Static and dynamic behavior of the cluster phase in LaMn_(0.6)Co_(0.4)O)3

摘要

A non-uniform magnetic state in the perovskite system LaMn_(0.6)Co_(0.4)O)3 has been studied by magnetometric methods. Two ferromagnetic (FM) critical temperatures are observed. The first one at T_(C1)217 K is ascribed to the onset of FM state in the Mn~(4+)/Co~(2+)-ordered domains (phase 1), while the disordered regions containing Mn~(4+), Mn~(3+), Co~(3+) and Co~(2+) valence states (phase 2) remain in the paramagnetic state. Below T_(C2)≈147 K a long-range FM arrangement develops also in certain part of phase 2, but the major part of phase 2 is formed by polydisperse FM clusters. This second transition is accompanied by a huge increase of coercivity and by occurrence of long-time relaxation. Finally, between 40 and 65 K, a frequency-dependent maximum in the imaginary part of the ac susceptibility is observed, which can be ascribed to the relaxation of small clusters behaving superparamagnetically.