Nonmagnetic shell in surfactant-coated FePt nanoparticles

Self-assemblies of chemically-synthesized 3 nm FePt nanoparticles show a dramatic increase of the saturation magnetization from about $\text{210\hspace{0.17em}}{\mathrm{emu/cm}}^3$ in the as-prepared state to about $\text{850\hspace{0.17em}}{\mathrm{emu/cm}}^3,$ when annealed above $\text{400\hspace{0.05em}°}\mathrm{C}$ for 30 min. This increase correlates directly with the decomposition of the surfactant coating above $\text{400\hspace{0.05em}°}\mathrm{C}.$ The low magnetization state is attributed to the formation of a nonmagnetic shell (dead layer) due to oxygen bonds of the polar end group surfactant of oleic acid/oleylamine with Fe. Magnetic analysis in the superparamagnetic regime suggests a 2.7-nm-diameter FePt particle size with core–shell structure of 1.7-nm-diameter FePt magnetic core and $\text{∼0.5\hspace{0.17em}}\mathrm{nm}$ nonmagnetic shell.