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The Fokker-Planck equation is used to calculate the time decay of the magnetization for a system of two interacting particles. This approach is valid for all values of the energy barriers compared to the thermal energy, whereas the usual approach, using the Néel formula for the relaxation rate, is limited to large energy barriers. The effect of interactions can be described by an effective volume, which is proportional to the interaction strength for weakly interacting particles in zero applied field, and becomes equal to twice the particle volume for strongly interacting particles (if the ratio of the anisotropy field over the saturation magnetization is not too large). Results are also shown in the case of a small applied field.