Due to the outstanding magnetic properties, Nd–Fe–B based permanent magnets have been widely applied in various fields ranging from sensors, hybrid vehicles, wind generators to electronic devices.^[1,2] The continuously growing demand of Nd–Fe–B magnets has led to the overspending of the closely-relied Pr, Nd, Dy, and Tb metals, whereas, high abundant La and Ce are overstocked. In order to balance the use of rare earth resources, La or Ce substitution for Nd to prepare RE–Fe–B (RE, rare earth) magnets have attracted worldwide attention again.^[3–6] Moreover, the extraction and purifying processes of these elements are very complex and harmful to the environmental security. Therefore, in terms of the environment protection and balanced utilization of rare earth resources, misch metal (MM) with natural ratio^[7] (26–29 wt.% La, 49–53 wt.% Ce, 4–6 wt.% Pr, and 15–17 wt.% Nd) from Bayan Obo mine should be directly used to fabricate RE–Fe–B magnets in China.

As shown in the previous studies on Nd–La–Fe–B magnets,^[8–11] besides the inferior intrinsic magnetic properties of La₂Fe₁₄B to Nd₂Fe₁₄B, there exist troubles in the formation of 2:14:1 tetragonal phase and phase stability at high temperature as well. Hitherto the application of La in RE–Fe–B magnetic material is seriously restrained. Different from La, although Ce is easy to form Ce₂Fe₁₄B phase, yet CeFe₂ phase is prone to emerge when the Ce substitution for Nd exceeds 30 at.%.^[12,13] On the contrast, for the magnets with La–Ce or MM doping in RE–Fe–B system, CeFe₂ phase can be suppressed effectively and more amount of La enters into 2:14:1 phase than that with individual La or Ce substitution.^[14] Hence, substitution of MM, composed of nearly 80% La and Ce, for Nd to fabricate RE–Fe–B magnets is believed to be beneficial to improve the overall magnetic performance.

In addition, due to the relatively lower intrinsic magnetic properties of La₂Fe₁₄B (saturation magnetization μ₀M_s = 1.38 T, magnetocrystalline field H_A = 2069 kA/m, and Curie temperature T_c = 530 K) and Ce₂Fe₁₄B (μ₀M_s = 1.17 T, H_A = 1592 kA/m, and T_c = 424 K) compared to Nd₂Fe₁₄B (μ₀M_s = 1.60 T, H_A = 5809 kA/m, and T_c = 585 K),^[15,16] the magnetic properties of La, Ce, or MM doping single main phase (SMP) RE–Fe–B magnets deteriorate rapidly with increasing La, Ce, or MM content.^[12,17,18] Recently, a novel process known as double main phase (DMP) process has been proposed^[3,19–23] to fabricate RE–Fe–B sintered magnets by blending two kinds of powders of different compositions, such as Pr–Nd–Fe–B and La, Ce, or MM based RE–Fe–B, respectively. As a result, the remanence B_r = 1.240 T, the intrinsic coercivity H_cj = 716.2 kA/m, and the maximum magnetic energy product (BH)_max = 292.0 kJ/m³ can be obtained at a high Ce substitution level of 45 wt.%.^[24] By DMP method, when MM/RE = 30.3 at.%, the sintered RE–Fe–B magnets have H_cj = 566 kA/m and (BH)_max = 326 kJ/m³.^[22] However, in the MM-based sintered magnets prepared by DMP process, there is no significant enhancement of coercivity as observed in Ce-based sintered magnets fabricated by the same process. And the RE–Fe–B magnetic material with La, Ce, or MM content higher than 50 at.% is rarely investigated so far, the amount of La, Ce, or MM substitution for Nd in RE–Fe–B magnets is usually below 50 at.% in the previous research. Magnetic properties of the sintered RE–Fe–B magnets with a large amount of MM by DMP process need to be further improved to meet the application requirements.

In this work, MM is directly employed to fabricate RE–Fe–B sintered magnets. A Pr–Nd–Fe–B component and an MM–Fe–B component are designed to prepare the RE–Fe–B magnets with DMP process, in which the ratios of MM to RE are 50 at.% and 70 at.%, respectively. The same nominal composition magnets are also manufactured with SMP process for comparison.

