In this study, we synthesized Mn65Ga35-δBδ (δ = 0–10) alloys using the melt-spinning technique. In the as-quenched state, the alloys consisted of the D019 and Mn8Ga5 phases in the as-quenched state. After heat treatments, the Mn65Ga35 alloy consisted of the D022 phase, whereas the Mn65Ga30B5 and Mn65Ga25B10 alloys were mainly composed of the D019 phase. The magnetization of the Mn65Ga30B5 and Mn65Ga25B10 alloys was smaller than that of the Mn65Ga35 alloy, but the Mn65Ga30B5 and Mn65Ga25B10 alloys exhibited higher coercivity than the Mn65Ga35 alloy. The highest coercivity of 8.7 kOe was measured at room temperature in the Mn65Ga30B5 alloy with the fined grains of the D019 phase.
With the increasing concern about environmental issues, it is essential to reduce greenhouse gas emissions from all sources, including automobiles and other forms of transportation. In this regard, the increased use of electric vehicles is essential for a sustainable future. Nd-Fe-B-based rare-earth permanent magnets, which are extensively applied in electric vehicles, have consequently become key materials in the modern world.1–3 The supply of rare-earth elements, which are considered to be critical resources for high technology, is currently a focus of concern.4–6 The problem of the rare-earth elements is the balance of supply and demand.7–9 Some rare-earth elements are consequently produced in larger amounts than required, and these elements are simply stockpiled. For example, although Nd is necessary for Nd-Fe-B-based rare-earth permanent magnets, it is impossible to refine only this particular rare-earth element from the rare-earth ore. Due to the recent fluctuations in rare-earth prices, demands exists in industrial sectors for the realization of rare-earth-free permanent magnets.10,11
Mn-Ga alloys exhibit promising hard magnetic properties.12–14 Many intermetallic compounds have been reported in Mn-Ga alloys, such as the hexagonal D019 phase, tetragonal L10 phase (P4/mmm), and tetragonal D022 phase (I4/mmm).15,16 The most extensively studied phases of the intermetallic compounds are the L10 and D022 phases.17–24 Both phases possess high magnetocrystalline anisotropy, which is a prerequisite for high coercivity. High-coercivity magnets have been produced in Mn-Ga alloys with the tetragonal DO22 phase.25,26 However, due to the low magnetization of the Mn-Ga alloys and the high price of Ga,27 Mn-Ga alloys are not yet suitable for permanent magnets. One possibility to overcome the problems of Mn-Ga alloys is the addition of a third element to the alloy. Since B belongs to the same chemical group as Ga in the periodic table, substitution on the Ga sites of the DO22 Mn-Ga phase with small amounts of B was examined. The addition of B can change the magnetocrystalline anisotropy of the DO22 Mn-Ga phase and improve the magnetic properties of Mn-Ga alloys. In this paper, we present our experimental investigation on the structures and magnetic properties of Mn-Ga alloy with the DO22 phase and samples with the substitution of amounts of B.
Mn65Ga35-δBδ (δ = 0–10) alloy ingots were produced in an Ar atmosphere by induction melting of the elemental materials with a purity 99.9% for Mn, 99.9% for Ga, and 99.5% for B. Rapidly quenched specimens were produced by the melt-spinning technique with a wheel velocity of 50 m/s in an Ar atmosphere, and subsequently annealed at temperatures of 673–1073 K for 0.5 h in an Ar atmosphere. The crystallographic structure was determined using a MiniFlex600 X-ray diffractometer (XRD: Rigaku). The microstructures were observed using a JEM-2100 transmission electron microscope (TEM: JEOL). After the ribbons had been mixed with epoxy resin, the magnetic properties of the specimens were measured at room temperature using a BHV-525RSCM vibrating sample magnetometer (VSM: Riken Denshi).
RESULTS AND DISCUSSION
Figure 1 shows the XRD patterns of the as-quenched melt-spun ribbons of the Mn65Ga35-δBδ (δ = 0–10) alloys. Various intermetallic compounds exist in Mn-Ga alloys, and the phase diagrams of these alloys are therefore relatively complex and have not been fully elucidated. However, the details of the Mn-rich portion of Mn-Ga phase diagrams have been well studied and elucidated by K. Minakuchi et al.15 According to the Mn-Ga phase diagram, the equilibrium phase in the Mn65Ga35 alloy at high temperatures is the Mn8Ga5 (σ) and D019 (ε) phases, but the equilibrium phase at room temperature is the L10 (γ) phase. The L10 phase forms from the Mn8Ga5 and D019 phases (Mn8Ga + D019 → L10) at 1033 K. However, no diffraction peaks of the L10 phase were found in the XRD pattern of the Mn65Ga35 alloy. The high-temperature Mn8Ga5 and D019 phases were retained in the as-quenched specimen without the formation of the L10 phase. It is known that B addition to binary alloys promotes amorphous phase formation.28 Although the XRD patterns of the Mn65Ga30B5 and Mn65Ga25B10 alloys are different from that of the Mn65Ga35 alloy, diffraction peaks of the Mn8Ga5 and D019 phases are found in the XRD patterns. This suggests that the addition of B to the Mn65Ga35 alloy was not enough to obtain the amorphous phase in the Mn-Ga alloy.
