Competing magnetic phases in mixed-valent manganese oxide perovskites

Abstract

Measurement of specific heat ${C}_{p}(T)$ below $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ of melt-grown samples of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.3)$ and ${R}_{0.7}{A}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ $(R=\text{rare}\text{\ensuremath{-}}\text{earth},A=\text{alkaline}\text{\ensuremath{-}}\text{earth})$ with room-temperature tolerance factor $0.950\ensuremath{\leqslant}t\ensuremath{\leqslant}0.996$ have been supplemented by transport and magnetic measurements. Comparison of the phase diagram of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ with that of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ and the evolution with $t$ of the ${R}_{0.7}{A}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ family illustrate the sensitivity to $t$ of the crossover from localized to itinerant behavior of the $\ensuremath{\sigma}$-bonding electrons and support the model of two magnetic phases in the crossover compositional range that has been used to account for the colossal magnetoresistance (CMR) phenomenon found in these oxides. A vanishing of the specific-heat anomaly at the Curie temperature ${T}_{c}$ and the magnetic data at crossover are typical of a spin glass, and a broad hump in ${C}_{p}(T)$ below a ${T}_{h}>{T}_{c}$, where there is no anomaly at the ${T}_{c}$ signal ferromagnetic ordering within isolated pockets of a hole-rich, conductive $\mathrm{O}*$ minority phase at ${T}_{h}$. On cooling through ${T}_{N}$ of the antiferromagnetic matrix, the spins freeze at a spin-glass temperature ${T}_{g}$ in zero magnetic field $H$ if the ferromagnetic phase does not percolate; the ferromagnetic phase grows in an applied $H$, and a modest $H$ converts the spin glass to a bulk ferromagnet with a Curie temperature ${T}_{c}\ensuremath{\approx}{T}_{g}$, where the ferromagnetic phase grows to beyond percolation. As $x$ increases in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$, a ferromagnetic-insulator ${\mathrm{O}}^{\ensuremath{''}}$ phase having a charge ordering and a different orbital ordering than the parent ${\mathrm{O}}^{\ensuremath{'}}$ phase percolates below a ${T}_{g}\ensuremath{\approx}{T}_{c}$, and the minority ${\mathrm{O}}^{\ensuremath{'}}$ phase remains paramagnetic until it becomes antiferromagnetic below a ${T}_{M}<{T}_{c}\ensuremath{\approx}{T}_{g}$. In the interval $0.15<x<0.25$, an orbitally disordered conductive, ferromagnetic vibronic phase appears in a narrow temperature interval ${T}_{\mathrm{oo}}<T<{T}_{c}$, the majority phase transforming to the charge and orbitally ordered ${\mathrm{O}}^{\ensuremath{''}}$ phase below a ${T}_{\mathrm{oo}}$. At $x\ensuremath{\geqslant}0.25$, the system transforms from a polaronic paramagnetic to a ferromagnetic metal at ${T}_{c}$.