Electron magnetic resonance in La_1−xCa_xMnO₃ (x=0.18, 0.20, 0.22): Crossing through the boundary between ferromagnetic insulating and metallic ground states

Electron magnetic resonance (EMR) measurements were employed for studying the magnetic state of La_1−xCa_xMnO₃ crystalline samples with x=0.18, 0.20, and 0.22. It is shown that EMR spectra of La_0.82Ca_0.18MnO₃ and La_0.8Ca_0.2MnO₃ become drastically different from those of La_0.78Ca_0.22MnO₃ below the Curie points of these compounds. Such spectra of La_0.82Ca_0.18MnO₃ and La_0.8Ca_0.2MnO₃ below TC are constituted of low-intensity doublet and relatively intense singlet signals attributed to ferromagnetic insulating and ferromagnetic metallic phases, respectively. The spectra of La_0.78Ca_0.22MnO₃ have a shape of a broad asymmetric singlet signal similar to that observed in homogeneous ferromagnetic metallic manganites. The EMR data obtained on La_1−xCa_xMnO₃ 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.