EPR line shape and magnetometry—chances and pitfalls

Electron paramagnetic resonance (EPR) may permit chemical identification of paramagnetic defects and their atomic arrangement within a crystalline solid. EPR, being a spectroscopic method, can also be used to evaluate the number of paramagnetic centres of microscopically identified contributions to the magnetic susceptibility in contrast to conventional susceptometry which yields only integral information. Complications may arise, however, in the evaluation as exemplified in this paper. We discuss two aspects: (i) the occurrence of spin excitation mechanisms other than magnetic dipole transitions and (ii) the specific shape of the resonance line. In conducting samples (i), the spin resonance absorption due to a spin-dependent electric conductivity can easily exceed the absorption originating from magnetic dipole transitions. The Rashba coupling leads to (a) spin excitations by an electric current and (b) dependence of the conductivity on spin polarization, which varies at the resonance, and thus causes a polarization signal. There is also a dispersive contribution to the EPR signal which can be attributed to the spin–orbit interaction as well. In addition, we review some mechanisms of line broadening or line shift (ii) leading to the occurrence of very long tails of the resonance line. Such non-uniform distributions can lead to a large error in the evaluation of the integrated amplitude of electron spin resonance