Ferroelectric materials have a permanent dipole moment, or polarization, like their ferromagnetic counterparts. However, in ferroelectrics, the dipole moment is electric rather than magnetic and so can be oriented using electric fields and not magnetic ones. This means that electrically digital information can be stored in ferroelectric thin films, something that might be exploited for making high-density computer memory chips.

Until now, researchers manipulated ferroelectric domains using highly charged, sharp metallic tips contacted with the surface of the material to create the strong, reorienting electric field. However, this physical contact often damaged the ferroelectric or the metallic tip.

No physical contact

Now, a team led by Xiaozhou Liao has overcome this problem by using an electron beam produced by an electron gun. The electron gun does not physically contact the ferroelectric material directly and the technique can be used to manipulate a domain just 5 nm across. This distance is 10 times smaller than that possible with conventional methods.

The researchers performed their experiments on thin films of hexagonal yttrium manganite. They began by measuring and mapping the polarization pattern over a set of ferroelectric domains in the material using a transmission electron microscope. They then used the microscope’s 200 nm wide electron beam to illuminate a portion of one domain.

Polarization switch stable for a month

The team observed that the interaction of the electrons with the thin films created an electric field that points radially outwards from the beam centre towards its edges. This field was strong enough to reverse the polarization direction of the domains within the beam’s trajectory.

And that is not all: the polarization switch (which could be used to store one bit of information) remained stable for a month.

Secondary electrons and oxygen vacancies

Liao and colleagues say that they are not exactly sure how the electron beam switches polarization in the ferroelectric material they studied. However, one explanation is that the beam may produce secondary electrons and oxygen vacancies as it interacts with the thin film.

“These electrons and vacancies generate positive charges with an omnidirectional electric field in the area illuminated by the electron beam,” explains Liao. “On the side where the electric field direction is opposite to the polarization of the ferroelectric thin film, the electric field can redirect the local polarization and therefore manipulate ferroelectric nanodomains. We are able to reverse (or undo) this local polarization change by laterally shifting the electron beam so that an opposite electron field is applied.”

Towards more efficient ways of manipulating nanodomains

Although more work is needed to improve the technique and fully understand how it works, the researchers say that it could be used to produce memory devices based on ferroelectric materials that store information at densities 100 times higher than today’s memories.

The team, describing its work in Physical Review Letters DOI:http://dx.doi.org/10.1103/PhysRevLett.117.027601, says that it is now busy studying how a switched single domain affects the stability of nearby domains. This will become particularly important as domain dimensions reduce (or bit density increases), Liao tells nanotechweb.org. “We are also looking at how combined external stimuli, such as mechanical loading and local heating, affect ferroelectric domain structures,” he adds. “This may lead to a more efficient way of manipulating nanoscale domains in the future.”