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α-Fe alloys
Journal of Alloys and Compounds 459 (2008) 41–44
Effect of pulsed magnetic field treatment on the magnetic properties for nanocomposite Nd2Fe14B/␣-Fe alloys Shiyan Zhang, Hui Xu ∗ , Xiaohua Tan, Jiansen Ni, Xueling Hou, Yuanda Dong Institute of Material Science, Shanghai University, Shanghai 200072, PR China Received 27 March 2007; received in revised form 29 April 2007; accepted 30 April 2007 Available online 3 May 2007
Abstract Nanocomposite alloys with the compositions of Nd8.5 Fe77.6−x Co5 Zr2.7 Gax B6.2 (x = 0, 0.3, 0.6, 0.8, 1.0) were prepared by melt spinning. It was found that a small amount of 0.6 at.% Ga addition is beneficial to improve the hard magnetic properties of the alloy. The melt-spun Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 ribbons were annealed in a pulsed magnetic field. The results revealed that the remanence Jr and the maximum energy product (BH)max were obviously improved when annealed below the Curie temperature of the alloy. The best magnetic properties of Jr = 0.975 T, 3 ◦ i Hc = 582 kA/m, and (BH)max = 131 kJ/m were obtained for the ribbons annealed at 300 C in a pulsed magnetic field. The (BH)max was enhanced by 23.8% compared with that of the ribbons without magnetic annealing. It provides a new way to improve the magnetic properties of permanent magnets produced by pulsed magnetic annealing at temperatures below the Curie point of the alloys. © 2007 Elsevier B.V. All rights reserved. Keywords: Permanent magnets; Nanostructured materials; High magnetic field; Magnetic properties
1. Introduction Recently, nanocomposite magnetic materials consisting of hard and soft magnetic phases have been extensively investigated for permanent magnet development due to their enhanced remanence, potentially high energy product and low rare earth cost [1–3]. However, the maximum energy product of nanocomposite permanent magnets obtained experimentally up to now is significantly lower than that predicted by theory [3–6]. The discrepancy is primarily attributed to difficulties in obtaining the optimum microstructures employed for the theoretical models. A uniform microstructure and a well-coupled interface between soft and hard phases are essential conditions for well-performing nanostructured materials [7]. Since the enhanced remanence in nanocomposite magnets is obtained by exchange coupling between hard and soft magnetic grains, the grain boundaries between magnetically hard and soft phases play an important role in the magnetic hardening mech-
∗
Corresponding author. Tel.: +86 21 56337887; fax: +86 21 56335353. E-mail address: [email protected] (H. Xu).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.299
anism and exchange coupling. The addition of other elements is an effective way to explore the role of the intergranular phase. It was reported that In and Ga, both elements with low melting points, could enhance the wet-ability of the grain boundary phase [8,9]. Magnetic annealing is another effective method to optimize the microstructure and enhance the magnetic properties of permanent alloys. Yang and co-workers found that the magnetic field treatment reduced the grain size and induced a uniform distribution of magnetic phases for nanocomposite Nd2 Fe14 B/Fe3 B magnets [10–12]. Chiriac et al. reported that magnetic annealing enhanced the coercivity and increased the remanence ratio by 13% for Nd8 Fe77 Co5 B6 CuNb3 alloy [13,14]. Kato et al. found obvious enhancements in i Hc by high-magnetic field annealing in sintered Nd–Fe–B magnets, which is attributed to a fieldinduced change in interface matching between Nd2 Fe14 B and Nd-rich intergranular phases [15,16]. Crystallographic alignment and magnetically anisotropic behavior were induced by magnetic annealing in nanocomposite (NdPrDy)2 Fe14 B/␣-Fe magnets [17]. Ji et al. demonstrated that the magnetic heattreatment near the Curie point of the Nd2 Fe14 B phase resulted in a significant improvement in the coercivity and remanence ratio
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S. Zhang et al. / Journal of Alloys and Compounds 459 (2008) 41–44