The alloys with the nominal composition of [(Pr, Nd)_{1 – x}MM_x]_13.8Fe_balM_1.5B_5.9 (x = 0, 0.5, 0.7, and 1.0), where M refers to Cu, Al, Co, and Nb elements, were prepared by the induction melting and strip casting under high purity argon atmosphere. The raw materials are iron of a purity > 99.5 wt.%, Fe–B alloy with 79.56 wt.% Fe and 19.78 wt.% B, and Pr–Nd alloy of 24.46 wt.% Pr and 75.4 wt.% Nd, as well as M metals and MM of a purity > 99.5%. MM is one kind of the misch metals from Bayan Obo ore with a composition of 28.63 wt.% La, 50.13 wt.% Ce, 4.81 wt.% Pr, 16.28 wt.% Nd, and < 0.05 wt.% the others. Subsequently, the alloy strips were pulverized to powders of average particle size of 3.0–3.5 μm by hydrogen decrepitating and jet-milling process successively. For DMP process, both (Pr, Nd)_13.8Fe_balM_1.5B_5.9 and MM_13.8Fe_balM_1.5B_5.9 powders were blended together according to the preset nominal compositions of [(Pr, Nd)_0.5MM_0.5]_13.8Fe_balM_1.5B_5.9 and [(Pr, Nd)_0.3MM_0.7]_13.8Fe_balM_1.5B_5.9, respectively. Then, the powders for DMP and SMP processes, respectively, were aligned and compacted under a magnetic field of 2 T and a pressure of ∼ 10 MPa, and followed by an isostatic pressing at 200 MPa, and then by sintering at 1020–1060 °C for 2 h. Two-stage annealing of the as-sintered magnets were performed for 2 h at 900 °C and 450–500 °C, respectively. Both sintering and annealing processes were operated in vacuum of ∼ 1 × 10^–3 Pa.

The permanent magnetic properties of the sintered magnets were tested by the NIM-2000 precision measuring system for hard magnets. The temperature dependence of magnetization, the minor loops, and the recoil curves were measured by a Quantum Design VersaLab at the temperature range of 300–800 K and by a SQUID_VSM at the range of 200–380 K respectively. The microstructure and elements distribution were observed by a SUPRA55 scanning electron microscope (SEM) equipped with energy dispersive x-ray detector (EDX). Elemental concentration mapping was characterized by an EPMA-1720 electron probe micro-analyzer with wavelength dispersive x-ray spectrometer (WDS).

Figure 1(a) presents the room-temperature demagnetization curves for [(Pr, Nd)_{1 – x}MM_x]_13.8Fe_balM_1.5B_5.9 (x = 0.0, 0.5, 0.7, and 1.0) sintered magnets prepared by SMP and DMP methods, respectively. The sharp decrease of (BH)_max from 382.5 kJ/m³ to 95.70 kJ/m³ and the remarkable diminishment of H_cj from 1219 kA/m to 111.8 kA/m are observed with increasing MM substitution from x = 0.0 to 1.0. The magnetic performance of x = 1.0 magnets is better than those reported in the literatures,^{[20, 22, 25–27]} as illustrated in Fig. 2. Meanwhile, it can be seen that significant enhancements of coercivity, remanence, and maximum magnetic energy product are obtained for magnets with DMP process compared with the SMP ones. For x = 0.5, compared with magnets with SMP process of B_r = 1.245 T, H_cj = 358.8 kA/m, and (BH)_max = 242.2 kJ/m³, the magnetic performances of magnets with DMP process are B_r = 1.278 T, H_cj = 657.9 kA/m, and (BH)_max = 289.8 kJ/m³. For x = 0.7, H_cj increases from 232.1 kA/m for magnets with SMP process to 371.9 kA/m for magnets with DMP process, B_r also increases from 1.197 T to 1.241 T, and (BH)_max is significantly enhanced from 161.4 kJ/m³ to 253.3 kJ/m³. It should be noted, as shown in Fig. 1(a), that the permanent magnetic properties of the magnets of x = 0.7 with DMP process are even better than those of the magnets of x = 0.5 with SMP process unexpectedly, in consideration of the fact of 20 at.% more MM adopted. Figure 1(b) shows the x-ray diffraction (XRD) patterns of the powders from the sintered magnets with SMP and DMP processes. It is obvious that all the samples exhibit the tetragonal 2:14:1 phase. But there exists CeFe₂ phase in magnets with SMP process that is harmful to both B_r and H_cj, and then (BH)_max. In one word, when the amount of MM substitution is larger than 50 at.%, magnets with DMP process have relatively better magnetic properties than those with SMP process.

