- Email: [email protected]
Fe hybrid magnets
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 303 (2006) e410–e414 www.elsevier.com/locate/jmmm
Effect of host and added alloy composition on magnetic properties of Nd2Fe14B+Nd2Fe14B/Fe hybrid magnets H.W. Kwona,, Y. Zhangb, G.C. Hadjipanayisb a
School of Materials Science and Engineering, Pukyong National University, San 100, Yondang-Dong, Nam-Gu, Busan 608-739, Republic of Korea b University of Delaware, Newark, DE 19711, USA Available online 17 February 2006
Abstract The effect of the host and added alloy composition on the magnetic properties of the hybrid magnet consisting of Nd2Fe14B single phase and Nd2Fe14B/Fe nanocomposite phases was studied using the NdxFe93.5xGa0.5B6 (x ¼ 13:5 and 11.8) host alloy and NdxFe93xNb1B6 (x ¼ 6 and 9) nanocomposite added alloys. Magnetic performance of the die-upset hybrid magnet decreased with increasing the added nanocomposite alloys. The die-upset hybrid magnet containing added nanocomposite alloy with lower Nd-content had higher performance with respect to the magnets containing the added nanocomposite alloy with higher Nd-content. The die-upset hybrid magnets containing higher Nd-content host alloy had better performance with respect to the magnets with lower Nd-content host alloy. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Ww Keywords: Permanent magnet; Nd–Fe–B magnet; Nanocomposite; Hybrid magnet; Die-upset
1. Introduction Nanocomposite magnets exploiting the exchange-coupling through the grain boundary between magnetically hard and soft nanosize grains are attracting a great interest in the permanent magnetic field due to their enhanced remanence [1–3]. Most of the nanocomposite materials are isotropic in nature, in which the hard magnetic grains are randomly oriented. In order to exploit fully the potential of nanocomposite magnet, a texture in the hard magnetic grains is necessary. Much research efforts have been made to develop an effective means of achieving a full texture in the hard grains. However, the present state of the art of the texture development is far from a satisfaction. Recently, a hybridization technique [4,5], in which the R2Fe14B/a-Fe (R ¼ Nd, Pr) nanocomposite material is hybridized with a Nd2Fe14B single phase material, has been proposed. The anisotropic hybrid magnet is prepared by hot pressing and subsequent die-upsetting of the blend of R-rich Nd2Fe14B Corresponding author. Tel.: +82 51 620 1641; fax: +82 51 624 0746.
E-mail address: [email protected] (H.W. Kwon). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.056
single phase alloy and R-lean R2Fe14B/a-Fe (R ¼ Nd, Pr) nanocomposite alloy melt-spun ribbons. The die-upset hybrid magnet has an anisotropic nature, and this is thought to be due to a texture in the R-rich Nd2Fe14B single-phase material region formed during the die-upsetting. The texture is a key factor for improving an overall magnetic performance, and that is believed to be influenced crucially by the composition of the host and added alloys. The aim of the present study is therefore to provide knowledge on the effect of the host and added alloy composition on the magnetic properties of the hybrid magnet consisting of Nd2Fe14B single phase and Nd2Fe14B/Fe nanocomposite phases. 2. Experimentals In the present study two types of rare-earth-rich NdxFe93.5xGa0.5B6 (x ¼ 13:5 and 11.8) alloys were used as a Nd2Fe14B single-phase host alloy. The Nd11.8Fe81.7Ga0.5B6 alloy is near Nd2Fe14B stoichiometric and the Nd13.5Fe80Ga0.5B6 alloy has an excessive Nd. Two types of rare-earthlean NdxFe93xNb1B6 (x ¼ 6 and 9) alloys were used as a
ARTICLE IN PRESS H.W. Kwon et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e410–e414
