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.

In strained thin films, one may observe non-classical behaviors, labeled as: longitudinal-transversal (lt) transversal (t) Laurent Baudry, Igor A. Luk’yanchuk, and Anna Razumnaya, Phys. Rev. B 91, 144110 (2015) https:/doi.org/10.1103/PhysRevB.91.144110

Different polarization reversal mechanisms are illustrated below and corresponding current signatures are shown in right panel.

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.

(A) Sketch of the experimental setup. The ferroelectric cell (orange) is grown on the substrate (blue) and is sandwiched between the two electrodes (green). The electric field produced by the voltage operates the polarization orientation. (B) and (C) The c-phase possessing two stable states, c+ and c− of polarization vector, P. (D) to (F) aa-phase : one stable state, a, and two metastable states, c+ and c− . (G) to (J) r phase: two stable states, r+ and r−, and two metastable states, c+ and c− . The lower sub-panels display the positions of the corresponding polarization states in the minima of the energy relief (yellow spheres) and the respective logical quantum (loq)-numbers. Laurent Baudry, Igor Lukyanchuk, & Valerii M. Vinokur, Scientific Reports 7, 42196 (2017) (https://doi.org/10.1038/srep42196)

Polarization-field dependence tuned by Temperature and Misfit strain in Lead Titanate thin films multilevel memory cells at room temperature

Misfit strain–temperature phase diagram of multibit switching regimes in PbTiO3 Ferroelectric Multibit Cell. The domains corresponding to different switching regimes are shown as color sectors. The insets display calculated topologically different 4-states hysteresis loops that are realized at room temperature (the room temperature is marked by the dashed yellow line).

Topologically different multibit hysteresis loops. The number of the panels indicate the region of the phase diagram shown above. The maps of the switching logical operations are shown in the right bottom corners of the respective panels. Laurent Baudry, Igor Lukyanchuk, & Valerii M. Vinokur, Scientific Reports 7, 42196 (2017) (https://doi.org/10.1038/srep42196)