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Effect of nitrogen content on structure and magnetic properties of Nd16Fe84−xBxNy alloys prepared by mechanical alloying
Journal of Alloys and Compounds 309 (2000) 172–175
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Effect of nitrogen content on structure and magnetic properties of Nd 16 Fe 842x B x N y alloys prepared by mechanical alloying W. Liu*, Z.D. Zhang, X.K. Sun, J.F. He, H. Tang, B.Z. Cui, X.G. Zhao International Center for Materials Physics, Institute of Metal Research, Academia Sinica, Shenyang 110015, PR China Received 27 April 2000; accepted 16 June 2000
Abstract The structure and magnetic properties of Nd 16 Fe 842x B x Ny alloys prepared by mechanical alloying using pyrolytic boron nitride (p-BN) as starting material have been investigated. By increasing the content of boron and nitrogen, the magnetic main phase changes from Nd 2 Fe 17 to Nd 2 Fe 14 BNd , and finally transforms into the paramagnetic Nd 1.1 Fe 4 B 4 phase. The nitrogen and boron contents determine the component of phases and magnetic properties. The enhancement of the Curie temperature of the Nd 2 Fe 14 BNd phase originates from the increase of the interstitial nitrogen content in the Nd 2 Fe 14 B lattice. 2000 Elsevier Science S.A. All rights reserved. Keywords: Structure; Magnetic properties; Mechanical alloying PACS: 75.50Bb; 75.60Gm; 75.60Jp; 81.40Rs
1. Introduction Since Coey and Sun discovered a new family of rare earth–transition metal (R–T) intermetallic nitrides R 2 Fe 17 N 32d by means of a gas–solid reaction [1], other series such as RFe 11 TiNd and R 2 Fe 14 BN x compounds have been reported [2–5]. In our recent work [6], the quaternary interstitial nitrides R 2 Fe 14 BN 0.1 (R5Nd and Sm) were synthesized by arc melting. It was found that B could be replaced by pyrolytic boron nitride (p-BN) without an extra nitrogenization treatment during the synthesizing process. The breaking of the B–N bonding by arc-melting results in the combination of the atomic B and N with rare earth and transition-metal atoms and consequently the formation of the interstitial R 2 Fe 14 BN 0.1 compound [6]. Mechanical alloying (MA) has served as a novel method of synthesis of new rare-earth permanent magnets, after the pioneering work of Schultz et al. [7]. MA and subsequent heat treatment have achieved rather high values of coercivity in some R–T alloy systems such as SmFe 7 Nd [8], Sm–Fe–Ti [9] and Dy–Fe–C [10]. Although p-BN can be decomposed completely into nitrogen and boron only at about 2773 K [11], the p-BN compound can be partially decomposed by milling until an amorphous phase con*Corresponding author. Fax: 186-24-2389-1320. E-mail address: [email protected] (W. Liu).
taining B and N is formed according to our previous work [12]. Mechanical alloying and subsequent annealing allow the milled p-BN to combine with pure Nd and Fe powders to form the Nd 2 Fe 14 BNd alloys [12]. It is interesting to investigate further the formation and magnetic properties of quaternary Nd 2 Fe 14 BNd compounds [13]. In this paper, the effect of N content on the structure and magnetic properties of Nd 16 Fe 842x B x N y alloys prepared by mechanical alloying is reported.
2. Experimental details Powders of elemental Nd, Fe and as-milled p-BN with purity higher than 99.5% were mixed according to the composition of Nd x Fe 842x B x Ny where y represents the N content in raw materials. A B atom was replaced by a p-BN molecule and the N content was not taken into account. The powder mixture of 10 g was sealed in hardened steel vials with steel balls of 12 mm diameter in a high-purity argon-filled glove box. Mechanical alloying of the mixtures was carried out using a self-made highenergy ball mill which was rotated in two dimensions perpendicular to the horizontal plane. The rotation speed of the mill was chosen to be 800 rpm. The ball-to-powder weight ratio was 20:1. The MA powders were annealed at 1023 K for 30 min in a vacuum furnace directly connected
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )01091-4
W. Liu et al. / Journal of Alloys and Compounds 309 (2000) 172 – 175
to a closed glove box. X-ray diffraction (XRD) analysis of the powder samples was conducted using CuKa radiation. The content of nitrogen in the MA samples was determined with a TC-436 oxygen–nitrogen determinator. The AC susceptibility was measured at temperatures ranging from 300 to 773 K, with an AC field of 16 A m 21 and a frequency of 1.13 kHz. The powders were embedded in epoxy resins to form magnetically isotropic magnets. The magnetic properties were measured at room temperature using a pulsed magnetometer at fields up to 15 T. The dilution effect was neglected and the density of the powdered samples was taken to be 7.6 g cm 23 .
