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Influence of glass coating thickness and metallic core diameter on GMI effect of glass-coated Co68Fe4.5Si13.5B14 amorphous microwires
Journal of Magnetism and Magnetic Materials 323 (2011) 1712–1716
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of glass coating thickness and metallic core diameter on GMI effect of glass-coated Co68Fe4.5Si13.5B14 amorphous microwires Zhihao Zhang, Beiyu Li, Weilong Cui, Jianxin Xie n Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Xueyuan Road 30, 100083 Beijing, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 November 2010 Received in revised form 17 December 2010 Available online 15 February 2011
Influences of the size factors (glass coating thickness and metallic core diameter) of microwires on GMI effects of the glass-coated Co68Fe4.5Si13.5B14 amorphous microwires were investigated. The results indicated that the GMI effect of the microwires with the same glass coating thickness or the same metallic core diameter was initially increased to a peak and then decreased with an increase in the diameter or the thickness. The glass coating thickness and the metallic core diameter corresponding to the maximum GMI varied with metallic core diameter and glass thickness, respectively. The GMI effect of the microwires with the same geometric size varied remarkably under different cooling rates. Such effect was ascribed to the microstructural changes of the metallic core wire under different cooling rates. The influence of the glass coating thickness on the GMI effect of the microwire was attributed to the synthetical actions of crystallization enthalpy (degree of disorder) and the internal stress. & 2011 Elsevier B.V. All rights reserved.
Keywords: Glass-coated amorphous microwire GMI effect Glass coating thickness Metallic core diameter
1. Introduction Amorphous alloys possess a widespread application prospect in the magnetic sensors [1,2] because their Giant Magneto Impedance (GMI) effects have higher magnetic sensitivity than Giant Magneto Resistive (GMR) effects, and preferable temperature stability. At present, an increasing attention is being paid to the design, preparation and application of the materials with GMI effect. The Taylor–Ulitovsky method can be used to produce continuously amorphous microwires with the diameters about 2–30 mm. Both the capability of anticorrosion and high temperature resistance of the microwires can be enhanced due to its glass coat [3], which enables the microwires to work under harsh conditions. The geometric size (the microwires diameter, the ratio of glass thickness and metallic core diameter) of glass-coated amorphous microwires directly affects their internal stress [3,4], and the performance of the microwires is related to the special magnetic domain structure resulting from the internal stress. Therefore, the influences of size on the GMI of amorphous or nanocrystalline microwires have been extensively investigated [5–7]. Furthermore, revealing the influence of geometric size on the properties of glass-coated amorphous microwires is helpful to control their service performance accurately.
n
Corresponding author. Tel./fax: + 86 1062332254. E-mail address: [email protected] (J. Xie).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.02.001
In the present work, the influences of glass coating thickness and metallic core diameter on GMI effect of the glass-coated Co68Fe4.5Si13.5B14 amorphous microwires were investigated. The influences of the glass coating thickness of the microwires on the cooling rate and crystallization enthalpy were also discussed.
2. Experiment Co68Fe4.5Si13.5B14 glass-coated amorphous microwires with different dimensions were prepared by using Taylor–Ulitovsky technology and the precise control method of melting bath temperature, glass tube and drawing speed [8,9], as shown in Fig. 1(a). In Fig. 1(a), the metallic core diameters (from top to down) are 5, 6, 11 mm, and the corresponding glass cover thickness are 3, 7, 9 mm, respectively. The XRD spectrum of the microwires is shown in Fig. 1(b), which exhibits typical diffuse scattering peak. This implies that the metallic core wire is amorphous structure. GMI was measured by an Agilent 4294A impedance analyzer. The steady magnetic field was generated by the Helmholtz coil of 250 mm in diameter. The magnetic field intensity ranged from 0 to 160 Oe. Drive alternating current (5 mA) passed via the sample axial filaments during measurement. The current direction was parallel to the steady magnetic field direction. The samples of microwire were placed in the east–west direction. All the measurements were carried out at room temperature. The metallic core diameter and glass coating thickness of the microwires were 10–25 and 5–25 mm, respectively, and the total
Z. Zhang et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1712–1716
1713
400
glass
1MHz 11MHz 21MHz 31MHz 41MHz 47MHz 49MHz 51MHz 61MHz
350
metallic
300 (ΔZ/Z) (%)
250 200 150 100
d = 16 μm, t = 23 μm
50 0 -50 -50
50 µm
-30
10
-10
30
50
H (Oe) Fig. 2. GMI effect varied with magnetic field and frequency (d for metallic core diameter, t for glass coated thickness).
