From the superparamagnetic to the magnetically ordered state in systems of transition metal clusters embedded in matrices

One of the interests in systems of ultrafine particles embedded in matrices is to adjust the size of the nanoparticles and the mean distance between them separately to study cluster interactions in a controlled environment. The synthesis of iron and cobalt clusters embedded in an insulating or metallic matrix has 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 continuously adjust the atomic concentrations of the clusters in the matrix from 3 to 70%. In situ electrical conductivity measurements confirmed a 3D percolation threshold of around 25%. The typical size distribution of embedded clusters determined from HRTEM observations revealed nanocrystallized grains with a very narrow dispersion in size. Two local environments emerge from EXAFS simulations: core atoms in the cluster with the density of bulk and surface atoms with a dilated parameter. In agreement with structural considerations, we clearly observed by magnetoresistance and magnetization measurements versus temperature and concentration a magnetic percolation threshold of around 25% for Co clusters in a Ag matrix (maximum of 12% GMR) corresponding to the transition from the superparamagnetic to the magnetically ordered state.