3. Results and discussion Fig. 1 shows X-ray diffraction patterns of MA Nd 16 Fe 842x B x N y alloys annealed at 1023 K for 30 min. A large amount of Nd 2 Fe 17 , the N-containing Nd-rich phase with fcc structure and a small amount of Nd 2 Fe 14 B Nd phase coexist in the samples with 1#x#2. By increasing the content of the as-milled p-BN, the amount of the Nd 2 Fe 17 phase decreases while that of the Nd 2 Fe 14 B phase increases gradually. When x54, nearly single phase Nd 2 Fe 14 BNd is found together with some amounts of a N-containing Nd-rich phase and a small amount of
Fig. 1. X-ray diffraction patterns of MA Nd 16 Fe 842x B x N y alloys annealed at 1023 K for 30 min.
173
Nd 2 Fe 17 . For 6#x#8, the Nd 2 Fe 17 phase disappears, and a Nd 2 Fe 14 B-based phase, NdN and a possibly Nd-rich phase coexist with a-Fe in the sample. When x$10, the Nd 2 Fe 14 B-based phase disappears, and a large amount of NdN, a-Fe and traces of the Nd 1.1 Fe 4 B 4 phase are observed. In our previous work [12], the p-BN was completely decomposed into N and B after mechanical alloying in the presence of Nd. As soon as the nitrogen emerges due to p-BN decomposition, it easily combines with Nd to form the NdN phase. It is clear that when the Nd content is fixed, a small addition of p-BN leads to the formation of Nd-rich and Nd 2 Fe 17 phase. A large addition of p-BN results in the formation of a large amount of NdN and the free B atoms, and thus to the formation of a-Fe and a B-rich phase, Nd 1.1 Fe 4 B 4 . The board diffraction peaks of NdN in Fig. 1 may originate from overlapping of the peaks of NdN and the Nd-rich phase, because they are very close to each other. The temperature dependence of the AC initial susceptibility of the MA Nd 16 Fe 842x B x N y alloys annealed at 1023 K for 30 min is displayed in Fig. 2. For the sample with 1#x#2, Nd 2 Fe 17 and Nd 2 Fe 14 B phases are observed with Curie temperatures of about 343 K and 585 K, respectively. When x54, the Curie temperature of the hard magnetic Nd 2 Fe 14 B phase is about 595 K. Meanwhile, the
Fig. 2. Temperature dependence of the AC initial susceptibility for the MA Nd 16 Fe 842x B x N y alloys annealed at 1023 K for 30 min.
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W. Liu et al. / Journal of Alloys and Compounds 309 (2000) 172 – 175
trace of Nd 2 Fe 17 phase is hardly seen due to the very small signal. For 6#x#8, only the Curie temperature of Nd 2 Fe 14 B phase is observed. The Curie temperature of the Nd 2 Fe 14 BNd phase increases with the content of nitrogen in the samples. The slight increase of the Curie temperature suggests that a small amount of nitrogen is introduced into the lattice of the Nd 2 Fe 14 B phase after milling and subsequent annealing, in good agreement with our recent result [6]. Based on these facts, it may be assumed that the Curie temperature linearly depends on the nitrogen content and the composition of the hard magnetic phase with x58 is close to Nd 2 Fe 14 BN 0.25 [12]. The Nd 1.1 Fe 4 B 4 phase is paramagnetic above room temperature according to Bezinge’s work [14]. Therefore, no Curie temperature is found in Fig. 2 for the samples with x$10. All the results of the initial susceptibility measurements are in good agreement with the analysis of the X-ray diffraction patterns. The relationship between the nitrogen and boron contents in the MA Nd 16 Fe 842x B x N y alloys annealed at 1023 K for 30 min is given in Fig. 3. It can be seen from Fig. 3 that the nitrogen content increases linearly with increasing content of B for x$2. Most of the nitrogen in the sample combines with Nd to form NdN; meanwhile, a small quantity of nitrogen is added as an interstitial atom into the lattice of the hard Nd 2 Fe 14 B phase. According to the above-mentioned results, the quantity of nitrogen as an interstitial atom increases with increasing nitrogen content for x#8. In addition, the too high nitrogen content for x.8 inevitably leads to the formation of NdN, and the residual Nd combines with B and Fe to form the Nd 1.1 Fe 4 B 4 phase. It is concluded that a suitable nitrogen (and boron) content
Fig. 3. The relationship between the nitrogen and the B contents in the MA Nd 16 Fe 842x B x N y alloys annealed at 1023 K for 30 min.
