Spin-phonon interaction increased by compressive strain in antiferromagnetic MnO thin films

Abstract

MnO thin films with various thicknesses and strains were grown on MgO substrates by pulsed laser deposition technique, then characterized using x-ray diffraction and infrared reflectance spectroscopy. Films grown on (0 0 1)-oriented MgO substrates exhibit homogenous biaxial compressive strain which increases as the film thickness is reduced. For that reason, in paramagnetic phase, the frequency of doubly-degenerate phonon increases with the strain, and splits below Néel temperature TN due to the magnetic-exchange interaction. The phonon splitting in the MnO (0 0 1) films is 20% larger than that of the bulk MnO. Films grown on (1 1 0)-oriented MgO substrates exhibit a huge phonon splitting already at room temperature due to the anisotropic in-plane compressive strain. Below TN, the lower-frequency phonon splits in the IR spectra and the higher-frequency phonon strongly hardens in AFM phase; these features are evidences for a spin-order-induced structural phase transition from tetragonal to a lower symmetry phase. Total phonon splitting is 55 cm−1 in (1 1 0)-oriented MnO film, which is more than twice the value in bulk MnO, but since the splitting is present already in paramagnetic phase, we cannot clearly correlate it with the value of exchange coupling constant. Nevertheless, at least observation of enhanced phonon splitting in strained MnO (0 0 1) films show that the exchange coupling could be enhanced by the compressive strain which supports recent theoretical predictions published by Wan et al (2016 Sci. Rep. 6 22743) and Fischer et al (2009 Phys. Rev. B 80 014408).