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Structure and magnetic properties of Nd–Fe–B–Ti prepared by mechanical alloying
Journal of Magnetism and Magnetic Materials 184 (1998) 101—105
Structure and magnetic properties of Nd—Fe—B—Ti prepared by mechanical alloying Zhi-Dong Zhang, Wei Liu*, X.K. Sun, Xin-guo Zhao, Qun-feng Xiao, Yu-cheng Sui, Tong Zhao Institute of Metal Research, Academia Sinica, Shenyang 110015, People’s Republic of China Received 12 June 1997; received in revised form 3 November 1997
Abstract The structure and magnetic properties of Nd Fe B and Nd Fe B Ti alloys prepared by mechanical x 92~x 8 10 76 8~y 6`y alloying (MA) and subsequent annealing have been studied. It has been found that the exchange interaction between a soft a-Fe and a hard magnetic Nd Fe B phase results in a significant enhancement of the remanence for x(12. The 2 14 best values of 4pM "8.1 kGs, H "21.1 kOe and (BH) "14.1 MGOe are obtained for the series of Nd Fe B 3 * # .!9 x 92~x 8 with x"16. For the series of Nd Fe B Ti , when 0)y)6, the main phase Nd Fe B is formed and accom10 76 8~y 6`y 2 14 panied by Nd O , Fe Ti and Nd-rich phases. In the present experimental investigation, the occupation state of Ti in the 2 3 2 MA samples for 0)y)6 has been considered. The intrinsic coercivity reaches up to more than 11 kOe for 4)y)6. It is evident that the addition of Ti is favourable for the enhancement of the intrinsic coercivity in Nd—Fe—B alloys with a low level of Nd, which is mainly due to the coexistence of the Nd-rich phase and the hard magnetic phase with the structure of Nd Fe B containing Ti, combined with the suppression of a-Fe, for the samples with 0)y)6. ( 1998 2 14 Elsevier Science B.V. All rights reserved. PACS: 75.50.Bb; 75.60.Gm; 75.60.Jp; 81.40.R Keywords: Permanent magnets; Mechanical alloying; Remanence; Coercivity
1. Introduction In recent years, it was reported that nanocomposite magnets exhibit enhanced remanence in an isotropic state due to exchange coupling between their nanocrystalline grains of hard and soft magnetic phases [1,2]. For an isotropic Nd Fe B 10 84 6 * Corresponding author. Fax: #86 24 389 1320; e-mail: [email protected].
melt-spun material a remanence value of over 1 T was reported by Manaf et al. [3]. However, these magnets have somewhat reduced coercivity. The appropriate addition of Ti led to the enhancement of coercivity in Nd—Fe—B alloys with a low level of Nd and B contents by rapid quenching [4,5]. Schultz et al. reported that mechanical alloying (MA) and a subsequent solid-state reaction can be a route to form Nd—Fe—B magnets [6]. By this method some nanostructured magnets with main phase such as
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metastable SmFe N or Sm (Fe,Ti) phases are easy 7 d 5 17 to be formed after milling and subsequent annealing, with very high coercivities [7,8]. All the results previously reported urge us to study the role of Ti addition in the Nd—Fe—B alloys by mechanical alloying. In this paper, the effects of adding Ti on the phase structure and the magnetic properties of Nd—Fe—B alloys with a low level of Nd and B contents have been studied, using high-energy ball milling followed by a suitable heat treatment.
2. Experimental details The element powders with the purities of 99.6, 99, 99.5 and 99.5 for Fe, Ti, Nd and B elements, respectively, were mixed according to the compositions of Nd Fe B (x"8—16) and Nd Fe B Ti x 92~x 8 10 76 8~y 6`y (y"0—8). Mechanical alloying of the mixtures was performed under an argon atmosphere for 5 h. The MA powdered samples were annealed at 600—900°C for 30 min in a vacuum furnace which was directly connected to a closed glove box. X-ray diffraction (XRD) analysis of the powder samples was conducted using Cu K radiation with a Rigaku D/Maxa rA diffractometer equipped with a graphite crystal monochromator. Initial AC susceptibility measurement can be operated in the temperature range 300—1000 K with an AC field of 16 A/m and frequency of 1.13 kHz, which was applied to determine the Curie temperature and to estimate preliminarily the possible magnetic phases in the samples. For the magnetic measurements at room temperature, the powders were embedded in epoxy resin to form magnetically isotropic magnets. The magnetic properties were measured using a pulsed magnetometer in fields up to 15 T. The magnetization was related to the amount of magnetic powders, and the demagnetization factor of MA powder bonded samples due to the dilutional effect was approximately determined as 0.28 by experiments and the density of the samples was assumed to be 7.6 g/cm3.
