Dynamical magneto-electric effects : probe and theory of hybride excitations – DYMAGE

The heart of the project is based on the use of complementary spectroscopic techniques such as inelastic neutron scattering, identifying the magnetic / nuclear nature of the observed excitations and their dispersion in reciprocal space and synchrotron based THz spectroscopy enabling the identification of the excitation rules of these modes in order to, ultimately, identify magnetoelectric excitations. Finally, the project includes the application of external parameters (magnetic and electric fields, pressure) on these excitations for their handling.

The prospective part on new materials has progressed (tasks 1 and 2). The compounds provided in the project as well as new compounds were synthesized and characterized (magnetization measurements, specific heat, electrical polarization, X-ray and neutron diffraction). Some prospective materials have proven to be non multiferroic / magnetoelectric and / or difficult to synthesize in the form of single crystals (Co-based honeycomb samples, delafossites) while new materials (pyroxene) were obtained, one of which is very promising for the study of electromagnons (SrMnGe2O6).
Spectroscopic measurements (inelastic neutron scattering and THz spectroscopy synchrotron) were performed on compounds of the hexagonal family RMnO3 (R = Y, Er, Ho). These experiments highlight dramatic effects in the magnetic excitations due to the magnetic coupling between the two magnetic species present in the material (rare earth and Mn). A software devoted to the calculation of the influence of this magnetic coupling was developed.
THz spectroscopy measurements and inelastic neutron scattering were also conducted on the iron langasite with Ta instead of Nb, to refine our understanding of this system in which we confirm the discovery of a magneto-electric excitation a new type. At this stage of interpretation, a very successful collaborative work is underway between experimentalists and theoreticians (symmetry absed phenomenological approaches and ab-initio calculations).

Attempts to apply an electric field in situ during THz spectroscopic measurements at SOLEIL have not been successful so far. However, neutron diffraction measurements under high magnetic field were conducted in the multiferroic CuO establishing its complex phase diagram. The continuation of these studies in the dynamic regime is forseen. Finally, the present possibility to apply pressure when measuring at SOLEIL motivated us to undertake studies with pressure as an external parameter. We focused on YMn2O5 in which an electric polarization is induced by pressure. We characterized the magnetic order associated with this transition by neutron diffraction. We plan to determine the modification of the magnetoelectric excitations associated with this phase transition by THz spectroscopy. This will require the optimization of the experimental setup to reach lower temperatures than are currently achievable.

1. L. Chaix, S. de Brion, S. Petit, R. Ballou, L.-­--P. Regnault, J. Ollivier, J.-­--B. Brubach, P. Roy, J. Debray, P. Lejay, A. Cano, E. Ressouche, and V. Simonet, “Magneto-­-- to electroactive transmutation of spin waves in ErMnO3”, Phys. Rev. Lett. 112, 137201 (2014).
2. C. Toulouse, J. Liu, Y. Gallais, M-­--A. Measson, A. Sacuto and M. Cazayous, L. Chaix, V. Simonet, S. de Brion, L. Pinsard-­--Godart, F. Willaert, J. B. Brubach and P. Roy, S. Petit, “Lattice and spin excitations in multiferroic h-­--YMnO3”, Phys. Rev. B 89, 94415 (2014).
3. M. Deutsch, I. Mirebeau, et al, “Evolution of the mazgnetic structure of YMn2O5 under pressure”, submitted to PRB
4. A high-­--pressure polymorph of LuFe2O4 with room temperature antiferromagnetic order F. Damay, M. Poienar, M. Hervieu, A. Guesdon, J. Bourgeois, T. Hansen, E. Elkai¨m, J. Haines, P. Hermet, L. Konczewicz, T. Hammouda, J. Rouquette and C. Martin, submitted to PRB

A strong interest is presently raised in material science by (i) the phenomena involving cross coupling between electric and magnetic degrees of freedom, to be precise magnetoelectric effects, and (ii) multiferroicity, which most often refers to the coexistence of electric and magnetic orders occurring either successively or simultaneously. The dynamical magnetoelectrical effect, whose interpretation is still far from complete, has been detected so far in the form of spin-lattice coupled excitations called electromagnons, but only in a few multiferroics. An incentive to investigate these, besides better understanding the fundamental mechanisms, comes from the recent proposal to use wave-like excitations in magnets as a mean to carry and process information as light waves in photonics, a route towards magnonics. Spin-wave-based devices could operate in the THz regime and have excellent integration in spintronics. The electric field control of spin waves is also particularly attractive for this kind of applications. A deep insight into the microscopic mechanisms however is mandatory to develop the correct strategy for the search and synthesis of the materials appropriate for the applications.

The DYMAGE ANR project will focus on the dynamical magnetoelectric effect from both experimental and theoretical sides. It will be conducted by teams most actively involved in the topic, with complementary expertises including theory and combining a wide spectrum of experimental means. More precisely, we propose several approaches:
(1) the use of complementary spectroscopic tools, part of which have to be technically developed: synchrotron TeraHertz spectroscopy that probes the excitations of matter by an electromagnetic radiation; Inelastic neutron scattering, ideally suited to identify lattice and magnetic excitations and their cross correlation and for probing their whole dispersion;
(2) the investigation of the dynamical effects under external parameters such as electric and magnetic fields. Particularly interesting is the possibility of inducing electromagnons in non ferroelectric magnets by the application of static electric fields, and more generally the impact of these fields on the spin wave excitations;
(3) a very strong contribution from theoreticians, for the prospective part in the identification and prediction of the magnetoelectric dynamical properties of new classes of materials, in particular non ferroelectric, on which the experimental works (1) and (2) will be performed. Theoretical work will also intervene in the interpretation of the experimental results obtained on these samples.

The proposed compounds are the members of the hexagonal RMnO3 familly, antiferromagnetic and ferroelectric, which show indication for the presence of electromagnons; the Fe langasite, which has recently been shown to support a new type of magnetoelectric hybrid excitation; some ferrotoroidic/magnetoelectric materials in which the possibility of specific dynamical magnetoelectric effects also might be anticipated. This is the case of several compounds presenting a honeycomb lattice of transition metal atoms, MnPS3 which has been identified as ferrotoroidic, members of the magnetoelectric family BaM2(XO4)2 (M=Co, Ni and X=P, As, V) and the Na3Co2SbO6 oxide, suspected to show ferrotoroidicity. Obviously, the sample synthesis and characterization part of the project is an important preliminary to the dynamical studies and will be fulfilled thanks to the devoted tools already available in the four laboratories.

It is expected that the work proposed in this project will allow unambiguously characterize the magnetoelectric excitations and discerning the selection rules for their observation by the different experimental means. The ultimate goal is to achieve a complete understanding of the microscopic mechanisms at the origin of these excitations and to contribute substantially to the emerging magnonic activity.