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Microstructure and magnetic properties of NdFeB alloys by co-doping alnico elements
Physics Letters A 383 (2019) 125878
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Physics Letters A www.elsevier.com/locate/pla
Microstructure and magnetic properties of NdFeB alloys by co-doping alnico elements Sajjad Ur Rehman, Qingzheng Jiang, Lunke He, Houdong Xiong, Kai Liu, Lei Wang, Munan Yang, Zhenchen Zhong ∗ Jiangxi Key Laboratory for Rare Earth Magnetic Materials and Devices/Institute for Rare Earth Magnetic Materials and Devices, School of Materials Science and Engineering, Jiangxi University of Science and Technology, Ganzhou, China
a r t i c l e
i n f o
Article history: Received 29 May 2019 Received in revised form 6 July 2019 Accepted 2 August 2019 Available online 7 August 2019 Communicated by M. Wu Keywords: NdFeB Magnetic properties Exchange interactions Nanostructure Phase transition temperature
a b s t r a c t Elements of alnico 8 are added into Nd-Fe-B alloys fabricated by rapid solidification method. It is observed that the magnetic properties at high temperature improved by small addition of alnico elements. The Curie temperature of the alloys increased from 580 K for standard alloy to 639 K by 20% addition of alnico 8 elements. The spin reorientation temperature decreased from 133 K to 104 K. The TEM analysis showed that elements of alnico 8 refine the microstructure of Nd-Fe-B ribbons. The STEM analysis confirmed the heterogeneous distribution of Nd, Fe, Cu, Al, Ni and homogeneous distribution of Ti, Nb and Co. The boundaries of nano grains contain more than 70% ferromagnetic elements, ensuring strong inter-grain coupling among the grains. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Rare earth (RE) based permanent magnets (SmCo, NdFeB) have superior magnetic properties in terms of coercivity Hcj and energy density (BH)max , and are widely used in traction motors, wind energy turbines, biomedical instruments, NMR, MRI, CT scans and miniaturized electronic devices [1,2]. Magnetic materials play very critical role in advanced devices and various kinds of motors and power generators. They enable the conversion of electrical energy into mechanical energy and vice versa. The basic function of a permanent magnet is to provide external field in a magnetic circuit of an electric machine. The (BH)max represents the performance capability of a permanent magnet, i.e. it is the measure of the energy of a magnet in magnetic circuit [3]. The miniaturization of modern day instruments including motors and power generators has been possible due to the high energy density of Nd-Fe-B type magnets. To obtain high (BH)max in a permanent magnet, large Br must be maintained in absence of external magnetic field, and at the same time, it must resist demagnetization from the electric or magnetic circuits and thermal demagnetization from the operating temperature. Therefore, Hcj is of utmost importance for motor applications because demagnetization continuously occurs due to
*
Corresponding author. E-mail address: [email protected] (Z. Zhong).
https://doi.org/10.1016/j.physleta.2019.125878 0375-9601/© 2019 Elsevier B.V. All rights reserved.
demagnetization of electric and magnetic circuits, and from heating during operation [1]. Because of the above mentioned extrinsic properties, the RE permanent magnet brushless motors are much more efficient than induction motors because of (a) no electric energy is observed by the field excitation system, (b) higher torque and output power per unit volume, and (c) better dynamic performance [4]. The current consumption of high performance PMs are expected to grow because the demand for the aforementioned devices is expanding year by year. To meet the diverse applications, magnetic materials should posses high extrinsic properties such as Hcj , Br , and (BH)max which are satisfied by RE based permanent magnets for all room temperature applications [3]. In some applications, like wind turbine generators and electrical vehicles, the RE based PMs need to operate at around 160◦ C. At such high temperature the coercive field reduces to less than half a Tesla (0.5 T) which is too low to be used in electrical vehicles and wind turbines [5]. From technological point of view, it is therefore, necessary to develop Nd-Fe-B based magnets that could operate at high temperature with reasonably high Hcj and (BH)max . The RE elements such as Nd, Pr, Dy and Tb provide strong spin orbital coupling for high magnetocrystalline anisotropy (H A ) in REFe-B type magnets while 3d metals like Fe and Co are responsible for high spontaneous magnetization (M s ), and high Curie temperature (T C ) [6]. The H A of heavy RE elements is high; therefore, they ensure high Hcj in the ternary RE2 Fe14 B compounds. The partial substitution of Nd by Dy/Tb improves the Hcj and thermal stability
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Fig. 1. Demagnetization curves of S0 - S4 ribbons, (a) at 300 K, and (b) at 400 K.
and makes the alloys capable of operating at temperatures beyond 160◦ C [7]. However, the antiparallel coupling of the magnetic moments of heavy RE (Dy/Tb) and Fe moments in RE2 Fe14 B compound reduce the Br values resulting in a low (BH)max [8]. Another, rather commonly used, approach to improve the thermal stability of RE-Fe-B alloys is to increase the T C by partial Co substitution for Fe, but the magnetic properties of the ternary alloys decrease by Co substitution [9]. Furthermore, the substitution of resource critical heavy RE elements and Co increase the price of the magnets [10]. In addition to Co substitution for Fe and Dy/Tb substitution for Nd, different additive elements have been tried for improving the microstructure related properties, Hcj , Br and (BH)max . The additive elements are broadly divided into high melting point elements and low melting point elements [11]. The low melting point elements, such as Al, Ga, Cu and Sn are known to modify the gain boundary phase and increase the Hcj by magnetically decoupling the grains [12–16]. The high melting point elements Nb, Ti, W, Cr, V, Mo, Hf act as refractory elements, which refine the grains and suppress the formation of deleterious α -Fe phase in Nd-Fe-B type alloys. [16–22]. The high melting point elements are believed to form intergranular borides which in turn act as pinning centers for improving the Hcj [23]. Some reports even suggest that the high melting point elements are helpful for improving the thermal stability [24]. The single doping of Al, Co, Cu, Ti and Nb are known to improve the microstructure and magnetic properties of Nd-Fe-B type alloys as summarized above. However, the combined addition of these elements on intrinsic and extrinsic properties of Nd-Fe-B alloys is rarely studied. Some recent papers report the effect of adding multi elements on magnetic properties of RE based magnetic materials. Bian et al. [25] studied the effects of alnico 5 (Fe48.00 Al15.58 Ni12.54 Co21.40 Cu2.48 ) on SmCo5 ribbons, while Qin et al. [26] studied the combined effects of Al, Ni, Co and Cu on microstructure and magnetic properties of Nd-Fe-B ribbons by varying the B content from 5.7 to 9.7 at.%. Contrary to alnico 5, which contains only low melting point elements (Al, Ni, Cu, Co, Fe), alnico 8 contains both high melting point elements like Ti and Nb and low melting point elements like Al, Ni, Co and Cu. It is expected that the addition of alnico 8 elements would be beneficial for improving the magnetic properties of Nd-Fe-B alloys. In the current manuscript, we have systematically investigated the synergetic effects of alnico 8 elements (Al = 7.0, Ni = 13.0, Co = 34.0, Ti = 6.0, Cu = 4.0, Nb = 1.0 and Fe = 35.0 wt.%) on the magnetic properties, phase transition temperatures (T C and T sr ) and microstructures of Nd-Fe-B ribbons. The distribution of these elements in the alloys is studied by Cs corrected STEM elemental mapping and line profiling.
