1. Introduction

The Sm₅Fe₁₇ phase is a ferromagnetic phase in the binary Sm-Fe system [1,2]. It is a metastable phase and its formation has been reported only in sputtered films [3,4]. Through the intensive studies to produce the Sm₅Fe₁₇ phase as bulk materials, it was found that the Sm₅Fe₁₇ phase can be obtained by annealing of amorphous melt-spun ribbons [5]. Sm₅Fe₁₇ melt-spun ribbon shows a large coercivity exceeding 25 kOe and a remanence of 40 emu/g, which is significantly lower than the remanence of Nd-Fe-B magnets. For the application in the motors of electric vehicles and wind-turbine generators, the temperature of the magnets rises to nearly 473 K due to the evolution of eddy currents. Nd-Fe-B magnets cannot be used for the motors and instead Nd-Dy-Fe-B magnets, which the Dy was added to increase the coercivity, are applied. At such a high temperature, the magnetic properties of the Sm₅Fe₁₇ melt-spun ribbon were almost comparable to those of the Nd-Fe-B melt-spun ribbon (The results are discussed later in the results and discussion section). Thus, the high coercivity magnets are required for the high temperature applications. Since the Sm₅Fe₁₇ melt-spun ribbon possesses the high coercivity, it can be suitable for a hard magnet component in the nanocomposite magnets, which is the most promising candidate for the new types of the permanent magnets. In any case, the increase of the remanence of the Sm₅Fe₁₇ melt-spun ribbon is almost always beneficial. There have been several efforts to increase the remanence of Sm₅Fe₁₇-type melt-spun ribbons, but the reported values are not yet satisfactory [6,7,8]. It is known that the magnetic properties of Sm-Co-based magnets can be improved by the addition of B to the Sm-Co-based alloy [9,10,11]. In this study, small amounts of B were added to Sm₅Fe₁₇ melt-spun ribbon in order to increase the remanence. The structures and magnetic properties of the Sm-Fe and Sm-Fe-B melt-spun ribbons were then examined.

2. Materials and Method

Sm₅Fe₁₇ and Sm₅Fe₁₇B_x (x = 0.5-1.5) alloy ingots were induction melted in an argon atmosphere in a quartz crucible having an orifice of 0.6 mm in diameter at the bottom. The molten metal was ejected through the orifice with argon onto a chromium-plated copper wheel rotating at a surface velocity of 40 ms⁻¹. The resultant melt-spun ribbons were annealed under an argon atmosphere at temperatures between 773 K and 1073 K for 1 h. The phases in the specimens were examined by X-ray diffraction (XRD) using Cu Kα radiation. The microstructures of the specimens were examined using a transmission electron microscope (TEM) after ion beam thinning. The thermomagnetic properties of the specimens were examined in a vacuum using a vibrating sample magnetometer (VSM) with an applied field of 500 Oe. The magnetic properties of the specimens were measured by the VSM with a maximum applied field of 25 kOe after premagnetization in a pulsed field of 70kOe. Some of the specimens were further examined by VSM using a superconducting magnet with a maximum applied field of 100 kOe.

3. Results and Discussion

3.1. Sm-Fe melt-spun ribbon

The Sm₅Fe₁₇ melt-spun ribbons were amorphous and exhibited a low coercivity value, less than 1 kOe. It is known that crystalline phases can be produced by rapid solidification processing and subsequent heat treatment [6]. In the previous studies [7,8], the annealed samples consisted of the Sm₅Fe₁₇ phase but also contained some other phase such as the SmFe₁₂ phase and the SmFe₃ phase. As the result of the extensive study of the annealing conditions, the optimal annealing condition to obtain the Sm₅Fe₁₇ phase was established in this paper. Figure 1 shows the XRD patterns of the annealed Sm-Fe melt-spun ribbons. The amorphous melt-spun ribbon should be annealed at relatively high heating rates of 0.5-1.0 K/s to 873-973 K and then kept for 1 h. As shown in Figure 1,the optimal annealed specimens consisted of the Sm₅Fe₁₇ (hexagonal) phase. However, the Sm-Fe ribbon annealed at 1073 K consisted of the SmFe₃ (rhombohedral) phase together with the Sm₅Fe₁₇ phase.