Figure 2 shows the comparison of (BH)_max of [(Pr, Nd)_{1 – x}MM_x]_13.8Fe_balM_1.5B_5.9 (x = 0.0, 0.5, 0.7, and 1.0) magnets with those of some MM based sintered magnets reported in the literatures.^{[20, 22, 25–29]} Generally, a dramatic drop of (BH)_max occurs when x exceeds 0.5 for all the sintered magnets except the one fabricated by DMP process in this work. It is worthy to note, as mentioned previously, that the sintered magnets of x = 1.0, i.e., MM_13.8Fe_balM_1.5B_5.9 magnets in this work show the highest (BH)_max to date, it is reasonable to conclude that the excellent magnetic performances of the sintered magnets by DMP process in this work probably could be attributed to the superior magnetic properties of the original alloy of MM_13.8Fe_balM_1.5B_5.9.

Due to the similar lattice parameters of the [(Pr, Nd)_{1 – x}MM_x]_13.8Fe_balM_1.5B_5.9 magnets, it is difficult to identify the main phase components by XRD solely. Hence, the temperature dependence of magnetization for the [(Pr, Nd)_{1 – x}MM_x]_13.8Fe_balM_1.5B_5.9 (x = 0.5 and 0.7) magnets with DMP process was measured, as shown in Fig. 3, with an applied magnetic field of 23.8 kA/m in a temperature range of 300–700 K. The plots of d σ_m / d T vs. temperature T, which are employed to determine the Curie temperatures of the magnets, are displayed in red lines in Fig. 3. In magnets by DMP process, three Curie temperatures, labeled as T_c1, T_c2, and T_c3, respectively, are observed for both x = 0.5 and 0.7. With the content of MM increasing from 0.5 to 0.7, the Curie temperatures of T_c1, T_c2, and T_c3 decrease from 526.2 K, 568.0 K, and 603.0 K to 521.0 K, 558.2 K, and 589.1 K, respectively. The value of T_c2 is close to T_c of the magnets by SMP process in our previous study,^[30] but T_c1 and T_c2 are similar to those of MM₂(Fe, Co)₁₄B and (Pr, Nd)₂(Fe, Co)₁₄B, respectively. Since the Curie temperature of magnets intensively depends on the composition,^[31] the occurrence of three Curie temperatures indicates that there should be three magnetic phases with different La, Ce, Pr, and Nd as well as Fe concentration in the magnets with DMP process. It is probably resulted from the element diffusion among the main grains during the sintering process, and a kind of construction with multiple main phases is synthesized in the magnets during the DMP preparation process, which is completely different from the sintered magnets with SMP process.

Figure 4 illustrates the back-scattered images of [(Pr, Nd)_{1 – x}MM_x]_13.8Fe_balM_1.5B_5.9 (x = 0.5 and 0.7) magnets prepared by SMP and DMP processes, respectively. The dark gray region in Fig. 4 is RE₂Fe₁₄B matrix phase, and the bright region corresponds to RE-rich intergranular phase. It can be seen that magnets with DMP process (Figs. 4(b) and 4(d)) have more continuous RE-rich phase layers surrounding the main grains compared with the magnets with SMP process (Figs. 4(a) and 4(c)). On the contrast, for the magnets with SMP process, the RE-rich phase is prone to aggregate in grain boundary triple junctions. It is well known that the nucleation of reversed domains is prone to occur at the tripe junctions, thus the lower coercivity is found in magnets with SMP but not DMP process. In addition, as shown in Figs. 4(b) and 4(d), most of the grains show a clear contrast between core and shell for the magnets with DMP process. The shell region is darker than the core region, suggesting elements of smaller average atomic numbers existing in the shell rather than in the core, it implies that Pr and Nd are left in the core region, meanwhile, La and Ce diffuse to the shell region, therefore the grains in the DMP magnets are composed of a magnetically soft shell around a magnetically hard core region. With higher Pr and Nd contents, the core region bears both high magnetocrystalline anisotropy field and the saturation magnetization, compared to the homogeneous magnets with SMP process. All above factors are beneficial to yield better magnetic properties for magnets with DMP process.

