Doping effects on the phase separation in perovskite La0.67-xBixCa0.33MnO3

Abstract

Effects of Bi, Cr, and Fe doping on phase separation of La0.67Ca0.33MnO3 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 (FM) phase and charge-ordered antiferromagnetic phase. As a consequence, two subsequent magnetic transitions at ∼120 K and ∼230 K are resulted. A strong coupling between the coexisted phases is assumed, which is responsible for the insensitivity of Tc(L), the higher Curie temperature, to Bi doping after the appearance of phase separation, and consistent with the discontinuous variation of Tc(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 ∼100 neighboring unit cells, for the high-moment state, and ∼60 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 ∼90%. 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 ∼4.6% Bi. In contrast, both Cr doping and magnetic field promote the FM order. 1% Cr correspond to a field of ∼4.5 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 ∼0.96 meV/Mn and ∼1.3 meV/Mn, respectively.