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Flux stability in nanocomposite magnets
Journal of Magnetism and Magnetic Materials 203 (1999) 304}306
Flux stability in nanocomposite magnets H. Fukunaga *, S. Hayashida , Y. Kanai , F. Yamashita Department of Electrical and Electronic Engineering, Faculty of Enginerring, Nagasaki University, Nagasaki 852-8521, Japan Matsushita Electric Industry Co. Ltd., Osaka 574, Japan
Abstract The stability of #ux in Nd}Fe}B based nanocomposite magnets was studied and it was found that nanocomposite magnets have a high stability of #ux in spite of their small coercivity. The observed high stability can be attributed to a square demagnetization curve at an elevated temperature and a small irreversible susceptibility in the nanocomposite magnets. 1999 Elsevier Science B.V. All rights reserved. Keywords: Nanocomposite magnets; Stability of #ux; Flux loss; Coercivity; Susceptibility
1. Introduction Nanocomposite magnets are one of the most attractive magnets because they have the possibility of a high remanence and a large maximum energy product [1]. The presence of soft phases, however, decreases coercivity and may deteriorate the stability of magnetic #ux in the magnets. In this contribution, we report the #ux stability of Nd}Fe}B based nanocomposite magnets and compare it with that of a conventional isotropic Nd}Fe}B magnet.
2. Experimental procedure Five cylindrical resin-bonded magnets (NCC1} NCC5), 5 mm in diameter and 4.1 mm in height, were prepared from Nd}Fe}B based nanocomposite powder. A bonded magnet (RQC1) was also prepared from conventional rapidly quenched Nd}Fe}B powder. The prepared magnets were magnetized under a pulse "eld of 6.4 MA/m, and the variation of their #ux due to an exposure at an elevated temperature, ¹ , was measured # by the sample-extraction method as a function of the
* Corresponding author. Tel. #81-958-47-6825; fax:#81958-46-7379. E-mail address: [email protected] (H. Fukunaga)
exposure time, t . The irreversible #ux loss, FL, due to the exposure was calculated from FL"(U !U )/U , (1) where U and U are the #ux values measured at room temperature before and after the exposure. The main magnetic properties of the prepared magnets are listed in Table 1.
3. Results and discussion A 1 h exposure of the magnets caused a signi"cant FL and a successive exposure decreased the #ux linearly with the logarithm of t , as shown in the inset of Fig. 1. Thus, we named the "rst large #ux loss as `the initial #ux lossa and the subsequent linear change in the #ux as `the long-term #ux lossa. As shown in Fig. 1, the initial #ux loss, FL , in the nanocomposite magnets decreased with increasing coercivity at room temperature, H , and the values of FL in NCC4 and NCC5 were smaller than that of RQC1 in spite of their small values of H . A temperature rise up to ¹ during the measurement of FL reduces coercivity and then causes local magnetization reversal by the demagnetizing "eld. Assuming that the magnetization reversed locally is not recovered in the original direction after the temperature is decreased to room temperature, ¹ , FL is given by 0+ FL "1!+I (¹ )/I (¹ ),;+I (¹ )/I (¹ ),, (2) 1 02 1 #6 5 #6 5 02
0304-8853/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 2 5 6 - 5
H. Fukunaga et al. / Journal of Magnetism and Magnetic Materials 203 (1999) 304}306
305
Table 1 Magnetic properties of prepared magnets at room temperature Magnet
No.
H (kA/m)
I (T) P
Nanocomposite
NCC1 NCC2 NCC3 NCC4 NCC5
247 290 338 427 539
0.86 0.76 0.76 0.65 0.58
RQ Nd}Fe}B
RQC1
758
0.68
Fig. 2. Correlation between calculated and measured #ux losses. Fig. 1. Initial #ux loss as a function of coercivity at room temperature. The inset indicates the #ux loss as a function of the exposure time.
where I (¹) and I (¹) are the saturation magnetization 1 5 and the magnetization at the working point measured at the temperature ¹, respectively. The calculated values of FL agreed roughly with those measured, as seen in Fig. 2, which suggests that the above assumption is valid for the magnets under study. Considering Eq. (2) and the comparison of I (¹ )/I (¹ ) values for these mag5 #6 5 0+ nets revealed that the small FL values in NCsC4 and C5 can be attributed to their large I (¹ )/I (¹ ) 5 #6 5 0+ values. This result is consistent with our previous numerical calculation indicating that a demagnetization curve of a nanocomposite magnet becomes square with increasing temperature [2]. The long-term #ux loss was evaluated by calculating the #ux loss per logarithm of t , *FL/ln(t ). *FL/ln(t ) of the nanocomposite magnets decreased with increasing H , as shown in Fig. 3; the values of *FL/ln(t ) in NCsC4 and C5 at 1203C and the values of *FL/ln(t ) in NCC5 at 803C are smaller than those of RQC1 at the corresponding temperatures, in spite of their small values of H . Extrapolating the FL vs. t curve, the long-term #ux loss during 10 years in NCC5 is expected to be less than 1% at 1203C. Magnetic after-e!ect is considered to be responsible for the observed long-term #ux loss. The decrease of the magnetization due to magnetic after-e!ect per logarithm of t *I /ln(t ), has been reported [3] by the equation *I /ln(t )"(1!Ns /k )s S , 4
(3)
Fig. 3. Flux loss per logarithm of exposure time as a function of coercivity at room temperature.
