Influence of Hard/Soft Layer Ordering on Magnetization Reversal of Bimagnetic Nanoparticles: Implications for Biomedical/Theranostic Applications

Published

June 14, 2023

Author(s)

Corisa Kons, Kathryn L. Krycka, Joshua Robles, Nikolaos Ntallis, Manuel Pereiro, Manh-Huong Phan, Hariharan Srikanth, Julie A. Borchers, Dario A. Arena

Abstract

We investigate the spatial distribution of spin orientation in magnetic nanoparticles consisting of hard and soft magnetic layers. The nanoparticles are synthesized in a core / shell spherical morphology where the magnetically hard, high anisotropy layer is CoFe₂O₄ (CFO) while the lower anisotropy material is Fe₃O₄ (FO). The nanoparticles have a mean diameter of ∼9.2 - 9.6 nm and are synthesized as two variants: a conventional hard / soft core / shell structure with a CFO core / FO shell (CFO@FO) and the inverted structure FO core / CFO shell (FO@CFO). High resolution electron microscopy confirms the coherent spinel structure across the core / shell boundary in both variants while magnetometry indicates the nanoparticles are superparamagnetic at 300 K and develop a considerable anisotropy at reduced temperatures. Low temperature M vs. H loops suggest a multi-step reversal process. Small angle neutron scattering (SANS) with full polarization analysis reveals a considerable alignment of the spins perpendicular to the field even in 1.56 T. The perpendicular magnetization is surprisingly correlated from one nanoparticle to the next, though the interaction is of limited range. More significantly, the SANS data reveal a pronounced difference in the reversal process of the magnetization parallel to the field for the two nanoparticle variants. For the CFO@FO nanoparticles, the core and shell magnetizations appear to track each other through the coercive region, while in the FO@CFO variant the softer Fe₃O₄ core reverses before the higher anisotropy CoFe₂O₄ shell, consistent with expectations from micromagnetics modeling. These results highlight the interplay between interfacial exchange coupling and anisotropy as a means to tune the composite properties of the nanoparticles.

Citation

ACS Applied Nano Materials

Volume

6

Issue

13

Pub Type

Journals

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DOI Link

Keywords

Polarized SANS, Core/Shell Nanoparticles, Exchange Coupling, Magnetization Revesal

Citation

Kons, C. , Krycka, K. , Robles, J. , Ntallis, N. , Pereiro, M. , Phan, M. , Srikanth, H. , Borchers, J. and Arena, D. (2023), Influence of Hard/Soft Layer Ordering on Magnetization Reversal of Bimagnetic Nanoparticles: Implications for Biomedical/Theranostic Applications, ACS Applied Nano Materials, [online], https://dx.doi.org/10.1021/acsanm.3c00510 (Accessed May 3, 2024)