Semester

Summer

Date of Graduation

2006

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Mohindar S. Seehra.

Abstract

In this dissertation, 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 (XRD) are compared with the results from TEM. From the analysis of the XRD line intensities, the particle size dependence of the Debye-Waller factors for Ni and O atoms are derived. It is found that the Debye-Waller factors of nickel and oxygen atoms in smaller particles are larger than those in bulk NiO.;For the coated and uncoated NiO nanorods of 5 nm diameter, variations of the magnetization M with temperature T (5 K to 370 K) and temperature variations of the EMR (electron magnetic resonance) spectra were measured to determine the respective blocking temperatures TB(m) and TB(EMR). The following differences are noted: (1) TB(m) is reduced from 230 K (uncoated) to 85 K(coated) for H = 25 Oe; (2) Decrease of TB(m) with H is weaker and the ratio TB(EMR)/T B(m) is smaller for the uncoated particles. These differences are due to stronger interparticle interaction present in the uncoated particles.;Temperature variation (5 K-300 K) of the AC magnetic susceptibilities (chi' and chi") at various frequencies f (0.1-10,000 Hz) are reported for the coated and uncoated 5 nm diameter nanorods of NiO. Using the peak in chi' as the blocking temperature TB, it is observed that TB increases with increasing f. The data for the two samples fit the Vogel-Fulcher law: f = f0exp[-Ea/k(TB-T0)] with f 0 = 9.2 x 1011 Hz, Ea/k = 1085 K and T0 = 162 K (0 K) for the uncoated (coated) particles. This shows that T0 provides a good measure of the effects of interparticle interactions on magnetic relaxation and that these interactions are essentially eliminated with the OA coating.;For all the particles, measurements of M versus T (5 K-370 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 for D 10 nm, TP follows the same trend as for the coated NP. These differences are interpreted in terms of significant interparticle interaction. The data of M vs. the applied field H for T > TP are fit to the modified Langevin function: M = M0 L (muPH/kBT) + chiaH, to determine the magnetic moment muP per particle as a function of size D. The variation of muP with size D is interpreted in terms of the fraction of spins on the surface layer of the particles which contribute to mu P. It is observed that this fraction varies as 1/D reaching nearly 100 % for the 5 nm particles. From the temperature dependence of M0 and extrapolating to M0 → 0, the Neel temperatures TN for various sizes are determined. TN for NiO nanoparticles is found to decreases rapidly with decrease in size for D < 10 nm.

Download

Share

COinS