2. Experimental procedure Precursor ingots with nominal composition Nd29.2 Fe69.8-x (alnico)x B (x = 0–20 wt.%) were fabricated by electric arc melting. Alnico contains typical composition of alnico 8 (Al = 7.0, Ni = 13.0, Co = 34.0, Ti = 6.0, Cu = 4.0, Nb = 1.0 and Fe = 35.0 wt.%). To obtain maximum homogeneity, elements with purity more than 99.95% were melted five to six times under pure Ar gas protection. The alloys containing 0, 5, 10, 15 and 20 wt.% alnico were marked as S0 , S1 , S2 , S3 and S4 , respectively. The content of ferromagnetic elements (Co + Ni + Fe) decreased from 69.8 wt.% for S0 to 68.9 wt.% for S1 , 68.0 wt.% for S2 , 67.1 wt.% for S3 and 66.2 wt.% for S4 . The ingots were melt-spun onto Cu crucible rotating at angular wheel speed varying between 16-24 ms−1 . The alloys fabricated with wheel speed 19 and 20 m/s exhibited good magnetic properties, therefore, alloys prepared with wheel speed 20 m/s were selected for detailed investigation. The crystal structures were analyzed using PANAlytical X ray diffractometer with Cu Kα radiation by obtaining the patterns from 20-90◦ in 2θ range. The XRD patterns were analyzed by Rietveld refinement [27] using Rietica and Jade software. Virgin samples of about 2.2 to 2.30 mm in width and 3.0 to 4.0 mm in length were selected for magnetic measurements. The phase transition temperatures and magnetic properties were measured by physical property measurement system, PPMS (Quantum Design, USA) by applying magnetic field of 0.1 T and 5 T respectively. The M-T curves were obtained in the temperature range 10-300 K for measuring spin reorientation temperature (T sr ), while the M-T curves were obtained in temperature range 300-750 K to find the T C . Transmission electron microscopy (TEM, Tecnai G2 F20) was used for microstructure analysis while elemental distributions in the grains and at grain boundaries of the ribbons were analyzed by applying Cs corrected STEM elemental mapping and line profiling. 3. Results and discussion The second quadrant demagnetization curves of S0 - S4 ribbons are depicted in Fig. 1. The standard alloy without addition of alnico (S0 ) depicts magnetic properties of Hcj = 15.73 kOe, Br = 8.08 kG and (BH)max = 13.97 MGOe which are comparable or better than the reported values for similar composition [11,28]. The Br of the alloys slightly improved from 8.08 kG to 8.18 kG for S1 ribbon, and then decreased with further addition of alnico to finally reached at 7.90 kG for S4 ribbon. While the saturation magnetization (μo Ms ) of the alloys, by applying 5.0 T field, decreased from 13.0 kG for the S0 alloy to 12.2 T, 12.1 kG, 12.1kG and 11.5 kG for S1 , S2 , S3 and S4 alloys, respectively. It is important to note that the Ms /Mr values increased from 0.62 for S0 alloy to 0.66, 0.68,
S.U. Rehman et al. / Physics Letters A 383 (2019) 125878
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Fig. 2. (a) The XRD patterns of S0 - S4 ribbons, (b) the Rietveld refinement of S2 ribbon. Table 1 Temperature coefficients of remanence (α ) and temperature coefficient of coercivity (β ) of S0 - S4 ribbons. Sample
α (%/K)
β (%/K)
S0 S1 S2 S3 S4
−0.13 −0.12 −0.12 −0.12 −0.12
−0.47 −0.42 −0.41 −0.39 −0.36
0.66 and 0.66 for S1 , S2 , S3 and S4 alloys, respectively, which indicates enhanced inter-grain interactions among the nano grains of alnico containing alloys. The Hcj increases from 15.73 kOe to 16.1 kOe for S1 and S2 ribbons and then decreases to 14.57 kOe for S4 ribbon. The (BH)max increases from 13.97 to 14.6 for S2 ribbon and then slightly decreases. However, the S4 ribbon depicts better (BH)max than S0 ribbon although both the Br and Hcj of S4 are lower than the Br and Hcj of S0 . The better (BH)max value of the S4 ribbon is due to the better squareness of the demagnetization curve, which in turn is attributed to the refined microstructure. The individual doping of alnico constituents initially increase the coercivity, and later the coercivity reduces with higher amount of doping [12,16–18,29]. Similar pattern is followed when the elements are co-doped. There may be two reasons for this decrease; (a) the ideal composition of 2:14:1 phase is altered, (b) some nonmagnetic phases form in the alloy ribbons which form nucleation centers for reverse domains. In order to investigate the effects of alnico elements on the thermal stability, the M-H curves were obtained at 400 K. The demagnetization curves obtained at 400 K are shown in Fig. 1(b). All the alnico containing alloys have better Hcj than the alloy without alnico. It is further noted that the Hcj of S3 and S4 ribbons is lower than S0 at 300 K, while the Hcj of both the ribbons is significantly higher than the S0 ribbon at 400 K. It clearly indicates that the thermal stability is greatly improved by alnico addition. The temperature coefficient of coercivity (β ) and temperature coefficient of remanence (α ) were calculated by using the equation [30]:
β=
α=
H c j (T 1 ) − H c j (T 0 ) H c j ( T 0 ) ( T 1− T 0 ) B r (T 1 ) − B r (T 0 ) B r ( T 0 ) ( T 1− T 0 )
× 100%
× 100%
(A) (B)
where T 0 = 300 K and T 1 = 400 K . The values of β decrease gradually from −0.47%/K for S0 ribbon to −0.36%/K for S4 ribbon, while α decreases slightly from −0.13%/K for S0 to −0.12%/K for S1 ∼ S4 ribbons. The α and β values are shown in Table 1.
