Ferroelectric materials have a permanent dipole moment, like their ferromagnetic counterparts. However, in ferroelectrics, the dipole moment is electric and not magnetic and so can be oriented using electric fields rather than magnetic ones. This opens up a host of novel device applications because it allows electrically digital information to be stored in ferroelectric thin films, something that might be exploited for making computer memory chips. However, the problem is that these materials generally lose their ferroelectricity as they become thinner, which obviously limits their usefulness in nanoelectronics.

The researchers, led by Chang-Beom Eom at the University Wisconsin-Madison, have now found that a thin film of a normally non-polar material can be made polar by taking advantage of naturally existing tiny polar nanoregions. “This happens when the film is made so thin that its whole volume is occupied by these nanoregions,” explains Eom. “When they are electrically aligned in one direction, this leads to a net polarization – and the material becomes ferroelectric.”

The flexoelectric effect

The team’s new discovery stems from their earlier work in which they discovered naturally occurring polar nanoregions in strontium titanate (SrTiO3) films and crystals, which are neither polar nor ferroelectric. “We found that we could reverse polarization in this material without any applied voltage, by simply exerting pressure on the film through the tip of an atomic force microscope,” says Alexei Gruverman at the University of Nebraska-Lincoln. “Such voltage-free ferroelectric switching is possible thanks to the ‘flexoelectric effect’, whereby a mechanical strain gradient induces electrical polarization.”

The researchers wanted to see if the same thing happened in much thinner films of the non-polar material.

“We found that we could induce a large flexoelectric effect in this material, but only if the films were very thin,” Eom and Gruverman tell nanotechweb.org. “To our surprise, we saw that these films behaved almost like ferroelectric ones – that is they could be polarized not only by applying mechanical strain to them but also with an applied voltage, and that this polarization was stable. The striking discovery was that the thinner the film, the more stable the polarization, and ferroelectrics typically tend to behave in the opposite way.”

Individual polar nanoregions occupy whole film volume

We immediately connected this observation with our previous work on polar nanoregions, they say, and can now clearly explain how the strain-free ultrathin films of otherwise non-ferroelectric SrTiO3 become ferroelectric, says Eom. “As mentioned, when the film’s thickness becomes as small as individual polar nanoregions (which are several nanometres across), the whole volume of the film is occupied by them and the film starts to behave like a ferroelectric.”

The researchers performed ferroelectric measurements, piezoresponse force microscopy (PFM) and scanning transmission electron microscopy on their samples to confirm their results.

SrTiO3 is an important building block for oxide electronics and has superconducting, two-dimensional gas and magnetic properties and so is useful for a range of nanoelectronics device applications. It also appears to be a good material for solar cells.

Eom and Gruverman say that they do not yet know whether the effect they have observed is unique to strontium titanate but hope that it is valid for other perovskite dielectrics, in which polar nanoregions could be controlled by carefully engineering defect structures in these materials. If this is the case, nanoscale devices in which ferroelectricity is coupled to other properties like magnetism might be possible.

The team, which includes researchers from Pennsylvania State University, Korea Institute of Materials Science, Temple University, Pohang University of Science and Technology, University of California-Santa Barbara and Boise State University details its work in Science DOI: 10.1126/science.aaa6442.