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Growth, magnetic and spin polarized vacuum tunneling properties of ultrathin ferromagnetic films

Authors Niu, Yuran
Issue Date 2010
Summary Vacuum tunneling in field emission from sharp W tips coated with ultrathin ferromagnetic films attracts interest because it could potentially generate bright, coherent spin polarized electron beams. Investigations of spin polarized field emission (SPFE) also may contribute to a better understanding of the spin polarized tunneling process in spin polarized scanning tunneling microscopy. SPFE has been achieved from ultrathin film Fe- and Co-coated W(001) and W(111) tips in their spontaneously magnetized state at 300K. In each case, a transverse spin polarization component is detected. A modest magnitude of spin polarization up to 35% was obtained. For (001) tips, the azimuthal orientation of polarization showed a strong preference for alignment with the transverse low-index crystallographic directions, i.e. the <110> for Co and <100> for Fe. In contrast, the polarization direction for W(111) tips exhibited only a weak preferential alignment with the set of <1̅10> and <112̅> tip directions due to the competing influence of magnetocrystalline anisotropy and tip morphology on tip magnetization. Superparamagnetic fluctuations of the tip magnetization are evident in the polarization direction of emitted electrons when film coatings are very thin. This superparamgentic behavior effectively imposes a limit to spin polarized vacuum tunneling applications in the very thin film limit. This limit can be overcome by cooling very thin film coated tips below 300K or by increasing the volume of the emitting domain at the tip apex through increasing the film thickness. At marginally larger thickness, long-term stability of the polarization magnitude and direction is observed at 300K. A method for changing the stable spin polarization direction is also presented that exploits spontaneous flipping of the tip magnetization at elevated temperature. A new instrument for measuring the spin-resolved field emission energy distribution (SPFEED) was also developed. Its design, simulation, control and operation are presented as well as initial SPFEED results for Fe/W(001) tips. In order to aid our understanding of the polarized emission from Fe/W(001) tips, the growth and magnetic properties of ultrathin Fe films on a W(001) single crystal surface were also investigated. These investigations were carried out using low energy electron microscopy (LEEM) and spin polarized LEEM (SPLEEM). The three key growth regimes were observed — rough growth of highly-strained pseudomorphic (ps) films at room temperature, kinetically-limited layer-by-layer growth of ps films at intermediate temperature and Stranski-Krastanov (SK) growth of three-dimensional (3D) crystallites on top of a thermodynamically stable 2 monolayer (ML) thick wetting layer at higher temperature. In the SK mode, a metastable 4 ps ML Fe film forms before the nucleation of 3D Fe islands and consequent dewetting of the top two unstable layers. Regardless of growth temperature, ferromagnetic order is detected in 3 ML and 4 ML films but not in 2 ML films at 300K. We observe in-plane magnetization with easy axis along the substrate <110> and <100> directions in 3 ML films and along the <100> directions in 4 ML and thicker films. Large magnetic domains are observed in metastable 4 ML films grown at intermediate temperature, which break up into smaller domains due to the spontaneous formation of dislocations when thickness is increased. This trend is slightly delayed when strain is partially relieved by roughness during growth of Fe films at room temperature. Depending on the size and shape of an isolated Fe island that forms in the SK growth regime, the magnetic domain structure can be vortex, quasisingle domain with flux closure ends or single domain.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2010
Language English
Format Thesis
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