Effects of D-strain, g-strain, and dipolar interactions on EPR linewidths of the molecular magnets Fe-8 and Mn-12

Electron paramagnetic resonance 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 Lande 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.