了各层之间的理想耦合，这将首次在该项目中实现。实现这一点的一个要求是对两种材料的外延生长进行复杂的控制。此外，界面处的强电场充当可控的、可切换的Rashba场，并改变界面属性，如磁界面各向异性或界面上的Dzyaloshinskii Moriya相互作用，我们将寻求实验证实。

Multiferroic materials showing simultaneously magnetic and ferroelectric ordering offer a unique possibility to change the direction of the magnetization by an electric field. Multiferroic BiFeO3 (BFO) being a ferroelectric and an antiferromagnet will be used in combination with a ferromagnetic overlayer to achieve a direct exchange coupling of the ordered Fe spins in the G-type antiferromagnet to the corresponding spatial ordering of the Fe atoms in the double perovskite. Rotating the ferroelectric polarization direction in the multiferroic enforces rotation of its antiferromagnetic direction. Due to the exchange coupling at the interface the magnetization direction of the ferromagnet follows. This enables a switching of the ferromagnet with extremely low dissipation, as only the small charging currents of the capacitor structure need to flow.Up to now this vision could not be realized in a reproducible way at room temperature as most often it was attempted to achieve an exchange coupling to "conventional, simple" ferromagnets as metallic cobalt or mixed valent manganites. However, at an ideal interface the net exchange coupling between the spins of the antiferromagnet, that show alternating to the left and the right, to the spins of the ferromagnet, that show all to the left (right), vanishes. Such only a residual coupling at antiferromagnetic domain walls or interface roughnesses remains.In the ordered structure of the double perovskites, as e.g. Sr2FeMoO6, the Fe and Mo atoms occupy atomic positions in an alternating fashion. So at an interface always Fe atoms of the double perovskite can couple to left pointing Fe spins of the antiferromagnet, while the right pointing Fe spins of the antiferromagnet are neighbours to non magnetic Mo atoms. This promises an ideal coupling between the layers that will be achieved in this project for the first time. A requirement for the realization is a sophisticated control of the epitaxial growth of both materials.Furthermore the strong electric fields at the interface act as a controllable, switchable Rashba field and modify the interface properties as e.g. the magnetic interface anisotropy or the Dzyaloshinskii Moriya interaction at the interface for which we will seek experimental confirmation.