- Email: [email protected]
Anisotropic Nd–Fe–B thick film magnets
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 303 (2006) e375–e378 www.elsevier.com/locate/jmmm
Anisotropic Nd–Fe–B thick film magnets M. Nakanoa,, S. Satoa, H. Fukunagaa, F. Yamashitab a
Faculty of Engineering, Nagasaki University, 1-14 Bunkyo-Machi, Nagasaki, 852-8521, Japan b Motor R&D Laboratory, Panasonic, 7-1-1 Morofuku, Daito, Osaka 574-0044, Japan Available online 13 March 2006
Abstract The substrate heating system was applied to the high-speed pulsed laser deposition (PLD) method and it was clarified that anisotropic Nd–Fe–B thick film magnets can be obtained under the high deposition rate of larger than 20 mm/h. The obtained coercivity (HCJ), remanence (Br) and (BH)max of an anisotropic film magnets were approximately 500 kA/m, 0.75 T and 75 kJ/m3, respectively. The values of remanence and (BH)max were improved compared with those for the isotropic PLD-made thick films. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Ww Keywords: Pulsed laser deposition; Thick film magnets; Nd–Fe–B; Anisotropic film magnets
1. Introduction In order to advance a size reduction in electronic devices such as milli-size motors and micro-actuators, Nd–Fe–B film magnets thicker than 10 mm have been prepared by the sputtering method [1–5]. It was also confirmed that a substrate heating method enabled us to obtain anisotropic sputtering-made Nd–Fe–B samples [1–6]. For example, Uehara [5] reported superior magnetic properties of a Nd–Fe–B/Ta multiplayered thick film. (HCJ: 1210 kA/m, Jr: 1.24 T, (BH)max : 279 kJ/m3). However, it is generally said that the sputtering method has a difficulty in realizing high deposition rate. Although Kapitanov et al. [2] reported anisotropic Nd–Fe–B film magnets with thickness of 30–300 mm under a high deposition rate of 20–40 mm/h, (1) the detail conditions of the sputtering method with a high deposition rate, (2) the relationship between magnetic properties and the film thickness, and (3) the evaluation method of magnetization were not shown clearly. On the other hand, we have already reported Nd–Fe–B thick films by a PLD method with a deposition rate of 20–40 mm/h, and obtained isotropic magnets with the coercivity, remanence and (BH)max values of approxiCorresponding author. Tel./fax: +81 95 819 2555.
E-mail address: [email protected] (M. Nakano). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.240
mately 800 kA/m, 0.6 T, and 55 kJ/m3, respectively, through the post annealing process [7,8]. In addition, we succeeded in appling a 200 mm thick Nd–Fe–B film on a Fe substrate to a milli-size motor, and reported that the motor rotates at 15,160 rpm under no-load test and has torque constant of 0.0236 Nm/A at the gap of 0.1 mm between a rotor and a stator [9]. A further increase in the remanence is considered to be an effective way to improve the characteristics of the motor. Namely, preparation of anisotropic Nd–Fe–B films is a promising method for enhancing remanence. Previously, it has been reported that the PLD method under an optimum substrate heating condition enables us to obtain anisotropic Nd–Fe–B films [10]. The film thickness, however, was thinner than 1 mm. In this study, we applied a substrate heating system to the high-speed PLD method and tried to obtain anisotropic Nd–Fe–B film magnets prepared under a high deposition rate of 20–40 mm/h. Resultantly, the obtained remanence of 0.75 T was higher than that of previously reported PLDmade isotropic Nd–Fe–B thick film magnets [7,8]. 2. Experimental In order to compensate loss of metallic Nd due to evaporation as well as oxidation, the nominal composition
ARTICLE IN PRESS M. Nakano et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e375–e378
of targets was set to Nd2.4Fe14B. The targets include a larger amount of Nd than the stoichiometric composition. These targets were ablated with a Nd-YAG pulse laser (l ¼ 355 nm) at the repetition rate of 30 Hz, and the distance between a target and a Ta substrate was fixed at 10 mm. Before the ablation, the chamber was evacuated down to approximately 104 Pa with a molecular turbo pump. Deposition time was fixed at 1 h, and a Ti sublimation pump was used as an auxiliary pump during the deposition. Fig. 1 shows a diagram of a substrate heating system which was made of Ta sheet. Electric current flowed through the Ta sheet, and a sample on a substrate was annealed by Joule heat during deposition. The substrate