published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 1997. / Includes bibliographical references (leaf 218-227).

published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 1995. / Includes bibliographical references.

The initial part of this investigation was to determine the ternary isothermal sections for the Cr-Co-Cu system and the Crâ Mm-Cu system at lOOO°C. and 975°C., respectively. This was done by vacuum melting the Cr-Co~Cu alloys and annealing these alloys in an inert atmosphere of a helium-hydrogen gas mixture. The Cr-Mn-Cu alloys were melted under a helium atmosphere and annealed in evacuated Vycor tubes. By the use of metallographic techniques and x-ray analysis the phases present in each alloy were determined and the two ternary isotherml sections constructed. It was found that copper had a very limited solid solubility in the sign phase of both systems. The sigma field in the Cr-Co-Cu 1000°C. ternary isothermal comes into equilibrium with the terminal solid solutions of chromium, cobalt, and copper. In the Cr-Mn-Cu ternary isothermal section sigma comes into equilibrium with the terminal solid solution of chromium and cobalt and the face-centered cubic solid solution of copper in manganese. From these data the electron vacancy scheme(16) may not be extended to include copper due to the limited solid solubility of copper in sigma. Also, the extent of copper solubility in the sigmas cannot be related to the solubility of the copper in cobalt or manganese. / Master of Science

Diss. (sammanfattning) Uppsala : Univ., 2003. / Härtill 12 uppsatser.

No description available.

No description available.

No description available.

Development of the faster and denser magnetic memory storage elements has been an active area of research since early 20th century. The path of research on magnetization manipulation began with firstly changing the magnetization state of a medium in an external magnetic field, then heating of a medium and magnetizing with a permanent magnet was explored, while the latest efforts have been focused on switching the magnetization only by a polarized laser light. Nowadays due to the technological advancement of lasers and material fabrication methods, the search and development process of magnetic memory elements is much faster. The implementation of such technologies, however, relies on finding suitable magnetic materials which would allow for a fast magnetization writing and read-out processes and would remain magnetized, even with the reduced dimensions. Ferrimagnetic rare Earth - transition metal (RE-TM) alloys have been used for fabricating magneto-optical recording media already since the 1990’s. Relatively recently, in 2007, it was demonstrated that the ferrimagnetic GdFeCo alloy magnetization state can be switched using only circularly polarized laser light. Hence, ferrimagnetic RE-TMalloys could be suitable candidates for all-optical light-induced magnetization switching (AOS), without any external magnetic field. Another combination of RE-TM alloys that was shown to exhibit AOS is ferrimagnetic amorphous alloys containing terbium and cobalt (Tb:Co). They have attracted attention due to their strong out-of-plane magnetic anisotropy, high magneto-optical activity and amorphicity, which makes them attractive from a fabrication point of view since a variety of substrates and buffer layers could be used for growing such layers. In this Thesis, TbCo alloys are investigated in order to examine how the magnetic, optical and magneto-optical properties could be tuned by varying the elemental ratio and film thickness. The main question that was addressed here was whether such a system is suitable for fabrication of nanosized magnetic elements as the building blocks for the magnetic memory applications. TbCo alloys were prepared as thin films by magnetron co-sputtering method onto different substrates and buffer layers. Films were characterized using a variety of techniques such as an ion beam analysis, an x-ray reflectivity and diffraction, and magneto-optical characterization techniques. It was observed that the properties of such alloys depend not only on the Tb:Co ratio but also on the film thickness and an underlying buffer layer. Magnetization compensation point, at which the magnetization of a film is zero, as in an antiferromagnet, can be modified depending on the buffer layer. All-optical switching (AOS) of magnetization experiments were performed on the fabricated samples. It was determined that AOS with at least 50-100 laserpulses can be achieved for the films grown directly onto fused silica substrates and with the compositions above the magnetization compensation point at room temperature, in the range of 24 - 30 at.% Tb. In the Outlook, the initial efforts of patterning the films into the arrays of nanosized elements are presented. It is demonstrated that after the lithographic patterning of the films, the resulting nanosized elements remained out-of-plane magnetized. In this work it is shown that the ferrimagnetic TbCo alloy system is a potential candidate material for bothfacilitating AOS and the fabrication of arrays of nanomagnets. Combining the TbCo alloys,which show AOS, together with a suitable buffer layer and patterning the hybrid structure,could enable selective element-by-element magnetization switching for the magnetic memorystorage devices.