# Microstructure, Magnetic Properties, and Application of FINEMET-Type Alloys with Co Addition
## 3 Results and Discussion  
The investigated Fe\({}_{83-x}\)Co\({}_{x}\)Nb\({}_{3}\)B\({}_{13}\)Cu\({}_{1}\) (x = 6, 25, 58) alloys in the as-quenched state were fully amorphous, which was confirmed by X-ray diffraction and Mossbauer spectroscopy measurements. X-ray diffraction patterns of the as-cast alloys show only broad bumps [25]. For nanocrystalline alloys obtained by annealing at 750 K for 10 min, sharp peaks corresponding to the crystalline phase are detected in X-ray images (Figure 1). Amorphicity of investigated materials in the as-quenched state was also tested by Mossbauer spectroscopy. Mossbauer spectra for these samples consist of sextets with very broad and overlapped lines typical of an amorphous state [26]. Transmission Mossbauer spectra obtained at room temperature for nanocrystalline alloys are presented in Figure 2. The sextets with sharp lines related to \(\alpha\)-FeCo crystalline grains superimposed on the broad subspectra corresponding to the remaining intergranular amorphous phase are visible. In the hyperfine magnetic field distributions P(B) of these samples, the appearance of additional high field components was assigned to this crystalline phase. The crystalline \(\alpha\)-FeCo phase in nanocrystalline HITPERM-type alloys obtained by the conventional annealing of amorphous ribbons exhibits a long-range order of atoms [14]. However, its composition depends on the Co concentration in amorphous precursors and annealing conditions. The presence of the crystalline phase in alloys annealed at 750 K for 10 min was also confirmed by TEM images. For example, in Figure 3, micrographs and the corresponding electron diffraction pattern (an insert) of Fe\({}_{58}\)Co\({}_{25}\)Nb\({}_{3}\)B\({}_{13}\)Cu\({}_{1}\) alloy are shown. The presence of crystalline grains of about 10-30 nm in diameter embedded in the amorphous matrix can be observed. These grains are distributed irregularly, and in some regions, conglomerates of grains separated by the amorphous matrix are observed. The electron diffraction pattern, in addition to the broad rings that come from the amorphous phase, contains very bright reflexes, with the distances from the center of the pattern being related to the reciprocal of the interplane distances in \(\alpha\)-FeCo phase. Figure 1: XRD patterns for Fe\({}_{83-x}\)Co\({}_{x}\)Nb\({}_{3}\)B\({}_{13}\)Cu\({}_{1}\) (x = 6 (a), 25 (b), and 58 (c)) alloys annealed at 750 K for 10 min. presented. The shape of these loops is typical for soft ferromagnetic materials and indicates a low value of the coercive field of less than 125 A/m. The characteristic parameters of the tested materials determined from B-H loops (Figure 4) are shown in Table 1. It can be seen that H\({}_{c}\), as well as B\({}_{s}\), varies with the change in Co content. However, in the studied materials annealed at 750 K for 10 min, no relationship between the saturation magnetic flux density B\({}_{s}\) and the coercive field was observed, which presents a general trade-off in FINEMET soft magnetic materials [27]. Figure 5 shows the dependence of core losses on maximum induction. For comparison, the P(B\({}_{\rm max}\)) curve (a) for the Fe\({}_{83}\)Nb\({}_{3}\)B\({}_{13}\)Cu\({}_{1}\) alloy annealed under the same heat treatment conditions was added. The studied alloys do not show large differences in the magnitude of losses, especially in the induction range up to 0.5 T. Above this induction value, the smallest losses were obtained for Fe\({}_{58}\)Co\({}_{25}\)Nb\({}_{3}\)B\({}_{13}\)Cu\({}_{1}\) alloy. It is worth noting that nanocrystalline Fe-Co-Nb-B-Cu alloys show good thermal stability of the magnetic properties. As can be seen in Figure 6, the core losses do not change for all of the investigated alloys at temperatures up to 500 K. Above this temperature, the losses increase sharply for the Fe\({}_{83}\)Nb\({}_{3}\)B\({}_{13}\)Cu\({}_{1}\) alloy and slightly for the Fe\({}_{77}\)Co\({}_{6}\)Nb\({}_{3}\)B\({}_{13}\)Cu\({}_{1}\) and Fe\({}_{25}\)Co\({}_{58}\)Nb\({}_{3}\)B\({}_{13}\)Cu\({}_{1}\) alloys.