Brown University Team Eliminates 'Dead Layer' in Multiferroics, Opening Path to Low-Power AI Chips

A film of bismuth ferrite just a few angstroms thick, roughly the width of a handful of atoms, is still electrically polarized and still magnetic. That result, from a team at Brown University, is what the researchers describe as the end of the "dead layer." It matters because that problem has kept a promising class of materials out of computer chips.

The materials are called multiferroics. Their atomic structure is built from repeating units in which positive and negative charges sit on opposite sides, which gives the material an electrical polarization. A small voltage can reverse that polarization by swapping the positions of the charges. That offers a way to store ones and zeros using very little electricity. Because the material's magnetism is tied to its polarization, the stored information can also be read back magnetically. The combination could allow memory and logic components that need far less energy than current technology.

The obstacle has been size. Modern chips need exceptionally thin layers, and multiferroics have traditionally stopped working properly at those dimensions. Scientists trace the breakdown to two effects. The atomic structure becomes distorted, which disrupts the arrangement and chemical environment the properties depend on. And internal electric fields interfere with the polarization until it can no longer be maintained. Together, they wipe out both the ferroelectric and the magnetic behavior, leaving the dead layer.

The Brown team, led by Lucas Caretta, an assistant professor of engineering, attacked both causes. They deposited bismuth ferrite, a widely studied multiferroic, onto a foundation of dysprosium scandate, a compound derived from rare earth elements. Its crystal structure is slightly smaller than that of bismuth ferrite, and the mismatch squeezes the bismuth ferrite under compression, which helps preserve the atomic structure its properties require. They also added a second, metallic layer designed to counteract the unwanted electric fields that would otherwise undermine the polarization.

The result, Caretta said, is that the dead layer is gone. "We show that you can make this material basically as thin as you want -- down to a few angstroms -- and it's still ferroelectric, and it's still magnetic," he said. The findings were published in the journal Nature Materials.

The team also found something they had not simply been aiming to preserve. The engineered layers caused the bismuth ferrite to show altermagnetism, a form of magnetism that combines useful traits of ferromagnetism and antiferromagnetism. Caretta explained the appeal. Ferromagnets produce strong signals that are easy to read, but they are relatively slow, and their stray magnetic fields make dense packing on a chip difficult. Antiferromagnets are much faster and avoid stray fields, but their signals are much harder to detect. Altermagnets, he said, are "kind of the best of both worlds": fast, densely integrated, and still able to give a strong electrical signal for readout.

The work arrives as artificial intelligence drives rising demand for computing power, which has made lower-energy hardware an increasingly important goal. Caretta believes the findings could bring multiferroics closer to practical use in low-power memory and computing devices. The technology still needs further development before it could appear in commercial chips. "We know the underlying properties are already there in the bulk materials," he said. "The trick is learning how to engineer them into a form that's useful -- and sometimes even better than what nature gives us."

The research was funded by the U.S. Army Research Office, the National Science Foundation and the U.S. Department of Energy. What to watch next is whether the approach can be developed into something manufacturable in commercial chips, which the researchers say has not yet happened.