Intensity (a.u.)
from 1 to 61 MHz. It means that the characteristic frequency of the microwire is 49 MHz. In the present work, the following experimental data were all measured at the frequency of 49 MHz. As the curves of GMI vs magnetic field was symmetric at zero magnetic field, the GMI performance of the microwires in the positive magnetic field with scope of 0–50 Oe was discussed.
50
60
70 2θ (°)
80
90
100
Fig. 1. The macro-morphology of microwires by optical microscope (a) and the XRD spectrum (b).
length was 25 mm. Both ends of the glass coat of the sample with 5 mm in length were removed by mechanical cracking method, i.e., the actual measurement length was 15 mm. The GMI effect can be determined by impedance ratio DZ/Z [10]:
DZ=Z ¼
ZðHÞZðHmax Þ 100% ZðHmax Þ
ð1Þ
where Z(H) is the impedance corresponding to any magnetic field, Z(Hmax) is the impedance of the material when the maxim magnetic field is applied.
3. Experimental results and discussion 3.1. The characteristic frequency of the microwire Due to the difference both in composition and process during preparation of microwires, the characteristic frequencies of various microwires, i.e., the frequency corresponding to the maximum GMI effect, changed. For obtaining the characteristic frequency of the glass-coated Co68Fe4.5Si13.5B14 amorphous microwire, the impedances of the microwire were measured under scanning frequency of 1–61 MHz and magnetic field of 100 to +100 Oe, as shown in Fig. 2. When the magnetic field is more than 50 Oe or less than 50 Oe, the GMI effects varied very slightly. Therefore, the curves of GMI vs magnetic field with scope of 50 to 50 Oe were shown in Fig. 2. Fig. 2 indicates that the maximum GMI effect could be obtained at about 49 MHz when the scanning frequency ranged
3.2. The effect of metallic core diameter on GMI When the glass coat thickness (t) of the microwires remained unchanged, both the GMI effects and the magnetic field corresponding to the maximum GMI (GMImax) varied with the metallic core diameter (d), as shown in Fig. 3(a). For easy analysis, the curves of GMImax vs the diameter d under various thickness t are given in Fig. 3(b). GMImax initially increased to a peak and then decreased with an increase in the metallic core diameter (d). The maximum value of the GMI (GMImax) depended on the diameter of the metallic core (dp), such as t¼13 mm, dp ¼20 mm, GMImax ¼482%; t¼ 17 mm, dp ¼16 mm, GMImax ¼ 421%; t ¼20 mm, dp ¼ 22 mm, GMImax ¼481%. However, Qin et al. [7] indicated that the GMI effect of the Co70.3Fe3.7B10Si13Cr3 glass-coated amorphous microwires with glass cover thickness (t) of 3 mm and metallic core diameter (d) of 15–30 mm monotonously increased with the diameter; Chiriac et al. [11] reported that the GMI effect of the Co68.15Fe4.35Si12.5B15 glass-coated amorphous microwires with d of 5–29 mm and t of 11 mm also monotonously increased with d, which disagreed with the results of the present work. Chiriac et al. [12] proposed that GMI effect was proportional to the magnetic anisotropic constants Ky. Ky first increased and then decreased when the metallic core diameter increased. This implies that GMI effect should not be monotonously increased with the diameter. Furthermore, Di et al. [13] indicated that the magnetic properties of the Co-based amorphous microwires exhibited an inflection point as diameter increased. When the diameter increased, both the coercive force and axial remanence ratio of the microwires first decreased and then increased, but the radial remanence ratio firstly increased and then decreased. In conclusion, when the glass coating thickness remained unchanged, the GMI effect of the glass-coated Co68Fe4.5Si13.5B14 amorphous microwire exhibited an inflection point reasonably (as shown in Fig. 3). Before the inflection point, the GMI effect
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Z. Zhang et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1712–1716
475
d 275 175
[ΔZ/Z] max (%)
ΔZ/Z (%)
375
t = 13μm t = 17μm t = 20μm
550
11μm 13μm 16μm 18μm 27μm t = 17μm
75
450 350 250 150 50
-25
-50 -5
5
15
25 35 H (Oe)
45
55
10
5
15
25
20
30
35
d (μm)
Fig. 3. GMI effect on the metallic core diameter with the wires of the same glass cover thickness: (a) GMI effect varies with magnetic field and (b) GMI effect varied with metallic core diameter.
800 d = 15 μm
d = 20 μm
d = 16 μm
600
d = 20 μm
v (106 K • s-1)
[ΔZ/Z]max (%)
d = 16 μm
3.5
400
2.5
1.5 200
0.5 0
0 5
10
15
20
25
30
5
10
15 t (μm)
20
25
30
Fig. 4. The effect of glass cover thickness on GMI effect of the wires with the same metallic core diameter.
