MAGNETOELASTIC COUPLING IN CaMn7O12

The mixed manganese oxide CaMn7O12 is a multiferroic material [1] with a distorted perovskite structure [2]. The crystal structure [3] and the magnetic ordering [4] of CaMn7O12 have been studied by using high resolution SR diffraction and high resolution neutron diffraction. These diffraction studies show a modulation of the atomic positions in CaMn7O12 at temperatures below 250 K and magnetic structure modulation below TN = 90 K [5,6]. The modulation of atomic positions has been described by using a quantitative model with a propagation vector (0,0,qp) [5] which gives good agreement with the results of both SR and neutron diffraction studies [6]. The neutron diffraction studies of CaMn7O12 show a modulated magnetic ordering below the Neel temperature TN = 90 K. The modulation of the magnetic structure is described with a propagation vector (0,0,qm). Both magnetic and positional modulations are coupled together. The modulation length of the atomic positions (Lp) and of the magnetic moment modulation length (Lm) fulfill the relation Lm= 2 Lp at temperatures between 50 K and the Neel temperature TN. The relation between modulation lengths Lp and Lm as well as minimum and maximum of the lattice constant c clearly show the magnetoelastic coupling in CaMn7O12 [1]. Below 50 K, there is a magnetic phase transition which is associated with important changes of the magnetic modulation length and also with the increase of magnetoelectric coupling [1].