Spin Coupling in Magnetic Core - Shell Nanoparticles

Magnetic nanoparticles (NPs) have garnered much interest due to the unique properties that emergewhen compared to their bulk counterparts. In particular, core-shell (CS) NPs offer the ability to tailor the magnetic properties of the system through careful selection of core and shell constituent materials. In this work, the magnetic ordering of CS NPs with contrast in core/shell features (moment, anisotropy, coercivity, etc.) have been investigated using a combination of X-ray magnetic circular dichroism spectroscopy (XMCD) and polarized small angle neutron scattering (SANS). First, temperature-dependent analyses of XMCD and SANS studies were carried out for CS NPs featuring a core composed of metallic a-Fe paired with an Fe-oxide shell, and a variant NP structure with a reduced a-Fe core followed by a partial void layer surrounded by an Fe-oxide shell (CVS). Next, the effect magnetically hard and soft materials have on spin coupling throughout a NP wasexplored by means of fully polarized SANS analyses of NPs with a hard CoFe2O4-core and soft Fe3O4-shell (core @ shell, CFO@FO) as well as the inverted structure with a Fe3O4-core and CoFe2O4-shell (FO@CFO). Large anisotropy differences between the core and shell will play a significant role in the degree of spin canting across the NP. The existence of a Bragg peak for neutron scattering perpendicular to the applied magnetic field (H) suggests the presence of spin canting in both CFO/FO NP variants with the peak persisting up to applied fields of 0.4 T for CFO@FO and 0.1 T for FO@CFO NPs. Whereas prior worked sought to find a balance between anisotropy and saturation magnetization, thefinal project focused on keeping anisotropy in the core constant for various shell anisotropy values all while maintaining a constant moment. This was achieved by using NPs with a FO core and doping a CFO shell with increasing amounts of Zn; recent work has shown that while saturation magnetization is similar for various doping levels the shell anisotropy increases linearly with Co concentration. The magnetic ordering was explored using polarized SANS to probe spin distributions throughout the NP as a function of Zn doping concentration