3. Results and discussion In order to compare the magnetic properties of MA Nd—Fe—B—Ti samples with those of Nd—Fe—B
Fig. 1. Dependences of magnetic properties and M /M of MA 3 4 Nd Fe B annealed at 750°C for 30 min on Nd content. x 92~x 8
samples, at first, the magnetic properties of Nd—Fe—B alloys prepared by mechanical alloying and followed by annealing are studied. The Ndcontent dependences of the magnetic properties and M /M in the series of Nd Fe B annealed 3 4 x 92~x 8 at 750°C for 30 min are shown in Fig. 1. It is shown that the remanence of the samples increases with decreasing Nd content for x(12 and the ratio M /M ranges from 0.58 to 0.68. Therefore, the 3 4 exchange interaction between the soft a-Fe and the hard magnetic Nd Fe B phase results in an en2 14 hancement of the remanent magnetization. Even the ratio M /M of the samples for x*12 is above 3 4 0.55. It seems that the exchange interaction between nanocrystalline grains of the hard magnetic phases also leads to a slightly small remanence enhancement despite the presence of a Nd-rich phase at grain boundaries. The best values, 4pM "14.0 kGs, 4pM "8.1 kGs, H "21.1 kOe 4 3 * # and (BH) "14.1 MGOe, are obtained at x"16. .!9 The coercivity increases slightly for x)12, and rises rapidly with a further increase of the Nd content, as is expected. X-ray diffraction patterns of Nd Fe B and 10 82 8 Nd Fe B Ti (0)y)8) powders pre10 76 8~y 6`y pared by MA and annealed at 750°C for 30 min are shown in Fig. 2. Comparing the patterns
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Fig. 2. X-ray diffraction patterns of MA Nd Fe B and Nd Fe B Ti (0)y)8) powders annealed at 750°C for 30 min. 10 82 8 10 76 8~y 6`y
of Nd Fe B Ti powders with that of 10 76 8~y 6`y Nd Fe B powder in Fig. 2, it can be seen that 10 82 8 the content of a-Fe decreases with increasing Ti content. Hence, a lower a-Fe content in Nd Fe B Ti alloys can be obtained more 10 76 8~y 6`y easily than that in Nd Fe B alloy. For y"0, 10 82 8 the TbCu -type structure reported in Ref. [9] is not 7 observed from the X-ray data of Fig. 2. This is mainly due to the different methods to prepare the samples. Instead, the Nd Fe B phase coexists with 2 14 some a-Fe, and small amounts of Nd O and an 2 3
Nd-rich phase emerge in this sample. With increasing Ti content and decreasing B content, Fe Ti is 2 also formed for 2)y)4 besides the phases mentioned above. A nearly single-phase Nd Fe B ma2 14 terial is obtained for y"6, in which a small amount of Nd O , an Nd-rich phase and Fe Ti are 2 3 2 included. Nd Fe B and the 1 : 7 phase with the 2 14 TbCu structure are formed in coexistence with 7 a noticeable amount of Fe Ti and Nd-rich phase 2 for y"7. When the Ti content reaches 14 at%, three phases, namely 1 : 7, Fe Ti and Nd-rich 2
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phase, coexist as shown in Fig. 2. The black-box symbol in Fig. 2 stands for the Nd-rich phase, and these peaks belong to an FCC structure according to the analysis of the X-ray reflections of the MA samples. It was reported that an FCC structured Nd-rich intergranular phase stabilized by oxygen with a composition of nearly 95 at% Nd and 5 at% Fe existed in the sintered or die-upset R—Fe—B alloys. The lattice parameter of this phase is about 5.2 A_ [10,11]. Thus, a reasonable inference is that, this Nd-rich phase exists in the MA Nd—Fe—B—Ti alloy, too. As can be seen from Fig. 2 the X-ray