Tiny Electrical Vortexes Bridge Gap Between Ferroelectrics and Ferromagnets

Ferromagnetic materials have a self-generating magnetic field and ferroelectric materials generate their own electrical field. Although electric and magnetic fields are related, physics tells us that ferromagnets and ferroelectrics are very different classes of material. Now, the discovery by a team led by researchers at the University of Warwick in the UK of a complex electrical ‘vortex’-like pattern that mirrors its magnetic counterpart suggests they could actually be two sides of the same coin.

Reported in a paper in Nature, these results provide the first evidence of a process in ferroelectric materials comparable to the Dzyaloshinskii–Moriya interaction (DMi) in ferromagnets. This particular interaction plays a pivotal role in stabilizing topological magnetic structures, such as skyrmions, and it might be crucial for potential new electronic technologies exploiting their electrical analogues.

Bulk ferroelectric crystals have been used for many years in a range of technologies, including sonar, audio transducers and actuators. All these technologies exploit the crystals’ intrinsic electric dipoles and the inter-relationship between their crystal structure and applied fields.

For this study, the scientists created a thin film of the ferroelectric lead titanate sandwiched between layers of the ferromagnet strontium ruthenate, each about 4nm thick – only twice the thickness of a single strand of DNA.

While the atoms of the two materials form a single continuous crystal structure, in the ferroelectric lead titanate layer the electric polarization would normally form multiple ‘domains’, like a honeycomb. These domains can only be observed with state-of-the-art transmission electron microscopy and x-ray scattering.

But when the researchers examined the structure of the combined layers, they saw that the domains in the lead titanate were actually a complex topological structure of lines of vortexes, spinning alternately in different directions. Almost identical behavior has been seen in ferromagnets, where it is known to be generated by the DMi.

“If you look at how these characteristics scale down, the difference between ferromagnetism and ferroelectricity becomes less and less important,” said lead author Marin Alexe, a professor in Warwick’s Department of Physics. “It might be that they will merge at some point in one unique material. This could be artificial and combine very small ferromagnets and ferroelectrics to take advantage of these topological features. It’s very clear to me that we are at the tip of the iceberg as far as where this research is going to go.”

“Realising that in ferroelectrics dipolar textures that mimic their magnetic counterpart to such a degree ensures further research into the fundamental physics that drives such similarities,” said co-author Dorin Rusu, a postgraduate student at the University of Warwick. “This result is not a trivial matter when you consider the difference in the origin and strengths of the electric and magnetic fields.”

The existence of these vortexes had previously been theorised, but it took the use of cutting-edge transmission electron microscopes at the University of Warwick, as well as the use of synchrotrons at four other facilities, to accurately observe them. These techniques allowed the scientists to measure the position of every atom to a high degree of certainty.

“Electron microscopy is a game-changing technique in understanding these topological structures,” said co-author Ana Sanchez, a professor at the University of Warwick. “It is the key tool in revealing the ins and outs of these novel materials, using a subatomic beam of electrons to generate images of internal structure.”

This story is adapted from material from the University of Warwick, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.