A full-time PhD position is available in the department of Inorganic Theoretical Chemistry (CTI) at the Institute of Chemical Sciences of Rennes (ISCR, University of Rennes 1) for a talented and ambitious student. The position is fully funded by the French Research National Agency (ANR).
Magnetoelectric (ME) multiferroics (MF), which combine electric and magnetic dipole orders, are multifunctional materials with a high potential in new technologies. It can be used to reduce computer memory energy consumption, to improve magnetic field sensors or in spintronic applications.1,2 Two classes have been defined based on the mechanism promoting the spontaneous polar order.1,2
Type I ME-MF, known as proper ferroelectrics, are based on displacive mechanism (lone-pair, geometric and charge ordering), while type II ME-MF, also named spin-driven ferroelectrics (SDF), are improper ferroelectrics in the sense that the polar state is driven by a magnetic transition. Unfortunately, too few ME crystals are known to date, and even less could lead to industrial applications. Indeed, they usually give a small response (electric polarization, PS) and need low functioning temperature to exhibit the desired ME-MF properties. Two main routes to overcome these problems have been considered in the past: (1) the elaboration of composite materials and (2) the search of new crystals exhibiting improved ME-MF properties.
We aim at the investigation of a series of compounds (known to be ME-MF or not) having the following specifications: (1) large magnetic exchange couplings in order to reach high temperature functioning and (2) magnetic frustrations to have a strong ME-MF coupling mechanism based on exchange-striction and thus large ferroelectric polarization (Ps). Cuprates are ideal candidates for these two reasons and interestingly exhibit both types I and II ME-MFs.3-6 The present project combine advanced experimental techniques (X-ray, neutron and Raman techniques under pressure, magnetometry, dielectric measurements...) and state-of-the-art calculations (density functional theory (DFT), multireference wavefunction (WFT) calculations and Monte-Carlo (MC) simulations).
The present PhD thesis is part of an ANR project named HTHPCM which combines advanced experimental techniques (X-ray, neutron and Raman techniques under pressure, magnetometry, dielectric measurements...) and state-of-the-art calculations (density functional theory (DFT), multireference wavefunction (WFT) calculations and Monte-Carlo (MC) simulations). The main goal of this project is the search of high-temperature and high-polarization cuprate multiferroics (HTHPCM), i.e. (1) with a large ME-MF coupling and thus large PS, (2) operating at room-temperature (RT), and (3) showing an electric-field magnetization reversal. Our focus is cuprate MFs.
1. Why are there so few magnetic ferroelectrics? Hill, N. A., J. Phys. Chem. B 104, 6694 (2000)
2. Revival of the magnetoelectric effect. Fiebig, M., J. Phys. D 38, R123 (2005)
3. Multiferroic and magnetoelectric materials. Eerenstein, W. et al., Nature 442, 759 (2006)
4. Multiferroics: a magnetic twist for ferroelectricity. Cheong, S. W. & Mostovoy, M., Nat. Mater. 6, 13 (2007) 5. Application of magnetoelectrics. Scott, J. F. J. Mater. Chem. 22, 4567 (2012)
6. The Electric and Optical Behavior of BaTiO3 Single-Domain Crystals. Merz, W. J., Phys. Rev. 76, 1221 (1949) 7. Room-temperature spin-spiral multiferroicity in high-P CuO, Rocquefelte X. et al., Nat. Comm. 4, 2511 (2013) 8. Theoretical Investigation of the Magnetic Exchange Interactions in Copper(II) Oxides under Chemical and Physical Pressures, X. Rocquefelte et al., Sci. Rep. 2, 759 (2012)
9. Comment on “High-Tc Ferroelectricity Emerging from Magnetic Degeneracy in Cupric Oxide”, X. Rocquefelte et al. PRL 107, 239701 (2011)
10. Short-range magnetic order and temperature-dependent properties of cupric oxide, X. Rocquefelte et al., J. Phys.: Cond. Mat. 22, 045502 (2010)
Profile of the candidate
The PhD will be in charge of the DFT calculations to generate all structural models under chemical and physical pressure. He will also estimate the elastic and vibrational properties (phonon calculations) and estimate the electric polarization. In addition, he will do two or three stays of 2 months in the PHELIQS laboratory) to respectively, (1) participate to the elaboration of single crystals, and to do the complete structural characterization, (2) learn how to setup high-pressure experiments and (3) to participate eventually to a high-pressure experimental campaign. The selected student will have a solid experience in solid state science and DFT calculations. Refs 7-10 illustrate the theoretical strategy we will use during the PhD.
The PhD project will start in October 2020. Applications are already open and candidates shall contact both supervisors by e-mail, with a CV and a motivation letter, including clear description of previous Master internship(s).