Figure 1. XRD patterns of (a) the Sm-Fe melt-spun ribbon and the specimens annealed at (b) 773 K, (c) 873 K, (d) 973 K, and (e) 1073 K.

DownLoad: Full-Size ImgPowerPoint

Figure 2 shows the dependence of the coercivity of the Sm₅Fe₁₇ melt-spun ribbons on the annealing temperature. The specimens annealed at 873 K and 973 K showed high coercivity values. According to the results of the XRD studies, thoseannealed melt-spun ribbon consists of the Sm₅Fe₁₇ phase. This indicates that the observed high coercivity in the annealed melt-spun ribbon is due to the existence of the Sm₅Fe₁₇ phase.

Figure 2. Dependence of the coercivity of the Sm-Fe melt-spun ribbons on the annealing temperature.

DownLoad: Full-Size ImgPowerPoint

The hysteresis loops of the Sm₅Fe₁₇ melt-spun ribbon and the specimen annealed at 973 K are shown in Figure 3. Since the applied field of 25 kOe is far lower than the field required to fully saturate the Sm₅Fe₁₇ melt-spun ribbon, the hysteresis curve is not closed (i.e., it is a minor loop), and it is not symmetrical with respect to either coordinate. The actual coercivity of the annealed specimen was measured as 40 kOe using a superconducting magnet with a maximum applied field of 100 kOe.

Figure 3. Hysteresis loops of (a) the Sm-Fe melt-spun ribbon and (b) the specimen annealed at 973 K. The demagnetization curve of the annealed specimen measured using a superconducting magnet with a maximum applied field of 100 kOe is also shown in the insert.

DownLoad: Full-Size ImgPowerPoint

Figure 4 shows the hysteresis loops of the Sm₅Fe₁₇ melt-spun ribbon and the Nd₁₅Fe₇₇B₈ melt-spun ribbon measured at 473 K under the applied magnetic field of 20 kOe. Although the Sm₅Fe₁₇ melt-spun ribbon ribbons showed a much smaller remanence than the Nd₁₅Fe₇₇B₈ melt-spun ribbon at room temperature, it exhibits a much smaller difference in remanence at 473 K. Therefore, the magnetic properties of the Sm₅Fe₁₇ melt-spun ribbon are comparable to those of Nd₁₅Fe₇₇B₈ melt-spun ribbon at 473 K.

Figure 4. Hysteresis loops of (a) the Nd₁₅Fe₇₇B₈ melt-spun ribbon and (b) the Sm₅Fe₁₇ melt-spun ribbon measured at 473 K. The high-temperature measurements were made by VSM under a maximum applied magnetic field of 20 kOe.

DownLoad: Full-Size ImgPowerPoint

3.2. Sm-Fe borides

The Sm₅Fe₁₇B_x (x = 0.5-1.5) melt-spun ribbon showed a low coercivity, as was the case for the Sm₅Fe₁₇ melt-spun ribbon. Figure 5 shows the dependence of the coercivity of the Sm₅Fe₁₇B_x (x = 0.5-1.5) melt-spun ribbons on the annealing temperature. The coercivity of the Sm₅Fe₁₇B_x (x = 0.5-1.5) alloys shows a similar temperature dependence, but their maximum coercivity, achieved by annealing at 973 K, decreases with increasing B content of the alloy. This indicates that the addition of B to the Sm₅Fe₁₇ alloy results in a decrease in coercivity.

Figure 5. Dependence of the coercivity of the Sm₅Fe₁₇B_x (x = 0.5-1.5) melt-spun ribbons on the annealing temperature.