Fig. 5. The dependence of cooling rate on glass coated thickness.
increased with the metallic core diameter. After the point, the GMI effect decreased as the metallic core diameter was increased.
increments were decreased gradually. Such increases of the radial stress and circumferential stress led to the increase both in the stress anisotropy energy [15], which enhanced GMI effect of microwire. This implies that if only taking the role of internal stress into account, GMI effect increases with the glass coating thickness and the inflection point in Fig. 4 is impossible. Therefore, it is proposed that the inflection point is related to the change of cooling rate of the microwires due to the difference in their coating thickness. The average cooling rate of core wires with the same metallic core diameter (d) can be determined by the following equation [16]:
3.3. The effect of glass coating thickness on GMI Fig. 4 shows that the maximum GMI (GMImax) varied with the glass coating thickness (t), as the metallic core diameter (d) was 15, 16 and 20 mm, respectively. GMImax initially increased to a peak and then decreased with an increase in glass thickness, which was similar to those change with the metallic core diameter. Furthermore, when t changes, the maximum GMI ratio correspond with the glass cover thickness tmax is different for the microwires with different d. For example, when t¼15 mm, tmax ¼12 mm, GMImax ¼392%; d¼16 mm, tmax ¼23 mm, GMImax ¼411; d¼20 mm, tmax ¼15 mm, GMImax ¼ 734%. In the present work, a relatively large internal stress was induced in the metallic core microwire due to the special preparation. Larin et al. [14] indicated that the internal stress mainly consisted of quenching stress owing to the difference in temperatures of glass and metallic core as well as thermoelastic stress resulting from the difference in the expansion coefficients of glass and metallic core. The order of the thermoelastic stress was GPa, and the quenching stress was small enough to be neglected when the internal stress of the microwire was taken into account. The stress calculation formula provided by Larin [14] indicated that for the metallic core with constant diameter, both the radial compression stress and circumferential stress of the microwire increased with the glass coating thickness, and the stress
Vt ¼ aðT2 T k Þðd þtÞ=½rm d2 Cm þ rg Cg ðdt þt 2 Þ
ð2Þ
Based on Eq. (2), the relationship between the average cooling rate and the glass coating thickness of the microwires were obtained (as shown in Fig. 5). It indicates that for the microwires with a diameter of 16 mm, the average cooling rate of the core wires decreased from 3.176 106 to 1.466 106 K s 1 (reduced by 54%) when the thickness increased from 6 to 27 mm. In the case of the microwires with a diameter of 20 mm, the average cooling rate of the core wires decreased from 2.414 106 to 1.278 106 K s 1 (reduced by 47%). It is apparent that the thickness of the microwire is inversely proportional to the cooling rate when the diameter is constant. For analyzing the influence of cooling rate on the GMI effect of microwires, the GMI effect of the microwires with the same geometric size prepared under different cooling rates was
Z. Zhang et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1712–1716
measured as shown in Fig. 6. The XRD analysis showed that both of the two microwires in Fig. 6 exhibited amorphous structure. It is indicated that the GMI effect of the microwire prepared by water cooling was greater than that of the microwire by air cooling under the external magnetic field of 0–50 Oe. The maximum GMI effects of microwires with different cooling rates are listed in Table 1. It can be seen that the glass-coated amorphous microwires with the same diameter prepared by larger cooling rate exhibited greater GMI effect. Based on the above analysis, the internal stress of microwires depends primarily on the thermal elastic stress and the quenching stress is small enough to be ignored. As a result, the cooling conditions have little impact on the internal stress. From Fig. 6 and Table 1, the internal stress of the microwires with same geometric size, glass coating thickness and core material were almost same. The apparent difference in GMI effect of the
450 400 d = 19μm, t = 6μm
350
air cooling
(ΔZ/Z) (%)
300
water cooling
250 200 150 100 50 0 -5
5
15
25 H (Oe)
35
1715
microwires under different cooling rates was not caused by the difference in the internal stress. Fig. 7 shows the DSC results of the microwires with different glass coating thickness and cooling rates. It was found that both of the water-cooling and air-cooling microwires with the same core diameter but different glass coating thickness had the same point of crystallization peaks and similar curves, which indicated that the cooling rate did not change the crystallization kinetics of the amorphous microwires basically. However, the areas of the exothermic peak of the DSC curves were different. Fig. 7 shows that the area (the S region) of the microwire with smaller glass coating thickness under water-cooling was larger than that of the microwire with larger glass thickness under air-cooling. The physical meaning of exothermic peak area, as the concept of enthalpy in thermodynamics, is the heat released by amorphous alloy per unit mass during crystallization process [17]. The larger the area is, the greater the crystallization enthalpy is. This implies that atomic configuration of alloy with a larger exothermic peak area is of a higher degree of disorder, and vice versa [18]. Furthermore, the magnetic properties of the alloy are very sensitive to its atomic structure [19]. Therefore, the local atomic structure of the amorphous alloy can be varied by changing cooling rate, which further affects its magnetic properties. The above analysis shows that the cooling rate had a great impact on the GMI effect of microwires with the same geometric size. Such effect was mainly attributed to the changes of the microwires’ microstructure caused by different cooling rates. Internal stress of the core wires increased with the glass coating thickness, and the crystallization enthalpy of core wires decreased with a reduction in the cooling rate (i.e. lower degree of disorder). Based on the combination of the above two effects the peak of GMI effect firstly increases and then decreases with an increase in the glass thickness, as shown in Fig. 4.
