Chips: an ultrathin film keeps its magnetic properties
· BehindLines newsroom
Researchers at Brown University in the United States say they have removed a major obstacle to building chips that use very little energy. According to the ScienceDaily text, which republishes a Brown press release, their technique makes it possible to produce bismuth ferrite films only a few angstroms thick without losing their essential electrical and magnetic properties.
The "dead layer" problem
Multiferroic materials pair electrical and magnetic behaviours that a very small voltage can drive jointly. According to the text, they could make tomorrow's electronics far more energy efficient, including in the face of growing demand from artificial intelligence, by serving to build memory and logic components.
The main obstacle is this: when these materials are thinned enough to fit inside such devices, they generally lose the electrical and magnetic behaviour that gives them their value. Scientists call it the "dead layer" effect. Two phenomena cause it, according to the text: the atomic structure becomes distorted, and internal electric fields disturb the polarization to the point where it can no longer hold.
A layered structure
The team, led by Lucas Caretta, who teaches engineering at Brown as an assistant professor, deposited the bismuth ferrite on a base of dysprosium scandate, a rare-earth compound. The crystal lattice of that base is slightly smaller, which compresses the bismuth ferrite and helps preserve its atomic structure. A second metallic layer counterbalances the unwanted electric fields.
"We have established that the dead layer can be got rid of. This material can be made practically as thin as one wishes, down to a few angstroms, and it remains ferroelectric and magnetic," says Lucas Caretta, quoted in the text. The results are published in the journal Nature Materials.
An unexpected form of magnetism
According to the text, the process also brings out altermagnetism, a state that combines strengths of ferromagnetism and antiferromagnetism. According to Lucas Caretta, ferromagnetic materials are relatively slow and their stray fields prevent packing them closely on a chip, whereas antiferromagnetic materials are faster but reading their signals is far harder; altermagnetic materials can be fast, compact and deliver a strong electrical read-out signal.
The abstract of the Nature Materials article speaks of "signatures" of emergent altermagnetism. For films four unit cells thick, it reports a monoclinic phase with collinear magnetic order of the altermagnetic d-wave kind, backed by measurements of magnetic circular dichroism and by photoemission that resolves spin.
Why it matters
According to Lucas Caretta, these results could move multiferroic materials nearer to being used in practice for energy-frugal memory and computing hardware. The text specifies, however, that the technology still has to be developed before it can be built into commercial chips; it adds that the research shows a long-standing limit of materials can be overcome. Only the abstract of the Nature Materials article was accessible in the sources consulted.