MI Column 1

The birth of amorphous alloy wires and
business applications for security sensor tags

In 1972, the American company Allied Chemical Co., Ltd. (later Allied) announced that it had begun providing samples of "Dream Alloy Amorphous" worldwide. The author also obtained a sample as soon as possible; a thin silver-white ribbon (amorphous ribbon) 2 mm wide, 30 microns thick and 15 cm long. First of all, its mechanical properties were prominent. It is a razor blade-like elastic body. The tensile strength is also very high (300 kg/mm2) and is at least as strong as piano wire. This amorphous ribbon was immediately examined by a university in the United States for its electromagnetic properties, and it was announced at the Society of Applied Magnetics that it is a very excellent soft magnetic material because it is not crystalline. It was expected to be applied to magnetic cores of power transformers with low power loss and high-frequency radio electronic circuit components. Since the samples we obtained were so-called iron-based amorphous ribbons and magnetic materials whose magnetic properties were stress-sensitive, we devised various high-sensitivity stress sensor electronic circuits in our laboratories and presented them at an electrical systems academic conference on electrical systems. This dream alloy; amorphous alloy was realized as a FePC foil by the piston-anvil super-quenching method by Prof. Pol. Duez of the California Institute of Technology in the United States over a 10-year period from 1960 to 1970. The lab staff were soon able to create a continuous amorphous ribbon using a single roll method of heat conduction, such as steel or brass, or a twin roll method, which was commercialized by the first company.

In 1976, an Allied Chemical researcher R. C. O'Handley created a wire-like amorphous alloy about 40 centimeters long with a V-shaped narrow groove on the surface of a single roll. He found that domain walls propagated in the wire length direction with this amorphous wire. The propagation properties of this domain wall were analyzed in detail by J. C. L Bishop at Sheffield University in the following year.

In Japan, this Amorphous Alloy Wire attracted attention, and Professor Itsuo Osaka of Osaka University devised a method to create an Amorphous Alloy Wire by placing water inside a spinning roll and rotating it, then injecting the molten alloy into the water with a quartz pipe nozzle. This new method of making amorphous wire (underwater spinning method) was immediately improved to practical levels in Professor Ken Masumoto's Laboratory in Tohoku University in which Unitika Central Laboratory members also participated to make long wires with a uniform circular cross-section, and these were announced together with samples at the RQ8 International Conference in 1981.

In 1982, at the recommendation of Professor Masumoto, the Unitika Central Research Laboratory brought this long amorphous wire (FeSiBC) to the Kyushu Institute of Technology, where the author was working at the time, and it was decided to search for applications. Immediately after investigating its AC magnetization characteristics, we found a peculiar phenomenon not found in conventional magnetic materials. This was a phenomenon in which a domain wall propagates over a long distance in the wire length direction due to a weak magnetic field. We therefore decided to call this the "large Barkhausen effect." It has a high sensitivity about 100 times higher than that of the U.S. Wiegand wire, which had become available even in Japan at that time, and moreover, it had the dreamlike magnetic property of the "Dream Alloy Wire", that which propagates over a few meters uniformly. Immediately afterwards, I received an international call from F. B. Humphrey, a Professor at the University of Boston, USA. . It fit the magnetic material hebhad been searching for. The professor immediately came to Japan and began joint experiments at Kyushu Institute of Technology. Intense experimentation was carried out in a laboratory with no air conditioning and the constant threat of heat stroke. Immediately after the professor’s return to the U.S., an executive group from Sensormatic Ltd, a U.S. security sensor venture, visited Unitika Co., Ltd., negotiated business and signed contracts, and began business with amorphous wire supermarket shoplifting prevention security sensor tags.

The technologies for this security sensor system, including the activation-deactivation concept, are of great interest, but I shall refer to them in a separate article due to paper constraints. The social necessity of this system was very inconceivable in Japanese society at the time of the 1980s, when public ethics were high. The amorphous wire security sensor tag business was not widely known during the years between 1985-2007 due to the confidentiality of technologies, but as an application of magnetic technologies, it was a hidden long-term hit supported by the unique functions of amorphous wires.

Starting in 2014, this amorphous wire (metal fiber) business was moved from Unitika Corporation to Aichi Steel Corporation as reported by the media. After security sensor tags, amorphous wires are mass-produced by two companies of Aichi Steel Corporation and ROHM Corporation, mainly for electronic compass MI elements for smartphones, by changing composition, shape (thinning of wires) and effect (magnetic impedance effect of outer shell).


I. 機械的特性

  • (1) 結晶質合金でないため、機械的に弱い結晶粒界がなく機械的強度が高い。線引き加工されたアモルファスワイヤの引張強度は約400 kg/mm2 あり、ピアノ線より強靭である。
  • (2) また結晶面のすべりがないため塑性変形がなく、強靭弾性体である。弾性率は95%である。微結晶の存在のため5%の塑性率がある。この僅かな塑性率のため線引き加工ができ、多数回のダイヤモンド線引き加工で、as castの 130 μm径から約11μm径まで線引きされる。
  • (3) 結晶化温度は約550℃である。ただし長時間での微結晶化の可能性を考慮し、長時間使用は200℃以下が望ましい。

II. 電気特性

  • (4) 結晶構造がないため荷電子帯が明確でなく電導電子の走行が平滑でないため電気抵抗率が130μΩ-cmと高い。(3%SiFeで約60μΩ-cm、NiFeで45μΩ-cm)このため、電子回路部品として小型化できる。

III. 磁気特性

  • (5) 結晶磁気異方性がないため、磁化回転が容易である。磁化特性は基本的に磁歪エネルギーで決定される。Coリッチの零磁歪アモルファスワイヤ(CoFeSiB)は微弱な磁界で磁化されるので、高感度磁気センサの磁心に使用される。
  • (6) アモルファスワイヤの超急冷凝固過程の残留応力の分布により、磁区構造は、ワイヤ長さ方向(軸方向)に磁気異方性をもつ内心部と、ワイヤ円周方向に磁気異方性をもつ外殻部に大別される。(ハンフリー磁区モデル)

IV. 化学特性

  • (7) アモルファスワイヤは結晶粒界がないので、基本的には耐食性が高い。Co系アモルファスワイヤはステンレス鋼より耐食性が高い。MI素子は耐食処理が不要であり、より小型のセンサ素子である。一方、原因は不明であるが、鉄系アモルファスワイヤは錆び易い。このため耐食性が必要な場合は、Crの添加が必要である。