- Email: [email protected]
Temperature dependence of the magneto-impedance effect
~4
Journalof magnetism
and
ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 162-163
magnetic materials
Temperature dependence of the magneto-impedance effect H. Chiriac*, C.S. Marinescu, T.-A. Ovfiri National Institute oj'R&D fi)r Technical Physics. 47 Mangeron Blvd., 6600 lasi 3, Romania
Abstract The temperature dependence of the magneto-impedance effect in CoFeSiB amorphous wires is investigated in the temperature range between the room and Curie temperatures. A maximum value of 0.65 was found for the reduced impedance change ratio, its frequency dependence displaying two peaks, that reflect the changes of the magnetization dynamics in the circumferential direction as the frequency increases. 4 1999 Elsevier Science B.V. All rights reserved.
Kevwords. Magneto-impedance effect: Dynamic magnetization processes; Amorphous wires
The magneto-impedance (MI) effect has been extensively studied in the last few years, and the largest MI response was reported in Co-based amorphous wires with nearly zero magnetostriction [1]. The aim of this paper is to investigate the temperature dependence of the MI effect in in-rotating-water quenched C068.15Fe4.asSiI2.sBa5 amorphous wires with 120 gm in diameter. The MI response of such wires was studied in temperature range between the room and Curie temperatures. In order to analyze the magnitude of the M! effect in the above-mentioned materials, we used the reduced impedance change ratio, A Z / Z , defined as A Z / Z = [Zm,x -- Z ( H = 750A m - 1)]/Z ..... 750A m 1 being the maximum value of the applied DC field H, and Zmax the maximum value of the impedance, that corresponds to the anisotropy field Hk of the wires. MI measurements were performed at amplitudes of the AC driving current IAC from 5 to 15 mA, the current frequency ranging between 10 kHz and 10 MHz. These measurements were correlated with circumferential permeability measurements performed at low frequencies, its temperature dependence being also investigated. Fig. 1 illustrates the frequency dependence of A Z / Z , with the temperature as a parameter, for an amplitude of the AC driving current of 15 mA. The curves that corre-
* Corresponding author. Tel.: + 40-32-130680: fax: + 40-32231132: e-mail: [email protected].
spond to temperatures below 150'C, display a single peak in the low-frequency range, at about 70 kHz. A Z / Z first increases with the temperature, reaching a maximum value of 0.65 at 50~C, and then it decreases, being almost null near the Curie point. This behaviour is well reflected by the temperature dependence of the circumferential permeability, Ito, which is shown in Fig. 2. Thus, a slight increase of the temperature determines a reversible relaxation of internal stresses induced during rapid solidification, that determines a reduction of the circumferential magnetoelastic anisotropy, and consequently an improvement of the wire's soft magnetic properties without changing its specific circumferential magnetic structure, which is the most favourable for a sensitive MI response [1]. Consequently, the circumferential permeability increases, and the magnetic penetration depth, 6,,, decreases, according to 6m = (p/zr.f. iLo)1/2, where p is the resistivity and f the frequency, and the M! increases. An increase of the temperature over 50'C~ determines further stress relief, but the domain structure simultaneously suffers deterioration. This fact determines a decrease of the circumferential permeability associated to the circumferential magnetization process achieved through domain wall movements, also reflected in a decrease of A Z / Z , since 6m increases, and consequently the MI decreases. For temperatures over 150C, the curves that describe the frequency dependence of A Z / Z , display two peaks, the first one at ~ 70kHz, and a second one at
0304-8853/99/$ - see front matter ~ 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 7 0 2 - 1
H. Chiriac et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 162-163 0.8
4~- 20°C ]
0.6
--41--- 50 oC -~ 100oC 150~C / ~ - - 200 ~c " ~ 250eC --Y¢---300 °C 350eC
/
.,11
~
0.4q
--380°C
-.-
0.C
10
100
1000
10000
Frequency (kHz) Fig. 1. Frequency dependence of the reduced impedance change ratio of C068.15Fe4.35Si12.sB15 amorphous wires, with the temperature as a parameter; IAC = 15 mA.
70000
6OOO0
~
40000
'4
311000
~
20000
r)
10000
0
0
' SO
100
150
200
250
300
350
400
450
Temperature (~C) Fig. 2. Temperature dependence of the circumferential permeability at low frequencies for Co6s.15Fe,,.35Silz.sB15 amorphous wires.
