Correlated spin canting in ordered core-shell Fe3O4/MnxFe3−xO4 nanoparticle assemblies

Abstract

Polarization-analyzed small-angle neutron-scattering methods are used to determine the spin arrangements and experimental length scales of magnetic correlations in ordered three-dimensional assemblies of $\ensuremath{\sim}7.4$-nm-diam core-shell ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}/{\mathrm{Mn}}_{x}{\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{O}}_{4}$ nanoparticles. In moderate to high magnetic fields, the assemblies display a canted magnetic structure where the canting direction is coherent from nanoparticle to nanoparticle, in contrast to the less extended, more single-particle-like behavior for similar ferrite assemblies. The observed magnetic scattering is modeled by assuming that the interparticle dipolar coupling combined with Zeeman effects in a field leads to nanoparticle domains with preferred net spin alignments relative to packing symmetry axes. Over a range of fields and temperatures, the model qualitatively explains the observed scattering anomalies in terms of clusters that vary in area and thickness, highlighting the complex structures adopted in real, dense nanoparticle systems. The clusters often have a strong two-dimensional magnetic character which is attributed to structural stacking faults and the resulting influence of interparticle dipolar interactions for these magnetically soft nanoparticles.