Phd Thesis Proposal 2015
Name of laboratory : Laboratoire de Physique des Solides
Address : Centre Scientifique d’Orsay, Bâtiment 510, 91405 Orsay CEDEX
Name of thesis advisor : Pascale Foury / Victor Balédent
E-mail : pascale.foury@u-psud.fr Phone number : 01 69 15 60 55
Web Page :
Source of funding :
Doctoral School : PIF
Title : Magneto-electric coupling in RMn2O5 multiferroics.
Abstract :
Multiferroicity is defined by the concomitant presence of several orders (like magnetism and
ferroelectricty) and a coupling between them. Such a remarquable property opens a wide
range of technological applications due to the possibility of controling the magnetization
through an electric field and reciprocally. One can quote the writing of magnetic information
(up and down spins as 0 and 1 bits) on hard disk drive using electric field instead of magnetic
fiels as it stands at present. This enable a considerable economy of energy and a significant
increase of the writing speed.
However, these applications are limited by the low temperature at which the strongest
couplings are observed, and by the absence of theoretical description for a microscopic
mechanism in most of these materials. This is the case for the family of compounds RMn2O5,
R standing as a rare earth element. We here propose to study the coupling between the several
degrees of freedom in these specific materials, namely the atomic structure, magnetism, and
dielectric properties. The aim is to identify the relevant theoretical model(s) to explain the
microscopic origin of the magneto-electric coupling. Among the proposed microscopic
mechanisms, here are the most commonly quoted as best candidates :
- the Dzyaloshinskii-Moriya interaction : in presence of non colinear magnetic moments (such
as a helicoidal spin order), this leads to a small displacement of the ligand (generally the
oxygen), moving the barycenter of the negative charges away from the positive charges
barycenter. This breaking of the inversion symmetry would induce a macroscopic electric
polarization.
- magnetostriction : due to magnetic frustration (unsatisfied magnetic interactions due to
geometrical arrangement of magnetic sites), the system may handle small displacement of the
magnetic sites in order to relax the frustration and then minimize its energy. These structural
changes may break the inversion symmetry and induce an electrical polarization.
- other more exotic models like spin dependent hybridization, valence changes are also
mentioned in the literature.
This PHD work will focus on mostly unstudied compounds with rare earth having large ionic
radius (R=Pr, Nd or Sm). It appears from preliminary measurements that their properties are
strongly peculiar compared to the known other family members. This offers the unique
possibility to investigate further the role of the rare earth on general properties and
particularly on the magneto-electric coupling. Experimental techniques intended to be used
are : X ray scattering (in synchrotron and laboratory);neutron scattering for the structural and
magnetic structures and dielectric constant and pyroelectric measurements in the laboratory. It
is also planed to look at the influence of external parameters (electric field, magnetic field or
pressure) at a longer term.
List significant keywords : Correlated electrons, multiferroicity
List required skills : phase transitions, magnetism