Dipolar-coupled moment correlations in clusters of magnetic nanoparticles

Here, we resolve the nature of the moment coupling between 10-nm dimercaptosuccinic acid–coated magnetic
<br />nanoparticles. The individual iron oxide cores were composed of >95% maghemite and agglomerated to
<br />clusters. At room temperature the ensemble behaved as a superparamagnet according to Mössbauer and magnetization
<br />measurements, however, with clear signs of dipolar interactions. Analysis of temperature-dependent
<br />ac susceptibility data in the superparamagnetic regime indicates a tendency for dipolar-coupled anticorrelations
<br />of the core moments within the clusters. To resolve the directional correlations between the particle moments
<br />we performed polarized small-angle neutron scattering and determined the magnetic spin-flip cross section
<br />of the powder in low magnetic field at 300 K. We extract the underlying magnetic correlation function of
<br />the magnetization vector field by an indirect Fourier transform of the cross section. The correlation function
<br />suggests nonstochastic preferential alignment between neighboring moments despite thermal fluctuations, with
<br />anticorrelations clearly dominating for next-nearest moments. These tendencies are confirmed by Monte Carlo
<br />simulations of such core clusters.