Helical screw type magnetic structure of the multiferroic CaMn7O12 with low Cu-doping

The modulated crystal structure and modulated magnetic ordering of the multiferroic CaCuxMn7-xO12 is studied by analysing neutron and synchrotron-radiation (SR) powder diffraction data with a model based on the magnetic super-space group R31'(00 gamma)ts. Both atomic position modulations and magnetic modulations are described with the modulation vector (0, 0, q). The magnetic ordering is a screw-type circular helix where the magnetic moments are perpendicular to the c direction. The temperature dependence of the modulation vector length and the ordered magnetic moments of Mn3+ and Mn4+ ions is given between T = 50 K and the Neel temperature T-N similar or equal to 90 K. The atomic position modulation length L-p and the magnetic modulation length L-m fulfil the relation L-m = 2L(p) at all temperatures between 50 K and T-N.