N, M in quad. - Pi // Ox
The incident polarisation vector is chosen parallel to the X axis, i.e. parallel to the scattering vector Q. The direction of the scattered polarisation is shown in red.
When N and M (in green) are in quadrature, the polarisation rotates around the magnetic interaction vector M, i.e. around the Y axis in the example shown. The angle of rotation depends on the ratio of the amplitudes of N and M (e.g. π/2 when M=N). We can see that the cross-section does not necessarily depend on the relative amplitudes of the nuclear and magnetic structure factors.