reflections of the Nd-rich phase is shifted to lower angles, which may indicate that a part of Ti has entered the Nd-rich phase and results in the expansion of the lattice. Temperature dependences of AC susceptibility of MA Nd Fe B and 10 82 8 Nd Fe B Ti annealed at 750°C for 30 min 10 76 8~y 6`y are given in Fig. 3. From there, it is obvious that Nd Fe B-type phases with Curie temperature of 2 14 about 320°C are observed for 0)y)6. AC susceptibility measurement gives two Curie temperatures for y"7, corresponding to two magnetic phases having TbCu -type and the Nd Fe B-type 7 2 14 structures, respectively. When y"8, one Curie temperature of the 1 : 7 phase is detected. All the results of AC susceptibility measurements are in good agreement with the analysis of X-ray diffraction patterns. It is worth studying the occupation state of Ti in Nd Fe B Ti alloys for 10 76 8~y 6`y 0)y)6. As is observed from X-ray data, no evidence of element Ti can be found whereas a small amount of Fe Ti is visible. At the same time, 2 Ti-containing magnetic phases, such as TbCu -, 7 Th Zn - and ThMn -type structures, are not 2 17 12 found for 0)y)6, according to X-ray diffraction patterns and AC susceptibility measurements. It is seen from Fig. 2 that the X-ray diffraction peaks of the Nd Fe B phase are also shifted to lower 2 14 angles with increasing Ti content as a result of the lattice expansion. The lattice parameters of Nd Fe B calculated by analysing the X-ray dif10 82 8 fraction pattern are a"8.799 A_ and c"12.198 A_ , in agreement with those of Nd Fe B (a"8.80 A_ 2 14 and c"12.20 A_ ) reported by Herbst et al. [12]. The lattice parameters of the sample with y"0, corresponding to the composition of Nd Fe B Ti , are a"8.799 A_ and c"12.199 A_ . 10 76 8 6
Fig. 3. Temperature dependences of AC susceptibility of MA Nd Fe B and Nd Fe B Ti annealed at 750°C for 10 82 8 10 76 8~y 6`y 30 min.
Thus, it is concluded that Ti does not enter into the lattice of the main phase Nd Fe B for a relatively 2 14 low content of Ti, because the lattice parameters are almost unchanged. However, when y"6, the measured lattice parameters of the main phase of the alloy corresponding to the composition Nd Fe B Ti are a"8.822 A_ and c"12.24 A_ . 10 76 2 12 The larger values of a and c in the MA sample with a relatively high Ti content compared with the reported results of Nd Fe B imply that Ti may 2 14 partly occupy the sites of the tetragonal Nd Fe B 2 14 structure. It is well known that the atomic radius of B is much smaller than that of Ti. Usually, Ti can occupy the Fe sites in Nd Fe B. According to our 2 14 knowledge, it is difficult to believe that the single B site is occupied by Ti. Particularly, although only 2 at% of B and a large amount of Ti exist in the sample for y"6, a nearly single-phase Nd Fe B 2 14 is formed. For the present MA sample, it is
Zhi-Dong Zhang et al. / Journal of Magnetism and Magnetic Materials 184 (1998) 101–105
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that the addition of appropriate Ti is favourable for obtaining higher intrinsic coercivity in MA Nd Fe B Ti series, which is mainly due to 10 76 8~y 6`y the coexistence of the Nd-rich phase and the hard magnetic Nd Fe B phase, both containing Ti, 2 14 combined with the suppression of a-Fe, when 0)y)6. Therefore, an appropriate addition of Ti led to the enhancement of coercivity in Nd—Fe—B alloys with a low level of Nd.
