Studies of ultrathin magnetic films for advanced storage applications

Date of Completion

January 2003

Keywords

Physics, Condensed Matter|Engineering, Materials Science

Degree

Ph.D.

Abstract

For a thorough understanding of tunneling magneto-resistance (TMR) and exchange anisotropy, the local chemical and magnetic environment near the surface and interfaces of magnetic thin films creeds characterization. This thesis comprises a series of studies that investigate ultra-thin magnetic films using various electron spectroscopy techniques with that necessity in mind. ^ First, we investigate the chemical and magnetic properties of FM/AlO x bilayers. We find that the FM layer polarizes the states of the AlOx layer. The sign of the induced polarization is sensitive to the type of FM material. These findings challenge our understanding of TMR in magnetic tunneling junctions with similar films. It is assumed AlO x only acts as a non-magnetic tunneling barrier and the tunneling current's spin-polarization is an intrinsic property of the FM layers. The existence of magnetism in the AlOx layer suggests new models that include its contribution to this tunneling spin-polarization. ^ Next, we investigate the chemical and magnetic properties of Fe3O4/AlOx bilayers. We find the growth of Fe3O4 is dependent upon the substrate heterostructure, and minority-spin electrons dominate the tunneling current in junctions with similar films. This result is understood from band-structure calculations that suggest Fe3O4 is a half-metallic ferromagnet with only minority-spin states at the Fermi Level. These findings are in contrast to all previous TMR studies of AlOx-based junctions, where the tunneling current is dominated by majority-spin electrons. ^ Finally, we investigate the role of non-interfacial AFM layers in exchange biasing of ferromagnetic (FM) films, which is characterized as a shift in the hysteresis loop along the field axis of the FM film in contact with an AFM film. We demonstrate exchange anisotropy in Fe films exchange biased by NiO/CoO heterostructures is sensitive to the magnetic anisotropy, thickness and depth from the Fe/AFM interface of the CoO layer. These results suggest exchange anisotropy is not just an interface effect and a “depth” dependent model, where all the AFM layers are considered, is a more accurate picture. We argue the length scale of this “depth” dependence is inversely related to the strength of the magnetic anisotropy of the AFM film. ^

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