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Magnetic field induced anisotropy in nanocrystalline FeCuNbSiB alloys
MaterialsScience and Engineering, A181/A182 (1994) 876-879
876
Magnetic field induced anisotropy in nanocrystalline Fe-Cu-Nb-Si-B alloys Giselher Herzer Vacuumschmelze GmbH, D-63450 Hanau (Germany) Abstract Magnetic field annealing of nanocrystallized glassy Fe-Cu-Nb-Si-B alloys induces a uniaxial anisotropy, the easy axis being parallel to the direction of the applied field. The induced anisotropy energy K u reaches an equilibrium vaiue characteristic of the individual alloy composition if the magnetic field is applied during the formation of the nanocrystalline state. Typical magnitudes of induced anisotropy energy range from about Ku = 10 J m-3 to Ku = 100 J m-3. Analysis of the experimental data suggests that the anisotropy is mainly induced in the precipitated b.c.c. FeSi grains and that its magnitude is determined by the fraction and silicon content of crystallites.
1. Introduction Nanocrystalline Fe-Cu-Nb-Si-B alloys are produced by rapid quenching, giving an amorphous ribbon. Subsequent heat treatment above the crystallization temperature produces a homogeneous ultrafine grain structure of b.c.c. FeSi with grain sizes of typically 10-15 nm and random texture [1]. Owing to the small grain size, the local magnetocrystalline anisotropy is randomly averaged out by exchange interaction [2, 3]. The structural phases present lead to a low or vanishing saturation magnetostriction [1, 4]. Both the suppressed magnetocrystalline anisotropy and the low magnetostriction give rise to superior soft magnetic properties comparable with those of permalloys or near-zero-magnetostrictive cobalt-base amorphous alloys. Like in other soft magnetic materials, magnetic field annealing induces a uniaxial anisotropy, the easy axis being parallel to the magnetic field applied during the heat treatment [5]. This allows the shape of the hysteresis loop to be changed according to the demands of various applications. The induced anisotropy energy K u depends on both the annealing conditions and the alloy composition. The influence of annealing conditions has been discussed recently for nanocrystalline Fe73.sfulNb3Si13.sB 9 by Yoshizawa and Yamauchi [6]. The present work complements these investigations by showing the effect of alloy composition within the Fe-Cu-Nb-Si-B system.
2. Experimental details Amorphous ribbons of composition FebalCUaNbb a=0.5-2 at.%, b = 2 - 5 at.%,
Sitz_xlBx with
0921-5093/94/$7.00
SSD10921-5093(93)05623-W
z = 18.5-23.5 at.% and x = 5-14 at.% were prepared by rapid quenching from the melt. The ribbons, typically 15 mm wide and 20/am thick, were wound into toroidal cores with 16 mm inner and 22 mm outer diameters and annealed in a protecting nitrogen atmosphere. The typical annealing condition chosen was 1 h at 540 °C. This treatment in most cases yields a nanocrystalline microstructure close to the quasiequilibrium state characteristic of each alloy composition. To induce a uniaxial anisotropy, a transverse magnetic field about 2 kA cm-1 in strength was applied during the heat treatment. The induced anisotropy energy density K u was determined at room temperature from the anisotropy field of the d.c. hysteresis loop.
3. Results Figure 1 shows the induced anisotropy energy K u as a function of the field annealing temperature T.a. The results given here for FeTa.sCulNb3Sila.sB 9 are also representative of the other alloy compositions. In the first annealing series the material was subjected to a 1 h pre-anneal treatment at 540 °C without a magnetic field to obtain the nanocrystalline state. The magnetic field was then applied in a second step at lower annealing temperatures between 300 and 540 °C. In this twostep treatment the resulting induced anisotropy depends sensitively on the annealing temperature Ta and the annealing time ta (cf. ref. 6). In a second annealing series the magnetic field was applied during the transformation from the amorphous to the nanocrystalline state. In this case the field induced anisotropy reaches a maximum value which proved to be relatively insensitive (within about + 10%) to the precise annealing conditions (typically Ta=500© 1994 - Elsevier Sequoia. All rights reserved
G. Herzer /
MagneticfieM induced anLsotropy
2503
~-
mental error (about _+3%) when the copper and niobium contents are changed between a = 0.5-2 at.% and b = 2-5 at.% respectively.
