Surface-modified FeCo nanoparticles: magnetic dilution and structure-magnetism relationships.

FeCo nanoparticles with controlled composition and tailored surface chemistry were synthesized via an ultrasound-assisted co-precipitation method and subsequently functionalized with zirconium oxide (ZrO2), polyethylene glycol, and chitosan using precipitation, adsorption, and surface coupling strategies, respectively. Structural and morphological analyses by X-ray diffraction and electron microscopy confirm the formation of crystalline body-centered cubic (bcc) FeCo nanoparticles with preserved core structure after surface modification, while energy-dispersive X-ray spectroscopy verifies the successful incorporation of constituent elements without detectable impurity phases. Magnetic measurements show that pristine FeCo nanoparticles exhibit a high saturation magnetization (M s) of ∼187 emu g-1 with typical soft magnetic behavior at room temperature. After surface modification, all samples retain their intrinsic ferromagnetic characteristics; however, a systematic reduction in M s is observed. This decrease is attributed to magnetic dilution from non-magnetic shells together with interfacial spin disorder at the FeCo surface. Among the investigated coatings, ZrO2 induces the strongest reduction in saturation magnetization, consistent with its higher density and more effective surface passivation. This effect is quantitatively described by an effective magnetically inactive thickness (δ mag), which accounts for both the physical coating layer and interfacial magnetic suppression. A nearly linear dependence between M s and δ mag is observed, suggesting that magnetic dilution is governed by a progressively growing interfacial inactive region rather than abrupt phase changes. These results demonstrate that surface engineering provides a robust route to tune the balance between magnetic performance and interfacial stability in FeCo nanoparticles without altering their fundamental magnetic nature. The combination of high magnetization, structural robustness, and controllable surface chemistry highlights their potential for advanced magnetic and bio-related applications.