Electron magnetic resonance inLa1−xCaxMnO3(x=0.18,0.20,0.22): Crossing through the boundary between ferromagnetic insulating and metallic ground states

Abstract

Electron magnetic resonance (EMR) measurements were employed for studying the magnetic state of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ crystalline samples with $x=0.18,$ 0.20, and 0.22. It is shown that EMR spectra of ${\mathrm{La}}_{0.82}{\mathrm{Ca}}_{0.18}{\mathrm{MnO}}_{3}$ and ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_{3}$ become drastically different from those of ${\mathrm{La}}_{0.78}{\mathrm{Ca}}_{0.22}{\mathrm{MnO}}_{3}$ below the Curie points of these compounds. Such spectra of ${\mathrm{La}}_{0.82}{\mathrm{Ca}}_{0.18}{\mathrm{MnO}}_{3}$ and ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_{3}$ below ${T}_{C}$ are constituted of low-intensity doublet and relatively intense singlet signals attributed to ferromagnetic insulating and ferromagnetic metallic phases, respectively. The spectra of ${\mathrm{La}}_{0.78}{\mathrm{Ca}}_{0.22}{\mathrm{MnO}}_{3}$ have a shape of a broad asymmetric singlet signal similar to that observed in homogeneous ferromagnetic metallic manganites. The EMR data obtained on ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ were compared with the recent results on magnetic, conductive, and nuclear magnetic resonance properties of these compounds and discussed in the context of transition from electronically separated phase to a homogeneous state with increasing Ca content.