Ionic size effect in charge-orderedLa0.5Ca0.5MnO3

Abstract

Magnetization, resistivity, and neutron diffraction measurements have been carried out on a series of samples ${\mathrm{La}}_{0.5\ensuremath{-}x}{\mathrm{Y}}_{x}{\mathrm{Ca}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ ($x=0$, 0.1, 0.18), to study the influence of ionic radii in charge-ordered manganites. ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ exhibits ferromagnetic (FM) transition at ${T}_{C}\ensuremath{\sim}230\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and antiferromagnetic (AFM) transition at ${T}_{N}\ensuremath{\sim}170\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Our experiments suggest presence of ferromagnetic clusters in charge-ordered (CO) AFM matrix at $T<{T}_{N}$. The CO state melts under the influence of magnetic field 5 T and a large (99% at 50 K) magnetoresistance (MR) is observed. Substituting with $\mathrm{Y}$ leads to decrease in unit cell volume and increase in substitution induced disorder ${\ensuremath{\sigma}}^{2}$ due to ionic size mismatch between $\mathrm{Y}$ and La. Phase separation effects as a result of the disorder are observed in all the samples studied. Significant changes in long-range magnetic ordering and MR behavior are observed as a result of substitution of La with $\mathrm{Y}$. For $x=0.1$ long-range AFM ordering is suppressed while FM clusters grow in size. Additionally, the MR is found to be negligibly small in comparison with $x=0$. This and other behavior observed as a result of disorder are discussed within the context of phase separation effects in charge ordered systems.