Nd2Fe14B/Fe nanocomposite phase added alloy. The Nd6Fe87Nb1B6 alloy may have more amount of a-Fe in the nanocomposite form compared to the Nd9Fe84Nb1B6 alloy. The alloys were prepared by an arc-melting using the high purity component elements, and were melt-spun using Cu wheel with 30 m/s speed. The melt-spun alloy ribbons were crushed into powders, and powders were mixed thoroughly with the desired ratio. The mixed powder was then compacted in vacuum at 750 1C with a pressure up to 410 MPa and subsequently die-upset with height reduction of 80%. Magnetic characterization of the compacted and dieupset magnets was performed at 300 K along the direction parallel to the pressing direction in the compacting and dieupsetting. The cubic shape specimens (1 1 1 mm3) were pre-magnetized with dc field of 50 kOe in a SQUID and the room temperature demagnetization curve was measured using a combination of the SQUID and VSM. 3. Results and discussion Fig. 1 shows the magnetic property variation of the compacted and die-upset hybrid magnets consisting of Nd13.5Fe80Ga0.5B6 host alloy and NdxFe93xNb1B6 (x ¼ 6 and 9) added alloy with varying wt% of the added alloy. 16 compacted 4πMr (kG)
die-upset 12
8 16
iHc (kOe)
x=6
e411
For the compacted magnet the remanence increases with increasing the added alloys up to 20 wt% and then decreases slightly. The initial remanence increase in the hybrid compacted magnet may be due to the increased amount of added alloy. The added alloys are nanocomposites and have an enhanced remanence due to exchangecoupling between the Nd2Fe14B and a-Fe phases. The slight remanence decrease in remanence at higher amount of added nanocomposite alloys may be attributed to the lower density of the compact. Density of the hybrid compact decreased slightly with increasing the added alloys. The effect of chemical composition of the added nanocomposite alloy on the remanence of the hybrid compacted magnets can also be seen in Fig. 1. Remanence of the compacted hybrid magnet containing Nd6Fe87Nb1B6 added alloy shows consistently higher value with respect to the magnet containing Nd9Fe84Nb1B6 added alloy. This may be due to the higher enhanced remanence of the Nd6Fe87Nb1B6 nanocomposite alloy, which contains more a-Fe. For the die-upset hybrid magnet the remanence was greatly improved compared to that of the compacted magnet and it decreased with increasing the added alloys. The highly improved remanence may be due to the texture developed in the Nd13.5Fe80Ga0.5B6 host alloy region during the dieupsetting. As can be seen in Fig. 2 the microstructure of the Nd13.5Fe80Ga0.5B6 host alloy region in the die-upset hybrid magnet showed different feature from that of the same host alloy region in the compacted magnet. While the microstructure of the compacted magnet consisted of polyhedral equi-axed grains with around 100 nm diameters, the microstructure of the die-upset magnet consisted of elongated plate-like grains around 50 nm thick and 100–300 nm long. Selected area electron diffraction (SAD) showed that the plate-like grains in the host alloy region in the die-upset magnet had a preferred orientation, in which the easy magnetization (c-axis) is oriented along the
x=9
12 8 4
(BH) max (MGOe)
50 40 30 20 10 0 -10
0
10
20 30 Added alloy (wt%)
40
50
60
Fig. 1. Magnetic property variations of the compacted and die-upset hybrid magnets consisting of Nd13.5Fe80Ga0.5B6 and NdxFe93xNb1B6 (x ¼ 6 and 9) melt-spun ribbons as a function of wt% of the added R-lean nanocomposite alloy.
Fig. 2. TEM photographs showing the microstructure of the compacted and die-upset hybrid magnet consisting of 85 wt% Nd13.5Fe80Ga0.5B6 and 15 wt% Nd9Fe84Nb1B6: (a) host (a0 ) added alloy regions in compacted magnet (b) host (b0 ) added alloy regions in die-upset magnet.
ARTICLE IN PRESS H.W. Kwon et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e410–e414
300 200 Intensity (a. u.)
