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

• Applicant: Professor Dr. Gerhard Jakob
• Subject Area: Experimental Condensed Matter Physics ; Synthesis and Properties of Functional Materials
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 358671374

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.

• DFG Programme: Research Grants