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SmFe12 compounds with a tetragonal ThMn12-type crystal structure is expected to be a promising candidate for new high-performance permanent magnet materials due to the excellent intrinsic magnetic properties of the Sm(Fe0.8Co0.2)12compound (saturation magnetization: μ0Ms = 1.78 T, anisotropy field: μ0HA = 12 T, and Curie temperature: TC = 879 K) [1]. However, it is well known that the SmFe12 phase is thermo-dynamically unstable in the bulk state, and some Fe sites must be substituted by stabilizing elements such as Al, Ti, V, Co, Nb, and Zr. In addition, it has been reported that a columnar structure in which SmFe12 grains are surrounded by grain boundary phases results in a high coercivity (μ0Hc = 1.2 T) in B-doped Sm(Fe-Co)12 thin films [2], and the coercivity has been successfully increased to 1.8 T by depositing an Al cap layer on Sm(Fe-Co)12-B thin films [3]. Recently, some results have been reported on the improvement of the magnetic properties of Sm-Fe-Co based alloys by the addition of stabilizing elements. For example, the coercivity of Sm-Fe-Co-B-Nb alloys can be increased by enriching Nb and B at the grain boundaries through optimal heat treatment [4]. In addition, it has been reported that partial substitution of Sm sites in Sm(Fe-Co)12 compounds with Zr element increases the saturation magnetization [5]. In this study, in order to further improve the magnetic properties of Sm(Fe0.8Co0.2)12-B thin films, Nb or Zr was deposited as a capping layer on the Sm(Fe-Co)12-B thin films, and then the changes in the crystal structure and magnetic properties by post-annealing at varying annealing temperatures were also investigated in detail.The samples were prepared by using an ultra-high vacuum magnetron sputtering system. First, a V buffer layer with a thickness of 20 nm was deposited on a MgO(100) single crystal substrate at substrate temperature of 400ºC, and then a Sm(Fe-Co)12-B layer of 100 nm was deposited. Subsequently, Nb or Zr cap layer was deposited with varying thicknesses. Then, post annealing was performed at an annealing temperature (Ta) between 400 and 700ºC for 1 h. Finally, a V cover layer of 10 nm was deposited to prevent oxidation. The crystal structure was analyzed by X-ray diffraction (XRD), and the magnetic properties were evaluated by using a superconducting quantum interference device (SQUID) magnetometer.From the XRD patterns, the (002) and (004) peaks of the ThMn12-type crystal structure were clearly observed in Sm(Fe-Co)12-B thin films. The intensity of these peaks was confirmed to decrease when Ta exceeded 500ºC, but was recovered by the deposition of a Nb cap layer. In addition, the peak intensity of the α-Fe phase, which increased at high-temperature annealing, was sufficiently decreased by the deposition of the Nb cap layer. As a result, the coercivity of the film without a cap layer was decreased to 0.09 T at Ta = 600ºC, but the film with the Nb cap layer maintained a high coercivity of approximately 1.0 T even when Ta was increased to 600ºC. Furthermore, in the sample without a cap layer, the easy axis of magnetization varied from perpendicular direction to in-plane direction at Ta = 600ºC, however, by depositing the Nb cap layer, perpendicular magnetic anisotropy was obtained up to Ta = 650ºC. Therefore, it was confirmed that the deposition of a Nb layer as a capping layer improved the thermal stability of Sm(Fe-Co)12-B thin films.

References
[1] Hirayama, Y. K. Takahashi, S. Hirosawa, and K. Hono, Scr. Mater., 138, 62-65 (2017).
[2] Sepehri-Amin, Y. Tamazawa, M. Kambayashi, G. Saito, Y. K. Takahashi, D. Ogawa, T. Ohkubo, S. Hirosawa, M. Doi, T. Shima, and K. Hono: Acta Mater., 194, 337 (2020).
[3] Sepehri-Amin, N. Kulesh, Y. Mori, T. Ohkubo, K. Hono, and T. Shima, Scr. Mater., 242, 115955 (2024).
[4] Kurosawa, M. Matsuura, S. Sakurada, and S. Sugimoto, J. Magn. Magn. Mater., 556, 169414 (2022).
[5] Tozman, Y. K. Takahashi, H. Sepehri-Amin, D. Ogawa, S. Hirosawa, and K. Hono, Acta Mater., 178, 114-121 (2019).

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• Creative Commons BY Attribution 4.0 International
  

キーワード: REPM2025, SmFe12-based compound, ThMn12-type crystal structure, Element addition

会議: REPM2025 (2025-07-27 - 2025-07-31)

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原稿種別: 論文以外のデータ

MDR DOI: https://doi.org/10.48505/nims.5664

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更新時刻: 2025-09-11 16:30:36 +0900

MDRでの公開時刻: 2025-09-11 16:20:10 +0900