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Résumé |
Manganese oxides AMn7O12 (A=Na, Ca, La) with quadruple perovskite structure display charge, spin and magnetic orderings not observed in simple perovskites. Here we study the magnetic structure of the single-valent compounds LaMn7O12 and BiMn7O12, the quadruple perovskite counterpart of simple perovskites LaMnO3 and BiMnO3, respectively. By means of a neutron diffraction study, we found that in both compounds the A’ and B ions order antiferromagnetically (AFM) at low temperatures in an independent fashion. However, in spite of the common Mn3+ properties, the magnetic structures are strikingly different in the two compounds. In particular, in LaMn7O12 the structure of the B-site is of C-type, which contrasts the A-type structure reported for LaMnO3. On the other hand, in BiMn7O12, the AFM structure of the B-site is characterized by two distinct propagation vectors (1/2,0,1/2) and (0,0,0). We discuss the origin of these differences in terms of the polar properties of the Bi 6s2 lone pair, which gives a first evidence of coupling between magnetic and ferroelectric order parameters. |
