Nanocrystalline soft magnetic materials from binary alloy precursors with high saturation magnetization

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.