Acknowledgements This work has been supported by the National Natural Science Foundation of China projects 59571014, 59421001 and 59725103, and the Science and Technology Commissions of Liaoning and Shenyang. Fig. 4. Composition dependences of magnetic properties of MA Nd Fe B Ti alloys annealed at 750°C for 30 min. 10 76 8~y 6`y
suggested that Ti might occupy part of the Nd and/or Fe sites, while some B sites may be empty, or two B atoms at the g sites could be replaced by Ti. This might explain preliminarily why the main phase with the Nd Fe B-type structure can form 2 14 in the limit of vanishing B content. Fig. 4 exhibits the dependences of magnetic properties on composition in MA Nd Fe B Ti 10 76 8~y 6`y alloys annealed at 750°C for 30 min. It can be seen that the intrinsic coercivity H first increases with * # Ti content, reaches a maximum of 11.6 kOe for y"4, and then decreases with further increase of Ti content. For 0)y)6 the remanence 4pM 3 and the maximum energy product (BH) .!9 decrease slightly with increasing Ti content and therefore decreasing B content. The best values, of 4pM "7.3 kGs and (BH) "11.4 MGOe are 3 .!9 obtained for y"6, together with an intrinsic coercivity of 11 kOe. For the samples with y'6, the magnetic properties decrease rapidly, which is mainly due to the existence of the Nd(Fe,Ti) phase 7 with an easy plane magnetization. From Fig. 1, the intrinsic coercivity of Nd Fe B alloy 10 82 8 is only 4.6 kOe in comparison and it is clear
References [1] R. Skomski, J.M.D. Coey, Phys. Rev. B 48 (21) (1993) 15812. [2] T. Schrefl, H. Kronmu¨ller, H.A. Davies, J. Magn. Magn. Mater. 127 (1993) L273. [3] A. Manaf, M. Al-Khafaji, P.Z. Zhang, H.A. Davies, R.A. Buckley, W.M. Rainforth, J. Magn. Magn. Mater. 128 (1993) 307. [4] Tsung-Shune Chin, Cheng-Hao Lin, Sen-Huan Huang, Jiunn-ming Yau, Tsung-Yao Chu, Cherng-Dean Wu, Jpn. J. Appl. Phys. 31 (1992) 3323. [5] J.M. Yao, T.S. Chin, S.K. Chen, J. Appl. Phys. 76 (1994) 7071. [6] L. Schultz, J. Wecker, E. Hellstern, J. Appl. Phys. 61 (1987) 3583. [7] W. Liu, Q. Wang, X.K. Sun, X.G. Zhao, T. Zhao, Z.D. Zhang, Y.C. Chuang, J. Magn. Magn. Mater. 131 (1994) 413. [8] 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. 134 (1994) 197. [9] T. Kiyomiya, H. Kohmura, K. Matsui, Proc. 10th Int. Workshop on Rare-Earth Magnets and Their Application, Kytot, Japan (Society of Non-traditional Technology, Tokyo, 1989) vol. II, p. 551. [10] R. Ramesh, J.K. Chen, G. Thomas, J. Appl. Phys. 61 (1987) 2993. [11] J. Fidler, K.G. Knoch, H. Kronmu¨ller, G. Schneider, J. Mater. Res. 4 (1989) 806. [12] J.F. Herbst, J.J. Croat, W.B. Yelon, J. Appl. Phys. 57 (1985) 4086.
Structure and magnetic properties of Nd—Fe—B—Ti prepared by mechanical alloying Zhi-Dong Zhang, Wei Liu*, X.K. Sun, Xin-guo Zhao, Qun-feng Xiao, Yu-cheng Sui, Tong Zhao Institute of Metal Research, Academia Sinica, Shenyang 110015, People’s Republic of China Received 12 June 1997; received in revised form 3 November 1997
Abstract The structure and magnetic properties of Nd Fe B and Nd Fe B Ti alloys prepared by mechanical x 92~x 8 10 76 8~y 6`y alloying (MA) and subsequent annealing have been studied. It has been found that the exchange interaction between a soft a-Fe and a hard magnetic Nd Fe B phase results in a significant enhancement of the remanence for x(12. The 2 14 best values of 4pM "8.1 kGs, H "21.1 kOe and (BH) "14.1 MGOe are obtained for the series of Nd Fe B 3 * # .!9 x 92~x 8 with x"16. For the series of Nd Fe B Ti , when 0)y)6, the main phase Nd Fe B is formed and accom10 76 8~y 6`y 2 14 panied by Nd O , Fe Ti and Nd-rich phases. In the present experimental investigation, the occupation state of Ti in the 2 3 2 MA samples for 0)y)6 has been considered. The intrinsic coercivity reaches up to more than 11 kOe for 4)y)6. It is evident that the addition of Ti is favourable for the enhancement of the intrinsic coercivity in Nd—Fe—B alloys with a low level of Nd, which is mainly due to the coexistence of the Nd-rich phase and the hard magnetic phase with the structure of Nd Fe B containing Ti, combined with the suppression of a-Fe, for the samples with 0)y)6. ( 1998 2 14 Elsevier Science B.V. All rights reserved. PACS: 75.50.Bb; 75.60.Gm; 75.60.Jp; 81.40.R Keywords: Permanent magnets; Mechanical alloying; Remanence; Coercivity
1. Introduction In recent years, it was reported that nanocomposite magnets exhibit enhanced remanence in an isotropic state due to exchange coupling between their nanocrystalline grains of hard and soft magnetic phases [1,2]. For an isotropic Nd Fe B 10 84 6 * Corresponding author. Fax: #86 24 389 1320; e-mail: [email protected].
