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New routes to design vertically aligned multiferroic nanocomposites

Abstract : Multiferroic materials including magnetoelectric materials that combine magnetic and ferroelectric orders have attracted great attention due to a possible strain-mediated coupling leading to potential applications in memories, sensors, detectors, spintronic and microwave devices. The number of single-phase multiferroic materials operating at room temperature being limited, we are exploring artificially designed multiferroic nanostructures consisting of ferroelectric and ferrimagnetic oxides. Current work is focused on strain-mediated magnetoelectric effect, which allows to generate a spontaneous polarization or magnetization by an applied magnetic field (direct ME effect) and electric field (converse ME effect) respectively. ME effects can be observed at room temperature through interface and strain interaction in two-phase multiferroic nanocomposites. The combination of piezoelectric materials PbZr0.52Ti0.48O3 (PZT), Ba0.7Sr0.3TiO3 (BSTO), BaTiO3 (BTO) and magnetostrictive CoFe2O4 (CFO) materials have been intensively studied in multiferroic nanocomposites. The community has been able to demonstrate large magnetoelectric coupling at room temperature in epitaxial thin films, so called 2-2 connectivity system, but a key limitation in epitaxially grown thin films is a substrate imposed clamping effect limiting thin film’s strain. Designing innovative architectures is a challenge in the field of multiferroic nanocomposites. Our work is focused on vertically aligned multiferroic nanostructures, so called (1-3) connectivity nanocomposites, where one-dimensional ferrimagnetic CoFe2O4 nanostructures (1) are embedded into three-dimensional PZT, BTO and BSTO layers (3). New routes were considered to design three kinds of materials: i) vertically aligned CFO nanowire arrays surrounded by PZT nanotubes embedded into alumina membranes; ii) vertically aligned CFO nanopillar arrays embedded in thin BTO, BSTO and PZT layers supported on Si substrates; ii) 3-D interconnected CFO nanowire networks embedded in a thick PZT matrix. The objectives of the present work are to control the oxidation of metallic CoFe2 nanowires and nanopillars to control the morphology and density of CFO nanostructures, to control the resistivity and dielectric losses of the nanocomposites at the interface region, and to increase the magnetoelectric coupling of the multiferroic nanocomposites by increasing the interfacial surface area between the two ferroic phases.The first geometry we are developing is a deposition by sol-gel dip impregnation of PZT nanotube arrays into self-supported porous alumina membranes, followed by an electrodeposition and thermal oxidation of CoFe2 nanowire arrays within PZT nanotubes. The second architecture we are focusing on is a deposition by RF magnetron sputtering of BSTO and BTO layers and by sol-gel dip coating of PZT layers onto vertically aligned CoFe2 and CoFe2O4 nanopillar arrays supported on Si substrates. The CoFe2 oxidation is conducted in-situ during the BSTO and BTO sputter deposition. Free-standing CoFe2 nanopillar arrays are obtained by electrodeposition into anodized alumina nanoporous structures and chemical dissolution of alumina templates. The last geometry is prepared using a combination of electrodeposition into self-supported porous polymer membranes and sol-gel processes. The PZT-CFO nanostructures are prepared using impregnation of thick PZT layers into self-supported CoFe2 3D nanowire networks on Si substrates by sol-gel method and their simultaneous oxidation during PZT layers crystallization. Specific attention was focused on interfaces through microstructural and morphological evaluations of nanocomposites using XRD, HRSEM, TEM and EDS characterizations. The performances of the nanocomposites were evaluated using magnetic, dielectric, ferroelectric and ME measurements, an alternating gradient magnetometer, impedance analyser, PFM and the ME susceptometer operated inside PPMS were utilized, respectively.
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Submitted on : Thursday, March 15, 2018 - 6:18:07 PM
Last modification on : Thursday, September 3, 2020 - 4:36:42 AM
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Sergey Basov. New routes to design vertically aligned multiferroic nanocomposites. Material chemistry. Université de Bordeaux; Université catholique de Louvain (1970-..), 2018. English. ⟨NNT : 2018BORD0007⟩. ⟨tel-01735390⟩

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