Magnetoelectric coupling through Electromagnons in Multiferroic compounds – EMMA

We measure different iron borat compound with different rare earth to observe the influence of this rare earth in the magnetism. So we mesure by infrared these samples to try to observe the electromagnons.

We have observed a new peak at low frequency and low temperature along the a axis. We have also observe a phonon mode along c with a behavior of soft mode signature of the ferroelctricity..

Determine the nature of the new peak with measurement under magnetic field and inelastique neutron scattering.

The results will be presented in the next LEES conference in June.

The present project dwells on the recent surge of interest in a family of very attractive materials in which both ferroelectric and magnetic orders are present. These materials, known as multiferroics, have attracted much attention worldwide because of their large magnetoelectric effects as compared with previously studied materials. Multiferroics open a myriad of possibilities in spintronics applications as tuning the polarization direction with a magnetic field and/or change the magnetization direction via an applied voltage. A particularly exciting prospect in the field of spintronics is to use the wave like excitations of a magnetic material as a means to transmit and process information. This technology named magnonics relies on the control of spin waves just as optical waves are manipulated in photonics technologies. One of the project goals is to electrically control the particles signature of the magnetoelectric coupling for a better understanding of this coupling in perspective of application in new magnonic devices providing a framework to realize low power and non-volatile magnetoelectric devices.
There are two classes of multiferroics. Those of type I, such as BiFeO3, which show almost independent ferroelectric and magnetic orders with a small magnetoelectric coupling, and the type II multiferroics, such as TbMnO3, where ferroelectricity is induced by magnetism.
The magnetic ferroelectricity in this type is intimately tied to an enhanced magnetoelectric coupling and the formation of new excitations named electromagnons. Nevertheless, the mechanism underneath this ferroelectric order is largely controversial. Two major microscopic theoretical approaches exist to explain the formation of the ferroelectric order. One attempt to explain the formation of ferroelectricity through an ionic displacement induced by a spiral magnetic order, the other one through an electronic cloud displacement created by spin currents.
The project is focused on the study of a wide range of type II multiferroic materials, with infrared and neutron scattering. Because of ability of infrared scattering to probe excitations with electric charge as phonons and more interestingly electromagnons, this technique is uniquely suited to determine the charge transfer between these excitations at the phase transition and the unique method to identify the ferroelectric phase transition. Moreover, using neutron scattering, which probes the magnon and phonon dispersion curves, we will have a powerful overview of multiferroic excitations. In this case, while most of the studies have been focused on the static magnetoelectric effects, we specifically aim at probing the dynamical coupling between spin and lattice degrees of freedom. By study these materials under various external parameters such as magnetic and electric field, we will be able to tune both magnetic and ferroelectric orders and simultaneously study their impact on lattice and spin degrees of freedom in search for novel dynamical effects. Indeed, the electromagnons might be unique to this type of materials.
The basic issues we want to address are: (i) What is the origin of the improper ferroelectricity? (ii) What is the role, if any, of the crystalline lattice? Is the magnetoelectric coupling a direct interaction between charges and spin degrees of freedom or does the lattice plays a mingling role? (iii) Is the electromagnon a fundamental new excitation or a hybrid magnon-phonon particle? What is the effect of external fields upon these fundamental excitations?
The outcome of our proposal is to create a comprehensive understanding of the fundamental excitation in multiferroics, determining those that are the cause and those that are the consequence of the dielectric and magnetic properties interplay. We aim at settling definite constraints on the proposed theories for the magnetoelectric coupling and the improper ferroelectricity in these systems.