Journal of Materials Science & Technology  2019 , 35 (9): 1877-1885 https://doi.org/10.1016/j.jmst.2019.05.007

Orginal Article

Improving the hard magnetic properties by intragrain pinning for Ta doped nanocrystalline Ce-Fe-B alloys

J.S. Zhanga, W. Lia, X.F. Liaoa, H.Y. Yua, L.Z. Zhaoab, H.X. Zenga, D.R. Penga, Z.W. Liua*

a School of Materials Science and Engineering, South China University of Technology, Guangzhou, 510640, China
b Innovative Center for Advanced Materials (ICAM), Hangzhou Dianzi University, Hangzhou, 310012, China

Corresponding authors:   ∗Corresponding author.E-mail address: zwliu@scut.edu.cn (Z.W. Liu).

Received: 2019-01-31

Revised:  2019-02-3

Accepted:  2019-03-8

Online:  2019-09-20

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

To develop Ce based permanent magnets with high performance/cost ratio, Ta doping is was employed to enhance the magnetic performance of Ce-Fe-B alloys. For melt spun Ce17Fe78-xTaxB6 (x = 0-1) alloys, the coercivity Hc increases from 439 to 553 kA/m with increasing x value from 0 to 0.75. Microstructure characterizations indicate that Ta doping is helpful for grain refinement. A second phase of TaB2 is observed in Ce17Fe77.25Ta0.75B6 alloy, which acts as the pinning center of the magnetic domains, resulting in the change of coercivity mechanism from nucleation type to nucleation + pinning type. The micromagnetic simulation confirms that non-magnetic particles within hard magnetic phase can increase the demagnetization field around them and it is crucial for preventing the further magnetization reverse by pinning effect. Take the advantage of Ta doping for enhancing the coercivity, Ce content of Ce-Fe-B alloy can be further cut down to increase the remanence Jr due to the reduced volume fraction of CeFe2 phase and increased Fe/Ce ratio. As a result, a good combination of magnetic properties with Hc = 514 kA/m, Jr = 0.49 T, and the maximum energy product (BH)max = 36 kJ/m3 have been obtained in Ce15Fe79.25Ta0.75B6 alloy. It is expected that the present work can serve as a useful reference for designing new permanent magnetic materials with low-cost.

Keywords: Melt-spinning ; Permanent magnets ; Second phase ; Coercivity mechanism ; Thermal stability

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J.S. Zhang, W. Li, X.F. Liao, H.Y. Yu, L.Z. Zhao, H.X. Zeng, D.R. Peng, Z.W. Liu. Improving the hard magnetic properties by intragrain pinning for Ta doped nanocrystalline Ce-Fe-B alloys[J]. Journal of Materials Science & Technology, 2019, 35(9): 1877-1885 https://doi.org/10.1016/j.jmst.2019.05.007

1. Introduction

Rare-earth (RE) permanent magnets are widely used in various industrial products like motors in EV (electric vehicles), HEV (hybrid electric vehicles) and wind turbines [1]. A large amount of RE elements like Pr, Nd, Dy, and Tb have been employed for preparing the high-performance RE-Fe-B magnets. The increasing marketing demand for permanent magnets raises the cost of those critical elements. Hence, much attention has been forced to look for the possibility of cost reduction of RE-Fe-B magnets by element substitution, waste recycling or other approaches. In fact, compared to looking for the permanent magnets with extremely high properties, reducing the cost of the magnets is in a sense more important for industry. Coey [2] suggested that it is important to seek a new, low-cost material with the maximum energy product lying in the gap between ferrite (<38 kJ/m3) and Nd-Fe-B magnets (>200 kJ/m3).