Fig. 3. High temperature M-T curves of S0 - S4 ribbons.
The XRD patterns of S0 – S4 ribbons are shown in Fig. 2 (a). The Rietveld refinement was applied using Rietica software to analyze the XRD patterns. Fig. 2 (b) depicts the details of the refinement applied to S2 ribbon. It can be observed that the majority of characteristic peaks are coming from Nd2 Fe14 B (2:14:1) phase. The major peaks have been identified and indexed. There is a sharp high peak of α -Fe overlapping (006) peak of main phase at ∼44.6◦ . This sharp peak decreases significantly in S2 ribbon and completely disappears in S3 and S4 ribbons suggesting that the doping of alnico elements is beneficial for suppressing the formation of α -Fe soft phase. Furthermore, the peaks are broadened and shortened with the co-doping of alnico elements indicating the grain refinement behavior of alnico elements in Nd-Fe-B. The grain refinement is mainly attributed to the presence of small amount of Ti and Nb which have been reported as refractory elements in these types of alloys [17]. The average grain size of the ribbon alloys decreased from ∼92 nm for standard alloys (S0 ) to ∼50 nm, ∼36 nm, ∼31 nm and ∼27 nm for S1 , S2 , S3 and S4 ribbon alloys, respectively. The lattice constants of the main phase calculated by Rietveld refinement increase slightly from a = 8.7843 Å, c = 12.188 Å for S0 ribbon to a = 8.7985 Å, c = 12.202 Å for the S4 ribbon. For stable magnetic properties at high temperature, high T C is required in Nd-Fe-B type alloys. To investigate the effects of alnico addition on the T C of Nd-Fe-B alloys, the M-T curves were obtained in the temperature range 300–750 K. Fig. 3 depicts the M-T curves of S0 - S4 ribbons. The T C of the alloys increased from 580 K for S0 to 639 K for S4 ribbon. The elements, Co and Ni are known to increase the T C of Nd-Fe-B type alloys [20]. Co is known to completely substitute Fe ending in Nd2 Co14 B ternary compound increasing the T C significantly. For the low concentration range the T C of the Nd2 (Fe1-x Cox )14 B increases with a dramatic range
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Fig. 4. Low temperature M-T curves of S0 ∼ S4 ribbons, the inset shows dM/dT-T curves of S0 and S4 ribbons.
of 10 K per atomic percent substitution of Co for Fe [30]. Ni has also been reported to increase the T C of Nd-Fe-B type alloys, while Al, Nb, Ti, Cu reduce the T C of the ternary compound [17,30,31]. Therefore, the increase in T C is attributed to the Co and Ni atoms which replace the Fe atoms in 2:14:1 phase [30]. The spin reorientation temperature, T sr, is another phase transition in Nd-Fe-B type alloys which occurs due to the decrease of the first order anisotropy constant K1 with decreasing T, and tilting from c-axis to the cone configuration with cone angle of 30◦ at 4.2 K. Nd2 Fe14 B phase has a T sr = 135 K [32]. It is known that the T sr , being intrinsic property, changes when the composition of the 2:14:1 phase is altered [33]. The M-T curves showing T sr of the investigated alloys are shown in Fig. 4. The inset in Fig. 4 shows the dM/dT-T curve of S0 and S4 ribbons. The substitution of Fe by alnico elements has changed the T sr from 133 K for S0 to 104 K for S4 ribbon. Thus the addition of alnico elements has effectively increased the working temperature range at high temperature by increasing the T C from 580 K to 639 K (Fig. 3), and at low temperature by reducing the T sr from 133 K to 104 K (Fig. 4). Intergranular exchange interaction via the magnetic moments at the grain interfaces is described by Henkel plot, defined in mathematical equation as follows [34,35]:
δ M ( H ) = [M d ( H ) − M r (∞) + 2M r ( H )] / M r (∞)
(C)
where M d (H ) is the reduced magnetization and M r (H ) is the reduced remanent magnetization as a function of external field, and Mr (∞) is the remanent magnetization corresponding to the maximum applied external field. Accordingly to Stoner-Wohlfarth theory [35], non-interacting single and randomly oriented particles have reduced remanence of 0.5. All the investigated alloys in the present study, with or without alnico, have Mr (H) values higher than 0.75 indicating strong inter-grain interactions. Furthermore, it is reported that the exchange interactions become stronger by decreasing the grain size. When the grain size is below 40 nm, a very strong exchange coupling among the grains occurs [36–38]. With the addition of alnico alloys the intergranular exchange interactions enhanced as indicated by the positive peaks in Fig. 5. In addition to grain refinement, the enhancement in intergranular exchange interactions is attributed to the uniform distribution of Co in the alloys which helps to couple the grains. The non-uniform distribution of Ni which segregates near the Nd rich phase, as shown in Fig. 7, also helps to couple the nanocrystalline grains. The superior magnetic properties of nanocrystalline magnets are mainly attributed to the superior intrinsic magnetic properties like high spontaneous magnetization (Ms ) and strong magnetocrystalline anisotropy (HA ). The magnetic properties Hcj , Br , and (BH)max also depend on the microstructure of the alloys. The grain
Fig. 5. Henkel plots of S0 - S4 ribbons.
