Low-field magnetic properties of ceramic La 1- x Ca x MnO 3 (0⩽ x ⩽0.4) are investigated between T = 5 and 310 K. The paramagnetic-ferromagnetic transition is observed in all the samples. The dependence of the Curie temperature, T C , on x is described within a model of spin polarons associated with electronic localization. Critical behaviour of the susceptibility χ -1 ( T ) ~ ( T - T C ) γ is observed for T > T C , with the critical exponents γ = 1.20±0.05 and γ* = 1.64±0.06 below and above the composition x ≈0.18 corresponding to the Mn 4+ ion concentration c ≈ 0.23, respectively. For the compound with x = 0.3 no temperature hysteresis of the resistivity is observed in magnetic fields between 0 and 8 T. In all the samples the field-cooled 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. The relaxation rate reaches a maximum near a wait time t W ~ 10 3 s. The time dependence of TRM can be described with a stretched exponential law, as in spin or cluster glasses in conditions where the observation time is comparable with t W .
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Laiho et al. (2000) studied this question.
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