Ferroelectric materials have a spontaneous dipole moment which can point up or down. This means that they can be used to store information, just like magnetic bits on a hard disk. The advantage of ferroelectric memory bits is that they can be written at a low voltage and power, while magnetic bits require large currents to create a magnetic field for switching, and so need more power. The disadvantage of ferroelectrics is that the aligned dipoles are only stable in fairly large groups, so as the size of the crystals reduces, the dipole moment eventually disappears.
“Reducing the size of ferroelectric materials has been a research topic for more than 20 years” said Prof Beatriz Noheda, Professor of Functional Nanomaterials at UG. “Some eight years ago, a breakthrough was announced by the Nanoelectronic Materials Laboratory in Dresden, Germany. They claimed that hafnium oxide thin films were ferroelectric when thinner than ten nanometres and that thicker films actually lost their ferroelectric properties. This went against everything we knew, so most scientists were skeptical, including me,” she said.
Some of the skepticism was because the ferroelectric hafnium samples used in these studies were polycrystalline and showed multiple phases, obscuring any clear fundamental understanding of such an unconventional phenomenon.
Noheda and her group wanted to study these crystals by growing clean (single-phase) films on a substrate. Using X-ray scattering and high-resolution electron microscopy techniques, they observed that thin films under 10nm thick grow in an entirely unexpected and previously unknown polar structure, which is necessary for ferroelectricity. Combining these observations with transport measurements, they confirmed that the material was indeed ferroelectric. “In the substrate that we used, the atoms were a little bit closer than those in hafnium oxide, so the hafnium crystals would be a little strained”, said Prof Noheda.
A key discovery was that the crystal structure changed when the layers exceeded 10nm, reproducing the results of the Dresden lab as a result. “We used a totally different method, but we reached similar conclusions. This confirmed that ferroelectricity in nanosized hafnium oxide crystals is indeed real and unconventional. And that begged the question: why does this happen?” she said.
This led the team to study the phase diagrams of hafnium oxide. At a very small size, particles have a very large surface energy, creating pressures of up to 5 gigapascals in the crystal. “This pressure, along with the substrate-imposed strain, induces a polar phase, which is in line with the observation that these crystals are ferroelectric,” she said.
One more important finding is that, in contrast to the thin films in Dresden, the new crystals do not need a ‘wake-up’ cycle to become ferroelectric. “The previously studied thin films only became ferroelectric after going through a number of switching cycles. This increased the suspicion that ferroelectricity was some sort of artefact. We now believe that the wake-up cycles were necessary to align the dipoles in “unclean” samples grown via other techniques. In our material, the alignment is already present in the crystals.”
This confirms that hafnium oxide is ferroelectric at the nanoscale, allowing very small bits to be built from the material, with the added advantage that they switch at low voltage. The particular substrate used in the study is magnetic, and this combination of magnetic and ferroelectric bits brings an extra degree of freedom, allowing each bit to store double the information. Now that the mechanism of nanosized ferroelectricity is clear, it seems likely that other simple oxides could have similar properties. Noheda expects that together, this will spark a lot of new research.
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