We show that reducing the crystal size of ͓Mn 12 O 12 ͑O 2 C 6 H 5 ͒ 16 ͑H 2 O͒ 4 ͔ single-molecule magnets from 11.5 to 0.4 m strongly affects the molecular magnetic anisotropy and magnetic-relaxation rates. The effective activation energy for the spin reversal of the standard clusters decreases by 13% with decreasing size, whereas it remains approximately constant for the "fast relaxing" species. The pre-exponential factor 0 increases with decreasing crystal size for both. The observed decrease in the effective energy barrier for the slow relaxing species seems to be associated with the existence of a distribution of second-order transverse anisotropy terms, centered on E = 0, which broadens as the crystal size decreases. By contrast, the expected changes in the axial anisotropy parameter D with decreasing crystal size are too small to account for the change in U. The different effects that the reduction in crystal size has on the fast and slow relaxing species are discussed.

We use neutron diffraction to probe the magnetization components of a crystal of Mn12 single-molecule magnets. Each of these molecules behaves, at low temperatures, as a nanomagnet with spin S = 10 and strong anisotropy along the crystallographic c axis. The application of a magnetic field H(perpendicular) perpendicular to c induces quantum tunneling between opposite spin orientations, enabling the spins to attain thermal equilibrium. For T approximately < 0.9(1) K, this equilibrium state shows spontaneous magnetization, indicating the onset of ferromagnetism. These long-range magnetic correlations nearly disappear for mu0H(perpendicular) approximately > 5.5 T, possibly suggesting the existence of a quantum critical point.

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