Detailed single-crystal EPR line shape measurements for the single-molecule magnets Fe8Br and Mn12-acetate -: art. no. 224410

It is shown that our multi-high-frequency (40-200 GHz) resonant cavity technique yields distortion-free high-field electron paramagnetic resonance (EPR) spectra for single-crystal samples of the uniaxial and biaxial spin S=10 single-molecule magnets (SMM's) [Mn12O12(CH3COO)(16)(H2O)(4)].2CH(3)COOH.4H(2)O and [Fe8O2(OH)(12)(tacn)(6)]Br-8.9H(2)O. The observed line shapes exhibit a pronounced dependence on temperature, magnetic field, and the spin quantum numbers (M-S values) associated with the levels involved in the transitions. Measurements at many frequencies allow us to separate various contributions to the EPR linewidths, including significant D strain, g strain, and broadening due to the random dipolar fields of neighboring molecules. We also identify asymmetry in some of the EPR line shapes for Fe-8 and a previously unobserved fine structure to some of the EPR lines for both the Fe-8 and Mn-12 systems. These findings prove relevant to the mechanism of quantum tunneling of magnetization in these SMM's.