melt-spun material a remanence value of over 1 T was reported by Manaf et al. [3]. However, these magnets have somewhat reduced coercivity. The appropriate addition of Ti led to the enhancement of coercivity in Nd—Fe—B alloys with a low level of Nd and B contents by rapid quenching [4,5]. Schultz et al. reported that mechanical alloying (MA) and a subsequent solid-state reaction can be a route to form Nd—Fe—B magnets [6]. By this method some nanostructured magnets with main phase such as
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metastable SmFe N or Sm (Fe,Ti) phases are easy 7 d 5 17 to be formed after milling and subsequent annealing, with very high coercivities [7,8]. All the results previously reported urge us to study the role of Ti addition in the Nd—Fe—B alloys by mechanical alloying. In this paper, the effects of adding Ti on the phase structure and the magnetic properties of Nd—Fe—B alloys with a low level of Nd and B contents have been studied, using high-energy ball milling followed by a suitable heat treatment.
2. Experimental details The element powders with the purities of 99.6, 99, 99.5 and 99.5 for Fe, Ti, Nd and B elements, respectively, were mixed according to the compositions of Nd Fe B (x"8—16) and Nd Fe B Ti x 92~x 8 10 76 8~y 6`y (y"0—8). Mechanical alloying of the mixtures was performed under an argon atmosphere for 5 h. The MA powdered samples were annealed at 600—900°C for 30 min in a vacuum furnace which was directly connected to a closed glove box. X-ray diffraction (XRD) analysis of the powder samples was conducted using Cu K radiation with a Rigaku D/Maxa rA diffractometer equipped with a graphite crystal monochromator. Initial AC susceptibility measurement can be operated in the temperature range 300—1000 K with an AC field of 16 A/m and frequency of 1.13 kHz, which was applied to determine the Curie temperature and to estimate preliminarily the possible magnetic phases in the samples. For the magnetic measurements at room temperature, the powders were embedded in epoxy resin to form magnetically isotropic magnets. The magnetic properties were measured using a pulsed magnetometer in fields up to 15 T. The magnetization was related to the amount of magnetic powders, and the demagnetization factor of MA powder bonded samples due to the dilutional effect was approximately determined as 0.28 by experiments and the density of the samples was assumed to be 7.6 g/cm3.
3. Results and discussion In order to compare the magnetic properties of MA Nd—Fe—B—Ti samples with those of Nd—Fe—B
Fig. 1. Dependences of magnetic properties and M /M of MA 3 4 Nd Fe B annealed at 750°C for 30 min on Nd content. x 92~x 8
samples, at first, the magnetic properties of Nd—Fe—B alloys prepared by mechanical alloying and followed by annealing are studied. The Ndcontent dependences of the magnetic properties and M /M in the series of Nd Fe B annealed 3 4 x 92~x 8 at 750°C for 30 min are shown in Fig. 1. It is shown that the remanence of the samples increases with decreasing Nd content for x(12 and the ratio M /M ranges from 0.58 to 0.68. Therefore, the 3 4 exchange interaction between the soft a-Fe and the hard magnetic Nd Fe B phase results in an en2 14 hancement of the remanent magnetization. Even the ratio M /M of the samples for x*12 is above 3 4 0.55. It seems that the exchange interaction between nanocrystalline grains of the hard magnetic phases also leads to a slightly small remanence enhancement despite the presence of a Nd-rich phase at grain boundaries. The best values, 4pM "14.0 kGs, 4pM "8.1 kGs, H "21.1 kOe 4 3 * # and (BH) "14.1 MGOe, are obtained at x"16. .!9 The coercivity increases slightly for x)12, and rises rapidly with a further increase of the Nd content, as is expected. X-ray diffraction patterns of Nd Fe B and 10 82 8 Nd Fe B Ti (0)y)8) powders pre10 76 8~y 6`y pared by MA and annealed at 750°C for 30 min are shown in Fig. 2. Comparing the patterns
