High Frequency Electron Paramagnetic Resonance Studies of Single Molecule Magnets

Single crystal samples of different single-molecule magnets (SMMs) were studied by High Frequency Electron Paramagnetic Resonance (HFEPR) in order to characterize their magnetic properties. Multiple studies have been conducted to determine the spin Hamiltonian parameters for different systems, including various Mn and Ni based SMMs. The use of single crystals allows studies in multiple orientations: when the sample's easy axis of magnetization is aligned parallel (perpendicular) to the external field, the axial (transverse) anisotropy parameters can be determined. Variable temperature studies allow the determination of the sign of the axial anisotropy parameters, as well as the energy difference between the ground state and excited states. Multiple orientation studies in a Zn system that is lightly doped with Ni allow determination of the anisotropy parameters for a single Ni$^{+2}$ ion. These results can then be compared to those for the tetranuclear nickel complex consisting of 4 Ni$^{+2}$ ions, enabling an explanation for the fast magnetic quantum tunneling in this system. Finally, analysis of absorption peak linewidths enables characterization of the disorder in SMMs. All of these studies have contributed to a better understanding of the magnetic quantum tunneling behavior in these materials.