Asymmetric lineshape due to inhomogeneous broadening of the crystal-field transitions in ${\mathrm{Mn}}_{12}$ acetate single crystals

The lineshape of crystal-field transitions in oriented single crystals of ${\mathrm{Mn}}_{12}$ acetate molecular magnets is determined by the magnetic history. The absorption lines are symmetric and Gaussian for the nonmagnetized state obtained by zero-field cooling. In the magnetized state which is reached when the sample is cooled in a magnetic field, however, they are asymmetric even in the absence of an external magnetic field. These observations are quantitatively explained by inhomogeneous symmetrical (Gaussian) broadening of the crystal-field transitions combined with a contribution of off-diagonal components of the magnetic susceptibility to the effective magnetic permeability. This can be qualitatively understood as a redistribution of the interacting modes superposed in the total inhomogeneously broadened line which dynamically occurs via the magnetic medium, i.e., via different components of its susceptibility depending on the magnetic state.