[en] A detailed study of step-like metamagnetic transitions in polycrystalline Pr_0.75Na_0.25Mn_1-xFe_xO₃ (0 ≤ x ≤ 0.30) is reported. It shows that the charge-orbit ordering antiferromagnetic (COOAF) phase in the phase-separated materials is responsible for the step-like metamagnetic transition with extremely sharp steps (width <0.01 T). The fraction of antiferromagnet II (AFII) with larger antiferromagnetic exchange interaction than COOAF is proved to be the effective factor of critical fields (H_c). A model of anisotropy barrier for up/down thermal blocking spins has been proposed to explain the sharp step-like transitions in polycrystalline manganites for the first time. The metamagnetic transitions at 2 K (below the spin blocking temperature T_B) and high temperatures are compared. Both sorts of metamagnetic transitions originate from COOAF phase. And the temperature is responsible for the difference between them