Persistence of ferroelectricity above the Curie temperature at the surface of Pb(Zn1/3Nb2/3)O3-12%PbTiO3

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).Relaxor-based ferroelectrics have been known for decades to possess a relatively thick surface layer ("skin") that is distinct from its interior. Yet while there is consensus about its existence, there are controversies about its symmetry, phase stability, and origin. In an attempt to clarify these issues, we have examined the surface layer of PZN-12%PT. While the bulk transitions from a ferroelastically twinned tetragonal ferroelectric state with in-plane polarization to a cubic paraphase at Tc=200C, the skin layer shows a robust labyrinthine nanodomain structure with out-of-plane polarization that persists hundreds of degrees above the bulk Curie temperature. Cross-sectional transmission electron microscopy analysis shows that the resilience of the skin's polarization is correlated with a compositional imbalance: lead vacancies at the surface are charge-compensated by niobium enrichment; the excess of Nb5+ - a small ion with d0 orbital occupancy - stabilizes the ferroelectricity of the skin layer.We acknowledge financial support from the Spanish Ministerio de Ciencia e Innovación (Contracts No. MAT2010-17771, No. MAT2011-29081-C02, and No. FIS2013-48668- C2-1-P), and the Generalitat de Catalunya (Project 2014 SGR 1216). N.D. thanks the Spanish Ministerio de Ciencia e Innovación for the Ramon y Cajal Research Grant (No. RYC-2010-06365), and G.C. acknowledges an ERC Starting Grant (Project reference: ERC-SG-308023). ICN2 acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295).Peer Reviewe

Elastic and anelastic relaxations in the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3: I. Strain analysis and a static order parameter

The structural evolution of Pb(Mg 1/3Nb 2/3)O 3 (PMN) has been reviewed in terms of characteristic temperatures, length scales and timescales, with a view to considering the overall relaxor behaviour from the perspectives of strain and elasticity. A conventional analysis of lattice parameter data in terms of spontaneous strain and strain/order parameter coupling shows that even though a normal phase transition does not occur the relaxor ordering process is accompanied by a significant volume strain which follows the pattern of a static order parameter evolving according to that expected for a tricritical phase transition with T c350K. This matches the evolution of the intensity of the elastic central peak in neutron scattering spectra, and reflects the development of static (or quasistatic) polar nanoregions (PNRs) as if by a mean-field phase transition. Use of a Landau free energy expansion, which includes order parameter components to describe ferroelectric contributions and an order parameter to describe cation ordering together with their formal coupling with strain, then allows the pattern of elastic softening expected for a cubic rhombohedral phase transition to be anticipated. The extent to which observed softening differs from this static mean-field pattern serves to highlight the additional roles of local heterogeneity and relaxation dynamics in determining the relaxor properties of PMN. 2012 IOP Publishing Ltd.Peer Reviewe

Direct Visualization of Anti-Ferroelectric Switching Dynamics via Electrocaloric Imaging

The large electrocaloric coupling in PbZrO allows using high-speed infrared imaging for visualizing anti-ferroelectric switching dynamics via the associated temperature change. It is found that in ceramic samples of homogeneous temperature and thickness, switching is fast due to the generation of multiple nucleation sites, with devices responding in the millisecond range. By introducing gradients of thickness, however, it is possible to change the dynamics to propagation limited, whereby a single-phase boundary sweeps across the sample like a cold front, at a speed of ≈20 cm s. Additionally, introducing thermostatic temperature differences between two sides of the sample enables the simultaneous generation of a negative electrocaloric effect on one side and a positive one on the other, yielding a Janus-like electrocaloric response.The authors acknowledge financial support to ICN2, which is funded by the CERCA programme/Generalitat de Catalunya and by the Severo Ochoa programme of the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. SEV-2017-0706). The authors also acknowledge the support of Plan Nacional (MINECO, Grant Nos. MAT2016-77100-C2-1-P and BES-2016-077392), as well as the Agencia Estatal de Investigacion (Grant No. PID2019-108573GB-C21). R.F. and E.D. thank the Luxembourg National Research Fund (FNR) for funding part of this research through the projects CAMELHEAT/C17/MS/11703691/Defay. This work was also supported in part by the Spanish Ministry of Science, Innovation and Universities under the HIPERCELLS project (RTI2018-098392-B-I00), the Regional Government of the Generalitat de Catalunya under Grant Nos. 2017 SGR 1384 and 2017 SGR 00579. This work was also supported by the National Science Centre, Poland, within the Project No. 2016/21/B/ST3/02242

Switchable tribology of ferroelectrics

Artificially induced asymmetric 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 where down domains having lower friction coefficient than up domains can be used as smart masks as they show slower wear rate than up domains. 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 universal across ferroelectrics with different chemical composition 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 establish that ferroelectrics are electrically tunable tribological materials at the nanoscale for versatile applications.Peer ReviewedPostprint (author's final draft

