The origin of wasp-waisted shape of magnetization hysteresis loops in 3D CoxFe3-xO4 nanoassemblies

Magnetic nanoassemblies magnetize to saturation at low magnetic fields thereby experiencing cooperative magnetic couplings between their building blocks. It is well known that doping Co2+ into iron oxide nanoparticles effectively enhances magnetic anisotropy and coercivity of particles. The magnetization hysteresis loops of binary metal oxides often reveal intriguing features. As such, CoxFe3-xO4 nanoparticles show peculiar non-continuous magnetization reversal processes at zero magnetic fields, the origin of which is still controversial. This abrupt jump in the magnetization is known as wasp-waisted hysteresis loops and its first encounter dates back a few decades ago. Becker et al. have proposed that this anomalous behavior is the result of a superposition of hysteresis loops of grains with different magnetic coercivities. Since then several underlying mechanisms including the co-existence of soft-hard magnetic phases, magnetic dipole-dipole interactions, and spin canting have been reported. We recently synthesized nano-assembled nanoparticles with a complex flower-shape configuration. We found that this unique assembly of Co0.82Fe2.18O4 petal shape nanocrystallites into 3D flower-like nanoassemblies exhibits a wasp-waisted shape of magnetization hysteresis loops that is nonexistent in single-core NPs. In order to shed light on the origin of this peculiar magnetization process and test our hypotheses, we aim to perform magnetic SANS (SANSPOL) measurements at Larmor.