Rachel Desfeux

Spray coating of the ferroelectric polymer P(VDF-TrFE) appears as an attractive approach for the fabrication of electromechanical transducers. However, it is important to elucidate how the crystallinity and the associated piezoelectric properties depend on the coating thickness and additives. To this aim, we have spray-coated various substrates both with pure and nanocomposite films of 70–30% copolymers in a broad thickness range (200 nm–30 μm). Using X-ray diffraction, differential scanning calorimetry, Raman spectroscopy and atomic force microscopy, we show that the obtained films are dense and homogeneous with ca. 50% crystallinity, which consists of a major polar phase, with slight alterations in the sub-micrometer thickness regime. Robust piezoelectricity and ferroelectricity are revealed at room temperature through both local hysteresis loops and lithography experiments using the piezoresponse force microscopy technique. After poling, the piezoelectric d₃₃ coefficient displays values up to −19 and −11 pC N⁻¹ for the pure copolymer and the composite, respectively. For a 33 vol% load of inorganic spin-crossover nanofillers, the switching properties are substantially improved and a coercive voltage of in situ integration of high quality piezopolymer films into complex transducer geometries for sensing, actuating and energy harvesting purposes.

Organic multiferroic tunnel junctions (OMFTJs) with multi-resistance states have been proposed and have drawn intensive interests due to their potential applications, for example in memristor and spintronics based synapse devices. The ferroelectric control of spin polarization at a ferromagnet/ferroelectric organic (FE-Org) interface by electrically switching the ferroelectric polarization of the FE-Org has been recently realized. However, there is still a lack of understanding of the transport properties in OMFTJs, especially the interplay between the ferroelectric domain structure in the organic barrier and the spin polarized electron tunneling through the barrier. Here, we report on a systematic study of the temperature dependent transport behavior in La _0.6 Sr _0.4 MnO ₃ /PVDF/Co OMFTJs. It is found that thermal fluctuation of the ferroelectric domains play an important role on the transport properties. When T > 120 K, the opposite temperature dependence of resistance for up and down ferroelectric polarization states results in a rapid diminishing of the tunneling electroresistance. These results contribute to the understanding of the transport properties for designing high performance OMFTJs for memristor and spintronics applications.

We report on the fabrication of an organic multiferroic tunnel junction (OMFTJ) based on an organic barrier of the Poly(vinylidene fluoride) (PVDF):Fe₃O₄ nanocomposite. By adding Fe₃O₄ nanoparticles into the PVDF barrier, we found that the ferroelectric properties of the OMFTJ are considerably improved compared to those with a pure PVDF barrier. This can lead to tunneling electroresistances of about 450% at 10 K and 100% at room temperature (RT), which are much higher than those of the pure PVDF based device (70% at 10 K and 7% at RT). OMFTJs based on the PVDF:Fe₃O₄ nanocomposite could open new functionalities in smart multiferroic devices via the interplay of the magnetism of nanoparticles with the ferroelectricity of the organic barrier.

The relaxed epitaxial growth of Ni thin films has been successfully performed on both (100)- and (110)-oriented SrTiO ₃ substrates by pulsed laser deposition.

Artificial [(Nd₂Ti₂O₇)₄/(SrTiO₃)_n]₁₀ superlattices were epitaxially grown. Local conductivity and misleading PFM signals were mainly attributed to the oxygen vacancies in the SrTiO₃ layers and to the mixed valence state of the titanium.