Date Awarded

1999

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Physics

Advisor

William J Kossler

Abstract

The discovery of tunneling magneto-resistance has led to a great deal of interest in the study of ferromagnet-insulator-ferromagnet (FIF) systems due to potential sensor and magnetic storage applications. An analysis of the band structure of the 3d ferromagnets shows that the conduction electrons become spin polarized by the molecular field. The transmission coefficient of these electrons across a tunneling gap therefore depends upon the relative alignment of the molecular field between the two ferromagnets.;In this work the manufacture of such tunneling gaps through compression molding of powdered ferromagnetic iron with a high performance polyimide has been studied for the first time. The percent change in the resistance with applied magnetic field depends critically on the volume percentage of ferromagnetic material in the composite. A peak in the tunneling magnetoresistance (TMR) occurs at a volume concentration just beneath the percolation threshold of the ferromagnetic material. The change in resistance relative to the resistance at zero field, DeltaR/R0, obtains a room temperature peak value of -4.5% at 20% iron volume concentration.;Granular conducting systems near the percolation threshold are also subject to variable range hopping (VRH) conduction. The charging energy of small metallic grains results in an energy barrier for the acceptance of an additional electron. Electronic conduction requires thermal activation over this barrier along with tunneling through the insulating regime. The result of these two combined processes is a temperature dependent tunneling distance and a conductivity of the form ln sigma ∝ T-x, with 1/4 ≤ x ≤ 1/2.;The theoretical development and experimental measurements of TMR and VRH in iron polyimide nanocomposites are thoroughly developed and analyzed in this work. Ferromagnet particle size and band structure effects on TMR are also explored in an effort to optimize the material for sensor applications.

DOI

https://dx.doi.org/doi:10.21220/s2-qmwv-9x40

Rights

© The Author

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