Origin of the fast magnetization tunneling in tetranuclear nickel single-molecule magnets

Several tetranuclear nickel(II) single-molecule magnets (SMMs) have been prepared with the general composition of [Ni(hmp)(ROH)X](4) S, where hmp(-) is the monoanion of 2-hydroxymethylpyridine, X- is either Cl- or Br- and S is the solvate molecule. Magnetization versus magnetic field hysteresis loops for these Ni-4 SMMs show that there is a relatively fast rate of magnetization tunneling (small coercive field) and, in certain cases, an exchange bias present. Detailed measurements have been carried out in order to determine the origin of the fast magnetization tunneling. High-field electron paramagnetic resonance (HFEPR) data were collected on a single crystal of [Zn(hmp)(dmb)Cl](4) doped with a small amount of Ni(II), where, dmb is 3,3-dimethyl-1-butanol. These variable-frequency/temperature data give values of the single-ion zero-field splitting parameters D-i and E-i, and the orientations of these interactions, for the single Ni-II ions in a Zn3Ni complex doped into a Zn-4 crystal. HFEPR data were also obtained at many frequencies and temperatures for a single crystal of isostructural [Ni(hmp)(dmb)Cl](4). Rotation of the single crystal such that the external field is positioned in the hard plane clearly establishes that the transverse zero-field interaction B-4(4) is the cause of the fast magnetization tunneling in the S = 4 ground state of this SMM. The magnitude of B-4(4) and the Ni-4 D value can be related to the directionality and magnitude of the D-i and E-i interactions at the individual Ni-II ions, determined for the doped crystal. The microenvironments and ligand dynamics were probed by means of a single-crystal X-ray structure at 12 K and by heat capacity data. (c) 2005 Elsevier Ltd. All rights reserved.