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

► Theory of EPR lineshape due to interactions with paramagnetic ions in the vicinity. ► Existence of significant internal magnetic fields due to PM ions in HFHF EPR. ► The resulting EPR line shapes becomes distorted, or even split in HFHF EPR. ► EPR lineshapes in MnSO4⋅H2O powder and K3CrO8 single-crystal exhibit the above effects.