size reduction improves the Hcj due to reducing the stray field between the grains [39]. The TEM micrographs of the selected alloys are depicted in Fig. 6 (a), (b) and (c). The micrograph of S0 ribbon shown in Fig. 6(a) has an average grain size of ∼87 nm which decreased to ∼44 nm in Fig. 6(b) for S1 and to ∼31 nm in Fig. 6(c) for S2 ribbon. The grain size below ∼250 nm is reported to be single domain in Nd-Fe-B type alloys [28], so the nano grains of all the ribbons shown in Fig. 6 are in the purview of single domain. The optimization of microstructure of the grain boundaries and triple junctions are vital for improving the magnetic properties of Nd-Fe-B type magnets. The Hcj of melt spun ribbons depends on the Nd content of the Nd-Fe-B alloys, and the Nd rich phase forms at the grain boundaries in the composition range richer in Nd than the stoichiometry of the Nde2 Fe14 B phase [40]. The continuous thin layer of Nd rich phase surrounding the Nd-Fe-B grains is an important microstructural feature for high Hcj of Nd-Fe-B magnets [41]. By using quantitative 3D probe tomography, it was reported that the thin layer is ferromagnetic in sintered and HDDR type magnets [41]. The composition of grain boundaries and triple junctions cannot be ascertained by simple TEM. Therefore, Cs corrected STEM elemental mapping was applied to investigate the elemental distribution in the alloy. The results for selected alloy are depicted in Fig. 7 (a-i). It can be observed that Co, Nb and Ti distribute uniformly in the grain boundaries and inside the grains. While the distribution of Fe, Nd, Cu, Ni, and Al is heterogeneous. The NdCu rich phase is accumulated at the triple junctions of Nd-Fe-B grains as pointed by arrows in Fig. 7 (c) and (f), and it is reported that, in sintered magnets, this phase diffuses into the grain boundaries during post sinter annealing and improves the coercivity [42]. This type of phase is also reported to improve the Hcj of SmCo5 type alloys by forming intermetallic compounds along the grain boundaries which act as domain pinning sites [43]. The line profile drawn across the two adjacent grains is shown in Fig. 8. The content of ferromagnetic elements at the grain boundary is ∼71 at.% (58% Fe, 7% Ni, 6% Co). These results show that there is significant amount of ferromagnetic elements Fe, Co and Ni at the grain boundary and triple junction. With such high content of ferromagnetic elements at grain boundaries, it would not be possible to effectively decouple the grains. So, there should exist a strong exchange coupling between the grains. The coupling becomes stronger with the addition of alnico alloys as can be seen in the Henkel plots in Fig. 5. 3DAP and Lorenz microscopy observation also suggest that the grain boundaries of melt spun ribbons should be ferromagnetic [44]. However the grain boundaries are rich in Nd (∼24 at.%), and hence provide strong pinning force for improving the Hcj .
S.U. Rehman et al. / Physics Letters A 383 (2019) 125878
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Fig. 6. TEM micrographs of selected ribbons (a) S0 , (b) S1 , (c) S2 .
Fig. 7. STEM elemental mapping of S2 ribbon.
4. Conclusions The elements of alnico 8 were added to Nd-Fe-B alloys and the magnetic properties were investigated. The magnetic properties of the alloys were improved for optimized addition of alnico. The optimum magnetic properties of Hcj = 16.1 kOe, Br = 8.1 kG and (BH)max = 14.6 MGOe were obtained in S2 ribbon. The T C of the alloys increased from 580 K for S0 to 639 K for S4 ribbon, while the T sr decreased from 133 to 104 K. The TEM analysis showed that the grains are refined as a result of alnico addition. The Cs
corrected STEM elemental mapping and line profiling confirmed that Co, Nb and Ti distribute homogeneously while other elements distribute heterogeneously in the alloy. The line profile obtained by Cs corrected STEM showed that there is more than 70 at.% ferromagnetic elements (Fe, Ni, Co) in the grain boundary of the nano grains. The intergranular grains interactions enhanced by the addition of alnico elements which is attributed to the refined nano grains and favorable distribution of Fe, Co and Ni atoms. This work might be useful for the development of Dy/Tb free NdFeB alloys for high temperature applications with improved T C .
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Fig. 8. STEM line profile of S2 ribbon.
Acknowledgements Sajjad Ur Rehman dedicates this research work to the memory of his father. This work was financially supported by the National Natural Science Foundation of China (Grant # 51564037, 51661011 and 51671097) and outstanding PhD dissertation project of Jiangxi University of Science and Technology, Ganzhou, China (Grant no. 3105500032-Sajjad Ur Rehman). References [1] O. Gutfleisch, M.A. Willard, E. Brück, C.H. Chen, S.G. Sankar, J.P. Liu, Adv. Mater. 23 (7) (2011) 821. [2] S.U. Rehman, Q. Jiang, L. He, M. Ghazanfar, W. Lei, X. Hu, S.U. Awan, S. Ma, Z.C. Zhong, J. Magn. Magn. Mater. 466 (2018) 377. [3] J.M.D. Coey, IEEE Trans. Magn. 47 (12) (2011) 4671. [4] J.F. Gieras, M. Wing, Permanent Magnet Motors Technology – Design and Application, 2nd edition, Marcel Dekker, New York, USA, 2002. [5] A.K. Pathak, M. Khan, K.A. Cschneidner, R.W. McCallum, L. Zhou, K. Sun, K.W. Dennis, C. Zhou, F.E. Pinkerton, M.J. Kramer, V.K. Pecharsky, Adv. Mater. 27 (2015) 2663. [6] B.D. Cullity, C.D. Graham, Introduction to Magnetic Materials, John Wiley & Sons, 2011. [7] T.G. Woodcock, Y. Zhang, G. Hrkac, G. Ciuta, N.M. Dempsey, T. Schrefl, O. Gutfleische, D. Givord, Scr. Mater. 67 (2012) 536. [8] T.H. Kim, S.R. Lee, H.J. Kim, M.W. Lee, T.S. Jang, Acta Mater. 93 (2015) 95. [9] A.K. Pathak, M. Khan, K.A. Gschneidner, R.W. McCallum, L. Zhou, K. Sun, M.J. Kramer, V.K. Pecharsky, Acta Mater. 103 (2016) 211. [10] D. Sander, S. Valenzuela, D. Makarov, C. Marrows, E. Fullerton, P. Fischer, J. McCord, P. Vavassori, S. Mangin, P. Pirro, B. Hillebrands, A. Kent, T. Jungwirth, O. Gutfleisch, C.-G. Kim, A. Berger, J. Phys. D, Appl. Phys. 50 (2017) 363001. [11] Q.Z. Jiang, M.L. Zhong, Q.C. Quan, J.S. Zhang, Z.C. Zhong, J. Alloys Compd. 688 (2016) 363. [12] A.S. Kim, F.E. Camp, J. Appl. Phys. 79 (1996) 5035. [13] K. Morimoto, N. Katayama, H. Akamine, M. Itakura, J. Magn. Magn. Mater. 324 (2012) 3723. [14] T.H. Kim, S.R. Lee, K.H. Bae, H.J. Kim, M.W. Lee, T.S. Jang, Acta Mater. 133 (2017) 200. [15] T.T. Sasaki, T. Ohkubo, Y. Takada, T. Sato, A. Kato, Y. Kaneko, K. Hono, Scr. Mater. 113 (2016) 218. [16] S. Pandian, V. Chandrasekaran, G. Markandeyulu, K.J.L. Lyer, K.V.S. Rama Rao, J. Appl. Phys. 92 (2002) 6082. [17] Q. Quan, L. Zhang, Q.Z. Jiang, W. Lie, Q. Zeng, X. Hu, L. Wang, X. Yu, J. Du, G. Fu, R. Liu, Z.C. Zhong, J. Magn. Magn. Mater. 442 (2017) 377.
