MAGNETISM OF FREE TRANSITION METAL AND RARE EARTH CLUSTERS

Abstract

When ferromagnetic (monodomain) transition metal clusters pass through a magnetic field gradient, they deflect towards increasing magnetic field. For transition metal clusters with a size between N=10 and N=400 atoms, the observable effective magnetic moment (similar to magnetization), measured from the cluster deflection, scales with magnetic field, cluster size and inverse of cluster vibrational temperature. The measurements are in quantitative agreement with a picture in which the cluster moments are subject to rapid orientational fluctuations and explore the whole distribution of magnetic moment projections on the field axis on the time scale of the experiment. Intrinsic magnetic moments per atom in excess of the bulk values are obtained. While transition metal clusters show a size independent behavior of the magnetic properties down to N=20, similar to what was observed previously for transition metal clusters in matrices, rare earth clusters are quite sensitive to symmetry (anisotropy) and exhibit dramatic variations in their magnetic behavior as a function of size. These size-specific variations of the magnetic behavior of clusters have never been seen before. Except for some “magic numbers”, for which the statistical interpretation still holds, an anomalous spreading of the deflection profile is observed. This spreading is due to a strong coupling of the magnetic moment with the cluster body. When the moment is locked to the lattice by strong crystal field anisotropies, the rotational temperature starts to play an important role in the interpretation of experimental data. This distinct behavior points to the fact that 3d and 4f ferromagnets react quite differently to a confined geometry. This dissimilarity is due in part to a different relative importance of magnetic anisotropy energy and exchange energy. It is found that Gd and Tb clusters retain their magnetic order for temperatures well above their bulk Curie temperatures. Three aspects of cluster magnetic properties can be determined by molecular beam experiments: (i) The effective magnetic moment of a cluster (equivalent to magnetization in a laboratory reference frame), (ii) the intrinsic quasi ground state properties in the reference frame of the particle, such as the magnetic moment per atom and the temperature dependence of the order parameter, and (iii) the dynamics of the cluster as a whole. While the vibrational temperature T vib is almost an experimental input, the rotational temperature T rot can be inferred in the case of strong anisotropy.