Effects of Bi, Cr, and Fe doping on phase separation of La 0.67 Ca 0.33 MnO 3 have been experimentally studied. As proved by the electron-spin resonance and neutron-diffraction studies, partial replacement of La by Bi causes the simultaneous occurrence of ferromagnetic ͑FM͒ phase and charge-ordered antiferromagnetic phase. As a consequence, two subsequent magnetic transitions at ϳ120 K and ϳ230 K are resulted. A strong coupling between the coexisted phases is assumed, which is responsible for the insensitivity of T c (L), the higher Curie temperature, to Bi doping after the appearance of phase separation, and consistent with the discontinuous variation of T c (L) with Cr doping. As expected, the substitution of Cr for Mn in this case promotes the FM order, but its effects are significantly different for the two magnetic states. Each Cr drives ϳ100 neighboring unit cells, for the high-moment state, and ϳ60 unit cells, for the low-moment state, into the FM state. Two definite processes can be identified for the melting of the charge-ordered phase. The FM fraction increases rapidly in the initial stage of Cr doping, and then slowly when the FM population exceeds ϳ90%. This could be a common feature of the phase-separated system suffering from random-phase fluctuation according to a theoretical analysis. Exactly opposite effects on phase constituent are produced by Cr doping and Bi doping, and 1% Cr are equivalent to ϳ4.6% Bi. In contrast, both Cr doping and magnetic field promote the FM order. 1% Cr correspond to a field of ϳ4.5 T for the low-moment state and 6 T for the high-moment state, reducing the energy difference between the charge ordering and the FM states by ϳ0.96 meV/Mn and ϳ1.3 meV/Mn, respectively.

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Partial replacement of La by Bi in La0.67Ca0.33MnO3 depresses the ferromagnetic order of the compound and leads to a stepwise magnetic behavior characterized by two subsequent transitions at ∼ 120 K and ∼ 225 K. Based on a combined study of the electron spin resonance spectra and the thermal and isothermal magnetization, a complete scenario for the phase separation that causes the complex behavior has been given. It is found that the paramagnetic phase exists in a wide temperature region even below the ferromagnetic onset at ∼ 225 K. It coexists with the ferromagnetic phase below ∼ 240 K and with both the ferromagnetic and antiferromagnetic phases below ∼ 190 K with a reduced but significant fraction. The magnetization jump at ∼ 120 K is possibly a conversion of the remaining paramagnetic phase into the ferromagnetic phase.

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