From single-molecule magnetism to long-range ferromagnetism in Hpyr[Fe 17O16(OH)12(py)12Br4]Br4

The molecular magnet Hpyr[Fe 17O16OH12py12Br4Br4] “Fe17” has a well-defined cluster spin ground state of S=35/2 at low temperatures and an axial molecular anisotropy of only D ≈−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 Fe17, as determined by unpolarized neutron diffraction experiments performed on a polycrystalline sample of deuterated Fe17 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.