pressing direction. This microstructure is similar to that of the typical Nd–Fe–B die-upset magnet (MQIII-type) [6,7]. The equi-axed grains in the host alloy region in the compacted magnet oriented randomly. No evidence for a texture was observed in the added nanocomposite alloy region in the both compacted and die-upset magnets. The texture developed in the Nd13.5Fe80Ga0.5B6 host alloy region in the die-upset magnet leads to an anisotropic nature in die-upset hybrid magnet, thus the die-upset hybrid magnet has highly improved remanence with respect to the compacted hybrid magnet. As the amount of added nanocomposite alloy increases, the volume of anisotropic host alloy region with texture decreases, hence the overall remanence of the die-upset hybrid magnet decreases. It can also be seen that the remanence of the die-upset hybrid magnets containing Nd6Fe87Nb1B6 added alloy showed consistently higher value with respect to the magnets containing Nd9Fe84Nb1B6 added alloy. This may be due to the higher enhanced remanence of the added Nd6Fe87Nb1B6 nanocomposite alloy. The coercivity variation of the compacted and die-upset hybrid magnets consisting of Nd13.5Fe80Ga0.5B6 and NdxFe93xNb1B6 (x ¼ 6 and 9) with varying wt% of the added nanocomposite alloy was also shown in Fig. 1. For both the compacted and die-upset magnets the coercivity decreased with increasing the added nanocomposite alloys. Coercivity of the die-upset magnet was drastically deteriorated (o5 kOe) when the amount of added R-lean alloy exceeded 20 wt%. The coercivity deterioration with increasing the added nanocomposite alloy is attributed to the increased amount of nanocomposite alloy which has a far lower coercivity compared to the host alloy. The effect of chemical composition of the added nanocomposite alloy on the coercivity of the compacted and die-upset magnets was also shown in Fig. 1. The coercivity of the compacted and die-upset magnets containing Nd6Fe87Nb1B6 nanocomposite alloy showed consistently higher value with respect to the magnets containing the Nd9Fe84Nb1B6 nanocomposite alloy. This result is somewhat unexpected, because the added nanocomposite alloy with less Nd content (6 at%) may have a lower coercivity as compared to the nanocomposite alloy with more Nd (9 at%). The reason for this is not fully understood. It can be suggested, however, that the difference in the initial state of the melt-spun added alloy ribbons may play a key role in this matter (see Fig. 3). It can also be found that the coercivity of the die-upset magnet is consistently and significantly lower than that of the compacted magnet. This reduced coercivity may be due to the texture developed during the die-upsetting. The maximum energy product of the die-upset hybrid magnets decreases with increasing the added nanocomposite alloys, and this is simply due to the reduction of the remanence and coercivity of the magnet. In addition to the reduction in the remanence and coercivity the squareness of the second quadrant of the demagnetization curve may also play a role in part for reducing the maximum energy product. As can be seen in Fig. 4 the squareness of
Nd2Fe14B
Nd9Fe84Nb1B6
α-Fe
100 200 Nd6Fe87Nb1B6 α-Fe 100
0
30
35
40 2 θ (°)
45
50
Fig. 3. XRD spectra of the melt-spun ribbons with different composition.
15
(b)
(a)
10
(c')
4πM (kG)
e412
5
(b') 0
-15
-10
(c) -5 0 5 Applied field (kOe)
10
15
Fig. 4. Demagnetization curves of the hybrid magnets consisting of Nd13.5Fe80Ga0.5B6 host alloy and various amount of added alloys: (a) 0 wt% added alloy, (b) 15 wt% Nd6Fe87Nb1B6, (b0 ) 15 wt% Nd9Fe84Nb1B6, (c) 30 wt% Nd6Fe87Nb1B6 and (c0 ) 30 wt% Nd9Fe84Nb1B6.
demagnetization curve of the die-upset hybrid magnet was deteriorated with increasing the amount of added nanocomposite alloy. The effect of host alloy composition on the magnetic properties of the hybrid magnet was examined using two different types of host alloys NdxFe93.5xGa0.5B6 (x ¼ 11:8 and 13.5), and the results were shown in Fig. 5. As can be seen the hybrid magnets containing higher Nd-content (13.5 at%) host alloy have higher coercivity with respect to the magnets with lower Nd-content (11.8 at%) host alloy. It is worth noting that the die-upset magnet containing higher Nd-content host alloy has much higher remanence with respect to the magnet containing lower Nd-content host alloy. This can be attributed to the better texture developed in the Nd13.5Fe80Ga0.5B6 host alloy region rather than in the Nd11.8Fe81.7Ga0.5B6 host alloy region in the die-upset magnet. It is known [8–11] that in the typical
ARTICLE IN PRESS H.W. Kwon et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e410–e414
15
e413
15 compacted
compacted
10
4πM (kG)
4πM (kG)
10
5
5 (a)
(a) (b)
(b)
0
0 die-upset
die-upset
10
(a)
4πM (kG)
4πM (kG)
10
(a) 5
(b) 5
(b) 0 -20
-15
-10
-5 0 5 Applied field (kOe)
10
15
20
0 -20
-15
-10
-5 0 5 Applied field (kOe)
10
15
20
Fig. 5. Demagnetization curve of the die-upset hybrid magnets consisting of 85 wt% NdxFe93.5xGa0.5B6 (x ¼ 13:5 and 11.8) host alloys and 15 wt% Nd9Fe84Nb1B6 added alloy: (a) x ¼ 13:5 and (b) x ¼ 11:8.
