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 ₁₆N₂ has been viewed as a controversial material or mystery before 2000. We have started to work on this material since 2003. Besides reporting a systematic experimental study on Fe₁₆N₂ thin films and confirmed its giant saturation magnetization and large anisotropy constant [5], we proposed a "cluster + atom" model based on the fi rst -principles calculation to explain the giant saturation magnetization of Fe₁₆N₂ [6]. This model is supported by the discovery of partially localized 3d electrons in Fe₁₆N₂ [5]. After those validations, Fe₁₆N₂, with a magnetic flux density as high as 2.9T and an anisotropy constant up to 1.0-1.8 MJ/m 3 [7] has been expected to be one of possible ram -earth -free magnet candidates. Moreover, Fe₁₆N₂ has combined features of low cost with most redundant elements on earth, environment -friendly and theoretically two times higher energy product than the current market available rare earth magnets, as shown in Fig. 1. In this paper, we introduce three approaches, including an ion implantation approach, a strained -wire method and a ball milling method, which we developed in past six years, for the synthesis of bulk Fe₁₆N₂ magnets. By using a nitrogen ion implantation approach [8], we successfully synthesized free-standing Fe₁₆N₂ foils with a coercivity of up to 1910 Oe and a magnetic energy product of up to 20 MGOe at room temperature. An integrated synthesis technique was developed, including a direct foil-substrate bonding step, an ion implantation step and a two-step post-annealing process. With the tunable capability of the ion implantation fluence and energy, a microstructure with grain size 25-30 nm is constructed on the FeN foil sample with the implantation fluence of 5×10^{12/cm2. To the best of our knowledge, this could be the first experimental evidence of the existence of a giant saturation magnetization, an obviously large coercivity with a magnetic energy product of up to 20 MGOe in a bulk -type FeN magnet sample. Ball milling is one of the other approaches to prepare the Fe₁₆N₂ Powder [9]. Shock compaction using a gas gun was used to compact the powder into a dense disk shape. We experimentally demonstrated that the volume ratio of the Fe₁₆N₂ phase is 70% and that it is stable under shock compaction, without obvious phase decomposition. This approach presents the possibility of mass producing bulk permanent magnets using Fe₁₆N₂ with enhanced magnetic properties. Furthermore, we proposed and demonstrated a novel synthesis method for bulk anisotropic Fe₁₆N₂ magnet, named as the "strained wire method" [10]. Based on this method, an anisotropic bulk iron nitride magnet with 9 MGOe was achieved for the first time.}