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First-principle Calculations Reveal How High-performance Magnets Are Achieved by Interstitial Nitrog
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First-principle Calculations Reveal
How High-performance Magnets Are Achieved
by Interstitial Nitrogen

Fig. 1:Lattice structure of rare-earth magnet NdFe11TiN and change in the electron density distribution due to interstitial nitrogenation. The electron density increases in the red region.

Materials for high performance magnets are required to have large magnetization and strong coercivity. The origin of coercivity still remains to be a fundamental problem in materials science, but is known to be well correlated with the magnitude of magnetic anisotropy of each magnetic material. It is known that NdFe11TiN and related materials have high iron density and can become a high-performance magnet. Recent first-principles calculations revealed mechanisms leading to change of magnetization and anisotropy of NdFe11TiN when Fe is partially replaced with Ti or when N is interstitially introduced. In particular, the enhancement of magnetization and magnetic anisotropy in the presence of interstitial N are well understood in terms of the corresponding change in electronic states.

More than 50 % of total electric power is consumed by electric motors in Japan. Therefore, high power permanent magnets are now being used for various purposes, including hard disks, air conditioning, washing machines, etc. In particular, the demand for hybrid cars or electric vehicles and wind turbine generators is rapidly expanding. The performance of a permanent magnet is characterized by its magnetization and coercivity, the latter of which is closely related to magnetic anisotropy. Permanent magnets with a high figure of merit have usually been realized by materials called rare-earth magnets. They are based on Fe or Co and contain a small amount of rare-earth elements, which tend to enhance the magnetic anisotropy. Among them the so-called neodymium magnets (Nd2Fe14B) have been the strongest magnets for the last 30 years. Unfortunately, however, neodymium magnets become less effective at around 200 degrees Celsius, which is common under the operation conditions of vehicles. This problem can be overcome by partly replacing neodymium with dysprosium, which belongs to the so-called minor metals. However, this tends to reduce the magnetization itself, lowering the figure of merit. Therefore, the development of high-performance magnets and the theoretical understanding of how to enhance their figure of merit are both currently attracting a lot of attention.

Among rare-earth magnets, RFe12-xMx (R is a rare-earth element and M is called stabilizing element) having a ThMn12 type lattice structure may become a high-performance magnet when x is sufficiently small, thus realizing the high density of iron. In the case of NdFe11Ti, for example, R can be neodymium, which is relatively abundant on the earth, and high iron density with x=1 can be stabilized using Ti as M. It has been observed that interstitial nitrogenation enhances both magnetization and magnetic anisotropy.

Recent calculations from a first-principles densityfunctional scheme clarified the effects of Ti and N on the magnetism of NdFe11TiN [1]. The strong magnetic anisotropy in rare-earth magnets are usually ascribed to 4f electrons of rare-earth elements. In fact, the spatial distribution of 4f electrons in crystal is slightly deviated from spherical symmetry and its direction is determined by the crystal field determined by other electrons and nuclei.

The calculations show that bond charge is formed between neighboring neodymium and nitrogen atoms by N doping, resulting in the increase of the electron density in the c axis direction of a neodymium atom as shown in Fig. 1. Due to Coulomb repulsion, the distribution of Nd-4f electrons becomes flattened in the ab plane, leading to strong magnetic anisotropy. This result was semi-quantitatively confirmed by calculations of the corresponding crystal-field parameter. Furthermore, 2p levels of N lie lower than 3d levels of Fe and anti-bonding hybrids of these levels appear in the vicinity of the Fermi level. The magnetization is enhanced through the spinconfiguration dependence of their occupancy.

On the other hand, when Fe is partially replaced with substitutional titanium, unoccupied Ti-3d levels are formed, leading to an appreciable amount of reduction in the magnetization according to the Friedel sum rule. All these results show the possibility to be able to change the magnetic properties through hybridization. Thus, various new high-power magnetic compounds are highly likely to be realized through the appropriate substitution and doping of elements.

References

[1] T . Miyake, K. Terakura, Y. Harashima, H. Kino, and S. Ishibashi, J. Phys. Soc. Jpn. 83, 043702 (2014).

 
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