Recently, the abundant and inexpensive RE elements like La and Ce have been employed for developing permanent magnets with high performance/price ratio [3,4]. Ce is the only RE element, which has mixed valences of trivalent (3+) 4f1 and tetravalent (4+) 4f0 electronic states [5,6]. There is also a large volume change (15%-17%) between the γ-Ce and α-Ce phases or between the β-Ce and α-Ce phases because the localized 4f electrons can be at least partially delocalized and enter into the valence band of Ce [7]. Although the intrinsic magnetic properties of Ce2Fe14B compound are inferior to those of Nd2Fe14B, they are sufficient for producing the magnets with hard magnetic properties higher than ferrites. Herbst et al. [8] obtained the magnetic properties in melt spun Ce17Fe78B6 alloy with intrinsic coercivity iHc = 429.9 kA/m, remanence Jr = 0.49 T, and maximum energy product (BH)max = 33 kJ/m3. To further improve its performance, it is essential to modify the composition, control the phase constitution, or refine the microstructure. Our previous work [9] suggested that, in ternary Ce-Fe-B alloy, Ce2Fe14B phase precipitated first during rapid quenching, but α-Fe phase precipitated first from the amorphous matrix during annealing. In addition, the CeFe2 phase behaves as a soft magnetic phase below 230(±2) K in Ce-Fe-B alloys [10,11]. A certain amount of CeFe2 phase has been suggested to be beneficial to high coercivity in sintered (Nd,Ce)-Fe-B based magnets due to its weak wettability with the hard phase for smoothing the grain boundaries (GBs) [12,13]. However, it was also found that in the magnets with high Ce content, a great amount of RE-rich phases gathered at the triangle GBs (TGBs) due to the formation of CeFe2 phase, which is not beneficial to coercivity. [14] In the Ce-Fe-B sample, it was confirmed that the CeFe2 phase embedded in main phase can act as the nucleation sites for domain reversal. [15] The grain boundaries become rather rough for the alloy containing only Ce as the RE element [16], which reduces the exchange decoupling between the grains and decreases the coercivity.

For the composition modification of Ce-Fe-B alloys, Co substitution for Fe was used to increase the Curie temperature Tc of Ce2Fe14B phase [17]. Zr and Hf doping could reduce the grain size and optimize the microstructure. Ga doping could also improve the magnetic properties and Tc due to the increase of Ce3+ proportion [18]. We recently reported that Si substitution for Fe can not only suppress the formation of CeFe2 phase, but also significantly improve the coercivity and thermal stability of Ce17Fe78-xSixB6 (x = 0-3.0) alloys. [19] On the other hand, another transition metal Ta has been frequently employed in soft magnetic materials for improving their thermal stability and glass forming ability. For sintered Nd-Fe-B magnets, the 5d elements Ta or W show a low solubility and form the precipitates within the hard magnetic 2:14:1 phase, which probably act as domain wall pinning sites [20]. With the addition of 2 at.% Ta into (NdDy)-(FeCo)-B alloys, the effectively improved room-temperature coercivity and thermal stability have been obtained by Chin et al [21]. We also reported that Ta plays an important role in producing a good combination of magnetic properties in Nd9Fe86-xTaxB5 alloys [22]. Most recently, Rehman et al. [23] reported that the addition of Ta in minor quantity could enhance the coercivity of melt-spun ribbons and improve the temperature coefficient of coercivity (β) from -0.41%/K for Nd13.5Fe80.5B6 to -0.27%/K for Nd13.5Fe79.9B6Ta0.6 in the temperature range of 300-400 K.

However, although the beneficial effects of Ta doping on the magnetic properties of Nd-Fe-B alloys have been studied, there is no report on the Ta doped Ce-Fe-B alloys until now. As we know, the Ce-Fe-B alloy shows different behavior from Nd-Fe-B. For example, the moment-carrying Ce+3, the existence of CeFe2, element segregation and phase segregation in Ce-Fe-B alloys have been commonly confirmed. Thus, it is important to clearly understand the role of the refractory element Ta in optimizing the hard magnetic Ce-Fe-B alloy in order to develop potential low-cost Ce-Fe-B based magnets. In this work, the effects of Ta doping associated with Ce reduction on the phase constitution, microstructure, magnetic properties, and coercivity mechanism of melt spun Ce17Fe78B6 alloy were systematically investigated.

2. Experimental

A series of Ta-doped ingots with nominal compositions of Ce17-yFe78-x+yTaxB6 (x = 0-1, y = 0-5) were prepared by arc melting under argon atmosphere using the raw materials of Ce, Fe, Fe-B and Ta with purity higher than 99.8%. Each ingot was re-melted fo5 five times to obtain a homogeneous composition. The alloy ribbons were prepared by melt spinning using various wheel speeds between 18 and 24 m/s. The optimal hard magnetic properties for each composition were found to be obtained at an optimized wheel speed about 22 m/s. These optimal ribbons were selected for various characterizations. The phase constitution of the ribbons was characterized by X-ray diffraction (XRD, Philip X-pert) using Cu- (alpha) radiation. The ribbons were milled into powders with size <100 μm by hand to reduce the effects of anisotropy. The XRD patterns were refined using the Maud software. The microstructure was examined by transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN 200 kV). The magnetic properties of as-spun alloys were tested by the physical property measurement system (PPMS-9, Quantum Design, USA) equipped with a vibrating sample magnetometer (VSM) using the maximum magnetic field of 5 T. The micromagnetic simulation was employed in this work for understanding the effects of non-magnetic particles on the demagnetization of the alloys and the details are described in Section 3.3.