55
45
4. Conclusions
Fig. 6. The effect of cooling rates on the GMI effect of microwires.
Table 1 The maximum impedance ratio of microwires measured in different cooling conditions. Size of microwires (mm)
t¼9 t ¼ 11 t¼6 t¼8
Air-cooled
Water-cooled
17 20 324 381
38 64 425 442
0.4
0.4 1
Tx
0.2
0.2
1 – water cooling 2 – air cooling d = 13μm, t = 9μm
1
Tx 2
0 -0.2 -0.4
mW/mg
mW/mg
d ¼ 13, d ¼ 13, d ¼ 19, d ¼ 23,
The maximum value of the GMI/GMImax(%)
(1) The GMI effect of the Co68Fe4.5Si13.5B14 microwires with the same glass coating thickness or metallic core diameter initially increased to a peak and then decreased with an increase in the diameter or the thickness. The glass coating thickness tmax and the metallic core diameter dp corresponding to the maximum GMI varied with d and t, respectively. When t was 13, 17 and 20 mm, dp was 20, 16 and 22 mm, respectively. When d was 13, 15, 20 mm, tmax was 20, 23 and 15 mm, respectively. (2) The GMI effect of the microwires with the same geometric size varied remarkably under different cooling rates. Such effect was mainly ascribed to the microstructural changes of the metallic core wire under different cooling rates.
S 1- d = 16μm, t = 6μm 2- d = 16μm, t = 23μm
400
450 500 T (°C)
2
-0.2
S
-0.4 heating rate: 10°C/min
heating rate: 10°C/min -0.6 350
0
550
600
-0.6 350
400
450
500 T (°C)
550
600
Fig. 7. The DSC curves of microwires with different glass coated thickness (a) and different cooling rates (b).
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Z. Zhang et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1712–1716
(3) Based on the synthetical actions of crystallization enthalpy (degree of disorder) and the internal stress, the peak of GMI effect firstly increases and then decreases with an increase in the glass coating thickness.
[3] [4] [5] [6] [7] [8]
Acknowledgements
[9] [10]
Authors acknowledge support from the Foundation of Key Laboratory of Advanced Materials and Manufacturing Technologies of Ministry of Education of China. We also thank Dr. Y.B. Jiang for discussion. References [1] L.V. Panina, D.P. Makhnovskiya, K. Mahri, J. Magn. Magn. Mater 272–276 (2004) 1452. [2] S. Yabukami, H. Mawatari, N. Horikoshi, Y. Murayama, T. Ozawa, K. Ishiyama, K.I. Arai, J. Magn. Magn. Mater 290–291 (2005) 1318.
