Fe16N2: from a 40-year mystery of magnetic materials to one of promises for rare-earth-free magnets.

摘要

Fe₁₆N₂ is one of the most promising rare-earth-free magnet candidates with high magnetic energy product. Iron nitride magnet is of great interest as a magnetic material for applications at relatively low temperature (<;150 <;°C) ranging from magnets in hard disk drives for data storage and in all kinds of electrical motors, wind turbines, and other power generation machines. A perspective review on our research work on bulk Fe₁₆N₂ compound permanent magnet in past years is presented on the aspects of material processing and magnetic characterizations. Specifically, we will introduce and discuss our effort to prepare bulk Fe₁₆N₂ compound permanent magnet by using three different approaches, including an ion implantation method, a ball milling method and a strained-wire method. A feasibility of free-standing iron nitride foils with magnetic energy product up to 20 MGOe was successfully demonstrated based on an ion implantation method. Based on our theoretical and experimental progress, we believe that Fe₁₆N₂ compound permanent magnet is currently in an accelerating step to be an alternative magnet candidate. TECHNICAL RESULTS AND DISCUSSION: During past decades, several permanent magnet materials were discovered, especially those based on rare-earth intermetallic compounds [1,2,3]. The key fi gure of merit of permanent magnets is the energy product (BH) . Figure 1 lists the development in the maximum magnetic energy product (BH) at room temperature of market -available hard magnetic materials so far [4] and our predicted value for iron nitride magnet. It is interesting to note that this value, starting from 1MGOe for steels discovered during the early part of last century, increasing to 3MGOe for ferrites, and finally that peaks at 56MGOe for ueodymium-iron-boron magnets during the past twenty years. However, new magnets with more abundant and less economically-limited and environmentally -restricted elements is highly demanded to supplement rare earth magnets [3]. At the same time, the saturation magnetization of rare earth magnets may not be high enough to satisfy the requirements for the applications of electric machines. One of basic function of permanent magnets used in electric vehicles and wind turbines is to provide magnetic flux. This function requires a higher saturation magnetization as well as an appropriate coercivity to against self -demagnetization. The most ideal permanent magnet should have the following features: (1) be composed by abundant and environment friendly elements; (2) large saturation magnetization; (3) large energy product; (4) reasonable high coercivity; (BH)_max has doubled every 12 years during the 20th century mainly with the progress due to improvements in coercivity [4]. Next generation permanent magnet would be expected with a higher remanent magnetization while with a reasonable coercivity. Fe ₁₆N_展开▼