Abstract
A brief survey of the recent advances in Fe-based nanocrystalline soft magnetic alloys has shown that the saturation magnetization (Js) of these alloys is governed by the mass fraction, rather than the atomic fraction, of the nonmagnetic additives. Thus, the ultimate limit of Js in the alloys prepared by nano-crystallization of amorphous precursors is expected in an Fe-B binary system where amorphization by rapid quenching takes place with the lowest mass fraction of glass forming elements in Fe-based systems. We will demonstrate that nano-crystallization is possible in this binary system when the precursor amorphous phase is annealed ultra-rapidly. While the grain size after conventional annealing for amorphous Fe-B alloys is too large for the exchange softening effect, a small grain size well below the exchange length is obtained after annealing with a heating rate of 103 – 104 K/s. This results in magnetically soft nanostructures with Fe content up to 97.2 wt. %, leading to a high Js ≥ 1.9 T with a small coercivity (Hc) between 3.8 and 6.4 A/m. An addition of Co to nc-Fe87B13 results in a higher Js of 2.0 T with a slight increase of Hc to 9.3 A/m. The soft magnetic properties of these ultra-rapidly annealed alloys (named HiB-Nanoperm) is well understood by the random anisotropy model. The formation of nano-meter scale microstructures in a simple binary system unlocks previously unavailable alloy design strategies in nanostructured systems which is not only relevant to magnetic materials but, also to structural materials.
Highlights
It has been three decades since Finemet, the first example of nanocrystalline soft magnetic alloys, was reported by Yoshizawa et al.1 at the MMM/Intermag meeting in Vancouver in 1988
We have demonstrated that simple (Fe,Ni)-B4 and Fe-B5 binary amorphous precursors lead to a magnetically soft nanostructure after ultra-rapid annealing
The removal of EM additives is effective in enhancing the saturation magnetization and high Js well above those of Finemet and Nanoperm has been reported for the EM-free alloys
Summary
It has been three decades since Finemet, the first example of nanocrystalline soft magnetic alloys, was reported by Yoshizawa et al. at the MMM/Intermag meeting in Vancouver in 1988. The magnetic softness in nanocrystalline alloys is due to the exchange-averaging effect of the local anisotropy (K1). Nanocrystalline soft magnetic alloys stand out with a great potential of realizing both high saturation magnetization (Js) and excellent soft magnetic properties In spite of this potential, Js in nanocrystalline alloys remains lower than that of Si steels due to the use of nonmagnetic additives such as Cu and other early transition metals (e.g. Zr and Nb) for the formation of nanostructures from amorphous precursors. We will discuss the effects of additives on the enhancement of Js and reduction of Hc in the Fe-B alloys along with the grain size dependence of the coercivity in the Fe-B based nanocrystalline alloys
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