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Structure of the amorphous phase during crystallization of Fe73.5Cu1Nb3Si13.5B9 alloy
ELSEWIER
Journal of Magnetism and Magnetic Materials 138 (1994) 94-98
Structure of the amorphous phase during crystallization of Fe,,Cu,Nb3Si,,,B, alloy Yang Huisheng
a,*, Tu Guochao b, Xiong Xiaotao a, Xu Zuxiong a, Ma Ruzhang a
aDeparhnent of Material Physics, Uniuersity of Science and Technology Beijing, Beijing 100083, China b Metallurgical
In&h&e of Shoudu Iron and Steel Company, Beijing 100083, China
Received 5 November 1993; in final revised form 31 May 1994
Abstract Mossbauer spectroscopy and the positron annihilation technique have been used to investigate the structure of the B s nanocrystalline alloy annealed at different temperatures. The results show that amorphous phase of Fe,,~,Cu,Nb,Si,,,, there is a weak ferromagnetic amorphous boundary with a low hyperfine field. On other hand, the results of positron annihilation indicate that a large amount of vacancy-like defects appears in the amorphous phase at higher temperature. The results of magnetic measurements indicate that the magnetic properties of nanocrystalline alloy are closely related to the density of defects.
1. Introduction 13.5B9 alloy after optimal annealFe,,.,Cu,Nb,Si ing has large magnetization, high permeability, low coercivity and power loss [l-4]. It has attracted much attention as a new type of soft magnetic material. At first, much work concentrated on the formation of the nanocrystalline structure and the effect of grain size on the magnetic properties. More recent results [2,5] have shown that the magnetic properties change considerably with annealing temperature, although the crystalline phase shows no obvious change. Therefore, the magnetic properties of the boundary phase and its effects on the magnetic properties of the alloy began to be mentioned [6-91. It has been known that the excellent soft magnetic
* Corresponding author. Present address: Group 207, Institute of Physics, Chinese Academy of Science, Beijing, China. 0304-8853/94/$07.00 0 1994 Elsevier Science B.V. AII rights reserved SSDI 0304-8853(94)00415-N
properties will degrade when the amorphous phase transforms into a paramagnetic phase above 300°C. This implies that the amorphous phase plays an important role in the magnetic properties of the nanocrystalline alloy. The hyperfine parameters of MGssbauer spectra can provide much information on all phases in the alloy, especially the amorphous phase, and the positron annihilation parameters are sensitive to the defects. For this reason, they are used to investigate the structure of the amorphous phase of the nanocrystalline alloy.
2. Experiments 2.1. The crystallization process Si,,, B, amorphous alloy
of Fe,,,
The experimental Fe,,,,Cu,Nb,Si,,,,B, phous ribbon was prepared by the single-roller
Cu, Nb,
amormelt-
H. Yang et al. /Journal
58.00
78.00
95
of Magnetism and Magnetic Materials 138 (1994) 94-98
90100
Wdeg) Fig. 1. X-ray diffraction patterns for Fe,3,&u,Nb3Si,,,,B, annealed at different temperatures (0, a-Fe(S), 0, Fe,B).
alloy
quenching method. Its typical cross section was 10 mm X 18 km and crystallization temperature (measured by DTA at lO”C/min) was 500°C. The ribbon was annealed at different temperatures for 30 min; the X-ray diffraction patterns are shown in Fig. 1. Fe,,,,Cu,Nb,Si,,,5B, amorphous alloy begins to crystallize at 480°C and precipitates a-Fe(%) solid solution (crystalline phase). As the annealing temperature increases, the volume ratio of crystalline to amorphous phases gradually increases, and finally, at 630°C the Fe,B phase and other boron compounds
1
I
I
I
I
I
I
-6
-4
-2
0
2
4
6
mm/S
Fig. 3. Mijssbauer spectra for samples annealed at different temperatures: (a) as-quenched, (b) 480°C (c) 51O”C, (d) 700°C.
