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.

Unraveling the magnetic softness in Fe–Ni–B-based nanocrystalline material by magnetic small-angle neutron scattering

“…This phenomenon has been successfully modeled within the framework of the random anisotropy model (RAM) (Herzer, 1989(Herzer, , 1990(Herzer, , 2007Suzuki et al, 1998), and becomes effective when the average grain size D is smaller than the ferromagnetic exchange length L 0 ¼ ' 0 ðA ex =K 1 Þ 1=2 , where A ex is the exchange-stiffness constant and ' 0 is a proportionality factor of the order of unity which reflects the symmetry of K 1 . In this regime, the RAM predicts that the coercivity H C scales as H C / ðD=L 0 Þ n , where n = 3 or n = 6 depending on the nature of the magnetic anisotropy [see, for example, the work by Suzuki et al (1998Suzuki et al ( , 2019 for details]. Therefore, an improvement of the magnetic softness comes about by either reducing D and/or increasing L 0 .…”

Nanocrystalline Soft Magnetic Iron-Based Materials from Liquid State to Ready Product

“…The Fe 87 B 13 alloy was obtained at a high heating rate of 3 K•s −1 [123]. It follows from the Figure that the saturation magnetization of the nanocrystalline alloy decreases in proportion to the mass fraction of non-magnetic elements [122].…”

“…In nanocrystalline alloys with high saturation magnetization, Si is not used or is used as a small additive. To inhibit grain growth, Zr and B [118], P and B [119,120] or B alone [121,122] are used. To obtain finer grain an increase of heating rate is used during annealing [89,122,123].…”

“…To inhibit grain growth, Zr and B [118], P and B [119,120] or B alone [121,122] are used. To obtain finer grain an increase of heating rate is used during annealing [89,122,123]. Rapid heating at elevated temperatures allows the number of Cu-clusters to almost double [124], and a grain is formed whose size is about 30% smaller than with slow heating.…”

Structural, Thermal and Magnetic Analysis of Fe75Co10Nb6B9 and Fe65Co20Nb6B9 Nanostructured Alloys

“…The addition of Co serves two purposes. On the one hand, it helps to maximize the magnetic moment (increasing the saturation magnetization) [ 8 ] because Fe-Co alloys are at the top of the Slater–Pauling curve. On the other hand, Co increases the Curie temperature (>1000 K) of both the residual disordered grain boundaries and the nanocrystals [ 9 ].…”