Fig. 6. Comparison of the demagnetization curves of the compacted and die-upset: (a) hybrid and (b) single alloy magnets having an identical overall composition of Nd12.7Fe80.8Ga0.4Nb0.1B6. The hybrid magnet consists of 90 wt% Nd13.5Fe80Ga0.5B6 and 10 wt% Nd6Fe87Nb1B6 alloys.
die-upset Nd–Fe–B magnet (MQIII-type) the texture becomes poorer as the Nd-content in the alloy decreases towards the Nd2Fe14B stoichiometric composition. The ultimate goal of the hybridization is to improve an overall magnetic performance by blending a Nd2Fe14B single phase host alloy with high coercivity and Nd2Fe14B / a-Fe nanocomposite alloy with higher magnetization. It would seem, however, that the hybridization deteriorates the overall magnetic performance considering the fact that the die-upset hybrid magnet had much poorer magnetic performance compared to the die-upset magnet prepared from the Nd13.5Fe80Ga0.5B6 host alloy alone. It should be noted here that the hybrid magnet and the Nd13.5Fe80Ga0.5B6 single alloy magnet have different overall compositions. It is interesting, therefore, to compare the magnetic properties of the hybrid and single alloy magnets having an identical overall chemical composition. Fig. 6 shows the demagnetization curves of the compacted and die-upset magnets made from the hybrid and single alloys having an identical overall composition. The hybrid magnet contains 90 wt% Nd13.5Fe80Ga0.5B6 and 10 wt% Nd6Fe87Nb1B6 alloys and its overall composition is Nd12.7Fe80.8Ga0.4Nb0.1B6, which is identical with the composition of single alloy. It can be seen that the hybrid magnets have significantly higher coercivity than the single alloy counterpart magnets in both the compacted and die-upset states. The host alloy with higher
Nd content (13.5 at% Nd) may maintain higher coercivity in the hybrid magnet compared to the single alloy with less Nd content (12.7 wt% Nd). The die-upset hybrid magnet has much higher remanence with respect to the single alloy counterpart magnet. A better texture developed in the host alloy region in the die-upset hybrid magnet may be responsible for the higher remanence. It is known [7,8] that in the typical single alloy die-upset Nd–Fe–B magnet the texture becomes poorer as the Nd content decreases. Therefore, a poor texture is expected from the single alloy die-upset magnet, and this is evidenced by the fact that the remanence of die-upset single alloy magnet is not improved from that of the compacted magnet.
4. Conclusion Magnetic performance of the die-upset hybrid magnet decreased with increasing the added nanocomposite alloys, and a more profound deterioration was found when the amount of nanocomposite alloy exceeded 20 wt%. The dieupset hybrid magnet containing added nanocomposite alloy with lower Nd-content had higher performance with respect to the magnets containing the added nanocomposite alloy with higher Nd-content. The die-upset hybrid magnets containing higher Nd-content host alloy had
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higher performance with respect to the magnets with lower Nd-content host alloy. Acknowledgements This work was supported by the Defence Advanced Research Projects Agency (DARPA) and the Pukyong National University Research Fund. References [1] R. Coehoorn, D.B. de Mooij, J.P.W.B. Duchateau, K.H.J. Buschow, J. Phys. Colloq. C 8 (49) (1988) 669.
[2] E.F. Kneller, R. Hawig, IEEE Trans. Mag. 27 (1991) 3588. [3] R. Skomski, J.M.D. Coey, Phys. Rev. B 48 (1993) 15812. [4] A.M. Gabay, Y. Zhang, G.C. Hadjipanayis, Appl. Phys. Lett. 85 (2004) 446. [5] H.W. Kwon, G.C. Hadjipanayis, IEEE Trans. Mag. 41 (2005) 3856. [6] R.W. Lee, Appl. Phys. Lett. 46 (1985) 790. [7] T. Saito, M. Fujita, T. Kuji, K. Fukuoka, Y. Syono, J. Appl. Phys. 83 (1998) 6390. [8] L. Li, C.D. Graham, IEEE Trans. Magn. 28 (1992) 2130. [9] L.H. Lewis, T.R. Thurston, V. Pachanathan, U. Wildgruber, D.O. Welch, J. Appl. Phys. 82 (1997) 3430. [10] P. Tenaud, A. Chamberaid, V. Vanoni, Solid State Commun. 63 (1987) 303. [11] M. Leonowicz, H.A. Davies, Mater. Lett. 19 (1994) 275.