for nanocomposite Nd10 Fe83 B6 In alloy [8,18]. The reason is due to the presence of indium atoms which changed the boundary features and strengthened the exchange coupling between the adjacent grains. However, the previous studies were focused on the effect of the static magnetic field in a low magnitude range. Compared with the static magnetic field the pulsed magnetic field has much more advantages, such as higher magnitude with over 5 T and lower cost. However, few works applying a pulsed magnetic field to permanent materials were carried out up to now. In present work, the effect of Ga addition on the magnetic properties of melt-spun Nd8.5 Fe77.6−x Co5 Zr2.7 Gax B6.2 alloys was investigated on the basis of our previous studies [19–21]. Especially, the magnetic properties for the given alloy annealed near the Curie point using a pulsed magnetic field were investigated. The influence of magnetic annealing on magnetic properties and exchange coupling for nanocomposite Nd2 Fe14 B/␣-Fe alloys will be discussed. 2. Experimental Ingots with nominal composition Nd8.5 Fe77.6−x Co5 Zr2.7 Gax B6.2 (x = 0, 0.3, 0.6, 0.8, 1.0) were prepared by arc-melting the elements Nd, Fe, Co, Zr, Ga and Fe–B alloy under a high-purity Ar atmosphere. The ingots were remelted four times to ensure homogeneity. Small pieces of the arc-melted buttons were melt-spun using a single roller Cu wheel at a wheel speed of 15 m/s. The asspun ribbons were annealed in the temperature range of 200–600 ◦ C for 10 min with and without an external pulsed magnetic field of 5 T. The external applied field was fixed along the direction of the ribbon’s long axis and the frequency was set to five times per min. X-ray diffraction scans were performed for phase identification on melt-spun and annealed samples using a D/max-2550 diffractometer with Cu K␣ radiation. Magnetic properties were measured by vibrating sample magnetometer (VSM) with a maximum applied field of 1.8 T. Curie temperature of the alloy was measured by thermo-gravimetric apparatus (TG).
Fig. 1. Magnetic properties as a function of Ga content for melt-spun Nd8.5 Fe77.6−x Co5 Zr2.7 Gax B6.2 (x = 0–1.0) ribbons.
3. Results and discussions
Based on the above results, the Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 alloy was selected to further study. Fig. 3 illustrates the TG curve of the Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 ribbon at a heating rate of 20 ◦ C/min. It can be found that the Curie temperature of the Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 alloy is 322 ◦ C. The melt-spun Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 ribbons were annealed in the temperature range of 200–600 ◦ C for 10 min
Fig. 1 illustrates the magnetic properties as a function of Ga content for melt-spun Nd8.5 Fe77.6−x Co5 Zr2.7 Gax B6.2 (x = 0, 0.3, 0.6, 0.8, 1.0) ribbons. It can be seen that the intrinsic coercivity increases gradually with increasing Ga content. The remanence has a maximum value of 0.890 T at x = 0.6 and then decreases with further Ga addition. The optimal magnetic properties of Jr = 0.890 T, i Hc = 482 kA/m, and (BH)max = 88 kJ/m3 were obtained for the sample with 0.6 at.% Ga addition. The demagnetization curves of melt-spun ribbons with various Ga content are shown in Fig. 2. All samples show a smooth demagnetization curve and exhibit single-phase hard magnetic behavior. Furthermore the Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 sample exhibits obvious remanence enhancement, indicating a finer and uniform distributed grain size. Thus appropriate Ga (0.6 at.%) addition is beneficial to improve the magnetic properties of meltspun Nd8.5 Fe77.6 Co5 Zr2.7 B6.2 alloy. This is consistent with the results reported by Ping et al. [22] and Harland and Davies [23]. The reason is that Ga atoms could enrich on the boundaries of adjacent grains and inhibit the grain growth. Excessive amount of Ga addition may deteriorate the remanence due to non-magnetic dilution by non-magnetic Ga atoms.
Fig. 2. Demagnetization curves for melt-spun Nd8.5 Fe77.6−x Co5 Zr2.7 Gax B6.2 (x = 0, 0.6, 1.0) ribbons at room temperature.