temperature was varied from 373 to 1173 K. In-plane and perpendicular M–H loops were measured with a vibrating sample magnetometer (VSM). The analyses of crystal structure were carried out with an Xray diffractometer. Thickness was measured with a digital micrometer to 70.5 mm. 3. Results and discussion The control of substrate temperature is a key point to obtain anisotropic Nd–Fe–B films [9]. Therefore, we investigated the in-plane and perpendicular magnetic properties of the films fabricated under various substrate temperatures. Fig. 2 shows the in-plane and perpendicular coercivity as a function of the substrate temperature Ts. When Ts was lower than 650 K, the films did not exhibit distinguishable coercivity. We also confirmed that the Xray diffraction patterns of the films deposited at 350 and 650 K did not have any peaks originating from crystal structures. The extremely low coercivity of the films deposited at temperatures lower than 650 K can be attributed to the amorphous state of the films. The deposited films were crystallized when the heating temperature was higher than 680 K. The coercivity, however, decreased significantly with increase in the temperature beyond 1000 K. The heating temperature of 873 K lead to the maximum in-plane and perpendicular coercivity values,
Substrate heating system (Ta sheet)
Substrate
Plume
Target Pulse Laser Fig. 1. Diagram of PLD method with a substrate heating system. Electric current flowed through the Ta sheet, and a sample on a substrate was annealed by Joule heat during deposition.
500
Coercivity [kA/m]
e376
in-plane perpendicular
400 300 200 100 0
400
600 800 1000 Ts ; Substrate temperature [K]
1200
Fig. 2. Coercivity of PLD-made Nd–Fe–B films as a function of substrate temperature. The heating temperature of 873 K lead to the maximum in-plane and perpendicular coercivity values, approximately 520 and 490 kA/m, respectively.
approximately 516 and 484 kA/m, respectively. The abovementioned values are smaller by approximately 300 kA/m than that reported in Ref. [7,8]. We considered that the reduction in coercivity is attributed to the degraded microstructure in the deposited films for the substrate heating during PLD process. Further investigation on the microstructure is needed for the PLD-made thick film magnets. Fig. 3 shows M–H loops of the above-mentioned film at 873 K. The correction of the demagnetization effect is not carried out. The in-plane and perpendicular coercivity values were almost the same. In isotropic film, the magnetization of the perpendicular loop was lower than that of the in-plane one by demagnetization effect. However, in the film prepared by the substrate heating method, the magnetization of the perpendicular loop was higher than that of the in-plane one. In order to investigate the origins of a high magnetization value, X-ray diffraction patterns of the Nd–Fe–B film were observed and were shown in Fig. 4. Here, in the figure, the pattern of the previously reported isotropic film is also shown [8]. The peak intensities corresponding to the c-plane such as (0 0 4), (0 0 6), (0 0 8) and (1 0 5) for the film prepared in this study are stronger than those for the isotropic film. This result indicates that usage of the substrate heating system enables us to obtain anisotropic Nd–Fe–B thick film magnets in the high-speed PLD method with the deposition rate of 20–40 mm. Therefore, we can conclude that a high-speed deposition of anisotropic Nd–Fe–B thick films was achieved and that the substrate heating is effective in preparation of anisotropic film magnets under high deposition rate. Fig. 5 shows a demagnetization curve measured in the perpendicular direction for the film deposited at 873 K. The demagnetizing effect was corrected by using the demagnetizing factor of 1.0. From the figure, the remanence and (BH)max values are deduced as 0.75 T and 75 kJ/m3,
ARTICLE IN PRESS M. Nakano et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e375–e378
Magnetization [T]
Thickness 23.6 [µm] In-plane Hc : 516 [kA/m] Perpendicular
-2000
1
1 PLD-made Nd-Fe-B film magnet Substrate temperature : 873 K
0.8
0.6 B [T]
Hc : 484 [kA/m]
e377
-1000
0
1000 2000 Applied field [kA/m]
0.4
0.2
-600
-500
-400
-1
(008) Sample A
Sample B
30
40
50
-200
-100
0
Fig. 5. Demagnetization curve of a film deposited at the substrate temperature of 873 K. The curve was measured in the perpendicular direction and corrected by using the demagnetization factor of 1.0.