DownLoad: Full-Size ImgPowerPoint

Figure 6 shows the hysteresis loops of the Sm₅Fe₁₇B_x (x = 0.5-1.5) melt-spun ribbons annealed at 973 K. For comparison, the hysteresis loop of the Sm₅Fe₁₇ melt-spun ribbon annealed at 973 K is shown in the each figure. It is found that the remanence of the Sm₅Fe₁₇B_x (x = 0.5-1.5) alloys is higher than that of the Sm₅Fe₁₇ alloy even though the coercivity of the Sm₅Fe₁₇B_x (x = 0.5-1.5) alloys is much smaller than that of the Sm₅Fe₁₇ alloy. The remanence increases with increasing B content from 40 emu/g for the Sm₅Fe₁₇ alloy to 65 emu/g for the Sm₅Fe₁₇B_1.0 alloy and then decreases to 60 emu/g for the Sm₅Fe₁₇B_1.5 alloy. The remanence of 65 emu/g exhibited by the annealed Sm₅Fe₁₇B_1.0 alloy is much higher than that of the annealed Sm₅Fe₁₇ melt-spun ribbon.

Figure 6. Hysteresis loops of the Sm₅Fe₁₇B_x (x = 0.5-1.5) melt-spun ribbons annealed at 973 K. For comparison, the hysteresis loop of the Sm₅Fe₁₇ melt-spun ribbon annealed at 973 K is also shown in the each figure.

DownLoad: Full-Size ImgPowerPoint

The Sm₅Fe₁₇ and Sm₅Fe₁₇B_x (x = 0.5-1.5) melt-spun ribbons annealed at 973 K were examined by XRD and thermomagnetic studies to evaluate the differences in the coercivity value. Figure 7 shows the XRD patterns of the annealed Sm-Fe-B melt-spun ribbons. The diffraction peaks of the Sm₅Fe₁₇ alloy were determined to be the Sm₅Fe₁₇ phase. Virtually the same XRD patterns were obtained from the Sm₅Fe₁₇B_0.5 alloy. This indicates that the small addition of B to the Sm₅Fe₁₇ alloy did not alter the XRD pattern. On the other hand, the XRD pattern of the Sm₅Fe₁₇B_1.0 alloy is quite different from that of the Sm₅Fe₁₇ alloy. The diffraction peaks of the Sm₅Fe₁₇ alloy were determined to be the Sm₂Fe₁₄B and SmFe₂ phases. The XRD pattern of the Sm₅Fe₁₇B_1.5 alloy is similar to that of the Sm₅Fe₁₇B_1.0 alloy, indicating that the Sm₅Fe₁₇B_1.5 alloy also consisted of the Sm₂Fe₁₄B (tetragonal) and SmFe₂ (cubic) phases. These results reveal that the large addition of B to the Sm₅Fe₁₇ alloy resulted in the formation of the Sm₂Fe₁₄B and SmFe₂ phases instead of the Sm₅Fe₁₇ phase, and hence the decrease in coercivity.

Figure 7. XRD patterns of the annealed melt-spun ribbons: (a) Sm₅Fe₁₇, (b) Sm₅Fe₁₇B_0.5, (c) Sm₅Fe₁₇B_1.0, and (d) Sm₅Fe₁₇B_1.5 alloys.

DownLoad: Full-Size ImgPowerPoint

Figure 8 shows the thermomagnetic curves of the annealed Sm-Fe-B melt-spun ribbons. The thermomagnetic curve of the Sm₅Fe₁₇ alloy exhibits one magnetic transition at around 550 K, which corresponds to the Curie temperature of the Sm₅Fe₁₇ phase. Unlike in the case of the XRD studies, the thermomagnetic curve of the Sm₅Fe₁₇B_0.5 alloy is quite different from that of the Sm₅Fe₁₇ alloy. The thermomagnetic curve exhibits three magnetic transitions at around 550 K, 620 K, and 680 K. It has been reported that the Curie temperatures of the Sm₂Fe₁₄B phase and SmFe₂ phase are 616 K and 675 K, respectively [12,13,14]. Therefore, the Sm₅Fe₁₇B_0.5 alloy contains the Sm₂Fe₁₄B and SmFe₂ phases together with the Sm₅Fe₁₇ phase. The thermomagnetic curve of the Sm₅Fe₁₇B_1.0 alloy exhibits two magnetic transitions at around 620 K and 680 K, indicating that the Sm₅Fe₁₇B_1.0 alloy has no Sm₅Fe₁₇ phase. The Sm₅Fe₁₇B_1.5 alloy also exhibits two magnetic transitions at around 620 K and 680 K, which correspond to the Curie temperatures of the Sm₂Fe₁₄B and SmFe₂ phases. This is consistent with the results of the XRD studies.