Fig. 4. The composition dependence of the magnetic properties of MA Nd 16 Fe 842x B x N y alloys annealed at 1023 K for 30 min.
(x56) is needed for formation of a certain amount of Nd 2 Fe 14 B-based phase. Fig. 4 shows the composition dependence of the magnetic properties of MA Nd 16 Fe 842x Bx Ny alloys annealed at 1023 K for 30 min. The intrinsic coercivity i Hc reaches a maximum of 9.2 kOe at x54, which is higher than that of MA Nd 8 (Fe,Co) 88 B 4 and Nd 8 Fe 87 B 4 X (X5Cu, Zr, Si) alloys [15,16]. Although almost the same B content exists in the alloys, the mechanisms of coercivity and phase constitutions are very different. We attribute the high coercivity of the present alloy to the N-containing Nd-rich phase and Nd 2 Fe 14 BNd phase. The low coercivities in the MA Nd 8 (Fe,Co) 88 B 4 and Nd 8 Fe 87 B 4 X (X5Cu, Zr, Si) nanocomposite magnets are due to the existence of a large amount of a-Fe around the Nd 2 Fe 14 B-type phase. The remanence 4p Mr and the maximum energy product (BH ) max of the alloy in which about 6 at.% Nd combines with N to form NdN and in which the magnetic property is due to the Nd 2 Fe 14 BNd are a maximum at x56. The coercivity of the sample with x56 is slightly higher that of the MA Nd 12.6 Fe 81.4 B 6 alloy [17]. The somewhat disappointing magnetic properties for the samples with lower B and N contents are mainly due to the existence of a large amount of Nd 2 Fe 17 phase with an easy plane magnetization. On the other hand, the formation of a large amount of NdN, a-Fe and a small amount of paramagnetic Nd 1.1 Fe 4 B 4 phases due to the higher B and N content also causes the magnetic properties to decrease. In conclusion, p-BN powder can be used as the source of B and N to synthesise Nd 2 Fe 14 BNd -based alloys. Under fixed Nd content, if the content of nitrogen and boron is
W. Liu et al. / Journal of Alloys and Compounds 309 (2000) 172 – 175
low (x#2), Nd 2 Fe 17 and N-containing Nd-rich phases with fcc structure are formed, accompanied by a small amount of the Nd 2 Fe 14 BNd phase. A higher nitrogen content (x$10) leads to the formation of NdN and a-Fe together with traces of the Nd 1.1 Fe 4 B 4 phase. The Curie temperature of the Nd 2 Fe 14 BNd phase increases with increasing nitrogen content for x,8. The intrinsic coercivity reaches a maximum of 9.2 kOe at x54. The appropriate nitrogen and boron contents for 4#x#6 are necessary for optimizing the magnetic properties.
Acknowledgements This work has been supported by the National Natural Sciences Foundation of China under Project Nos. 59725103, 59571014 and 59831010 and the Science and Technology Commissions of Liaoning and Shenyang
References [1] J.M.D. Coey, H. Sun, J. Magn. Magn. Mater. 86 (1990) L251. [2] Y.C. Yang, X.D. Zhang, L.S. Kong, Q. Pan, S.L. Ge, Appl. Phys. Lett. 58 (1991) 2042.
