An important part of our activity has been devoted to the study of small size ferroelectric systems.
Work on this topic has been carried out in collaboration with teams from Amiens University, Cambridge, Rostov on Don, and Argonne National Lab
Device applications mainly concern memory storage devices for which reduction of energy operating costs and improvement of storage capacity are highly desirable and actively sought.
Device operation crucially relies on the properties of electric polarization. The key point is that, in thin films and nanostructures, polarization behavior may strongly differ from that in bulk materials.
Striking results :
- We demonstrated the existence of non conventional switching mechanisms in some ferroelectric systems
- We proposed multibit memory cells based on multiaxial ferroelectric thin film under certain temperature and strain conditions.
New polarization switching mechanisms
We paid particular attention to polarization switching mechanisms.
We have demonstrated the existence of different homogeneous switching regimes in strained thin ferroelectric films. In addition to the conventional longitudinal switching mechanism (for which polarization vector always keeps the same direction), other mechanisms involving the rotation of polarization are also possible.
Towards Multibit Ferroelectric Memory Cells
We have investigated the polarization-field dependence in ultrathin films and found behaviors very different from the classical hysteresis loop, with two opposite polarization states at zero field, which constitutes the basic cell of existing ferroelectric memory. We have demonstrated that by means of misfit strain and/or temperature, one can tune the shape of the polarization response to the electric field and induce exotic sequences of multistable states. That should enable to design ternary or quaternary memory cells. Such multilevel cells can be realized using ultrathin films of ferroelectric oxides such as the prototypical PbTiO3, which appears promising for room temperature operations.
Multistable polarization states in the ferroelectric film.