Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 2 / Section B / 21 / g / Solution

Below the ordering temperature, coherent spin correlations produce magnetic Bragg intensity. Depending on the magnetic propagation vector, this can appear as additional peaks or as added intensity at positions already occupied by nuclear reflections. A magnetic unit cell larger than the chemical one gives extra reciprocal-space positions; a zero-propagation-vector canted structure need not do so.

Magnetic intensity generally decreases at large scattering vector through the magnetic form factor and depends on moment components perpendicular to the scattering vector. Above the transition the long-range magnetic Bragg contribution disappears, though short-range correlations can leave diffuse scattering. The nuclear pattern remains approximately unchanged unless magnetic ordering also drives a structural distortion.