Phase separation and isotope effect in the ferromagnetic insulating state of the Pr_1-xCa_xMnO₃ system (0.2<x<0.33)

The magnetic and transport properties of the Pr_1-xCa_xMnO₃ system were studied at the characteristic points of the phase diagram corresponding to the ferromagnetic insulating and charge-ordered antiferromagnetic states. The magnetization M and resistivity ρ of the ceramic samples with x=0.2, 0.25, 0.27, 0.29, 0.30, and 0.33 were measured at temperatures T=4–300K and applied magnetic fields H up to 7 T. It was shown that the Curie temperature T_C exhibits a nonmonotonic change with x. The maximum value of T_C was achieved at x=0.25. The isotope substitution ¹⁶O⃗¹⁸O was performed for the samples with x=0.2,0.25, and 0.30. A pronounced isotope effect in T_C was found. A noticeable drop in the activation energy E_a below the Curie temperature and a decrease of resistivity in the magnetic field both in the ferromagnetic insulator phase and in the phase-separated region at x>0.3 were observed. Special attention is paid to the possible manifestations of the phase separation in the ferromagnetic insulating state.