Journal of Magnetism and Magnetic Materials 303 (2006) e410–e414 www.elsevier.com/locate/jmmm
Effect of host and added alloy composition on magnetic properties of Nd2Fe14B+Nd2Fe14B/Fe hybrid magnets H.W. Kwona,, Y. Zhangb, G.C. Hadjipanayisb a
School of Materials Science and Engineering, Pukyong National University, San 100, Yondang-Dong, Nam-Gu, Busan 608-739, Republic of Korea b University of Delaware, Newark, DE 19711, USA Available online 17 February 2006
Abstract The effect of the host and added alloy composition on the magnetic properties of the hybrid magnet consisting of Nd2Fe14B single phase and Nd2Fe14B/Fe nanocomposite phases was studied using the NdxFe93.5xGa0.5B6 (x ¼ 13:5 and 11.8) host alloy and NdxFe93xNb1B6 (x ¼ 6 and 9) nanocomposite added alloys. Magnetic performance of the die-upset hybrid magnet decreased with increasing the added nanocomposite alloys. The die-upset hybrid magnet containing added nanocomposite alloy with lower Nd-content had higher performance with respect to the magnets containing the added nanocomposite alloy with higher Nd-content. The die-upset hybrid magnets containing higher Nd-content host alloy had better performance with respect to the magnets with lower Nd-content host alloy. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Ww Keywords: Permanent magnet; Nd–Fe–B magnet; Nanocomposite; Hybrid magnet; Die-upset
1. Introduction Nanocomposite magnets exploiting the exchange-coupling through the grain boundary between magnetically hard and soft nanosize grains are attracting a great interest in the permanent magnetic field due to their enhanced remanence [1–3]. Most of the nanocomposite materials are isotropic in nature, in which the hard magnetic grains are randomly oriented. In order to exploit fully the potential of nanocomposite magnet, a texture in the hard magnetic grains is necessary. Much research efforts have been made to develop an effective means of achieving a full texture in the hard grains. However, the present state of the art of the texture development is far from a satisfaction. Recently, a hybridization technique [4,5], in which the R2Fe14B/a-Fe (R ¼ Nd, Pr) nanocomposite material is hybridized with a Nd2Fe14B single phase material, has been proposed. The anisotropic hybrid magnet is prepared by hot pressing and subsequent die-upsetting of the blend of R-rich Nd2Fe14B Corresponding author. Tel.: +82 51 620 1641; fax: +82 51 624 0746.
E-mail address: [email protected] (H.W. Kwon). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.056
single phase alloy and R-lean R2Fe14B/a-Fe (R ¼ Nd, Pr) nanocomposite alloy melt-spun ribbons. The die-upset hybrid magnet has an anisotropic nature, and this is thought to be due to a texture in the R-rich Nd2Fe14B single-phase material region formed during the die-upsetting. The texture is a key factor for improving an overall magnetic performance, and that is believed to be influenced crucially by the composition of the host and added alloys. The aim of the present study is therefore to provide knowledge on the effect of the host and added alloy composition on the magnetic properties of the hybrid magnet consisting of Nd2Fe14B single phase and Nd2Fe14B/Fe nanocomposite phases. 2. Experimentals In the present study two types of rare-earth-rich NdxFe93.5xGa0.5B6 (x ¼ 13:5 and 11.8) alloys were used as a Nd2Fe14B single-phase host alloy. The Nd11.8Fe81.7Ga0.5B6 alloy is near Nd2Fe14B stoichiometric and the Nd13.5Fe80Ga0.5B6 alloy has an excessive Nd. Two types of rare-earthlean NdxFe93xNb1B6 (x ¼ 6 and 9) alloys were used as a
ARTICLE IN PRESS H.W. Kwon et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e410–e414