‘““Ii.i ,O*W
Ta (“C)
Fig. 2. Dependence of grain size (D, 0) of the crystalline phase and the crystallization fraction (P, A) on annealing temperature.
form. On the other hand, the grain size, calculated by the Sherrer method [lo], remains almost constant about at 10 nm when annealing between 480 and 630°C as shown in Fig. 2, but it increases rapidly above 630°C. This implies that the grain size is closely related to the formation of boride [ll]. In addition, the crystallization fraction was calculated approximately by the integrated intensity ratio of the (200) peak of a-Fe(Si) to the amorphous diffuse peak, as shown in Fig. 2.
H. Yang et al. /Journal
96
of Magnetism and Magnetic Materials 138 (1994) 94-98
-3.0JoFig. 6. Dependence
0
10
30
20
0
Ta (‘C)
of the parameter
S on annealing
temperature.
40
H,,(T) Fig. 4. Hyperfine field distribution as-quenched, (b) 480°C, (c) 510°C.
of the amorphous
phase: (a)
2.2. Results of Miissbauer spectroscopy The room-temperature Miissbauer spectra for samples annealed at different temperatures are shown in Fig. 3, which indicates total spectra and sub-spectra of the amorphous phase. The as-quenched sample gives a typical amorphous spectrum. The samples annealed at temperatures between 480 and 630°C precipitate partially a-Fe(%) solid solution only, which can be fitted by five subspectra. Finally, the samples annealed at 700°C for 10 h are completely transformed into a-Fe&) solid solution, Fe,B and Nb-rich boride with paramagnetic character. On the other hand, for all samples, the hyperfine field distribution of the amorphous phase includes two components: a lower hyperfine field component and a higher one. As the annealing temperature increases, the ratio of the low-field to the high-field component
s20.0
i-7
becomes larger. As a result, the average hyperfine field of the amorphous phase decrease rapidly after crystallization occurs, as shown in Figs. 4 and 5, which indicates that the amorphous phase is inhomogeneous. Therefore, the low-field component may correspond to the boundary, which contains more nonferromagnetic atoms such as Nb, Cu and B, and the high one to the residual amorphous phase, the composition of which is similar to that of the asquenched alloy. 2.3. Results of Diippler positron annihilation
500
I300
700
Ta (“C)
Fig. 5. Dependence of the average hyperfine phous phase on annealing temperature.
field of the amor-
spectrum
of
The parameter S of Doppler broadening spectrum of positron annihilation is related to the density of vacancy-like defects. The higher the density, the larger is the value of S [12]. The dependence of S on annealing temperature for Fe,,&u,Nb,Si,,,,B, alloy is shown in Fig. 6. Here, S = (S - S,,)/S,, where S,, is the parameter S for the as-quenched samples. When partial crystallization occurs, S obviously decreases and has a minimum at about 530°C but when the annealing temperature is above 550°C S increases rapidly, indicating that the density of vacancy-like defects in the sample becomes larger. 2.4. Soft magnetic properties
“:::jL
broadening
of the samples
The annealing temperature dependence of the soft magnetic properties of Fe,,,,Cu,Nb,Si,,,,B, alloy are shown in Fig. 7. The as-quenched sample had lower permeability and higher coercivity. As the annealing temperature increases, the coercivity decreases and the permeability increases, and espe-
H. Yang et al. /Journal
of Magnetism and Magnetic Materials 138 (1994) 94-98
r
100.0
-80.0
2
-60.0
-2 -
-40.0
2
_~20.0
k g a
ii
0.00
0.0 400
450
500
550
600
co.0
650
Ta (‘C) Fig. 7. Dependence of the magnetic properties of Fe,,,,Cu,Nb, Si,,,,B, alloy on annealing temperature. 0 coercivity (H,); A permeability; and 0 power loss at a field of 5 Oe and frequency 2 kHz.
