Zhang, Wei; (1999) Pulsed laser ablation and deposition of magnetic materials. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

This thesis concentrates on the growth by PLD of GMR metallic granular materials and CMR metallic manganite oxides for magnetic recording industry applications. The focus of this work is on the growth of high-quality perovskite-type manganite oxides films on Si substrates. The possibility of growing multilayers with a gradual change in lattice constant to ensure better lattice matching to the top layer (manganite oxides) is shown by depositing LaCaMnO with CeO2 and YBaCuO/CeO2 as a buffer layer on Si substrates. The lattice effect, produced by ion A-site and B-site doping in AA'BO3 (A = La, Nd and Y, A' = Ca, Sr, and Pb, B = Mn and Fe) perovskite oxides, on the electrical, magnetic and structural properties has been systematically investigated. The temperature associated with the peak resistivity and magnetoresistance level can be markedly changed by varying the trivalent ion composition and the introduction of Fe ions at the B-site (Mn). The introduction of stress, produced by the deposition of LaCaMnO films on Si substrates and on CeO2 coated Si, can also significantly modify the transport and magnetoresistance behaviour of these films. Both resistivity and magnetoresistance were strongly suppressed upon the application of stress. Elemental transfer from the target to substrate during PLD process was investigated. The composition of the films is found to be strongly dependent on substrate temperature (Ts) with the calcium and oxygen content being significantly reduced at high Ts (> 700 degree C). The kinetic energies of the various ions in the laser generated plume were also examined using Mass Spectroscopy. Preferential re-sputtering of these surface Ca-rich segregated samples, driven by a mixture of high energy ion irradiation and thermally- activated processes, can be proposed to explain the incongruent transfer during PLD process. Phase segregation has been directly produced in as-deposited Co-Ag granular films, unlike in film deposition by sputtering, where post-annealing is required to achieve the required phase segregation. The magnetoresistance behaviour was found to be strongly sensitive to the film composition and preparation conditions, particularly post-annealing. Thus the optimal choice of parameters to maximise the magnetoresistance could be determined.

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