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 and logic devices.

Like their magnetic counterparts, ferroelectrics also have domain walls (atomically sharp topological defects that separate regions of uniform polarization), but they are 10 to 100 times smaller. “This makes them the ultimate nanoscale controllable feature in solid materials,” explains team leader Jan Seidel.

“Ferroelectric domain walls (FEDWs) are typically just 1 nm in width (which is also around 10 times smaller than current silicon electron CMOS structures). Thanks to their extremely small size, structural and symmetry changes can occur at FEDWs and these changes can drastically alter a material’s properties. In fact, we can think of the FEDW as being a completely different material from the surrounding bulk.”

Electrode arrangement is critical

“This is the key point we exploit in our memory cells,” he tells nanotechweb.org. “FEDWs are much more electrically conducting than the material surrounding the wall (the bulk), which is itself insulating. Since the electric dipoles in ferroelectrics can be reoriented by external electric fields (an applied voltage), we can create, erase or relocate FEDWs in a material using these fields. By creating and erasing walls we can form or take away conductive channels in which data is stored, thus creating the Is and 0s of binary logic.”

The researchers made their non-volatile memory from thin films of the ferroelectric material BiFeO3 (BFO). Using electron-beam nanolithography, they patterned Pt/Ti metal electrodes onto the film, which allowed them to apply an electric field in the plane of the film. This particular electrode arrangement is critical since the field applied between the electrodes allows them to stabilize the DW on the nanoscale.

“Such electrode geometry is tailored to the specific material being studied and knowing the best type of arrangement to choose is only possible by having characterized the material’s intricate properties beforehand, something that has taken us more than 10 years for the case of BFO,” explains Seidel.

FEDWs can store data on multiple levels

The specially designed in-plane arrangement of the electrodes also allows us to encode and retrieve information using moderate electric fields, he adds. The device thus works using little energy. The low-voltage, pulse-based readout of the written states is non-destructive too.

And that is not all: the FEDWs can store data on multiple levels because a series of sequentially distinct resistance states can be tuned in a stepwise manner by precisely controlling the length of the DWs. Data storage densities in these FEDW memory devices are therefore much improved compared to traditional binary bits.

“While researchers have intensely studied magnetic domain walls over the last decade or more for memory and logic applications, work on FEDWs is more recent but it is now catching up,” explains Seidel. “Much of the work at the moment is fundamental research but proposals for commercialization are being discussed and actively explored. I think our work lays some of the groundwork for potential industrial adoption of the ideas and concepts described in our study.”

The team, reporting its work in Science Advances DOI: 10.1126/sciadv.1700512, says that it would now like to scale up its memory concept to the real device scale, explore other similar materials to optimize memory performance and ideally better understand the inner structure of FEDWs.