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Fig. 2. X-ray diffraction patterns of MA Nd Fe B and Nd Fe B Ti (0)y)8) powders annealed at 750°C for 30 min. 10 82 8 10 76 8~y 6`y
of Nd Fe B Ti powders with that of 10 76 8~y 6`y Nd Fe B powder in Fig. 2, it can be seen that 10 82 8 the content of a-Fe decreases with increasing Ti content. Hence, a lower a-Fe content in Nd Fe B Ti alloys can be obtained more 10 76 8~y 6`y easily than that in Nd Fe B alloy. For y"0, 10 82 8 the TbCu -type structure reported in Ref. [9] is not 7 observed from the X-ray data of Fig. 2. This is mainly due to the different methods to prepare the samples. Instead, the Nd Fe B phase coexists with 2 14 some a-Fe, and small amounts of Nd O and an 2 3
Nd-rich phase emerge in this sample. With increasing Ti content and decreasing B content, Fe Ti is 2 also formed for 2)y)4 besides the phases mentioned above. A nearly single-phase Nd Fe B ma2 14 terial is obtained for y"6, in which a small amount of Nd O , an Nd-rich phase and Fe Ti are 2 3 2 included. Nd Fe B and the 1 : 7 phase with the 2 14 TbCu structure are formed in coexistence with 7 a noticeable amount of Fe Ti and Nd-rich phase 2 for y"7. When the Ti content reaches 14 at%, three phases, namely 1 : 7, Fe Ti and Nd-rich 2
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phase, coexist as shown in Fig. 2. The black-box symbol in Fig. 2 stands for the Nd-rich phase, and these peaks belong to an FCC structure according to the analysis of the X-ray reflections of the MA samples. It was reported that an FCC structured Nd-rich intergranular phase stabilized by oxygen with a composition of nearly 95 at% Nd and 5 at% Fe existed in the sintered or die-upset R—Fe—B alloys. The lattice parameter of this phase is about 5.2 A_ [10,11]. Thus, a reasonable inference is that, this Nd-rich phase exists in the MA Nd—Fe—B—Ti alloy, too. As can be seen from Fig. 2 the X-ray reflections of the Nd-rich phase is shifted to lower angles, which may indicate that a part of Ti has entered the Nd-rich phase and results in the expansion of the lattice. Temperature dependences of AC susceptibility of MA Nd Fe B and 10 82 8 Nd Fe B Ti annealed at 750°C for 30 min 10 76 8~y 6`y are given in Fig. 3. From there, it is obvious that Nd Fe B-type phases with Curie temperature of 2 14 about 320°C are observed for 0)y)6. AC susceptibility measurement gives two Curie temperatures for y"7, corresponding to two magnetic phases having TbCu -type and the Nd Fe B-type 7 2 14 structures, respectively. When y"8, one Curie temperature of the 1 : 7 phase is detected. All the results of AC susceptibility measurements are in good agreement with the analysis of X-ray diffraction patterns. It is worth studying the occupation state of Ti in Nd Fe B Ti alloys for 10 76 8~y 6`y 0)y)6. As is observed from X-ray data, no evidence of element Ti can be found whereas a small amount of Fe Ti is visible. At the same time, 2 Ti-containing magnetic phases, such as TbCu -, 7 Th Zn - and ThMn -type structures, are not 2 17 12 found for 0)y)6, according to X-ray diffraction patterns and AC susceptibility measurements. It is seen from Fig. 2 that the X-ray diffraction peaks of the Nd Fe B phase are also shifted to lower 2 14 angles with increasing Ti content as a result of the lattice expansion. The lattice parameters of Nd Fe B calculated by analysing the X-ray dif10 82 8 fraction pattern are a"8.799 A_ and c"12.198 A_ , in agreement with those of Nd Fe B (a"8.80 A_ 2 14 and c"12.20 A_ ) reported by Herbst et al. [12]. The lattice parameters of the sample with y"0, corresponding to the composition of Nd Fe B Ti , are a"8.799 A_ and c"12.199 A_ . 10 76 8 6
Fig. 3. Temperature dependences of AC susceptibility of MA Nd Fe B and Nd Fe B Ti annealed at 750°C for 10 82 8 10 76 8~y 6`y 30 min.
