| Summary: |
This project aims at inducing high magnetoelectric coupled dynamical multiferroicity in RFeO3, R=Sm,Nd,Eu,Gd,Tb,Dy, thin films at room conditions, through
nonlinear phononics. We will take advantage of the epitaxial strain effect on spin-phonon and phonon-phonon couplings, and the phonon propagation by
atomically connected interfaces, towards the THz-electric field control of properties.
We propose to first perform a systematic characterization of center-zone phonon/magnon spectra of those RFeO3 thin films, to ascertain the level of
substrate/film connectivity, and study their magnetic/polar properties under applied magnetic field (up to 7T) in a broad temperature range (4-300K). Thereafter,
we will able to selectively excite optical phonons in nonlinear regime by ultrafast THz pulses to induce hidden magnetic/polar states through phononmagnetism changes and resonant magnon excitation, to reach a substantial enhancement of the magnetoelectric coupling at room conditions. For this purpose,
epitaxial RFeO3 (R=Sm,Nd,Gd,Tb,Dy) thin films will be deposited onto atomically terminated (001)-oriented LaAlO3 and SrTiO3 substrates. They exhibit FeO6
rotational/R-oscillations phonons and spin structure coupling, whose strength depends on R-cation (Annex A), allowing to tailor the energy landscape. It is worth
to stress that magnon frequencies lie close to those of optical phonons that control time-dependent transient magnetization. From their diverse magnetic
properties and spin-phonon coupling properties, those compounds are representative for the whole RFeO3 family.
The preliminary XRD results, obtained by the team in epitaxial SmFeO3 and NdFeO3 thin films (thickness below 100 nm) deposited by PLD in non-terminated
(001)-oriented LaAlO3 substrates, evidence that the films exhibit the same preferential growth direction along the b-axis, sharing rather similar structures.
Moreover, high mass differences between the moving atoms, and low-frequency optical phonon  |
Summary
This project aims at inducing high magnetoelectric coupled dynamical multiferroicity in RFeO3, R=Sm,Nd,Eu,Gd,Tb,Dy, thin films at room conditions, through
nonlinear phononics. We will take advantage of the epitaxial strain effect on spin-phonon and phonon-phonon couplings, and the phonon propagation by
atomically connected interfaces, towards the THz-electric field control of properties.
We propose to first perform a systematic characterization of center-zone phonon/magnon spectra of those RFeO3 thin films, to ascertain the level of
substrate/film connectivity, and study their magnetic/polar properties under applied magnetic field (up to 7T) in a broad temperature range (4-300K). Thereafter,
we will able to selectively excite optical phonons in nonlinear regime by ultrafast THz pulses to induce hidden magnetic/polar states through phononmagnetism changes and resonant magnon excitation, to reach a substantial enhancement of the magnetoelectric coupling at room conditions. For this purpose,
epitaxial RFeO3 (R=Sm,Nd,Gd,Tb,Dy) thin films will be deposited onto atomically terminated (001)-oriented LaAlO3 and SrTiO3 substrates. They exhibit FeO6
rotational/R-oscillations phonons and spin structure coupling, whose strength depends on R-cation (Annex A), allowing to tailor the energy landscape. It is worth
to stress that magnon frequencies lie close to those of optical phonons that control time-dependent transient magnetization. From their diverse magnetic
properties and spin-phonon coupling properties, those compounds are representative for the whole RFeO3 family.
The preliminary XRD results, obtained by the team in epitaxial SmFeO3 and NdFeO3 thin films (thickness below 100 nm) deposited by PLD in non-terminated
(001)-oriented LaAlO3 substrates, evidence that the films exhibit the same preferential growth direction along the b-axis, sharing rather similar structures.
Moreover, high mass differences between the moving atoms, and low-frequency optical phonons with high Born effective charge tensor tailor the amplitude of
the driven phonon mode. The combination of these factors enhances both ME effect and induced magnetization [17].
Once detailed experimental characterization of film structure, optical phonons and magnons is obtained, the phonon-driven reconfiguration of the energy
landscape will be analyzed through DFT calculations and symmetry considerations. The correspondence between energy landscape and phonon amplitude will
be ascertained towards stimulating both polar distortions and spin structure reconfiguration. The results will allow to select the best phonons and experimental
conditions to achieve both coupled polar lattice distortions and magnetic states.
Epitaxial thin films will be deposited by PLD. High quality RFeO3 targets are already available. (001)-oriented LaAlO3 and SrTiO3 substrates will be purchased
and simple chemical etching will be used to obtain terminated surfaces [26]. These substrates are suitable for transmission optical measurements as they
are transparent in the 400-800nm range (Annex B). Moreover, LaAlO3 [001] direction matches the optical axis, avoiding birefringence effects in optical
Research plan and methods
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measurements, while SrTiO3 being cubic is optically isotropic. The proposed (001)-orientation for both substrates also facilitates a chemical etched LaO surface
for LaAlO3, or SrO surface for SrTiO3 that ensures FeO6/AlO6 or FeO6/TiO6 octahedral film/substrate connectivity.
The structure, preferential growth orientation, strain state and morphology of the as-prepared thin films will be studied by XRD, XRR, pole figures, Raman
spectroscopy and imaging, SEM and AFM. Film thickness will be kept below 80 nm, ensuring non-relaxed film structures.
The best films will be then characterized by high resolution electron microscopy and associated spectroscopic techniques. The obtained results are of crucial
importance, as both film structure and substrate/films interface quality are key parameters to ensure success in tailoring lattice/magnetic excitations, and to
yield information regarding phonon/magnon symmetries, interface mediated connectivity and distortions (Annex B).
Once accomplished the tasks referred to above, the project will be divided into two main steps:
A- Systematic studies of Raman scattering, THz absorption and FTIR spectroscopic measurements will be carried out versus strain, temperature and applied
magnetic/electric fields, in collaboration with Dr S. Kamba, Institute of Physics, Czech Academy of Sciences, towards the characterization of zone-center phonon
and/or magnon spectra. This will give a full picture of phonon-phonon/spin-phonon coupling, Born effective charge tensor, principal refractive indices, and
optical phonons/symmetry-allowed lattice distortions relation. Complementary dielectric, polar and magnetic measurements will be done to characterize
physical properties. The obtained results will be key inputs for theoretical models to unravel coupling mechanisms, and other crucial ingredients for future work.
DFT calculations will be carried out versus applied fields, epitaxial strain, and coupling strength.
B. The next step concerns the study of dynamical induced states. We will take advantage of the knowledge already obtained from the coupling between spin
structure and non-degenerated FeO6 rotational (tilt) and R-oscillations lattice phonons (Annex A).
Two different approaches are envisaged:
a) ultrafast tuning of the magnetic structure by driving optical phonons. The symmetry-allowed lowest order coupling is the quadratic-linear term between two
infrared-active and one Raman-active phonons. As the coupling between the magnetic potential and the phonon amplitude depends on the R-cation, we will be
able to play with different atomic motions to drive different magnetic states, associated with different polar lattice distortions.
b) resonant magnon excitation by optically driven phonons. In this case, the excitation involves two perpendicularly linearly polarized infrared-active phonons
coupled with a low lying magnon. The time-dependent electric polarization driven by the excitation of polar phonons induces a magnetization whose amplitude
depends on the excited phonon amplitudes and on the phonon frequency difference. The latter one will enable us to improve the use of high energy THz pulses
to achieve enough high magnetization amplitude, provided that the excited phonon frequencies are close together, and their frequency difference lies near the
frequency of a magnon excitation |