where N, s s , k , and S are the demagnetizing fac 4 tor, the reversible susceptibility, the irreversible susceptibility, and the after-e!ect constant, respectively. The superscript, prime, indicates that the corresponding values is an apparent value a!ected by the demagnetizing "eld. From Eq. (2) *FL/ln(t ) is expected to correlate to (1!Ns /k )s . *FL/ln(t ) is roughly proportional to (1!Ns / k )s , as seen in Fig. 4. This result suggests that S of the 4 nanocomposite magnets is roughly in agreement in magnitude with that of RQC1, and is consistent with the previous report [4]. Therefore the small *FL/ln(t ) values in NCsC4 and C5 can be attributed to their small irreversible susceptibility.
306
H. Fukunaga et al. / Journal of Magnetism and Magnetic Materials 203 (1999) 304}306
observed. The high stability of #ux was attributed to a square demagnetization curve and a small irreversible susceptibility at an elevated temperature.
References
Fig. 4. Correlation between #ux loss per logarithm of exposure time and (1!Ns /k )s .
4. Conclusions In conclusion, taking into account the small coercivity of nanocomposite magnets, high stability of #ux was
[1] E.F. Kneller, R. Hawig, IEEE Trans. Magn. 27 (1991) 3588. [2] H. Fukunaga, H. Inoue, N. Kitajima, Appl. Electromagnetics Mater (JSAEM, 1995) p. 169. [3] K.-H. MuK ller, D. Eckert, A. Handerson, S. Wirth, L.L. Martinez, Proceedings of the Eight International Symposium on Magnetic Anisotropy and Coercivity in RE-TM Alloys, 1989 p. 179. [4] H. Nishio, Meeting Records of IEE Japan, MAG-96-208, 1996.
Flux stability in nanocomposite magnets H. Fukunaga *, S. Hayashida , Y. Kanai , F. Yamashita Department of Electrical and Electronic Engineering, Faculty of Enginerring, Nagasaki University, Nagasaki 852-8521, Japan Matsushita Electric Industry Co. Ltd., Osaka 574, Japan
Abstract The stability of #ux in Nd}Fe}B based nanocomposite magnets was studied and it was found that nanocomposite magnets have a high stability of #ux in spite of their small coercivity. The observed high stability can be attributed to a square demagnetization curve at an elevated temperature and a small irreversible susceptibility in the nanocomposite magnets. 1999 Elsevier Science B.V. All rights reserved. Keywords: Nanocomposite magnets; Stability of #ux; Flux loss; Coercivity; Susceptibility
1. Introduction Nanocomposite magnets are one of the most attractive magnets because they have the possibility of a high remanence and a large maximum energy product [1]. The presence of soft phases, however, decreases coercivity and may deteriorate the stability of magnetic #ux in the magnets. In this contribution, we report the #ux stability of Nd}Fe}B based nanocomposite magnets and compare it with that of a conventional isotropic Nd}Fe}B magnet.
2. Experimental procedure Five cylindrical resin-bonded magnets (NCC1} NCC5), 5 mm in diameter and 4.1 mm in height, were prepared from Nd}Fe}B based nanocomposite powder. A bonded magnet (RQC1) was also prepared from conventional rapidly quenched Nd}Fe}B powder. The prepared magnets were magnetized under a pulse "eld of 6.4 MA/m, and the variation of their #ux due to an exposure at an elevated temperature, ¹ , was measured # by the sample-extraction method as a function of the
* Corresponding author. Tel. #81-958-47-6825; fax:#81958-46-7379. E-mail address: [email protected] (H. Fukunaga)
exposure time, t . The irreversible #ux loss, FL, due to the exposure was calculated from FL"(U !U )/U , (1) where U and U are the #ux values measured at room temperature before and after the exposure. The main magnetic properties of the prepared magnets are listed in Table 1.
