One-dimensional nanostructures that coaxially combine several phases of distinct physical properties are the focus of extensive research, because of their potential applications in various fields such as electronic devices, ultra high-density memories, microwave devices, spintronics devices and various sensors, among other things.

In particular, the ability to combine two or more materials with different magnetic and electric properties in a radial structure opens up new perspectives to elaborate multiferroic composite materials and to allow a precise engineering of the desirable material properties which is essential for the future new multifunctional devices (1-4). In this regard, the coexistence and coupling between the ferromagnetic and ferroelectric properties may lead to the attractive electric control of the magnetic properties (or vice versa).

In the framework of a collaboration between the Université catholique de Louvain and Université Bordeaux I, we have recently developed a multi-step synthesis method combining electrochemical synthesis, sol-gel and annealing processes to successfully fabricate multiferroic nanowire arrays embedded in a dielectric nanoporous template.

The goals of the present PhD project may be summarized as follows:

1. To establish a reliable and controllable synthesis of high-density arrays of coaxial nanocables, consisting of ferromagnetic nanowires surrounded by ferroelectric nanotube sheaths, within anodic aluminium oxide membranes. Since this synthesis method is very versatile we plan to combine very different materials, including ferroelectrics (BaTiO3, PZT), 3d-ferromagnetic metals and alloys as well as magnetic oxides (ferrites). As a whole, we believe that this synthesis approach will allow us to precisely control the size, interwire distance, morphology and material of the core-shell nanowire arrays.

2. To determine the magnetoelectric coefficient arising from a mechanical coupling between the magnetostrictive and piezoelectric phases in such core-shell nanocables. The magnetoelectric effect, defined as the variation of the dielectric polarization response under an applied magnetic field or conversely by the induced magnetization under an external electric field, is expected to be quite large in arrays of vertically aligned core-shell nanowires having large interface area.

3. To pave the way for developing innovative devices. We plan to develop microwave devices (such as rf filters or circulator for example) for frequency agile applications based on a correlation between the ferromagnetic and ferroelectric properties. Also, metamaterials based on aligned arrays of metallic nanowires in a dielectric matrix have recently received considerable interest (5). We also plan to design new negative-index metamaterials based on arrays of coaxial multiferroic nanowires. These systems may also exhibit a double negative effective permittivity and permeability properties for some frequency ranges.

Expected outcomes

By combining electrochemical and sol-gel processes, it is possible to elaborate core/shell multiferroic nanowires that are very difficult to obtain using conventional lithographic techniques and to make large arrays of nanocables with compositions, morphologies and geometrical parameters controlled to a large extent. When considering the fabrication process, this project will also address the challenges of versatile synthesis of core-shell multiferroic nanowires to tune accurately the dimensions andcompositionthus the magnetic and electric properties of nanocable array. Multiferroic nanostructured composites combining different functionalities are currently a system of considerable interest that may lead to an expanding field of applications. The proposed research is expected to further develop capabilities for breakthrough innovation, by making the demonstration of multiferroic core-shell nanowired devices with enhanced magnetic/electrical tunability of their operating frequency. There is a clear potential to expand applicability of research activity in multiferroic nanocables to make it highly attractive for developing tunable microwave devices and new metamaterial structures.

(1) Zheng et al, Science 303, 661 (2004)
(2) Xie et al, Nanoscale 3, 3152 (2011)
(3) Johnson et al, Appl. Phys. Lett. 99, 182901 (2011)
(4) Narayanan et al, Nanoletters 12, 3025 (2012)
(5) Burgos et al, Nature Materials 9, 407 (2010)

Project Partners and their roles

Institute of Condensed Matter and Nanosciences, Université Catholique Louvain:
fabrication of nanoporous alumina templates - synthesis of ferromagnetic nanowires by electrodeposition – structural, composition and near field microscopy characterizations -  SQUID magnetometry – rf measurements with voltage and/or magnetic field bias.

Institute for Condensed Matter Chemistry Bordeaux, University Bordeaux :
synthesis of ferroelectric nanotubes using various impregnation processes - electron microscopy - electron tomography for 3D reconstruction - interfaces studies (STEM-EDX and HRTEM).

Industry Partner: Schneider Electric, Grenoble, France