Beats in the Magnetic Modulation of Multiferroic CaMn7O12

The modulated magnetic ordering and the atomic position modulation are studied in the case of the multiferroic CaCu x Mn 7- x O 12 by neutron and SR powder diffraction methods. Below the magnetic transition temperature of T N2 =50 K both orderings are described by using the magnetic superspace group R31'(00γ)ts. For the magnetic ordering two different incommensurate modulation vectors are used: (0,0, q 1 m ) and (0,0, q 2 m ). For the atomic position modulation only one modulation vector is needed (0,0, q p ). The magnetic ordering is of helical screw-type showing beats in the magnetic moment spatial distribution. The values of q 1 m and q 2 m change with temperature below T N2 =50 K but the average value fulfil the relation (1/2)( q 1 m + q 2 m ) = (1/2) q p = const. This relation underlines the importance of the magnetoelastic coupling in CaCu x Mn 7- x O 12 compounds below T N2 =50 K.