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Quantum Fluctuations and Long-Range Order in Molecular Magnets

We review our studies of the effect of transverse fields on the susceptibility and magnetization of single crystals of the prototype single molecule magnet (SMM), Mn$_{12}$-acetate, and of a new high-symmetry variant, Mn$_{12}$-acetate-MeOH. SMM single crystals can exhibit long range ferromagnetic order associated with intermolecular dipole interactions. Transverse fields increase quantum spin fluctuation and quantum tunneling of the magnetization suppressing long range order. However, we have found that suppression of the Curie temperature by a transverse field in Mn$_{12}$-acetate is far more rapid than predicted by the Transverse-Field Ising Ferromagnetic Model (TFIFM). It appears that solvent disorder in Mn$_{12}$-acetate that results in an intrinsic distribution of small discrete tilts of the molecular magnetic easy axis from the global easy axis of the crystal ($\approx \pm 1^\circ$) gives rise to a distribution of random-fields that further suppresses long-range order. Semiquantitative agreement with the predictions of a Random-Field Ising Ferromagnet Model is found. Subsequent susceptibility studies we have conducted of the high symmetry Mn$_{12}$ variant, Mn$_{12}$-acetate-MeOH, with the same spin structure and similar lattice constants but without the same solvent disorder as Mn$_{12}$-acetate, agrees with the TFIFM. An important implication of our studies is that long-range order in these two chemically very similar SMMs are described by distinct physical models.