Detailed single-crystal EPR line shape measurements for the single-molecule magnets Fe8Br and Mn12-acetate

It is shown that our multi-high-frequency (402200 GHz) resonant cavity technique yields distortion-free high-field electron paramagnetic resonance ~EPR! spectra for single-crystal samples of the uniaxial and biaxial spin S510 single-molecule magnets ~SMM’s! @Mn12O12(CH3COO)16(H2O)4#•2CH3COOH•4H2O and @Fe8O2(OH)12(tacn)6#Br8•9H2O. 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 Fe8 and a previously unobserved fine structure to some of the EPR lines for both the Fe8 and Mn12 systems. These findings prove relevant to the mechanism of quantum tunneling of magnetization in these SMM’s.