cially after crystallization, the changes become violent. At 53O”C, the coercivity reaches its minimum and the permeability its maximum. Finally, when the annealing temperature is above 550°C the coercivity rises and the permeability declines rapidly. The excellent soft magnetic properties of the nanocrystalline alloy degrade completely. During this process, the tendency of the power loss is the inverse of that of the permeability. 3. Discussion After partial crystallization, Fe,3,5Cu,Nb,Si,,,~B, amorphous alloy can transform into two phases: (1) a crystalline phase, a-Fe&) solid solution, and (2) an amorphous phase, including a newly created boundary and a residual amorphous region where crystallization has not occurred. As we know, in Fe-CuNb-Si-B alloy, Si can easily dissolve in o-Fe to form a solid solution, but Nb and B hardly dissolve in o-Fe, so that they are dispelled from the crystallized region into the boundary and then probably interact to form relatively stable clusters because the difference in electronegativity between them is very large. Above all, an Nb- and B-rich boundary appears and it is relatively weakly ferromagnetic, corresponding to the low-field component, while the uncrystallized region is probably associated with the high-field component. In addition, with crystallization enhancing, the volume ratio of the boundary to the uncrystallized region increases, leading to an increasing ratio of low-field to high-field components. Meanwhile, the average hyperfine field of the amorphous phase decreases, as shown in Fig. 5.
97
According to the results of Leighly [12], the large amount of vacancy-like defects existing in the boundary may be a positron trap. When the grain size is smaller than 0.5 pm, most positrons would annihilate in the boundary. Furthermore the positrons in Fe,,,&Ju1Nb3Si,,,~B, nanocrystalline alloy would act in the same way. For the as-quenched sample, the melt-quenching creates a large amount of vacancylike defects such as the free volume, leading to a high value of the parameter S. After crystallization, crystalline grains with more perfect structure precipitate and the density of defects decreases so that S decreases. On the other hand, for Fe,&u,Nb, Si,,,sB, nanocrystalline alloy, the volume fraction of the boundary is so high that the structure of boundary is very important. When the amealing temperature is above 550°C Nd and B atoms segregated in the boundary interact and form more stable clusters before the Nb-B compound appears at 700°C. This may result in the increasing density of defects in boundary. Of course, the parameter S increases again. Recently, Herzer [5] used the random anisotropy model to interpret successfully the behavior of the magnetic properties of nanocrystalline alloy prepared by crystallization, and derived the relation of coercivity to the grain size (D), the anisotropy constant (K), and the exchange stiffness (A), i.e. H, =pcK4D6/(Js pi =pPJ;.A3/(
*A”) /+,K4 *D”),
where H, is the coercivity, pi is the permeability, D is the grain size, and A is the exchange stiffness, which is relative to the coupling intensity between neighboring grains. From this relation, it can be seen that the coercivity of nanocrystalline alloy increases with increasing grain size D of o-Fe(Si) and decreasing exchange stiffness A. As shown in Fig. 2, for the samples annealed in the temperature range 510-630°C the grain size of o-Fe&) remains constant at about 10 nm, and the crystallization fraction changes slowly from 63% to 80%. But the average hyperfine field of the amorphous phase obviously decreases; this behavior is similar to that of the exchange stiffness A. According to the above relation, the coercivity should increase monotanously with increasing annealing temperature in this range, but at 530°C there are in fact a minimum for the coercivity and a maximum for the permeability.
98
H. Yang et al. /Journal
of Magnetism and Magnetic Materials 138 (1994) 94-98
Comparing with Fig. 6, the behaviors of the coercivity and the S parameter are similar. Perhaps this implies that the magnetic properties of nanocrystalline alloy are closely related to the density of defects. For the samples annealed in this temperature range, it has been confirmed that the magnetization is performed through the domain wall movement 161. As we know, the moving domain wall interacts with the defects and is subjected to a hindering force. As a result, the lowest density of defects at 530°C creates the smallest coercivity. If the annealing temperature is above 6OO”C, the Nb-B compounds with paramagnetism form in the boundary, as described. They interact with the moving domain wall as magnetic defects so seriously that the coercivity increases significantly, as shown in Fig. 7. On the other hand, the crystallites of a-Fe(Si) are surrounded by the boundary with weak ferromagnetism. In this case, the domain wall is pinned by the boundary and the neighbouring grains are separated by the boundary so that direct ferromagnetic coupling does not occur. The magnetic behavior of the nanocrystalline alloy is the same as that of the nanocrystalline particles, which are compacted together. In this field, some results have been investigated [6,9].
