Wachstum und Charakterisierung des Tunnelmagnetowiderstand-Mehrschichtsystems Co-Al 2 O 3 -CoAl und das Studium spinfrustrierter Vortices in Ringstrukturen
This work discusses two major topics: The study of a Co-Al2O3-CoAl(100) TMR model system and the spin frustration in magnetic rings. CoAl(100) crystallizes in a CsCl-type structure. The scanning tunneling microscopy (STM) image displays flat and large terraces with a width of 40-100 nm separated by steps with a height of 0.28 nm. This agrees with the lattice constant of CoAl and represents a double atomic step of the CsCl structure. The ABAB stacking sequence is disturbed at the surface. The disturbance is caused by antisite atom segregation of Co atoms to the nominal Al sublattice in the topmost surface plane. Adsorption of O2 to the intermetallic alloy CoAl(100) allows the preparation of thin alumina films that can serve as a tunnel barrier in the tunneling magnetoresistance (TMR) model system. Exposure to oxygen at 300 K leads to the growth of amorphous oxide. The electron energy loss (EEL) spectra exhibit two losses at 640 und 890 cm-1. The self-limiting thickness of the amorphous alumina at saturation amounts to 0.7 nm. For this film a band gap of 3.7 eV was found. A new phenomenon shows up, namely the segregation-induced magnetism of CoAl(100). During the oxidation process Al segregates to the oxide-gas phase interface. The relative enrichment of Co in the crystal is sufficient to produce ferromagnetism of CoAl at 300 K. Oxidation at elevated temperatures leads to the growth of well-ordered alumina. At first, we study the oxidation at 800 K. The oxide is characterized by Fuchs-Kliewer modes at around 430, 630, 780 and 920 cm-1, which is indicative for the growth of a well-ordered oxide. LEED shows a two-domain (2x1) structure. We conclude that a well-ordered teta-Al2O3 grows on CoAl(100). Due to Al missing chains in the topmost layer STM exhibits streaks that are separated by 0.564 nm, which agrees with the lattice constant of the oxide. The thickness of the oxide film amounts to around 1 nm, and the band gap amounts to 4.4 eV. A thermal stability of the oxide film up to 1300 K was found. Co exhibits Volmer-Weber-type growth on both amorphous and teta-Al2O3. This is a consequence of the huge difference in the surface free energy of Co and that of the oxide. On teta-Al2O3, already after the deposition of 0.08 nm Co, clusters with a mean diameter of 2 nm and a mean height of 0.7 nm can be found. After deposition of 0.6 nm Co the surface is fully covered. Finally, after depositing 3 nm of Co, clusters between 1 and 4.5 nm show up. Due to the interaction with the underlying oxide these clusters are partly oriented in chains. The thermal stability of the Co/Al2O3/CoAl TMR model system is limited by diffusion of Co into the sample. The diffusion sets in at 500 K. Interacting rings in direct contact, arranged in a tri-ring structure, exhibit a magnetic reversal process that is strongly affected by the direction of the applied external field. At saturation, all three rings yield an onion state, where the magnetization follows the two halves of a ring. Reducing the field leads to nucleation of vortices and vortex cores. However, the formation of vortices is frustrated due to the interaction between the rings within the tri-ring structure. In particular, different intermediate magnetization states can be selected by changing the orientation of the applied field. While the preferred reversal mode of vortex nucleation is determined by the angle of the applied field with respect to the tri-ring orientation, the stability of the intermediate vortex states can be improved by decreasing the width of the rings.
info:eu-repo/classification/ddc/530, Physik, Auger-Spektroskopie, Rastertunnelmikroskopie, Elektronen-Energieverlustspektroskopie, Magnetooptischer Kerr-Effekt, LEED, Flussschlauch, MFM, Oxidation, Al2O3, Vortex, CoAl
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