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Microstructure and magnetic properties of NdFeB alloys by co-doping alnico elements Sajjad Ur Rehman, Qingzheng Jiang, Lunke He, Houdong Xiong, Kai Liu, Lei Wang, Munan Yang, Zhenchen Zhong ∗ Jiangxi Key Laboratory for Rare Earth Magnetic Materials and Devices/Institute for Rare Earth Magnetic Materials and Devices, School of Materials Science and Engineering, Jiangxi University of Science and Technology, Ganzhou, China
a r t i c l e
i n f o
Article history: Received 29 May 2019 Received in revised form 6 July 2019 Accepted 2 August 2019 Available online 7 August 2019 Communicated by M. Wu Keywords: NdFeB Magnetic properties Exchange interactions Nanostructure Phase transition temperature
a b s t r a c t Elements of alnico 8 are added into Nd-Fe-B alloys fabricated by rapid solidification method. It is observed that the magnetic properties at high temperature improved by small addition of alnico elements. The Curie temperature of the alloys increased from 580 K for standard alloy to 639 K by 20% addition of alnico 8 elements. The spin reorientation temperature decreased from 133 K to 104 K. The TEM analysis showed that elements of alnico 8 refine the microstructure of Nd-Fe-B ribbons. The STEM analysis confirmed the heterogeneous distribution of Nd, Fe, Cu, Al, Ni and homogeneous distribution of Ti, Nb and Co. The boundaries of nano grains contain more than 70% ferromagnetic elements, ensuring strong inter-grain coupling among the grains. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Rare earth (RE) based permanent magnets (SmCo, NdFeB) have superior magnetic properties in terms of coercivity Hcj and energy density (BH)max , and are widely used in traction motors, wind energy turbines, biomedical instruments, NMR, MRI, CT scans and miniaturized electronic devices [1,2]. Magnetic materials play very critical role in advanced devices and various kinds of motors and power generators. They enable the conversion of electrical energy into mechanical energy and vice versa. The basic function of a permanent magnet is to provide external field in a magnetic circuit of an electric machine. The (BH)max represents the performance capability of a permanent magnet, i.e. it is the measure of the energy of a magnet in magnetic circuit [3]. The miniaturization of modern day instruments including motors and power generators has been possible due to the high energy density of Nd-Fe-B type magnets. To obtain high (BH)max in a permanent magnet, large Br must be maintained in absence of external magnetic field, and at the same time, it must resist demagnetization from the electric or magnetic circuits and thermal demagnetization from the operating temperature. Therefore, Hcj is of utmost importance for motor applications because demagnetization continuously occurs due to
*
Corresponding author. E-mail address: [email protected] (Z. Zhong).
https://doi.org/10.1016/j.physleta.2019.125878 0375-9601/© 2019 Elsevier B.V. All rights reserved.
demagnetization of electric and magnetic circuits, and from heating during operation [1]. Because of the above mentioned extrinsic properties, the RE permanent magnet brushless motors are much more efficient than induction motors because of (a) no electric energy is observed by the field excitation system, (b) higher torque and output power per unit volume, and (c) better dynamic performance [4]. The current consumption of high performance PMs are expected to grow because the demand for the aforementioned devices is expanding year by year. To meet the diverse applications, magnetic materials should posses high extrinsic properties such as Hcj , Br , and (BH)max which are satisfied by RE based permanent magnets for all room temperature applications [3]. In some applications, like wind turbine generators and electrical vehicles, the RE based PMs need to operate at around 160◦ C. At such high temperature the coercive field reduces to less than half a Tesla (0.5 T) which is too low to be used in electrical vehicles and wind turbines [5]. From technological point of view, it is therefore, necessary to develop Nd-Fe-B based magnets that could operate at high temperature with reasonably high Hcj and (BH)max . The RE elements such as Nd, Pr, Dy and Tb provide strong spin orbital coupling for high magnetocrystalline anisotropy (H A ) in REFe-B type magnets while 3d metals like Fe and Co are responsible for high spontaneous magnetization (M s ), and high Curie temperature (T C ) [6]. The H A of heavy RE elements is high; therefore, they ensure high Hcj in the ternary RE2 Fe14 B compounds. The partial substitution of Nd by Dy/Tb improves the Hcj and thermal stability
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Fig. 1. Demagnetization curves of S0 - S4 ribbons, (a) at 300 K, and (b) at 400 K.
and makes the alloys capable of operating at temperatures beyond 160◦ C [7]. However, the antiparallel coupling of the magnetic moments of heavy RE (Dy/Tb) and Fe moments in RE2 Fe14 B compound reduce the Br values resulting in a low (BH)max [8]. Another, rather commonly used, approach to improve the thermal stability of RE-Fe-B alloys is to increase the T C by partial Co substitution for Fe, but the magnetic properties of the ternary alloys decrease by Co substitution [9]. Furthermore, the substitution of resource critical heavy RE elements and Co increase the price of the magnets [10]. In addition to Co substitution for Fe and Dy/Tb substitution for Nd, different additive elements have been tried for improving the microstructure related properties, Hcj , Br and (BH)max . The additive elements are broadly divided into high melting point elements and low melting point elements [11]. The low melting point elements, such as Al, Ga, Cu and Sn are known to modify the gain boundary phase and increase the Hcj by magnetically decoupling the grains [12–16]. The high melting point elements Nb, Ti, W, Cr, V, Mo, Hf act as refractory elements, which refine the grains and suppress the formation of deleterious α -Fe phase in Nd-Fe-B type alloys. [16–22]. The high melting point elements are believed to form intergranular borides which in turn act as pinning centers for improving the Hcj [23]. Some reports even suggest that the high melting point elements are helpful for improving the thermal stability [24]. The single doping of Al, Co, Cu, Ti and Nb are known to improve the microstructure and magnetic properties of Nd-Fe-B type alloys as summarized above. However, the combined addition of these elements on intrinsic and extrinsic properties of Nd-Fe-B alloys is rarely studied. Some recent papers report the effect of adding multi elements on magnetic properties of RE based magnetic materials. Bian et al. [25] studied the effects of alnico 5 (Fe48.00 Al15.58 Ni12.54 Co21.40 Cu2.48 ) on SmCo5 ribbons, while Qin et al. [26] studied the combined effects of Al, Ni, Co and Cu on microstructure and magnetic properties of Nd-Fe-B ribbons by varying the B content from 5.7 to 9.7 at.%. Contrary to alnico 5, which contains only low melting point elements (Al, Ni, Cu, Co, Fe), alnico 8 contains both high melting point elements like Ti and Nb and low melting point elements like Al, Ni, Co and Cu. It is expected that the addition of alnico 8 elements would be beneficial for improving the magnetic properties of Nd-Fe-B alloys. In the current manuscript, we have systematically investigated the synergetic effects of alnico 8 elements (Al = 7.0, Ni = 13.0, Co = 34.0, Ti = 6.0, Cu = 4.0, Nb = 1.0 and Fe = 35.0 wt.%) on the magnetic properties, phase transition temperatures (T C and T sr ) and microstructures of Nd-Fe-B ribbons. The distribution of these elements in the alloys is studied by Cs corrected STEM elemental mapping and line profiling.
