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 T_B 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 M_s. [ABSTRACT FROM AUTHOR]