Ferroelectricity in ultrathin perovskite films

Abstract

We report studies of ferroelectricity in ultra-thin perovskite films with realistic electrodes. The results reveal stable ferroelectric states in thin films less than 10Å thick with polarization normal to the surface. Under short-circuit boundary conditions, the screening effect of realistic electrodes and the influence of real metal/oxide interfaces on thin film polarization are investigated. Our studies indicate that metallic screening from the electrodes is affected by the difference in work functions at oxide surfaces. We demonstrate this effect in ferroelectric PbTiO3 and BaTiO3 films.The effect of size on thin-film ferroelectricity has been known for a long time, but has not been completely understood. Initial experiments and mean field calculations based on the Landau theory suggested that below a critical correlation volume 1 of electrical dipoles between 10-100 nm 3 , ferroelectricity vanishes due to intrinsic size effects. 2,3 For thin films with the polar axis perpendicular to the surface, incomplete compensation of surface charges creates a depolarizing field that has been shown to further reduce the polarization stability. 4,5 Recently, however, monodomain ferroelectric phases have been observed in very thin films, below ten unitcells thick. 6,7,8 Furthermore, Fong et al. showed that ferroelectric phases can be stable down to ∼12Å (three unit cells) in PbTiO 3 films by forming 180 • stripe domains, suggesting that no fundamental thickness limit is imposed by the intrinsic size effect in thin films. 9 This idea has been corroborated by ab initio calculations carried out on perovskite films, which tell that no critical thickness exists for polarization parallel to the surface 10 and that polarization perpendicular to the surface can exist in films three unit-cells thick if the depolarization field is artificially removed. 11,12 On the other hand, it has been found that the depolarization field plays a dominant role in reducing polarization normal to the surface and depressing ferroelectric transition temperatures in thin films. In a continuum model by Batra et al. 4 which includes the depolarization effect, a critical thickness of 100Å for perovskite films was analytically derived, assuming a Thomas-Fermi screening length of 1Å for the metal electrodes. Also, a recent first-principles calculation revealed that BaTiO 3 films with SrRuO 3 electrodes lose ferroelectricity below ∼24Å (6 unit cells), 13 thus suggesting that a minimum thickness limit exists for useful ferroelectric films. While indeed a minimum film thickness must be influenced by the polarization of the ferroelectrics and the screening length of the electrodes, 14 it is not yet clear whether the depolarizing field can ever be completely removed by realistic electrodes on ultrathin films, nor how monodomain thin film ferroelectricity is affected by the choice of electrodes and by the interactions at the metal/oxide interface.In this paper, we provide answers to these questions.In particular, we investigate how the critical thickness varies ...

Multi-ferroic and magnetoelectric materials and interfaces

“…The presence of interfaces imposes restrictions on the atomic displacements since the atoms at the boundary of the FE are bonded to the electrodes. This effect was demonstrated in KNbO 3 FE films placed between SrRuO 3 or Pt electrodes [160], as well as BaTiO 3 with SrRuO 3 [161,162], Fe [116] and Pt [163] electrodes. First-principles transport calculations for Pt/BaTiO 3 /Pt FTJs show that the interface transmission function [164] of the Pt/BaTiO 3 interface differs by a factor of three depending on the polarization direction [159].…”

Ferroelectric or non-ferroelectric: Why so many materials exhibit “ferroelectricity” on the nanoscale

“…Correspondingly, in the beginning of XXI century the main model for ferroelectric screening has become external screening by surface ionic charges, as explored by SPM, 74,76 X-ray 78,228 and density functional theory. 235 The direct visualization of the mobile surface charges was performed using time-resolved Kelvin Probe Microscopy. 236,237 The necessary consequence of the ionic screening model is that polarization switching necessitates the redistribution of the screening charges, and overall polarization dynamics is now controlled both by ferroelectric and ionic subsystems.…”

Effect of spin-dependent screening on tunneling electroresistance and tunneling magnetoresistance in multiferroic tunnel junctions

“…1 A FTJ consists of two metal electrodes separated by a nm-thick ferroelectric barrier which allows electron tunneling through it. Recent experimental 2-4 and theoretical [5][6][7][8] studies of perovskite ferroelectric oxide thin films have demonstrated that ferroelectricity persists down to a nanometer scale, which makes it possible to use ferroelectrics as functional tunnel barriers in FTJs. Contrary to ferroelectric capacitors where leakage currents are detrimental to the device performance, 9 the conductance of a FTJ is the functional characteristic of the device.…”