163
ferential permeability, associated to the circumferential magnetization process achieved through spin rotations, since at these high frequencies, only the rotational circumferential permeability determines the magnitude of the MI effect, through the same mechanism of the magnetic penetration depth [2]. The appearance of the second peak over a certain value of the temperature is related to the reduction of the circumferential permeability associated to the domain wall movements over that specific temperature (see Fig. 2), that also indicates a change in the dynamics of the magnetization from domain wall movements to spin rotations. Thus, the second peak appears due to the increase of the rotational circumferential permeability at these higher frequencies, since the axial DC field helps the rotation of the circumferentially disposed magnetic moments [3]. Consequently, the corresponding magnetic penetration depth decreases, and the MI response gains larger sensitivity. Thus, the investigation of the temperature dependence of the MI effect brings information about the magnetization dynamics in the circumferential direction. Summarizing, the MI response of CoFeSiB amorphous wires increases at temperatures slightly larger than the room temperature, and then it decreases, vanishing at temperatures close to the Curie point. The shape of the curves representing the frequency dependence of A Z / Z depends on temperature, displaying a single peak at temperatures below 150°C, and two peaks above this value. The first peak is related to the increase of the circumferential permeability associated to the circumferential magnetization process achieved through domain wall movements, while the second one to the increase of the circumferential permeability associated to the circumferential magnetization process achieved through spin rotations. The investigation of the temperature dependence of the MI effect allows an analysis of the changes that occur in the dynamics of magnetization in the circumferential direction, and reveal the importance of spin rotations in a sensitive MI response at high frequencies.
References
frequencies over 500 kHz. The first peak also reflects the increase of the circumferential permeability associated to the circumferential magnetization process achieved through domain wall movements, due to the circumferential field produced by the AC driving current. The second peak reflects the increase of the rotational circum-
[1] L.V. Panina, K. Mohri, K. Bushida, M. Noda, J. Appl. Phys. 76 (1994) 6198. [2] H. Chiriac, T.-A. Ovfiri, C.S. Marinescu, IEEE Trans. Magn. 33 (1997) 3352. [3] H. Chiriac, F. Vinai, T.-A. 6v~ri, C.S. Marinescu, F. Barariu, P. Tiberto, Mater. Sci. Eng. A 226-228 (1997) 646.
Journalof magnetism
and
ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 162-163
magnetic materials
Temperature dependence of the magneto-impedance effect H. Chiriac*, C.S. Marinescu, T.-A. Ovfiri National Institute oj'R&D fi)r Technical Physics. 47 Mangeron Blvd., 6600 lasi 3, Romania
Abstract The temperature dependence of the magneto-impedance effect in CoFeSiB amorphous wires is investigated in the temperature range between the room and Curie temperatures. A maximum value of 0.65 was found for the reduced impedance change ratio, its frequency dependence displaying two peaks, that reflect the changes of the magnetization dynamics in the circumferential direction as the frequency increases. 4 1999 Elsevier Science B.V. All rights reserved.
Kevwords. Magneto-impedance effect: Dynamic magnetization processes; Amorphous wires
The magneto-impedance (MI) effect has been extensively studied in the last few years, and the largest MI response was reported in Co-based amorphous wires with nearly zero magnetostriction [1]. The aim of this paper is to investigate the temperature dependence of the MI effect in in-rotating-water quenched C068.15Fe4.asSiI2.sBa5 amorphous wires with 120 gm in diameter. The MI response of such wires was studied in temperature range between the room and Curie temperatures. In order to analyze the magnitude of the M! effect in the above-mentioned materials, we used the reduced impedance change ratio, A Z / Z , defined as A Z / Z = [Zm,x -- Z ( H = 750A m - 1)]/Z ..... 750A m 1 being the maximum value of the applied DC field H, and Zmax the maximum value of the impedance, that corresponds to the anisotropy field Hk of the wires. MI measurements were performed at amplitudes of the AC driving current IAC from 5 to 15 mA, the current frequency ranging between 10 kHz and 10 MHz. These measurements were correlated with circumferential permeability measurements performed at low frequencies, its temperature dependence being also investigated. Fig. 1 illustrates the frequency dependence of A Z / Z , with the temperature as a parameter, for an amplitude of the AC driving current of 15 mA. The curves that corre-
* Corresponding author. Tel.: + 40-32-130680: fax: + 40-32231132: e-mail: [email protected].