1h field annealing at T a / ~ / / = ~
<
/
2o
877
/ /
15 Q.
Y
/
4. Discussion
J
o
10..Dj
..~.-" ~
5
~
J
1h 540°C without field + 4h field annealing at Ta
..~
a60
200
J
460
56o
660
Field Annealing Temperature, Ta, in °C
Fig. 1. Field induced anisotropy K u of Fe7~ 5CulNb~Sil35B , as a function of the field annealing temperature 7~,.
12@ 03
<
z = 18.5 at%
loo~"
80
e.
60
~ x-
x - - - - ~
/
x
20.5 at% 22.5 at%
~
20 ~
0
< r - - 23.5 at%
4
1'4 Boron content, x, in at%
Fig. 2. Field induced anisotropy K u of an Fe,)~ :CulNb~Si :_,,B, alloy series annealed for 1 h at 540 °C in a magnetic field (the numbers on the curves indicate the total silicon and b o r o n contents).
600 °C, t~ > 0.5 h and cooling rates from 0.2 to 20 K min-~). The induced anisotropy achieved in this way can thus be considered as characteristic of the individual alloy compositions. The most prominent influence of alloy composition is due to the silicon and boron contents. Figure 2 shows the variation in K u with metalloid content for an Fe~,_:CulNb3Sii:_,.)B x alloy series annealed for 1 h at 540 °C in a transverse magnetic field. The resulting K u values range from about 10 J m 3 to 100 J m 3. The effect of the copper and niobium concentrations on K u is only minor (provided that the copper and niobium contents are chosen such that the nanocrystalline state is formed at all). For example, in Fq,~,ICu,,Nb~,SiL~.sB~ annealed for 1 h at 540 °C, the induced anisotropy does not change within experi-
As reported by Yoshizawa and Yamauchi [5] the results of X-ray diffraction and transmission electron microscopy give no evidence that there is any change in microstructure or any orientation of the b.c.c, grains due to magnetic field annealing. This indicates that the field induced anisotropy in nanocrystalline FeCuNbSiB is of similar origin to that in other soft magnetic alloys, i.e. atomic ordering of both magnetic and nonmagnetic atoms (el ref. 7). The induced anisotropy is generally very sensitive to the composition and can only be induced if the field annealing temperature is below the Curie temperature. The microstructure of nanocrystalline FeCuNbSiB alloys essentially consists of two magnetic phases [2, 4, 8]: (1) b.c.c. Fe.,~ ,Si, grains and (2) a still amorphous minority matrix with a composition close to stoichiometric (Fe~ ,,Nb,)eB. The Curie temperatures T~. of the b.c.c, grains typically range from about 600 °C to 700 °C, while the Curie points of the residual amorphous matrix are significantly lower and range from about 2()0 °C to 400 °C [2, 4]. The present field annealing temperature of 540 °C is thus below the 7~. of the b.c.c, grains, but clearly above the T~ of the amorphous matrix. This indicates that the anisotropy is primarily induced in the b.c.c. FeSi grains. A significant contribution from the amorphous matrix, in particular during cooling down to room temperature, can be largely ruled out since variation in the cooling rate between about 0.2 and 2() K min t produced only minor changes. A further understanding of the K u formation is obtained when the K u data are analysed in terms of the volume fraction Vcr and the silicon content of the b.c.c. FeSi grains. Figure 3 shows these microstructural data for an Fev3.sCulNb~Si,22.5 ,:B~ alloy series annealed for 1 h at 540 °C. The data points can be well reproduced theoretically (see full lines in Fig. 3) from the balance of atomic concentrations assuming the following crystallization reaction FeCuNbSiB --* Vc~a-Fe I ySiy +(1 - vcr)(Fe ~__,Nb,)2B
(1)
This indicates that the nucleation and growth of the b.c.c, grains proceeds until the residual amorphous matrix is enriched with boron such that its composition is close to stoichiometric (Fe~ ,Nb,)eB. The crystalline fraction is thus mainly determined by the boron
G. Herzer /
878
Magneticfield induced anisotropy "22
~,
z.~ 18.5 at%
-2O
0.9-
0.8
200"
o~ -18 ~ n
[]
150
u
~ 0.7~ .