175
[3] X.D. Zhang, Q. Pan, S.Z. Dong, S.L. Ge, Y.C. Yang, J.L. Yang, B.S. Zhang, Y.F. Ding, C.T. Ye, Acta Physica Sinica 2 (1993) 537. [4] X.D. Zhang, Q. Pan, S.L. Ge, Y.C. Yang, J.L. Yang, Y.F. Ding, B.S. Zhang, C.T. Ye, L. Jin, Solid State Commun. 83 (1992) 231. ¨ [5] X.C. Kou, T.S. Zhao, R. Grossinger, H. Kirchmayr, X. Li, F.R. de Boer, Phys. Rev. B 46 (1992) 204. [6] Z.D. Zhang, W. Liu, D. Zhang, X.M. Jin, X.G. Zhao, Q.F. Xiao, J. Phys. Cond. Matt. 11 (1999) 3951. [7] L. Schultz, J. Wecker, E.J. Hellstern, J. Appl. Phys. 61 (1987) 3583. [8] W. Liu, Q. Wang, X.K. Sun, X.G. Zhao, T. Zhao, Z.D. Zhang, Y.C. Chuang, J. Magn. Magn. Mater. 131 (1994) 197. [9] J.L. Yang, Q. Wang, X.K. Sun, G.Y. Zeng, M. Chen, W. Liu, X.G. Zhao, T. Zhao, Z.D. Zhang, J. Magn. Magn. Mater. 132 (1994) 197. [10] Y.C. Sui, Z.D. Zhang, Q.F. Xiao, W. Liu, X.G. Zhao, T. Zhao, Y.C. Chuang, J. Phys. Cond. Matt. 9 (1997) 9985. [11] A.W. Moore, J. Crystal Growth 106 (1990) 6. [12] W. Liu, Z.D. Zhang, X.K. Sun, J.F. He, X.G. Zhao, J. Phys D: Appl. Phys. 32 (1999) 1591. [13] Z.D. Zhang, W. Liu, J.P. Liu, D.J. Sellmyer, J. Phys. D: Appl. Phys. (2000) (in press). [14] A. Bezinge, H.F. Braun, J. Muller, K. Yvon, Solid State Commun. 55 (1985) 131. [15] W.F. Miao, J. Ding, P.G. McCormick, R. Street, J. Appl. Phys. 82 (1997) 4439. [16] P. Crespo, V. Neu, L. Schultz, J. Phys. D: Appl. Phys. 30 (1997) 2298. [17] M. Jurczyk, J.S. Cook, S.J. Collocott, J. Alloys Comp. 217 (1995) 65.
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Effect of nitrogen content on structure and magnetic properties of Nd 16 Fe 842x B x N y alloys prepared by mechanical alloying W. Liu*, Z.D. Zhang, X.K. Sun, J.F. He, H. Tang, B.Z. Cui, X.G. Zhao International Center for Materials Physics, Institute of Metal Research, Academia Sinica, Shenyang 110015, PR China Received 27 April 2000; accepted 16 June 2000
Abstract The structure and magnetic properties of Nd 16 Fe 842x B x Ny alloys prepared by mechanical alloying using pyrolytic boron nitride (p-BN) as starting material have been investigated. By increasing the content of boron and nitrogen, the magnetic main phase changes from Nd 2 Fe 17 to Nd 2 Fe 14 BNd , and finally transforms into the paramagnetic Nd 1.1 Fe 4 B 4 phase. The nitrogen and boron contents determine the component of phases and magnetic properties. The enhancement of the Curie temperature of the Nd 2 Fe 14 BNd phase originates from the increase of the interstitial nitrogen content in the Nd 2 Fe 14 B lattice. 2000 Elsevier Science S.A. All rights reserved. Keywords: Structure; Magnetic properties; Mechanical alloying PACS: 75.50Bb; 75.60Gm; 75.60Jp; 81.40Rs
1. Introduction Since Coey and Sun discovered a new family of rare earth–transition metal (R–T) intermetallic nitrides R 2 Fe 17 N 32d by means of a gas–solid reaction [1], other series such as RFe 11 TiNd and R 2 Fe 14 BN x compounds have been reported [2–5]. In our recent work [6], the quaternary interstitial nitrides R 2 Fe 14 BN 0.1 (R5Nd and Sm) were synthesized by arc melting. It was found that B could be replaced by pyrolytic boron nitride (p-BN) without an extra nitrogenization treatment during the synthesizing process. The breaking of the B–N bonding by arc-melting results in the combination of the atomic B and N with rare earth and transition-metal atoms and consequently the formation of the interstitial R 2 Fe 14 BN 0.1 compound [6]. Mechanical alloying (MA) has served as a novel method of synthesis of new rare-earth permanent magnets, after the pioneering work of Schultz et al. [7]. MA and subsequent heat treatment have achieved rather high values of coercivity in some R–T alloy systems such as SmFe 7 Nd [8], Sm–Fe–Ti [9] and Dy–Fe–C [10]. Although p-BN can be decomposed completely into nitrogen and boron only at about 2773 K [11], the p-BN compound can be partially decomposed by milling until an amorphous phase con*Corresponding author. Fax: 186-24-2389-1320. E-mail address: [email protected] (W. Liu).
