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The molecular magnet Hpyr[Fe17O16(OH)(12)(py)(12)Br-4]Br-4 ("Fe-17") has a well-defined cluster spin ground state of S=35/2 at low temperatures and an axial molecular anisotropy of only D similar or equal to -0.02 K. Dipolar interactions between the molecular spins induce long-range magnetic order below 1.1 K. We report here the magnetic structure of Fe-17, as determined by unpolarized neutron diffraction experiments performed on a polycrystalline sample of deuterated Fe-17 in zero applied magnetic field. In addition, we report bulk susceptibility, magnetization, and specific heat data. The temperature dependence of the long-range magnetic order has been tracked and is well accounted for within mean-field theory. Ferromagnetic order along the crystallographic c axis of the molecular spins, as determined by the neutron diffraction experiments, is in agreement with ground-state dipolar energy calculations.
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