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Effect of heat treatment on the magnetic aftereffect in Fe-based amorphous alloys
N ELSEVIER
Journal of Magnetism and Magnetic Materials 163 (1996) 327-330
~ H journalof magnetism and ~iJ magnetic JRrll materials
Effect of heat treatment on the magnetic aftereffect in Fe-based amorphous alloys Qixian Ba *, Jing Zhi, Guiyi Zeng, Guilan Liu Department of Materials Science and Engineering, Northeastern University, Shenyang 110006, China
Received 10 October 1995; revised 30 April 1996
Abstract The magnetic aftereffect has been measured in Fe-based amorphous alloys in both the as-quenched and annealed states. It has been found that aging at 150°C is unfavorable for the stability of Fe-based amorphous alloys. However, annealing above the Curie temperature leads to a remarkable suppression of the aftereffect. The effect of heat treatment on the magnetic aftereffect is discussed using current theory. Keywords: Fe-based amorphous alloy; Magnetic aftereffect; Heat treatment
1. Introduction The magnetic aftereffect is a major impediment in the application of amorphous soft magnetic alloys. It is therefore important to seek a method for suppressing the magnetic aftereffect in order to improve the stability of amorphous materials during application [1,21. The structure and magnetic properties of amorphous alloys depend on their chemical compositions and the preparation techniques, and also on subsequent heat treatment [3-5]. Thus, the stability of amorphous magnetic materials may be improved by heat treatment. Among the various amorphous alloys, Fe-based alloys have the largest magnetic aftereffect at room temperature [4]. It is of great practical significance to suppress or remove the magnetic aftereffect by heat treatment. In the present work, the
* Corresponding author.
magnetic properties and magnetic aftereffect of a series of FeSiBCu alloys were investigated in the as-quenched state and various heat-treated states in order to find a method for preparing Fe-based amorphous materials with excellent soft magnetic properties and with high magnetic stability. The physical origin of the magnetic aftereffect in amorphous alloys is also discussed. 2. Experimental Ribbons of (Feo.s0Sio.05Bo.15)10o_xCu x amorphous alloys, with x = 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2 and 2.5 at%, 10 m m wide and 40 I-Lm thick, were prepared by the melt-quenching technique. The crystallization temperature, Tx, was determined by differential scanning calorimetry (DSC) at a heating rate of 10°C/rain. The Curie temperature, Tc, was measured by means of the Hopkinson effect method. The ac initial permeability, /~o, was measured using the voltage-ampere method at a frequency of 50 Hz at
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S0304-8853(96)00341-1
328
Q. Ba et al. /Journal of Magnetism and Magnetic Materials 163 (1996) 327-330 104
room temperature. The magnetic aftereffect was measured in a magnetic field of 1 kHz at room temperature. The magnetic aftereffect, DA, is evaluated by the following expression: DA = A/~//~ =
/ ~ ( t l ) -- ~ ( t 2 )
(t2)
× 100%
@
(1)
10-~
/ ' /
w h e r e /z(t 1) and /z(t 2) are permeabilities measured
at 5 and 200 s after demagnetization, respectively. For the magnetic measurements, amorphous ribbons about 300 cm long were wound into toroidal samples with an average diameter of 1.8 cm. Two groups of samples of (Fe0.80Si0.osB0.1s)loo_xCu x ( x = 0-2.5) amorphous alloys were prepared. For one group, the ac initial permeability /z o and the magnetic aftereffect DA were first measured, followed by aging at 150°C for 32 h. Then /xo and DA in the aged samples were measured. The other group of samples were first annealed at Ta = 430°C (T~ < T~ < Tx) for 30 rain in a nitrogen atmosphere and furnace-cooled to room temperature, and /x 0 and DA were measured. These samples were then aged at 150°C for 32 h, and /x 0 and DA were again measured. The saturation magnetostriction As was measured by means of the small angle magnetization rotation (SAMR) method at room temperature for the asquenched samples and the samples annealed at various temperatures iva in the range 70-450°C for 2 h in a nitrogen atmosphere.
