N, M in phase - Pi // Oy
The incident polarisation vector is chosen parallel to the Y axis, i.e. perpendicular to the scattering vector Q. The direction of the scattered polarisation is shown in red.
When N and M (in blue) are in phase, the polarisation rotates in the plane defined by the incident polarisation and the magnetic interaction vector, i.e. remains parallel to Y in the example shown. The angle of rotation, when Pi is not parallel to M, 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.