Gas phase synthesis of magnetic nanocomposite materials and Its application in anisotropic-exchange-spring magnets.

Abstract

The improvement of the properties of magnetic materials has reached a critical stage since it is more and more difficult to discover new materials to achieve higher performance. Nanocomposite material is a new generation of materials, which combines different materials in nano-scale to generate unachievable properties from single phase materials. Nanocomposite materials have very broad applications including permanent magnets, nanocrystalline magnetic soft materials, solar cells, biomedical materials and devices. In this study, a novel gas phase synthesis process is developed for fabrication of nanocomposite materials. This technique represents a general process and is applicable to most materials. The synthesis system consists of a magnetron-sputtering-based gas-phase nanoparticle deposition source, conventional thin film sputtering sources and a modified substrate holder with external magnetic field generator. Anisotropic exchange-spring magnet with nanocomposite structure, based on a FePt/Fe-Ni system, has been successfully fabricated for the first time in this research work, through using the novel nanocomposite fabrication process. FePt/Fe-Ni anisotropic exchange-spring nanocomposite consists of exchange coupled L10 FePt hard phase and Fe-Ni soft phase. It has greatly enhanced the energy product compared to a single phase of FePt or Fe-Ni. This structure also has a defined easy-axis, thereby improving its energy product from the isotropic case (124%). Systematic magnetic and structural studies have been performed. The optimized hard phase volume ratio is found experimently for the first time to be around 10%, which confirms the theoretical finding by Skomski and Coey 15 years ago. Studies on materials with ultra-high crystalline anisotropy such as L10 FePt, SmCo5 and materials with potential ultra-high saturation magnetization such as Fe16N2 are also included.