3. R e s u l t s a n d d i s c u s s i o n
The magnetic properties of amorphous alloys are very sensitive to heat treatment. Fig. 1 shows the dependence of the ac initial permeability on heat treatment in FeSiBCu amorphous alloys. The permeability /x 0 generally decreases after aging at low temperature, but improves notably after annealing above Tc. The improvement is more pronounced for alloys containing lower or higher copper contents, and improvements in /% of up to an order of magnitude can be attained. It is possible that the decrease in /z 0 arising from aging at low temperature results from an induced magnetic anisotropy, and that the enhancement in /% is due to the relaxation of inter-
q
~
102 0
1,0 x, at%Cu
2.0
Fig. 1. Effect of heat treatment on the ac initial permeability of (FeSiB)100_xCu x amorphous alloys. O as-quenched, • aged at 150°C for 32 h, ~ annealed at 430°C for 30 min.
nal stresses. For amorphous soft magnetic materials, the following relation holds [5]: ix o cz I 2 / ( a K ÷ /3As o" ) ,
(2)
where a and 13 are constants. It is considered that the saturation magnetization I s and magnetostriction As are hardly influenced by annealing [4]. The results for the FesoSisB15 alloy shown in Fig. 2 confirm that the value of As is independent of annealing temperature. Hence, the changes in both the magnetic anisotropy K and the internal stresses ~r due to heat treatment, have to be taken into account when studying the effects of annealing on /z 0. For amorphous materials, K originates mainly from the induced magnetic anisotropy. There is almost no change in the internal stresses after low-temperature aging at 150°C [3], but when annealing is performed
36
0 O
()
0
0
()
@
)
0
A 3O 0
200 Ta, "C
400
Fig. 2. Variation in the magnetostrictive coefficient of Feso Sis B 15 amorphous alloy with annealing temperature.
Q. Ba et al. / Journal of Magnetism and Magnetic Materials 163 (1996) 327-330 12
0"-' k
-I
0
1. o
2.0 x, atMCu
Fig. 3. Changes in the magnetic aftereffect of (FeSiB)100_xCu x amorphous alloys after aging at 150°C for 32 h. O as-quenched, • aged.
in the ferromagnetic state, the magnetic anisotropy induced by the local magnetization and internal stresses can cause an increase in K1, which will lead to a decrease in /x0. With annealing above Tc, the internal stresses can almost be removed, and there is no influence of the local magnetization because the paramagnetic state is involved. Thus, /x0 is greatly improved. But during slow cooling from above Tc to room temperature, a newly induced magnetic anisotropy emerges as the spontaneous magnetization occurs. The largest induced magnetic anisotropy has been observed in the alloy containing 1 at% Cu [6]. Thus, the effect of annealing on the magnetic permeability depends on the alloy composition, as shown in Fig. 1. Aging at low temperature not only deteriorates magnetic properties of the FeSiBCu alloy system, but is also unfavorable for their magnetic stability. Fig. 3 shows that in the (FeSiB)100_xCu x (x = 0-2.5) alloys the magnetic aftereffect A/z//x increases after aging at 150°C for 32 h, in comparison with that in the as-quenched state. It is likely that the locally induced magnetic anisotropy in these alloys enhances the instability of the structure, which contributes to the stabilization potential of domain walls [7,9]. Thus, the magnetic aftereffect increases. Fig. 4 shows the magnetic aftereffect in the amorphous alloys in three states: as-quenched, annealed, and annealed plus aged. It can be seen that in the alloys with copper contents higher than 0.4 at% the magnetic aftereffect is completely suppressed. The magnetic aftereffect in the alloys with lower copper
329
contents (x < 0.4) decreases remarkably after annealing above Tc, and a further drop occurs after subsequent aging at 150°C for 32 h. This result is of great practical significance, in that it indicates that for Fe-based amorphous alloys with poor magnetic stability, the magnetic aftereffect can be suppressed by annealing at high temperature above Tc followed by slow cooling. The magnetic properties are also improved. According to the theory proposed by Allia [4], the magnetic aftereffect in amorphous alloys originates from the magnetostrictive coupling between shear stress defects and the spontaneous magnetization, and it may be expressed by A tx/ Ix = A N T 1 2 2 --l~eff('I" ) , kBT IsHe
(3)
where A is a constant; N T is the number of effective defects at a given temperature T; k B is the Boltzmann constant; I s is the saturation magnetization; H e is the amplitude of the ac magnetic field applied in measuring the magnetic aftereffect; and (~.2) expresses the second moment of the shear stress. For Fe-based amorphous alloys, A~ff may be written as 8"rr 2 ~ 2 2 Aeff - - ~ - ( 1 + (rn)Cve) As ,
(4)
where CF~ is a constant, As is the saturation magnetostrictive coefficient, and ( m ) is the average value of the ordering parameter m i. ( m ) increases with increasing disorder. The effect of annealing on the
10
D.._I~/•\ O./tD~ ,o/
0
•~
1.
J
2.0 x,
at%Cu
Fig. 4. Comparison of the magnetic aftereffect of amorphous (FeSiB)100_xCu x alloys in different states. • as-quenched, O annealed, tD annealed plus aged.