-16 ~
<
6 14 -E
.~ I00
z = 23.5 at%
~0.6" 5(? 0.5
4
6
8
1()
1'2
1~4
16
10
Boron content, x, in at%
Fig. 3. Fraction v~r and
silicon content of the b.c.c. FeSi grains of nanocrystaltine Fe73.sCulNb3Si22.5_xBx (annealed for 1 h at 540 °C) as a function of the boron content. The experimental data points were determined from thermomagnetic analysis (cf. refs. 2 and 4); the full lines are calculated from the balance of atomic concentrations.
content, while niobium seems to stabilize the amorphous matrix in order to prevent the formation of borides. Since there is practically no induced anisotropy contribution from the residual amorphous matrix, the anisotropy induced locally in the FeSi grains, i.e. Ku(FeSi), is given by Ku(FeSi) = Ku/vcr
(2)
where K u is the experimentally determined anisotropy of the nanocrystalline material. Figure 4 shows Ku(FeSi) vs. the silicon content of the a-FeSi grains. This representation gives a considerably clearer view of the alloying effect. Thus, the induced anisotropy in nanocrystalline FeTCu-Nb-Si-B alloys is primarily determined by the silicon content and the fraction of b.c.c, grains. These structural parameters themselves depend mainly on the total boron and silicon content while, within certain limits, the copper and niobium content seems to be less significant. The latter result is indicated by the insensitivity of K u to detailed copper and niobium concentrations. The dependence of Ku/Vc, on the silicon content in the b.c.c, grains is comparable with that observed for conventional a-FeSi single crystals [9, 10]. For example, K u is reported [9] to show a maximum at about 10 at.% Si and then to decrease rapidly towards values near zero for 25 at.% Si (Fe3Si). The magnitudes of the present data are situated in between the reported values [9, 10]. A more quantitative comparison, however, is difficult since the literature data [9, 10] reveal a relatively large scatter, owing mostly to different crystal
8
Fe-Cul Nb3 Si (z-x) B-x annealed lh at 540,(3 in a magnetic field 10
12
14
"~+ ~
16
18
_ ~
=
20
22
24
Si-content of the ct-FeSi grains in at*/.
Fig. 4. Field induced anisotropy K u normalized to the crystalline fraction Vcr as a function of the silicon content in the b.c.c. FeSi grains of nanocr:ystalline F e 9 6 _ zCu 1Nb3Si(z- x/Bx•
orientations and measuring problems. In particular, the relatively large magnetocrystalline anisotropy K~ (several thousand joules per cubic metre [10]) makes it difficult to separate experimentally the field induced anisotropy in FeSi single crystals. There is no such problem in the present case since KI is averaged out by exchange interaction to values below a few joules per cubic metre [3]. The formation of the field induced anisotropy in a-FeSi has been proposed to arise from the directional ordering of the nearest and next-nearest silicon-atom pairs [9]. Thus, in terms of N&l's theory [11], the decrease in K u can be understood from the increasing long-range order of the silicon atoms (Fe3Si superlattice structure) with increasing silicon content since the available lattice sites for directional ordering decrease simultaneously. The induced anisotropy in a-FeSi is also reported to decrease again for low silicon contents [9]. The behaviour of nanocrystalline Fe-Cu-Nb-Si-B alloys with low silicon contents is not yet quite clear, because it was difficult to produce a homogeneous nanocrystalline structure for low silicon contents. The reason is that the corresponding alloys simultaneously reveal a high boron content. This is necessary for glass formation but at the same time favours the formation of borides on crystallization. However, a homogeneous a-Fe grain structure can be obtained in the FeZrCuB system [12]. Preliminary investigations of nanocrystalline F e 8 6 C u l Z r 6 B 7 yield a field induced anisotropy of K J v c , ~ 50 J m -3, indicating that Ku indeed seems to decrease again for low silicon contents.
G. Herzer
/
Magnetic field induced anisotropy
879
5. Conclusions
Acknowledgment
Magnetic field annealing of nanocrystalline F e - C u - N b - S i - B alloys induces a uniaxial magnetic anisotropy parallel to the magnetic field applied during the heat treatment. When the magnetic field is applied during the transformation from the originally amorphous to the nanocrystalline state, the induced anisotropy energy K u reaches a maximum value which is relatively insensitive to the detailed annealing conditions and is thus characteristic of the individual alloy composition. Analysis of the experimental data leads to the following conclusions. (1) The anisotropy is primarily induced in the b.c.c. FeSi grains, probably by atomic pair ordering of the silicon atoms. A significant contribution from the residual amorphous matrix can be largely ruled out. (2) The magnitude of K, is determined by the fraction Ver and the silicon content of the b.c.c. FeSi grains. In particular, K~ significantly decreases with increasing silicon content of the b.c.c, grains, which can be attributed to the increasing long-range order (Fe~Si superlattice structure) of the silicon atoms. (3) In terms of the total alloy composition, the induced anisotropy mainly depends on the silicon and boron contents. This indicates that the crystalline fraction and composition themselves are mainly determined by these two elements. This is also supported by the balance of atomic concentrations which reproduces well the microstructural features.
