Static and dynamic behavior of the cluster phase in LaMn0.6Co0.4O3

A non-uniform magnetic state in the perovskite system LaMn0.6Co0.4O3 has been studied by magnetometric methods. Two ferromagnetic (FM) critical temperatures are observed. The first one at TC1≈217 K is ascribed to the onset of FM state in the Mn4+/Co2+-ordered domains (phase 1), while the disordered regions containing Mn4+, Mn3+, Co3+ and Co2+ valence states (phase 2) remain in the paramagnetic state. Below TC2≈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.