Spin-phonon coupling in epitaxial Sr0.6Ba0.4MnO3 thin films

Spin-phonon coupling is investigated in epitaxially strained $\mathrm{S}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{a}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ thin films with perovskite structure by means of microwave (MW) and infrared (IR) spectroscopy. In this work we focus on the $\mathrm{S}{\mathrm{r}}_{0.6}\mathrm{B}{\mathrm{a}}_{0.4}\mathrm{Mn}{\mathrm{O}}_{3}$ composition grown on ${(\mathrm{LaAl}{\mathrm{O}}_{3})}_{0.3}{(\mathrm{S}{\mathrm{r}}_{2}\mathrm{AlTa}{\mathrm{O}}_{6})}_{0.7}$ substrate. The MW complex electromagnetic response shows a decrease in the real part and a clear anomaly in the imaginary part around 150 K. Moreover, it coincides with a $17%$ hardening of the lowest-frequency polar phonon seen in IR reflectance spectra. In order to further elucidate this phenomenon, low-energy muon-spin spectroscopy was carried out, signaling the emergence of antiferromagnetic order with N\'eel temperature (${T}_{N}$) around 150 K. Thus, our results confirm that epitaxial $\mathrm{S}{\mathrm{r}}_{0.6}\mathrm{B}{\mathrm{a}}_{0.4}\mathrm{Mn}{\mathrm{O}}_{3}$ thin films display strong spin-phonon coupling below ${T}_{N}$, which may stimulate further research on tuning the magnetoelectric coupling by controlling the epitaxial strain and chemical pressure in the $\mathrm{S}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{a}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ system.