Defects as a Mechanism for Asymmetric Lineshapes of Electron Paramagnetic Resonance Spectra for the Molecular Magnet Mn12

We study the effect of defects on the lineshapes of Electron Paramagnetic Resonance (EPR) spectra for the molecular magnet Mn12 when an applied field along the crystal c axis is swept with the frequency held fixed. Defects in a sample may cause local rotations of the magnetization easy axes of the molecules. Perturbation theory with a single-spin Hamiltonian expressed in terms of spin operators along the molecular axes, indicates that these local rotations produce asymmetric EPR spectra, that the direction of the asymmetry depends on the energy well within which the EPR transition occurs, and that its magnitude depends on the frequency, the energy level, and the distribution of the tilt angle. To predict observable spectra, we calculate the probability distribution functions for the resonant fields with distributions in the tilt angle, the uniaxial anisotropy parameter, and the Landé g-factor. We also discuss optimal conditions under which the asymmetry may be observed experimentally. Knowledge of the asymmetries in the spectra should enable 1 one to estimate the width of the distribution of easy-axis directions. This information may be relevant to a distribution in tunnel splittings for Mn12. PACS numbers: 75.50.Xx, 76.30.-v, 75.45.+j, 61.72.Hh Typeset using REVTEX 2