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. (orig.)