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Temperature dependence of the coercivity of sintered R17Fe83−xBx magnets (R = Pr and Nd)
316
Journal of Magnetism and Magnetic Materials 70 (1987) 316 318 North-Itolland, Amsterdam
TEMPERATURE
DEPENDENCE
OF THE COERCIVITY
OF S I N T E R E D RITFes3_.B~ M A G N E T S (R = Pr A N D Nd) M. S A G A W A , S. H I R O S A W A , Y. O T A N I *, H. M I Y A J I M A * and S. C H I K A Z U M I
*
Sumitomo Special Metals Co. Ltd., Osaka 618, Japan
• Department of Physics, Facul(v of Science and Technology Keio Uniuersi(v. Yokohama 223. Japan
Temperature dependence of the coercivity of B-rich sintered magnets of the composition RwFes3 , B, (R = Pr and Nd) with different microscopic features have been measured in order to study what aspects of the sintered magnet and how they influence the coercivity-anisotropy relation.
1. Introduction
3. Results and discussion
In the N d 2 F % 4 B - t y p e sintered magnets, coercivity is governed by nucleation of reversed dom a i n s [1]. Several researchers have p r o p o s e d that the intrinsic coercivity, H~, of sintered R - F e B magnets, where R is a rare earth, is related to the a n i s o t r o p y field, HA, and the s p o n t a n e o u s magnetization, I~, of the matrix R z F % 4 B phase through
T h e r o o m t e m p e r a t u r e coercivity of N d w F%3 ,B~, /*oH d, increases substantially with increasing the B content, from 1.3 T for x = 8 to 2.1 T for x = 37. The t e m p e r a t u r e d e p e n d e n c e of t, oHcl of NdlvFes3 , B , is shown in fig. 1. The values of ~oH~l of the magnets with x = 17 a n d 30 show a m o n o t o n i c increase with decreasing temperature, while the t t ~ value of the magnet with x = 37 exhibits a n o m a l o u s downfall at low temperatures. This is due to a c o m b i n a t i o n of two effects, namely, m a g n e t i c o r d e r i n g of the B-rich p h a s e (Ndl.lFe4B4) at 15 K and an increasing p a r a m a g n e t i c susceptibility of this phase when t e m p e r a t u r e a p p r o a c h e s to this t e m p e r a t u r e from above.
~*on~, =
c~,, n , , - N 1 ,
(1)
where c a n d N are c o n s t a n t s [2-5]. However, the m e a n i n g of c a n d N are not well u n d e r s t o o d . In this paper, we e x a m i n e how the c o n s t a n t s c a n d N vary a c c o r d i n g to changes in metallurgical aspects of the magnet. F o r this purpose, we have chosen B-rich R w F % 3 ,.B~ sintered magnets, where R = Pr a n d N d , in which n o n m a g n e t i c R~+~Fe4B4 g r a d u a l l y separates the RzFe=4B grains with increasing B content.
Ndz7Fe83-xBx
l\
2. Experimental procedures T h e same p r o c e d u r e s r e p o r t e d in ref. [1] are a p p l i c a b l e to the B-rich sintered magnets. After sintering, all samples were h e a t - t r e a t e d in v a c u u m at their o p t i m u m t e m p e r a t u r e s , 870 K for N d - F e B magnets and 850 K for Pr F e - B , which were f o u n d to be c o m m o n in each system regardless of the B content. T h e average grain size in the direction parallel to the c-axis weighed by volume was o b t a i n e d from K e r r m i c r o g r a p h s to be 16 ~ m for Pr~yFe75B s sintered at 1373 K for 1.5 h, whereas it increased up to 20 ~ m when the sintering time was 12 h.
~,
x=17
%
x=30 x =37
,.\
°o
'
26o
'
460
600
T (K)
Fig. 1. Temperature dependence of the intrinsic coercivity of
0 3 0 4 - 8 8 5 3 / 8 7 / $ 0 3 . 5 0 ~:) Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
NdwF%3 B , magnets.