films under high deposition rate is strongly required. Accordingly, it was clarified that further improvement in the magnetic properties for the films under high deposition rate is strongly required. 4. Conclusion
(004)
Intensity [arb.unit]
Nd2O3
(105)
Nd2Fe14B
(006)
Fig. 3. In-plane and perpendicular M–H loops of a Nd–Fe–B film deposited on Ta substrate at 873 K. Although the coercivity values of the both loops are almost the same, the magnetization in the perpendicular direction is higher than that of the in-plane loop. The correction of the demagnetization effect is not carried out.
-300 H [kA/m]
60
We applied a heating substrate method to the high-speed PLD system, and succeeded in preparing a Nd–Fe–B thick film magnet with perpendicular magnetic anisotropy, whose coercivity, remanence and (BH)max of a thick film magnet were approximately 500 kA/m, 0.75 T and 75 kJ/m3, respectively. Although the high-speed PLD method comprising of a heating substrate system is promising to obtain anisotropic Nd–Fe–B thick film magnets under high deposition rate, further improvement in the magnetic properties for PLDmade thick film magnets is necessary.
2θ [degree] Fig. 4. X-ray diffraction patterns of Nd–Fe–B films deposited on Ta substrate. Sample A was deposited on a substrate heated at 873 K. Sample B was deposited without substrate heating and crystallized by postannealing at 923 K [7]. The sample B has an isotropic structure.
respectively. These values were larger by approximately 0.15 T and 20 kJ/m3, respectively, than those for the isotropic film reported previously [7,8]. The magnetic properties for the PLD-made anisotropic thick films, however, were inferior compared with those for anisotropic film magnets prepared by sputtering method [4–6]. In addition, it is considered that the obtained thick films with such a poor demagnetization loop have difficulty in an application where thermal stability and shape effects become important. Accordingly, it was clarified that further improvement in the magnetic properties for the
Acknowledgements This work was supported in part by the president’s discretionary fund. This work was also supported in part by the Ministry of Education, Science, Sports and Culture of Japan under a Grand-in-Aid (no. 16080214 and no. 16686022). References [1] S. Yamashita, J. Yamasaki, M. Ikeda, N. Iwabuchi, J. Appl. Phys. 70 (1991) 6627. [2] B.A. Kapitanov, N.V. Kronilov, Ya.L. Linetsky, V.Yu. Tsvetkov, J. Magn. Magn. Mater 127 (1993) 289. [3] T. Araki, T. Honda, In: Reports of the IEE Japan Research Meeting, vol. MAG-97-70, 1997, p. 7 (in Japanese). [4] T. Okuda, A. Sugimura, O. Eryu, L. Serrona, N. Adachi, I. Sakamoto, A. Nakanishi, J. Appl. Phys. 42 (2003) 6859. [5] M. Uehara, J. Magn. Soc. Japan 28 (2004) 1043 (in Japanese).
ARTICLE IN PRESS e378
M. Nakano et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e375–e378
[6] L. Serrona, A. Sugimura, N. Adachi, T. Okuda, H. Ohsato, I. Sakamoto, A. Nakanishi, M. Motokawa, D.H. Ping, K. Hono, Appl. Phys. Lett. 82 (2003) 1751. [7] M. Nakano, S. Tsutsumi, H. Fukunaga, IEEE Trans. Magn. 38 (2002) 2913. [8] M. Nakano, R. Katoh, H. Fukunaga, S. Tutumi, F. Yamashita, IEEE Trans. Magn. 39 (2003) 2863.
[9] M. Nakano, R. Kato, H. Fukunaga, F. Yamashita, IEEJ. Trans. FM 124 (10) (2004) 892 (in Japanese). [10] S. Fahler, U. Hannemann, V. Neu, V. Hoffmann, B. Holzapfel, L. Schultz, Proceedings of the 17th International Workshop on RareEarth Magnets and Their Applications, 2002, p. 449.