330
Q. Ba et al. / Journal of Magnetism and Magnetic Materials 163 (1996) 327-330
magnetic aftereffect can be explained by means of the above theory. Since, to a great extent, I~ and As remain unchanged after annealing, and (~.2> is not influenced by structural relaxation [8], annealing effects are determined by the changes in N T and ( m ) after annealing. As pointed out by Allia, N T and ( m ) in the annealed state are 73% and 17% of those of the corresponding samples in the as-quenched state, respectively. Hence, the decrease in the magnetic aftereffect is caused b y both the decrease in structural defects and the increased ordering in the annealed alloys. However, this is not enough to explain all of the phenomena observed in the experiment. Considering Anderson's model of two-level systems, Kronmiiller proposed that after annealing the amplitude of the magnetic aftereffect decreases due to annealing of the total number of two-level systems [1,9]. Although according to this theory, the magnetic aftereffect should be affected by the annealing treatment, it is difficult to explain perfectly the nearly complete suppression of the magnetic aftereffect observed in this work on this basis alone. Further work on this aspect is in now progress in our laboratory.
4. Conclusions
(1) Annealing above the Curie temperature can greatly reduce or completely suppress the magnetic aftereffect in amorphous FeSiBCu alloys, and the magnetic properties are improved. (2) Low-temperature aging alone is unfavorable for the stability of FeSiBCu alloys, and should be avoided.
References [1] H. Kronmiiller, Philos. Mag. B 48 (1983) 127. [2] J. Degro, P. Vojtanik, J. Filipensky and P. Duhaj, J. Magn. Magn. Mater. 117 (1992) 251. [3] F. Alves and J.C. Rerron, J. Magn. Magn. Mater. 112 (1992) 337. [4] P. Allia and F. Vinai, Phys. Rev. B 33 (1986) 422. [5] Qixian Ba, Kaiyuan Ho, Liming Wang and Yujun Fu, J. Northeastern University of Technology 10 (1989) 269 (in Chinese). [6] Qixian Ba and Jing Zhi, J. Magn. Magn. Mater. 154 (1996) 245. [7] H. Kronmtiller, J. Magn. Magn. Mater. 41 (1984) 366. [8] T. Egami and K. Sroloviz, J. Phys. F 12 (1982) 2141. [9] H. Kronmiiller, Phys. Status Solidi (b) 127 (1985) 531.
Journal of Magnetism and Magnetic Materials 163 (1996) 327-330
~ H journalof magnetism and ~iJ magnetic JRrll materials
Effect of heat treatment on the magnetic aftereffect in Fe-based amorphous alloys Qixian Ba *, Jing Zhi, Guiyi Zeng, Guilan Liu Department of Materials Science and Engineering, Northeastern University, Shenyang 110006, China
Received 10 October 1995; revised 30 April 1996
Abstract The magnetic aftereffect has been measured in Fe-based amorphous alloys in both the as-quenched and annealed states. It has been found that aging at 150°C is unfavorable for the stability of Fe-based amorphous alloys. However, annealing above the Curie temperature leads to a remarkable suppression of the aftereffect. The effect of heat treatment on the magnetic aftereffect is discussed using current theory. Keywords: Fe-based amorphous alloy; Magnetic aftereffect; Heat treatment
1. Introduction The magnetic aftereffect is a major impediment in the application of amorphous soft magnetic alloys. It is therefore important to seek a method for suppressing the magnetic aftereffect in order to improve the stability of amorphous materials during application [1,21. The structure and magnetic properties of amorphous alloys depend on their chemical compositions and the preparation techniques, and also on subsequent heat treatment [3-5]. Thus, the stability of amorphous magnetic materials may be improved by heat treatment. Among the various amorphous alloys, Fe-based alloys have the largest magnetic aftereffect at room temperature [4]. It is of great practical significance to suppress or remove the magnetic aftereffect by heat treatment. In the present work, the
* Corresponding author.
