Συσχέτιση δομής και μαγνητισμού σε νέα μαγνητικά υλικά νανοδιαστάσεων

Abstract

A combined structural, magnetic, magneto?optic and magnetotransport study of magnetic nanocomposite materials in the form of multilayers was carried out in the framework of the current Doctorate Thesis. The main objective was to get a detailed and systematic knowledge of the properties of the nanostructures that arise from the combination of the 3d magnetic metals with the 5d non?magnetic metals. More specifically, the magnetic elements Co & Ni were combined with the metals Pt, Au & Cr. Systems such as Ni/Pt, Co/Pt, Co/Au, CoCr/Pt were synthesized. From the aspect of physics, the goal of this work was the growth and characterisation of those novel magnetic materials, in order to clarify the mechanisms that determine their magnetic properties. Apart from the importance that these structures present due to the appearance of new phenomena from the view of basic physics, these materials may constitute some good candidates for future materials that concern the prosperous technology of magnetic and magnetooptic recording as well as the technology based on the phenomenon of giant magnetoresistance. The magnetic multilayers were grown in an ultra high vacuum chamber. The method of the synthesis was the evaporation with the use of electron beam. Moreover, patterned substrates made by the lithography technique were used, so as to be appropriate for the fabrication of patterned nanostructures. In particular, the Ni/Pt system represented the model system for the study of the behavior of the 3d/5d nanostructures. It showed excellent multilayer configuration with negligible interdiffusion. The influence of the substrate on the growth mode was examined. The magnetic features of the materials were studied toward to the growth parameters, while most of the samples showed a tendency for perpendicular magnetic anisotropy. This system also constituted the starting point for the whole study of the magnetooptic phenomenon and its dependence from various parameters. The fabrication of the Co/Au system proved to be successful regarding to the quality of its multilayer structure, since the relevant studies marked out almost square texture profiles. This system showed a tendency for perpendicular anisotropy depending on the decrease of the thickness of the Co layers and on the increase of the thickness of the Au layers. The appearance of enhanced magnetooptic response at about 3 eV led to the investigation of the spin?orbit coupling of the Au atoms. Measurements by the XMCD method (x?ray magnetic circular dichroism) demonstrated that Au presents an induced magnetic moment. Such phenomenon for Au has not been published until now elsewhere. Another object of the research of the multilayers was to examine their behavior approaching their natural limit, which is the monolayer. For this purpose the Co/Pt system was used. The result was strong perpendicular anisotropy and a very large value of the magnetooptic response mainly for samples with only one or two Co atomic layers. The synthesized samples are suggested for future magnetooptic recording means. The last part of this doctorate thesis included the implementation of the idea of the combination of two promising magnetic means. Firstly, there were the CoCr alloys and on the other hand there was the Co/Pt system. The aim was to fabricate CoCr/Pt multilayer structures. Accordingly to the literature, such structures have not been synthesized by many researchers. The nominal ratio between Co and Cr was the same in each layer. There was an effort to correlate the structure of the samples with the magnetic properties. A perpendicular magnetic anisotropy was derived for the samples with the extra?thin layers, which was amplified by the decrease of the temperature.