[11] [12] [13] [14] [15] [16] [17] [18] [19]
H. Chiriac, T. Aovari, Prog. Mater. Sci. 40 (1996) 333. B. Tian, J.J. Jiang, P. Liang, Electron. Components Mater. 27 (2008) 36. Y.J. Di, B. Jia, J.J. Jiang., Acta Mater. Compos. Sin. 26 (2009) 116. F.X. Qin, H.X. Peng, M.H. Phan., Mater. Sci. Eng. B 167 (2010) 129. F.X. Qin, H.X. Peng, M.H. Phan., Sol. State Commun. 150 (2010) 114. C.D. Wang, Z.H. Zhang, J.X. Xie., J. Univ. Sci. Technol. Beijing 31 (2009) 1038. C.D. Wang, F.M. Wang, Z.H. Zhang, J.X. Xie., J. Univ. Sci. Technol. Beijing 31 (2009) 1436. S.N. Kane, A. Gupta, T. Kulik, L. Kraus, J. Magn. Magn. Mater. 254–255 (2003) 498. H. Chiriac, S. Corodeanu, M. Tibu, T.A. Ovari, IEEE Trans. Magn. 42 (2007) 2977. H. Chiriac, M. Tibu, V. Dobrea, J. Magn. Magn. Mater. 290–291 (2005) 1142. Y.J. Di, J.J. Jiang, H.H. He, J. Inorg. Mater. 22 (2007) 1187. A.V. Torcunov, S.A Baranov, V.S Larin, J. Magn. Magn. Mater. 196–197 (1999) 835. H. Chiriac, T.A. Ovari, A. Zhukov, J. Magn. Magn. Mater. 254–255 (2003) 469. V.S. Larin, A.V. Torcunov, A. Zhukov, J. Gonzalez, M. Vazquez, L. Panina, J. Magn. Magn. Mater. 249 (2002) 39. Y. Zhou, X. Fan, Analysis of Materials, China Machine Press, Beijing, 1999. Z.J. Yan, The Microstructure Evolution during Plastic Deformation, Shanghai, 2006. Z.Z. Yuan, L. Yao, S.L. Bao, Rare Metal Mater. Eng. 38 (2009) 999.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of glass coating thickness and metallic core diameter on GMI effect of glass-coated Co68Fe4.5Si13.5B14 amorphous microwires Zhihao Zhang, Beiyu Li, Weilong Cui, Jianxin Xie n Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Xueyuan Road 30, 100083 Beijing, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 November 2010 Received in revised form 17 December 2010 Available online 15 February 2011
Influences of the size factors (glass coating thickness and metallic core diameter) of microwires on GMI effects of the glass-coated Co68Fe4.5Si13.5B14 amorphous microwires were investigated. The results indicated that the GMI effect of the microwires with the same glass coating thickness or the same metallic core diameter was initially increased to a peak and then decreased with an increase in the diameter or the thickness. The glass coating thickness and the metallic core diameter corresponding to the maximum GMI varied with metallic core diameter and glass thickness, respectively. The GMI effect of the microwires with the same geometric size varied remarkably under different cooling rates. Such effect was ascribed to the microstructural changes of the metallic core wire under different cooling rates. The influence of the glass coating thickness on the GMI effect of the microwire was attributed to the synthetical actions of crystallization enthalpy (degree of disorder) and the internal stress. & 2011 Elsevier B.V. All rights reserved.
Keywords: Glass-coated amorphous microwire GMI effect Glass coating thickness Metallic core diameter
1. Introduction Amorphous alloys possess a widespread application prospect in the magnetic sensors [1,2] because their Giant Magneto Impedance (GMI) effects have higher magnetic sensitivity than Giant Magneto Resistive (GMR) effects, and preferable temperature stability. At present, an increasing attention is being paid to the design, preparation and application of the materials with GMI effect. The Taylor–Ulitovsky method can be used to produce continuously amorphous microwires with the diameters about 2–30 mm. Both the capability of anticorrosion and high temperature resistance of the microwires can be enhanced due to its glass coat [3], which enables the microwires to work under harsh conditions. The geometric size (the microwires diameter, the ratio of glass thickness and metallic core diameter) of glass-coated amorphous microwires directly affects their internal stress [3,4], and the performance of the microwires is related to the special magnetic domain structure resulting from the internal stress. Therefore, the influences of size on the GMI of amorphous or nanocrystalline microwires have been extensively investigated [5–7]. Furthermore, revealing the influence of geometric size on the properties of glass-coated amorphous microwires is helpful to control their service performance accurately.
n
Corresponding author. Tel./fax: + 86 1062332254. E-mail address: [email protected] (J. Xie).
0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.02.001
In the present work, the influences of glass coating thickness and metallic core diameter on GMI effect of the glass-coated Co68Fe4.5Si13.5B14 amorphous microwires were investigated. The influences of the glass coating thickness of the microwires on the cooling rate and crystallization enthalpy were also discussed.
