The reverse shift of the EPR line of paramagnetic centers coupled to species with a fast paramagnetic relaxation

The EPR spectrum of the spin 1/2 paramagnetic centers with a relatively slow relaxation is considered in the case when they are coupled via the Heisenberg exchange interaction to partners which have short times of the longitudinal and transverse paramagnetic relaxation. Under these conditions only the EPR line of paramagnetic centers with a relatively slow relaxation is detectable in experiment. The shape of this line is analyzed by solving numerically kinetic equations for the spin density matrix for simple model systems. Depending on a ratio between the exchange integral and the paramagnetic relaxation rates of partner spins, the EPR line shifts in opposite directions. For moderate relaxation rates, as the relaxation rates decrease, the EPR line shifts toward the gravity center of the total EPR spectrum. In the case of extremely fast relaxation, as the relaxation rates decrease, the reverse shift of the EPR line is expected, the line shifts away from the gravity center of the total EPR spectrum. This type of the non-monotonous line shift was experimentally observed for the monocrystal of [CuNd2(C4O4)4(H2O)16] · 2H2O when relaxation rates were changed by temperature variation.