Nd2Fe14B/Fe nanocomposite phase added alloy. The Nd6Fe87Nb1B6 alloy may have more amount of a-Fe in the nanocomposite form compared to the Nd9Fe84Nb1B6 alloy. The alloys were prepared by an arc-melting using the high purity component elements, and were melt-spun using Cu wheel with 30 m/s speed. The melt-spun alloy ribbons were crushed into powders, and powders were mixed thoroughly with the desired ratio. The mixed powder was then compacted in vacuum at 750 1C with a pressure up to 410 MPa and subsequently die-upset with height reduction of 80%. Magnetic characterization of the compacted and dieupset magnets was performed at 300 K along the direction parallel to the pressing direction in the compacting and dieupsetting. The cubic shape specimens (1 1 1 mm3) were pre-magnetized with dc field of 50 kOe in a SQUID and the room temperature demagnetization curve was measured using a combination of the SQUID and VSM. 3. Results and discussion Fig. 1 shows the magnetic property variation of the compacted and die-upset hybrid magnets consisting of Nd13.5Fe80Ga0.5B6 host alloy and NdxFe93xNb1B6 (x ¼ 6 and 9) added alloy with varying wt% of the added alloy. 16 compacted 4πMr (kG)
die-upset 12
8 16
iHc (kOe)
x=6
e411
For the compacted magnet the remanence increases with increasing the added alloys up to 20 wt% and then decreases slightly. The initial remanence increase in the hybrid compacted magnet may be due to the increased amount of added alloy. The added alloys are nanocomposites and have an enhanced remanence due to exchangecoupling between the Nd2Fe14B and a-Fe phases. The slight remanence decrease in remanence at higher amount of added nanocomposite alloys may be attributed to the lower density of the compact. Density of the hybrid compact decreased slightly with increasing the added alloys. The effect of chemical composition of the added nanocomposite alloy on the remanence of the hybrid compacted magnets can also be seen in Fig. 1. Remanence of the compacted hybrid magnet containing Nd6Fe87Nb1B6 added alloy shows consistently higher value with respect to the magnet containing Nd9Fe84Nb1B6 added alloy. This may be due to the higher enhanced remanence of the Nd6Fe87Nb1B6 nanocomposite alloy, which contains more a-Fe. For the die-upset hybrid magnet the remanence was greatly improved compared to that of the compacted magnet and it decreased with increasing the added alloys. The highly improved remanence may be due to the texture developed in the Nd13.5Fe80Ga0.5B6 host alloy region during the dieupsetting. As can be seen in Fig. 2 the microstructure of the Nd13.5Fe80Ga0.5B6 host alloy region in the die-upset hybrid magnet showed different feature from that of the same host alloy region in the compacted magnet. While the microstructure of the compacted magnet consisted of polyhedral equi-axed grains with around 100 nm diameters, the microstructure of the die-upset magnet consisted of elongated plate-like grains around 50 nm thick and 100–300 nm long. Selected area electron diffraction (SAD) showed that the plate-like grains in the host alloy region in the die-upset magnet had a preferred orientation, in which the easy magnetization (c-axis) is oriented along the
x=9
12 8 4
(BH) max (MGOe)
50 40 30 20 10 0 -10
0
10
20 30 Added alloy (wt%)
40
50
60
Fig. 1. Magnetic property variations of the compacted and die-upset hybrid magnets consisting of Nd13.5Fe80Ga0.5B6 and NdxFe93xNb1B6 (x ¼ 6 and 9) melt-spun ribbons as a function of wt% of the added R-lean nanocomposite alloy.
Fig. 2. TEM photographs showing the microstructure of the compacted and die-upset hybrid magnet consisting of 85 wt% Nd13.5Fe80Ga0.5B6 and 15 wt% Nd9Fe84Nb1B6: (a) host (a0 ) added alloy regions in compacted magnet (b) host (b0 ) added alloy regions in die-upset magnet.
ARTICLE IN PRESS H.W. Kwon et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e410–e414
300 200 Intensity (a. u.)