taining B and N is formed according to our previous work [12]. Mechanical alloying and subsequent annealing allow the milled p-BN to combine with pure Nd and Fe powders to form the Nd 2 Fe 14 BNd alloys [12]. It is interesting to investigate further the formation and magnetic properties of quaternary Nd 2 Fe 14 BNd compounds [13]. In this paper, the effect of N content on the structure and magnetic properties of Nd 16 Fe 842x B x N y alloys prepared by mechanical alloying is reported.
2. Experimental details Powders of elemental Nd, Fe and as-milled p-BN with purity higher than 99.5% were mixed according to the composition of Nd x Fe 842x B x Ny where y represents the N content in raw materials. A B atom was replaced by a p-BN molecule and the N content was not taken into account. The powder mixture of 10 g was sealed in hardened steel vials with steel balls of 12 mm diameter in a high-purity argon-filled glove box. Mechanical alloying of the mixtures was carried out using a self-made highenergy ball mill which was rotated in two dimensions perpendicular to the horizontal plane. The rotation speed of the mill was chosen to be 800 rpm. The ball-to-powder weight ratio was 20:1. The MA powders were annealed at 1023 K for 30 min in a vacuum furnace directly connected
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )01091-4
W. Liu et al. / Journal of Alloys and Compounds 309 (2000) 172 – 175
to a closed glove box. X-ray diffraction (XRD) analysis of the powder samples was conducted using CuKa radiation. The content of nitrogen in the MA samples was determined with a TC-436 oxygen–nitrogen determinator. The AC susceptibility was measured at temperatures ranging from 300 to 773 K, with an AC field of 16 A m 21 and a frequency of 1.13 kHz. The powders were embedded in epoxy resins to form magnetically isotropic magnets. The magnetic properties were measured at room temperature using a pulsed magnetometer at fields up to 15 T. The dilution effect was neglected and the density of the powdered samples was taken to be 7.6 g cm 23 .
3. Results and discussion Fig. 1 shows X-ray diffraction patterns of MA Nd 16 Fe 842x B x N y alloys annealed at 1023 K for 30 min. A large amount of Nd 2 Fe 17 , the N-containing Nd-rich phase with fcc structure and a small amount of Nd 2 Fe 14 B Nd phase coexist in the samples with 1#x#2. By increasing the content of the as-milled p-BN, the amount of the Nd 2 Fe 17 phase decreases while that of the Nd 2 Fe 14 B phase increases gradually. When x54, nearly single phase Nd 2 Fe 14 BNd is found together with some amounts of a N-containing Nd-rich phase and a small amount of
Fig. 1. X-ray diffraction patterns of MA Nd 16 Fe 842x B x N y alloys annealed at 1023 K for 30 min.
173
Nd 2 Fe 17 . For 6#x#8, the Nd 2 Fe 17 phase disappears, and a Nd 2 Fe 14 B-based phase, NdN and a possibly Nd-rich phase coexist with a-Fe in the sample. When x$10, the Nd 2 Fe 14 B-based phase disappears, and a large amount of NdN, a-Fe and traces of the Nd 1.1 Fe 4 B 4 phase are observed. In our previous work [12], the p-BN was completely decomposed into N and B after mechanical alloying in the presence of Nd. As soon as the nitrogen emerges due to p-BN decomposition, it easily combines with Nd to form the NdN phase. It is clear that when the Nd content is fixed, a small addition of p-BN leads to the formation of Nd-rich and Nd 2 Fe 17 phase. A large addition of p-BN results in the formation of a large amount of NdN and the free B atoms, and thus to the formation of a-Fe and a B-rich phase, Nd 1.1 Fe 4 B 4 . The board diffraction peaks of NdN in Fig. 1 may originate from overlapping of the peaks of NdN and the Nd-rich phase, because they are very close to each other. The temperature dependence of the AC initial susceptibility of the MA Nd 16 Fe 842x B x N y alloys annealed at 1023 K for 30 min is displayed in Fig. 2. For the sample with 1#x#2, Nd 2 Fe 17 and Nd 2 Fe 14 B phases are observed with Curie temperatures of about 343 K and 585 K, respectively. When x54, the Curie temperature of the hard magnetic Nd 2 Fe 14 B phase is about 595 K. Meanwhile, the
Fig. 2. Temperature dependence of the AC initial susceptibility for the MA Nd 16 Fe 842x B x N y alloys annealed at 1023 K for 30 min.