magnetic properties and magnetic aftereffect of a series of FeSiBCu alloys were investigated in the as-quenched state and various heat-treated states in order to find a method for preparing Fe-based amorphous materials with excellent soft magnetic properties and with high magnetic stability. The physical origin of the magnetic aftereffect in amorphous alloys is also discussed. 2. Experimental Ribbons of (Feo.s0Sio.05Bo.15)10o_xCu x amorphous alloys, with x = 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2 and 2.5 at%, 10 m m wide and 40 I-Lm thick, were prepared by the melt-quenching technique. The crystallization temperature, Tx, was determined by differential scanning calorimetry (DSC) at a heating rate of 10°C/rain. The Curie temperature, Tc, was measured by means of the Hopkinson effect method. The ac initial permeability, /~o, was measured using the voltage-ampere method at a frequency of 50 Hz at
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pll S0304-8853(96)00341-1
328
Q. Ba et al. /Journal of Magnetism and Magnetic Materials 163 (1996) 327-330 104
room temperature. The magnetic aftereffect was measured in a magnetic field of 1 kHz at room temperature. The magnetic aftereffect, DA, is evaluated by the following expression: DA = A/~//~ =
/ ~ ( t l ) -- ~ ( t 2 )
(t2)
× 100%
@
(1)
10-~
/ ' /
w h e r e /z(t 1) and /z(t 2) are permeabilities measured
at 5 and 200 s after demagnetization, respectively. For the magnetic measurements, amorphous ribbons about 300 cm long were wound into toroidal samples with an average diameter of 1.8 cm. Two groups of samples of (Fe0.80Si0.osB0.1s)loo_xCu x ( x = 0-2.5) amorphous alloys were prepared. For one group, the ac initial permeability /z o and the magnetic aftereffect DA were first measured, followed by aging at 150°C for 32 h. Then /xo and DA in the aged samples were measured. The other group of samples were first annealed at Ta = 430°C (T~ < T~ < Tx) for 30 rain in a nitrogen atmosphere and furnace-cooled to room temperature, and /x 0 and DA were measured. These samples were then aged at 150°C for 32 h, and /x 0 and DA were again measured. The saturation magnetostriction As was measured by means of the small angle magnetization rotation (SAMR) method at room temperature for the asquenched samples and the samples annealed at various temperatures iva in the range 70-450°C for 2 h in a nitrogen atmosphere.
3. R e s u l t s a n d d i s c u s s i o n
The magnetic properties of amorphous alloys are very sensitive to heat treatment. Fig. 1 shows the dependence of the ac initial permeability on heat treatment in FeSiBCu amorphous alloys. The permeability /x 0 generally decreases after aging at low temperature, but improves notably after annealing above Tc. The improvement is more pronounced for alloys containing lower or higher copper contents, and improvements in /% of up to an order of magnitude can be attained. It is possible that the decrease in /z 0 arising from aging at low temperature results from an induced magnetic anisotropy, and that the enhancement in /% is due to the relaxation of inter-
q
~
102 0
1,0 x, at%Cu
2.0
Fig. 1. Effect of heat treatment on the ac initial permeability of (FeSiB)100_xCu x amorphous alloys. O as-quenched, • aged at 150°C for 32 h, ~ annealed at 430°C for 30 min.
nal stresses. For amorphous soft magnetic materials, the following relation holds [5]: ix o cz I 2 / ( a K ÷ /3As o" ) ,
(2)
where a and 13 are constants. It is considered that the saturation magnetization I s and magnetostriction As are hardly influenced by annealing [4]. The results for the FesoSisB15 alloy shown in Fig. 2 confirm that the value of As is independent of annealing temperature. Hence, the changes in both the magnetic anisotropy K and the internal stresses ~r due to heat treatment, have to be taken into account when studying the effects of annealing on /z 0. For amorphous materials, K originates mainly from the induced magnetic anisotropy. There is almost no change in the internal stresses after low-temperature aging at 150°C [3], but when annealing is performed
36
0 O
()
0
0
()
@
)
0
A 3O 0
200 Ta, "C
400
Fig. 2. Variation in the magnetostrictive coefficient of Feso Sis B 15 amorphous alloy with annealing temperature.
Q. Ba et al. / Journal of Magnetism and Magnetic Materials 163 (1996) 327-330 12
0"-' k
-I
0
1. o
2.0 x, atMCu
Fig. 3. Changes in the magnetic aftereffect of (FeSiB)100_xCu x amorphous alloys after aging at 150°C for 32 h. O as-quenched, • aged.
