NIMS team develops new magnetic compound with lower neodymium content

20 October 2014

A research group led by Dr. Kazuhiro Hono at Japan’s National Institute for Materials Science (NIMS) has synthesized a new magnetic compound which requires a lower amount of rare earth element than the currently used neodymium iron boron compound.

The ratio of neodymium, iron and nitrogen in the new compound (NdFe₁₂N is 1:12:1). Its neodymium concentration is 17% of the entire mass compared to 27% for the neodymium iron boron compound known as Nd₂Fe₁₄B, the main component used in the strongest permanent magnets. Furthermore, the intrinsic hard magnetic properties of the new material were found to be superior to those of Nd₂Fe₁₄B.

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In particular, the Curie temperature of the new compound is higher than that of Nd₂Fe₁₄B by 200 ˚C. This feature is particularly attractive for the application to traction motors of hybrid and pure electric vehicles, where the operation temperature exceeds 150 ˚C. The results of this research are published in Scripta Materialia.

Temperature dependent anisotropy field and magnetization of the new NdFe₁₂Nx compound. For comparison, the values of Nd₂Fe₁₄B compound are also shown. Source: NIMS. Click to enlarge.

Neodymium permanent magnets are the strongest magnets used in many industrial applications such as hard disk drives, cell phones, hybrid electric vehicles and wind turbines. Recently, their usage is increasing rapidly due to the growth of the production of hybrid and electric vehicles.

Rare earth elements such as neodymium and dysprosium are essential components of these magnets. Since China dominates the production of rare earth metals, there is some concern regarding the stable supply of rare earth elements for the increasing demand of these magnets. This has led to worldwide intensive research to reduce the content of rare earth elements in high-performance permanent magnets (e.g., earlier post, earlier post).

The neodymium permanent magnet, invented by Dr. Masato Sagawa in 1982, is composed of the Nd₂Fe₁₄B compound and some minor nonmagnetic components. No magnetic compound has been able to challenge this compound during the last 32 years.

A different type of magnetic compound, NdFe₁₁TiN, has been known to exist in the long history of rare earth permanent magnet research. However, the alloying of the non-magnetic Ti was necessary to stabilize this compound, causing its magnetic properties to be inferior to those of Nd₂Fe₁₄B; therefore, not much attention has been paid to it.

Since the compound becomes unstable without Ti, no one has succeeded in synthesizing it only with magnetic elements. In this new study, the NdFe₁₂N compound was synthesized successfully without non-magnetic Ti.

The NdFe₁₂Nx compound with a ThMn₁₂ structure (space group I4/mmm) was successfully synthesized by nitriding an NdFe₁₂ layer grown on a W underlayer on a single-crystalline MgO(001) substrate. The c-axis expanded from 0.480 to 0.492 nm while the a-axis showed a slight contraction from 0.852 to 0.849 nm after the nitriding. Excellent intrinsic hard magnetic properties of μ0Ms ≈ 1.66 ± 0.08 T, μ0Ha ≈ 8 T, and Tc ≈ 550 °C, which are superior to those of Nd₂Fe₁₄B, were obtained.

The NdFe₁₂ thin film was grown on a tungsten layer using a sputtering method, and then the NdFe₁₂N was obtained by nitriding the NdFe₁₂ layer. The small lattice misfit with the tungsten layer made the growth of the unstable NdFe12N compound possible. Although the compound was processed in the form of a thin film, they found it can be grown to the thickness of 350 nm.

Looking ahead, the team will seek to establish a processing route to synthesize a large quantity of powders of this compound and to manufacture bulk magnets.

This research was performed at the Elements Strategy Initiative Center for Magnetic Materials (ESICMM), which was established at NIMS in the scheme of Elements Strategy Initiative Project by MEXT, Japan. The work was in part supported by the Japan Science and Technology Agency, CREST.

Resources

• Y. Hirayama, Y.K. Takahashi, S. Hirosawa, K. Hono (2014) “NdFe₁₂N_x hard-magnetic compound with high magnetization and anisotropy field,” Scripta Materialia doi: 10.1016/j.scriptamat.2014.10.016