Journal of Magnetism and Magnetic Materials 303 (2006) e375–e378 www.elsevier.com/locate/jmmm
Anisotropic Nd–Fe–B thick film magnets M. Nakanoa,, S. Satoa, H. Fukunagaa, F. Yamashitab a
Faculty of Engineering, Nagasaki University, 1-14 Bunkyo-Machi, Nagasaki, 852-8521, Japan b Motor R&D Laboratory, Panasonic, 7-1-1 Morofuku, Daito, Osaka 574-0044, Japan Available online 13 March 2006
Abstract The substrate heating system was applied to the high-speed pulsed laser deposition (PLD) method and it was clarified that anisotropic Nd–Fe–B thick film magnets can be obtained under the high deposition rate of larger than 20 mm/h. The obtained coercivity (HCJ), remanence (Br) and (BH)max of an anisotropic film magnets were approximately 500 kA/m, 0.75 T and 75 kJ/m3, respectively. The values of remanence and (BH)max were improved compared with those for the isotropic PLD-made thick films. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Ww Keywords: Pulsed laser deposition; Thick film magnets; Nd–Fe–B; Anisotropic film magnets
1. Introduction In order to advance a size reduction in electronic devices such as milli-size motors and micro-actuators, Nd–Fe–B film magnets thicker than 10 mm have been prepared by the sputtering method [1–5]. It was also confirmed that a substrate heating method enabled us to obtain anisotropic sputtering-made Nd–Fe–B samples [1–6]. For example, Uehara [5] reported superior magnetic properties of a Nd–Fe–B/Ta multiplayered thick film. (HCJ: 1210 kA/m, Jr: 1.24 T, (BH)max : 279 kJ/m3). However, it is generally said that the sputtering method has a difficulty in realizing high deposition rate. Although Kapitanov et al. [2] reported anisotropic Nd–Fe–B film magnets with thickness of 30–300 mm under a high deposition rate of 20–40 mm/h, (1) the detail conditions of the sputtering method with a high deposition rate, (2) the relationship between magnetic properties and the film thickness, and (3) the evaluation method of magnetization were not shown clearly. On the other hand, we have already reported Nd–Fe–B thick films by a PLD method with a deposition rate of 20–40 mm/h, and obtained isotropic magnets with the coercivity, remanence and (BH)max values of approxiCorresponding author. Tel./fax: +81 95 819 2555.
E-mail address: [email protected] (M. Nakano). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.240
mately 800 kA/m, 0.6 T, and 55 kJ/m3, respectively, through the post annealing process [7,8]. In addition, we succeeded in appling a 200 mm thick Nd–Fe–B film on a Fe substrate to a milli-size motor, and reported that the motor rotates at 15,160 rpm under no-load test and has torque constant of 0.0236 Nm/A at the gap of 0.1 mm between a rotor and a stator [9]. A further increase in the remanence is considered to be an effective way to improve the characteristics of the motor. Namely, preparation of anisotropic Nd–Fe–B films is a promising method for enhancing remanence. Previously, it has been reported that the PLD method under an optimum substrate heating condition enables us to obtain anisotropic Nd–Fe–B films [10]. The film thickness, however, was thinner than 1 mm. In this study, we applied a substrate heating system to the high-speed PLD method and tried to obtain anisotropic Nd–Fe–B film magnets prepared under a high deposition rate of 20–40 mm/h. Resultantly, the obtained remanence of 0.75 T was higher than that of previously reported PLDmade isotropic Nd–Fe–B thick film magnets [7,8]. 2. Experimental In order to compensate loss of metallic Nd due to evaporation as well as oxidation, the nominal composition
ARTICLE IN PRESS M. Nakano et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e375–e378
of targets was set to Nd2.4Fe14B. The targets include a larger amount of Nd than the stoichiometric composition. These targets were ablated with a Nd-YAG pulse laser (l ¼ 355 nm) at the repetition rate of 30 Hz, and the distance between a target and a Ta substrate was fixed at 10 mm. Before the ablation, the chamber was evacuated down to approximately 104 Pa with a molecular turbo pump. Deposition time was fixed at 1 h, and a Ti sublimation pump was used as an auxiliary pump during the deposition. Fig. 1 shows a diagram of a substrate heating system which was made of Ta sheet. Electric current flowed through the Ta sheet, and a sample on a substrate was annealed by Joule heat during deposition. The substrate temperature was varied from 373 to 