Visualizing high-temperature spin dynamics in La1-xCaxMnO3 from a mapping of EPR linewidth and g-factor

We report the measurements of electron paramagnetic resonance (EPR) on powder samples of La1-xCaxMnO3 (LCMO) at the commensurate carrier concentrations of x = N/8 (N = 1,2,3,4,5,6, and 7) within the temperature range 100 K≤T≤450 K. It is found that the mapping of the EPR linewidth ΔH and the g-factor shows an electron-hole asymmetry in the paramagnetic insulating regime of the LCMO phase diagram. The drop of linewidth ΔH with decreasing temperature in the mapping resembles the contour of the ferromagnetic transition curve in the LCMO phase diagram, while the isothermal ΔH becomes narrowest at x = 3/8 due to the predominant double exchange interaction. By comparing the small-polaron model with spin-spin relaxation mechanism, we demonstrate that the latter should dominate the paramagnetic insulating regime of the LCMO phase diagram