M. Sagawa et al. / Coercivi(v of sintered R 1z F e ss
The correlation between H~ and H A in the Nd~TFe~3_~B~ sintered magnets is examined according to eq. (1) which is rewritten as t . t o H ~ i / l ~ = c ~ o H A / I ~ -- N . Accordingly, values for c and N can be determined by plotting I ~ o H ~ l / I ~ against ~ o H A / I ~ if the plot lies on a straight line. Fig. 2 shows the ~ t o H ~ i / 1 ~ versus the I . t o H A / I ~ plot for the NdlvFes3 ~B~ magnets. The values for H A have been taken from ref. [6] and contain only the K~ term. It is clearly recognized that there does not exist a linearity between the two quantities. This is an indication that the contribution of the K 2 term is already innegligible in the temperature range investigated. Such a contribution of the K 2 term to the magnetization reversal process in real magnets can not be derived from theoretical prediction for a critical field for a magnetization reversal via a coherent magnetization rotation even if the K 2 term is taken into account [3,4], One may suspect the plausibility of using eq. (1). However, the validity of using eq. (1) in cases in which K~ is dominant can be verified clearly when we study the P r - F e - B system. Fig. 3 shows the t z o H ~ l / I s vs. I I o H A / I ~ plot for the PrlTFe75B ~ and Pr~vFe53B30 magnets, both of which were optimally processed, together with the plot for Pr~vFevsB~ with an enlarged grain size (20 ~tm) which was produced by a prolonged sintering for 12 h. What should be noted, is the good linearity of the plot which supports the validity of 1.5
7
Nd17Fe83-xBx
/.o
G x=8
J /
÷ x=17 • x=30
1.0
/
A x=37
:o
:
0.5
*
/
/
,
,
j/ #
,/"///."*f/.,,'/~,
,
•/ef,,~, 2--
3 ,fJ.oHA/Is
Fig. 2. The #0Hal versus #oHA/I~ plot for NdlvFe83_~B~ magnets.
~ B~
magnets
317
1.5r
1 / J PrlTFes3B3o 1373K, 1.Sh
1.0
c=0.45
"." PrI7Fe75B8 1373K, 1.5h
N= 1.21
J
I
Pr17Fe75B8 1373K, 12h
;J" o
--
~"
Z/
3
.-" -'"
'
4
"''"
N=O.615 5
fi
,~oHA/Is
Fig. 3. The /~oHd versus I.ZoHA/I~ plot for the Pr~TFe75B~ and Pr17Fe53B30 magnets.
eq. (1). Comparision of PrwFe53B30 and Pr17Fe75B 8 in fig. 3 demonstrates that the isolation of the R2Fel4 B grains by nonmagnetic R I +,Fe4B 4 leads to increases in both the c and N values, whereas comparison of different sintering time for PrlTFevsB ~, which resulted in different grain sizes, and presumably different smoothness of grain boundaries reveals that both c and N values decrease appreciably with prolonging the sintering time. The increase of c with increasing the degree of separation between RzFe~4B grains, which have been realized either by making the average grain size small or by increasing the amount of the R~ +,Fe4B4 phase, is reasonable because, by doing so, a volume governed by a nucleation site with the least nucleation field becomes gradually small. The value of the nucleation field itself, i.e., the c value of each grain, is presumably related to the nature of magnetically soft regions such as structurally and chemically altered layer on the envelope of each grain where, conceivably, the magnetic anisotropy changes gradually. The value of N is undoubtedly interconnected with the shape of magnetic grains. In the magnet in which R 1+,Fe4B 4 is only a minority phase, the N I S term is governed mainly by the stray fields created by neighboring grains, especially in the course of magnetization reversal where magnetic poles of neighboring grains oppose with each other. However, in the B-rich magnets, the self-demagne-
318
M, Sagawa et al. / Coercit~itv of sintered R i ~["e x.¢
tization field, which can be fairly large in the vicinity of the sharp irregularity of the outer shape of the grains, becomes important. If one can prepare a magnet consisting of the R2Fe~4B grains with a small grain size, maintaining the grain shape more round and more smooth than that obtained in the present B-rich magnets, a very high coercivity may be realized. References [1] M. Sagawa, S, Fujimura, N. Togawa, H. Yamamoto and Y. Matsuura, J. Appl. Phys. 55 (1984) 2083.
, B~ magnets
[2] S. Hirosawa, K. Tokuhara, Y. Matsuura, H. Yamamoto, S. Fujimura and M. Sagawa, J. Magn. Magn. Mat. 61 (1986) 363. [3] G. Herzer, W. Fernangel and E. Adler, J. Magn. Magn. Mat. 58 (1986) 48. [4] K.-D. Durst and H. Kronmiiller, Proc. of the 8th Intern. Workshop on Rare Earth Magnets and their Applications (Univ. Dayton, 1985) p. 725. [5] E. Adler and P. Hamann, ibid, p. 747. [6] S. Hirosawa, Y. Matsuura, H. Yamamoto, S. Fujimura, M. Sagawa and H. Yamauchi, Japan. J. Appl. Phys. 24 (1985) L803.
