Inter-particle magnetic correlations and fluctuations in assemblies of Fe3O4 nanoparticles

Magnetite (Fe3O4) nanoparticles (NPs) are increasingly used in biomedical applications such as drug & gene delivery, hyperthermia, or MRI. While the structural and magnetic properties of bulk Fe3O4 is well known [1], knowledge is still lacking about the spatio-temporal magnetic behavior of collections of Fe3O4 NPs. We here investigate the inter-particle magnetic correlations within the NP assembly throughout the superparamagnetic transition using x-ray resonant magnetic scattering (XRMS) [2]. By exploiting the light polarization, we extract the local inter-particle magnetic order. We show a dependence on particle size, suggesting an enhancement of magnetic couplings for bigger particles. [3] Additionally, we show a model based on NP chains, which we use to fit the XRMS data, suggesting ferromagnetic ordering at high external magnetic field, and the emergence of antiferromagnetic ordering near remanence. [4] Finally we investigate the slow dynamics of magnetic fluctuations between NPs by using photon correlation spectroscopy and show a drastic change through the superparamagnetic transition.
[1] Verwey, Nature 144, 327 (1939);
[2] Kortright et al., Phys. Rev. B 71, 012402 (2005)
[3] Chesnel et al., Magnetochemistry 4, 42-58 (2018)
[4] Rackham et al., AIP Advances 9, 035033 (2019)