The fabrication of new nanostructured materials is a rapidly growing field with strong possibilities of applications. Particular attention is actually devoted to overcome the superparamagnetic limit of nanoparticles by means of the controlled manipulation of the shape and magnetocrystalline anisotropies and to the development of composite elements for magnetic memories and sensors. The activity of the institute in that field is specifically dedicated to the synthesis and to investigate the magnetic properties of atomic scale nanostructured systems. Effects of the geometry, shape, chemical composition and dimensionality on to magnetic properties are investigated by means of innovative methods of synchrotron radiation spectroscopy and magnetometry at ELETTRA and ESRF laboratories. In particular It was recently demonstrated that X-ray circular magnetic dichroism allows to measure the magnetic momentum and the magnetic anisotropy of diluted systems with concentrations down to 1012 atomi/cm2. During the 2002, several aspects of the magnetic interactions and of the magnetic properties on the atomic and nanometric scale have been examined by means these new investigation methods. Here it is shortly summarized a outstanding result.
As the dimensionality of the system is reduced, the magnetic order tends to reduce because of magnetic fluctuations thermically activated. The theoretical models predict the impossibility of stabilizing the long range ferromagnetic order of a linear chain at any temperature. But, such models do not include the effect of kinetic barriers that obstacle the evolution of the system toward its own thermodynamic equilibrium, nor the interaction with the substrate that sustains the chains. By means of X-ray magnetic dichroism measurements it was possible to demonstrate the existence of ferromagnetic order in either long and short range in monoatomic chains of Co, assembled by means of self-organizing processes on a Pt substrate. These monoatomic chains result made of segments of ferromagnetically coupled and thermically fluctuating atoms that, due to the barriers of magnetocrystalline anisotropy, evolve, under a certain critical temperature, in a ferromagnetically ordered state. The Co monoatomic chains show orbital magnetic moments and magnetic anisotropy energies much more high than the metallic Co in thin films or either in the bulk.
Focus