S. Zhang et al. / Journal of Alloys and Compounds 459 (2008) 41–44
Fig. 3. Thermogravimetry curve of the Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 alloy at a heating rate of 20 ◦ C/min.
with a pulsed magnetic field of 5 T. The magnetic properties as a function of temperature are shown in Fig. 4. For comparison, the magnetic properties for the sample annealed without an external field are also presented. It can be seen that the sample annealed without magnetic field exhibits a slight increase in i Hc with increasing annealing temperature. The Jr and (BH)max increase initially with increasing annealing temperature and then decrease when the temperature exceeds 400 ◦ C. It is known that quenched ribbons are prone to form crystal lattice stress and aberrance stress, which probably suppress the magnetic moments alignment. By maintaining the ribbons to a temper-
Fig. 4. Magnetic properties of Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 ribbons annealed at various temperature for 10 min with and without a magnetic field of 5 T.
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ature below 400 ◦ C, the internal stresses existing within the samples are relieved, which results in an increased exchange coupling interaction between the magnetically phases and corresponding improvement of coercivity and remanence. Grains become coarse and uneven, especially for the ␣-Fe phase, which deteriorates magnetic properties when annealing temperature exceeds 400 ◦ C. However, when the samples were annealed in a pulsed magnetic field, the Jr increases significantly, up to a maximum of 0.975 T at 300 ◦ C, and then decreases with further increasing annealing temperature. The (BH)max shows a similar behavior with that of remanence. The best magnetic properties of Jr = 0.975 T, i Hc = 582 kA/m, and (BH)max = 131 kJ/m3 can be obtained for Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 ribbon annealed with a magnetic field. It is noted that the magnetic properties for the samples annealed around 300 ◦ C with a pulsed field of 5 T are larger than that of the samples annealed without a magnetic field, especially for Jr and (BH)max . The (BH)max is enhanced by 23.8% compared with that of the sample without magnetic annealing. The demagnetization curves of Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 samples annealed with and without an external field at 300 ◦ C are illustrated in Fig. 5. It can be seen that the magnetic annealing could improve not only the remanence and coercivity, but also the squareness of the hysteresis loop. It is suggested that exchange coupling between the hard and soft magnetic phases could be enhanced by the magnetic field annealing. This phenomena provides a new way to improve the magnetic properties of permanent materials. It is interesting to note that a maximum of Jr and (BH)max appear at an annealing temperature of around 300 ◦ C which is near the Curie temperature of the alloy (Tc = 322 ◦ C). Taking into account that when the annealing temperature is lower than the Curie temperature of the alloy, the Nd2 Fe14 B phase is ferromagnetic, and the magnetic field may promote the exchange
Fig. 5. Demagnetization curves for Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 samples annealed with and without a magnetic field at 300 ◦ C.
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S. Zhang et al. / Journal of Alloys and Compounds 459 (2008) 41–44
4. Conclusion An appropriate addition of Ga can increase the magnetic properties of melt-spun Nd8.5 Fe77.6−x Co5 Zr2.7 Gax B6.2 alloys due to the grain refinement. After annealing in a pulsed magnetic field at temperatures below the Curie point of the alloys, the magnetic properties of nanocomposite Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 alloy were clearly improved. It provides a new way to improve the magnetic properties of permanent materials produced by pulsed magnetic annealing at temperatures below the Curie point of the alloys. Acknowledgement This work was sponsored by the National Natural Science Foundation of China (Grant No. 50671059). References Fig. 6. XRD patterns of Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 samples annealed with and without a magnetic field at 300 ◦ C.
coupling between magnetic phases. On the other hand, the melting of Ga-rich phases at the grain boundaries may be favorable for the grain rotation and phase-distribution adjustment when the alloy annealed at 300 ◦ C. The X-ray diffraction patterns for Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 samples with and without a magnetic field at 300 ◦ C are shown in Fig. 6. No obvious (h k 0) diffraction peaks of Nd2 Fe14 B phase were found for magnetic annealing sample, which indicated that no strong (0 0 l) c-axis texture in the long direction of the sample was induced by the magnetic field. From the selected inset patterns, it is found that the relative intensities of some diffraction planes of the Nd2 Fe14 B phase, such as (2 2 2), (3 1 1) and (2 0 4), have some changes. This demonstrated that the complete orientation of Nd2 Fe14 B phase along the long axis had not taken place. It was presumed that the independent magnetization reversal of the soft phase was inhibited by the external field, which improves the shape of the second quadrant demagnetization curve and strengthens the exchange coupling interaction between magnetically hard and soft phases. The obvious improvement of Jr may be attributed to the alignment of the Nd2 Fe14 B crystallites along the external applied magnetic field, and correspondingly the (BH)max reached to a maximum value after magnetic annealing at 300 ◦ C. In other words, pulsed magnetic annealing at temperatures below the Curie point of the alloys could effectively improve the magnetic properties of Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 alloy.
