From Nd2Fe14B to NdFe12Nx

AUTHOR: Xuan Xu

Our daily environment is significantly dependent on the materials with outstanding magnetic properties. Hard magnetic materials have been successfully exploited with enhanced energy densities (BH)_max, starting from ≈1 MGOe for steels discovered during the early part of the century, increasing to ≈3 MGOe for hexagonal ferrites, and finally peaking at ≈56 MGOe for neodymium-iron-boron magnets in the past thirty years. This historical evolution spanning nearly 100 years of these permanent magnets is shown in Figure 1 [1]. Recently, much more attention is paid on rare-earth permanent magnets due to the growing demand for energy efficient technologies. These magnets are very efficient at various operating temperatures. The excellent magnetic properties of NdFeB-based magnets result from the Nd₂Fe₁₄B phase which is characterized by a high saturation magnetization (µ₀M_S) of 1.6 T, a high anisotropy field (µ₀H_a) of ~7.5 T and a Curie temperature (T_c) of 321 ℃ [2, 3]. So Nd₂Fe₁₄B based magnets are often chosen for small and light applications.

Figure 1. Development in the energy density (BH)_max at room temperature of hard magnetic materials in the 20^th century and the percentage sales for 2010 of the major permanent magnets in the world, from Ref. [1]

A hard magnetic compound that is suitable for a permanent magnet application must have a sufficiently large anisotropy field (H_a). H_a is the theoretical limit of coercivity for spins coherent rotation, while the coercivities (H_c) of industrially processed bulk magnets have never exceeded H_a/3. This is becausee a certain fraction of nonmagnetic phase (>10%) must be present in the bulk magnets in order to have a heterogeneous microstructure which  induces the coercive force. Many papers have pointed out the poor thermal stability of Nd₂Fe₁₄B, this results  in a very rapid decrease of coercivity with temperature. Another serious problem is the poor corrosion resistance which comes from the Nd-rich grain boundary phase and  is very sensitive to humidity [4]. Therefore, finding a new hard magnetic compound, whose saturation magnetization is higher than that of Nd₂Fe₁₄B and with a sufficient magnetic hardness of H_c > H_a/3 is of great interest [5]. Additionally, the worldwide concern for rare-earth resources has triggered the development of new high-performance permanent magnets with lower rare-earth element content and may result in higher anti-corrosion property. During the rare-earth crisis of 2011, the European Union (EU) investigated several possible solutions to alleviate the shortages of rare-earth resources. In the DEMETER project, researchers are working on  the synthesis of novel hard permanent magnets with lower rare-earth element content.

Since 1990, the discovery of the nitrogenation effect on the relevant rare earth-iron intermetallics has attracted great interest in the study of rare earth nitrides [6-8]. The study of nitrogenation effect reveal that the interstitial nitrogen dissolution indeed enhances the magnetic properties: µ₀H_a, µ₀M_S and T_c. It is quite logical that the nitrogenation of Nd₂Fe₁₄B may improve the magnetic properties. However, experimental results suggested [9] that interstitial modification of Nd₂Fe₁₄B using N indeed resulted in an increase in T_c of 60 K, but with a reduction in M_s and anisotropy field.Other studies even have showed that nitrogenation may lead directly to  decomposition [10, 11]. So nitrogenation of Nd₂Fe₁₄B fails to enhance its magnetic properties. At the same time R₂Fe₁₇N₃ (2:17, R = rare-earth element) [12, 13] and RFe_12-xM_xN (1:12, R= rare-earth element, and M = Mo, Ti and V) [7, 14, 15] rare-earth/iron compounds have received extensive attention. These nitrides exhibit high intrinsic magnetic properties which are comparable with those of Nd₂Fe₁₄B, or even better in terms of Curie temperature [16] and the corrosion stability [17]. The average increase in T_c caused by introducing nitrogen into RFe_12-xM_xN is ~ 200 K [18] which is desirable for these magnets applications in the high-temperature operation conditions. Among these RFe_12-xM_xN alloys, NdFe_12-xM_xN magnets show a higher saturation magnetization [7, 16]. Furthermore, the relative content of the rare-earth element Nd in NdFe_12-xM_xN (~ 7.6 at.%) is much lower compared to other rare-earth permanent magnets such as R₂Fe₁₇N₃ and Nd₂Fe₁₄B (~ 12.5 at.%) magnets. However, RFe₁₂ compounds are metastable which can only be obtained by replacing the Fe with a third element M (M = Mo, Ti and V). As a consequence, µ₀M_S of NdFe_12-xM_xN compounds is lower than that of Nd₂Fe₁₄B, due to the substitution  of a non-magnetic element for Fe to stablize the ThMn₁₂ structure. For example, M. Akayama [19] and S. Suzuki [20] reported that µ₀M_S of NdFe₁₁TiN and NdFe₇Co₃TiN_1.3 is, respectively, 1.48 T and 1.50 T, which are lower than that of Nd₂Fe₁₄B (1.60 T). If x in NdFe_12-xM_xN compounds can be reduced, large magnetization will be achieved which has been approved by T. Miyake et al [21] theoretically via first-principles study. Although the pure NdFe₁₂ phase is metastable, recently, Hirayama et al [5, 22]. obtained the NdFe₁₂N_x phase via co-sputtering of the Nd and Fe on textured W (110) substrates and subsequent annealing in nitrogen atmosphere (Figure 2 left). They acheived excellent intrinsic hard magnetic properties of µ₀M_S ≈ 1.66 ± 0.008 T, µ₀H_a ≈ 8 T and T_c ≈ 550 ℃ (Figure 2 right) that are superior to those of Nd₂Fe₁₄B.

Fig. 2 Left: STEM/EDS image of (a) NdFe₁₂ and (b) NdFe₁₂N_x; Right: (a) Anisotropy field (µ₀H_a) and (b) saturation magnetization (µ₀M_S) as a function of temperature along with those of Nd₂Fe₁₄B, from Ref. [5]

Although co-sputtering, the method for preparing NdFe₁₂N_x compounds Hirayama used, is technically interesting, it is a procedure that is very costly due to the required high vaccum conditions and  relatively low deposition rates that makes it difficult to prepare thick film coatings. In contrastn, the electrochemical reduction is quite attractive due to its simplicity, high deposition rates, and its  ability to make single- and multi-layers of different metal or alloy, ease of scale-up to large surface area and the applicability to wide variety of different shapes and surface geometries. Unfortunately, there are no recent reports on the electrodeposition of NdFe₁₂ phase. However, some articles on electrodeposition of NdFeB magnets have already been published which provide some guidelines for the synthesis of NdFe₁₂ phase through electrochemistry. If NdFe₁₂ alloy could be well electrodeposited on suitable substrates first, then the nitrogen could be introduced in-situ via the use of N-containing regents or ex-situ via post annealing in N₂ atmosphere at 400-500 ℃ to form NdFe₁₂N_x.

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