Electron correlations on a mesoscopic scale: Magnetic properties of transition metal telluride cluster compounds

We report the magnetic properties of a new class of materials: Ni₉{Te}₆ⁿ⁺$ and ${Co}₆Te₈ⁿ⁺ with n=0, 1, 2. These cluster compounds, which can be charged by chemical means from neutral to 2+, provide a unique and novel way to change the Fermi level. For most charge states we observe quenching of the spin and orbital moments at low temperatures, accompanied by a large value for the temperature independent susceptibility of the ground state. The generic presence of low-energy magnetic excitations in these compounds indicates that these systems exhibit strong electron correlations and form mesoscopic analogs of the mixed-valence--heavy-fermion compounds.