PhD / Ab initio calculations of multiferroic interface properties

Ab initio calculations of the conductivity mechanisms at spinel ferrite – ferroelectric perovskite interfaces

R. Arras (CEMES-CNRS, Toulouse, remi.arras@cemes.fr), L. Calmels (lionel.calmels@cemes.fr)

The thesis project aims at studying the electronic, magnetic and structural properties of MFe₂O₄/BaTiO₃ (M = Fe, Co, Ni,…) interfaces.

Transition metal oxides are attractive materials, from a fundamental as well as a technological point of view. These compounds can indeed display complex phase diagrams and be therefore multifunctional. The association of spinel ferrites, which are ferrimagnetic, and BaTiO₃, a well-known ferroelectric oxide with a perovskite structure, is interesting to form extrinsic multiferroics. These materials are particularly interesting due to their high order temperature, which is necessary for room temperature applications. Their chemical composition can finally be controlled, on the one hand to tune their physical properties and, on the other hand to select non-toxic and recyclable heterostructures.

During this thesis, the student will perform a numerical study of the mechanisms responsible of a conductivity change in ferrite layers, in the vicinity of their interface with BaTiO₃. It was indeed experimentally demonstrated that the oxidation state of Fe cations could be modified by applying a bias voltage with a piezoresponse-force-microscope tip, consequently creating the emergence of local conducting channels [1,2]. Ab initio calculations would help to understand how these channels form, by verifying different hypotheses such as the migration of defects (oxygen vacancies [3], etc.) or the possibility of having inter-cation charge exchanges [4]. A second objective of this thesis will be to calculate the multiferroic properties and the magnetoelectric coupling [5] induced at such interfaces. A detailed study to describe the most precisely the interface effect and characteristics (electrical polarization state, interface termination, tetragonal distortions as a function of the thickness, etc.) will be critical to obtain a general understanding of the studied systems.

The candidate is expected to have a strong interest in fundamental physics and chemistry of condensed mater, and to have advanced knowledge in solid physics. Good communication skills and the willing to work with experimentalists is essential. Basics of programming and using numerical methods for electronic structure calculations will be considered as a plus.

This project benefits of a funding from the French national agency (ANR). The starting period is flexible (ideally, September-October 2020).

References:
[1] N. Jedrecy, et al., ACS Appl. Mater. Interfaces, 10, 28003 (2018).
[2] T. Aghavnian, Couplages magnéto-électriques dans le système multiferroïque artificiel BaTiO₃/CoFe₂O₄, Thèse de doctorat (2016).
[3] R. Arras, et al., Appl. Phys. Lett. 100, 032403 (2012).
[4] R. Arras et L. Calmels, Phys. Rev. B, 045411 (2014).
[5] R. Arras et S. Cherifi-Hertel, ACS Appl. Mater. Interfaces 11, 34399 (2019).