Strain enhanced ferroelectric properties of multiferroic BiFeO3/SrTiO3 superlattice structure prepared by radio frequency magnetron sputtering

•BiFeO3/SrTiO3 (BFO/STO) superlattice structure was fabricated with rf sputtering.•X-ray result shows the high quality of the BFO/STO superlattice structure.•An epitaxial relation between the film and substrate was conformed.•Strain of the BFO sublayer increase with decreasi the STO sublayer thickness.•A strong effect of the strain state on the ferroelectric properties was observed.

Asymmetric superlattice structures consisting of SrTiO3 (STO) and multiferroic BiFeO3 (BFO) sublayers were deposited on a Nb-doped STO (001) substrate with a radio frequency magnetron sputtering system. For an investigation of the effect of strain on the microstructures, ferroelectric properties and piezoelectric properties resulting from a difference of lattice parameters of the BFO sublayer, varying the thickness of the STO sublayer in a range 1–2.5 nm introduced interfacial strain into the superlattice. The epitaxial relation and strain state of films were clearly observed with a synchrotron as a source of radiation. The results of X-ray diffraction measurements clearly showed that the decrease of strain was related to the thickness of the STO substrate. A separate diffraction signal of the superlattice structure with thickness ≧ 1.75 nm of the STO sublayer was found, indicating a large strain relaxation in these superlattice films. The remnant polarization values Pr, piezoelectric coefficient d33 and leakage properties of the BFO/STO superlattice thin films that improved with increasing in-plane strain provide strong evidence of a strain-enhancement effect. The BFO/STO superlattice with a well defined crystal structure and a large strain state showed a large Pr, greater than a BFO single layer of the same thickness.