Dipolar-coupled moment correlations in clusters of magnetic nanoparticles

Abstract

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

Correlated spin canting in ordered core-shellFe3O4/MnxFe3−xO4Ijiri1, Krycka2, Hunt-Isaak3 et al. 2019Phys. Rev. B341310View full textAdd to dashboardCite

“…Dipolar interactions were also critical in Monte Carlo simulations guided by magnetometry data, probing the crossover from two dimensional (2D) to threedimensional (3D) magnetic behavior in ordered collections of γ-Fe 2 O 3 nanoparticles 15 . More recent work on disordered, agglomerated clusters of iron oxide nanoparticles has revealed a tendency for anti-parallel alignment from dipolar coupling 16 . From an applications standpoint, dipolar interactions between nanoparticles are being investigated for the resulting effects on hyperthermia 17,18 as well as for the collective domain structures formed from nanoparticle aggregation in vivo 19 .…”

Magnetic structure factor of correlated moments in small-angle neutron scattering

“…Details regarding the synthesis and characterization of the iron oxide particles can be found in Ref. 46. A representative transmission electron microscopy (TEM) image of the particles is shown in Fig.…”

“…1(b) displays the isothermal magnetization curve M (H), normalized to the sat-uration magnetization M S (which is around 330 kA/m at 300 K) of the particle powder. According to integral magnetization measurements and Mössbauer spectroscopy, the clustered iron-oxide nanoparticles show at room temperature superparamagnetic behavior (vanishing magnetization at zero field) but with signs for cooperative magnetic correlations 46 . This finding is an indication that the superparamagnetic relaxations (i.e., Néel relaxation) of the particle moments is slowed down by dipole-dipole interactions [47][48][49][50] .…”

Supraferromagnetic correlations in clusters of magnetic nanoflowers

“…29 When introduced into tumors, it is safe to assume that the nanoflowers will agglomerate to clusters, and thus interparticle interactions will be relevant. 30 In Bender et al 31 we could show for homogeneous superparamagnetic nanoparticles a predominance for antiferromagnetic-like moment correlations within particle clusters via polarized small-angle neutron scattering (SANS). In this work we use the same approach to determine the nature of the moment coupling within a powder of iron oxide a) Electronic mail: philipp.bender@uni.lu.…”

“…The nuclear SANS results indicate that within the particle powder large clusters with sizes > 160 nm exist. arXiv:1907.02752v2 [cond-mat.mes-hall] 24 Sep 2019 vestigate magnetic nanoparticle ensembles 31,34,35. A homogeneous magnetic field H was applied perpendicular to the neutron beam (H ⊥ k) with a field amplitude of µ 0 H = 2 mT, which was necessary to maintain the neutron beam polarization.Figs.…”