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Temperature effect on the MI ratio of Co68.15Fe4.35Si12.5B15 amorphous wires
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 290–291 (2005) 1075–1077 www.elsevier.com/locate/jmmm
Temperature effect on the MI ratio of Co68.15Fe4.35Si12.5B15 amorphous wires O´. Monteroa,, D. Garcı´ aa, V. Raposoa, H. Chiriacb, J. I´n˜igueza a
Applied Physics Department, University of Salamanca, Plza. Merced s/n, 37008 Salamanca, Spain b National Institute of R&D for Technical Physics, 47 Mangeron Blvd., 700050 Iasi, Romania Available online 15 December 2004
Abstract Study of temperature effects on magneto impedance (MI) ratio in Co68.15Fe4.35Si12.5B15 amorphous wires is here studied. Amorphous wires of 130 mm in diameter were prepared by in-rotating-water quenching technique. During MI measurements as a function of temperature, a decrease of MI ratio has been observed as the number of temperature sweeps increased. These wires have been subjected to different temperature sweeps beginning in 80 K until 300 K and slowly increasing the range until 400 K in order to detect a critical temperature. Such critical temperature has been found close to 375 K. The decrease of MI ratio is due to thermal diffusion and a change of the energy of the amorphous state as a consequence of the fact that the amorphous state is a meta stable state. It is believed that, at that temperature, atom diffusion and stress annealing take place and the state moves to a low-energy state following a dynamics in which the time when the sample is subjected to diffusion is important. r 2004 Elsevier B.V. All rights reserved. PACS: 75.40.Gb; 75.50.Kj; 75.60.d Keywords: Amorphous wires; Magneto impedance; Temperature effects
1. Introduction The magneto impedance (MI) was discovered years ago in amorphous samples and has a great interest due to its possible application as magnetic sensors [1–3]. MI effect originates in the skin effect as a consequence of the changes in the penetration depth induced by the static external field through modification of the transverse permeability. The skin depth is defined as d ¼ ðr=pmf f Þ1=2 ; Corresponding author. Tel.: +34 923 29 45 00; fax: +34 923 29 45 84. E-mail address: [email protected] (O´. Montero).
(1)
where r is the resistivity of the material, mf the circular permeability and f the frequency of the AC current flowing through the wire. The impedance of the sample depends on the penetration depth according to [4] Z ¼ RDC ka
J 0 ðkaÞ ; J 1 ðkaÞ
(2)
where J are Bessel functions, k ¼ (1j)/d and RDC is the electrical resistance. When the frequency is high enough the skin effect plays an important role reducing the effective section of the sample and then increasing the impedance. So, the change of impedance is a consequence of the modification of the skin depth penetration with the external field.
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.457
ARTICLE IN PRESS O´. Montero et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1075–1077
Hysteresis loops for magnetic characterization have also been fulfilled.
The MI ratio can be defined as ZðHÞ ZðH max Þ 100: ZðH max Þ
(3)
In this work, it has been studied the influence of the temperature in the MI percentage. We have chosen the composition of the wire among the ones which give the highest percentage of MI due to its nearly zero magnetostriction constant [1]. The internal stresses distributions created during quenching coupled with magnetostriction generate a relative small circular anisotropy and a circular domain pattern which enhances the value of the circular permeability [5]. This aspect is observed in the plots of MI versus temperature where a maximum of MI is observed in temperatures related with a maximum in the circular permeability [6,7].
