Spin Ordering in Bi-Magnetic (Core / Shell) Nanoparticles With Varying Anisotropy

Abstract

Chemically-synthesized magnetic nanoparticles (MNPs) consisting of different combinations of spinel ferrites (γ-Fe2O3, Fe3O4, MnFe2O4, CoFe2O4) provide a platform for developing ensembles of nanostructures whose magnetic properties are distinct from their bulk counterparts. We report on studies of MNPs composed of high-anisotropy CoFe2O4 combined with relatively high-moment, low anisotropy Fe3O4. We examined two variations of MNPs: a CoFe2O4 core with a Fe3O4 shell and the inverted structure (Fe3O4 core / CoFe2O4 shell). Both variants had a core diameter of 3-3.5 nm and an overall diameter of ∼10 nm and we investigated their properties using magnetometry, small-angle neutron scattering (SANS), and Monte Carlo simulations. The magnetometry indicates that both MNP variants are superparamagnetic at room temperature with a considerable coercivity enhancement below the blocking temperature (~250 K) accompanied by a multi-step path towards magnetization reversal (Fig 1-b). Fully spin-polarized SANS measurements show that magnetic order perpendicular to the applied field direction is strong at remanence; this spin configuration is quenched upon application of a small field (Fig. 1-a). Spin alignment parallel to the field develops at higher fields. Monte Carlo simulations indicate that the emergence of dipolar-coupled long range order may be associated with the steps observed in the field hysteresis loops. Modelling of the SANS distributions, informed by the Monte Carlo simulations, to investigate the spin alignment in the core and shell, as demonstrated for metallic Fe core / Fe oxide shell MNPs [1], for both MNP variants is currently underway. The results demonstrate the value of combining modeling of fully-spin polarized SANS data with numerical simulations of ensembles of macrospins to illuminate the complex spin ordering across a range of length scales. The authors acknowledge the support of Univ. of South Florida Nexus Initiative (UNI) Award, US Department of Energy, Office of Basic Energy Sciences (Award DE-FG02-07ER46438) and the Knut and Alice Wallenberg Foundation (Grant 2018.0060). Access to spin filters on NG-7 SANS was provided by CHRNS, a partnership between NIST and the NSF (No. DMR-1508249).