Anomalous magnetic behavior in La2/3Ca1/3MnO3 near the critical point: stable clusters and crossover to uniform ferromagnetism

A detailed study of the magnetization NI(H, T) of bulk ceramic La2.3Ca1.3Mn O3 (T-c = 267 K) in the paramagnetic phase is presented. Near T-c (T - T-c < 30 K) first-order phase transition features are found. At low magnetic fi elds (H: 0.3-50 Oe), a sequence of step and plateaus in the effective Curie constant C(T) = M(T - T-c)/H is found, accompanied by temperature hysteres is. At higher fields, maxima in dM/dH are found at a characteristic field, H-c(T), that increases almost linearly with T - T-c up to a critical point (T-c* approximate to 292 K, H-c approximate to 15 kOe). These features agre e with the results of the Landau theory of phase transition with negative M -4 coefficient in the energy expansion (for first order). On the other hand , the crossover field between the H-2 and classical H-2/3 magneto resistanc e regimes corresponds closely to H-c(T) near T-c but does not reach an end point at least up to 297 K (32 kOe). (C) 2001 Published by Elsevier Science B.V.