2. Experimental procedure Precursor ingots with nominal composition Nd29.2 Fe69.8-x (alnico)x B (x = 0–20 wt.%) were fabricated by electric arc melting. Alnico contains typical composition of alnico 8 (Al = 7.0, Ni = 13.0, Co = 34.0, Ti = 6.0, Cu = 4.0, Nb = 1.0 and Fe = 35.0 wt.%). To obtain maximum homogeneity, elements with purity more than 99.95% were melted five to six times under pure Ar gas protection. The alloys containing 0, 5, 10, 15 and 20 wt.% alnico were marked as S0 , S1 , S2 , S3 and S4 , respectively. The content of ferromagnetic elements (Co + Ni + Fe) decreased from 69.8 wt.% for S0 to 68.9 wt.% for S1 , 68.0 wt.% for S2 , 67.1 wt.% for S3 and 66.2 wt.% for S4 . The ingots were melt-spun onto Cu crucible rotating at angular wheel speed varying between 16-24 ms−1 . The alloys fabricated with wheel speed 19 and 20 m/s exhibited good magnetic properties, therefore, alloys prepared with wheel speed 20 m/s were selected for detailed investigation. The crystal structures were analyzed using PANAlytical X ray diffractometer with Cu Kα radiation by obtaining the patterns from 20-90◦ in 2θ range. The XRD patterns were analyzed by Rietveld refinement [27] using Rietica and Jade software. Virgin samples of about 2.2 to 2.30 mm in width and 3.0 to 4.0 mm in length were selected for magnetic measurements. The phase transition temperatures and magnetic properties were measured by physical property measurement system, PPMS (Quantum Design, USA) by applying magnetic field of 0.1 T and 5 T respectively. The M-T curves were obtained in the temperature range 10-300 K for measuring spin reorientation temperature (T sr ), while the M-T curves were obtained in temperature range 300-750 K to find the T C . Transmission electron microscopy (TEM, Tecnai G2 F20) was used for microstructure analysis while elemental distributions in the grains and at grain boundaries of the ribbons were analyzed by applying Cs corrected STEM elemental mapping and line profiling. 3. Results and discussion The second quadrant demagnetization curves of S0 - S4 ribbons are depicted in Fig. 1. The standard alloy without addition of alnico (S0 ) depicts magnetic properties of Hcj = 15.73 kOe, Br = 8.08 kG and (BH)max = 13.97 MGOe which are comparable or better than the reported values for similar composition [11,28]. The Br of the alloys slightly improved from 8.08 kG to 8.18 kG for S1 ribbon, and then decreased with further addition of alnico to finally reached at 7.90 kG for S4 ribbon. While the saturation magnetization (μo Ms ) of the alloys, by applying 5.0 T field, decreased from 13.0 kG for the S0 alloy to 12.2 T, 12.1 kG, 12.1kG and 11.5 kG for S1 , S2 , S3 and S4 alloys, respectively. It is important to note that the Ms /Mr values increased from 0.62 for S0 alloy to 0.66, 0.68,
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Fig. 2. (a) The XRD patterns of S0 - S4 ribbons, (b) the Rietveld refinement of S2 ribbon. Table 1 Temperature coefficients of remanence (α ) and temperature coefficient of coercivity (β ) of S0 - S4 ribbons. Sample
α (%/K)
β (%/K)
S0 S1 S2 S3 S4
−0.13 −0.12 −0.12 −0.12 −0.12
−0.47 −0.42 −0.41 −0.39 −0.36
0.66 and 0.66 for S1 , S2 , S3 and S4 alloys, respectively, which indicates enhanced inter-grain interactions among the nano grains of alnico containing alloys. The Hcj increases from 15.73 kOe to 16.1 kOe for S1 and S2 ribbons and then decreases to 14.57 kOe for S4 ribbon. The (BH)max increases from 13.97 to 14.6 for S2 ribbon and then slightly decreases. However, the S4 ribbon depicts better (BH)max than S0 ribbon although both the Br and Hcj of S4 are lower than the Br and Hcj of S0 . The better (BH)max value of the S4 ribbon is due to the better squareness of the demagnetization curve, which in turn is attributed to the refined microstructure. The individual doping of alnico constituents initially increase the coercivity, and later the coercivity reduces with higher amount of doping [12,16–18,29]. Similar pattern is followed when the elements are co-doped. There may be two reasons for this decrease; (a) the ideal composition of 2:14:1 phase is altered, (b) some nonmagnetic phases form in the alloy ribbons which form nucleation centers for reverse domains. In order to investigate the effects of alnico elements on the thermal stability, the M-H curves were obtained at 400 K. The demagnetization curves obtained at 400 K are shown in Fig. 1(b). All the alnico containing alloys have better Hcj than the alloy without alnico. It is further noted that the Hcj of S3 and S4 ribbons is lower than S0 at 300 K, while the Hcj of both the ribbons is significantly higher than the S0 ribbon at 400 K. It clearly indicates that the thermal stability is greatly improved by alnico addition. The temperature coefficient of coercivity (β ) and temperature coefficient of remanence (α ) were calculated by using the equation [30]:
β=
α=
H c j (T 1 ) − H c j (T 0 ) H c j ( T 0 ) ( T 1− T 0 ) B r (T 1 ) − B r (T 0 ) B r ( T 0 ) ( T 1− T 0 )
× 100%
× 100%
(A) (B)
where T 0 = 300 K and T 1 = 400 K . The values of β decrease gradually from −0.47%/K for S0 ribbon to −0.36%/K for S4 ribbon, while α decreases slightly from −0.13%/K for S0 to −0.12%/K for S1 ∼ S4 ribbons. The α and β values are shown in Table 1.
