In complex systems, the magnetic structure has a large number of degrees of freedom (typically the three components of the magnetic moment on several inequivalent atoms), and the d-spacing range available to observe Bragg peaks is limited due to fall-off of the magnetic form factor.
High-resolution cold-neutron powder diffraction excels when multiple nearly overlapping Bragg peaks occur at long d-spacing. In this case, sheer flux is not sufficient to extract all the available information, and much better results can be obtained with a high-resolution diffractometer such as Wish, even at the cost of losing some flux.
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For further information please see: LC Chapon, P Manuel et al, Neutron News 22 (2011) 22-25.