This work has been financially supported by the Bundesministerium fiir Forschung und Technologie (BMFT).
References 1 Y. Yoshizawa, S. Oguma and K. Yamauchi, J. Appl. Phys., 64 (1988) 6044. 2 G. Herzer, IEEE Trans. Magn., 25(1989) 3327. 3 G. Herzer, IEEE Trans. Magn., 26(1990) 1397. 4 G. Herzer, Proc. Int. Symp. on 3d Transition-Semimetal Thin Films, 5-8 March, Sendai, 1991, Japan Society for the Promotion of Science 131 Committee (Thin Film), Sendai, 199l,p. 130. 5 Y. Yoshizawa and K. Yamauchi, IEEE Trans. Magn., 25 (1989) 3324. 6 Y. Yoshizawa and K. Yamauchi, IEEE Translation J. Magn., ,5(1990) 1070. 7 H. Fujimori, in F. E. Luborsky (ed.), Amorphous Metallic Alloys, Butterworths Monographs in Materials, London, 1983, Chapter 16 and references cited therein. 8 K. Hono, K. Hiraga, Q. Wang, A. Inoue and T. Sakurai, Acta Metall. Mater., 40 (1992) 2137. 9 K. Sixtus, Z. Angew. Phys., 14(1962) 241; 28(1970) 270. 10 H. Gengnagel and H. Wagner, Z. Angew Phys., 8 ( 1961 ) 174. 11 L. Ndel, J. Phys. Radium, 15 (1954) 225. 12 K. Suzuki, A. Makino, A. Inoue and T. Masumoto, J. Appl. Phys., 70(1991)6232.
876
Magnetic field induced anisotropy in nanocrystalline Fe-Cu-Nb-Si-B alloys Giselher Herzer Vacuumschmelze GmbH, D-63450 Hanau (Germany) Abstract Magnetic field annealing of nanocrystallized glassy Fe-Cu-Nb-Si-B alloys induces a uniaxial anisotropy, the easy axis being parallel to the direction of the applied field. The induced anisotropy energy K u reaches an equilibrium vaiue characteristic of the individual alloy composition if the magnetic field is applied during the formation of the nanocrystalline state. Typical magnitudes of induced anisotropy energy range from about Ku = 10 J m-3 to Ku = 100 J m-3. Analysis of the experimental data suggests that the anisotropy is mainly induced in the precipitated b.c.c. FeSi grains and that its magnitude is determined by the fraction and silicon content of crystallites.
1. Introduction Nanocrystalline Fe-Cu-Nb-Si-B alloys are produced by rapid quenching, giving an amorphous ribbon. Subsequent heat treatment above the crystallization temperature produces a homogeneous ultrafine grain structure of b.c.c. FeSi with grain sizes of typically 10-15 nm and random texture [1]. Owing to the small grain size, the local magnetocrystalline anisotropy is randomly averaged out by exchange interaction [2, 3]. The structural phases present lead to a low or vanishing saturation magnetostriction [1, 4]. Both the suppressed magnetocrystalline anisotropy and the low magnetostriction give rise to superior soft magnetic properties comparable with those of permalloys or near-zero-magnetostrictive cobalt-base amorphous alloys. Like in other soft magnetic materials, magnetic field annealing induces a uniaxial anisotropy, the easy axis being parallel to the magnetic field applied during the heat treatment [5]. This allows the shape of the hysteresis loop to be changed according to the demands of various applications. The induced anisotropy energy K u depends on both the annealing conditions and the alloy composition. The influence of annealing conditions has been discussed recently for nanocrystalline Fe73.sfulNb3Si13.sB 9 by Yoshizawa and Yamauchi [6]. The present work complements these investigations by showing the effect of alloy composition within the Fe-Cu-Nb-Si-B system.
