Effects of D-strain, g-strain, and dipolar interactions on EPR linewidths of the molecular magnets Fe8 and Mn12

Electron paramagnetic resonance (EPR) measurements on single crystals of the molecular magnets Fe$_8$ and Mn$_{12}$ reveal complex nonlinearities in the linewidths as functions of energy eigenstate, frequency, and temperature. Using a density-matrix equation with distributions of the uniaxial anisotropy parameter $D$, the Land\'{e} $g$ factor, and dipolar interactions, we obtain linewidths in good agreement with experiments. Our study shows that the distribution in $D$ is common to the examined molecular magnets Fe$_8$ and Mn$_{12}$ regardless of the qualities of the samples. This could provide the basis for a proposed tunneling mechanism due to lattice defects. The distribution in $g$ is also quite significant for Mn$_{12}$.