Magnetic relaxation measurements in the hysteretic regime of the field induced transition from Antiferromagnetic (AFM) to ferromagnetic (FM) phase are carried out on a single crystal (Sm_0.95 Nd_0.05)_0.98MnO₃. The characteristic energy barriers are found to be dependent on the fraction of the FM phase, independent of the magnetic history. The results will be interpreted based on the simulations performed to obtain local field distributions due to dipolar fields for different FM fractions.