THEORY FOR SPIN RELAXATION IN SMALL MAGNETIC METAL-CLUSTERS

We discuss the magnetic properties of small neutral transition-metal clusters like Fe(n) and Co(n) deduced from Stern-Gerlach deflection experiments. We claim that the asymmetric Stern-Gerlach deflection profiles are due to a transfer from electronical angular momentum to the cluster rotation, allowing for a depopulation of the high energy magnetic levels. For finite temperatures we consider two limitng cases. First, the cluster magnetization is assumed to be tied to the random orientation of the cluster easy axes due to the lattice anisotropy. This causes a surprisingly small magnetization for small external magnetic fields. For larger fields and also for increasing temperatures the magnetization is released from the cluster geometry and allowed to align itself parallel to the field. In the second case the clusters are treated as an ensemble of superparamagnetic particles. Here, the effect of the anisotropy is less visible. The cluster lattice anisotropy per atom is expected to decrease for increasing cluster size. Preliminary results support this.