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Register a new userChemical control of polarization in thin strained films of a multiaxial ferroelectric: phase diagrams and polarization rotation
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Added by
Anna Nickolaevna Morozovska
Publication date
2021
fields
Physics
and research's language is
English
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The emergent behaviors in thin films of a multiaxial ferroelectric due to an electrochemical coupling between the rotating polarization and surface ions are explored within the framework of the 2-4 Landau-Ginzburg-Devonshire (LGD) thermodynamic potential combined with the Stephenson-Highland (SH) approach. The combined LGD-SH approach allows to describe the electrochemical switching and rotation of polarization vector in the multiaxial ferroelectric film covered by surface ions with a charge density dependent to the relative partial oxygen pressure. We calculate the phase diagrams and analyze the dependence of polarization components on the applied voltage, and discuss the peculiarities of quasi-static ferroelectric, dielectric and piezoelectric hysteresis loops in thin strained multiaxial ferroelectric films. The nonlinear surface screening by oxygen ions makes the diagrams very different from the known diagrams of e.g., strained BaTiO3 films. Quite unexpectedly we predict the appearance of the ferroelectric reentrant phases. Obtained results point on the possibility to control the appearance and features of ferroelectric, dielectric and piezoelectric hysteresis in multiaxial FE films covered by surface ions by varying their concentration via the partial oxygen pressure. The LGD-SH description of a multiaxial FE film can be further implemented within the Bayesian optimization framework, opening the pathway towards predictive materials optimization.
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The ideal intrinsic barriers to domain switching in c-phase PbTiO_3 (PTO), PbZrO_3 (PZO), and PbZr_{1-x}Ti_xO_3 (PZT) are investigated via first-principles computational methods. The effects of epitaxial strain on the atomic structure, ferroelectric response, barrier to coherent domain reversal, domain-wall energy, and barrier to domain-wall translation are studied. It is found that PTO has a larger polarization, but smaller energy barrier to domain reversal, than PZO. Consequentially the idealized coercive field is over two times smaller in PTO than PZO. The Ti--O bond length is more sensitive to strain than the other bonds in the crystals. This results in the polarization and domain-wall energy in PTO having greater sensitivity to strain than in PZO. Two ordered phases of PZT are considered, the rock-salt structure and a (100) PTO/PZO superlattice. In these simple structures we find that the ferroelectric properties do not obey Vergards law, but instead can be approximated as an average over individual 5-atom unit cells.
After decades of searching for robust nanoscale ferroelectricity that could enable integration into the next generation memory and logic devices, hafnia-based thin films have appeared as the ultimate candidate because their ferroelectric (FE) polarization becomes more robust as the size is reduced. This exposes a new kind of ferroelectricity, whose mechanism still needs to be understood. Towards this end, thin films with increased crystal quality are needed. We report the epitaxial growth of Hf0.5Zr0.5O2 (HZO) thin films on (001)-oriented La0.7Sr0.3MnO3/SrTiO3 (STO) substrates. The films, which are under epitaxial compressive strain and are predominantly (111)-oriented, display large FE polarization values up to 34 {mu}C/cm2 and do not need wake-up cycling. Structural characterization reveals a rhombohedral phase, different from the commonly reported polar orthorhombic phase. This unexpected finding allows us to propose a compelling model for the formation of the FE phase. In addition, these results point towards nanoparticles of simple oxides as a vastly unexplored class of nanoscale ferroelectrics.
An oxide heterostructure made of manganite bilayers and ferroelectric perovskites is predicted to lead to the full control of magnetism when switching the ferroelectric polarizations. By using asymmetric polar interfaces in the superlattices, more electrons occupy the Mn layer at the $n$-type interface side than at the $p$-type side. This charge disproportionation can be enhanced or suppressed by the ferroelectric polarization. Quantum model and density functional theory calculations reach the same conclusion: a ferromagnetic-ferrimagnetic phase transition with maximal change $>90%$ of the total magnetization can be achieved by switching the polarizations direction. This function is robust and provides full control of the magnetizations magnitude, not only its direction, via electrical methods.
The crystal structure of BiFeO3/BaxSr1-xTiO3 (BFO/BST) heterostructures with x = 0.2, 0.6 and 0.8, grown on single-crystal MgO (001) substrate was investigated by x-ray diffraction and Raman spectroscopy in order to determine the influence of mismatch-induced strains and spontaneous polarization in BST buffer layers on BFO layers. The lattice parameter of the BFO layers was shown to decrease with increasing concentration of Ba ions, despite the increasing in-plain lattice parameters of tetragonal unit cells of BST layers. The rhombohedral angle of the crystal structure of BFO layers demonstrates an increase towards the ideal cubic perovskite structure with the appearance of the built-in electric field, induced by the spontaneous polarization in buffer layers. This result provides a remarkable tool for the control of polarization in BFO layers and other ferroelectric films in general, by changing the built-in electric field from ferroelectric buffer layer without changing a single crystal substrate.
The direction of ferroelectric polarization is prescribed by the symmetry of the crystal structure. Therefore, rotation of the polarization direction is largely limited, despite the opportunity it offers in understanding important dielectric phenomena such as piezoelectric response near the morphotropic phase boundaries and practical applications such as ferroelectric memory. In this study, we report the observation of continuous rotation of ferroelectric polarization in order-disorder type LiNbO3 thin films. The spontaneous polarization could be tilted from an out-of-plane to an in-plane direction in the thin film by controlling the Li vacancy concentration within the hexagonal lattice framework. Partial inclusion of monoclinic-like phase is attributed to the breaking of macroscopic inversion symmetry along different directions and the emergence of ferroelectric polarization along the in-plane direction.
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