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), Mn12-acetate, and of a new high-symmetry variant, Mn12-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 Mn12-acetate is far more rapid than predicted by the Transverse-Field Ising Ferromagnetic Model (TFIFM). It appears that solvent disorder in Mn12-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 (≈±1°) 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 Mn12 variant, Mn12-acetate–MeOH, with the same spin structure and similar lattice constants but without the same solvent disorder as Mn12-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.

Effect of easy axis tilts on magnetic order. In zero field a crystal with defects and a crystal in which there are easy axis tilts (red spins) will order at nearly the same temperature. However, in an applied transverse field, the spins of the tilted molecules experience a field along their Ising axis. When this field becomes comparable to the intermolecular dipolar field, these spins are frozen and do not order. This leads to an effective spin dilution that results in the decrease of susceptibility and transition temperature.Figure optionsDownload as PowerPoint slide