Muon spin relaxation study ofLa1−xCaxMnO3

Abstract

We report predominantly zero-field muon spin relaxation measurements in a series of Ca-doped ${\mathrm{LaMnO}}_3$ compounds which includes the colossal magnetoresistive manganites. Our principal result is a systematic study of the spin-lattice relaxation rates ${1/T}_1$ and magnetic order parameters in the series ${\mathrm{La}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_3,x=0.0,\hspace{0.5em}0.06,\hspace{0.5em}0.18,\hspace{0.5em}0.33,\hspace{0.5em}0.67,$ and 1.0. In ${\mathrm{LaMnO}}_3$ and ${\mathrm{CaMnO}}_3$ we find very narrow critical regions near the Néel temperatures $T_N$ and temperature independent ${1/T}_1$ values above $T_N.$ From the ${1/T}_1$ in ${\mathrm{LaMnO}}_3$ we derive an exchange integral $J=0.85\mathrm{}\hspace{0.5em}\mathrm{meV}$ which is consistent with the mean-field expression for $T_N.$ All of the doped manganites except ${\mathrm{CaMnO}}_3$ display anomalously slow, spatially inhomogeneous spin-lattice relaxation below their ordering temperatures. In the ferromagnetic (FM) insulating ${\mathrm{La}}_{0.82}{\mathrm{Ca}}_{0.18}{\mathrm{MnO}}_3$ and ferromagnetic conducting ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_3$ systems we show that there exists a bimodal distribution of $\mu \mathrm{SR}$ rates ${\lambda }_f$ and ${\lambda }_s$ 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.