Theory of EPR lineshape in samples concentrated in paramagnetic spins: effect of enhanced internal magnetic field on high-field high-frequency (HFHF) EPR lineshape.

A theoretical treatment is provided for the calculation of EPR (electron paramagnetic resonance) lineshape as affected by interactions with paramagnetic ions in the vicinity. The internal fields seen by the various paramagnetic ions due to interactions with paramagnetic ions in their vicinity, as well as the resulting lineshapes, become quite significant at high magnetic fields required in high-frequency (HFHF) EPR. The resulting EPR signals for the various ions are therefore characterized by different g-shifts and lineshapes, so that the overall EPR lineshape, which is an overlap of these, becomes distorted, or even split in HFHF EPR, from that observed at lower frequencies. The observed EPR lineshapes in MnSO(4)⋅H(2)O powder and K(3)CrO(8) single-crystal samples have been simulated here taking into account g-shifts and modified lineshapes. These simulations show that in these samples, concentrated in paramagnetic spins, the position and lineshapes of EPR signals are significantly modified in HFHF EPR involving very high magnetic fields.