Finite-Size Effects and Spin-Dependent Transport in Magnetite Nanoparticles
Kai Liu, L. Zhao, P. Klavins, Frank E. Osterloh, and Daniel P. Hewitt
University of California, Davis, US
Keywords: magnetite, spin-dependent tunneling, superparamagnetic, finite-size, magnetization
Nanoparticles of magnetite (Fe3O4) have been synthesized using an aqueous precipitation technique. The particles are spherical in shape, with an average size of 8-9 nm and a small size distribution, revealed by transmission electron microscopy and atomic force microscopy. A single-phase Fe3O4 cubic spinel structure and the small size of the nanoparticles are confirmed by powder x-ray diffraction. Dispersed nanoparticles displays superparamagnetic behavior above 150K over a 30s time scale. The corresponding magnetic interaction volume indicates clustering of the nanoparticles. The blocking behavior is modified by the packing density, or the average inter-particle distance, of the particle assembly. Comparing to bulk magnetite, the saturation magnetization in nanoparticles decreases faster with increasing temperatures, still according to T^3/2, due to enhanced contributions from surface magnetizations. In a compressed pellet form, the electron transport is by tunneling through adjacent particles. The resistivity increases exponentially with decreasing temperature, however, no clear evidence of a Verwey transition is observed between 110K and 300K. Negative magnetoresistance, -8.6% at 200K and ?4.5% at 300K, has been observed. This is due to the field-induced alignment of the nanoparticles magnetization directions.
NSTI Nanotech 2003 Conference Technical Program Abstract