Magnetic field impulse (7 T amplitude 10 ms duration) was found to affect microhardness of NaCl:Eu crystals at room temperature. Dimers (pairs of Eu2+ paramagnetic ions) were shown to be responsible for the crystal softening induced by magnetic field. Theoretical treatment of the magnetoplastic effect based on the spin dependence of processes resulting in transformation of the dimers in crystals is developed and applied to the description of the long-term magnetic memory. Activation energies of the dimer formation, E1=0.23±0.04 eV and decomposition, E2=0.33±0.06 eV were extracted from thermoactivation analysis of magnetic field controlled Eu2+ aggregation in 77–473 K temperature range.