Weak low-temperature polarity in a PbZr O3 single crystal

We report the existence of a low-temperature polar phase in PbZrO3 below 270 K. The temperature dependence of resultant polarization was assigned from pyroelectric current changes measured after poling the single crystal or ceramic in a DC electric field. This was observed in single crystals and ceramics and may have a connection with the presence of polar (ferrielectric) antiphase boundaries inside the antiferroelectric phase. The transition point is demonstrated by changes in domain structure and anomalies in dielectric permittivity and losses in as-grown single crystals and ceramic samples.This research was funded by the National Science Centre, Poland, with Grant No. 2020/37/B/ST3/02015.Peer reviewe

Supplemental Material of the article Weak low-temperature polarity in a PbZrO3 single crystal

Supplement Material 1 describes in detail the experimental techniques used. -- Supplement Material 2 presents a film showing changes in indicatrix orientations during the low-temperature transition in PbZrO3 single crystals.Weak low-temperature_supplementary_material_1.pdfPeer reviewe

Mechanical Tuning of LaAlO3/SrTiO3 Interface Conductivity

Sharma, P. et al.In recent years, complex-oxide heterostructures and their interfaces have become the focus of significant research activity, primarily driven by the discovery of emerging states and functionalities that open up opportunities for the development of new oxide-based nanoelectronic devices. The highly conductive state at the interface between insulators LaAlO3 and SrTiO3 is a prime example of such emergent functionality, with potential application in high electron density transistors. In this report, we demonstrate a new paradigm for voltage-free tuning of LaAlO3/SrTiO3 (LAO/STO) interface conductivity, which involves the mechanical gating of interface conductance through stress exerted by the tip of a scanning probe microscope. The mechanical control of channel conductivity and the long retention time of the induced resistance states enable transistor functionality with zero gate voltage.This research was supported by the Materials Research Science and Engineering Center (NSF grant DMR-0820521). The work at University of Wisconsin-Madison was supported by the NSF grant DMR-1234096 and the AFOSR grant FA9550-12-1-0342. G. C. acknowledges financial support from an European Research Council (ERC) Starting Grant. First-principles calculations were performed at the University of Nebraska Holland Computing Center and at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.N

Skin layer of BiFeO3 single crystals

4 páginas, 4 figuras.-- PACS numbers: 68.35.B-, 61.05.cp, 68.35.Rh, 75.85.+t.-- et al.A surface layer (“skin”) different from the bulk was found in single crystals of BiFeO3. Impedance analysis and grazing incidence x-ray diffraction reveal a phase transition at T*∼275±5 °C that is confined within the surface of BiFeO3. X-ray photoelectron spectroscopy and refraction-corrected x-ray diffraction as a function of incidence angle and photon wavelength indicate a reduced electron density and an elongated out-of-plane lattice parameter within a few nanometers of the surface. The skin will affect samples with large surface to volume ratios, as well as devices that rely on interfacial coupling such as exchange bias.We acknowledge financial support of the Spanish Ministerio de Ciencia e Innovación (PI201060E013), Consejo Superior de Investigaciones Científicas (PIE 200960I187), the German Science Foundation (Grant No. SFB762), Ministry of Education of Czech Republic (MSM0021620834), the Grant Agency of the Czech Republic (P204/11/P339), and of the EU (Project NAMASTE, 214499).Peer reviewe

AFM studies on BiFeO3 single crystals and thin films

Trabajo presentado a la Conferencia "Euro AFM Forum" celebrada en Zurich del 7 al 9 de septiembre del 2011, http://www.asylumresearch.com/News/News.shtml#EuroAFMForum.-- Presentado como póster a la Conferencia "BNC-b Research Meeting" celebrada en Barcelona el 14 de julio de 2011.Bismuth ferrite is a paradigmatic multiferroic material since it is both magnetic and a strong ferroelectric at room temperature [1]. This material shows many interesting phenomenolgies, among which we will focus on the discovery of a surface layer different from the bulk that was found in big single crystals of BiFeO3 [2] and the conductivity found in domain walls of BiFeO3 thin films [3].We thank the Ministerio de Ciencia e Innovación for the project MAT2010-1777. N.D also thanks the Ministerio de Ciencia e Innovación for a RyC grant and J.N thanks the CSIC for a JAE-Pre grant.Peer Reviewe

Local properties of the surface layer(s) of BiFeO3 single crystals

The surface of BiFeO3 single crystals has been characterized at the local level using several AFM-based techniques. We have observed the presence of two different epilayers showing electrical and mechanical properties different from those of the bulk: a ferroelectrically dead outer skin of 5 nm sitting upon a subsurface layer that displays an extremely fine pattern of hierarchical self-ordered nanodomains. Based on the size of the nanodomains and applying a Kittel-like analysis, we argue that the nanotwinned region should be confined in a layer less than a micron deep. The superficial phase transition at T 275 °C is restricted to the outer skin layer (the dead layer), while the nanotwinned layer is insensitive to this transition. In view of the photovoltaic properties and spin-dependent transport of domain walls in BiFeO3, the existence of nanodomains (and thus a high density of domain walls) in bulk single crystals is likely to be relevant for understanding their functional properties. © 2013 AIP Publishing LLC.This work has been supported by the Leverhulme Trust, the project MAT2010-17771 from the Spanish Government and DFG through SFB 762. N.D. wants to acknowledge the financial support from a RyC grant of the Spanish Ministerio de Economia y Competitividad. G.C. wants to acknowledge financial support from ICREA.Peer Reviewe