Journal of Magnetism and Magnetic Materials 70 (1987) 316 318 North-Itolland, Amsterdam
TEMPERATURE
DEPENDENCE
OF THE COERCIVITY
OF S I N T E R E D RITFes3_.B~ M A G N E T S (R = Pr A N D Nd) M. S A G A W A , S. H I R O S A W A , Y. O T A N I *, H. M I Y A J I M A * and S. C H I K A Z U M I
*
Sumitomo Special Metals Co. Ltd., Osaka 618, Japan
• Department of Physics, Facul(v of Science and Technology Keio Uniuersi(v. Yokohama 223. Japan
Temperature dependence of the coercivity of B-rich sintered magnets of the composition RwFes3 , B, (R = Pr and Nd) with different microscopic features have been measured in order to study what aspects of the sintered magnet and how they influence the coercivity-anisotropy relation.
1. Introduction
3. Results and discussion
In the N d 2 F % 4 B - t y p e sintered magnets, coercivity is governed by nucleation of reversed dom a i n s [1]. Several researchers have p r o p o s e d that the intrinsic coercivity, H~, of sintered R - F e B magnets, where R is a rare earth, is related to the a n i s o t r o p y field, HA, and the s p o n t a n e o u s magnetization, I~, of the matrix R z F % 4 B phase through
T h e r o o m t e m p e r a t u r e coercivity of N d w F%3 ,B~, /*oH d, increases substantially with increasing the B content, from 1.3 T for x = 8 to 2.1 T for x = 37. The t e m p e r a t u r e d e p e n d e n c e of t, oHcl of NdlvFes3 , B , is shown in fig. 1. The values of ~oH~l of the magnets with x = 17 a n d 30 show a m o n o t o n i c increase with decreasing temperature, while the t t ~ value of the magnet with x = 37 exhibits a n o m a l o u s downfall at low temperatures. This is due to a c o m b i n a t i o n of two effects, namely, m a g n e t i c o r d e r i n g of the B-rich p h a s e (Ndl.lFe4B4) at 15 K and an increasing p a r a m a g n e t i c susceptibility of this phase when t e m p e r a t u r e a p p r o a c h e s to this t e m p e r a t u r e from above.
~*on~, =
c~,, n , , - N 1 ,
(1)
where c a n d N are c o n s t a n t s [2-5]. However, the m e a n i n g of c a n d N are not well u n d e r s t o o d . In this paper, we e x a m i n e how the c o n s t a n t s c a n d N vary a c c o r d i n g to changes in metallurgical aspects of the magnet. F o r this purpose, we have chosen B-rich R w F % 3 ,.B~ sintered magnets, where R = Pr a n d N d , in which n o n m a g n e t i c R~+~Fe4B4 g r a d u a l l y separates the RzFe=4B grains with increasing B content.
Ndz7Fe83-xBx
l\
2. Experimental procedures T h e same p r o c e d u r e s r e p o r t e d in ref. [1] are a p p l i c a b l e to the B-rich sintered magnets. After sintering, all samples were h e a t - t r e a t e d in v a c u u m at their o p t i m u m t e m p e r a t u r e s , 870 K for N d - F e B magnets and 850 K for Pr F e - B , which were f o u n d to be c o m m o n in each system regardless of the B content. T h e average grain size in the direction parallel to the c-axis weighed by volume was o b t a i n e d from K e r r m i c r o g r a p h s to be 16 ~ m for Pr~yFe75B s sintered at 1373 K for 1.5 h, whereas it increased up to 20 ~ m when the sintering time was 12 h.
~,
x=17
%
x=30 x =37
,.\
°o
'
26o
'
460
600
T (K)
Fig. 1. Temperature dependence of the intrinsic coercivity of
0 3 0 4 - 8 8 5 3 / 8 7 / $ 0 3 . 5 0 ~:) Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
NdwF%3 B , magnets.
