Date of Graduation

Spring 2016

Degree

Master of Science in Materials Science

Department

Physics, Astronomy and Materials Science

Committee Chair

Robert Mayanovic

Keywords

hydrothermal, epitaxy, nanoparticle, transition metal, ferromagnetic, antiferromagnetic, hysteresis, exchange bias

Subject Categories

Materials Science and Engineering

Abstract

I used hydrothermal nano-phase epitaxy technique to synthesize well-ordered a-Cr2O3@a-MxCr2-xO3 (M = Co, Ni, Mn, Fe) inverted core-shell nanoparticles (NPs). This resulted in the formation of novel a-MxCr2-xO3 shells having ferromagnetic/ferrimagnetic (FM/FiM) spin ordering and an antiferromagnetic (AFM) a-Cr2O3 core structure. The combined results from Xray diffraction (XRD) and high resolution transmission electron microscoy (HRTEM) provide evidence of the presence of the corundum phase both in the shell and in the core regions. HRTEM results also show a sharp interface exhibiting epitaxial atomic registry of shell atoms over highly ordered core atoms whereas TEM-Energy dispersive spectroscopy analyses show that the M atoms reside in the shell regions. The Xray photoelectron spectroscopy (XPS) analyses of the NPs indicate the M transition metal is in +2 oxidation state. Magnetic measurements show well-developed hysteresis loops: the field cooled hysteresis loops reveal horizontal shifts in the applied field axis and vertical shifts in the magnetization axis, relative to the zero-field cooled hysteresis loops. This provides direct evidence for the exchange bias effect between the AFM a-Cr2O3 core and the FM/FiM a-MxCr2-xO3 shell. My first principles density functional theory calculations shows that the incorporation of Ni2+ in a-Cr2O3 results in localized FM ordering in the structure.

Copyright

© Mohammad Delower Hossain

Available for download on Friday, June 01, 2018

Open Access

Share

COinS