2. Experimental Co68.15Fe4.35Si12.5B15 wires with 130 mm in diameter have been prepared at the laboratory using the inrotating-water quenching technique, and samples taken for measurements were about 12 cm long. Real and imaginary parts of the impedance of the sample were obtained with a lock-in amplifier using a four-point technique in order to reduce voltage drops in cables. The lock-in provides in two points to the sample an AC current of 3 mA of amplitude and a frequency ranging from 500 Hz to 100 kHz, while the other two points are used to measure the impedance of the sample. The wire is placed inside a cryostat in order to study the temperature dependence. The magnetic field is achieved by a coil placed coaxially to the wire and around the area of the cryostat where the sample is. It is programmed to make a sweep in field from 194 to 194 Oe controlled by the lock-in. Meanwhile the sweep is applied, a couple of resistance and induction values are taken. This process is fulfilled at constant temperature inside a cryostat thanks to a temperature controller, which allows us to repeat it between the temperature of the cooling liquid, 77 K, and 450 K in steps of 5 K. Moreover, MI measurements in frequency have been achieved at room temperature proving that the effect of the skin penetration is very important at higher frequencies. The sample was placed inside a pair of Helmholtz coils controlled by a bipolar power supply which can make a field sweep from 180 to 180 Oe at room temperature. Magnetic shielding is achieved in order to isolate the system. Resistance and autoinduction of the sample were obtainded using a LCR bridge which provides a current of 1 mA and a frequency range from 100 kHz to 30 MHz using a four-points technique.
3. Results and discussion Three consecutive sweeps in temperature from 80 to 400 K have been made for a Co68.15Fe4.35Si12.5B15 wire as can be seen in Fig. 1. As must be noticed, the maximum of MI reported by Raposo et al. [7] coincides with the maximum of the circular permeability with temperature in the first measurement [6]. A clear reduction of the MI ratio as well as a displacement of the MI maximum in temperature is observed. This behaviour suggests that up to a certain temperature in the first sweep a slight increase of the temperature determines a reversible relaxation of internal stresses induced during rapid solidification, which determines a reduction of the circumferential magnetoelastic anisotropy and, consequently, an improvement of the soft magnetic properties of the wire without changing its specific circumferential magnetic structure [1]. 400 350
1st sweep 2nd sweep 3rd sweep
300 MI (%)
MIð%Þ ¼
250 200 150 100 50 0 80
130
180 230 280 Temperature (K)
330
380
Fig. 1. MI versus Temperature for three consecutive temperature sweeps.
0.8
0.4
µ0 M (T)
1076
0 -0.4
-0.2
0 -0.4
0.2
0.4 Tmax=325K Tmax=350K
-0.8 H (Oe)
Tmax=375K Tmax=400K
Fig. 2. Hysteresis loops after temperature processes.
ARTICLE IN PRESS O´. Montero et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1075–1077
1077
1700
MI (%)
1200 As-cast 80 K - 325K 80 K - 350 K 80 K - 375 K 80 K - 400 K
700
200 0
5
10
15 f (MHz)
20
25
30
Fig. 3. MI decrease with temperature.
Thus, the circumferential permeability increases due to the circumferential magnetisation process achieved through domain wall movements. But for temperatures higher than a critical temperature the activation of thermal processes of atom diffusion is expected and the relaxation becomes irreversible. Also stress annealing contributes to the atom diffusion effect activated by temperature and, altogether, make the MI maximums of the sucesive temperature sweeps decrease in ratio. The temperature observed for the first sweep peak is close to room temperature due to the fabrication processes. The rapid quenching technique allows the internal stresses and the atoms of the material to be quenched at nearly the low limit temperature for the quenching technique and this is the temperature of the water of the wheel used. Experiments suggest that the combination of stresses relaxation and the activation of atom diffusion contributes in the same temperature range to the MI maximum. This aspect can be revealed by after effect processes where, in the case of our Co68.15Fe4.35Si12.5B15 wire, the temperature of atom diffusion is close to the MI maximum one [7]. While the first temperature sweep is being fulfilled, stress annealing and atom diffusions begin to appear in the wire when passing the critical temperature. These processes last until the wire reaches again thermal equilibrium at room temperature before a new sweep in temperature is applied. It is a kind of slow quenching method which sets a new temperature for the MI maximum and a new temperature maximum of circular permeability. As can be seen in Fig. 1, this decrease in ratio and temperature is exponential and suggests that for temperatures higher than the one of the MI maximum there is no room for reversible relaxations in the wire but for thermal activated processes of atom diffusion. To be more precise, the temperature of this critical point is tried to be located by making temperature sweeps beginning in 80 K until 300 K and slowly increasing this limit in 25 K in sucesive measurements until 400 K. Hysteresis loops have been measured
