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Strain-Modulated Ferroelectricity in SrMnO3 Thin Films via In Situ Strain Engineering
SeongMin Park1,Chang Jae Yoon1,Gopinathan Anoop1,Tae Yeon Kim1,Young-Min Kim2,Ji Young Jo1
Gwangju Institute of Science and Technology1,Sungkyunkwan University2
Strain engineering especially in epitaxial films of complex oxides plays an important role in determining structural and physical characteristics. With the epitaxial strain, even paraelectric perovskite oxides can possess ferroelectricity that does not exist in a bulk state [1–3]. Density functional theory (DFT) has predicted that the ferroelectricity could be induced in SrMnO3, which is paraelectric cubic phase in bulk, exhibiting a high spontaneous polarization driven by off-centered Mn4+ ion under tensile strain larger than 1% . Moreover, an abrupt increase of spontaneous polarization exists around 2% misfit strain. The SrMnO3 (SMO) films have been experimentally studied grown on various substrates [4–8]; however, complicated growth condition and consequent crystalline characteristics depending on the substrate has been an obstacle to investigate a misfit strain effect onto the induced ferroelectricity of SrMnO3 films. Here, we report in-situ strain engineering of SrMnO3 film using a piezoelectric substrate, e.g., 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 (PMN-PT), in order to manipulate the spontaneous polarization in an SrMnO3 epitaxial thin film while applying voltage to the PMN-PT substrate. In this work, we show that epitaxially grown SrMnO3 films on (110)-oriented PMN-PT possess the spontaneous polarization along  direction which can be modulated by the epitaxial strain transfered from the piezoelectric strain according to applied voltage on substrate.
The SrMnO3 films with a thickness of approximately 100 nm were deposited onto La0.7Sr0.3MnO3 (LSMO) / (110)-oriented PMN-PT substrates using pulsed laser deposition (PLD) technique. The LSMO layers with a thickness of 20 nm were grown directly on the PMN-PT substrate to play roles of a bottom electrode as well as a buffer layer to reduce the large lattice mismatch between SMO and PMN-PT (~5.39%). The substrate temperature was kept at 700°C and oxygen partial pressure was controlled in a range from 1 to 200 mTorr. The Pt top electrodes with a size of 100 μm were deposited using an e-beam evaporator. We will discuss spontaneous polarization and crystallinity of films with respect to the misfit strain of films.
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