pressing direction. This microstructure is similar to that of the typical Nd–Fe–B die-upset magnet (MQIII-type) [6,7]. The equi-axed grains in the host alloy region in the compacted magnet oriented randomly. No evidence for a texture was observed in the added nanocomposite alloy region in the both compacted and die-upset magnets. The texture developed in the Nd13.5Fe80Ga0.5B6 host alloy region in the die-upset magnet leads to an anisotropic nature in die-upset hybrid magnet, thus the die-upset hybrid magnet has highly improved remanence with respect to the compacted hybrid magnet. As the amount of added nanocomposite alloy increases, the volume of anisotropic host alloy region with texture decreases, hence the overall remanence of the die-upset hybrid magnet decreases. It can also be seen that the remanence of the die-upset hybrid magnets containing Nd6Fe87Nb1B6 added alloy showed consistently higher value with respect to the magnets containing Nd9Fe84Nb1B6 added alloy. This may be due to the higher enhanced remanence of the added Nd6Fe87Nb1B6 nanocomposite alloy. The coercivity variation of the compacted and die-upset hybrid magnets consisting of Nd13.5Fe80Ga0.5B6 and NdxFe93xNb1B6 (x ¼ 6 and 9) with varying wt% of the added nanocomposite alloy was also shown in Fig. 1. For both the compacted and die-upset magnets the coercivity decreased with increasing the added nanocomposite alloys. Coercivity of the die-upset magnet was drastically deteriorated (o5 kOe) when the amount of added R-lean alloy exceeded 20 wt%. The coercivity deterioration with increasing the added nanocomposite alloy is attributed to the increased amount of nanocomposite alloy which has a far lower coercivity compared to the host alloy. The effect of chemical composition of the added nanocomposite alloy on the coercivity of the compacted and die-upset magnets was also shown in Fig. 1. The coercivity of the compacted and die-upset magnets containing Nd6Fe87Nb1B6 nanocomposite alloy showed consistently higher value with respect to the magnets containing the Nd9Fe84Nb1B6 nanocomposite alloy. This result is somewhat unexpected, because the added nanocomposite alloy with less Nd content (6 at%) may have a lower coercivity as compared to the nanocomposite alloy with more Nd (9 at%). The reason for this is not fully understood. It can be suggested, however, that the difference in the initial state of the melt-spun added alloy ribbons may play a key role in this matter (see Fig. 3). It can also be found that the coercivity of the die-upset magnet is consistently and significantly lower than that of the compacted magnet. This reduced coercivity may be due to the texture developed during the die-upsetting. The maximum energy product of the die-upset hybrid magnets decreases with increasing the added nanocomposite alloys, and this is simply due to the reduction of the remanence and coercivity of the magnet. In addition to the reduction in the remanence and coercivity the squareness of the second quadrant of the demagnetization curve may also play a role in part for reducing the maximum energy product. As can be seen in Fig. 4 the squareness of
Nd2Fe14B
Nd9Fe84Nb1B6
α-Fe
100 200 Nd6Fe87Nb1B6 α-Fe 100
0
30
35
40 2 θ (°)
45
50
Fig. 3. XRD spectra of the melt-spun ribbons with different composition.
15
(b)
(a)
10
(c')
4πM (kG)
e412
5
(b') 0
-15
-10
(c) -5 0 5 Applied field (kOe)
10
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Fig. 4. Demagnetization curves of the hybrid magnets consisting of Nd13.5Fe80Ga0.5B6 host alloy and various amount of added alloys: (a) 0 wt% added alloy, (b) 15 wt% Nd6Fe87Nb1B6, (b0 ) 15 wt% Nd9Fe84Nb1B6, (c) 30 wt% Nd6Fe87Nb1B6 and (c0 ) 30 wt% Nd9Fe84Nb1B6.
demagnetization curve of the die-upset hybrid magnet was deteriorated with increasing the amount of added nanocomposite alloy. The effect of host alloy composition on the magnetic properties of the hybrid magnet was examined using two different types of host alloys NdxFe93.5xGa0.5B6 (x ¼ 11:8 and 13.5), and the results were shown in Fig. 5. As can be seen the hybrid magnets containing higher Nd-content (13.5 at%) host alloy have higher coercivity with respect to the magnets with lower Nd-content (11.8 at%) host alloy. It is worth noting that the die-upset magnet containing higher Nd-content host alloy has much higher remanence with respect to the magnet containing lower Nd-content host alloy. This can be attributed to the better texture developed in the Nd13.5Fe80Ga0.5B6 host alloy region rather than in the Nd11.8Fe81.7Ga0.5B6 host alloy region in the die-upset magnet. It is known [8–11] that in the typical
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Fig. 5. Demagnetization curve of the die-upset hybrid magnets consisting of 85 wt% NdxFe93.5xGa0.5B6 (x ¼ 13:5 and 11.8) host alloys and 15 wt% Nd9Fe84Nb1B6 added alloy: (a) x ¼ 13:5 and (b) x ¼ 11:8.
