Expanding the limits of ferroelectrics towards a nano-sized electric memory

May 18, 2021 - by Santina Russo 

Today, electronic devices like computers or smartphones rely on two physical principles: electric materials to process information and magnetic materials to store it. However, magnetic data storage consumes a substantial amount of energy, since it requires magnetic fields generated from comparatively large magnets. In contrast, the generation of electric fields requires much less energy, and an electric memory could drastically reduce the energy demand of future electronic devices. 

That’s why Marie Curie Fellow Chiara Gattinoni and her colleagues in the materials theory group of ETH professor Nicola Spaldin investigate so-called ferroelectrics. As materials theorists, they aim to design new materials with functionalities that do not yet exist. In this respect, ferroelectrics are of particular interest because they possess a special feature: an inherent electric polarisation that can be reversed by an external electric field.

Two known phenomena contribute to this polarisation loss. One is the so-called depolarising field, which arises from opposite charges on the opposite surfaces of the material — a natural consequence of its dipole moment. These charges create an electric field that is contrary to the direction of the ferroelectric polarisation. The thinner the film, the closer these opposite charges come to one other, and, consequently, the stronger this destructive depolarising field becomes. The second phenomenon arises from the instability of the surface charges, per se. “In practice, the system has to find a way to compensate the surface charges in order to maintain the ferroelectric properties,” explains Gattinoni. Her DFT calculations indicate a way to achieve this even in thin films.

Thinking thinner 

Gattinoni found that, contrary to previous suppositions, the chemical bonding at the interface between the ferroelectric and the underlying metal — in this case a strontium ruthenate layer — does not have a major influence on maintaining polarisation. Instead, the electronic structure and therefore the electrostatic properties of the ferroelectric and the metal layer play the dominant role in the system. “This means that for controlling the polarisation stability, researchers need to engineer the electrostatic properties, rather than focus on the chemical bonding,” Gattinoni says. 

Her calculations also gave indications as to where such modifications would have the highest effect on charge compensation. According to the results, charge transfers at the interface between ferroelectric and metal, which had previously been assumed to be vital, really only play a minor role. In contrast, modifications at the ferroelectrics’ surface proved to be significantly more effective. Such modifications could consist of the introduction of structural defects to create vacancies that can adsorb the charges or of charge-adsorbing molecules in a device’s atmosphere. “Both of these methods can be used to stabilise the polarisation, even in very thin ferroelectric films,” Gattinoni says.