2. Experimental details Amorphous ribbons of composition FebalCUaNbb a=0.5-2 at.%, b = 2 - 5 at.%,
Sitz_xlBx with
0921-5093/94/$7.00
SSD10921-5093(93)05623-W
z = 18.5-23.5 at.% and x = 5-14 at.% were prepared by rapid quenching from the melt. The ribbons, typically 15 mm wide and 20/am thick, were wound into toroidal cores with 16 mm inner and 22 mm outer diameters and annealed in a protecting nitrogen atmosphere. The typical annealing condition chosen was 1 h at 540 °C. This treatment in most cases yields a nanocrystalline microstructure close to the quasiequilibrium state characteristic of each alloy composition. To induce a uniaxial anisotropy, a transverse magnetic field about 2 kA cm-1 in strength was applied during the heat treatment. The induced anisotropy energy density K u was determined at room temperature from the anisotropy field of the d.c. hysteresis loop.
3. Results Figure 1 shows the induced anisotropy energy K u as a function of the field annealing temperature T.a. The results given here for FeTa.sCulNb3Sila.sB 9 are also representative of the other alloy compositions. In the first annealing series the material was subjected to a 1 h pre-anneal treatment at 540 °C without a magnetic field to obtain the nanocrystalline state. The magnetic field was then applied in a second step at lower annealing temperatures between 300 and 540 °C. In this twostep treatment the resulting induced anisotropy depends sensitively on the annealing temperature Ta and the annealing time ta (cf. ref. 6). In a second annealing series the magnetic field was applied during the transformation from the amorphous to the nanocrystalline state. In this case the field induced anisotropy reaches a maximum value which proved to be relatively insensitive (within about + 10%) to the precise annealing conditions (typically Ta=500© 1994 - Elsevier Sequoia. All rights reserved
G. Herzer /
MagneticfieM induced anLsotropy
2503
~-
mental error (about _+3%) when the copper and niobium contents are changed between a = 0.5-2 at.% and b = 2-5 at.% respectively.
1h field annealing at T a / ~ / / = ~
<
/
2o
877
/ /
15 Q.
Y
/
4. Discussion
J
o
10..Dj
..~.-" ~
5
~
J
1h 540°C without field + 4h field annealing at Ta
..~
a60
200
J
460
56o
660
Field Annealing Temperature, Ta, in °C
Fig. 1. Field induced anisotropy K u of Fe7~ 5CulNb~Sil35B , as a function of the field annealing temperature 7~,.
12@ 03
<
z = 18.5 at%
loo~"
80
e.
60
~ x-
x - - - - ~
/
x
20.5 at% 22.5 at%
~
20 ~
0
< r - - 23.5 at%
4
1'4 Boron content, x, in at%
Fig. 2. Field induced anisotropy K u of an Fe,)~ :CulNb~Si :_,,B, alloy series annealed for 1 h at 540 °C in a magnetic field (the numbers on the curves indicate the total silicon and b o r o n contents).
600 °C, t~ > 0.5 h and cooling rates from 0.2 to 20 K min-~). The induced anisotropy achieved in this way can thus be considered as characteristic of the individual alloy compositions. The most prominent influence of alloy composition is due to the silicon and boron contents. Figure 2 shows the variation in K u with metalloid content for an Fe~,_:CulNb3Sii:_,.)B x alloy series annealed for 1 h at 540 °C in a transverse magnetic field. The resulting K u values range from about 10 J m 3 to 100 J m 3. The effect of the copper and niobium concentrations on K u is only minor (provided that the copper and niobium contents are chosen such that the nanocrystalline state is formed at all). For example, in Fq,~,ICu,,Nb~,SiL~.sB~ annealed for 1 h at 540 °C, the induced anisotropy does not change within experi-
