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

同时显示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。作为铁电和反铁磁体的多铁BiFeO3 （BFO）将与铁磁覆盖层结合使用，以实现g型反铁磁体中有序铁自旋与双钙钛矿中相应空间有序铁原子的直接交换耦合。在多铁氧体中旋转铁电极化方向会加强其反铁氧体方向的旋转。由于界面处的交换耦合，铁磁体的磁化方向如下。这使得铁磁体的开关具有极低的耗散，因为只有电容器结构的小充电电流需要流动。到目前为止，这一设想还不能在室温下以可重复的方式实现，因为大多数情况下，人们试图实现与“传统的、简单的”铁磁体（如金属钴或混合价锰铁）的交换耦合。然而，在理想的界面上，反铁磁体的自旋（显示左右交替）与铁磁体的自旋（显示全部向左（右））之间的净交换耦合消失了。只有反铁磁畴壁或界面粗糙度处的残余耦合仍然存在。在双钙钛矿的有序结构中，如Sr2FeMoO6， Fe和Mo原子以交替的方式占据原子位置。因此，在界面上，双钙钛矿的铁原子总是可以与反铁磁体的左旋铁原子偶联，而反铁磁体的右旋铁原子则与非磁性的Mo原子相邻。这保证了层之间的理想耦合，这将在该项目中首次实现。实现这一目标的必要条件是对两种材料的外延生长进行精密的控制。此外，界面处的强电场作为一个可控的、可切换的Rashba场，并改变界面性质，例如磁界面的各向异性或界面处的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.