Evolution of the ferromagnetic and nonferromagnetic phases with temperature in phase-separated La_1−xSr_xCoO₃ by high-field ¹³⁹La NMR

We have investigated microscopic magnetoelectronic phase separation and phase evolution with temperature in La_1−xSr_xCoO₃, for x in the range zero to 0.3, using high-field ¹³⁹La NMR. For x⩾0.1 three coexisting magnetic phases, ferromagnetic (FM), paramagnetic (PM), and spin glass (SG) or cluster glass are observed in the spectra over a wide temperature range. The temperature evolution of the phases shows some conversion of SG to FM phase occurs as the temperature is raised above 30 K. This temperature range coincides with the interval in which Co ions undergo a low-spin (LS) to intermediate-spin (IS) state transition in undoped or very lightly doped material. Measurements on undoped and lightly doped (x=0.03) samples provide information on line shape, nuclear relaxation rates, and the LS to IS transition of the Co ions in the PM material. At high temperatures approaching the maximum measured Curie temperature of ∼240 K, found for x⩾0.3, the FM phase signal changes and shifts significantly following the magnetization. Evidence for the existence of magnetic entities such as spin polarons coexisting with the PM phase is found at the higher temperatures.