The field of small atomic clusters has generated a great deal of attention since their properties are being discovered to be very different from bulk [1]. The review will describe the technology used to produce particles and investigate them. It is focused mostly on the magnetic properties of transition and rare-erath metal clusters. In addition to the native properties, the effect of the interaction with the medium will be explored.

The magnetic measurements of free clusters are usually carried out by passing size selected clusters through the gradient magnetic field region in a Stern-Gerlach experiment [2]. The analises of the cluster beam deflection allows one to restore the magnetic moment per atom. In this way the enhanced total magnetic moments have been observed in free Fe, Co, and Ni clusters. Since the magnito-ctrystalline anisotropy is relatively small, the thermally excited magnetization vector is decoupled from the lattice. In this situation, the transition metal clusters are superparamagnetic and become magnetized as they pass through the magnet to a degree determined by the field strength and temperature as described by the Langevin function. They always deflect towards the direction of the strong field.

Despite some progress in the understanding of transition metal clusters, the magnetic behaviour of rare-earth clusters remains an unsolved problem. Stern-Gerlach experiments on size selected clusters in beams indicate that their behaviour depends on size. While clusters of certain sizes deviate uniformly like transition metal clusters, for other sizes the beam spreads into a broad deflection. A nonuniform deflection can arise when the moment is fixed to the lattice, and is therefore unable to relax. At present it is believed that these two behaviours can be understood as being due to the differences in the anisotropy energy with size. However the comparision of the Gd cluster anisotropy energy with the thermal energy kBT for T=100K shows that the locked moment behaviour is possible only for large enough clusters, i.e., consisting of 400 atoms or more while in a Stern-Gerlach experiment the cluster sizes were in the range of 10-50.

Also there are some difficult problems in the free cluster physics related to the process used for producing clusters. In a Stern-Gerlach experiment the source cavity is filled with high pressure He and then the laser impuls evaporates the high energy Gd atoms from the Gd sample. The evaporated Gd atoms are cooled due to collisions with cold He atoms, stick to each other and come to equilibrium with He atmosphere. Thus a variaty of clusters of different sizes and shapes are produced. However the shape distribution function of a clusters of a certain size in unknown. This makes the modelling of the cluster magnetic structure difficult problem [3]. In addition the effect of He atmosphere on the cluster beam deflection was never explored. However the latter can lead to the decrease of the cluster beam deflection and thus effectively diminish the magnitude of the magnetic moment restored in a Stern-Gerlach experiment. The problems mentioned above remain unsolved untill now. In this way our report presents systematic results for magnetic clusters and formulates the unsolved problemes.

1. R.S.Berry, J.Burdett, A.W.Castelman, Jr.,Z.Phys. 26, (1993).
2. D.C.Douglass, A.J.Cox, J.P.Bucher, L.A.Bloomfield, Phys Rev B 47, 12874 (1993)
3. D.P.Pappas, A.P.Popov, A.V.Anisimov, B.V.Reddy, S.N.Khanna, Phys.Rev.Lett. 76 4332 (1996)