Determination of the exchange interaction between dissimilar copper ions by X- and Q-band electron paramagnetic resonance (EPR) in 3D Heisenberg ferromagnetic systems M2CuCl4 · 2H2O (M = Rb, K, NH4)

In the case of rather strong exchange, we have used a method already published by two of us in order to determine for the Rb compound J′/k = (0.28±0.03) K. In the case of rather weak exchange, we have used a method proposed by Hoffmann in order to determine J′/k = (0.050±0.002) K for the K compound and J′/k = (0.050±0.002) K for the NH4 compound