Abstract
Layered perovskite oxides A3B2O7 with Ruddlesden–Popper structures have attracted great attention due to their intriguing hybrid improper ferroelectricity. In this work, we prepared high-quality epitaxial Ca3Ti2O7 (CTO) thin films on Nb-SrTiO3 (STO) and Pt–Si substrates by pulsed laser deposition. The epitaxial relationship between the CTO thin film and Nb-STO substrate is [010]CTO//[010]Nb-STO, while the CTO film on Pt–Si is the polycrystalline growth with the preferred orientation of (010). The piezoelectric force microscopy image is attested to the stable existence of ferroelectricity in CTO films at room temperature with different growth patterns. A resistive switching behavior with at least two orders of magnitude is observed in CTO films driven by their ferroelectric polarization reversal. Moreover, a downward self-poling phenomenon is observed in CTO films on different substrates, and the flexoelectric effect induced by the strain gradient is supposed to be the main origin.
Highlights
INTRODUCTIONFerroelectric materials have been widely applied in electronic, sensing, and machinery products, including actuators, tunable microwave devices, and data storage devices. Improper ferroelectricity has appealed much interest due to its great potential applications in multiferroicity with strong magnetoelectric coupling at room temperature. The spontaneous electric polarization in the improper ferroelectricity can be induced by the non-polar mode, such as elastic instability, and is the second order parameter in the process of a phase transition. In 2011, Benedek and Fennie revealed a rich set of coupled structural, magnetic, and polar domains in A3B2O7 and ascribed the ferroelectricity to a combination of rotations and tilts of BO6 octahedral by first-principles calculations. It is known that a few orthorhombic A3B2O7 compounds have been formed in a polar structure, but due to the high energy barrier, the polarization switching has been rarely reported for these compounds
The hybrid improper ferroelectric has been experimentally confirmed in bulk crystals of Ca3Ti2O7 (CTO) and a small amount of Sr-doped Ca3Ti2O7.7 The saturation hysteresis loops and the ferroelectric domain structure with abundant charged domain walls have been observed at room temperature, and the ferroelectric domain switching path is thought to be through an intermediate phase with a−a0c0 tilting mode because of the unexpectedly low switching field measured experimentally compared to theoretical predictions
A series of CTO films with thicknesses in the range of 20–65 nm were obtained by adjusting the deposition time from 10 to 30 min, and the thickness for films was determined by x-ray reflectometry (XRR) using the same x-ray diffraction (XRD) setup and the program LEPTOS provided by Bruker AXS
Summary
Ferroelectric materials have been widely applied in electronic, sensing, and machinery products, including actuators, tunable microwave devices, and data storage devices. Improper ferroelectricity has appealed much interest due to its great potential applications in multiferroicity with strong magnetoelectric coupling at room temperature. The spontaneous electric polarization in the improper ferroelectricity can be induced by the non-polar mode, such as elastic instability, and is the second order parameter in the process of a phase transition. In 2011, Benedek and Fennie revealed a rich set of coupled structural, magnetic, and polar domains in A3B2O7 and ascribed the ferroelectricity to a combination of rotations and tilts of BO6 octahedral by first-principles calculations. It is known that a few orthorhombic A3B2O7 compounds have been formed in a polar structure, but due to the high energy barrier, the polarization switching has been rarely reported for these compounds. Shi et al prepared polycrystalline CTO thin films on Pt(111)/Ti/SiO2/Si(100) (Pt–Si) substrates by PLD and studied their ferroelectric domain switching, piezoelectric properties, and resistance switching behaviors.. Shi et al prepared polycrystalline CTO thin films on Pt(111)/Ti/SiO2/Si(100) (Pt–Si) substrates by PLD and studied their ferroelectric domain switching, piezoelectric properties, and resistance switching behaviors.11 Their experimental results of polarization reversal are not clear and evident for the proof of the ferroelectricity of CTO films at room temperature. A downward self-polarization forms in CTO films on different substrates with thickness ranging from 20 to 65 nm at room temperature, and the coupling between the polarization and strain gradient may be the main reason
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