174
W. Liu et al. / Journal of Alloys and Compounds 309 (2000) 172 – 175
trace of Nd 2 Fe 17 phase is hardly seen due to the very small signal. For 6#x#8, only the Curie temperature of Nd 2 Fe 14 B phase is observed. The Curie temperature of the Nd 2 Fe 14 BNd phase increases with the content of nitrogen in the samples. The slight increase of the Curie temperature suggests that a small amount of nitrogen is introduced into the lattice of the Nd 2 Fe 14 B phase after milling and subsequent annealing, in good agreement with our recent result [6]. Based on these facts, it may be assumed that the Curie temperature linearly depends on the nitrogen content and the composition of the hard magnetic phase with x58 is close to Nd 2 Fe 14 BN 0.25 [12]. The Nd 1.1 Fe 4 B 4 phase is paramagnetic above room temperature according to Bezinge’s work [14]. Therefore, no Curie temperature is found in Fig. 2 for the samples with x$10. All the results of the initial susceptibility measurements are in good agreement with the analysis of the X-ray diffraction patterns. The relationship between the nitrogen and boron contents in the MA Nd 16 Fe 842x B x N y alloys annealed at 1023 K for 30 min is given in Fig. 3. It can be seen from Fig. 3 that the nitrogen content increases linearly with increasing content of B for x$2. Most of the nitrogen in the sample combines with Nd to form NdN; meanwhile, a small quantity of nitrogen is added as an interstitial atom into the lattice of the hard Nd 2 Fe 14 B phase. According to the above-mentioned results, the quantity of nitrogen as an interstitial atom increases with increasing nitrogen content for x#8. In addition, the too high nitrogen content for x.8 inevitably leads to the formation of NdN, and the residual Nd combines with B and Fe to form the Nd 1.1 Fe 4 B 4 phase. It is concluded that a suitable nitrogen (and boron) content
Fig. 3. The relationship between the nitrogen and the B contents in the MA Nd 16 Fe 842x B x N y alloys annealed at 1023 K for 30 min.
Fig. 4. The composition dependence of the magnetic properties of MA Nd 16 Fe 842x B x N y alloys annealed at 1023 K for 30 min.
(x56) is needed for formation of a certain amount of Nd 2 Fe 14 B-based phase. Fig. 4 shows the composition dependence of the magnetic properties of MA Nd 16 Fe 842x Bx Ny alloys annealed at 1023 K for 30 min. The intrinsic coercivity i Hc reaches a maximum of 9.2 kOe at x54, which is higher than that of MA Nd 8 (Fe,Co) 88 B 4 and Nd 8 Fe 87 B 4 X (X5Cu, Zr, Si) alloys [15,16]. Although almost the same B content exists in the alloys, the mechanisms of coercivity and phase constitutions are very different. We attribute the high coercivity of the present alloy to the N-containing Nd-rich phase and Nd 2 Fe 14 BNd phase. The low coercivities in the MA Nd 8 (Fe,Co) 88 B 4 and Nd 8 Fe 87 B 4 X (X5Cu, Zr, Si) nanocomposite magnets are due to the existence of a large amount of a-Fe around the Nd 2 Fe 14 B-type phase. The remanence 4p Mr and the maximum energy product (BH ) max of the alloy in which about 6 at.% Nd combines with N to form NdN and in which the magnetic property is due to the Nd 2 Fe 14 BNd are a maximum at x56. The coercivity of the sample with x56 is slightly higher that of the MA Nd 12.6 Fe 81.4 B 6 alloy [17]. The somewhat disappointing magnetic properties for the samples with lower B and N contents are mainly due to the existence of a large amount of Nd 2 Fe 17 phase with an easy plane magnetization. On the other hand, the formation of a large amount of NdN, a-Fe and a small amount of paramagnetic Nd 1.1 Fe 4 B 4 phases due to the higher B and N content also causes the magnetic properties to decrease. In conclusion, p-BN powder can be used as the source of B and N to synthesise Nd 2 Fe 14 BNd -based alloys. Under fixed Nd content, if the content of nitrogen and boron is
W. Liu et al. / Journal of Alloys and Compounds 309 (2000) 172 – 175
low (x#2), Nd 2 Fe 17 and N-containing Nd-rich phases with fcc structure are formed, accompanied by a small amount of the Nd 2 Fe 14 BNd phase. A higher nitrogen content (x$10) leads to the formation of NdN and a-Fe together with traces of the Nd 1.1 Fe 4 B 4 phase. The Curie temperature of the Nd 2 Fe 14 BNd phase increases with increasing nitrogen content for x,8. The intrinsic coercivity reaches a maximum of 9.2 kOe at x54. The appropriate nitrogen and boron contents for 4#x#6 are necessary for optimizing the magnetic properties.
