Strain control of ferrimagnets

Summary

Magnetic random access memory (MRAM) using electrical current to switch magnetic layers has been on the market for a few years. It has the potential to replace much of the memory hierarchy in computer architectures but the device footprint and energy consumption are still too large to be competitive. To address the latter, there are several ongoing investigations into the use of electric field to manipulate nanomagnets, either by directly applying a gate voltage or indirectly via piezoelectric strain. Along with this has come an interest in the physics of interfaces of magnetic layers with other materials such as ferroelectrics, oxides and topological insulators. In previous work we have pioneered the piezoelectric strain control of ultrathin ferromagnetic layers with perpendicular magnetization (relevant to perpendicular magnetic recording, for example), and we have begun to explore the influence of strain at the surface of ferroelectric barium titanate on the magnetic domain structures in adjacent ferromagnetic layers. The use of a single crystal ferroelectric enables larger strain to be generated at the interface where it is needed, in contrast to commercial piezoelectric stressors where the surfaces are too rough to deposit high quality magnetic layers. The aim of this project is to replace the ferromagnet with a ferrimagnet, which has a smaller net magnetization making it promising, for example, for fast and efficient read/write operations, and which in the right composition has a peculiar sensitivity to strain. Ferrimagnet/ferroelectric structures will be grown in the Royce deposition facility at the University of Leeds, which combines several thin film growth techniques in one system, and characterised using state-of-the-art instruments in the Condensed Matter group. The goal will be to control the magnetic properties of the ferrimagnet reversibly by applying a voltage to the ferroelectric. The choice of materials will be guided by the principle of sustainability, ensuring that they are plentiful, as defined by the European Chemical Society in 2019. This requirement is already met by the constituent elements of barium titanate and some ferrimagnets too.

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