  We report the results of an unpolarized small-angle neutron scattering (SANS)
study on Mn-Zn ferrite (MZFO) magnetic nanoparticles with the aim to elucidate
the interplay between their particle size and the magnetization configuration.
We study different samples of single-crystalline MZFO nanoparticles with
average diameters ranging between 8 to 80 nm, and demonstrate that the smallest
particles are homogeneously magnetized. However, with increasing nanoparticle
size, we observe the transition from a uniform to a nonuniform magnetization
state. Field-dependent results for the correlation function confirm that the
internal spin disorder is suppressed with increasing field strength. The
experimental SANS data are supported by the results of micromagnetic
simulations, which confirm an increasing inhomogeneity of the magnetization
profile of the nanoparticle with increasing size. The results presented
demonstrate the unique ability of SANS to detect even very small deviations of
the magnetization state from the homogeneous one.
