Spin dynamics and magnetic phase diagram of La 1 − x Ca x MnO 3 ( 0 x 0 . 1 5 )

Spin dynamics of ceramic ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ with $x=0,$ 0.05, and 0.15 and the relative ${\mathrm{Mn}}^{4+}$ concentration $c=0.18--0.22$ is investigated by measurements of the ac susceptibility and the relaxation of the thermoremanent magnetization (TRM). The ${\ensuremath{\chi}}_{\mathrm{ac}}(T)$ function exhibits a transition from the paramagnetic to a mixed spin-glass and ferromagnetic phase. Below that is found a steep decrease and onset of the frequency dependence of ${\ensuremath{\chi}}_{\mathrm{ac}}(T),$ providing evidence for existence of a reentrant spin glass (RSG) phase. The dependence of the spin-glass freezing temperature, ${T}_{f},$ on f satisfies the conventional critical slowing-down law, $\ensuremath{\tau}/{\ensuremath{\tau}}_{0}{=(T}_{f}{/T}_{G}\ensuremath{-}{1)}^{\ensuremath{-}\mathrm{zv}}$ with $\ensuremath{\tau}=1/f,$ $zv=12\ifmmode\pm\else\textpm\fi{}2,$ ${\ensuremath{\tau}}_{0}\ensuremath{\sim}{10}^{\ensuremath{-}12}\mathrm{s}$ and the glass temperature, ${T}_{G},$ decreasing from 93 to 70.5 K when x is increased from 0 to 0.15. The relaxation rate $S(t)$ of TRM has a clear maximum near a wait time ${t}_{w}\ensuremath{\sim}(2\ensuremath{-}4)\ifmmode\times\else\texttimes\fi{}{10}^{3}\mathrm{s}$ and the function $S(T)$ shows a sharp maximum near ${T}_{G}.$ The plots of $S(T)$ exhibit a substantial decrease at $Tg{T}_{G}$ while for $Tl{T}_{G}$ they collapse into a single curve, as typical for mixed ${(T}_{G}lTl{T}_{C})$ and RSG phases, respectively. The observed spin dynamics agrees completely with the magnetic phase diagram (paramagnetic, mixed, and RSG phases with decreasing T) predicted recently for a system containing lattice disorder and competing superexchange and double exchange interactions.