Doctoral Dissertations

Date of Award

5-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Energy Science and Engineering

Major Professor

Ramakrishnan Kalyanaraman

Committee Members

Gerd J. Duscher, Syed K. Islam, Jagjit Nanda

Abstract

Amorphous oxides which are transparent and conducting find use in display devices and as top electrodes in energy applications, while those which are conducting and magnetic have the potential to be used in spintronics. With the fast approaching limit of Moore’s law, new materials are needed where the spin and polarization of the electrons are coupled. Despite progress in transparent conductors, materials selection is limited by the need to have wide optical bandgap and conduction via s-orbital. In contrast, search for new spintronics materials has picked up only in the last decade. Here we report the synthesis, characterization, and application of a new oxide made from Fe, Tb, and Dy - elements that do not conduct via the s-orbital. Thin films (<100 nm) of this oxide were synthesized by pulsed laser deposition (PLD) as a function of varying oxygen pressure, and by electron-beam evaporation as a function of the cation composition, and then annealed under different conditions. Films deposited at 5x10-8 Torr exhibited high optical transparency (>90%) and conductivity (~104 S/m). Films deposited at O2 pressures below 1x10-5 Torr were conductive (~104 S/m), magnetic (up to 480 emu/cc), and optically transparent, while the ones above 1x10-5 Torr were optically transparent but insulating and non-magnetic. Changes in the cation stoichiometry showed the films’ transition from being metallic to semiconducting with decreasing Fe content relative to the Lanthanides. However, when a very iron-rich film was annealed through several heating and cooling cycles in low vacuum, the film evolved into a semiconductor that was stable in ambient conditions and showed very high conductivity (2.8x105 S/m) and room temperature magnetism (380 emu/cc). The PLD deposited films were utilized as the ferromagnetic layer for magneto-electric coupling with bismuth ferrite as well as a top electrode for bismuth ferrite capacitors. A giant magneto-resistance (GMR) device made from the Fe-Tb-Dy-oxide and bismuth ferrite showed evidence of magneto-electric coupling at room temperature. The discovery of this oxide not only introduces new materials physics that could be explored and exploited to engineer new multifunctional materials, but the oxide itself proves to be very promising for spintronics device applications.

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