D-Strain g-STRAIN, and Dipolar Interactions in the Fe8 and Mn12 Single Molecule Magnets:. AN EPR Lineshape Analysis

We report high sensitivity, high field/frequency (up to 9 tesla/210 GHz) EPR measurements on the Fe₈ and Mn₁₂ single molecule magnets. Oriented single crystal studies, carried out in well defined experimental geometries, enable very accurate determinations of EPR lineshapes; we note that this is not possible when studying aligned powders or polycrystals. Analysis of individual resonances, which we can assign to known transitions, reveal complex nonlinearities in the linewidths as functions of energy eigenstate, frequency, and temperature. Our findings compare favorably with recent calculations which introduce distributions in the uniaxial anisotropy parameter D, the Landé g-factor, and dipolar interactions.