We have used a combination of x-ray magnetic circular dichroism (XMCD) spectroscopy and polarized small-angle neutron scattering (P-SANS) to investigate the distribution of magnetization in heterogenous magnetic nanoparticles (NPs) consisting of a metallic Fe core/Fe oxide shell (CS NPS) or Fe core/partial void layer/oxide shell (CVS NPs). Fe $L_{2,3}$ XMCD spectra were analyzed with a combination of experimental metallic Fe XMCD spectra and calculated $L$-edge spectra for the Fe cations in the oxide shell. Analyses of the temperature-dependent spectra indicate a weak variation of the relative contribution of the metallic and oxide contributions for the CS NPs, and a somewhat larger contribution from the metallic Fe core near the blocking temperature $T_B$ of the CVS NP ensemble. The P-SANS data also indicate a larger variation in the magnetization of the CVS NPs near $T_B$. Modeling of the spin-dependent neutron scattering reveals large variations in the radial magnetization distribution, with a region of reversed magnetization adjacent to the metallic core. Interfacial roughness may play a role in the development of this magnetization profile.

• Received 20 November 2019
• Revised 25 February 2020
• Accepted 2 March 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.034408