[1] R. Skomski, J.M.D. Coey, Phys. Rev. B 48 (1993) 15812. [2] R. Coehoorn, D.B. de Mooij, J.P.W.B. Duchateau, K.H.J. Buschow, J. Phys. C 49 (1988) 669. [3] S. Yang, X.P. Song, B.X. Gu, Y.W. Du, J. Alloys Compd. 394 (2005) 1. [4] M. Zhang, Z.D. Zhang, X.K. Sun, W. Liu, D.Y. Geng, X.G. Zhao, X.M. Jin, J. Alloys Compd. 364 (2004) 238. [5] H.A. Davies, Z.C. Wang, Mater. Sci. Eng. A 375–377 (2004) 78. [6] Y.Q. Wu, D.H. Ping, K. Hono, M. Hamano, A. Inoue, J. Appl. Phys. 87 (2000) 8658. [7] T. Schrefl, J. Fidler, H. Kronmuller, Phys. Rev. B 49 (1994) 6100. [8] Q.G. Ji, B.X. Gu, S.L. Tang, Y.W. Du, J. Magn. Magn. Mater. 257 (2003) 146. [9] B. Grieb, C. Pithan, T.Eh. Henig, G. Petzow, J. Appl. Phys. 70 (1991) 6354. [10] C.J. Yang, E.B. Park, J. Magn. Magn. Mater. 168 (1997) 278. [11] C.J. Yang, E.B. Park, J. Magn. Magn. Mater. 166 (1997) 243. [12] Y. Gao, J. Zhu, C. Yang, E.B. Park, J. Magn. Magn. Mater. 186 (1998) 97. [13] H. Chiriac, M. Marinescu, J. Appl. Phys. 83 (1998) 6628. [14] H. Chiriac, M. Marinescu, K.H.J. Buschow, F.R. de Boer, E. Bruck, J. Magn. Magn. Mater. 202 (1999) 22. [15] H. Kato, T. Miyazaki, M. Sagawa, K. Koyama, Proceedings of the 18th Workshop on High Performance Magnets & their Applications, Annecy, France, 2004, p. 84. [16] H. Kato, T. Miyazaki, M. Sagawa, K. Koyama, Appl. Phys. Lett. 84 (2004) 4230. [17] B.Z. Cui, M.Q. Huang, R.H. Yu, A. Kramp, J. Dent, D.D. Miles, S. Liu, J. Appl. Phys. 93 (2003) 8128. [18] Q.G. Ji, B.X. Gu, Y.W. Du, J. Appl. Phys. 88 (2001) 7230. [19] X.H. Tan, H. Xu, Q. Li, N.N. Qi, J.S. Ni, Y.D. Dong, J. Rare Earths 23 (2005) 24. [20] S.Y. Zhang, H. Xu, J.S. Ni, H.L. Wang, X.L. Hou, Y.D. Dong, Phys. B 393 (2007) 153. [21] S.Y. Zhang, H. Xu, J.S. Ni, H.L. Wang, Q. Bai, Y.D. Dong, J. Rare Earths 25 (2007) 74. [22] D.H. Ping, K. Hono, S. Hirosawa, J. Appl. Phys. 83 (1998) 7769. [23] C.L. Harland, H.A. Davies, J. Alloys Compd. 281 (1998) 37.
