Ellipsometry study of the infrared-active phonon modes in strained SrMnO3 thin films

We performed infrared and time-domain terahertz spectroscopic ellipsometry measurements of thin films of the perovskite antiferromagnetic insulator SrMnO3 that were grown by pulsed laser deposition (PLD) on LaAlO3, SrLaGaO4, and LSAT substrates which yield an epitaxial strain ranging from -0.3 to 1.7%. Taking these thin films as a representative example, we discuss the strategies for analyzing the ellipsometry spectra and extracting the information about the thin film dielectric response that can be equally applied to a variety of oxide based thin films and heterostructures. In particular, for the room temperature spectra we show that the three infrared-active phonon modes of the cubic perovskite structure of SrMnO3 undergo the expected softening with increasing tensile strain. For the SrMnO3 film on SrLaGaO4, we find that the low-energy (TO1) phonon mode reveals anomalous temperature dependence in the vicinity of the Neel temperature of about 170 K that signifies a strong spin-phonon coupling. For the SrMnO3 film on LSAT, we identify some irreversible changes of the infrared ellipsometry spectra that occur as the sample is heated to elevated temperature up to 560 K. These changes of the ellipsometry spectra have been attributed to a partial oxygen loss of the SrMnO3 thin film since they can be reverted with a post annealing treatment under high oxygen pressure.