Long-range ferromagnetism of Mn-12 acetate single-molecule magnets under a transverse magnetic field.

We use neutron diffraction to probe the magnetization components of a crystal of Mn-12 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 to perpendicular to c induces quantum tunneling between opposite spin orientations, enabling the spins to attain thermal equilibrium. For T less than or similar to 0.9(1) K, this equilibrium state shows spontaneous magnetization, indicating the onset of ferromagnetism. These long-range magnetic correlations nearly disappear for mu(0)H(perpendicular to)greater than or similar to 5.5 T, possibly suggesting the existence of a quantum critical point.