  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.