4. Conclusions With X-ray diffraction, Miissbauer and positron annihilation, the structure phous phase of Fe,,,5Cu,Nb,Si,,,5B,
spectroscopy of the amornanocrys-
talline alloy has been investigated. The results show that the structure changes with annealing temperature, and that Nb and B atoms, which hardly dissolve in o-Fe, segregate in the boundary and probably interact to form more stable clusters. This makes a weak ferromagnetic boundary with a low hyperfine field and a high density of defects. The amorphous phase plays an important role in the magnetic properties of the nanocrystalline alloy. The lower the density of defects, the smaller is the coercivity.
References [l] Y. Yoshizawa, S. Oguma and K. Yamauchi, J. Appl. Phys. 64 (1988) 6044. [2] T.H. Noh, M.B. Lee, H.J. Kim and I.K. Kang, J. Appl. Phys. 67 (1990) 5568. [3] Y. Yoshizawa and K. Yamauchi, Mater. Sci. Eng. A 133 (1991) 176. [4] K. Hono, A. Inoue and T. Saurai, Appl. Phys. Lett. 58 (1991) 2180. [5] G. Herzer, Mater. Sci. Eng. A 133 (1991) 1. [6] R. Schafer, A. Hubert and G. Herzer, J. Appl. Phys. 69 (1991) 5523. [7] G. Herzer, J. Mater. Eng. Performance 2 (1993) 193. [8] J. Jiangzhong, F.F. Aubertin, V. Gonser and H.R. Hilzinger, Z. Matallkde 82 (1991) 689. [9] X.Z. Zhou, A.H. Morrish, D.G. Naugle and R. Pan, J. Appl. Phys. 73 (1993) 6597. [lo] G.W. Fei, W.L. Zhong and Y.B. Qio, X-ray Diffraction for Single-Crystalline, Polycrystalline and Amorphous Materials (Shandong University Press, Shandong, 1989) p. 372 (in Chinese). [ll] H.S. Yang, S.L. Zhang, R.Z. Ma, et al., Chinese Sci. Bull. 38 (1993) 1843. [12] H.P. Leighly, Appl. Phys. 12 (1977) 217.
Journal of Magnetism and Magnetic Materials 138 (1994) 94-98
Structure of the amorphous phase during crystallization of Fe,,Cu,Nb3Si,,,B, alloy Yang Huisheng
a,*, Tu Guochao b, Xiong Xiaotao a, Xu Zuxiong a, Ma Ruzhang a
aDeparhnent of Material Physics, Uniuersity of Science and Technology Beijing, Beijing 100083, China b Metallurgical
In&h&e of Shoudu Iron and Steel Company, Beijing 100083, China
Received 5 November 1993; in final revised form 31 May 1994
Abstract Mossbauer spectroscopy and the positron annihilation technique have been used to investigate the structure of the B s nanocrystalline alloy annealed at different temperatures. The results show that amorphous phase of Fe,,~,Cu,Nb,Si,,,, there is a weak ferromagnetic amorphous boundary with a low hyperfine field. On other hand, the results of positron annihilation indicate that a large amount of vacancy-like defects appears in the amorphous phase at higher temperature. The results of magnetic measurements indicate that the magnetic properties of nanocrystalline alloy are closely related to the density of defects.
1. Introduction 13.5B9 alloy after optimal annealFe,,.,Cu,Nb,Si ing has large magnetization, high permeability, low coercivity and power loss [l-4]. It has attracted much attention as a new type of soft magnetic material. At first, much work concentrated on the formation of the nanocrystalline structure and the effect of grain size on the magnetic properties. More recent results [2,5] have shown that the magnetic properties change considerably with annealing temperature, although the crystalline phase shows no obvious change. Therefore, the magnetic properties of the boundary phase and its effects on the magnetic properties of the alloy began to be mentioned [6-91. It has been known that the excellent soft magnetic
* Corresponding author. Present address: Group 207, Institute of Physics, Chinese Academy of Science, Beijing, China. 0304-8853/94/$07.00 0 1994 Elsevier Science B.V. AII rights reserved SSDI 0304-8853(94)00415-N
properties will degrade when the amorphous phase transforms into a paramagnetic phase above 300°C. This implies that the amorphous phase plays an important role in the magnetic properties of the nanocrystalline alloy. The hyperfine parameters of MGssbauer spectra can provide much information on all phases in the alloy, especially the amorphous phase, and the positron annihilation parameters are sensitive to the defects. For this reason, they are used to investigate the structure of the amorphous phase of the nanocrystalline alloy.
