Synthesis and Characterization of Magnetic Nanoparticles growing in a Porous Alumina Template

C. L. LONDOÑO CALDERÓN; LAURA G. PAMPILLO; RICARDO MARTÍNEZ GARCÍA

Conferencia; SyncLight 2015: São Paulo School of Advanced Sciences on Recent Developments in Synchrotron Radiation; 2015

Magnetic nanostructures are currently of high scientific and technological interest in a variety of different fields. Some applications include ferrofluids, spintronics, directed assembly, as well as for imaging and therapeutic applications. The interest is due to the unique physical properties not seen in the bulk materials. Many different methods have been reported for obtaining magnetic nanostructures. Among them, template-assisted syntheses are methods for fabrication of magnetic nanomaterials which have received a special interest due to the capability of obtain 1D nanostructures like nanowires and nanotubes. These methods, involves the use of a nanoporous material (i.e alumina) which is filled by a physical or chemical process. Electrodeposition and infiltration methods are employed mainly in order to fill the cylindrical nanopores with magnetic materials. Less interest have received the fabrication of magnetic nanoparticles embedded in porous templates. The formation of continuous structures inside of the pores is the main reason. However, Kanamadi et. al [1] have suggested that infiltration methods in small pores produce discontinuous magnetic nanowires. The non-uniform nanopore size (along pore axis), the drying and decompositions of precursor solution are recognized as the main causes. The results are magnetic nanostructures 1D with a wide distribution of lengths and in some cases, formation of nanoparticle insulated. In this study, it is described a simple method for fabrication in situ of magnetic nanoparticles embedded into a porous Anodic Aluminum Oxide template (AAO). For this, an aqueous solution of the iron and cobalt salts was infiltrated in AAO template. During the process, vacuum was used to allow the pores to be filled. The precursor solution inside of pores was treated thermally for the magnetic phase formation. The structure of the as-prepared samples was studied by x-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The composition of the samples was analyzed by energy-dispersive x-ray spectroscopy (EDX). The magnetic properties were measured by SQUID in ZFC-FC mode at room temperature and at 5 K for hysteresis loops. The results confirm the presence of magnetic nanoparticles (cobalt ferrite), with crystallite size between 10 and 20 nm. Nanoparticles were embedded inside of AAO template with ellipsoidal shape (prolate). The ellipsoidal diameters along of the long and short axis are in good agreement with the crystallite size and the pore diameter of the template. Magnetocrystalline anisotropy is at least one order of magnitude larger than the shape anisotropy. That is why the hysteresis loops are the same when the sample is measured with applied field in parallel and perpendicular geometry at pore axis. ZFC-FC measurements reveal that the system is superparamagnetic, with a wide distribution of nanoparticles sizes and an average blocking temperature of 436 K.