Preparation of Hard Magnetic Nanostructures

The preparation methods and the essential properties of the three main types of hard nanostructures are presented in this section (i) in exchange-decoupled nanostructures, high coercivities are achieved, (ii) single-phase 

As described in Section 2, the coercivity of hard magnetic materials is generally determined by undesired structural defects. As the size of hard particles is progressively reduced, the coercive field tends to increase. This is explained by considering that the impact of a given defect extends to the volume of the concerned particle. It is reduced as the particle size is reduced. However, below a certain critical size, a reduction in coercivity is observed. The process used to reduce particle size inevitably introduces additional defects, which become dominant. 

This argument shows that the processes used for the production of very small particles must be carefully controlled in order that a minimum number of additional defects is introduced. Various sophisticated processes have been developed which allow nanoparticles to be prepared, showing more less controlled nanostructures. These processes may involve size reduction of larger particles (top-down approach) or direct nanoparticle growth (bottom-up approach). The most important of these processes are described in the following sections. 

Hydrogen decomposition desorption recombination (HDDR) process is the only top-down industrial process used for the preparation of coercive nanoparticles. This process applied to rare-earth transition-metal (RE-TM) alloys consists in heating the concerned alloy under hydrogen until it decomposes into a fine mixture of RE-hydride and TM. The hard magnetic phase is recombined with a much finer microstructure. This process was first developed to convert 100 microns sized cast Nd2Fei4B grains into 200-300 nm crystallites [18, 19]. Later, it has been applied to other RE-TM alloys [20, 21]. Recently, a new variation of this process has been developed towards developing texture in the final materials [22], It is briefly described below. 

Melt spinning is by excellence the bottom-up approach to prepare nanoparticles with typical grain size around 50 nm. 

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