Magnetic properties of NiO nanoparticles (NP) prepared by the sol-gel method in the size range D = 5 nm to 20 nm, with and without oleic acid (OA) coating, are reported. Transmission electron microscopy (TEM) studies show the morphology of the smaller particles to be primarily rod-like, changing over to nearly spherical shapes for D > 10 nm. Average sizes D of NP determined by x-ray diffraction are compared with the results from TEM. Measurements for the magnetization (M) versus temperature T ( 5 K to 350 K) in the zero-field cooled (ZFC) and field-cooled (FC) modes are used to determine the average blocking temperature Tp. For the OA coated particles, Tp increases with increase in size D as expected for superparamagnetic particles. However, for the uncoated NP, Tp decreases initially with increase in size D < 10 nm: but for D > 10 nm, Tp follows the same trend as for the coated NP. These differences are interpreted in terms of significant interparticle interaction present (absent) in the uncoated (coated) particles. The data of M vs. the applied field H for T > Tp are fit to the modified Langevin function: M = Mo (upH/kbT) + xaH, to determine the magnetic moment up per particle versus size D. The variation of up with D is interpreted in terms of the fraction of spinds on the surface layer of the particle which contribute to up. It is observed that this fraction varies as 1/D reaching nearly 100% for the 5 nm particles. The Neel temperature Tn decreases rapidly for D < 10 nm.
Citation: Physical Review B (Condensed Matter and Materials Physics)
Pub Type: Journals
magnetism, nanotechnology, particle interaction, size effects, transmission electron microscopy