Fig. 3. High temperature M-T curves of S0 - S4 ribbons.
The XRD patterns of S0 – S4 ribbons are shown in Fig. 2 (a). The Rietveld refinement was applied using Rietica software to analyze the XRD patterns. Fig. 2 (b) depicts the details of the refinement applied to S2 ribbon. It can be observed that the majority of characteristic peaks are coming from Nd2 Fe14 B (2:14:1) phase. The major peaks have been identified and indexed. There is a sharp high peak of α -Fe overlapping (006) peak of main phase at ∼44.6◦ . This sharp peak decreases significantly in S2 ribbon and completely disappears in S3 and S4 ribbons suggesting that the doping of alnico elements is beneficial for suppressing the formation of α -Fe soft phase. Furthermore, the peaks are broadened and shortened with the co-doping of alnico elements indicating the grain refinement behavior of alnico elements in Nd-Fe-B. The grain refinement is mainly attributed to the presence of small amount of Ti and Nb which have been reported as refractory elements in these types of alloys [17]. The average grain size of the ribbon alloys decreased from ∼92 nm for standard alloys (S0 ) to ∼50 nm, ∼36 nm, ∼31 nm and ∼27 nm for S1 , S2 , S3 and S4 ribbon alloys, respectively. The lattice constants of the main phase calculated by Rietveld refinement increase slightly from a = 8.7843 Å, c = 12.188 Å for S0 ribbon to a = 8.7985 Å, c = 12.202 Å for the S4 ribbon. For stable magnetic properties at high temperature, high T C is required in Nd-Fe-B type alloys. To investigate the effects of alnico addition on the T C of Nd-Fe-B alloys, the M-T curves were obtained in the temperature range 300–750 K. Fig. 3 depicts the M-T curves of S0 - S4 ribbons. The T C of the alloys increased from 580 K for S0 to 639 K for S4 ribbon. The elements, Co and Ni are known to increase the T C of Nd-Fe-B type alloys [20]. Co is known to completely substitute Fe ending in Nd2 Co14 B ternary compound increasing the T C significantly. For the low concentration range the T C of the Nd2 (Fe1-x Cox )14 B increases with a dramatic range
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Fig. 4. Low temperature M-T curves of S0 ∼ S4 ribbons, the inset shows dM/dT-T curves of S0 and S4 ribbons.
of 10 K per atomic percent substitution of Co for Fe [30]. Ni has also been reported to increase the T C of Nd-Fe-B type alloys, while Al, Nb, Ti, Cu reduce the T C of the ternary compound [17,30,31]. Therefore, the increase in T C is attributed to the Co and Ni atoms which replace the Fe atoms in 2:14:1 phase [30]. The spin reorientation temperature, T sr, is another phase transition in Nd-Fe-B type alloys which occurs due to the decrease of the first order anisotropy constant K1 with decreasing T, and tilting from c-axis to the cone configuration with cone angle of 30◦ at 4.2 K. Nd2 Fe14 B phase has a T sr = 135 K [32]. It is known that the T sr , being intrinsic property, changes when the composition of the 2:14:1 phase is altered [33]. The M-T curves showing T sr of the investigated alloys are shown in Fig. 4. The inset in Fig. 4 shows the dM/dT-T curve of S0 and S4 ribbons. The substitution of Fe by alnico elements has changed the T sr from 133 K for S0 to 104 K for S4 ribbon. Thus the addition of alnico elements has effectively increased the working temperature range at high temperature by increasing the T C from 580 K to 639 K (Fig. 3), and at low temperature by reducing the T sr from 133 K to 104 K (Fig. 4). Intergranular exchange interaction via the magnetic moments at the grain interfaces is described by Henkel plot, defined in mathematical equation as follows [34,35]:
δ M ( H ) = [M d ( H ) − M r (∞) + 2M r ( H )] / M r (∞)
(C)
where M d (H ) is the reduced magnetization and M r (H ) is the reduced remanent magnetization as a function of external field, and Mr (∞) is the remanent magnetization corresponding to the maximum applied external field. Accordingly to Stoner-Wohlfarth theory [35], non-interacting single and randomly oriented particles have reduced remanence of 0.5. All the investigated alloys in the present study, with or without alnico, have Mr (H) values higher than 0.75 indicating strong inter-grain interactions. Furthermore, it is reported that the exchange interactions become stronger by decreasing the grain size. When the grain size is below 40 nm, a very strong exchange coupling among the grains occurs [36–38]. With the addition of alnico alloys the intergranular exchange interactions enhanced as indicated by the positive peaks in Fig. 5. In addition to grain refinement, the enhancement in intergranular exchange interactions is attributed to the uniform distribution of Co in the alloys which helps to couple the grains. The non-uniform distribution of Ni which segregates near the Nd rich phase, as shown in Fig. 7, also helps to couple the nanocrystalline grains. The superior magnetic properties of nanocrystalline magnets are mainly attributed to the superior intrinsic magnetic properties like high spontaneous magnetization (Ms ) and strong magnetocrystalline anisotropy (HA ). The magnetic properties Hcj , Br , and (BH)max also depend on the microstructure of the alloys. The grain
Fig. 5. Henkel plots of S0 - S4 ribbons.
