  We examine theoretically electron paramagnetic resonance (EPR) lineshapes as
functions of resonance frequency, energy level, and temperature for single
crystals of three different kinds of single-molecule nanomagnets (SMMs):
Mn$_{12}$ acetate, Fe$_8$Br, and the $S=9/2$ Mn$_4$ compound. We use a
density-matrix equation and consider distributions in the uniaxial
(second-order) anisotropy parameter $D$ and the $g$ factor, caused by possible
defects in the samples. Additionally, weak intermolecular exchange and
electronic dipole interactions are included in a mean-field approximation. Our
calculated linewidths are in good agreement with experiments. We find that the
distribution in $D$ is common to the three examined single-molecule magnets.
This could provide a basis for a proposed tunneling mechanism due to lattice
defects or imperfections. We also find that weak intermolecular exchange and
dipolar interactions are mainly responsible for the temperature dependence of
the lineshapes for all three SMMs, and that the intermolecular exchange
interaction is more significant for Mn$_4$ than for the other two SMMs. This
finding is consistent with earlier experiments and suggests the role of
spin-spin relaxation processes in the mechanism of magnetization tunneling.

Materials Science