Researchers say that logic and memory functions of future electronic devices could shrink an order of magnitude (one or two nanometers, instead of tens of nanometers) if they can control domain walls, the transition zones in a material that separate regions of different magnetic, electric or other properties. And, by “control,” the researchers remarkably mean “writing,” “erasing” and moving the domains.
While studying bismuth ferrite (BiFe03), scientists at the Lawrence Berkeley National Laboratory and the University of California, Berkeley discovered an amazing property of domain walls. They found that within the material, between domains having different electrical polarization, the domain walls themselves – just a few nanometers wide – can be made to conduct electricity.
Ramamoorthy Ramesh of Berkeley Lab’s Materials Sciences Division, a professor in the Department of Materials Science and Engineering and the Department of Physics explains, “A domain wall is virtually a two-dimensional sheet through the material.” He says that besides being small, they can be moved, and thus offer great promise in future electronics design.
Ramesh says that the basic electrical and magnetic properties of a complex material such as bismuth ferrite are extremely sensitive to their environment. “Materials called multiferroics are an example of this kind of material, and bismuth ferrite is a prototypical multiferroic.”
A unique property of multiferroic materials is that they can have different properties within certain regions that are differently oriented, but not independent. “One reason we are looking at oxides like bismuth ferrite is because we can control one property by changing others,” says Ramesh. “These materials have a lot of personality.”
To make films of the material, researchers hit targets of ceramic oxides containing bismuth and iron with a laser pulse that causes a deposit to form on the substrate. They manipulate the substrate structure, temperature and atmosphere to get the sought after mix and phase.
Fifty to 200-nanometer films of bismuth ferrite have been grown substrates of strontium titanium oxide.
Bismuth ferrite has the crystal structure of perovskite. The bismuth ferrite films contain ferroelectric domains between 5 and 10 micrometers that can be mapped using a piezoresponse force microscope. Furthermore, the domain structure can be changed by using different scanning-probe mechanisms. For example, the PFM setup can switch the local polarization of the film by applying a large enough voltage.
“We worked with theorists to help us model the behavior we had observed and to understand the mechanism of the conduction,” says Ramesh. “What happens is that as the positions of the central iron atoms change crossing the domain wall, the polarization increases perpendicular to the domain wall – but at the same time goes to zero parallel to the wall, before increasing again,” says Martin. “This causes any free electrons in the vicinity to accumulate at the wall, where they can move along the wall itself.”
“Domain walls may be the ultimate nanoscale feature,” says Lane Martin of MSD. “They’re intrinsic to the material – they want to be there. And they’re only two nanometers wide! It’s like shoving a graphene sheet right down into a tough, insulating ceramic.”
For Ramesh, the discovery is big step on “the path to the Holy Grail of oxide materials, the challenge of creating and controlling metal-insulator transitions at interfaces in a material. It’s why DOE is so interested in this kind of research.”