[en] We investigate the low-temperature magnetic properties of intermetallic compounds Tb_1_−_xPr_x(Fe_0_._4Co_0_._6)_1_._8_8C_0_._0_5 (x = 0, 0.8, and 1) by detailed magnetization measurements. Obvious temperature- and field-induced irreversibilities suggest the coexistence of multiple magnetic phases. Sharp magnetization jumps across the antiferromagnetic to ferromagnetic transition are observed only in the Pr-containing samples, indicating that the behavior of the avalanche-like growth of ferromagnetic clusters is mainly related to the light lanthanide Pr ions. In addition, the time relaxation, field sweep rate, and cooling field dependence of magnetization jumps in the sample with x = 1 are consistent with those in the martensitic scenario. (paper)