2. Experiments 2.1. The crystallization process Si,,, B, amorphous alloy
of Fe,,,
The experimental Fe,,,,Cu,Nb,Si,,,,B, phous ribbon was prepared by the single-roller
Cu, Nb,
amormelt-
H. Yang et al. /Journal
58.00
78.00
95
of Magnetism and Magnetic Materials 138 (1994) 94-98
90100
Wdeg) Fig. 1. X-ray diffraction patterns for Fe,3,&u,Nb3Si,,,,B, annealed at different temperatures (0, a-Fe(S), 0, Fe,B).
alloy
quenching method. Its typical cross section was 10 mm X 18 km and crystallization temperature (measured by DTA at lO”C/min) was 500°C. The ribbon was annealed at different temperatures for 30 min; the X-ray diffraction patterns are shown in Fig. 1. Fe,,,,Cu,Nb,Si,,,5B, amorphous alloy begins to crystallize at 480°C and precipitates a-Fe(%) solid solution (crystalline phase). As the annealing temperature increases, the volume ratio of crystalline to amorphous phases gradually increases, and finally, at 630°C the Fe,B phase and other boron compounds
1
I
I
I
I
I
I
-6
-4
-2
0
2
4
6
mm/S
Fig. 3. Mijssbauer spectra for samples annealed at different temperatures: (a) as-quenched, (b) 480°C (c) 51O”C, (d) 700°C.
‘““Ii.i ,O*W
Ta (“C)
Fig. 2. Dependence of grain size (D, 0) of the crystalline phase and the crystallization fraction (P, A) on annealing temperature.
form. On the other hand, the grain size, calculated by the Sherrer method [lo], remains almost constant about at 10 nm when annealing between 480 and 630°C as shown in Fig. 2, but it increases rapidly above 630°C. This implies that the grain size is closely related to the formation of boride [ll]. In addition, the crystallization fraction was calculated approximately by the integrated intensity ratio of the (200) peak of a-Fe(Si) to the amorphous diffuse peak, as shown in Fig. 2.
H. Yang et al. /Journal
96
of Magnetism and Magnetic Materials 138 (1994) 94-98
-3.0JoFig. 6. Dependence
0
10
30
20
0
Ta (‘C)
of the parameter
S on annealing
temperature.
40
H,,(T) Fig. 4. Hyperfine field distribution as-quenched, (b) 480°C, (c) 510°C.
of the amorphous
phase: (a)
2.2. Results of Miissbauer spectroscopy The room-temperature Miissbauer spectra for samples annealed at different temperatures are shown in Fig. 3, which indicates total spectra and sub-spectra of the amorphous phase. The as-quenched sample gives a typical amorphous spectrum. The samples annealed at temperatures between 480 and 630°C precipitate partially a-Fe(%) solid solution only, which can be fitted by five subspectra. Finally, the samples annealed at 700°C for 10 h are completely transformed into a-Fe&) solid solution, Fe,B and Nb-rich boride with paramagnetic character. On the other hand, for all samples, the hyperfine field distribution of the amorphous phase includes two components: a lower hyperfine field component and a higher one. As the annealing temperature increases, the ratio of the low-field to the high-field component
s20.0
i-7
becomes larger. As a result, the average hyperfine field of the amorphous phase decrease rapidly after crystallization occurs, as shown in Figs. 4 and 5, which indicates that the amorphous phase is inhomogeneous. Therefore, the low-field component may correspond to the boundary, which contains more nonferromagnetic atoms such as Nb, Cu and B, and the high one to the residual amorphous phase, the composition of which is similar to that of the asquenched alloy. 2.3. Results of Diippler positron annihilation
500
I300
700
Ta (“C)
Fig. 5. Dependence of the average hyperfine phous phase on annealing temperature.
