Muon spin relaxation study of La1-xCaxMnO3 - art. no. 094408

We report predominantly zero-field muon spin relaxation measurements in a s eries of Ca-doped LaMnO3 compounds which includes the colossal magnetoresis tive manganites. Our principal result is a systematic study of the spin-lat tice relaxation rates 1/T-1 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 w e find very narrow critical regions near the Neel temperatures T-N and temp erature independent 1/T-1 values above T-N. From the 1/T-1 in LaMnO3 we der ive an exchange integral J = 0.85 meV which is consistent with the mean-fie ld expression for T-N. All Of the doped manganites except CaMnO3 display an omalously slow, spatially inhomogeneous spin-lattice relaxation below their ordering temperatures. In the ferromagnetic (FM) insulating La0.82Ca0.18Mn O3 and ferromagnetic conducting La0.67Ca0.33MnO3 systems we show that there exists a bimodal distribution of mu SR rates lambda (f) and lambda (s) ass ociated with relatively ''fast" and "slow" Mn fluctuation rates, respective ly. A physical picture is hypothesized for these FM phases in which the fas t Mn rates are due to overdamped spin waves characteristic of a disordered FM, and the slower Mn relaxation rates derive from distinct, relatively ins ulating regions in the sample. Finally, likely muon sites are identified, a nd evidence for muon diffusion in these materials is discussed.