3. Results and discussion

3.1. Magnetic properties of Ce17Fe78-xTaxB6 alloys with various Ta doping

The demagnetization curves and the composition dependent magnetic properties for Ce17Fe78-xTaxB6 (x = 0, 0.25, 0.5, 0.75 and 1.0) alloys are shown in Fig. 1. In Fig. 1(a), all curves are smooth and show single-phase-like behavior, which indicates that uniform microstructures have been obtained in all alloys. The remanence Jr, coercivity Hc and maximum energy products (BH)max of Ce17Fe78B6 alloy is 0.47 T, 439 kA/m and 35 kJ/m3, respectively, which are comparable to those reported by Herbst et al. [8]. Ta doping is beneficial to high coercivity. With increasing Ta concentration, the highest Hc of 553 kA/m is obtained in the alloy with x = 0.75. Further increase of Ta to 1 at.% leads to reduced coercivity, which is consistent with that of the excessive Ta substituted Nd-Fe-Ta-B alloys [22]. Nevertheless, Hc values of the Ta doped alloys are higher than that of Ta-free one, although Jr decreases slightly due to the dilution of the overall magnetization.

Fig. 1.   Demagnetization curves (a) and magnetic properties (b) of the as-spun Ce17Fe78-xTaxB6 alloys.

3.2. Phase structure and microstructure of Ce17Fe78-xTaxB6 alloys

The typical XRD refinement patterns for the optimized Ce17Fe78-xTaxB6 (x = 0‒0.75) alloys are shown in Fig. 2. Both Ce2Fe14B (2:14:1) phase and CeFe2 (1:2) phase are detected in all alloys. It was reported that the existence of CeFe2 phase could deteriorate the performance of hard magnets due to its paramagnetic properties above Tc = 230(±2) K [10,11]. Based on the Ce-rich composition, the RE-rich phase should exist in the grain boundary, but it is difficult to be identified by XRD. Fig. 3 displays that the lattice constants a and c of 2:14:1 phase and those of 1:2 phase show no big difference in different compositions, which indicates that Ta atoms do not enter into those two phases. With low content of Ta doping, Ta or boride Ta-B phase may not be detected directly by XRD unless it forms large inclusions in the alloys [23,24]. In the XRD patterns of the alloys with x = 0.75 and x = 1.00, however, the intensities of peaks marked with heart-shaped symbol show irregular change, which might be caused by Ta or Ta-B phase.

Fig. 2.   Refined XRD patterns of the as-spun Ce17-xFe78-x+yTaxB6 alloys.

Fig. 3.   (a) Dependences of lattice parameters a, c, c/a of the 2:14:1 phase, (b) lattice parameters a of 1:2 phase on the Ta content x for Ce17Fe78-xTaxB6 alloys.

Fig. 4 shows the TEM micrographs of Ce17Fe78-xTaxB6 alloys with x = 0 and x = 0.75. Bright-filed TEM images present the well-crystallized grains for both alloys. The microstructure of the alloy without Ta doping is rather non-uniform with grain size ranging from $\widetilde{1}$4 nm to $\widetilde{7}$4 nm (averaging $\widetilde{3}$3 nm). However, in the Ta doped alloy, the grain size distribution is refined with a range from $\widetilde{1}$5 to $\widetilde{6}$3 nm (averaging $\widetilde{3}$0 nm). In addition, more spherical grains are also observed in the Ta-containing alloy, which can lead to reduced value of effective demagnetization factor [25]. The result thus suggests that Ta prevents the grain growth by providing more nucleation sites as well as stabilizing amorphous matrix phase in Ta-substituted alloys, which is similar to the effects of Ta in bulk Nd-Fe-Ta-B direct cast magnets [26].

