Ferroelectricity and Self-Polarization in Ultrathin Relaxor Ferroelectric Films.

Peixian Miao,Aitian Chen,Yonggang Zhao,Huiyun Zhang,Meihong Zhu,Qiang Li,Nengneng Luo,Diyang Zhao,Meiqi Guo,Zhong Sun

doi:10.1038/srep19965

Abstract

We report ferroelectricity and self-polarization in the (001) oriented ultrathin relaxor ferroelectric PMN-PT films grown on Nb-SrTiO3, SrRuO3 and La0.7Sr0.3MnO3, respectively. Resistance-voltage measurements and AC impedance analysis suggest that at high temperatures Schottky depletion width in a 4 nm thick PMN-PT film deposited on Nb-SrTiO3 is smaller than the film thickness. We propose that Schottky interfacial dipoles make the dipoles of the nanometer-sized polar nanoregions (PNRs) in PMN-PT films grown on Nb-SrTiO3 point downward at high temperatures and lead to the self-polarization at room temperature with the assistance of in-plane compressive strain. This work sheds light on the understanding of epitaxial strain effects on relaxor ferroelectric films and self-polarization mechanism.

Highlights

(1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) is a very important system of relaxor ferroelectrics. 0.67PMN0.33 PT single crystals show large electromechanical coupling coefficient k33 (~94%) and piezoelectric constant d33 (~2800 pC/N)[20], and very large electromechanical coupling were observed in a 0.67PMN-0.33 PT film cantilever[21]
We found that ferroelectric domains change from the multi-domain state to the single domain state with decreasing film thickness (Later we will see that the dark color represents downward self-polarization)
Self-polarization in PMN-PT films is likely induced by the dipoles in PMN-PT/electrode interface, which is determined by the work functions of PMN-PT and the bottom electrode

Summary

Introduction

(1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) is a very important system of relaxor ferroelectrics. 0.67PMN0.33 PT single crystals show large electromechanical coupling coefficient k33 (~94%) and piezoelectric constant d33 (~2800 pC/N)[20], and very large electromechanical coupling were observed in a 0.67PMN-0.33 PT film cantilever[21]. We fabricated PMN-PT thin films grown on SrRuO3 (SRO)/ SrTiO3 (STO) structure by pulsed laser deposition (PLD) with thicknesses ranging from 10 to 600 nm, and found that with decreasing film thickness the ferroelectric domains change from the multi-domain state to the single domain state with downward self-polarization. In order to uncover the self-polarization mechanism, we fabricated ultrathin PMN-PT films grown on NSTO substrate with thickness ranging from 2 to 30 nm to analyze the variation of the depletion width of PMN-PT/NSTO barrier at different temperatures. We found that downward self-polarization forms in PMN-PT(4 nm)/NSTO sample at room temperature and the temperature of the dielectric response peak for the sample is higher than that for the single crystal. Resistance-voltage measurements and AC impedance analysis for PMN-PT (4 nm)/NSTO sample suggest that Schottky depletion width at high temperatures is smaller than film thickness. This work sheds light on the understanding of epitaxial strain effects on relaxor ferroelectric films, as well as self-polarization mechanism

Methods

Results

Conclusion