size reduction improves the Hcj due to reducing the stray field between the grains [39]. The TEM micrographs of the selected alloys are depicted in Fig. 6 (a), (b) and (c). The micrograph of S0 ribbon shown in Fig. 6(a) has an average grain size of ∼87 nm which decreased to ∼44 nm in Fig. 6(b) for S1 and to ∼31 nm in Fig. 6(c) for S2 ribbon. The grain size below ∼250 nm is reported to be single domain in Nd-Fe-B type alloys [28], so the nano grains of all the ribbons shown in Fig. 6 are in the purview of single domain. The optimization of microstructure of the grain boundaries and triple junctions are vital for improving the magnetic properties of Nd-Fe-B type magnets. The Hcj of melt spun ribbons depends on the Nd content of the Nd-Fe-B alloys, and the Nd rich phase forms at the grain boundaries in the composition range richer in Nd than the stoichiometry of the Nde2 Fe14 B phase [40]. The continuous thin layer of Nd rich phase surrounding the Nd-Fe-B grains is an important microstructural feature for high Hcj of Nd-Fe-B magnets [41]. By using quantitative 3D probe tomography, it was reported that the thin layer is ferromagnetic in sintered and HDDR type magnets [41]. The composition of grain boundaries and triple junctions cannot be ascertained by simple TEM. Therefore, Cs corrected STEM elemental mapping was applied to investigate the elemental distribution in the alloy. The results for selected alloy are depicted in Fig. 7 (a-i). It can be observed that Co, Nb and Ti distribute uniformly in the grain boundaries and inside the grains. While the distribution of Fe, Nd, Cu, Ni, and Al is heterogeneous. The NdCu rich phase is accumulated at the triple junctions of Nd-Fe-B grains as pointed by arrows in Fig. 7 (c) and (f), and it is reported that, in sintered magnets, this phase diffuses into the grain boundaries during post sinter annealing and improves the coercivity [42]. This type of phase is also reported to improve the Hcj of SmCo5 type alloys by forming intermetallic compounds along the grain boundaries which act as domain pinning sites [43]. The line profile drawn across the two adjacent grains is shown in Fig. 8. The content of ferromagnetic elements at the grain boundary is ∼71 at.% (58% Fe, 7% Ni, 6% Co). These results show that there is significant amount of ferromagnetic elements Fe, Co and Ni at the grain boundary and triple junction. With such high content of ferromagnetic elements at grain boundaries, it would not be possible to effectively decouple the grains. So, there should exist a strong exchange coupling between the grains. The coupling becomes stronger with the addition of alnico alloys as can be seen in the Henkel plots in Fig. 5. 3DAP and Lorenz microscopy observation also suggest that the grain boundaries of melt spun ribbons should be ferromagnetic [44]. However the grain boundaries are rich in Nd (∼24 at.%), and hence provide strong pinning force for improving the Hcj .
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Fig. 6. TEM micrographs of selected ribbons (a) S0 , (b) S1 , (c) S2 .
Fig. 7. STEM elemental mapping of S2 ribbon.
4. Conclusions The elements of alnico 8 were added to Nd-Fe-B alloys and the magnetic properties were investigated. The magnetic properties of the alloys were improved for optimized addition of alnico. The optimum magnetic properties of Hcj = 16.1 kOe, Br = 8.1 kG and (BH)max = 14.6 MGOe were obtained in S2 ribbon. The T C of the alloys increased from 580 K for S0 to 639 K for S4 ribbon, while the T sr decreased from 133 to 104 K. The TEM analysis showed that the grains are refined as a result of alnico addition. The Cs
corrected STEM elemental mapping and line profiling confirmed that Co, Nb and Ti distribute homogeneously while other elements distribute heterogeneously in the alloy. The line profile obtained by Cs corrected STEM showed that there is more than 70 at.% ferromagnetic elements (Fe, Ni, Co) in the grain boundary of the nano grains. The intergranular grains interactions enhanced by the addition of alnico elements which is attributed to the refined nano grains and favorable distribution of Fe, Co and Ni atoms. This work might be useful for the development of Dy/Tb free NdFeB alloys for high temperature applications with improved T C .
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Fig. 8. STEM line profile of S2 ribbon.
Acknowledgements Sajjad Ur Rehman dedicates this research work to the memory of his father. This work was financially supported by the National Natural Science Foundation of China (Grant # 51564037, 51661011 and 51671097) and outstanding PhD dissertation project of Jiangxi University of Science and Technology, Ganzhou, China (Grant no. 3105500032-Sajjad Ur Rehman). References [1] O. Gutfleisch, M.A. Willard, E. Brück, C.H. Chen, S.G. Sankar, J.P. Liu, Adv. Mater. 23 (7) (2011) 821. [2] S.U. Rehman, Q. Jiang, L. He, M. Ghazanfar, W. Lei, X. Hu, S.U. Awan, S. Ma, Z.C. Zhong, J. Magn. Magn. Mater. 466 (2018) 377. [3] J.M.D. Coey, IEEE Trans. Magn. 47 (12) (2011) 4671. [4] J.F. Gieras, M. Wing, Permanent Magnet Motors Technology – Design and Application, 2nd edition, Marcel Dekker, New York, USA, 2002. [5] A.K. Pathak, M. Khan, K.A. Cschneidner, R.W. McCallum, L. Zhou, K. Sun, K.W. Dennis, C. Zhou, F.E. Pinkerton, M.J. Kramer, V.K. Pecharsky, Adv. Mater. 27 (2015) 2663. [6] B.D. Cullity, C.D. Graham, Introduction to Magnetic Materials, John Wiley & Sons, 2011. [7] T.G. Woodcock, Y. Zhang, G. Hrkac, G. Ciuta, N.M. Dempsey, T. Schrefl, O. Gutfleische, D. Givord, Scr. Mater. 67 (2012) 536. [8] T.H. Kim, S.R. Lee, H.J. Kim, M.W. Lee, T.S. Jang, Acta Mater. 93 (2015) 95. [9] A.K. Pathak, M. Khan, K.A. Gschneidner, R.W. McCallum, L. Zhou, K. Sun, M.J. Kramer, V.K. Pecharsky, Acta Mater. 103 (2016) 211. [10] D. Sander, S. Valenzuela, D. Makarov, C. Marrows, E. Fullerton, P. Fischer, J. McCord, P. Vavassori, S. Mangin, P. Pirro, B. Hillebrands, A. Kent, T. Jungwirth, O. Gutfleisch, C.-G. Kim, A. Berger, J. Phys. D, Appl. Phys. 50 (2017) 363001. [11] Q.Z. Jiang, M.L. Zhong, Q.C. Quan, J.S. Zhang, Z.C. Zhong, J. Alloys Compd. 688 (2016) 363. [12] A.S. Kim, F.E. Camp, J. Appl. Phys. 79 (1996) 5035. [13] K. Morimoto, N. Katayama, H. Akamine, M. Itakura, J. Magn. Magn. Mater. 324 (2012) 3723. [14] T.H. Kim, S.R. Lee, K.H. Bae, H.J. Kim, M.W. Lee, T.S. Jang, Acta Mater. 133 (2017) 200. [15] T.T. Sasaki, T. Ohkubo, Y. Takada, T. Sato, A. Kato, Y. Kaneko, K. Hono, Scr. Mater. 113 (2016) 218. [16] S. Pandian, V. Chandrasekaran, G. Markandeyulu, K.J.L. Lyer, K.V.S. Rama Rao, J. Appl. Phys. 92 (2002) 6082. [17] Q. Quan, L. Zhang, Q.Z. Jiang, W. Lie, Q. Zeng, X. Hu, L. Wang, X. Yu, J. Du, G. Fu, R. Liu, Z.C. Zhong, J. Magn. Magn. Mater. 442 (2017) 377.
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