M. Sagawa et al. / Coercivi(v of sintered R 1z F e ss
The correlation between H~ and H A in the Nd~TFe~3_~B~ sintered magnets is examined according to eq. (1) which is rewritten as t . t o H ~ i / l ~ = c ~ o H A / I ~ -- N . Accordingly, values for c and N can be determined by plotting I ~ o H ~ l / I ~ against ~ o H A / I ~ if the plot lies on a straight line. Fig. 2 shows the ~ t o H ~ i / 1 ~ versus the I . t o H A / I ~ plot for the NdlvFes3 ~B~ magnets. The values for H A have been taken from ref. [6] and contain only the K~ term. It is clearly recognized that there does not exist a linearity between the two quantities. This is an indication that the contribution of the K 2 term is already innegligible in the temperature range investigated. Such a contribution of the K 2 term to the magnetization reversal process in real magnets can not be derived from theoretical prediction for a critical field for a magnetization reversal via a coherent magnetization rotation even if the K 2 term is taken into account [3,4], One may suspect the plausibility of using eq. (1). However, the validity of using eq. (1) in cases in which K~ is dominant can be verified clearly when we study the P r - F e - B system. Fig. 3 shows the t z o H ~ l / I s vs. I I o H A / I ~ plot for the PrlTFe75B ~ and Pr~vFe53B30 magnets, both of which were optimally processed, together with the plot for Pr~vFevsB~ with an enlarged grain size (20 ~tm) which was produced by a prolonged sintering for 12 h. What should be noted, is the good linearity of the plot which supports the validity of 1.5
7
Nd17Fe83-xBx
/.o
G x=8
J /
÷ x=17 • x=30
1.0
/
A x=37
:o
:
0.5
*
/
/
,
,
j/ #
,/"///."*f/.,,'/~,
,
•/ef,,~, 2--
3 ,fJ.oHA/Is
Fig. 2. The #0Hal versus #oHA/I~ plot for NdlvFe83_~B~ magnets.
~ B~
magnets
317
1.5r
1 / J PrlTFes3B3o 1373K, 1.Sh
1.0
c=0.45
"." PrI7Fe75B8 1373K, 1.5h
N= 1.21
J
I
Pr17Fe75B8 1373K, 12h
;J" o
--
~"
Z/
3
.-" -'"
'
4
"''"
N=O.615 5
fi
,~oHA/Is
Fig. 3. The /~oHd versus I.ZoHA/I~ plot for the Pr~TFe75B~ and Pr17Fe53B30 magnets.
eq. (1). Comparision of PrwFe53B30 and Pr17Fe75B 8 in fig. 3 demonstrates that the isolation of the R2Fel4 B grains by nonmagnetic R I +,Fe4B 4 leads to increases in both the c and N values, whereas comparison of different sintering time for PrlTFevsB ~, which resulted in different grain sizes, and presumably different smoothness of grain boundaries reveals that both c and N values decrease appreciably with prolonging the sintering time. The increase of c with increasing the degree of separation between RzFe~4B grains, which have been realized either by making the average grain size small or by increasing the amount of the R~ +,Fe4B4 phase, is reasonable because, by doing so, a volume governed by a nucleation site with the least nucleation field becomes gradually small. The value of the nucleation field itself, i.e., the c value of each grain, is presumably related to the nature of magnetically soft regions such as structurally and chemically altered layer on the envelope of each grain where, conceivably, the magnetic anisotropy changes gradually. The value of N is undoubtedly interconnected with the shape of magnetic grains. In the magnet in which R 1+,Fe4B 4 is only a minority phase, the N I S term is governed mainly by the stray fields created by neighboring grains, especially in the course of magnetization reversal where magnetic poles of neighboring grains oppose with each other. However, in the B-rich magnets, the self-demagne-
318
M, Sagawa et al. / Coercit~itv of sintered R i ~["e x.¢
tization field, which can be fairly large in the vicinity of the sharp irregularity of the outer shape of the grains, becomes important. If one can prepare a magnet consisting of the R2Fe~4B grains with a small grain size, maintaining the grain shape more round and more smooth than that obtained in the present B-rich magnets, a very high coercivity may be realized. References [1] M. Sagawa, S, Fujimura, N. Togawa, H. Yamamoto and Y. Matsuura, J. Appl. Phys. 55 (1984) 2083.
, B~ magnets
[2] S. Hirosawa, K. Tokuhara, Y. Matsuura, H. Yamamoto, S. Fujimura and M. Sagawa, J. Magn. Magn. Mat. 61 (1986) 363. [3] G. Herzer, W. Fernangel and E. Adler, J. Magn. Magn. Mat. 58 (1986) 48. [4] K.-D. Durst and H. Kronmiiller, Proc. of the 8th Intern. Workshop on Rare Earth Magnets and their Applications (Univ. Dayton, 1985) p. 725. [5] E. Adler and P. Hamann, ibid, p. 747. [6] S. Hirosawa, Y. Matsuura, H. Yamamoto, S. Fujimura, M. Sagawa and H. Yamauchi, Japan. J. Appl. Phys. 24 (1985) L803.