1173 K. In-plane and perpendicular M–H loops were measured with a vibrating sample magnetometer (VSM). The analyses of crystal structure were carried out with an Xray diffractometer. Thickness was measured with a digital micrometer to 70.5 mm. 3. Results and discussion The control of substrate temperature is a key point to obtain anisotropic Nd–Fe–B films [9]. Therefore, we investigated the in-plane and perpendicular magnetic properties of the films fabricated under various substrate temperatures. Fig. 2 shows the in-plane and perpendicular coercivity as a function of the substrate temperature Ts. When Ts was lower than 650 K, the films did not exhibit distinguishable coercivity. We also confirmed that the Xray diffraction patterns of the films deposited at 350 and 650 K did not have any peaks originating from crystal structures. The extremely low coercivity of the films deposited at temperatures lower than 650 K can be attributed to the amorphous state of the films. The deposited films were crystallized when the heating temperature was higher than 680 K. The coercivity, however, decreased significantly with increase in the temperature beyond 1000 K. The heating temperature of 873 K lead to the maximum in-plane and perpendicular coercivity values,
Substrate heating system (Ta sheet)
Substrate
Plume
Target Pulse Laser Fig. 1. Diagram of PLD method with a substrate heating system. Electric current flowed through the Ta sheet, and a sample on a substrate was annealed by Joule heat during deposition.
500
Coercivity [kA/m]
e376
in-plane perpendicular
400 300 200 100 0
400
600 800 1000 Ts ; Substrate temperature [K]
1200
Fig. 2. Coercivity of PLD-made Nd–Fe–B films as a function of substrate temperature. The heating temperature of 873 K lead to the maximum in-plane and perpendicular coercivity values, approximately 520 and 490 kA/m, respectively.
approximately 516 and 484 kA/m, respectively. The abovementioned values are smaller by approximately 300 kA/m than that reported in Ref. [7,8]. We considered that the reduction in coercivity is attributed to the degraded microstructure in the deposited films for the substrate heating during PLD process. Further investigation on the microstructure is needed for the PLD-made thick film magnets. Fig. 3 shows M–H loops of the above-mentioned film at 873 K. The correction of the demagnetization effect is not carried out. The in-plane and perpendicular coercivity values were almost the same. In isotropic film, the magnetization of the perpendicular loop was lower than that of the in-plane one by demagnetization effect. However, in the film prepared by the substrate heating method, the magnetization of the perpendicular loop was higher than that of the in-plane one. In order to investigate the origins of a high magnetization value, X-ray diffraction patterns of the Nd–Fe–B film were observed and were shown in Fig. 4. Here, in the figure, the pattern of the previously reported isotropic film is also shown [8]. The peak intensities corresponding to the c-plane such as (0 0 4), (0 0 6), (0 0 8) and (1 0 5) for the film prepared in this study are stronger than those for the isotropic film. This result indicates that usage of the substrate heating system enables us to obtain anisotropic Nd–Fe–B thick film magnets in the high-speed PLD method with the deposition rate of 20–40 mm. Therefore, we can conclude that a high-speed deposition of anisotropic Nd–Fe–B thick films was achieved and that the substrate heating is effective in preparation of anisotropic film magnets under high deposition rate. Fig. 5 shows a demagnetization curve measured in the perpendicular direction for the film deposited at 873 K. The demagnetizing effect was corrected by using the demagnetizing factor of 1.0. From the figure, the remanence and (BH)max values are deduced as 0.75 T and 75 kJ/m3,
ARTICLE IN PRESS M. Nakano et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e375–e378
Magnetization [T]
Thickness 23.6 [µm] In-plane Hc : 516 [kA/m] Perpendicular
-2000
1
1 PLD-made Nd-Fe-B film magnet Substrate temperature : 873 K
0.8
0.6 B [T]
Hc : 484 [kA/m]
e377
-1000
0
1000 2000 Applied field [kA/m]
0.4
0.2
-600
-500
-400
-1
(008) Sample A
Sample B
30
40
50
-200
-100
0
Fig. 5. Demagnetization curve of a film deposited at the substrate temperature of 873 K. The curve was measured in the perpendicular direction and corrected by using the demagnetization factor of 1.0.