Effect of pulsed magnetic field treatment on the magnetic properties for nanocomposite Nd2Fe14B/␣-Fe alloys Shiyan Zhang, Hui Xu ∗ , Xiaohua Tan, Jiansen Ni, Xueling Hou, Yuanda Dong Institute of Material Science, Shanghai University, Shanghai 200072, PR China Received 27 March 2007; received in revised form 29 April 2007; accepted 30 April 2007 Available online 3 May 2007
Abstract Nanocomposite alloys with the compositions of Nd8.5 Fe77.6−x Co5 Zr2.7 Gax B6.2 (x = 0, 0.3, 0.6, 0.8, 1.0) were prepared by melt spinning. It was found that a small amount of 0.6 at.% Ga addition is beneficial to improve the hard magnetic properties of the alloy. The melt-spun Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 ribbons were annealed in a pulsed magnetic field. The results revealed that the remanence Jr and the maximum energy product (BH)max were obviously improved when annealed below the Curie temperature of the alloy. The best magnetic properties of Jr = 0.975 T, 3 ◦ i Hc = 582 kA/m, and (BH)max = 131 kJ/m were obtained for the ribbons annealed at 300 C in a pulsed magnetic field. The (BH)max was enhanced by 23.8% compared with that of the ribbons without magnetic annealing. It provides a new way to improve the magnetic properties of permanent magnets produced by pulsed magnetic annealing at temperatures below the Curie point of the alloys. © 2007 Elsevier B.V. All rights reserved. Keywords: Permanent magnets; Nanostructured materials; High magnetic field; Magnetic properties
1. Introduction Recently, nanocomposite magnetic materials consisting of hard and soft magnetic phases have been extensively investigated for permanent magnet development due to their enhanced remanence, potentially high energy product and low rare earth cost [1–3]. However, the maximum energy product of nanocomposite permanent magnets obtained experimentally up to now is significantly lower than that predicted by theory [3–6]. The discrepancy is primarily attributed to difficulties in obtaining the optimum microstructures employed for the theoretical models. A uniform microstructure and a well-coupled interface between soft and hard phases are essential conditions for well-performing nanostructured materials [7]. Since the enhanced remanence in nanocomposite magnets is obtained by exchange coupling between hard and soft magnetic grains, the grain boundaries between magnetically hard and soft phases play an important role in the magnetic hardening mech-
∗
Corresponding author. Tel.: +86 21 56337887; fax: +86 21 56335353. E-mail address: [email protected] (H. Xu).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.04.299
anism and exchange coupling. The addition of other elements is an effective way to explore the role of the intergranular phase. It was reported that In and Ga, both elements with low melting points, could enhance the wet-ability of the grain boundary phase [8,9]. Magnetic annealing is another effective method to optimize the microstructure and enhance the magnetic properties of permanent alloys. Yang and co-workers found that the magnetic field treatment reduced the grain size and induced a uniform distribution of magnetic phases for nanocomposite Nd2 Fe14 B/Fe3 B magnets [10–12]. Chiriac et al. reported that magnetic annealing enhanced the coercivity and increased the remanence ratio by 13% for Nd8 Fe77 Co5 B6 CuNb3 alloy [13,14]. Kato et al. found obvious enhancements in i Hc by high-magnetic field annealing in sintered Nd–Fe–B magnets, which is attributed to a fieldinduced change in interface matching between Nd2 Fe14 B and Nd-rich intergranular phases [15,16]. Crystallographic alignment and magnetically anisotropic behavior were induced by magnetic annealing in nanocomposite (NdPrDy)2 Fe14 B/␣-Fe magnets [17]. Ji et al. demonstrated that the magnetic heattreatment near the Curie point of the Nd2 Fe14 B phase resulted in a significant improvement in the coercivity and remanence ratio
42
S. Zhang et al. / Journal of Alloys and Compounds 459 (2008) 41–44
for nanocomposite Nd10 Fe83 B6 In alloy [8,18]. The reason is due to the presence of indium atoms which changed the boundary features and strengthened the exchange coupling between the adjacent grains. However, the previous studies were focused on the effect of the static magnetic field in a low magnitude range. Compared with the static magnetic field the pulsed magnetic field has much more advantages, such as higher magnitude with over 5 T and lower cost. However, few works applying a pulsed magnetic field to permanent materials were carried out up to now. In present work, the effect of Ga addition on the magnetic properties of melt-spun Nd8.5 Fe77.6−x Co5 Zr2.7 Gax B6.2 alloys was investigated on the basis of our previous studies [19–21]. Especially, the magnetic properties for the given alloy annealed near the Curie point using a pulsed magnetic field were investigated. The influence of magnetic annealing on magnetic properties and exchange coupling for nanocomposite Nd2 Fe14 B/␣-Fe alloys will be discussed. 2. Experimental Ingots with nominal composition Nd8.5 Fe77.6−x Co5 Zr2.7 Gax B6.2 (x = 0, 0.3, 0.6, 0.8, 1.0) were prepared by arc-melting the elements Nd, Fe, Co, Zr, Ga and Fe–B alloy under a high-purity Ar atmosphere. The ingots were remelted four times to ensure homogeneity. Small pieces of the arc-melted buttons were melt-spun using a single roller Cu wheel at a wheel speed of 15 m/s. The asspun ribbons were annealed in the temperature range of 200–600 ◦ C for 10 min with and without an external pulsed magnetic field of 5 T. The external applied field was fixed along the direction of the ribbon’s long axis and the frequency was set to five times per min. X-ray diffraction scans were performed for phase identification on melt-spun and annealed samples using a D/max-2550 diffractometer with Cu K␣ radiation. Magnetic properties were measured by vibrating sample magnetometer (VSM) with a maximum applied field of 1.8 T. Curie temperature of the alloy was measured by thermo-gravimetric apparatus (TG).
