2004/12/30

Exchange-String Magnets

Bất cứ ai làm về từ đều nghe đến nam châm lò xo-trao đổi (Exchange-Spring magnets-ESM). Đối với người mới bắt đầu nghiên cứu thì có thể muốn hiểu nó là gì, đối với một số người khác thì cần những tài liệu tham khảo, như tôi hồi ở VN muốn tìm mấy cái tài liệu gốc được trích dẫn trong bất cứ bài nào về ESM mà không có, sau đây tôi tóm tắt về ESM và gửi ba bài báo được trích dẫn nhiều nhất (most cited) để những ai làm về ESM có thể có những tài liệu cần thiết nhất.

Vật liệu từ dạng khối có thể tạm phân chia thành hai loại, vật liệu từ cứng như SmCo, NdFeB (nam châm đất hiếm),… và vật liệu từ mềm như FeCo, FeNi,… Đặc điểm chung của chúng là, vật liệu từ cứng có lực kháng từ (Hc) cao nhưng từ độ bão hòa nhỏ (Ms) và ngược lại, vật liệu từ mềm có lực kháng từ nhỏ và từ độ bão hòa lớn. Một vật liệu từ cứng tốt phải thỏa mãn cả hai thông số Hc và Ms đều cao và do đó có thể đưa đến tích năng lượng cực đại BHmax là lớn nhất. Nếu ta chỉ thuần túy trộn hai loại bột tạo thành từ vật liệu từ cứng và từ mềm thì trên đường cong từ trễ sẽ có dạng nhảy bậc và do đó tích năng lượng cực đại sẽ thấp, đó là điều ta không mong muốn. Chính vì thế mà nam châm đàn hồi-trao đổi ra đời.

ESM được tạo thành từ hai vật liệu, một loại là vật liệu từ cứng và vật liệu từ mềm nhưng các vật liệu đó lại ở các dạng hạt nhỏ có kích thước nanomet (10-9 m). Do kích thước nhỏ như vậy nên giữa các hạt có tồn tại một tương tác gọi là tương tác trao đổi (exchange coupling - EC). Nhờ có tương tác này mà các hạt từ mềm không bị xoay một cách dễ dàng trong trường ngoài mà bị liên kết với các hạt từ cứng. Chính nhờ đó mà nam châm tạo thành từ hai vật liệu trên có được những ưu điểm của vật liệu từ cứng và từ mềm. Ngoài khía cạnh từ tính, các khía cạnh khác của ESC cũng nổi trội hơn, như độ ổn định trong môi trường có ô xy cũng tốt hơn, và đặc biệt là chúng có giá thành rẻ hơn nam châm đất hiếm.

Về thực nghiệm, ESM đầu tiên được một nhóm tác giả ở Hà Lan công bố vào năm 1988, họ tạo ESM bằng phương pháp nguội nhanh (melt-spun) một hợp kim gồm Nd, Fe, B và bằng phương pháp xử lí nhiệt, họ tạo được vật liệu tạo thành từ các hạt nano từ cứng NdFeB và từ mềm Fe3B [1].

Về mặt lí thuyết, có hai bài được trích dẫn nhiều nhất, bài đầu tiên [2] giải thích cơ chế của EC và ước tính độ lớn cực đại của các hạt nano từ. Các tác giả cho thấy độ lớn của các hạt từ mềm cỡ khoảng độ dày vách đô men là tốt nhất. Bài thứ hai [3] ước tính giá trị của tích năng lượng cực đại của ESM có thể đạt đến 1 MJ/m3 (tức là khoảng 120 MGOe), tuy vậy giá trị này cho đến nay, người ta vẫn chưa đạt được bằng thực nghiệm.

[1] R. Coehoorn, D. B. de Mooij, J. P. W. B. Duchateau, and K. H. J. Buschow, Novel Permanent Magnetic Materials Made by Rapid Quenching, J. de Phys., vol. 49, pp. C8 669, 1988.
[2] E. F. Kneller and R. Hawig, The exchange-spring magnet: a new material principle for permanent magnets, IEEE Trans. Magn., vol. 27, pp. 3588, 1991.
[3] R. Skomski and J. M. D. Coey, Giant energy product in nanostructured two-phase magnets, Phys. Rev. B, vol. 48, pp. 15812, 1993.

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