Figure 8. Thermomagnetic curves of the annealed Sm-Fe-B melt-spun ribbons.

DownLoad: Full-Size ImgPowerPoint

Figure 9 shows TEM micrographs of the annealed Sm₅Fe₁₇B_0.5 and Sm₅Fe₁₇B_1.5 melt-spun ribbons. The grain size of the annealed Sm₅Fe₁₇B_0.5 alloy was around 50 nm in diameter, which is almost comparable to the reported grain size of annealed Sm₅Fe₁₇ alloy [8]. Samarium and iron were detected in most of the grains of the annealed Sm₅Fe₁₇B_0.5 alloy, except for the centrally located grain that were rich in samarium but poor in iron. According to the results of the thermomagnetic studies, the specimenscontained some of the Sm₂Fe₁₄B (Sm11.7 at%) and SmFe₂ (Sm33.3 at%) phases together with the Sm₅Fe₁₇ (Sm22.7 at%) phase. Thus, the samarium-rich centrally located grain consisted of the SmFe₂ phase and the surrounding grains were either the Sm₂Fe₁₄B or Sm₅Fe₁₇ phase. The upper-left grain region, which was rich in iron but slightly poor in samarium, may be the Sm₂Fe₁₄B phase. In the TEM micrograph of the annealed Sm₅Fe₁₇B_1.5 alloy, an increase was seen in the amount of the SmFe₂ phase, where is poor in iron but rich in samarium. The grain size of the SmFe₂ phase was found to be larger than that in the annealed Sm₅Fe₁₇B_0.5 alloy. Since the Sm₅Fe₁₇B_1.5 alloy consisted of the Sm₂Fe₁₄B and SmFe₂ phases, the surrounding grains are considered to be the Sm₂Fe₁₄B phase. This confirms that the addition of B to Sm₅Fe₁₇ alloy results in the formation of the SmFe₂ and Sm₂Fe₁₄B phases, instead of the formation of a Sm₅Fe₁₇B_x phase. Since the Sm₂Fe₁₄B phase does not possess uniaxial anisotropy [12], the observed coercivity of the annealed Sm-Fe-B melt-spun ribbons is considered to be the fine SmFe₂ phase.

Figure 9. TEM micrographs of the (a) Sm₅Fe₁₇B_0.5 and (b) Sm₅Fe₁₇B_1.5 annealed melt-spun ribbons and corresponding X-ray mappings for iron and samarium.

DownLoad: Full-Size ImgPowerPoint

4. Conclusion

The structures and magnetic properties of Sm₅Fe₁₇B_x (x = 0-1.5) melt-spun ribbons annealed at 973 K were examined. The annealed Sm₅Fe₁₇ melt-spun ribbon consisted of the Sm₅Fe₁₇ phase and exhibited a high coercivity. The annealed Sm₅Fe₁₇B_0.5 melt-spun ribbon consisted of the Sm₅Fe₁₇ phase together with the Sm₂Fe₁₄B and SmFe₂ phases. In contrast, the annealed Sm₅Fe₁₇B_x (x = 1.0-1.5) melt-spun ribbons consisted of the Sm₂Fe₁₄B and SmFe₂ phases. The coercivity of the annealed Sm₅Fe₁₇B_x (x = 0-1.5) melt-spun ribbons decreased as the B content increased. On the other hand, the annealed Sm₅Fe₁₇B_x (x = 0.5-1.5) melt-spun ribbons exhibited a higher remanence than the Sm₅Fe₁₇ melt-spun ribbon.

Acknowledgments

The use of the facilities of the Materials Design and Characterization Laboratory at the Institute for Solid State Physics, the University of Tokyo, is gratefully acknowledged. This work was performed at High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University.

Conflict of Interest

All authors declare no conflicts of interest in this paper.