Fig. 1. Magnetic properties as a function of Ga content for melt-spun Nd8.5 Fe77.6−x Co5 Zr2.7 Gax B6.2 (x = 0–1.0) ribbons.
3. Results and discussions
Based on the above results, the Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 alloy was selected to further study. Fig. 3 illustrates the TG curve of the Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 ribbon at a heating rate of 20 ◦ C/min. It can be found that the Curie temperature of the Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 alloy is 322 ◦ C. The melt-spun Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 ribbons were annealed in the temperature range of 200–600 ◦ C for 10 min
Fig. 1 illustrates the magnetic properties as a function of Ga content for melt-spun Nd8.5 Fe77.6−x Co5 Zr2.7 Gax B6.2 (x = 0, 0.3, 0.6, 0.8, 1.0) ribbons. It can be seen that the intrinsic coercivity increases gradually with increasing Ga content. The remanence has a maximum value of 0.890 T at x = 0.6 and then decreases with further Ga addition. The optimal magnetic properties of Jr = 0.890 T, i Hc = 482 kA/m, and (BH)max = 88 kJ/m3 were obtained for the sample with 0.6 at.% Ga addition. The demagnetization curves of melt-spun ribbons with various Ga content are shown in Fig. 2. All samples show a smooth demagnetization curve and exhibit single-phase hard magnetic behavior. Furthermore the Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 sample exhibits obvious remanence enhancement, indicating a finer and uniform distributed grain size. Thus appropriate Ga (0.6 at.%) addition is beneficial to improve the magnetic properties of meltspun Nd8.5 Fe77.6 Co5 Zr2.7 B6.2 alloy. This is consistent with the results reported by Ping et al. [22] and Harland and Davies [23]. The reason is that Ga atoms could enrich on the boundaries of adjacent grains and inhibit the grain growth. Excessive amount of Ga addition may deteriorate the remanence due to non-magnetic dilution by non-magnetic Ga atoms.
Fig. 2. Demagnetization curves for melt-spun Nd8.5 Fe77.6−x Co5 Zr2.7 Gax B6.2 (x = 0, 0.6, 1.0) ribbons at room temperature.
S. Zhang et al. / Journal of Alloys and Compounds 459 (2008) 41–44
Fig. 3. Thermogravimetry curve of the Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 alloy at a heating rate of 20 ◦ C/min.
with a pulsed magnetic field of 5 T. The magnetic properties as a function of temperature are shown in Fig. 4. For comparison, the magnetic properties for the sample annealed without an external field are also presented. It can be seen that the sample annealed without magnetic field exhibits a slight increase in i Hc with increasing annealing temperature. The Jr and (BH)max increase initially with increasing annealing temperature and then decrease when the temperature exceeds 400 ◦ C. It is known that quenched ribbons are prone to form crystal lattice stress and aberrance stress, which probably suppress the magnetic moments alignment. By maintaining the ribbons to a temper-
Fig. 4. Magnetic properties of Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 ribbons annealed at various temperature for 10 min with and without a magnetic field of 5 T.
