Room Temperature Ferromagnetism in Iron Doped Zinc Oxide Nanoparticles

Lydia Marjorie Johnson, Boise State University

Abstract

The purpose of this work was to investigate the impact of crystallite size, surface properties, and transition metal dopants on the magnetic behavior of zinc oxide nanoparticles. To accomplish this, three experimental factors were independently controlled: undoped zinc oxide particle size, reaction solvent, and transition metal dopant concentration. Powders of nanocrystalline Zn_1-xFe_xO were chemically synthesized via two similar hydrolysis methods (solvent either diethylene glycol or ethanol), with dopant concentrations varying from x = 0 to x = 0.10 and the undoped (x = 0) samples synthesized in a range of sizes between 4 - 20 nm. To ensure statistically significant results, a total of 62 samples were independently synthesized and characterized. X-ray diffraction patterns confirmed the wurtzite phase of the host ZnO lattice, with no detected impurity phases, as well as revealing the excellent size selectivity of the synthesis methods employed. Transmission electron microscopy demonstrated the spherical morphology of the samples, and confirmed their highly crystalline nature. X-ray photoelectron spectroscopy studies placed the core level Fe 3p_1/2 peak for the doped samples at binding energies distinctly different from the Fe 3p_1/2 peak expected for the common iron oxides, underlining the phase purity ofthe prepared samples. Photoluminescence results showed a direct band gap, increasing with x from 3.35 eV for x = 0 up to 3.5 eV for x = 0.10. Surface studies of the materials’ zeta potentials as a function of pH revealed the cationic nature of the nanoparticles, with the magnitude of the zeta potential varying significantly based on synthesis reaction solvent. In magnetometry studies, the undoped ZnO samples showed little to no magnetization. Magnetic saturation values for the undoped samples ranged from M_s = 0 - 1 memu/g, and showed no dependence on nanoparticle size in the prepared 4 - 20 nm sizes. Conversely, Fe doped samples displayed ferromagnetic ordering, with saturation magnetization increasing systematically with dopant concentration. Samples prepared in ethanol displayed much stronger magnetization than samples prepared in diethylene glycol. Electron paramagnetic resonance studies confirmed the presence of ferromagnetic ordering in the doped samples, as well as revealing the nature of the dopant ions environments.