Domain Wall Orientations in Ferroelectric Superlattices Probed with Synchrotron X-Ray Diffraction

Ferroelectric domains in PbTiO$_3$/SrTiO$_3$ superlattices were studied using synchrotron X-ray diffraction. Macroscopic measurements revealed a change in the domain wall orientation from $\left\lbrace 100 \right\rbrace$ to $\left\lbrace 110 \right\rbrace$ crystallographic planes with increasing temperature. The temperature range of this reorientation depends on the ferroelectric layer thickness and domain period. Using a nanofocused beam, local changes in domain wall orientation within the buried ferroelectric layers were imaged, both in structurally uniform regions of the sample and near defect sites and argon ion etched patterns. Domain walls were found to exhibit preferential alignment with the straight edges of the etched patterns as well as with structural features associated with defect sites. The distribution of out-of-plane lattice parameters was mapped around one such feature, showing that it is accompanied by inhomogeneous strain and large strain gradients

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

Mapping Orthorhombic Domains with Geometrical Phase Analysis in Rare-Earth Nickelate Heterostructures

Most perovskite oxides belong to the Pbnm space group, composed by an anisotropic unit cell, A-site antipolar displacements and oxygen octahedral tilts. Mapping the orientation of the orthorhombic unit cell in epitaxial heterostructures that consist of at least one Pbnm compound is often required to understand and control the different degrees of coupling established at their coherent interfaces and, therefore, their resulting physical properties. However, retrieving this information from the strain maps generated with high-resolution scanning transmission electron microscopy can be challenging, because the three pseudocubic lattice parameters are very similar in these systems. Here, we present a novel methodology for mapping the crystallographic orientation in Pbnm systems. It makes use of the geometrical phase analysis algorithm, as applied to aberration-corrected scanning transition electron microscopy images, but in an unconventional way. The method is fast and robust, giving real-space maps of the lattice orientations in Pbnm systems, from both cross-sectional and plan-view geometries and across large fields of view. As an example, we apply our methodology to rare-earth nickelate heterostructures, in order to investigate how the crystallographic orientation of these films depends on various structural constraints that are imposed by the underlying single crystal substrates. We observe that the resulting domain distributions and associated defect landscapes mainly depend on a competition between the epitaxial compressive/tensile and shear strains, together with the matching of atomic displacements at the substrate/film interface. The results point towards strategies for controlling these characteristics by appropriate substrate choice.Comment: 32 pages, 5 figures, 2 table

Competition between Carrier Injection and Structural Distortions in Electron-Doped Perovskite Nickelate Thin Films

The discovery of superconductivity in doped infinite-layer nickelate thin films has brought increased attention to the behavior of the doped perovskite phase. Despite this interest, the majority of existing studies pertain to hole-doped perovskite rare-earth nickelate thin films, while most electron-doping studies have been performed on bulk materials so far. To tackle this imbalance, a detailed study that addresses doping of NdNiO thin films using A-site substitution is presented, using Pb as a dopant and taking advantage of its valence-skipping nature. Through a combination of complementary techniques including X-ray diffraction, transport measurements, X-ray absorption spectroscopy, electron energy-loss spectroscopy and scanning transmission electron microscopy, the valence of Pb in the NdPbNiO structure is confirmed to be 4+, and the behavior of the doped thin films is found to be controlled by a competition between carrier injection and structural distortions, which respectively reduce and increase the metal-to-insulator transition temperature. This work provides a systematic study of electron doping in NdNiO, demonstrating that A-site substitution with Pb is an appropriate method for such doping in perovskite rare-earth nickelate systems

Intrinsic magnetism in superconducting infinite-layer nickelates

The discovery of superconductivity in Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ [1] introduced a new family of layered nickelate superconductors that has now been extended to include a range of Sr-doping [2, 3], Pr or La in place of Nd [4-6], and the 5-layer Nd$_6$Ni$_5$O$_{12}$ [7]. A number of studies indicate that electron correlations are strong in these materials [8-14], and hence a central question is whether or not magnetism is present as a consequence of these interactions. Here we report muon spin rotation/relaxation studies of a series of superconducting infinite-layer nickelates. In all cases we observe an intrinsic magnetic ground state, regardless of the rare earth ion or doping, arising from local moments on the nickel sublattice. The coexistence of magnetism - which is likely to be antiferromagnetic and short-range ordered - with superconductivity is reminiscent of some iron pnictides [15] and heavy fermion compounds [16], and qualitatively distinct from the doped cuprates [17]

Ferroelectric domains in lead titanate heterostructures

This thesis focuses on the properties of thin films and multilayers of lead titanate (PbTiO3), an archetypal ferroelectric. Using off-axis radiofrequency magnetron sputtering, PbTiO3 thin films and superlattices are fabricated on different substrates, allowing control of the domain structures by tailoring electrostatic and mechanical boundary conditions. The domain structures and the macroscopic properties of these materials are then characterised using laboratory and synchrotron X-ray diffraction, piezoresponse force microscopy and dielectric impedance spectroscopy. The thesis initially focuses on synchrotron X-ray diffraction studies of ferroelectric domains in PbTiO3/SrTiO3 superlattices, periodic repetitions of ferroelectric PbTiO3 and dielectric SrTiO3 bilayers. It studies how the orientations of ferroelectric domain walls behave as a function of temperature across the ferroelectric phase transition and how they behave locally as a function of local strains and strain gradients (probed with a nanofocused X-ray beam), results that give more insight into their complex nature. The focus then turns to ferroelectric/metal superlattices (comprised of PbTiO3 and metallic SrRuO3 layers). It’s shown that even though the SrRuO3 layers remain metallic down to about four unit cells, the finite screening length at the interface between the metal and the ferroelectric creates a depolarising field which forces the PbTiO3 layers to adopt a polydomain configuration. These systems then allow us to study the macroscopic properties (e.g. electric permittivity) of ultrathin PbTiO3 layers, while at the same time making it possible to create novel domain configurations

Domain structure and dielectric properties of metal-ferroelectric superlattices with asymmetric interfaces

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Crystal growth and structure of a high temperature polymorph of Sr2TiO4

This data base contains the data shown in the figures of "Crystal growth and structure of a high temperature polymorph of Sr2TiO4 with tetrahedral Ti-coordination, and transition to the Ruddlesden-Popper tetragonal phase", published in CrystEngComm as doi:10.1039/D2CE00366

Crystal growth and structure of a high temperature polymorph of Sr₂TiO₄ with tetrahedral Ti-coordination, and transition to the Ruddlesden–Popper tetragonal phase

We have grown single crystals of a new polymorph of Sr 2 TiO 4 . It contains titanium in an unusual tetrahedral coordination and transforms to the Ruddlesden–Popper structure with an interesting orientational relationship. </p