Effects of D-strain, g-strain, and dipolar interactions on EPR linewidths of the molecular magnets ${\mathrm{Fe}}_8$ and ${\mathrm{Mn}}_{12}$

Electron paramagnetic resonance measurements on single crystals of the molecular magnets ${\mathrm{Fe}}_8$ and ${\mathrm{Mn}}_{12}$ 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 Landé 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 ${\mathrm{Fe}}_8$ and ${\mathrm{Mn}}_{12}$ 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 ${\mathrm{Mn}}_{12}.$