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Resumen del póster presentado al 4th Scientific Meeting of BNC-b Students (JPhD), celebrado en Bellatera (España) del 6 al 7 de junio de 2019., There is currently a vigorous research effort on the functional properties of ferroelectric domain walls. An important part of their appeal is that domain walls possess functional properties distinct from the host material. Their distinct functionality foment new concepts in electronic nanodevices where domain walls act as mobile two-dimensional elements. Among the many properties of domain walls, mechanical response appears to have been largely neglected. Our research is focused on the measurement on stiffness of domain walls. In particular, we have used Atomic Force Microscopy to investigate the difference in stiffness between domains and purely ferroelectric (non-ferroelastic) domain walls separating antiparallels 180o domains. Surprisingly, they have a distinct mechanical response, markedly softer than that of the domains they separate. Initially, Piezoresponse Force Microscopy (PFM) was used to image the polarized domains of the materials and consequently to identify different types of domain walls appearing in the materials. Thereafter, Contact Resonance Frequency (CRF) mode was used to identify the stiffness of the domain walls. In CRF, the stiffness of the material is determined from the measured resonance frequencies of the cantilever when the tip is in contact with the sample, as these contact resonance frequencies depend on the tip sample mechanical coupling and undergo distinct shifts when the tip is scanned over areas with different stiffness. Theoretical calculations, based also in flexoelectric effects, enhance our experimental results. However, flexoelectricity has a unique intrinsic effect, related to the fact that strain gradients are size-dependent property that scales in inverse proportion to size, therefore flexoelectricity can be huge at the nanoscale, affecting the stiffness of domain walls. Therefore, we also focus on this size effect comparing softness of domain walls in thin films and bulk materials.

Resumen del póster presentado a la 10th Conferencia Fuerzas y Túnel, celebrada en Girona (España) del 5 al 7 de septiembre de 2016., STO is one of the most widely studied materials in solid state physics and chemistry not only for its subtle and complex phase transitions but also for elucidating and exploring exotic new properties. Itoh et al. showed that substitution of 18O for 16O induces ferroelectricity at temperatures below 24 K. Despite extensive studies, the exact nature of the ferroelectric phase of SrTi18O3 (18STO) is not unambiguously determined and there are also huge difficulties to determine the crystallographic symmetry of the low temperature phase, due to three kinds of nanotwinning: tetragonal ferroelastic nanodomains below 105 K, rhombohedral nanoclusters below 40 K, and orthorhombic nanodomains below Tc =24 K. In the present study, the piezoelectric coefficient d33 of a 18STO single-crystal plate (whose edges were parallel to those of [001] cubic) were investigated at temperatures between 3.5K to 150K by Piezoresponse Force Microscopy (PFM). The experiments were performed using an Attocube AFM (AttoAFM) system controlled by Nanonics electronics. Due to the interferometric detection of cantilever motion in the attoAFM, out-of-plane polarization of piezo-electric domains can be directly obtained with picometer precision, but in-plane piezoelectric signal is only accessible indirectly via crosstalk effects. Experimental d33 coefficient behavior showed two regimes below and above 30K possibly related to the phase transition. No changes around the tetragonal-cubic transition near to 105K were observed., The authors would like to acknowledge the Mineco for financial support under project FIS2013-48668-C2-1-P.

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).

The experimental detection of negative capacitance in ferroelectrics rekindles hopes that the phenomenon could be used to further push the miniaturization of conventional transistors.

Trabajo presentado al American Physical Society March Meeting celebrado en Boston (US) del 27 de febrero al 2 de marzo de 2012.

Flexoelectricity (coupling between polarization and strain gradients) is a property of all dielectric materials that has been theoretically known for decades, but only relatively recently it has begun to attract experimental attention. As a consequence, there are still entire families of materials whose flexoelectric performance is unknown. Such is the case of antiferroelectrics: materials with an antiparallel but switchable arrangement of dipoles. These materials are expected to be flexoelectrically relevant because it has been hypothesised that flexoelectricity could be linked to the origin of their antiferroelectricity. In this work, we have measured the flexoelectricity of two different antiferroelectrics (PbZrO3 and AgNbO3) as a function of temperature, up to and beyond their Curie temperature. Although their flexocoupling shows a sharp peak at the antiferroelectric phase transition, neither flexoelectricity nor the flexocoupling coefficients are anomalously high, suggesting that it is unlikely that flexoelectricity causes antiferroelectricity., We acknowledge financial support from ICN2, which is funded by the CERCA Programme/Generalitat de Catalunya; ICN2 is supported by the Severo Ochoa programme of the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. SEV-2013-0295). We also acknowledge support from Plan Nacional (MINECO Grant No. MAT2016-77100-C2-1-P), ERC Starting Grant No. 308023, MINECO Grant No. BES-2016-077392, the National Science Centre of Poland (Project No. 2016/21/B/ST3/02242), as well as the Ministry of Science and Technology of China (Grant No. 2015CB654605).