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 210 emu/cm3 in the as-prepared state to about 850 emu/cm3, when annealed above 400 °C for 30 min. This increase correlates directly with the decomposition of the surfactant coating above 400 °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 ∼0.5 nm nonmagnetic shell.