Spin dynamics and magnetic phase diagram of La_1-xCa_xMnO₃ (0<~x<~0.15)

Spin dynamics of ceramic La_1-xCa_xMnO₃ with x=0, 0.05, and 0.15 and the relative Mn⁴⁺ concentration c=0.18–0.22 is investigated by measurements of the ac susceptibility and the relaxation of the thermoremanent magnetization (TRM). The χ_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 χ_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, τ/τ₀=(T_f/T_G-1)^-zv with τ=1/f, zv=12±2, τ₀∼10^-12s 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∼(2-4)×10³s and the function S(T) shows a sharp maximum near T_G. The plots of S(T) exhibit a substantial decrease at T>T_G while for T<T_G they collapse into a single curve, as typical for mixed (T_G<T<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.