The use of Nd-Fe-B magnets with Nd2Fe14B, which are used in high performance motors, is increasing. However, it is predicted that inexpensive Nd will become unavailable in near future, and there is a need to reduce the amount of Nd used and develop new magnets 1. Exchange-coupled nanocomposite magnets consisting of magnetic hard and soft nanoparticles, NPs have been proposed as candidate 2. The Sm(Fe0.8Co0.2)12 with ThMn12 structure has higher saturation magnetization, Ms, and uniaxial magnetocrystalline anisotropy, Ku than Nd2Fe14B and is expected to be a magnetic hard phase in the Fe-based system without Nd 3. We investigate the effect of the direction of the α-Fe coating surface and the volume fraction of α-Fe, VFe on the demagnetization process and the maximum energy product, (BH)max of exchange-coupled Sm(Fe0.8Co0.2)12 /α-Fe nanocomposite NPs using LLG simulations 4. Sm(Fe0.8Co0.2)12 NPs is as reference model #1. The nanocomposite NPs of Sm(Fe0.8Co0.2)12 coated with α-Fe on top/bottom, and both sides are model #2 and #3. For both # 2 and #3, α-Fe layer thickness is 1 nm ≤ tFe ≤ 7 nm. The size of NPs are x = y = 50 nm and z = 100 nm. The magnetic properties of Sm(Fe0.8Co0.2)12 and α-Fe were taken from references 3, 5-7. The cell volume is 1 nm3. External field, Hex is applied parallel to c-axis of Sm(Fe0.8Co0.2)12 along to z-axis of NPs. Fig. 1 shows VFe dependence of (BH)max for #2 and #3. The (BH)max of #1 was 630 kJ/m3 , while the (BH)max of #2 and #3 with optimized tFe were larger, 636 kJ/m3 for #2 with VFe = 4 % (tFe = 2 nm) and 657 kJ/m3 for #3 with VFe = 12 % (tFe = 3 nm). Fig. 2 shows distribution of demagnetizing field, Hd at Hex = 0 T for #2 and #3. The Hd acting on top/bottom surfaces of NPs was larger for #2, which was coated on top/bottom surfaces with α-Fe, which is larger than Ms of Sm(Fe0.8Co0.2)12, than for #3, which was coated on sides with α-Fe. Therefore, remanence and (BH)max of #3 are larger than those of #2. From the above discussion, it is clear that side coating of magnetic soft phase is effective in increasing (BH)max of Sm(FeCo)12/α-Fe NPs.
References:
1 S. Hirosawa, M. Nishino and S. Miyashita, Adv. Nat. Sci: Nanosci. Nanotechnol., 8, (2017) 013002.
2 R. Skomski and J. M. D. Coey, Phys. Rev. B, 48, (1993) 15812.
3 Y. Hirayama et al., Scripta Mater., 138, (2017) 62–65.
4 M. J. Donahue and D. G. Porter, “OOMMF User’s Guide, Version 1.0,” NISTIR 6376, National Institute of Standards and Technology, Gaithersburg, MD (Sept 1999). ; M. J. Donahue, D. G. Porter (2021), "OOMMF: Object Oriented MicroMagnetic Framework," https://nanohub.org/resources/oommf. (DOI: 10.21981/8RRA-5656).
5 Felix Jimenez-Villacorta and Laura H. Lewis, Chapter 7 "Advanced Permanent Magnetic Materials" Nanomagnetism, edited by J. Gonzalez (OCP Publishing Group) (2014).
6 D. Ogawa, T. Yoshioka et al., J. Magn. Magn. Mater., 497, (2020) 165965.
7 T. Fukazawa, H. Akai et al., J. Magn. Magn. Mater., 469, (2019) 296-301.

Fig. 1 VFe dependence of (BH)max for #2 and #3.

Fig. 2 Hd distribution on the top surface and the cross-section of NPs for #2 and #3 at Hex = 0 T.