Strain‐driven oxygen deficiency in multiferroic SrMnO3 thin films

Abstract

In the last decade, multiferroic materials exhibiting simultaneous magnetic and ferroelectric order with strong magnetoelectric coupling above room temperature have attracted great interest. This is motivated by the expectation of integrating them into low‐power spintronic nanodevices, where the magnetic order is controlled by low energy consuming electric fields instead of magnetic fields. Manganese‐based perovskite oxides are particularly promising for this purpose. Recently, it was suggested that strain‐driven multiferroicity associated with spin‐phonon coupling should arise in strained SrMnO 3 (SMO): under epitaxial strain a polar instability in the ferromagnetic phase leads to a substantial energy lowering, which stabilizes the ferromagnetic‐ferroelectric multiferroic phase over the bulk antiferromagnetic‐paraelectric phase [1]. Thus, the spontaneous polarization is driven by the off‐centering of the magnetic Mn 4+ cations from the MnO 6 octahedra and increases by expansion of the lattice. However, the SMO structure tends to incorporate oxygen vacancies, which makes it very difficult to synthesize fully stoichiometric SMO samples. Moreover, for SMO films, a small biaxial tensile strain of 2% is predicted to increase the oxygen vacancy concentration by one order of magnitude at room temperature [2]. Understanding the role of the oxygen non‐stoichiometry in this material is of fundamental importance for the development of multiferroic SMO thin films as perfectly stoichiometric highly resistive samples are required. Here, we focus on the structural and electronic properties of several SMO thin films grown on different substrates, namely LSAO, LAO, LSAT, STO and DSO, by using aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) and electron energy loss spectroscopy (EELS). The structural parameters of the SMO films are obtained by geometrical phase analysis of HAADF‐STEM images and are plotted as a function of nominal strain in Fig. 1. EELS data were acquired for all the samples: the obtained oxygen K‐ edge spectra show pronounced peak‐height changes and energy shifts as a function strain (Fig. 2). The experimental results are interpreted with the aid of the all‐electron density functional theory (DFT) code WIEN2k (see Fig. 3). Thus, the effect of oxygen vacancies on the O K and Mn L 2,3 electron energy‐loss near‐edge structures (ELNES) is theoretically investigated [3].