Electron paramagnetic resonance linewidths and line shapes for the molecular magnets Fe8 and Mn12

Abstract

We study theoretically electron paramagnetic resonance (EPR) linewidths for single crystals of the molecular magnets Fe8 and Mn12 as functions of energy eigenstates Ms, frequency, and temperature when a magnetic field along the easy axis is swept at fixed excitation frequency. This work was motivated by recent EPR experiments. To calculate the linewidths, we use density-matrix equations, including dipolar interactions and distributions of the uniaxial anisotropy parameter D and the Landé g factor. Our calculated linewidths agree well with the experimental data. We also examine the line shapes of the EPR spectra due to local rotations of the magnetic anisotropy axes caused by defects in samples. Our preliminary results predict that this effect leads to asymmetry in the EPR spectra.