Research－Teruo BITOH

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1. Fe-based nanocrystalline soft magnetic materials

Nanocrystalline soft magnetic materials consist of ultra-fine ferromagnetic crystallites with size of approximately 10 nm. Owing to strong ferromagnetic exchange coupling between the nanocrystallites, their magnetocrystalline anisotropy is averaged out and the materials exhibit good soft magnetic properties (Fig. 1).

Fig. 1 Relationship between crystalline size (D) and coercivity (H_c)

We are investing the microstructure and the magnetic properties of the nanocrystalline Fe-M-B (M = Zr, Hf, Nb) soft magnetic alloys. The nanocrystalline alloys are produced by the crystallizing the melt-spun (Fig. 2) amorphous phase (Fig. 3). The resultant alloys have a mixed structure consist of nanoscale α-Fe phase (crystalline size is approximately 10 nm) embedded in a ferromagnetic amorphous matrix (Fig. 4). The nanocrystalline Fe-M-B (M = Zr, Hf, Nb) soft magnetic alloys exhibit the high permeability as well as the high saturation magnetic flux density (Fig. 5).

Fig. 2 Melt-spinning apparatus
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Fig. 3 Schematic image of nanocrystallization

Fig. 4 TEM image of nanocrystalline Fe-Nb-B alloy

Fig. 5 Relationship between saturation magnetic flux density (B_s) and permeability (μ)

Related Publications

T. Bitoh, A. Makino and A. Inoue, “The Effect of Grain-Size Distribution on Coercivity in Nanocrystalline Soft Magnetic Alloys,” Journal of Magnetism and Magnetic Materials, vols. 272–276, pp. 1445–1446 (2004).
T. Bitoh, A. Makino, A. Inoue and T. Masumoto, “Random Anisotropy Model for Nanocrystalline Soft Magnetic Alloys with Grain-Size Distribution,” Materials Transactions, vol. 44, no. 10, pp. 2011–2019 (2003).

2. Fe-(Pt, Pd)-based nanocrystalline hard magnetic materials

The ordered L1₀ alloys (Fig. 6) such as Fe-Pt, Co-Pt, Fe-Pd have long been anticipated as materials for permanent magnets because of their large magnetocrystalline anisotropy and good corrosion resistance. However, near equiatomic FePt cast alloys do not exhibit high magnetic hardness because of their coarsening structure.

We are investing the microstructure and the magnetic properties of the nanocrystalline (Fe, Pt)-M-B (M = Zr, Hf, Ti) hard magnetic alloys. The simultaneous addition of transition metals and B to the Fe-Pt alloy remarkably decreases its melting temperature. The L1₀-FePt phase with a size of approximately 100 nm can be directly synthesized (Fig. 7) by rapidly quenching of the melt (Fig. 2) without allowing the formation of the disordered fcc phase, and the high coercivity of 688 kA/m, which is much higher than that of the annealed bulk Fe-Pt, is obtained for the melt-spun (Fe_0.50Pt_0.50)₇₈Zr₄B₁₈ alloy in an as-quenched state (Fig. 8).

Fig. 6 Unit cell of L1₀ ordered alloys

Fig. 7 TEM image of nanocrystalline Fe-Pt-Zr-B alloy

Fig. 8 Magnetization curve of melt-spun Fe-Pt-Zr-B alloy in an as-quenched state

Related Publications

T. Bitoh, M. Nakagawa and A. Makino, “Melting Temperature and Order-Disorder Transformation of Melt-Spun (Fe, Pt)-Zr-B Nanocrystalline Alloys,” Scripta Materialia, vol. 53, no. 4, pp. 429–434 (2005).
T. Bitoh, A. Makino and M. Nakagawa, “Microstructure and Hard Magnetic Properties of Directly Synthesized L1₀ (Fe_1–xPt_x)₇₈Zr₄B₁₈ Nanocrystalline Alloys by Melt-Spinning,” Journal of Applied Physics, vol. 97, no. 10, 10H307 (2005) (3 pages).

3. Fe-based bulk metallic glasses

Recently, a number of amorphous alloys exhibit a wide supercooled liquid region (ΔT_x) before crystallization. The appearance of the wide supercooled liquid region implies that the alloys have high resistance against crystallization. Consequently, these alloys with large ΔT_x values have been confirmed to have an extremely large glass-forming ability, which enables the production of bulk metallic glasses (Fig. 9).

