Effect of magnetic polydispersion in superparamagnetic colloids on neutron scattering line shapes

Neutron scattering profiles from superparamagnetic colloids have been reported by several authors seeking evidence of a difference between a single particle's spatial and magnetic sizes. Calculations are presented which show that the weighting factor present in summing scattering functions over the particle size distribution arising from size‐dependent particle magnetization can mask the single‐particle transforms sought for. Three experimental precautions are suggested: (1) study only samples of known narrow dispersion ( ΔD/D ≤ 30% for magnetite); (2) perform experiments at several values of magnetic field spanning several orders of magnitude; (3) measure both incident and final polarization states of the neutrons. Numerical calculations based on a model allow quantitatively experimentally acceptable limits of sample dispersion to be defined.