Thesis assignments

Hybrid improper ferroelectricity in multiferroic superlattices

Increasing demands for high-speed and low power computation and communication has increased interests in multiferroic materials, in which electric field can be used to control magnetization and vice versa. Combining the advantages of ferroelectric and magnetic random memories, four to eight logic states devices could be used to encode binary information in multiferroic devices [1].

Some materials present intrinsically this properties (BiFeO3 for instance), but the coupling at room temperature is usually weak and not suitable for practical applications. In this work, we propose to fabricate an artificial multiferroic phase, by means of superlattices combining perovskites showing anti-ferroelectric displacements. In these so-called “hybrid improper ferroelectrics”, the ferroelectric polarization and the magnetism are coupled through oxygen octahedral tiltings [2]–[5].

The studied system, BiFeO3/NdFeO3, has been previously explored theoretically by effective Hamiltonian numerical predictions [6]. The first objective of the work is to observe the predicted ferroelectrity in (BiFeO3)1/(NdFeO3)1 superlattices. If this is achieved, the student will set-up an experiment to explore the magneto-electric coupling in this superlattice. Finally, the structural properties of the superlattices will be analysed using XRD and TEM, and compared with both theoretical predictions and with films composed of the solid-solution (BixNd1-x)FeO3.

[1]  M. Bibes et A. Barthélémy, « Towards a magnetoelectric memory: Multiferroics », Nat. Mater., vol. 7, no 6, p. 425‑426, juin 2008.

[2]  J. M. Rondinelli et C. J. Fennie, « Octahedral Rotation-Induced Ferroelectricity in Cation Ordered Perovskites », Adv. Mater., vol. 24, no 15, p. 1961‑1968, avr. 2012.

[3]  N. A. Benedek, A. T. Mulder, et C. J. Fennie, « Polar octahedral rotations: A path to new multifunctional materials », J. Solid State Chem., vol. 195, p. 11‑20, nov. 2012.

[4]  N. A. Benedek et C. J. Fennie, « Hybrid Improper Ferroelectricity: A Mechanism for Controllable Polarization-Magnetization Coupling », Phys. Rev. Lett., vol. 106, no 10, mars 2011.

[5]  M. Stengel, C. J. Fennie, et P. Ghosez, « Electrical properties of improper ferroelectrics from first principles », Phys. Rev. B, vol. 86, no 9, sept. 2012.

[6]  B. Xu, D. Wang, H. J. Zhao, J. Íñiguez, X. M. Chen, et L. Bellaiche, « Hybrid Improper Ferroelectricity in Multiferroic Superlattices: Finite-Temperature Properties and Electric-Field-Driven Switching of Polarization and Magnetization », Adv. Funct. Mater., vol. 25, no 24, p. 3626‑3633, juin 2015.

Contact : Laura Bégon-Lours,