Thus, it is concluded that Ti does not enter into the lattice of the main phase Nd Fe B for a relatively 2 14 low content of Ti, because the lattice parameters are almost unchanged. However, when y"6, the measured lattice parameters of the main phase of the alloy corresponding to the composition Nd Fe B Ti are a"8.822 A_ and c"12.24 A_ . 10 76 2 12 The larger values of a and c in the MA sample with a relatively high Ti content compared with the reported results of Nd Fe B imply that Ti may 2 14 partly occupy the sites of the tetragonal Nd Fe B 2 14 structure. It is well known that the atomic radius of B is much smaller than that of Ti. Usually, Ti can occupy the Fe sites in Nd Fe B. According to our 2 14 knowledge, it is difficult to believe that the single B site is occupied by Ti. Particularly, although only 2 at% of B and a large amount of Ti exist in the sample for y"6, a nearly single-phase Nd Fe B 2 14 is formed. For the present MA sample, it is
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that the addition of appropriate Ti is favourable for obtaining higher intrinsic coercivity in MA Nd Fe B Ti series, which is mainly due to 10 76 8~y 6`y the coexistence of the Nd-rich phase and the hard magnetic Nd Fe B phase, both containing Ti, 2 14 combined with the suppression of a-Fe, when 0)y)6. Therefore, an appropriate addition of Ti led to the enhancement of coercivity in Nd—Fe—B alloys with a low level of Nd.
Acknowledgements This work has been supported by the National Natural Science Foundation of China projects 59571014, 59421001 and 59725103, and the Science and Technology Commissions of Liaoning and Shenyang. Fig. 4. Composition dependences of magnetic properties of MA Nd Fe B Ti alloys annealed at 750°C for 30 min. 10 76 8~y 6`y
suggested that Ti might occupy part of the Nd and/or Fe sites, while some B sites may be empty, or two B atoms at the g sites could be replaced by Ti. This might explain preliminarily why the main phase with the Nd Fe B-type structure can form 2 14 in the limit of vanishing B content. Fig. 4 exhibits the dependences of magnetic properties on composition in MA Nd Fe B Ti 10 76 8~y 6`y alloys annealed at 750°C for 30 min. It can be seen that the intrinsic coercivity H first increases with * # Ti content, reaches a maximum of 11.6 kOe for y"4, and then decreases with further increase of Ti content. For 0)y)6 the remanence 4pM 3 and the maximum energy product (BH) .!9 decrease slightly with increasing Ti content and therefore decreasing B content. The best values, of 4pM "7.3 kGs and (BH) "11.4 MGOe are 3 .!9 obtained for y"6, together with an intrinsic coercivity of 11 kOe. For the samples with y'6, the magnetic properties decrease rapidly, which is mainly due to the existence of the Nd(Fe,Ti) phase 7 with an easy plane magnetization. From Fig. 1, the intrinsic coercivity of Nd Fe B alloy 10 82 8 is only 4.6 kOe in comparison and it is clear
References [1] R. Skomski, J.M.D. Coey, Phys. Rev. B 48 (21) (1993) 15812. [2] T. Schrefl, H. Kronmu¨ller, H.A. Davies, J. Magn. Magn. Mater. 127 (1993) L273. [3] A. Manaf, M. Al-Khafaji, P.Z. Zhang, H.A. Davies, R.A. Buckley, W.M. Rainforth, J. Magn. Magn. Mater. 128 (1993) 307. [4] Tsung-Shune Chin, Cheng-Hao Lin, Sen-Huan Huang, Jiunn-ming Yau, Tsung-Yao Chu, Cherng-Dean Wu, Jpn. J. Appl. Phys. 31 (1992) 3323. [5] J.M. Yao, T.S. Chin, S.K. Chen, J. Appl. Phys. 76 (1994) 7071. [6] L. Schultz, J. Wecker, E. Hellstern, J. Appl. Phys. 61 (1987) 3583. [7] W. Liu, Q. Wang, X.K. Sun, X.G. Zhao, T. Zhao, Z.D. Zhang, Y.C. Chuang, J. Magn. Magn. Mater. 131 (1994) 413. [8] 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. 134 (1994) 197. [9] T. Kiyomiya, H. Kohmura, K. Matsui, Proc. 10th Int. Workshop on Rare-Earth Magnets and Their Application, Kytot, Japan (Society of Non-traditional Technology, Tokyo, 1989) vol. II, p. 551. [10] R. Ramesh, J.K. Chen, G. Thomas, J. Appl. Phys. 61 (1987) 2993. [11] J. Fidler, K.G. Knoch, H. Kronmu¨ller, G. Schneider, J. Mater. Res. 4 (1989) 806. [12] J.F. Herbst, J.J. Croat, W.B. Yelon, J. Appl. Phys. 57 (1985) 4086.