Room temperature magnetic properties of Mn-Ga-B melt-spun ribbons

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.

Mn₆₅Ga_35-δB_δ (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).

Figure 1 shows the XRD patterns of the as-quenched melt-spun ribbons of the Mn₆₅Ga_35-δB_δ (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 Mn₆₅Ga_35-δB_δ (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 Mn₆₅Ga_35-δB_δ (δ = 0–10) alloys was carried out. Figure 3(a) shows the annealing temperature dependence of the coercivity of the Mn₆₅Ga_35-δB_δ (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 Mn₆₅Ga_35-δB_δ (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 Mn₆₅Ga_35-δB_δ (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 Mn₆₅Ga_35-δB_δ (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 Mn₆₅Ga_35-δB_δ (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.

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.