The first ever observations of polar vortices in a ferroelectic material could find potential applications in ultracompact data storage and processing and the production of new states of matter. Credit: Berkeley Lab
New state of matter holds promise for ultracompact data storage, processing. The observation in a ferroelectric material of “polar vortices” that appear to be the electrical cousins of magnetic skyrmions holds intriguing possibilities for advanced electronic devices. These polar vortices, which were theoretically predicted more than a decade ago, could also “rewrite our basic understanding of ferroelectrics”.
“It has long been thought that rotating topological structures are confined to magnetic systems and aren’t possible in ferroelectric materials, but through the creation of artificial superlattices, we have controlled the various energies of a ferrolectric material to promote competition that lead to such new states of matter and polarization arrangements,” says Ramamoorthy Ramesh, Berkeley Lab.
Vortex structures occur in nature across a broad range of length scales, from galaxies and weather systems all the way down to the atomic scale of polar vortices in ferroics.
“Ferroelectric materials such as the materials used in this work
have produced a number of exciting emergent properties over the years, but these smoothly-rotating polar vortex structures really are different,” says Lane Martin. “I think if you surveyed the community many would shake their heads in disbelief at such structures, but it turns out there really is a tendency for vortex states to form in nature even in these polar systems. And, when one looks more broadly, vortex structures can occur across huge length scales – from galaxies and weather systems all the way down to 10s of atoms as in our case.”
Ferroic materials display unique electrical or magnetic properties – or both in the case of multiferroics. For example, the electrical field of a ferroelectric material can be polarized in favor of either a positive or negative charge with the application of an external electrical field. In a ferromagnetic material, the application of an external magnetic field aligns the spin of their charged particles, resulting in the material becoming a permanent magnet. In recent years, it was discovered that the application of an external magnetic field can also produce atom-sized cyclones of skyrmions, which act like baryon particles and can be moved coherently over macroscopic distances. These properties make skyrmions excellent candidates for spintronic applications.
“We believe the polar vortices we observed in ferroelectrics, when fully explored, have the potential to be topological states of matter that are similar to magnetic skyrmions,” Ramesh says. “The fact that our polar vortices can display emergent behavior in their electronic, optical, magnetic and other properties suggests that heretofore unexplored applications and functionalities could be possible.”
Ramesh, Martin and their collaborators worked with what has become a canonical system in the community, ultrafine layered structures built from lead titanate and strontium titante compounds controlled down to a few unit cells each, in which each unit cell is approximately 0.4nm thick. They -created superlattices that harbored a 3-way competition between elastic, electrostatic and gradient energies within the layers of lead titanate and strontium titanate >> gives rise to the polar vortices. A combination of scanning transmission electron microscopy (STEM) and X-ray diffraction studies were used observe and characterize the polar vortices.
“We’re observing a new state of matter and we have our work cut out for us in mapping and understanding how it evolves. We can imagine adding a magnetic spin component to similar superlattices and thus potentially paving a pathway to fundamentally demonstrate electric-field control of magnetism,” said Martin. http://newscenter.lbl.gov/2016/01/27/polar-vortices/
Researchers including (from left) Lane Martin, Anoop Damodaran, Ramamoorthy Ramesh, Christopher Nelson and Ajay Yadav have discovered polar vortices in a ferroelectric. (Photo by Roy Kaltschmidt)
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