field of the amor-
spectrum
of
The parameter S of Doppler broadening spectrum of positron annihilation is related to the density of vacancy-like defects. The higher the density, the larger is the value of S [12]. The dependence of S on annealing temperature for Fe,,&u,Nb,Si,,,,B, alloy is shown in Fig. 6. Here, S = (S - S,,)/S,, where S,, is the parameter S for the as-quenched samples. When partial crystallization occurs, S obviously decreases and has a minimum at about 530°C but when the annealing temperature is above 550°C S increases rapidly, indicating that the density of vacancy-like defects in the sample becomes larger. 2.4. Soft magnetic properties
“:::jL
broadening
of the samples
The annealing temperature dependence of the soft magnetic properties of Fe,,,,Cu,Nb,Si,,,,B, alloy are shown in Fig. 7. The as-quenched sample had lower permeability and higher coercivity. As the annealing temperature increases, the coercivity decreases and the permeability increases, and espe-
H. Yang et al. /Journal
of Magnetism and Magnetic Materials 138 (1994) 94-98
r
100.0
-80.0
2
-60.0
-2 -
-40.0
2
_~20.0
k g a
ii
0.00
0.0 400
450
500
550
600
co.0
650
Ta (‘C) Fig. 7. Dependence of the magnetic properties of Fe,,,,Cu,Nb, Si,,,,B, alloy on annealing temperature. 0 coercivity (H,); A permeability; and 0 power loss at a field of 5 Oe and frequency 2 kHz.
cially after crystallization, the changes become violent. At 53O”C, the coercivity reaches its minimum and the permeability its maximum. Finally, when the annealing temperature is above 550°C the coercivity rises and the permeability declines rapidly. The excellent soft magnetic properties of the nanocrystalline alloy degrade completely. During this process, the tendency of the power loss is the inverse of that of the permeability. 3. Discussion After partial crystallization, Fe,3,5Cu,Nb,Si,,,~B, amorphous alloy can transform into two phases: (1) a crystalline phase, a-Fe&) solid solution, and (2) an amorphous phase, including a newly created boundary and a residual amorphous region where crystallization has not occurred. As we know, in Fe-CuNb-Si-B alloy, Si can easily dissolve in o-Fe to form a solid solution, but Nb and B hardly dissolve in o-Fe, so that they are dispelled from the crystallized region into the boundary and then probably interact to form relatively stable clusters because the difference in electronegativity between them is very large. Above all, an Nb- and B-rich boundary appears and it is relatively weakly ferromagnetic, corresponding to the low-field component, while the uncrystallized region is probably associated with the high-field component. In addition, with crystallization enhancing, the volume ratio of the boundary to the uncrystallized region increases, leading to an increasing ratio of low-field to high-field components. Meanwhile, the average hyperfine field of the amorphous phase decreases, as shown in Fig. 5.
97
According to the results of Leighly [12], the large amount of vacancy-like defects existing in the boundary may be a positron trap. When the grain size is smaller than 0.5 pm, most positrons would annihilate in the boundary. Furthermore the positrons in Fe,,,&Ju1Nb3Si,,,~B, nanocrystalline alloy would act in the same way. For the as-quenched sample, the melt-quenching creates a large amount of vacancylike defects such as the free volume, leading to a high value of the parameter S. After crystallization, crystalline grains with more perfect structure precipitate and the density of defects decreases so that S decreases. On the other hand, for Fe,&u,Nb, Si,,,sB, nanocrystalline alloy, the volume fraction of the boundary is so high that the structure of boundary is very important. When the amealing temperature is above 550°C Nd and B atoms segregated in the boundary interact and form more stable clusters before the Nb-B compound appears at 700°C. This may result in the increasing density of defects in boundary. Of course, the parameter S increases again. Recently, Herzer [5] used the random anisotropy model to interpret successfully the behavior of the magnetic properties of nanocrystalline alloy prepared by crystallization, and derived the relation of coercivity to the grain size (D), the anisotropy constant (K), and the exchange stiffness (A), i.e. H, =pcK4D6/(Js pi =pPJ;.A3/(
*A”) /+,K4 *D”),
where H, is the coercivity, pi is the permeability, D is the grain size, and A is the exchange stiffness, which is relative to the coupling intensity between neighboring grains. From this relation, it can be seen that the coercivity of nanocrystalline alloy increases with increasing grain size D of o-Fe(Si) and decreasing exchange stiffness A. As shown in Fig. 2, for the samples annealed in the temperature range 510-630°C the grain size of o-Fe&) remains constant at about 10 nm, and the crystallization fraction changes slowly from 63% to 80%. But the average hyperfine field of the amorphous phase obviously decreases; this behavior is similar to that of the exchange stiffness A. According to the above relation, the coercivity should increase monotanously with increasing annealing temperature in this range, but at 530°C there are in fact a minimum for the coercivity and a maximum for the permeability.