As reported by Yoshizawa and Yamauchi [5] the results of X-ray diffraction and transmission electron microscopy give no evidence that there is any change in microstructure or any orientation of the b.c.c, grains due to magnetic field annealing. This indicates that the field induced anisotropy in nanocrystalline FeCuNbSiB is of similar origin to that in other soft magnetic alloys, i.e. atomic ordering of both magnetic and nonmagnetic atoms (el ref. 7). The induced anisotropy is generally very sensitive to the composition and can only be induced if the field annealing temperature is below the Curie temperature. The microstructure of nanocrystalline FeCuNbSiB alloys essentially consists of two magnetic phases [2, 4, 8]: (1) b.c.c. Fe.,~ ,Si, grains and (2) a still amorphous minority matrix with a composition close to stoichiometric (Fe~ ,,Nb,)eB. The Curie temperatures T~. of the b.c.c, grains typically range from about 600 °C to 700 °C, while the Curie points of the residual amorphous matrix are significantly lower and range from about 2()0 °C to 400 °C [2, 4]. The present field annealing temperature of 540 °C is thus below the 7~. of the b.c.c, grains, but clearly above the T~ of the amorphous matrix. This indicates that the anisotropy is primarily induced in the b.c.c. FeSi grains. A significant contribution from the amorphous matrix, in particular during cooling down to room temperature, can be largely ruled out since variation in the cooling rate between about 0.2 and 2() K min t produced only minor changes. A further understanding of the K u formation is obtained when the K u data are analysed in terms of the volume fraction Vcr and the silicon content of the b.c.c. FeSi grains. Figure 3 shows these microstructural data for an Fev3.sCulNb~Si,22.5 ,:B~ alloy series annealed for 1 h at 540 °C. The data points can be well reproduced theoretically (see full lines in Fig. 3) from the balance of atomic concentrations assuming the following crystallization reaction FeCuNbSiB --* Vc~a-Fe I ySiy +(1 - vcr)(Fe ~__,Nb,)2B
(1)
This indicates that the nucleation and growth of the b.c.c, grains proceeds until the residual amorphous matrix is enriched with boron such that its composition is close to stoichiometric (Fe~ ,Nb,)eB. The crystalline fraction is thus mainly determined by the boron
G. Herzer /
878
Magneticfield induced anisotropy "22
~,
z.~ 18.5 at%
-2O
0.9-
0.8
200"
o~ -18 ~ n
[]
150
u
~ 0.7~ .
-16 ~
<
6 14 -E
.~ I00
z = 23.5 at%
~0.6" 5(? 0.5
4
6
8
1()
1'2
1~4
16
10
Boron content, x, in at%
Fig. 3. Fraction v~r and
silicon content of the b.c.c. FeSi grains of nanocrystaltine Fe73.sCulNb3Si22.5_xBx (annealed for 1 h at 540 °C) as a function of the boron content. The experimental data points were determined from thermomagnetic analysis (cf. refs. 2 and 4); the full lines are calculated from the balance of atomic concentrations.
content, while niobium seems to stabilize the amorphous matrix in order to prevent the formation of borides. Since there is practically no induced anisotropy contribution from the residual amorphous matrix, the anisotropy induced locally in the FeSi grains, i.e. Ku(FeSi), is given by Ku(FeSi) = Ku/vcr
(2)
where K u is the experimentally determined anisotropy of the nanocrystalline material. Figure 4 shows Ku(FeSi) vs. the silicon content of the a-FeSi grains. This representation gives a considerably clearer view of the alloying effect. Thus, the induced anisotropy in nanocrystalline FeTCu-Nb-Si-B alloys is primarily determined by the silicon content and the fraction of b.c.c, grains. These structural parameters themselves depend mainly on the total boron and silicon content while, within certain limits, the copper and niobium content seems to be less significant. The latter result is indicated by the insensitivity of K u to detailed copper and niobium concentrations. The dependence of Ku/Vc, on the silicon content in the b.c.c, grains is comparable with that observed for conventional a-FeSi single crystals [9, 10]. For example, K u is reported [9] to show a maximum at about 10 at.% Si and then to decrease rapidly towards values near zero for 25 at.% Si (Fe3Si). The magnitudes of the present data are situated in between the reported values [9, 10]. A more quantitative comparison, however, is difficult since the literature data [9, 10] reveal a relatively large scatter, owing mostly to different crystal
8
Fe-Cul Nb3 Si (z-x) B-x annealed lh at 540,(3 in a magnetic field 10
12
14
"~+ ~
16
18
_ ~
=
20
22
24
Si-content of the ct-FeSi grains in at*/.
