Size dependence of magnetic parameters and surface disorder in magnetite nanoparticles

Magnetic properties of oleic acid/oleylamine coated magnetite nanoparticles of average diameter D=4, 6, 8, 10, and 12 nm are reported. The samples were characterized by transmission electron microscopy and x-ray diffraction (XRD) with XRD showing increasing disorder with decreasing D. Magnetization M vs temperature T data show the blocking temperature TB decreasing with decreasing D from T-B=38 K for 12 nm to T-B=6.5 K for D=4 nm. The saturation magnetization M-s at 2 K also decreases from M-s=62 emu/g for 12 nm to M-s=17 emu/g for 4 nm but the coercivity at 2 K increases with a decrease in D. It is shown that M-s fits M-s=92(1-2d/D)(3) with d=0.68 nm for D>4 nm and d=0.86 for D=4 nm. This equation is derived assuming a core-shell model with shell of thickness d consisting of disordered spins not contributing to Ms. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3055272]