Magnetic Reversal of Onion-Like Fe$_{3}$O$_{4}$\textbar MnO\textbar $\gamma $-Mn$_{2}$O$_{3}$ Core\textbar Shell\textbar Shell Nanoparticles

Magnetic nanoparticles offer potential for biomedical and data storage applications, especially with exchange bias to overcome the superparamagnetic limit. Here we study the role of an antiferromagnetic layer sandwiched between a soft ferrimagnetic core and hard ferrimagnetic shell. The nanoparticles studied consist of 3 nm (diameter) Fe$_{3}$O$_{\mathrm{4}}$ \textbar 50-60 nm thick MnO shell \textbar 5 nm thick $\gamma $-Mn$_{2}$O$_{3}$ shell [1]. Small-angle neutron scattering (SANS) probes both structural and magnetic morphology. SANS reveals that during reversal from 5 T to -5 T at 5 K, there is an increase in spins oriented perpendicular to the applied field. As the temperature is increased to 150 K (above the 123 K N\'{e}el temperature of MnO) evidence of an enhanced magnetism from within the MnO shell is observed. Finally, the scattering pattern shifts (indicating a change in the relative magnetism as a function of radius) between 5 K and 50 K. \\[4pt] [1] A. L\'{o}pez-Ortega \textit{et al}., Nanoscale 4, 5138 (2012); Salazar-Alvarez \textit{et al}., J. Am. Chem. Soc., 133, 16738 (2011)