between each slow increasing temperature sweep in order to determine changes in the magnetic properties of the wires. No significative changes were found as can be observed in Fig. 2. The most suitable way to conclude significative results were MI measurements ranging from 100 kHz to 30 MHz. By comparing the as-cast wire previously measured with the ones which had been temperature treated in the sucesive slow increasing temperature sweeps (Fig. 3), the activation temperature can be established to be close to 375 K. High MI ratio is achieved by minimizing contact resistances. From these facts it is clear that a relaxation of the magnetic properties of the wires appears after thermal treatments when passing relative low temperatures below melting temperature, showing a temperature limit for reversible relaxations and future applications based in these materials. If the temperature range of the MI measurements is increased, the MI curves are expected to reach faster the state shown in the last sweep in temperature, as well as a further decrease of the MI percentage at temperatures higher than 400 K due to the behaviour of the permeability [6]. References [1] L.V. Panina, K. Mohri, K. Bushida, M.D. Noda, J. Appl. Phys. 76 (1994) 6198. [2] R.S. Beach, A.E. Berkowitz, J. Appl. Phys. 76 (1994) 6209. [3] K. Morhi, K. Bushida, M. Noda, H. Yoshida, L.V. Panina, IEEE Trans. Magn. 31 (1995) 2455. [4] D.B. Popovic, Introductory Engineering Electromagnetics, Addison-Wesley, Philipines, 1971, p. 487. [5] D.-X. Chen, L. Pascual, F.J. Castan˜o, M. Vazquez, A. Hernando, IEEE Trans. Magn. 37 (2001) 994. [6] H. Chiriac, C.S. Marinescu, T.-A. O´va´ri, J. Magn. Magn. Mater. 196-197 (1999) 162. [7] V. Raposo, O. Montero, D. Garcı´ a, M. Zazo, J.I. In˜iguez, Proceedings of SMM16, Diisseldarf, Germany, September 9–12, 2003.
Journal of Magnetism and Magnetic Materials 290–291 (2005) 1075–1077 www.elsevier.com/locate/jmmm
Temperature effect on the MI ratio of Co68.15Fe4.35Si12.5B15 amorphous wires O´. Monteroa,, D. Garcı´ aa, V. Raposoa, H. Chiriacb, J. I´n˜igueza a
Applied Physics Department, University of Salamanca, Plza. Merced s/n, 37008 Salamanca, Spain b National Institute of R&D for Technical Physics, 47 Mangeron Blvd., 700050 Iasi, Romania Available online 15 December 2004
Abstract Study of temperature effects on magneto impedance (MI) ratio in Co68.15Fe4.35Si12.5B15 amorphous wires is here studied. Amorphous wires of 130 mm in diameter were prepared by in-rotating-water quenching technique. During MI measurements as a function of temperature, a decrease of MI ratio has been observed as the number of temperature sweeps increased. These wires have been subjected to different temperature sweeps beginning in 80 K until 300 K and slowly increasing the range until 400 K in order to detect a critical temperature. Such critical temperature has been found close to 375 K. The decrease of MI ratio is due to thermal diffusion and a change of the energy of the amorphous state as a consequence of the fact that the amorphous state is a meta stable state. It is believed that, at that temperature, atom diffusion and stress annealing take place and the state moves to a low-energy state following a dynamics in which the time when the sample is subjected to diffusion is important. r 2004 Elsevier B.V. All rights reserved. PACS: 75.40.Gb; 75.50.Kj; 75.60.d Keywords: Amorphous wires; Magneto impedance; Temperature effects
1. Introduction The magneto impedance (MI) was discovered years ago in amorphous samples and has a great interest due to its possible application as magnetic sensors [1–3]. MI effect originates in the skin effect as a consequence of the changes in the penetration depth induced by the static external field through modification of the transverse permeability. The skin depth is defined as d ¼ ðr=pmf f Þ1=2 ; Corresponding author. Tel.: +34 923 29 45 00; fax: +34 923 29 45 84. E-mail address: [email protected] (O´. Montero).
