Unusual Magnetic Spin Arrangements in Manganese Ferrite Nanoparticle Assemblies*

Magnetic nanoparticles are of interest for many applications, yet the relevant magnetic structures are often difficult to predict \textit{a priori} or even measure directly. In previous work, polarization analyzed small-angle neutron scattering (PASANS) measurements revealed an unusual core-shell magnetic structure for Fe$_3$O$_4$ nanoparticles governed primarily by the competition of exchange vs. Zeeman energy [1]. For CoFe$_2$O$_4$, the substantially larger anisotropy led to a uniform magnetic structure within each nanoparticle with sizeable canting in an applied magnetic field [2]. Here, we report new PASANS results investigating dense assemblies of 7.5 nm diameter manganese ferrite nanoparticles. For this system, we see a significant component of magnetic scattering perpendicular to the applied magnetic field, indicating important intra and now \textit{inter-particle} effects. These results are interpreted considering the expected weaker exchange and anisotropy for MnFe$_2$O$_4$ relative to Fe$_3$O$_4$ and CoFe$_2$O$_4$, respectively and the internal structure of the individual nanoparticles. [1] K.L. Krycka, et al., \textit{Phys. Rev. Lett.} \textbf{113}, 147203 (2014). [2] K. Hasz et al., \textit{Phys. Rev. B} \textbf{90}, 180405 (R) (2014).