Assembling of functionalized ferrite nanoparticles for magnetic and magneto-transport applications – MAGARRAYS

The preparation of controlled assemblies of magnetic nanoparticles (NPs) is a hot topic in Materials Science. From the point of view of applications, such functional ordered arrays of magnetic NPs with size-dependent properties should lead to magnetic and magneto-electronic nanodevices. Indeed films of magnetite NPs have been reported to display very interesting magneto-resistance values in comparison to conventional films elaborated by physical methods and could lead thus to the development of magnetoelectronic devices with enhanced magneto-transport properties. Also, in the field of high density storage media, the elaboration of arrays of non-interacting NPs could reduce the size of a bit cell from multiple grains to single grain and could result in dramatic increase in storage capacity. These nanograins should not interact magnetically, be not superparamagnetic, i.e. room temperature blocked single domain, be thermally stable and should display a high anisotropy constant and remanence. Among iron oxides, magnetite presents the most interesting magnetic and conducting properties due to the presence of iron cations in two valence states Fe2+ and Fe3+ in the inverse spinel structure. The high spin polarization of magnetite combined with its high Curie temperature (~850K) make that magnetite would be an attractive material for magnetoresistive sensors based on strong spin-selective tunnelling effect. Magnetite Fe3O4 in which some of the Fe2+ has been substituted by Co2+ (CoFe2O4) is a well-known hard magnetic material with high coercivity and moderate magnetization. These properties, along with their great physical and chemical stability, make CoFe2O4 NPs suitable for magnetic recording applications. In this context, the objective of the MAGARRAYS project is to elaborate stable arrays of functionalized/organically coated magnetite and cobalt ferrite nanoparticles (Nps) and to study their magnetic and magneto-transport properties as a function of the Nps properties and of the structuration parameters. The NPs will be either room temperature blocked or not single domains. The arrays will be formed by the Langmuir-Blodgett (LB) and Layer by Layer (LbL) techniques and the interparticle spacings tuned by designed dendron molecules grafted at the surface of NPs. This "bottom up" approach should open new possibilities to investigate the physical properties of structures at the nanometric scale. The elaboration of arrays of non interacting RT blocked single domain of magnetite and cobalt ferrite particles would lead to highly relevant films in the context of high density magnetic recording with a goal to reach 1 Terabit/inch2. The elaboration of monolayer and multilayer arrays of magnetite NPs and the study of their transport properties before and as a function of thermal treatments may lead to films displaying interesting magneto-resistance properties for spintronic applications. To fulfil these objectives, several scientific and technical points have to be addressed: - the synthesis of magnetite and cobalt ferrite NPs with different monodisperse sizes in the range 5 - 40 nm for magnetite and 5-100 nm for cobalt ferrite and to evaluate the critical size leading to RT blocked single domain NPs as a function of the synthesic methods. The latter are the co-precipitation technique followed or not by a hydrothermal treatment and the thermal decomposition of metal complex. - the synthesis of dendron molecules designed to be covalently bonded onto NPs, to tune the interparticle spacing and to bring terminal groups favouring the film formation owing to the deposition techniques (LB or LbL). - the grafting of the dendron molecules at the surface of nanoparticles in the conditions favouring a high grafting rate and a suspension stability of NPs in the adapted solvent. One important step is the grafting of hydrophobic and hydrophilic molecules to tune the hydrophobicity/hydrophilicity balance of NPs and improve the quality of films. - the elaboration of films with magnetite and cobalt ferrite NPs by the LB and LbL techniques with varied structuration parameters. - the correlation of the structuration parameters with the magnetic and magneto-transport properties of the films.