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

The origin of double-step magnetization reversal processes, so-called wasp-waist magnetization hysteresis loops, in single magnetic phase 3D cobalt ferrite nanoassemblies is still poorly understood. So far, this behavior has been mainly attributed to the co-existence of hard-soft magnetic phases and spin canting in nanoparticles. Here, we demonstrate the wasp-waisted magnetization loops in single-phase flower-like Co0.82Fe2.18O4 nanoassemblies that were synthesized by modifying the ligand chemistry. Combining magnetization hysteresis loops at different concentrations, degrees of dipolar interactions, and temperatures, energy dispersive X-ray and in-field Mössbauer spectroscopy, and small-angle neutron scattering, we propose that a combination of a strong dipolar field and spin disordered nano-building blocks, leading to soft magnetic phase at the grain boundaries, accounts for this anomalous and abrupt drop in magnetization in nanoassemblies. The nanoassemblies have a porous nanostructure with nanogaps between their nano-building blocks, as revealed from electron microscopy investigations. Small-angle neutron scattering studies reveal spin disorder at the surface and interfaces of the nano-building blocks. The strong dipolar field at the ensemble level is only achieved when particle colloidal suspensions are dried from high particle concentrations, indicating concentration-dependent nature of this behavior. Single-core nanoparticles with a comparable chemical composition, effective size, coercive field, and magnetization, but with a coherent crystal structure, do not reveal this peculiar behavior even at highest concentrations. This finding demonstrates the role that the assembly of nanoscale building blocks plays to give rise to this peculiar magnetization. Our study introduces organic capping ligands as a novel means to tune magnetization processes in nanoparticles and to initiate new applications, a novel role for organic ligands beyond giving nanoparticles colloidal stability.