New finding may lead to better memory chips

In ferroelectric memory the direction of molecules’ electrical polarisation serves as a 0 or a 1 bit. An electric field is used to flip the polarisation, which is how data is stored.

With his colleagues at University of Michigan and collaborators from Cornell University, Penn State University, and the University of Wisconsin, Madison, Xiaoqing Pan, a professor in the U-M Department of Materials Science and Engineering, has designed a material system that spontaneously forms small nano-size spirals of the electric polarisation at controllable intervals, which could provide natural budding sites for the polarisation switching and thus reduce the power needed to flip each bit.

“To change the state of a ferroelectric memory, you have to supply enough electric field to induce a small region to switch the polarisation. With our material, such a nucleation process is not necessary,” Pan said. “The nucleation sites are intrinsically there at the material interfaces.”

To make this happen, the engineers layered a ferroelectric material on an insulator whose crystal lattices were closely matched. The polarisation causes large electric fields at the ferroelectric surface that are responsible for the spontaneous formation of the budding sites, known as “vortex nanodomains”.

The researchers also mapped the material’s polarisation with atomic resolution, which was a key challenge, given the small scale. They used images from a sub-angstrom resolution transmission electron microscope at Lawrence Berkeley National Laboratory. They also developed image processing software to accomplish this.

“This type of mapping has never been done,” Pan said. “Using this technique, we have discovered unusual vortex nanodomains in which the electric polarisation gradually rotates around the vortices.”