Magnets play a major role in our rapidly developing world of technology. Electric motors and generators, transformers, data storage devices, MRI machines, cellphones, and NMR are some of the many applications for magnets. However, almost all the magnets currently being used have rare-earth heavy metals in them. Despite their high-energy product, the presence of rare-earth metals increases the cost significantly. Also, the processes involved in the mining of rare-earth metals are hazardous to the environment, and to all life forms. In the past few decades, oxide based magnets have gained a lot of attention as potential replacements for the rare-earth magnets. Oxide based magnetic nanocrystals are attracting a lot of attention as a potential replacement for rare-earth magnets. They are stable in ambient condition and their manufacturing cost is very low when compared to the rare-earth magnets. My work deals with the synthesis of core-shell magnetic structure for high frequency applications (Chapter 1) and the synthesis of high energy product magnetic nanocrystals (Chapter 2) and the synthesis of soft magnetic nanocrystals for high frequency measurement. NiZn ferrite, a soft oxide based magnet cannot be directly implied at high frequencies as they fail at the frequency which over the MHz range. On the other hand, BaZn ferrite is a Y-type magnets, which is robust at higher frequencies. Therefore, using the latter magnet as a protective shell for core material, made of former magnet, enables us to manufacture a cheap solution to the rare-earth magnets used in our cell phones and other devices that work on high frequency signals. On the other hand, successful coating of a very soft magnetic material on a hard-magnetic core increases the total energy product of the magnetic composite, which enhances its versatility. / Mechanical Engineering