Fig. 9 Fe-based bulk metallic glasses produced by Cu-mold casting

It is well know that Fe- and Co-based ordinary amorphous alloys exhibit good soft magnetic properties. The origin of the good soft magnetic properties is lack of long-range atomic order (the left figure in Fig. 3). This feature is also common in the bulk metallic glasses. It seems that therefore, the soft magnetic properties of the bulk metallic glasses are same as those of the ordinary amorphous alloys. Is this assumption correct?

We are investing the magnetic properties of the Fe-based bulk metallic glasses. The bulk metallic glasses exhibit lower coercivity (H_c) than that of the ordinary amorphous alloys (Fig. 10). The low H_c originates from the much higher packing density of the bulk metallic glasses than that of the ordinary amorphous alloys, which realizes the low density of the pinning centers for magnetic domain-walls. The good combination of high glass-forming ability and good soft magnetic properties indicates the possibility of future development as a new bulk glassy soft magnetic material.

Fig. 10 Relationship between density (ρ) and coercivity (H_c) of Fe-based bulk metallic glasses and ordinary amorphous alloys

Related Publications

T. Bitoh, A. Makino and A. Inoue, “Origin of Low Coercivity of (Fe_0.75B_0.15Si_0.10)_100–xNb_x (x = 1–4) Glassy Alloys,” Journal of Applied Physics, vol. 99, no. 8, 08F102 (2006) (3 pages).
T. Bitoh, A. Makino and A. Inoue, “Magnetization Process and Coercivity of Fe-(Al, Ga)-(P, C, B, Si) Soft Magnetic Glassy Alloys,” Materials Transactions, vol. 45, no. 4, pp. 1219–1227 (2004).
T. Bitoh, A. Makino and A. Inoue, “Origin of Low Coercivity of Fe-(Al, Ga)-(P, C, B, Si, Ge) Bulk Glassy Alloys,” Materials Transactions, vol. 44, no. 10, pp. 2020–2024 (2003).

The Fe-(Co, Ni) based bulk metallic glasses exhibit good soft magnetic properties, described above. However, the glass-forming ability of the soft magnetic bulk metallic glasses is inferior to that of the nonferrous alloys such as Pd-, Zr-, lanthanide-, and Mg-based alloys. It is well known that the main competition to prepare bulk metallic glasses is attributed to oxides and other inclusion in the molten metal which act as heterogeneous nucleation sites for crystallization. An approach to eliminate the inclusions is to heat and cool the molten metal while it is immersed in molten oxide flux. Recently, we have succeeded in the synthesis of the large soft magnetic [(Fe_0.5Co_0.5)_0.75B_0.20Si_0.05]₉₆Nb₄ bluk metallic glass specimens with the diameters up to 7.7 mm by water quenching the melt immersed in the molten flux of B₂O₃ (Fig. 11). The maximum diameter of the obtained specimens is approximately 1.5 times as large as the previous result for copper mold casting. This bulk specimen is the thickest of any soft magnetic bulk metallic glasses formed until now.

Fig. 11 Soft magnetic bulk metallic glasses prepared by B₂O₃ flux melting and water quenching technique

Related Publications

T. Bitoh, A. Makino, A. Inoue and A. L. Greer, “Large Bulk Soft Magnetic [(Fe_0.5Co_0.5)_0.75B_0.20Si_0.05]₉₆Nb₄ Glassy Alloy Prepared by B₂O₃ Flux Melting and Water Quenching,” Applied Physics Letters, vol. 88, no. 18, 182510 (2006) (3 pages).
T. Bitoh, A. Makino, A. Inoue and A. L. Greer, “Formation of Large Bulk [(Fe_0.5Co_0.5)_0.75B_0.20Si_0.05]₉₆Nb₄ Glassy Alloy by Flux Melting and Water Quenching,” Materials Research Society Symposium Proceedings, eds. E. Ma, S. Schuh, Y. Li and M. K. Miller (Boston, Materials Research Society, 2006) vol. 903E, Z05–18 (6 pages).

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Last modified on March 28th, 2019