spond to temperatures below 150'C, display a single peak in the low-frequency range, at about 70 kHz. A Z / Z first increases with the temperature, reaching a maximum value of 0.65 at 50~C, and then it decreases, being almost null near the Curie point. This behaviour is well reflected by the temperature dependence of the circumferential permeability, Ito, which is shown in Fig. 2. Thus, a slight increase of the temperature determines a reversible relaxation of internal stresses induced during rapid solidification, that determines a reduction of the circumferential magnetoelastic anisotropy, and consequently an improvement of the wire's soft magnetic properties without changing its specific circumferential magnetic structure, which is the most favourable for a sensitive MI response [1]. Consequently, the circumferential permeability increases, and the magnetic penetration depth, 6,,, decreases, according to 6m = (p/zr.f. iLo)1/2, where p is the resistivity and f the frequency, and the M! increases. An increase of the temperature over 50'C~ determines further stress relief, but the domain structure simultaneously suffers deterioration. This fact determines a decrease of the circumferential permeability associated to the circumferential magnetization process achieved through domain wall movements, also reflected in a decrease of A Z / Z , since 6m increases, and consequently the MI decreases. For temperatures over 150C, the curves that describe the frequency dependence of A Z / Z , display two peaks, the first one at ~ 70kHz, and a second one at
0304-8853/99/$ - see front matter ~ 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 7 0 2 - 1
H. Chiriac et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 162-163 0.8
4~- 20°C ]
0.6
--41--- 50 oC -~ 100oC 150~C / ~ - - 200 ~c " ~ 250eC --Y¢---300 °C 350eC
/
.,11
~
0.4q
--380°C
-.-
0.C
10
100
1000
10000
Frequency (kHz) Fig. 1. Frequency dependence of the reduced impedance change ratio of C068.15Fe4.35Si12.sB15 amorphous wires, with the temperature as a parameter; IAC = 15 mA.
70000
6OOO0
~
40000
'4
311000
~
20000
r)
10000
0
0
' SO
100
150
200
250
300
350
400
450
Temperature (~C) Fig. 2. Temperature dependence of the circumferential permeability at low frequencies for Co6s.15Fe,,.35Silz.sB15 amorphous wires.
163
ferential permeability, associated to the circumferential magnetization process achieved through spin rotations, since at these high frequencies, only the rotational circumferential permeability determines the magnitude of the MI effect, through the same mechanism of the magnetic penetration depth [2]. The appearance of the second peak over a certain value of the temperature is related to the reduction of the circumferential permeability associated to the domain wall movements over that specific temperature (see Fig. 2), that also indicates a change in the dynamics of the magnetization from domain wall movements to spin rotations. Thus, the second peak appears due to the increase of the rotational circumferential permeability at these higher frequencies, since the axial DC field helps the rotation of the circumferentially disposed magnetic moments [3]. Consequently, the corresponding magnetic penetration depth decreases, and the MI response gains larger sensitivity. Thus, the investigation of the temperature dependence of the MI effect brings information about the magnetization dynamics in the circumferential direction. Summarizing, the MI response of CoFeSiB amorphous wires increases at temperatures slightly larger than the room temperature, and then it decreases, vanishing at temperatures close to the Curie point. The shape of the curves representing the frequency dependence of A Z / Z depends on temperature, displaying a single peak at temperatures below 150°C, and two peaks above this value. The first peak is related to the increase of the circumferential permeability associated to the circumferential magnetization process achieved through domain wall movements, while the second one to the increase of the circumferential permeability associated to the circumferential magnetization process achieved through spin rotations. The investigation of the temperature dependence of the MI effect allows an analysis of the changes that occur in the dynamics of magnetization in the circumferential direction, and reveal the importance of spin rotations in a sensitive MI response at high frequencies.
References
frequencies over 500 kHz. The first peak also reflects the increase of the circumferential permeability associated to the circumferential magnetization process achieved through domain wall movements, due to the circumferential field produced by the AC driving current. The second peak reflects the increase of the rotational circum-
[1] L.V. Panina, K. Mohri, K. Bushida, M. Noda, J. Appl. Phys. 76 (1994) 6198. [2] H. Chiriac, T.-A. Ovfiri, C.S. Marinescu, IEEE Trans. Magn. 33 (1997) 3352. [3] H. Chiriac, F. Vinai, T.-A. 6v~ri, C.S. Marinescu, F. Barariu, P. Tiberto, Mater. Sci. Eng. A 226-228 (1997) 646.