98
H. Yang et al. /Journal
of Magnetism and Magnetic Materials 138 (1994) 94-98
Comparing with Fig. 6, the behaviors of the coercivity and the S parameter are similar. Perhaps this implies that the magnetic properties of nanocrystalline alloy are closely related to the density of defects. For the samples annealed in this temperature range, it has been confirmed that the magnetization is performed through the domain wall movement 161. As we know, the moving domain wall interacts with the defects and is subjected to a hindering force. As a result, the lowest density of defects at 530°C creates the smallest coercivity. If the annealing temperature is above 6OO”C, the Nb-B compounds with paramagnetism form in the boundary, as described. They interact with the moving domain wall as magnetic defects so seriously that the coercivity increases significantly, as shown in Fig. 7. On the other hand, the crystallites of a-Fe(Si) are surrounded by the boundary with weak ferromagnetism. In this case, the domain wall is pinned by the boundary and the neighbouring grains are separated by the boundary so that direct ferromagnetic coupling does not occur. The magnetic behavior of the nanocrystalline alloy is the same as that of the nanocrystalline particles, which are compacted together. In this field, some results have been investigated [6,9].
4. Conclusions With X-ray diffraction, Miissbauer and positron annihilation, the structure phous phase of Fe,,,5Cu,Nb,Si,,,5B,
spectroscopy of the amornanocrys-
talline alloy has been investigated. The results show that the structure changes with annealing temperature, and that Nb and B atoms, which hardly dissolve in o-Fe, segregate in the boundary and probably interact to form more stable clusters. This makes a weak ferromagnetic boundary with a low hyperfine field and a high density of defects. The amorphous phase plays an important role in the magnetic properties of the nanocrystalline alloy. The lower the density of defects, the smaller is the coercivity.
References [l] Y. Yoshizawa, S. Oguma and K. Yamauchi, J. Appl. Phys. 64 (1988) 6044. [2] T.H. Noh, M.B. Lee, H.J. Kim and I.K. Kang, J. Appl. Phys. 67 (1990) 5568. [3] Y. Yoshizawa and K. Yamauchi, Mater. Sci. Eng. A 133 (1991) 176. [4] K. Hono, A. Inoue and T. Saurai, Appl. Phys. Lett. 58 (1991) 2180. [5] G. Herzer, Mater. Sci. Eng. A 133 (1991) 1. [6] R. Schafer, A. Hubert and G. Herzer, J. Appl. Phys. 69 (1991) 5523. [7] G. Herzer, J. Mater. Eng. Performance 2 (1993) 193. [8] J. Jiangzhong, F.F. Aubertin, V. Gonser and H.R. Hilzinger, Z. Matallkde 82 (1991) 689. [9] X.Z. Zhou, A.H. Morrish, D.G. Naugle and R. Pan, J. Appl. Phys. 73 (1993) 6597. [lo] G.W. Fei, W.L. Zhong and Y.B. Qio, X-ray Diffraction for Single-Crystalline, Polycrystalline and Amorphous Materials (Shandong University Press, Shandong, 1989) p. 372 (in Chinese). [ll] H.S. Yang, S.L. Zhang, R.Z. Ma, et al., Chinese Sci. Bull. 38 (1993) 1843. [12] H.P. Leighly, Appl. Phys. 12 (1977) 217.