Fig. 6. Comparison of the demagnetization curves of the compacted and die-upset: (a) hybrid and (b) single alloy magnets having an identical overall composition of Nd12.7Fe80.8Ga0.4Nb0.1B6. The hybrid magnet consists of 90 wt% Nd13.5Fe80Ga0.5B6 and 10 wt% Nd6Fe87Nb1B6 alloys.
die-upset Nd–Fe–B magnet (MQIII-type) the texture becomes poorer as the Nd-content in the alloy decreases towards the Nd2Fe14B stoichiometric composition. The ultimate goal of the hybridization is to improve an overall magnetic performance by blending a Nd2Fe14B single phase host alloy with high coercivity and Nd2Fe14B / a-Fe nanocomposite alloy with higher magnetization. It would seem, however, that the hybridization deteriorates the overall magnetic performance considering the fact that the die-upset hybrid magnet had much poorer magnetic performance compared to the die-upset magnet prepared from the Nd13.5Fe80Ga0.5B6 host alloy alone. It should be noted here that the hybrid magnet and the Nd13.5Fe80Ga0.5B6 single alloy magnet have different overall compositions. It is interesting, therefore, to compare the magnetic properties of the hybrid and single alloy magnets having an identical overall chemical composition. Fig. 6 shows the demagnetization curves of the compacted and die-upset magnets made from the hybrid and single alloys having an identical overall composition. The hybrid magnet contains 90 wt% Nd13.5Fe80Ga0.5B6 and 10 wt% Nd6Fe87Nb1B6 alloys and its overall composition is Nd12.7Fe80.8Ga0.4Nb0.1B6, which is identical with the composition of single alloy. It can be seen that the hybrid magnets have significantly higher coercivity than the single alloy counterpart magnets in both the compacted and die-upset states. The host alloy with higher
Nd content (13.5 at% Nd) may maintain higher coercivity in the hybrid magnet compared to the single alloy with less Nd content (12.7 wt% Nd). The die-upset hybrid magnet has much higher remanence with respect to the single alloy counterpart magnet. A better texture developed in the host alloy region in the die-upset hybrid magnet may be responsible for the higher remanence. It is known [7,8] that in the typical single alloy die-upset Nd–Fe–B magnet the texture becomes poorer as the Nd content decreases. Therefore, a poor texture is expected from the single alloy die-upset magnet, and this is evidenced by the fact that the remanence of die-upset single alloy magnet is not improved from that of the compacted magnet.
4. Conclusion Magnetic performance of the die-upset hybrid magnet decreased with increasing the added nanocomposite alloys, and a more profound deterioration was found when the amount of nanocomposite alloy exceeded 20 wt%. The dieupset hybrid magnet containing added nanocomposite alloy with lower Nd-content had higher performance with respect to the magnets containing the added nanocomposite alloy with higher Nd-content. The die-upset hybrid magnets containing higher Nd-content host alloy had
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higher performance with respect to the magnets with lower Nd-content host alloy. Acknowledgements This work was supported by the Defence Advanced Research Projects Agency (DARPA) and the Pukyong National University Research Fund. References [1] R. Coehoorn, D.B. de Mooij, J.P.W.B. Duchateau, K.H.J. Buschow, J. Phys. Colloq. C 8 (49) (1988) 669.
[2] E.F. Kneller, R. Hawig, IEEE Trans. Mag. 27 (1991) 3588. [3] R. Skomski, J.M.D. Coey, Phys. Rev. B 48 (1993) 15812. [4] A.M. Gabay, Y. Zhang, G.C. Hadjipanayis, Appl. Phys. Lett. 85 (2004) 446. [5] H.W. Kwon, G.C. Hadjipanayis, IEEE Trans. Mag. 41 (2005) 3856. [6] R.W. Lee, Appl. Phys. Lett. 46 (1985) 790. [7] T. Saito, M. Fujita, T. Kuji, K. Fukuoka, Y. Syono, J. Appl. Phys. 83 (1998) 6390. [8] L. Li, C.D. Graham, IEEE Trans. Magn. 28 (1992) 2130. [9] L.H. Lewis, T.R. Thurston, V. Pachanathan, U. Wildgruber, D.O. Welch, J. Appl. Phys. 82 (1997) 3430. [10] P. Tenaud, A. Chamberaid, V. Vanoni, Solid State Commun. 63 (1987) 303. [11] M. Leonowicz, H.A. Davies, Mater. Lett. 19 (1994) 275.