in the ferromagnetic state, the magnetic anisotropy induced by the local magnetization and internal stresses can cause an increase in K1, which will lead to a decrease in /x0. With annealing above Tc, the internal stresses can almost be removed, and there is no influence of the local magnetization because the paramagnetic state is involved. Thus, /x0 is greatly improved. But during slow cooling from above Tc to room temperature, a newly induced magnetic anisotropy emerges as the spontaneous magnetization occurs. The largest induced magnetic anisotropy has been observed in the alloy containing 1 at% Cu [6]. Thus, the effect of annealing on the magnetic permeability depends on the alloy composition, as shown in Fig. 1. Aging at low temperature not only deteriorates magnetic properties of the FeSiBCu alloy system, but is also unfavorable for their magnetic stability. Fig. 3 shows that in the (FeSiB)100_xCu x (x = 0-2.5) alloys the magnetic aftereffect A/z//x increases after aging at 150°C for 32 h, in comparison with that in the as-quenched state. It is likely that the locally induced magnetic anisotropy in these alloys enhances the instability of the structure, which contributes to the stabilization potential of domain walls [7,9]. Thus, the magnetic aftereffect increases. Fig. 4 shows the magnetic aftereffect in the amorphous alloys in three states: as-quenched, annealed, and annealed plus aged. It can be seen that in the alloys with copper contents higher than 0.4 at% the magnetic aftereffect is completely suppressed. The magnetic aftereffect in the alloys with lower copper
329
contents (x < 0.4) decreases remarkably after annealing above Tc, and a further drop occurs after subsequent aging at 150°C for 32 h. This result is of great practical significance, in that it indicates that for Fe-based amorphous alloys with poor magnetic stability, the magnetic aftereffect can be suppressed by annealing at high temperature above Tc followed by slow cooling. The magnetic properties are also improved. According to the theory proposed by Allia [4], the magnetic aftereffect in amorphous alloys originates from the magnetostrictive coupling between shear stress defects and the spontaneous magnetization, and it may be expressed by A tx/ Ix = A N T 1 2 2 --l~eff('I" ) , kBT IsHe
(3)
where A is a constant; N T is the number of effective defects at a given temperature T; k B is the Boltzmann constant; I s is the saturation magnetization; H e is the amplitude of the ac magnetic field applied in measuring the magnetic aftereffect; and (~.2) expresses the second moment of the shear stress. For Fe-based amorphous alloys, A~ff may be written as 8"rr 2 ~ 2 2 Aeff - - ~ - ( 1 + (rn)Cve) As ,
(4)
where CF~ is a constant, As is the saturation magnetostrictive coefficient, and ( m ) is the average value of the ordering parameter m i. ( m ) increases with increasing disorder. The effect of annealing on the
10
D.._I~/•\ O./tD~ ,o/
0
•~
1.
J
2.0 x,
at%Cu
Fig. 4. Comparison of the magnetic aftereffect of amorphous (FeSiB)100_xCu x alloys in different states. • as-quenched, O annealed, tD annealed plus aged.
330
Q. Ba et al. / Journal of Magnetism and Magnetic Materials 163 (1996) 327-330
magnetic aftereffect can be explained by means of the above theory. Since, to a great extent, I~ and As remain unchanged after annealing, and (~.2> is not influenced by structural relaxation [8], annealing effects are determined by the changes in N T and ( m ) after annealing. As pointed out by Allia, N T and ( m ) in the annealed state are 73% and 17% of those of the corresponding samples in the as-quenched state, respectively. Hence, the decrease in the magnetic aftereffect is caused b y both the decrease in structural defects and the increased ordering in the annealed alloys. However, this is not enough to explain all of the phenomena observed in the experiment. Considering Anderson's model of two-level systems, Kronmiiller proposed that after annealing the amplitude of the magnetic aftereffect decreases due to annealing of the total number of two-level systems [1,9]. Although according to this theory, the magnetic aftereffect should be affected by the annealing treatment, it is difficult to explain perfectly the nearly complete suppression of the magnetic aftereffect observed in this work on this basis alone. Further work on this aspect is in now progress in our laboratory.
4. Conclusions
(1) Annealing above the Curie temperature can greatly reduce or completely suppress the magnetic aftereffect in amorphous FeSiBCu alloys, and the magnetic properties are improved. (2) Low-temperature aging alone is unfavorable for the stability of FeSiBCu alloys, and should be avoided.
References [1] H. Kronmiiller, Philos. Mag. B 48 (1983) 127. [2] J. Degro, P. Vojtanik, J. Filipensky and P. Duhaj, J. Magn. Magn. Mater. 117 (1992) 251. [3] F. Alves and J.C. Rerron, J. Magn. Magn. Mater. 112 (1992) 337. [4] P. Allia and F. Vinai, Phys. Rev. B 33 (1986) 422. [5] Qixian Ba, Kaiyuan Ho, Liming Wang and Yujun Fu, J. Northeastern University of Technology 10 (1989) 269 (in Chinese). [6] Qixian Ba and Jing Zhi, J. Magn. Magn. Mater. 154 (1996) 245. [7] H. Kronmtiller, J. Magn. Magn. Mater. 41 (1984) 366. [8] T. Egami and K. Sroloviz, J. Phys. F 12 (1982) 2141. [9] H. Kronmiiller, Phys. Status Solidi (b) 127 (1985) 531.