Fig. 4. Field induced anisotropy K u normalized to the crystalline fraction Vcr as a function of the silicon content in the b.c.c. FeSi grains of nanocr:ystalline F e 9 6 _ zCu 1Nb3Si(z- x/Bx•
orientations and measuring problems. In particular, the relatively large magnetocrystalline anisotropy K~ (several thousand joules per cubic metre [10]) makes it difficult to separate experimentally the field induced anisotropy in FeSi single crystals. There is no such problem in the present case since KI is averaged out by exchange interaction to values below a few joules per cubic metre [3]. The formation of the field induced anisotropy in a-FeSi has been proposed to arise from the directional ordering of the nearest and next-nearest silicon-atom pairs [9]. Thus, in terms of N&l's theory [11], the decrease in K u can be understood from the increasing long-range order of the silicon atoms (Fe3Si superlattice structure) with increasing silicon content since the available lattice sites for directional ordering decrease simultaneously. The induced anisotropy in a-FeSi is also reported to decrease again for low silicon contents [9]. The behaviour of nanocrystalline Fe-Cu-Nb-Si-B alloys with low silicon contents is not yet quite clear, because it was difficult to produce a homogeneous nanocrystalline structure for low silicon contents. The reason is that the corresponding alloys simultaneously reveal a high boron content. This is necessary for glass formation but at the same time favours the formation of borides on crystallization. However, a homogeneous a-Fe grain structure can be obtained in the FeZrCuB system [12]. Preliminary investigations of nanocrystalline F e 8 6 C u l Z r 6 B 7 yield a field induced anisotropy of K J v c , ~ 50 J m -3, indicating that Ku indeed seems to decrease again for low silicon contents.
G. Herzer
/
Magnetic field induced anisotropy
879
5. Conclusions
Acknowledgment
Magnetic field annealing of nanocrystalline F e - C u - N b - S i - B alloys induces a uniaxial magnetic anisotropy parallel to the magnetic field applied during the heat treatment. When the magnetic field is applied during the transformation from the originally amorphous to the nanocrystalline state, the induced anisotropy energy K u reaches a maximum value which is relatively insensitive to the detailed annealing conditions and is thus characteristic of the individual alloy composition. Analysis of the experimental data leads to the following conclusions. (1) The anisotropy is primarily induced in the b.c.c. FeSi grains, probably by atomic pair ordering of the silicon atoms. A significant contribution from the residual amorphous matrix can be largely ruled out. (2) The magnitude of K, is determined by the fraction Ver and the silicon content of the b.c.c. FeSi grains. In particular, K~ significantly decreases with increasing silicon content of the b.c.c, grains, which can be attributed to the increasing long-range order (Fe~Si superlattice structure) of the silicon atoms. (3) In terms of the total alloy composition, the induced anisotropy mainly depends on the silicon and boron contents. This indicates that the crystalline fraction and composition themselves are mainly determined by these two elements. This is also supported by the balance of atomic concentrations which reproduces well the microstructural features.
This work has been financially supported by the Bundesministerium fiir Forschung und Technologie (BMFT).
References 1 Y. Yoshizawa, S. Oguma and K. Yamauchi, J. Appl. Phys., 64 (1988) 6044. 2 G. Herzer, IEEE Trans. Magn., 25(1989) 3327. 3 G. Herzer, IEEE Trans. Magn., 26(1990) 1397. 4 G. Herzer, Proc. Int. Symp. on 3d Transition-Semimetal Thin Films, 5-8 March, Sendai, 1991, Japan Society for the Promotion of Science 131 Committee (Thin Film), Sendai, 199l,p. 130. 5 Y. Yoshizawa and K. Yamauchi, IEEE Trans. Magn., 25 (1989) 3324. 6 Y. Yoshizawa and K. Yamauchi, IEEE Translation J. Magn., ,5(1990) 1070. 7 H. Fujimori, in F. E. Luborsky (ed.), Amorphous Metallic Alloys, Butterworths Monographs in Materials, London, 1983, Chapter 16 and references cited therein. 8 K. Hono, K. Hiraga, Q. Wang, A. Inoue and T. Sakurai, Acta Metall. Mater., 40 (1992) 2137. 9 K. Sixtus, Z. Angew. Phys., 14(1962) 241; 28(1970) 270. 10 H. Gengnagel and H. Wagner, Z. Angew Phys., 8 ( 1961 ) 174. 11 L. Ndel, J. Phys. Radium, 15 (1954) 225. 12 K. Suzuki, A. Makino, A. Inoue and T. Masumoto, J. Appl. Phys., 70(1991)6232.