43
ature below 400 ◦ C, the internal stresses existing within the samples are relieved, which results in an increased exchange coupling interaction between the magnetically phases and corresponding improvement of coercivity and remanence. Grains become coarse and uneven, especially for the ␣-Fe phase, which deteriorates magnetic properties when annealing temperature exceeds 400 ◦ C. However, when the samples were annealed in a pulsed magnetic field, the Jr increases significantly, up to a maximum of 0.975 T at 300 ◦ C, and then decreases with further increasing annealing temperature. The (BH)max shows a similar behavior with that of remanence. The best magnetic properties of Jr = 0.975 T, i Hc = 582 kA/m, and (BH)max = 131 kJ/m3 can be obtained for Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 ribbon annealed with a magnetic field. It is noted that the magnetic properties for the samples annealed around 300 ◦ C with a pulsed field of 5 T are larger than that of the samples annealed without a magnetic field, especially for Jr and (BH)max . The (BH)max is enhanced by 23.8% compared with that of the sample without magnetic annealing. The demagnetization curves of Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 samples annealed with and without an external field at 300 ◦ C are illustrated in Fig. 5. It can be seen that the magnetic annealing could improve not only the remanence and coercivity, but also the squareness of the hysteresis loop. It is suggested that exchange coupling between the hard and soft magnetic phases could be enhanced by the magnetic field annealing. This phenomena provides a new way to improve the magnetic properties of permanent materials. It is interesting to note that a maximum of Jr and (BH)max appear at an annealing temperature of around 300 ◦ C which is near the Curie temperature of the alloy (Tc = 322 ◦ C). Taking into account that when the annealing temperature is lower than the Curie temperature of the alloy, the Nd2 Fe14 B phase is ferromagnetic, and the magnetic field may promote the exchange
Fig. 5. Demagnetization curves for Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 samples annealed with and without a magnetic field at 300 ◦ C.
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S. Zhang et al. / Journal of Alloys and Compounds 459 (2008) 41–44
4. Conclusion An appropriate addition of Ga can increase the magnetic properties of melt-spun Nd8.5 Fe77.6−x Co5 Zr2.7 Gax B6.2 alloys due to the grain refinement. After annealing in a pulsed magnetic field at temperatures below the Curie point of the alloys, the magnetic properties of nanocomposite Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 alloy were clearly improved. It provides a new way to improve the magnetic properties of permanent materials produced by pulsed magnetic annealing at temperatures below the Curie point of the alloys. Acknowledgement This work was sponsored by the National Natural Science Foundation of China (Grant No. 50671059). References Fig. 6. XRD patterns of Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 samples annealed with and without a magnetic field at 300 ◦ C.
coupling between magnetic phases. On the other hand, the melting of Ga-rich phases at the grain boundaries may be favorable for the grain rotation and phase-distribution adjustment when the alloy annealed at 300 ◦ C. The X-ray diffraction patterns for Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 samples with and without a magnetic field at 300 ◦ C are shown in Fig. 6. No obvious (h k 0) diffraction peaks of Nd2 Fe14 B phase were found for magnetic annealing sample, which indicated that no strong (0 0 l) c-axis texture in the long direction of the sample was induced by the magnetic field. From the selected inset patterns, it is found that the relative intensities of some diffraction planes of the Nd2 Fe14 B phase, such as (2 2 2), (3 1 1) and (2 0 4), have some changes. This demonstrated that the complete orientation of Nd2 Fe14 B phase along the long axis had not taken place. It was presumed that the independent magnetization reversal of the soft phase was inhibited by the external field, which improves the shape of the second quadrant demagnetization curve and strengthens the exchange coupling interaction between magnetically hard and soft phases. The obvious improvement of Jr may be attributed to the alignment of the Nd2 Fe14 B crystallites along the external applied magnetic field, and correspondingly the (BH)max reached to a maximum value after magnetic annealing at 300 ◦ C. In other words, pulsed magnetic annealing at temperatures below the Curie point of the alloys could effectively improve the magnetic properties of Nd8.5 Fe77 Co5 Zr2.7 Ga0.6 B6.2 alloy.
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