Magnetic interactions in transition metal clusters embedded in matrices prepared by LECBD technique

The synthesis of iron and cobalt clusters embedded in an insulating or metallic matrix have been realized by the co-deposition of both beams arriving at the same time on a 45 degrees-tilted substrate. The nanoparticles were produced from an intense cluster beam of selected size (centered around 300 atoms per cluster) produced from a laser vaporization source. We used a Knudsen cell to evaporate the matrix. From the ratio of the deposition rates of both beams, we were able to adjust continuously the atomic concentration of clusters in the matrix from 3 to 70%. The typical size distribution of embedded clusters determined from HRTEM observations on diluted samples revealed nanocrystallized-grains with a very narrow size dispersion. Moreover, magnetoresistance (MR) and magnetization measurements versus temperature and concentration were performed. The results show clearly a magnetic threshold around 25% Co-clusters content in matrix, corresponding to the transition from superparamagnetic to magnetically ordered state. This threshold is identical to the 3D-percolation threshold for connected particles. A maximum of MR ratio of 12% at 4.2 K was obtained for Co25%Ag75% granular film at the onset of non negligible magnetic interactions between Co-particles. The magnetic properties of our systems, strongly correlated to the nucleation processes, allowed us to determine the variation of the cluster radius as a function of the concentration and of the matrix nature.