2. Experiment Co68Fe4.5Si13.5B14 glass-coated amorphous microwires with different dimensions were prepared by using Taylor–Ulitovsky technology and the precise control method of melting bath temperature, glass tube and drawing speed [8,9], as shown in Fig. 1(a). In Fig. 1(a), the metallic core diameters (from top to down) are 5, 6, 11 mm, and the corresponding glass cover thickness are 3, 7, 9 mm, respectively. The XRD spectrum of the microwires is shown in Fig. 1(b), which exhibits typical diffuse scattering peak. This implies that the metallic core wire is amorphous structure. GMI was measured by an Agilent 4294A impedance analyzer. The steady magnetic field was generated by the Helmholtz coil of 250 mm in diameter. The magnetic field intensity ranged from 0 to 160 Oe. Drive alternating current (5 mA) passed via the sample axial filaments during measurement. The current direction was parallel to the steady magnetic field direction. The samples of microwire were placed in the east–west direction. All the measurements were carried out at room temperature. The metallic core diameter and glass coating thickness of the microwires were 10–25 and 5–25 mm, respectively, and the total
Z. Zhang et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1712–1716
1713
400
glass
1MHz 11MHz 21MHz 31MHz 41MHz 47MHz 49MHz 51MHz 61MHz
350
metallic
300 (ΔZ/Z) (%)
250 200 150 100
d = 16 μm, t = 23 μm
50 0 -50 -50
50 µm
-30
10
-10
30
50
H (Oe) Fig. 2. GMI effect varied with magnetic field and frequency (d for metallic core diameter, t for glass coated thickness).
Intensity (a.u.)
from 1 to 61 MHz. It means that the characteristic frequency of the microwire is 49 MHz. In the present work, the following experimental data were all measured at the frequency of 49 MHz. As the curves of GMI vs magnetic field was symmetric at zero magnetic field, the GMI performance of the microwires in the positive magnetic field with scope of 0–50 Oe was discussed.
50
60
70 2θ (°)
80
90
100
Fig. 1. The macro-morphology of microwires by optical microscope (a) and the XRD spectrum (b).
length was 25 mm. Both ends of the glass coat of the sample with 5 mm in length were removed by mechanical cracking method, i.e., the actual measurement length was 15 mm. The GMI effect can be determined by impedance ratio DZ/Z [10]:
DZ=Z ¼
ZðHÞZðHmax Þ 100% ZðHmax Þ
ð1Þ
where Z(H) is the impedance corresponding to any magnetic field, Z(Hmax) is the impedance of the material when the maxim magnetic field is applied.
3. Experimental results and discussion 3.1. The characteristic frequency of the microwire Due to the difference both in composition and process during preparation of microwires, the characteristic frequencies of various microwires, i.e., the frequency corresponding to the maximum GMI effect, changed. For obtaining the characteristic frequency of the glass-coated Co68Fe4.5Si13.5B14 amorphous microwire, the impedances of the microwire were measured under scanning frequency of 1–61 MHz and magnetic field of 100 to +100 Oe, as shown in Fig. 2. When the magnetic field is more than 50 Oe or less than 50 Oe, the GMI effects varied very slightly. Therefore, the curves of GMI vs magnetic field with scope of 50 to 50 Oe were shown in Fig. 2. Fig. 2 indicates that the maximum GMI effect could be obtained at about 49 MHz when the scanning frequency ranged
3.2. The effect of metallic core diameter on GMI When the glass coat thickness (t) of the microwires remained unchanged, both the GMI effects and the magnetic field corresponding to the maximum GMI (GMImax) varied with the metallic core diameter (d), as shown in Fig. 3(a). For easy analysis, the curves of GMImax vs the diameter d under various thickness t are given in Fig. 3(b). GMImax initially increased to a peak and then decreased with an increase in the metallic core diameter (d). The maximum value of the GMI (GMImax) depended on the diameter of the metallic core (dp), such as t¼13 mm, dp ¼20 mm, GMImax ¼482%; t¼ 17 mm, dp ¼16 mm, GMImax ¼ 421%; t ¼20 mm, dp ¼ 22 mm, GMImax ¼481%. However, Qin et al. [7] indicated that the GMI effect of the Co70.3Fe3.7B10Si13Cr3 glass-coated amorphous microwires with glass cover thickness (t) of 3 mm and metallic core diameter (d) of 15–30 mm monotonously increased with the diameter; Chiriac et al. [11] reported that the GMI effect of the Co68.15Fe4.35Si12.5B15 glass-coated amorphous microwires with d of 5–29 mm and t of 11 mm also monotonously increased with d, which disagreed with the results of the present work. Chiriac et al. [12] proposed that GMI effect was proportional to the magnetic anisotropic constants Ky. Ky first increased and then decreased when the metallic core diameter increased. This implies that GMI effect should not be monotonously increased with the diameter. Furthermore, Di et al. [13] indicated that the magnetic properties of the Co-based amorphous microwires exhibited an inflection point as diameter increased. When the diameter increased, both the coercive force and axial remanence ratio of the microwires first decreased and then increased, but the radial remanence ratio firstly increased and then decreased. In conclusion, when the glass coating thickness remained unchanged, the GMI effect of the glass-coated Co68Fe4.5Si13.5B14 amorphous microwire exhibited an inflection point reasonably (as shown in Fig. 3). Before the inflection point, the GMI effect
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Z. Zhang et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1712–1716
475
d 275 175
[ΔZ/Z] max (%)
ΔZ/Z (%)
375
t = 13μm t = 17μm t = 20μm
550
11μm 13μm 16μm 18μm 27μm t = 17μm
75
450 350 250 150 50
-25
-50 -5
5
15
25 35 H (Oe)
45
55
10
5
15
25
20
30
35
d (μm)
Fig. 3. GMI effect on the metallic core diameter with the wires of the same glass cover thickness: (a) GMI effect varies with magnetic field and (b) GMI effect varied with metallic core diameter.
