Tuning of the depolarization field and nanodomain structure in ferroelectric thin films

The screening efficiency of a metal-ferroelectric interface plays a critical role in determining the polarization stability and hence the functional properties of ferroelectric thin films. Imperfect screening leads to strong depolarization fields that reduce the spontaneous polarization or drive the formation of ferroelectric domains. We demonstrate that by modifying the screening at the metal-ferroelectric interface through insertion of ultrathin dielectric spacers, the strength of the depolarization field can be tuned and thus used to control the formation of nanoscale domains. Using piezoresponse force microscopy, we follow the evolution of the domain configurations as well as polarization stability as a function of depolarization field strength.Comment: 19 pages, 7 figure

Nanoscale domain engineering in SrRuO3_3 thin films

We investigate nanoscale domain engineering via epitaxial coupling in a set of SrRuO$_3$/PbTiO$_3$/SrRuO$_3$ heterostructures epitaxially grown on (110)$_o$-oriented DyScO$_3$ substrates. The SrRuO$_3$ layer thickness is kept at 55 unit cells, whereas the PbTiO$_3$ layer is grown to thicknesses of 23, 45 and 90 unit cells. Through a combination of atomic force microscopy, x-ray diffraction and high resolution scanning transmission electron microscopy studies, we find that above a certain critical thickness of the ferroelectric layer, the large structural distortions associated with the ferroelastic domains propagate through the top SrRuO$_3$ layer, locally modifying the orientation of the orthorhombic SrRuO$_3$ and creating a modulated structure that extends beyond the ferroelectric layer boundaries.Comment: 19 pages, 6 figures, supplementary materials. arXiv admin note: text overlap with arXiv:2304.0694

Full control of polarization in ferroelectric thin films using growth temperature to modulate defects

P.P. and C.W. acknowledge partial support by Swiss National Science Foundation Division II grant 200021_178782. L.R.D. acknowledges support from the US National Science Foundation under grant DMR‐1708615. L.W.M. acknowledges support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DE‐AC02‐05‐CH11231 (Materials Project program KC23MP) for the growth and study of defect structures in ferroic materials. A.B.N. gratefully acknowledges support from the Engineering and Physics Sciences Research Council (EPSRC) through grants EP/R023751/1 and EP/L017008/1.Deterministic control of the intrinsic polarization state of ferroelectric thin films is essential for device applications. Independently of the well-established role of electrostatic boundary conditions and epitaxial strain, the importance of growth temperature as a tool to stabilize a target polarization state during thin film growth is shown here. Full control of the intrinsic polarization orientation of PbTiO3 thin films is demonstrated-from monodomain up, through polydomain, to monodomain down as imaged by piezoresponse force microscopy-using changes in the film growth temperature. X-ray diffraction and scanning transmission electron microscopy reveal a variation of c-axis related to out-of-plane strain gradients. These measurements, supported by Ginzburg-Landau-Devonshire free energy calculations and Rutherford backscattering spectroscopy, point to a defect mediated polarization gradient initiated by a temperature dependent effective built-in field during growth, allowing polarization control not only under specific growth conditions, but ex-situ, for subsequent processing and device applications.Publisher PDFPeer reviewe

Switchable tribology of ferroelectrics

Switchable tribological properties of ferroelectrics offer an alternative route to visualize and control ferroelectric domains. Here, we observe the switchable friction and wear behavior of ferroelectrics using a nanoscale scanning probe—down domains have lower friction coefficients and show slower wear rates than up domains and can be used as smart masks. This asymmetry is enabled by flexoelectrically coupled polarization in the up and down domains under a sufficiently high contact force. Moreover, we determine that this polarization-sensitive tribological asymmetry is widely applicable across various ferroelectrics with different chemical compositions and crystalline symmetry. Finally, using this switchable tribology and multi-pass patterning with a domain-based dynamic smart mask, we demonstrate three-dimensional nanostructuring exploiting the asymmetric wear rates of up and down domains, which can, furthermore, be scaled up to technologically relevant (mm–cm) size. These findings demonstrate that ferroelectrics are electrically tunable tribological materials at the nanoscale for versatile applications.Peer ReviewedPostprint (published version

Assessing the Ubiquity of Bloch Domain Walls in Ferroelectric Lead Titanate Superlattices

The observation of unexpected polarization textures such as vortices, skyrmions, and merons in various oxide heterostructures has challenged the widely accepted picture of ferroelectric domain walls as being Ising-like. Bloch components in the 180° domain walls of PbTiO3 have recently been reported in PbTiO3/SrTiO3 superlattices and linked to domain wall chirality. While this opens exciting perspectives, the ubiquity of this Bloch component remains to be further explored. In this work, we present a comprehensive investigation of domain walls in PbTiO3/SrTiO3 superlattices, involving a combination of first- and second-principles calculations, phase-field simulations, diffuse scattering calculations, and synchrotron-based diffuse x-ray scattering. Our theoretical calculations highlight that the previously predicted Bloch polarization in the 180° domain walls in PbTiO3/SrTiO3 superlattices might be more sensitive to the boundary conditions than initially thought and is not always expected to appear. Employing diffuse scattering calculations for larger systems, we develop a method to probe the complex structure of domain walls in these superlattices via diffuse x-ray scattering measurements. Through this approach, we investigate depolarization-driven ferroelectric polarization rotation at the domain walls. Our experimental findings, consistent with our theoretical predictions for realistic domain periods, do not reveal any signatures of a Bloch component in the centers of the 180° domain walls of PbTiO3/SrTiO3 superlattices, suggesting that the precise nature of domain walls in the ultrathin PbTiO3 layers is more intricate than previously thought and deserves further attention

Ferroelectricity at the nanoscale : study of size effects in lead titanate thin films

Ce travail porte sur l'étude de l'évolution des propriétés physiques de matériaux ferroélectriques lorsque la taille de ces matériaux est réduite à l'échelle nanométrique. Cette étude représente un défi sur le plan expérimental puisque des films ultraminces de haute qualité cristalline sont nécessaires ainsi que des techniques adaptées à l'étude des propriétés physiques à cette échelle. C'est également un défi sur le plan théorique puisque des phénomènes nouveaux sont attendus à cette échelle et doivent être correctement interprétés. La tétragonalité de films minces de PbTiO3 a été mesurée par diffraction aux rayons x et aux photoélectrons. Sa diminution observée lorsque l'épaisseur des films est réduite, liée à une diminution de la polarisation, a permis de démontrer le rôle déterminant joué par l'écrantage imparfait de la polarisation ferroélectrique, responsable de l'apparition d'un champ dépolarisant et de la réduction de la polarisation dans les films très minces

InteractiveXRDFit: a new tool to simulate and fit X-ray diffractograms of oxide thin films and heterostructures

InteractiveXRDFit is a custom-made MATLAB program that calculates the X-ray diffracted intensity for oxide thin films and heterostructures based on a library of inorganic materials