Size dependence of magnetic parameters and surface disorder in magnetite nanoparticles

Abstract

Magnetic properties of oleic acid/oleylamine coated magnetite nanoparticles of average diameter D=4, 6, 8, 10, and 12nm 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 TB=38K for 12nm to TB=6.5K for D=4nm. The saturation magnetization Ms at 2K also decreases from Ms=62emu∕g for 12nm to Ms=17emu∕g for 4nm but the coercivity at 2K increases with a decrease in D. It is shown that Ms fits Ms=92(1−2d∕D)3 with d=0.68nm for D>4nm and d=0.86 for D=4nm. This equation is derived assuming a core-shell model with shell of thickness d consisting of disordered spins not contributing to Ms.

Physical Properties of Magnetite Nanoparticles Covered by 11-Mercaptoundecanoic Acid

“…According to the relation M s = 92(1 − 2d/D) 3 assuming core shell structure with M s = 92 emu/g for bulk magnetite and D = 7.1 nm as a mean particle diameter the value for thickness of shell was obtained as to be d = 1.03 nm. This value of shell thickness is higher but comparable with another values obtained for magnetite nanoparticles covered by oleic acid [10].…”

Size Control of Carbon Encapsulated Iron Nanoparticles by Arc Discharge Plasma Method

“…These results are in agreement with HRTEM observations. Similar trend and size dependency for iron saturation magnetization have been reported in the literature [47][48][49][50].…”

Core-Shell Magnetic Morphology of Structurally Uniform Magnetite Nanoparticles

“…Similarly, we cannot quantify boundary fuzziness or degree of overlap between core and shell, but we can conclude from the M 2 PERP scattering that magnetic shells of nearly uniform magnitude and direction exist. Thus, models involving a disordered outermost shell [6][7][8][9], a canted moment that extends throughout the entire volume of the particle, and ''hedgehoglike'' [12] or ''throttle'' [13] symmetry do not fit M 2 PERP and are excluded. The isolation of M 2 PERP demonstrated here is only possible using diffraction or other spatially sensitive techniques since moments quickly average to zero across the sample, as evidenced by an equivalence of M 2 Y and M 2 Z [22].…”

“…When the particles contain both ferromagnetic and antiferromagnetic phases, exchange bias effects may occur [2][3][4][5]. Surface disorder can lead to a magnetically dead layer or a spin glasslike phase [6][7][8][9]. Surfaces can also introduce a cubic anisotropy term when the material otherwise has uniaxial anisotropy [10].…”