Fig. 4.   Bright-field TEM image of the Ce17Fe78-xTaxB6 alloys with (a) x = 0 and (b) x = 0.75, insets in (a) and (b) is the histogram of grain sizes.

To clearly understand the distribution of the elements, the Ce17Fe77.25Ta0.75B6 alloy was examined by STEM HAADF (High Angle Annular Dark Field), which is sensitive to the atomic number of the element. As displayed in Fig. 5(b) and (c), the obvious contrast variation in the area marked in red box indicates the coexistence of Ce2Fe14B phase with high Fe content and intergranular phase with high Ce content. Fig. 5(d) shows Ta may distribute in both phases but slightly aggregate in CeFe2 phase or grain boundary.

Fig. 5.   Mapping of elements distributions in the Ce17Fe77.25Ta0.75B6 alloy: the image in STEM HAADF mode (a) and the distribution of (b) Ce element, (c) Fe element, and (d) Ta element.

Fig. 6 also shows the TEM microstructure of Ce17Fe77.25Ta0.75B6 alloy. The inserted table shows the EDS results of grains A, B, and C in Fig. 6(a). The Ce:Fe ratios are near to 1:2, 1:7, and 1:7 for grain A, grain B, and grain C, respectively, which should correspond to CeFe2 phase, Ce2Fe14B phase, and Ce2Fe14B phase, respectively. Fig. 6(b) and (e) are the high-resolution TEM (HRTEM) images for the selected grains A and C, respectively, in Fig. 6(a). Fig. 6(d) and (f) are the corresponding FFT graphs. The interplanar distances and the corresponding FFT patterns match well with Ce2Fe14B and CeFe2 phases. In particular, a second phase with grain size $\widetilde{4}$‒6 nm is observed in grain C, as shown in HRTEM images of Fig. 6(c) and (d). Based on the results of interplanar distance shown in Fig. 6(d) and the corresponding FFT pattern in Fig. 6(g), those grains could be indexed as the boride TaB2, which is consistent with the previous XRD result. The globular nano-sized TaB2 grains distributed in the 2:14:1 grains can prevent the domain wall propagation but reduce the magnetization. This microstructure explains that Ta element was detected in Ce2Fe14B phase but had no significant influence on lattice parameters. Similar to Zr2B phase [20] and HfB phase [27], TaB2 phase can also suppress the grain growth, which results in a fine microstructure. Although Ta element is also found in region A, the lattice parameter of 1:2 phase keeps constant, which suggests that TaB2 phase may also form in Region A. The other possibility is that Ta precipitates as amorphous phase, which is like Nb as a refractory element on the grain boundaries of Nd-Fe-B alloys [28].

Fig. 6.   TEM images of the Ce17Fe77.25Ta0.75B6 alloy: (a) Bright-field TEM image and the EDS results of spot A, B and C. The HRTEM images of grain A and grain C are shown in Fig. (b) and Fig. (c) (d). Fast Fourier Transformation (FFT) for the HRTEM images of the grain in Fig. (b), Fig. (c) and Fig. (d) are shown in Figs. (e), (f) and (g), respectively.

3.3. Micromagnetic simulation of nonmagnetic particles dispersed in hard phases

On the basis of above results, the refined microstructure and the non-magnetic particles within the 2:14:1 phase play a vital role in coercivity enhancement for Ta doping alloys. To qualitatively understand the specific effects of non-magnetic particles located in hard-magnetic phase, the micromagnetic simulation is carried out. The simulation model is set up by using OOMMF (Objective Oriented MicroMagnetic Frame work) software [29]. The model with size of 200 nm × 200 nm × 200 nm contains 64 irregularly shaped grains with average grain size of about 50 nm, as shown in Fig. 7(a). The mesh size used is 3 nm. For the alloy with pinning sites, some non-magnetic particles were placed inside 32 grains, and each grain contains two particles ($\widetilde{5}$ nm). The microstress and grain refinement induced by the nonmagnetic particle were not considered here. Fig. 7(b) shows the demagnetization curves for the magnets without and with non-magnetic particles. The coercivity of the magnets with non-magnetic particles is higher than that of the magnets without non-magnetic particles, indicating that non-magnetic particles inside the hard-magnetic grains can increase the coercivity. However, the demagnetization curve of the magnet with non-magnetic particles exhibits an earlier downtrend, which means that its magnetization reverses earlier. Fig. 7(b) inset shows the experimental curves, which is in very good agreement with the simulation result. Fig. 7(c) shows the distribution of demagnetizing fields on YZ plane at x = 130 nm. It is clear that the existence of the non-magnetic particles increases the demagnetizing fields in those regions near the particles (marked in red dotted circles). Due to the increased demagnetization field, the nucleation of reverse domain becomes easier. However, although the reverse starts earlier in the magnet with non-magnetic particles, the propagation of the reverse process is delayed due to the pinning effect, as confirmed by the magnetization distributions at the selected applied field of -880 kA/m and -1080 kA/m in Fig. 7(d). As a consequence, the magnet with intragrain particles exhibits higher coercivity. The results, thus, verified that the non-magnetic particles inside the hard magnetic grain can work as the pinning sites and enhance the coercivity.

