Jean-François Bobo

Bio: Jean-François Bobo is an academic researcher from University of Toulouse. The author has contributed to research in topics: Magnetization & Thin film. The author has an hindex of 21, co-authored 123 publications receiving 2287 citations. Previous affiliations of Jean-François Bobo include Saint-Gobain & Centre national de la recherche scientifique.

Electric-field control of exchange bias in multiferroic epitaxial heterostructures.

Abstract: The magnetic exchange between epitaxial thin films of the multiferroic (antiferromagnetic and ferroelectric) hexagonal YMnO3 oxide and a soft ferromagnetic (FM) layer is used to couple the magnetic response of the FM layer to the magnetic state of the antiferromagnetic one. We will show that biasing the ferroelectric YMnO3 layer by an electric field allows control of the magnetic exchange bias and subsequently the magnetotransport properties of the FM layer. This finding may contribute to paving the way towards a new generation of electric-field controlled spintronic devices.

Spin filtering through ferrimagnetic NiFe2O4 tunnel barriers

Abstract: We report experiments of spin-filtering through ultrathin insulating layers of the high Curie temperature ferrimagnetic oxide NiFe2O4 (NFO). The spin-filtering efficiency of electrons tunneling from a gold electrode through NFO is analyzed with a counter-electrode of La2∕3Sr1∕3MnO3 (LSMO). We measure a tunnel magnetoresistance of 40%–50% when the configuration of the magnetizations of the NFO and LSMO goes from parallel to antiparallel. This value corresponds to a spin-filtering efficiency of up to 22% by the NFO barrier. We discuss the sign and temperature dependence of the spin-filter effect and argue that our results show the potential of spinel ferrites for spin-filtering and injection.

Enhanced magnetic moment and conductive behavior in NiFe2O4 spinel ultrathin films

Abstract: Bulk $\mathrm{Ni}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ is an insulating ferrimagnet. Here, we report on the epitaxial growth of spinel $\mathrm{Ni}{\mathrm{Fe}}_{2}{\mathrm{O}}_{4}$ ultrathin films onto $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ single crystals. We will show that---under appropriate growth conditions---epitaxial stabilization leads to the formation of a spinel phase with magnetic and electrical properties that radically differ from those of the bulk material: an enhanced magnetic moment $({M}_{S})$---about 250% larger---and a metallic character. A systematic study of the thickness dependence of ${M}_{S}$ allows us to conclude that its enhanced value is due to an anomalous distribution of the Fe and Ni cations among the $A$ and $B$ sites of the spinel structure resulting from the off-equilibrium growth conditions and to interface effects. The relevance of these findings for spinel- and, more generally, oxide-based heterostructures is discussed. We will argue that this novel material could be an alternative ferromagetic-metallic electrode in magnetic tunnel junctions.

Liquid-crystalline and electron-deficient coronene oligocarboxylic esters and imides by twofold benzogenic Diels-Alder reactions on perylenes.

Abstract: Alkyl esters, imides and imido-esters of coronene-tri-, -tetraand-octacarboxylic acids are accessible by a twofold oxidative benzogenic Diels–Alder reaction. Alkyl acrylates add to perylene, and maleic alkyl imides react twice with perylene as well as with perylene-tetracarboxylictetraesters. Coronenes substituted with a greatly variable number of electron with drawing substituents are thus accessible, and di- and tetraimide derivatives are shown to be very pronounced electron-acceptor materials. The triand tetraalkyl esters and imidoesters self-assemble into columnar liquid-crystalline phases.

Multiferroic and magnetoelectric materials

Abstract: A ferroelectric crystal exhibits a stable and switchable electrical polarization that is manifested in the form of cooperative atomic displacements. A ferromagnetic crystal exhibits a stable and switchable magnetization that arises through the quantum mechanical phenomenon of exchange. There are very few 'multiferroic' materials that exhibit both of these properties, but the 'magnetoelectric' coupling of magnetic and electrical properties is a more general and widespread phenomenon. Although work in this area can be traced back to pioneering research in the 1950s and 1960s, there has been a recent resurgence of interest driven by long-term technological aspirations.

Physics and Applications of Bismuth Ferrite

Abstract: BiFeO3 is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. As a result, it has had an impact on the field of multiferroics that is comparable to that of yttrium barium copper oxide (YBCO) on superconductors, with hundreds of publications devoted to it in the past few years. In this Review, we try to summarize both the basic physics and unresolved aspects of BiFeO3 (which are still being discovered with several new phase transitions reported in the past few months) and device applications, which center on spintronics and memory devices that can be addressed both electrically and magnetically.

Multiferroics: progress and prospects in thin films.

Abstract: Multiferroic materials, which show simultaneous ferroelectric and magnetic ordering, exhibit unusual physical properties — and in turn promise new device applications — as a result of the coupling between their dual order parameters. We review recent progress in the growth, characterization and understanding of thin-film multiferroics. The availability of high-quality thin-film multiferroics makes it easier to tailor their properties through epitaxial strain, atomic-level engineering of chemistry and interfacial coupling, and is a prerequisite for their incorporation into practical devices. We discuss novel device paradigms based on magnetoelectric coupling, and outline the key scientific challenges in the field.

Ferromagnetic materials

Abstract: In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.