Coexistence of ferromagnetic and spin-glass phenomena in La1-xCaxMnO3 (0 <= x <= 0.4)

Low-field magnetic properties of ceramic La1-xCaMnO3 (0 less than or equal to x less than or equal to 0.4) are investigated between T = 5 and 310 K. T he paramagnetic-ferromagnetic transition is observed in all the samples. Th e dependence of the Curie temperature, Tc, on x is described within a model of spin polarons associated with electronic localization. Critical behavio ur of the susceptibility X-1(T) similar to (T - TC)(gamma) is observed for T > Tc, with the critical exponents gamma = 1.20 +/- 0.05 and gamma* = 1.64 +/- 0.06 below and above the composition x approximate to 0.18 correspondi ng to the Mn4+ ion concentration c approximate to 0.23, respectively. For t he compound with x = 0.3 no temperature hysteresis of the resistivity is ob served in magnetic fields between 0 and 8 T In all the samples the field-co oled and zero-field-cooled magnetizations deviate below T-C, the difference being approximately equal to the thermoremanent magnetization (TRM). Long- time relaxation of TRM in LCMO is observed for time scales up to 10(4) s. T he relaxation rate reaches a maximum near a wait time t(W) similar to 10(3) s. The time dependence of TRM can be described with a stretched exponentia l law, as in spin or cluster glasses in conditions where the observation ti me is comparable with t(W).