(1)
where r is the resistivity of the material, mf the circular permeability and f the frequency of the AC current flowing through the wire. The impedance of the sample depends on the penetration depth according to [4] Z ¼ RDC ka
J 0 ðkaÞ ; J 1 ðkaÞ
(2)
where J are Bessel functions, k ¼ (1j)/d and RDC is the electrical resistance. When the frequency is high enough the skin effect plays an important role reducing the effective section of the sample and then increasing the impedance. So, the change of impedance is a consequence of the modification of the skin depth penetration with the external field.
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.457
ARTICLE IN PRESS O´. Montero et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1075–1077
Hysteresis loops for magnetic characterization have also been fulfilled.
The MI ratio can be defined as ZðHÞ ZðH max Þ 100: ZðH max Þ
(3)
In this work, it has been studied the influence of the temperature in the MI percentage. We have chosen the composition of the wire among the ones which give the highest percentage of MI due to its nearly zero magnetostriction constant [1]. The internal stresses distributions created during quenching coupled with magnetostriction generate a relative small circular anisotropy and a circular domain pattern which enhances the value of the circular permeability [5]. This aspect is observed in the plots of MI versus temperature where a maximum of MI is observed in temperatures related with a maximum in the circular permeability [6,7].
2. Experimental Co68.15Fe4.35Si12.5B15 wires with 130 mm in diameter have been prepared at the laboratory using the inrotating-water quenching technique, and samples taken for measurements were about 12 cm long. Real and imaginary parts of the impedance of the sample were obtained with a lock-in amplifier using a four-point technique in order to reduce voltage drops in cables. The lock-in provides in two points to the sample an AC current of 3 mA of amplitude and a frequency ranging from 500 Hz to 100 kHz, while the other two points are used to measure the impedance of the sample. The wire is placed inside a cryostat in order to study the temperature dependence. The magnetic field is achieved by a coil placed coaxially to the wire and around the area of the cryostat where the sample is. It is programmed to make a sweep in field from 194 to 194 Oe controlled by the lock-in. Meanwhile the sweep is applied, a couple of resistance and induction values are taken. This process is fulfilled at constant temperature inside a cryostat thanks to a temperature controller, which allows us to repeat it between the temperature of the cooling liquid, 77 K, and 450 K in steps of 5 K. Moreover, MI measurements in frequency have been achieved at room temperature proving that the effect of the skin penetration is very important at higher frequencies. The sample was placed inside a pair of Helmholtz coils controlled by a bipolar power supply which can make a field sweep from 180 to 180 Oe at room temperature. Magnetic shielding is achieved in order to isolate the system. Resistance and autoinduction of the sample were obtainded using a LCR bridge which provides a current of 1 mA and a frequency range from 100 kHz to 30 MHz using a four-points technique.
3. Results and discussion Three consecutive sweeps in temperature from 80 to 400 K have been made for a Co68.15Fe4.35Si12.5B15 wire as can be seen in Fig. 1. As must be noticed, the maximum of MI reported by Raposo et al. [7] coincides with the maximum of the circular permeability with temperature in the first measurement [6]. A clear reduction of the MI ratio as well as a displacement of the MI maximum in temperature is observed. This behaviour suggests that up to a certain temperature in the first sweep a slight increase of the temperature determines a reversible relaxation of internal stresses induced during rapid solidification, which determines a reduction of the circumferential magnetoelastic anisotropy and, consequently, an improvement of the soft magnetic properties of the wire without changing its specific circumferential magnetic structure [1]. 400 350
1st sweep 2nd sweep 3rd sweep
300 MI (%)
MIð%Þ ¼
250 200 150 100 50 0 80
130
180 230 280 Temperature (K)
330
380
Fig. 1. MI versus Temperature for three consecutive temperature sweeps.
0.8
0.4
µ0 M (T)
1076
0 -0.4
-0.2
0 -0.4
0.2
0.4 Tmax=325K Tmax=350K
-0.8 H (Oe)
Tmax=375K Tmax=400K
Fig. 2. Hysteresis loops after temperature processes.