800 d = 15 μm
d = 20 μm
d = 16 μm
600
d = 20 μm
v (106 K • s-1)
[ΔZ/Z]max (%)
d = 16 μm
3.5
400
2.5
1.5 200
0.5 0
0 5
10
15
20
25
30
5
10
15 t (μm)
20
25
30
Fig. 4. The effect of glass cover thickness on GMI effect of the wires with the same metallic core diameter.
Fig. 5. The dependence of cooling rate on glass coated thickness.
increased with the metallic core diameter. After the point, the GMI effect decreased as the metallic core diameter was increased.
increments were decreased gradually. Such increases of the radial stress and circumferential stress led to the increase both in the stress anisotropy energy [15], which enhanced GMI effect of microwire. This implies that if only taking the role of internal stress into account, GMI effect increases with the glass coating thickness and the inflection point in Fig. 4 is impossible. Therefore, it is proposed that the inflection point is related to the change of cooling rate of the microwires due to the difference in their coating thickness. The average cooling rate of core wires with the same metallic core diameter (d) can be determined by the following equation [16]:
3.3. The effect of glass coating thickness on GMI Fig. 4 shows that the maximum GMI (GMImax) varied with the glass coating thickness (t), as the metallic core diameter (d) was 15, 16 and 20 mm, respectively. GMImax initially increased to a peak and then decreased with an increase in glass thickness, which was similar to those change with the metallic core diameter. Furthermore, when t changes, the maximum GMI ratio correspond with the glass cover thickness tmax is different for the microwires with different d. For example, when t¼15 mm, tmax ¼12 mm, GMImax ¼392%; d¼16 mm, tmax ¼23 mm, GMImax ¼411; d¼20 mm, tmax ¼15 mm, GMImax ¼ 734%. In the present work, a relatively large internal stress was induced in the metallic core microwire due to the special preparation. Larin et al. [14] indicated that the internal stress mainly consisted of quenching stress owing to the difference in temperatures of glass and metallic core as well as thermoelastic stress resulting from the difference in the expansion coefficients of glass and metallic core. The order of the thermoelastic stress was GPa, and the quenching stress was small enough to be neglected when the internal stress of the microwire was taken into account. The stress calculation formula provided by Larin [14] indicated that for the metallic core with constant diameter, both the radial compression stress and circumferential stress of the microwire increased with the glass coating thickness, and the stress
Vt ¼ aðT2 T k Þðd þtÞ=½rm d2 Cm þ rg Cg ðdt þt 2 Þ
ð2Þ
Based on Eq. (2), the relationship between the average cooling rate and the glass coating thickness of the microwires were obtained (as shown in Fig. 5). It indicates that for the microwires with a diameter of 16 mm, the average cooling rate of the core wires decreased from 3.176 106 to 1.466 106 K s 1 (reduced by 54%) when the thickness increased from 6 to 27 mm. In the case of the microwires with a diameter of 20 mm, the average cooling rate of the core wires decreased from 2.414 106 to 1.278 106 K s 1 (reduced by 47%). It is apparent that the thickness of the microwire is inversely proportional to the cooling rate when the diameter is constant. For analyzing the influence of cooling rate on the GMI effect of microwires, the GMI effect of the microwires with the same geometric size prepared under different cooling rates was
Z. Zhang et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1712–1716
measured as shown in Fig. 6. The XRD analysis showed that both of the two microwires in Fig. 6 exhibited amorphous structure. It is indicated that the GMI effect of the microwire prepared by water cooling was greater than that of the microwire by air cooling under the external magnetic field of 0–50 Oe. The maximum GMI effects of microwires with different cooling rates are listed in Table 1. It can be seen that the glass-coated amorphous microwires with the same diameter prepared by larger cooling rate exhibited greater GMI effect. Based on the above analysis, the internal stress of microwires depends primarily on the thermal elastic stress and the quenching stress is small enough to be ignored. As a result, the cooling conditions have little impact on the internal stress. From Fig. 6 and Table 1, the internal stress of the microwires with same geometric size, glass coating thickness and core material were almost same. The apparent difference in GMI effect of the
