Detailed single-crystal EPR line shape measurements for the single-molecule magnets ${\mathrm{Fe}}_8\mathrm{Br}$ and ${\mathrm{Mn}}_{12}-\mathrm{acetate}$

It is shown that our multi-high-frequency $\left(40\mathrm{}-200\hspace{0.5em}\mathrm{GHz}\right)$ resonant cavity technique yields distortion-free high-field electron paramagnetic resonance (EPR) spectra for single-crystal samples of the uniaxial and biaxial spin $S=10\mathrm{}$ single-molecule magnets (SMM’s) $\left[{\mathrm{Mn}}_{12}{\mathrm{O}}_{12}\right({\mathrm{CH}}_3\mathrm{COO}{)}_{16}\left({\mathrm{H}}_2\mathrm{O}{)}_4\right]\cdot 2{\mathrm{CH}}_3\mathrm{COOH}\cdot 4{\mathrm{H}}_2\mathrm{O}$ and $\left[{\mathrm{Fe}}_8{\mathrm{O}}_2\right(\mathrm{OH}{)}_{12}\left(\mathrm{tacn}{)}_6\right]{\mathrm{Br}}_8\cdot 9{\mathrm{H}}_2\mathrm{O}.$ 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 ${\mathrm{Fe}}_8$ and a previously unobserved fine structure to some of the EPR lines for both the ${\mathrm{Fe}}_8$ and ${\mathrm{Mn}}_{12}$ systems. These findings prove relevant to the mechanism of quantum tunneling of magnetization in these SMM’s.