MAGNETIC-PROPERTIES OF SMALL IRON SYSTEMS - FROM FERROMAGNETIC-RESONANCE OF PRECIPITATED PARTICLES IN SILICA TO STERN-GERLACH DEFLECTIONS IN MOLECULAR-BEAM

In silica glass doped with iron and melted under reducing conditions, spherical and non-interacting iron particles are formed by appropriate heat treatments in H-2 gas. Ferromagnetic resonance experiments show superparamagnetic behaviour for particles of sizes ranging from 9 to 1 5 nm with no modifications of the bulk Curie temperature or magnetocry stalline anisotropy energy. Magnetic moments of iron clusters are inve stigated from their Stern-Gerlach deflections in a molecular beam, wit h sizes ranging from 10 to 1000 atoms and temperatures ranging from 10 0 to 1000 K. Single-sided deflections are found indicating that spin r elaxation occurs within the isolated clusters. Superparamagnetic behav iour is observed when the cluster rotational frequency is much larger than the Larmor frequency of the cluster magnetic moment, mu, in the a pplied field. Marked decreases of mu are observed well below the bulk Curie temperature. At a given temperature, oscillations of mu are obse rved as a function of size that suggest the filling of layers of atoms in the cluster.