3. Results and discussion A 1 h exposure of the magnets caused a signi"cant FL and a successive exposure decreased the #ux linearly with the logarithm of t , as shown in the inset of Fig. 1. Thus, we named the "rst large #ux loss as `the initial #ux lossa and the subsequent linear change in the #ux as `the long-term #ux lossa. As shown in Fig. 1, the initial #ux loss, FL , in the nanocomposite magnets decreased with increasing coercivity at room temperature, H , and the values of FL in NCC4 and NCC5 were smaller than that of RQC1 in spite of their small values of H . A temperature rise up to ¹ during the measurement of FL reduces coercivity and then causes local magnetization reversal by the demagnetizing "eld. Assuming that the magnetization reversed locally is not recovered in the original direction after the temperature is decreased to room temperature, ¹ , FL is given by 0+ FL "1!+I (¹ )/I (¹ ),;+I (¹ )/I (¹ ),, (2) 1 02 1 #6 5 #6 5 02
0304-8853/99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 2 5 6 - 5
H. Fukunaga et al. / Journal of Magnetism and Magnetic Materials 203 (1999) 304}306
305
Table 1 Magnetic properties of prepared magnets at room temperature Magnet
No.
H (kA/m)
I (T) P
Nanocomposite
NCC1 NCC2 NCC3 NCC4 NCC5
247 290 338 427 539
0.86 0.76 0.76 0.65 0.58
RQ Nd}Fe}B
RQC1
758
0.68
Fig. 2. Correlation between calculated and measured #ux losses. Fig. 1. Initial #ux loss as a function of coercivity at room temperature. The inset indicates the #ux loss as a function of the exposure time.
where I (¹) and I (¹) are the saturation magnetization 1 5 and the magnetization at the working point measured at the temperature ¹, respectively. The calculated values of FL agreed roughly with those measured, as seen in Fig. 2, which suggests that the above assumption is valid for the magnets under study. Considering Eq. (2) and the comparison of I (¹ )/I (¹ ) values for these mag5 #6 5 0+ nets revealed that the small FL values in NCsC4 and C5 can be attributed to their large I (¹ )/I (¹ ) 5 #6 5 0+ values. This result is consistent with our previous numerical calculation indicating that a demagnetization curve of a nanocomposite magnet becomes square with increasing temperature [2]. The long-term #ux loss was evaluated by calculating the #ux loss per logarithm of t , *FL/ln(t ). *FL/ln(t ) of the nanocomposite magnets decreased with increasing H , as shown in Fig. 3; the values of *FL/ln(t ) in NCsC4 and C5 at 1203C and the values of *FL/ln(t ) in NCC5 at 803C are smaller than those of RQC1 at the corresponding temperatures, in spite of their small values of H . Extrapolating the FL vs. t curve, the long-term #ux loss during 10 years in NCC5 is expected to be less than 1% at 1203C. Magnetic after-e!ect is considered to be responsible for the observed long-term #ux loss. The decrease of the magnetization due to magnetic after-e!ect per logarithm of t *I /ln(t ), has been reported [3] by the equation *I /ln(t )"(1!Ns /k )s S , 4
(3)
Fig. 3. Flux loss per logarithm of exposure time as a function of coercivity at room temperature.
where N, s s , k , and S are the demagnetizing fac 4 tor, the reversible susceptibility, the irreversible susceptibility, and the after-e!ect constant, respectively. The superscript, prime, indicates that the corresponding values is an apparent value a!ected by the demagnetizing "eld. From Eq. (2) *FL/ln(t ) is expected to correlate to (1!Ns /k )s . *FL/ln(t ) is roughly proportional to (1!Ns / k )s , as seen in Fig. 4. This result suggests that S of the 4 nanocomposite magnets is roughly in agreement in magnitude with that of RQC1, and is consistent with the previous report [4]. Therefore the small *FL/ln(t ) values in NCsC4 and C5 can be attributed to their small irreversible susceptibility.
306
H. Fukunaga et al. / Journal of Magnetism and Magnetic Materials 203 (1999) 304}306
observed. The high stability of #ux was attributed to a square demagnetization curve and a small irreversible susceptibility at an elevated temperature.
References
Fig. 4. Correlation between #ux loss per logarithm of exposure time and (1!Ns /k )s .
4. Conclusions In conclusion, taking into account the small coercivity of nanocomposite magnets, high stability of #ux was
[1] E.F. Kneller, R. Hawig, IEEE Trans. Magn. 27 (1991) 3588. [2] H. Fukunaga, H. Inoue, N. Kitajima, Appl. Electromagnetics Mater (JSAEM, 1995) p. 169. [3] K.-H. MuK ller, D. Eckert, A. Handerson, S. Wirth, L.L. Martinez, Proceedings of the Eight International Symposium on Magnetic Anisotropy and Coercivity in RE-TM Alloys, 1989 p. 179. [4] H. Nishio, Meeting Records of IEE Japan, MAG-96-208, 1996.