ARTICLE IN PRESS O´. Montero et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1075–1077
1077
1700
MI (%)
1200 As-cast 80 K - 325K 80 K - 350 K 80 K - 375 K 80 K - 400 K
700
200 0
5
10
15 f (MHz)
20
25
30
Fig. 3. MI decrease with temperature.
Thus, the circumferential permeability increases due to the circumferential magnetisation process achieved through domain wall movements. But for temperatures higher than a critical temperature the activation of thermal processes of atom diffusion is expected and the relaxation becomes irreversible. Also stress annealing contributes to the atom diffusion effect activated by temperature and, altogether, make the MI maximums of the sucesive temperature sweeps decrease in ratio. The temperature observed for the first sweep peak is close to room temperature due to the fabrication processes. The rapid quenching technique allows the internal stresses and the atoms of the material to be quenched at nearly the low limit temperature for the quenching technique and this is the temperature of the water of the wheel used. Experiments suggest that the combination of stresses relaxation and the activation of atom diffusion contributes in the same temperature range to the MI maximum. This aspect can be revealed by after effect processes where, in the case of our Co68.15Fe4.35Si12.5B15 wire, the temperature of atom diffusion is close to the MI maximum one [7]. While the first temperature sweep is being fulfilled, stress annealing and atom diffusions begin to appear in the wire when passing the critical temperature. These processes last until the wire reaches again thermal equilibrium at room temperature before a new sweep in temperature is applied. It is a kind of slow quenching method which sets a new temperature for the MI maximum and a new temperature maximum of circular permeability. As can be seen in Fig. 1, this decrease in ratio and temperature is exponential and suggests that for temperatures higher than the one of the MI maximum there is no room for reversible relaxations in the wire but for thermal activated processes of atom diffusion. To be more precise, the temperature of this critical point is tried to be located by making temperature sweeps beginning in 80 K until 300 K and slowly increasing this limit in 25 K in sucesive measurements until 400 K. Hysteresis loops have been measured
between each slow increasing temperature sweep in order to determine changes in the magnetic properties of the wires. No significative changes were found as can be observed in Fig. 2. The most suitable way to conclude significative results were MI measurements ranging from 100 kHz to 30 MHz. By comparing the as-cast wire previously measured with the ones which had been temperature treated in the sucesive slow increasing temperature sweeps (Fig. 3), the activation temperature can be established to be close to 375 K. High MI ratio is achieved by minimizing contact resistances. From these facts it is clear that a relaxation of the magnetic properties of the wires appears after thermal treatments when passing relative low temperatures below melting temperature, showing a temperature limit for reversible relaxations and future applications based in these materials. If the temperature range of the MI measurements is increased, the MI curves are expected to reach faster the state shown in the last sweep in temperature, as well as a further decrease of the MI percentage at temperatures higher than 400 K due to the behaviour of the permeability [6]. References [1] L.V. Panina, K. Mohri, K. Bushida, M.D. Noda, J. Appl. Phys. 76 (1994) 6198. [2] R.S. Beach, A.E. Berkowitz, J. Appl. Phys. 76 (1994) 6209. [3] K. Morhi, K. Bushida, M. Noda, H. Yoshida, L.V. Panina, IEEE Trans. Magn. 31 (1995) 2455. [4] D.B. Popovic, Introductory Engineering Electromagnetics, Addison-Wesley, Philipines, 1971, p. 487. [5] D.-X. Chen, L. Pascual, F.J. Castan˜o, M. Vazquez, A. Hernando, IEEE Trans. Magn. 37 (2001) 994. [6] H. Chiriac, C.S. Marinescu, T.-A. O´va´ri, J. Magn. Magn. Mater. 196-197 (1999) 162. [7] V. Raposo, O. Montero, D. Garcı´ a, M. Zazo, J.I. In˜iguez, Proceedings of SMM16, Diisseldarf, Germany, September 9–12, 2003.