Figure 2 shows the magnetization curves of the as-quenched melt-spun ribbons of the Mn65Ga35-δBδ (δ = 0–10) alloys. The curves were measured at room temperature. The magnetization curve of the Mn65Ga35 alloy was paramagnetic, with the magnetization proportional to the applied magnetic field. Virtually the same magnetization curve was obtained from the Mn65Ga30B5 alloy, whereas a hysteresis curve was obtained from the Mn65Ga25B10 alloy. The coercivity of the Mn65Ga25B10 alloy was 4.8 kOe in the as-quenched state. It is known that Mn-Ga alloys with the DO22 phase exhibit high coercivity, but Mn-Ga alloys with the D019 phase also exhibit some coercivity.24,29 Thus, the origin of the observed coercivity in the Mn65Ga25B10 alloy is attributable to the existence of the D019 phase.
Mn-Ga alloys with the DO22 phase have been obtained by subjecting the alloys to isothermal annealing.12,26 Thus, isothermal annealing of the as-quenched melt-spun ribbons of the Mn65Ga35-δBδ (δ = 0–10) alloys was carried out. Figure 3(a) shows the annealing temperature dependence of the coercivity of the Mn65Ga35-δBδ (δ = 0–10) melt-spun ribbons. The coercivity of the Mn65Ga35 alloy increased from 2.2 kOe to a peak value of 5.3 kOe and then decreased as the annealing temperature further increased. The Mn65Ga30B5 and Mn65Ga25B10 alloys showed the same tendency, and the peak values, 8.7 kOe for the Mn65Ga30B5 alloy and 7.8 kOe for the Mn65Ga25B10 alloy, were obtained at 773 K. This indicates that the substitutions of B for Ga in the Mn65Ga35 alloy shifted the optimal annealing temperatures to a lower value and increased the coercivity.
The magnetization curves of the optimally annealed Mn65Ga35-δBδ (δ = 0–10) melt-spun ribbons are shown in Fig. 3(b). Although the optimally annealed Mn65Ga30B5 and Mn65Ga25B10 alloys exhibited higher coercivity than the optimally annealed Mn65Ga35 alloy, their saturation magnetization was lower. This suggests that the substitutions of B for Ga in the Mn65Ga35 alloy resulted in a decrease in the saturation magnetization. The saturation magnetization is an intrinsic property. Thus, the phase of the optimally annealed Mn65Ga35 alloy should be different from those of the optimally annealed Mn65Ga30B5 and Mn65Ga25B10 alloys.
To investigate the difference in the magnetic properties of the optimally annealed Mn65Ga35-δBδ (δ = 0–10) alloys: Mn65Ga35 alloy annealed at 973 K and Mn65Ga30B5 and Mn65Ga25B10 alloys annealed at 773 K, the structures and microstructures were examined by XRD and TEM. The results are shown in Figs. 4 and 5, respectively. The major phase in the optimally annealed Mn65Ga35 alloy was the D022 phase, with a grain size of around 1 μm. Therefore, the D022 phase in the optimally annealed Mn65Ga35 alloy gave rise to the observed coercivity of 5.3 kOe. The optimally annealed Mn65Ga30B5 alloy still contained some D022 phase, but the primary phase in both the optimally annealed Mn65Ga30B5 and Mn65Ga25B10 alloys was not the D022 phase but the D019 phase. It has been reported that the D019 phase is more stable than the D022 phase.17 The TEM studies revealed that both alloys consisted of fine grains. Therefore, the observed high coercivities of the optimally annealed Mn65Ga30B5 and Mn65Ga25B10 alloys were due to the fine grains of the D019 phase. In this case, the partial replacement of Ga with B in the Mn65Ga35 alloy stabilized the D019 phase instead of the D022 phase. The saturation magnetization of the Mn65Ga30B5 and Mn65Ga25B10 alloys with the D019 phase was therefore lower than that of the Mn65Ga35 alloy with the D022 phase (see Fig. 4).
The optimally annealed Mn65Ga35-δBδ (δ = 0–10) alloys investigated in this study exhibit high coercivity, but their saturation magnetization is still insufficient for their use as permanent magnets. The (BH)max of the Mn65Ga35 alloy was calculated to be 1.08 MGOe at room temperature. This value is almost comparable to that of Ba-ferrite magnets (1.0 MGOe).30 In practice, such Mn-Ga alloys could be used in developing nanocomposite magnets. Nanocomposite magnets are composed of soft magnetic and hard magnetic phases, where rare-earth magnets are used for the hard magnetic phase. Applying Mn-Ga-based alloys with high coercivity as the hard magnetic phase, new types of nanocomposite magnets that are not based on rare-earth elements can be realized. To produce such magnets in the future, it is still necessary to improve the magnetic properties of Mn-Ga-based alloys.
In this study, we investigated the structures and magnetic properties of as-quenched and annealed melt-spun ribbons of the Mn65Ga35-δBδ (δ = 0–10) alloys. The Mn65Ga35 alloy was composed of the D022 phase when annealed at 973 K, whereas the Mn65Ga30B5 and Mn65Ga25B10 alloys mainly consisted of the D019 phase when annealed at 773 K. The Mn65Ga35 alloy with the D022 phase showed a coercivity of 5.3 kOe, while the Mn65Ga30B5 and Mn65Ga25B10 alloys with the D019 phase exhibited coercivities of 8.7 kOe and 7.8 kOe, respectively. These values were measured at room temperature.
Although the alloys investigated in this study exhibited high coercivity, their saturation magnetization is still insufficient for application to permanent magnets. Further enhancement of the magnetic properties of such Mn-Ga-based alloys is therefore required.
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.
Conflict of Interest
The authors have no conflicts to disclose.
The data that support the findings of this study are available from the corresponding author upon reasonable request.