Europium oxide as a perfect electron spin filter
Name
174039990-MIT.pdf
Description
Full printable version
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8.96 MB
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Checksum (MD5)
20464f0760b3dc3ece4345e9d1a8438f
Author(s)
Santos, Tiffany S. (Tiffany Suzanne), 1980-
Advisor(s)
Jagadeesh Moodera.
Alternative Title
EuO as a perfect electron spin filter
Date Issued
2007
Publisher
Massachusetts Institute of Technology
Abstract
Essential to the emergence of spin-based electronics is a source of highly polarized electron spins. Conventional ferromagnets have at best a spin polarization P-50%. Europium monoxide is a novel material capable of generating a highly spin-polarized current when used as a tunnel barrier. EuO is both a Heisenberg ferromagnet (Tc=69 K) and a semiconductor. Exchange splitting of the conduction band creates different tunnel barrier heights for spin-up and spin-down electrons, thus filtering the spins during tunneling. High-quality EuO films at the monolayer level is necessary for efficient spin-filtering. Because non-ferromagnetic, insulating Eu20 forms more readily, growth of an ultra-thin, high-quality film is quite challenging, which restricted previous studies of EuO to bulk form. EuO films were grown by reactive thermal evaporation, and various thin film characterization techniques were employed to determine the structural, optical, and magnetic properties, even on the thickness scale needed for tunneling (<3 nm). The film properties closely matched those of bulk EuO, though the Tc for ultra-thin films was found to be reduced from bulk value, in agreement with theoretical prediction.
(cont.) Controlling the smoothness and chemical nature of the inter-faces between EuO and metallic electrodes was found to be of critical importance, as proven by careful interfacial chemical and magnetic analysis at the monolayer level, using x-ray absorption spectroscopy, magnetic circular dichroism, and diffuse x-ray resonance scattering techniques. EuO was successfully prepared as the barrier in Al/2.5 nm EuO/Y tunnel junctions. By fitting the current-voltage characteristics of these junctions to tunneling theory, exchange splitting in an ultra-thin layer of EuO was quantitatively determined for the first time, and complete spin filtering yielded total spin polarization, P=100%. In an alternative approach, P was directly measured using the superconducting Al electrode as a spin detector. Spin-filtering in EuO barriers was also observed in magnetic tunnel junctions (MTJs), in which a ferromagnetic electrode was the spin detector. In Cu/EuO/Gd MTJs a tunnel magnetoresistance (TMR) of 280% was measured by changing the relative alignment of magnetization of EuO and Gd, which is the largest TMR measured using a spin-filter barrier. Co/A1203/EuO/Y junctions, in which the A1203 barrier magnetically decoupled Co and EuO, also showed substantial TMR. Its matching band gap (1.1 eV) and compatibility with Si open up the novel possibility of using EuO to inject highly polarized spins into Si-based semiconductors.
Description
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.
Vita.
Includes bibliographical references (p. 93-103).
Subjects
Materials Science and Engineering.
MIT Department
Massachusetts Institute of Technology. Department of Materials Science and Engineering
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