Spin-phonon coupling is investigated in epitaxially strained $\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{B}{\mathrm{a}}_x\mathrm{Mn}{\mathrm{O}}_3$ 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 ${\left(\mathrm{LaAl}{\mathrm{O}}_3\right)}_{0.3}{\left(\mathrm{S}{\mathrm{r}}_2\mathrm{AlTa}{\mathrm{O}}_6\right)}_{0.7}$ 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 Néel 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 $\mathrm{S}{\mathrm{r}}_{1-x}\mathrm{B}{\mathrm{a}}_x\mathrm{Mn}{\mathrm{O}}_3$ system.

• Received 22 August 2016
• Revised 23 December 2016

DOI:https://doi.org/10.1103/PhysRevB.95.075126