Magnetodielectric coupling and phonon properties of compressively strained EuTiO3 thin films deposited on LSAT

TL;DR

This study investigates the magnetodielectric coupling and phonon behavior in compressively strained EuTiO3 thin films, revealing enhanced Neel temperature, phonon softening, and strong spin-phonon interactions influenced by magnetic fields.

Contribution

It provides new insights into the phonon properties and magnetodielectric effects in strained EuTiO3 thin films, supported by experimental infrared spectroscopy and first principles calculations.

Findings

Enhanced Neel temperature by 1 K under 0.9% compressive strain

Detection of a new polar phonon activated by phase transition

Observation of phonon softening and strong spin-phonon coupling in magnetic field

Abstract

Compressively strained epitaxial (001) EuTiO3 thin films of tetragonal symmetry have been deposited on (001) (LaAlO3)_0.29-(SrAl_{1/2}Ta_{1/2}O3)_0.71 (LSAT) substrates by reactive molecular-beam epitaxy. Enhancement of the Neel temperature by 1 K with 0.9% compressive strain was revealed. The polar phonons ofthe films have been investigated as a function of temperature and magnetic field by means of infrared reflectance spectroscopy. All three infrared active phonons show strongly stiffened frequencies compared to bulk EuTiO3 in accordance with first principles calculations. The phonon frequencies exhibit gradual softening on cooling leading to an increase in static permittivity. A new polar phonon with frequency near the TO1 soft mode was detected below 150 K. The new mode coupled with the TO1 mode was assigned as the optical phonon from the Brillouin zone edge, which is activated in infrared spectra due to an antiferrodistortive phase transition and due to simultaneous presence of polar and/or magnetic nanoclusters. In the antiferromagnetic phase we have observed a remarkable softening of the lowest-frequency polar phonon under an applied magnetic field, which qualitatively agrees with first principles calculations. This demonstrates the strong spin-phonon coupling in EuTiO3, which is responsible for the pronounced dependence of its static permittivity on magnetic field in the antiferromagnetic phase.