Fig. 7.   Simulation results on pinning and without pinning models. (a) Simulation model consisting of two non-magnetic grains within each Ce2Fe14B grain for pinning effects. (b) Demagnetization curves and their fitting line corresponding to the models w/ and w/o particles, the inset shows experimental MH curves for x = 0 and x = 0.75 alloys. (c) Demagnetizing field distribution in the section of x = 130 nm for both models in the saturation state. (d) Magnetization reversal behaviors in the section of x = 130 nm for both models at different reversed magnetic field marked in (a).

3.4. Coercivity mechanism analysis

The analysis of the coercivity mechanism based on the temperature dependent coercivity is employed for demonstrating the existence of pinning effect in Ce17Fe78-xTaxB6 (x = 0.75) alloy. Here, both the nucleation model and pinning model of coercivity were discussed. For the nucleation model, the coercivity is determined by the temperature dependent anisotropy constant K and saturation magnetization Ms [30,31]. The relationship is defined as:

μ0Hc(T)=αKαexμ0Ha(T)-NeffJs(T),

where αk describes the effect of imperfect grain surface on the crystal anisotropy, αex describes the effect of exchange coupling between the neighboring grains, Neff is the effective demagnetization factor due to the enhanced stray fields at the edges and corners of the grains.

The coercivity mechanism based on the pinning of the domain walls at defective positions in the crystals can be described by the relationship as:

Hc1/2(T)=b-c(T2/3),

where b is the intercept of the Hc1/2T2/3 line and c is the slope of the line [32].

Fig. 8(a) and (b) shows the plots of μ0Hc(T)/Js(T) vs μ0Ha(T)/Js(T) for Ce17Fe78B6 and Ce17Fe77.25Ta0.75B6 alloys, respectively. The fitting of the Ce17Fe78B6 alloy is nearly a straight line and the value of correlation coefficient R is almost equal to 1. For Ta substituted alloy, the straight line condition is not satisfied. Fig. 8(c), and (d) present the plot of [Hc(T)]1/2 vs [T(K)]2/3 for the alloy without and with Ta substitution, respectively. The linear fitting is not satisfied in both graphs. The results reveal that the coercivity mechanism of Ce17Fe78B6 alloy is a nucleation type, but that of the Ce17Fe77.25Ta0.75B6 alloy may be governed by both the nucleation and pinning. As we know, the coercivity mechanism for melt spun nanocrystalline Nd-Fe-B based alloys is complicated, which may be composition dependent [33]. In such case, another reason for the improved coercivity in 0.75 at.% Ta substituted alloy can be also attributed to the domain wall pinning, which might be originated from the increase of defect density and the pinning center caused by TaB2 phase.

Fig. 8.   Plot of μ0Hc(T)/Js(T) vs μ0Ha(T)/Js(T) for (a) Ce17Fe78B6 and (b) Ce17Fe77.25Ta0.75B6 alloys. Plot of [Hc(T)]1/2 vs [T(K)]2/3 for (c) Ce17Fe78B6 and (d) Ce17Fe77.25Ta0.75B6 alloys.