450 400 d = 19μm, t = 6μm
350
air cooling
(ΔZ/Z) (%)
300
water cooling
250 200 150 100 50 0 -5
5
15
25 H (Oe)
35
1715
microwires under different cooling rates was not caused by the difference in the internal stress. Fig. 7 shows the DSC results of the microwires with different glass coating thickness and cooling rates. It was found that both of the water-cooling and air-cooling microwires with the same core diameter but different glass coating thickness had the same point of crystallization peaks and similar curves, which indicated that the cooling rate did not change the crystallization kinetics of the amorphous microwires basically. However, the areas of the exothermic peak of the DSC curves were different. Fig. 7 shows that the area (the S region) of the microwire with smaller glass coating thickness under water-cooling was larger than that of the microwire with larger glass thickness under air-cooling. The physical meaning of exothermic peak area, as the concept of enthalpy in thermodynamics, is the heat released by amorphous alloy per unit mass during crystallization process [17]. The larger the area is, the greater the crystallization enthalpy is. This implies that atomic configuration of alloy with a larger exothermic peak area is of a higher degree of disorder, and vice versa [18]. Furthermore, the magnetic properties of the alloy are very sensitive to its atomic structure [19]. Therefore, the local atomic structure of the amorphous alloy can be varied by changing cooling rate, which further affects its magnetic properties. The above analysis shows that the cooling rate had a great impact on the GMI effect of microwires with the same geometric size. Such effect was mainly attributed to the changes of the microwires’ microstructure caused by different cooling rates. Internal stress of the core wires increased with the glass coating thickness, and the crystallization enthalpy of core wires decreased with a reduction in the cooling rate (i.e. lower degree of disorder). Based on the combination of the above two effects the peak of GMI effect firstly increases and then decreases with an increase in the glass thickness, as shown in Fig. 4.
55
45
4. Conclusions
Fig. 6. The effect of cooling rates on the GMI effect of microwires.
Table 1 The maximum impedance ratio of microwires measured in different cooling conditions. Size of microwires (mm)
t¼9 t ¼ 11 t¼6 t¼8
Air-cooled
Water-cooled
17 20 324 381
38 64 425 442
0.4
0.4 1
Tx
0.2
0.2
1 – water cooling 2 – air cooling d = 13μm, t = 9μm
1
Tx 2
0 -0.2 -0.4
mW/mg
mW/mg
d ¼ 13, d ¼ 13, d ¼ 19, d ¼ 23,
The maximum value of the GMI/GMImax(%)
(1) The GMI effect of the Co68Fe4.5Si13.5B14 microwires with the same glass coating thickness or metallic core diameter initially increased to a peak and then decreased with an increase in the diameter or the thickness. The glass coating thickness tmax and the metallic core diameter dp corresponding to the maximum GMI varied with d and t, respectively. When t was 13, 17 and 20 mm, dp was 20, 16 and 22 mm, respectively. When d was 13, 15, 20 mm, tmax was 20, 23 and 15 mm, respectively. (2) The GMI effect of the microwires with the same geometric size varied remarkably under different cooling rates. Such effect was mainly ascribed to the microstructural changes of the metallic core wire under different cooling rates.
S 1- d = 16μm, t = 6μm 2- d = 16μm, t = 23μm
400
450 500 T (°C)
2
-0.2
S
-0.4 heating rate: 10°C/min
heating rate: 10°C/min -0.6 350
0
550
600
-0.6 350
400
450
500 T (°C)
550
600
Fig. 7. The DSC curves of microwires with different glass coated thickness (a) and different cooling rates (b).
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Z. Zhang et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 1712–1716
(3) Based on the synthetical actions of crystallization enthalpy (degree of disorder) and the internal stress, the peak of GMI effect firstly increases and then decreases with an increase in the glass coating thickness.
[3] [4] [5] [6] [7] [8]
Acknowledgements
[9] [10]
Authors acknowledge support from the Foundation of Key Laboratory of Advanced Materials and Manufacturing Technologies of Ministry of Education of China. We also thank Dr. Y.B. Jiang for discussion. References [1] L.V. Panina, D.P. Makhnovskiya, K. Mahri, J. Magn. Magn. Mater 272–276 (2004) 1452. [2] S. Yabukami, H. Mawatari, N. Horikoshi, Y. Murayama, T. Ozawa, K. Ishiyama, K.I. Arai, J. Magn. Magn. Mater 290–291 (2005) 1318.
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