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

The modulated crystal structure and modulated magnetic ordering of the multiferroic CaCu_xMn_7−xO₁₂ is studied by analysing neutron and synchrotron-radiation powder diffraction data with a model based on the magnetic superspace group R31′(00γ)ts.