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/cm(3) in the as-prepared state to about 850 emu/cm(3), when annealed above 400 degreesC for 30 min. This increase correlates directly with the decomposition of the surfactant coating above 400 degreesC. 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 similar to0.5 nm nonmagnetic shell. (C) 2004 American Institute of Physics.