In order to further confirm the difference of the element distribution between magnets with SMP and DMP processes, the back-scattered electron image and corresponding mapping profile of La, Ce, Pr, Nd, and Fe elements are detected and illustrated in Figs. 5 and 6. For magnets with SMP process, the concentration distributions of La, Ce, Pr, and Nd within the main phase are uniform as shown in Fig. 5. Comparatively, as shown in Fig. 6, there are obvious rare earth element distribution heterogeneities both within one individual grain and between inter grains for magnets with DMP process. As a result of composition difference between two kinds of original powders of (Pr, Nd)_13.8Fe_balM_1.5B_5.9 and MM_13.8Fe_balM_1.5B_5.9, both Pr and Nd elements diffuse from (Pr, Nd)₂(Fe, M)₁₄B grains into MM₂(Fe, M)₁₄B grains, meanwhile, both La and Ce migrate oppositely. Core–shell structures, labeled as area A, are also obviously found in Figs. 6(b) and 6(c), La and Ce ions in MM have diffused into the surface layer of (Pr, Nd)₂(Fe, M)₁₄B grains, at the same time, Pr and Nd occupy the positions of La and Ce, as shown in labeled area B.

The plots of the reduced magnetization M/M_max, coercivity H_cj/H_cj,max, and remanence M_r/M_r,max, obtained from the minor hysteresis loops (the insets in Fig. 7), versus the reduced maximum applied field H/H_cj are shown in Fig. 7. For magnets with DMP process, M/M_max increases rapidly until the applied field H exceeds 0.5 H_cj, then follows by a steep rise when H is approaching to H_cj. This phenomenon may be resulted from the pinning effect of domain wall during the magnetization process. But the pinning effect is weaker than that found in the nanostructural magnetic materials, because H_cj/H_cj,max increases faster than M_r/M_r,max when H just exceeds H_cj. However, there is no such feature observed for magnets with SMP process. It may be attributed to heterogeneity of rare earth element distribution in magnets with DMP process, which leads to differences of the anisotropy fields in local regions. Due to the exchange couplings of these local regions, the magnetic properties of magnets with DMP process are enhanced considerably.

Recoil curves in demagnetization process are tested for both DMP and SMP [(Pr, Nd)_{1 – x}MM_x]_13.8Fe_balM_1.5B_5.9 (x = 0.5 and 0.7) magnets, as shown in Fig. 8. The recoil curves of the SMP and DMP magnets show the tadpole-like openness which is related to the irreversible magnetization. Compared with magnets with SMP process, the openness is relatively small in magnets with DMP process, which represents the larger magnetic hardness. The reversible specific susceptibility χ_rev/σ_s, as illustrated in Fig. 9, is defined as [σ_d(H) – σ_m(H)]/(σ_sH), where σ_d(H) and σ_m(H) are the values of specific magnetization after and before the removal of the applied reverse magnetic field H, respectively, and σ_s is the specific remanent magnetization. The deduced χ_rev/χ_s versus the applied field is shown in Fig. 9(b), which is proposed to reveal the strength of the exchange coupling.^[32] Compared with DMP magnets, the higher peak value of χ_rev/χ_s of the magnets with SMP process indicates that their magnetic moment is easy to be reversed under a lower applied field. The relative strong exchange coupling among grains with different anisotropy fields in local regions in magnets with DMP process strengthens the magnetic moment alignment after removing magnetizing field, and the uniformity of the magnetic moment reversal is enhanced.^[33] Combined with the fact that the grains of the main phase are perfectly aligned in the sintered magnets with DMP process verified by the XRD technique (not presented here), both B_r and H_cj of the magnets with DMP process are improved compared with the ones by SMP method.

In this work, the DMP and SMP processes are employed to fabricate the [(Pr, Nd)_{1 – x}MM_x]_13.8Fe_balM_1.5B_5.9 (x = 0.0, 0.5 0.7, and 1.0) sintered magnets with high MM content. The magnets prepared by DMP method exhibit much better magnetic properties than those by SMP method. When x is equal to 0.7, H_cj increases from 232.1 kA/m of magnets with SMP process to 371.9 kA/m of magnets with DMP process, B_r also increases from 1.197 T to 1.241 T, and (BH)_max is enhanced significantly from 161.4 kJ/m³ to 253.3 kJ/m³. In addition, magnets of high content of MM (x ≥ 0.5) with DMP process appears to be of multiple main phase construction. The enhancement of magnetic performance of the magnets with DMP process could be contributed to the strong exchange coupling among adjacent grains with different magnetocrystalline fields and the pinning effect.