Acknowledgements This work has been supported by the National Natural Sciences Foundation of China under Project Nos. 59725103, 59571014 and 59831010 and the Science and Technology Commissions of Liaoning and Shenyang
References [1] J.M.D. Coey, H. Sun, J. Magn. Magn. Mater. 86 (1990) L251. [2] Y.C. Yang, X.D. Zhang, L.S. Kong, Q. Pan, S.L. Ge, Appl. Phys. Lett. 58 (1991) 2042.
175
[3] X.D. Zhang, Q. Pan, S.Z. Dong, S.L. Ge, Y.C. Yang, J.L. Yang, B.S. Zhang, Y.F. Ding, C.T. Ye, Acta Physica Sinica 2 (1993) 537. [4] X.D. Zhang, Q. Pan, S.L. Ge, Y.C. Yang, J.L. Yang, Y.F. Ding, B.S. Zhang, C.T. Ye, L. Jin, Solid State Commun. 83 (1992) 231. ¨ [5] X.C. Kou, T.S. Zhao, R. Grossinger, H. Kirchmayr, X. Li, F.R. de Boer, Phys. Rev. B 46 (1992) 204. [6] Z.D. Zhang, W. Liu, D. Zhang, X.M. Jin, X.G. Zhao, Q.F. Xiao, J. Phys. Cond. Matt. 11 (1999) 3951. [7] L. Schultz, J. Wecker, E.J. Hellstern, J. Appl. Phys. 61 (1987) 3583. [8] W. Liu, Q. Wang, X.K. Sun, X.G. Zhao, T. Zhao, Z.D. Zhang, Y.C. Chuang, J. Magn. Magn. Mater. 131 (1994) 197. [9] J.L. Yang, Q. Wang, X.K. Sun, G.Y. Zeng, M. Chen, W. Liu, X.G. Zhao, T. Zhao, Z.D. Zhang, J. Magn. Magn. Mater. 132 (1994) 197. [10] Y.C. Sui, Z.D. Zhang, Q.F. Xiao, W. Liu, X.G. Zhao, T. Zhao, Y.C. Chuang, J. Phys. Cond. Matt. 9 (1997) 9985. [11] A.W. Moore, J. Crystal Growth 106 (1990) 6. [12] W. Liu, Z.D. Zhang, X.K. Sun, J.F. He, X.G. Zhao, J. Phys D: Appl. Phys. 32 (1999) 1591. [13] Z.D. Zhang, W. Liu, J.P. Liu, D.J. Sellmyer, J. Phys. D: Appl. Phys. (2000) (in press). [14] A. Bezinge, H.F. Braun, J. Muller, K. Yvon, Solid State Commun. 55 (1985) 131. [15] W.F. Miao, J. Ding, P.G. McCormick, R. Street, J. Appl. Phys. 82 (1997) 4439. [16] P. Crespo, V. Neu, L. Schultz, J. Phys. D: Appl. Phys. 30 (1997) 2298. [17] M. Jurczyk, J.S. Cook, S.J. Collocott, J. Alloys Comp. 217 (1995) 65.