3.5. Reducing Ce content in the Ce-Fe-B alloys via Ta doping

Due to the low anisotropy field of Ce2Fe14B compound, to achieve relatively high coercivity in Ce-Fe-B alloy, excessive Ce content is generally required. Herbst et al. [8] and Zhou et al. [34] found that for ternary Ce-Fe-B the alloys, 17% at.% and above Ce content is needed for relatively high coercivity and good combination of hard magnetic properties. Since Ta can increase the coercivity as demonstrated above, we managed to reduce Ce content in the Ta doped Ce-Fe-B alloys. Fig. 9 shows the demagnetization curves and composition dependent magnetic properties for Ce17-yFe77.25+yTa0.75B6 alloys. Although the coercivity Hc decreases monotonously with decreasing Ce content from y = 0 to y = 5, its values for y = 1-3 are still higher than that of Ce17Fe78B6 alloy (Fig. 1). At the same time, Jr is significantly enhanced by Ce reduction due to the increased Fe content. Fig. 10 shows the selected XRD patterns for Ce17-yFe77.25+yTa0.75B6 alloys with various Ce contents. The volume fraction of CeFe2 phase decreases with the decreasing y, and only 2:14:1 phase can be detected in Ce12Fe82.25Ta0.75B6 alloy. To minimize the remanence loss, the volume fraction of 2:14:1 phase and non-magnetic 1:2 phase need to be optimized. The elimination of the non-magnetic CeFe2 phase in Ce reduced alloys can increase the magnetization of the alloys, which further significantly enhance the remanence and energy product. As a result, good magnetic properties i.e. Hc = 514 kA/m, Jr = 0.49 T, (BH)max = 36 kJ/m3 and 448 kA/m, 0.54 T, 41 kJ/m3 are obtained for the Ce15Fe79.25Ta0.75B6 and Ce14Fe80.25Ta0.75B6 alloys, respectively.

Fig. 9.   Demagnetization curves (a) and magnetic properties (b) of the melt spun Ce17-yFe77.25+yTa0.75B6 alloys.

Fig. 10.   Refined XRD patterns of the as-spun Ce17-yFe77.25+yTa0.75B6 alloys.

Table 1 shows the magnetic properties of melt-spun Ce-Ta-Fe-B alloys obtained in this work and by other researchers. Ta doping shows advantage for improving the hard magnetic performance. The properties including Hc, Jr and (BH)max of Ce15Fe79.25Ta0.75B6 alloy with Ta doping and Ce reduction are superior to those of recently reported Ce17Fe78-xMxB6 alloys with Zr [35], Ga [18] or Hf [36] substitution.

Table 1   Magnetic properties of melt-spun Ce-(M)-Fe-B alloys in the current work and reported in other literature.

CompositionHcj (kA/m)Jr (T)(BH)max (kJ/m3)Annealing temperature (℃)Ref.
Ce-Fe-Ta-BCe17Fe78B64390.4735/This work
Ce17Fe77.25Ta0.75B65530.4532/This work
Ce15Fe79.25Ta0.75B65140.4936/This work
Ce14Fe80.25Ta0.75B64480.5441/This work
Ce13Fe81.25Ta0.75B63870.5947/This work
Ce-Fe-M-BCe17Fe77.25B6Ga0.754920.4319/[18]
Ce17Fe77.5B6Zr0.54300.4836788[35]
Ce17Fe77B6Hf4200.38//[36]
Ce3Fe12Co2B3900.5235/[17]

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4. Conclusion

The magnetic properties of Ce-Fe-B alloys have been improved by Ta doping and Ce reduction. The pinning effects of TaB2 phase for Ta substituted melt spun Ce17Fe78-xTaxB6 (x = 0-1) alloys have been verified in this work. Ta acts as a doping transition metal and has been demonstrated its important role in enhancing the coercivity by refining the microstructure, and inducing domain wall pinning effect. The highest Hc of 553 kA/m is obtained in Ce17Fe77.25Ta0.75B6 alloy, with Jr = 0.45 T and (BH)max =32 kJ/m3. For Ta substituted alloys, the Ce content can also be reduced, which helps to eliminate the CeFe2 phase. As a result, the remanence and energy product can be further enhanced. The Ce15Fe79.25Ta0.75B6 alloy exhibits pretty good magnetic properties with Hc = 514 kA/m, Jr = 0.49 T, and (BH)max = 36 kJ/m3. The present work shows that a minor addition of Ta is more effective in improving the magnetic properties of Ce-Fe-B alloys than other dopants, which is beneficial to designing the new and low-cost magnetic materials.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (No. 51774146) and the Guangzhou Municipal Science and Technology Program (No. 201605120111410).

The authors have declared that no competing interests exist.


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