films under high deposition rate is strongly required. Accordingly, it was clarified that further improvement in the magnetic properties for the films under high deposition rate is strongly required. 4. Conclusion
(004)
Intensity [arb.unit]
Nd2O3
(105)
Nd2Fe14B
(006)
Fig. 3. In-plane and perpendicular M–H loops of a Nd–Fe–B film deposited on Ta substrate at 873 K. Although the coercivity values of the both loops are almost the same, the magnetization in the perpendicular direction is higher than that of the in-plane loop. The correction of the demagnetization effect is not carried out.
-300 H [kA/m]
60
We applied a heating substrate method to the high-speed PLD system, and succeeded in preparing a Nd–Fe–B thick film magnet with perpendicular magnetic anisotropy, whose coercivity, remanence and (BH)max of a thick film magnet were approximately 500 kA/m, 0.75 T and 75 kJ/m3, respectively. Although the high-speed PLD method comprising of a heating substrate system is promising to obtain anisotropic Nd–Fe–B thick film magnets under high deposition rate, further improvement in the magnetic properties for PLDmade thick film magnets is necessary.
2θ [degree] Fig. 4. X-ray diffraction patterns of Nd–Fe–B films deposited on Ta substrate. Sample A was deposited on a substrate heated at 873 K. Sample B was deposited without substrate heating and crystallized by postannealing at 923 K [7]. The sample B has an isotropic structure.
respectively. These values were larger by approximately 0.15 T and 20 kJ/m3, respectively, than those for the isotropic film reported previously [7,8]. The magnetic properties for the PLD-made anisotropic thick films, however, were inferior compared with those for anisotropic film magnets prepared by sputtering method [4–6]. In addition, it is considered that the obtained thick films with such a poor demagnetization loop have difficulty in an application where thermal stability and shape effects become important. Accordingly, it was clarified that further improvement in the magnetic properties for the
Acknowledgements This work was supported in part by the president’s discretionary fund. This work was also supported in part by the Ministry of Education, Science, Sports and Culture of Japan under a Grand-in-Aid (no. 16080214 and no. 16686022). References [1] S. Yamashita, J. Yamasaki, M. Ikeda, N. Iwabuchi, J. Appl. Phys. 70 (1991) 6627. [2] B.A. Kapitanov, N.V. Kronilov, Ya.L. Linetsky, V.Yu. Tsvetkov, J. Magn. Magn. Mater 127 (1993) 289. [3] T. Araki, T. Honda, In: Reports of the IEE Japan Research Meeting, vol. MAG-97-70, 1997, p. 7 (in Japanese). [4] T. Okuda, A. Sugimura, O. Eryu, L. Serrona, N. Adachi, I. Sakamoto, A. Nakanishi, J. Appl. Phys. 42 (2003) 6859. [5] M. Uehara, J. Magn. Soc. Japan 28 (2004) 1043 (in Japanese).
ARTICLE IN PRESS e378
M. Nakano et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e375–e378
[6] L. Serrona, A. Sugimura, N. Adachi, T. Okuda, H. Ohsato, I. Sakamoto, A. Nakanishi, M. Motokawa, D.H. Ping, K. Hono, Appl. Phys. Lett. 82 (2003) 1751. [7] M. Nakano, S. Tsutsumi, H. Fukunaga, IEEE Trans. Magn. 38 (2002) 2913. [8] M. Nakano, R. Katoh, H. Fukunaga, S. Tutumi, F. Yamashita, IEEE Trans. Magn. 39 (2003) 2863.
[9] M. Nakano, R. Kato, H. Fukunaga, F. Yamashita, IEEJ. Trans. FM 124 (10) (2004) 892 (in Japanese). [10] S. Fahler, U. Hannemann, V. Neu, V. Hoffmann, B. Holzapfel, L. Schultz, Proceedings of the 17th International Workshop on RareEarth Magnets and Their Applications, 2002, p. 449.