Muon spin relaxation study of La1-xCaxMnO3

We report predominantly zero-field muon spin relaxation measurements in a series of Ca-doped LaMnO3 compounds which includes the colossal magnetoresistive manganites. Our principal result is a systematic study of the spin-lattice relaxation rates 1/T1 and magnetic order parameters in the series La1-xCaxMnO3,x=0.0, 0.06, 0.18, 0.33, 0.67, and 1.0. In LaMnO3 and CaMnO3 we find very narrow critical regions near the Néel temperatures TN and temperature independent 1/T1 values above TN. From the 1/T1 in LaMnO3 we derive an exchange integral J=0.85 meV which is consistent with the mean-field expression for TN. All of the doped manganites except CaMnO3 display anomalously slow, spatially inhomogeneous spin-lattice relaxation below their ordering temperatures. In the ferromagnetic (FM) insulating La0.82Ca0.18MnO3 and ferromagnetic conducting La0.67Ca0.33MnO3 systems we show that there exists a bimodal distribution of μSR rates λf and λs associated with relatively “fast” and “slow” Mn fluctuation rates, respectively. A physical picture is hypothesized for these FM phases in which the fast Mn rates are due to overdamped spin waves characteristic of a disordered FM, and the slower Mn relaxation rates derive from distinct, relatively insulating regions in the sample. Finally, likely muon sites are identified, and evidence for muon diffusion in these materials is discussed.