Phase separation and isotope effect in the ferromagnetic insulating state of thePr1−xCaxMnO3system(0.2<x<0.33)

Abstract

The magnetic and transport properties of the ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ 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 $\ensuremath{\rho}$ of the ceramic samples with $x=0.2,$ 0.25, 0.27, 0.29, 0.30, and 0.33 were measured at temperatures $T=4--300\mathrm{K}$ 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 ${}^{16}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{O}}}^{18}\mathrm{O}$ 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.