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Domain wall motion influence on the magnetoimpedance effect
ELSEVIER
~
Journalof
,~
and
magnetism magnetic materials
Journal of Magnetism and Magnetic Materials 196-197 (1999) 169-170
Domain wall motion influence on the magnetoimpedance effect J. Guti6rrez a'*, D. Atkinson b, P.T. Squire b, J.M.
Barandiar/m
a
aDepartamento de Electricidad y Electr6nica, Universidad del Pais Vaseo/EHU, Apartado 644, E-48080 Bilbao, Spain bDepartment of Physics, University o f Bath, Bath BA2 7A Y, UK
Abstract
The influence of domain wall movements in the giant magnetoimpedance measured in low magnetostrictive alloys has been studied. Samples were field annealed under a static field that was applied making an angle 0, varied from 90 ° to 0 °, with the longitudinal ribbon axis. Measurements were performed up to 10 MHz. Obtained results and theoretical predictions are compared and discussed. ~) 1999 Elsevier Science B.V. All rights reserved. Keywords: Magnetoimpedance; Domain wall motion; Soft ferromagnetic alloys
One of the most interesting phenomena observed in amorphous ribbons of soft ferromagnetic alloys is the frequency dependence of the giant magnetoimpedance (GMI) presented by low magnetostrictive alloys, usually Co-rich, CoFeSiB compositions [1]. In order to understand the G M I effect, it is necessary to consider the circumferential permeability, /~, of the sample in the form of wire. In ribbons, it corresponds to the transverse susceptibility, but the same symbol will be used throughout. Its square root is proportional to the impedance through the expression [2]. • pL ( 1 - i)L Z = (l -- t) ~-[~ 2lc x/zltP¢°#*'
where 6 is the skin depth penetration, p is the resistivity of the sample, and l and L are the ribbon width and length, respectively. This transverse susceptibility depends on the initial domain structure [3,4] and the magnetization process in the sample. Following previous phenomenological models [5,6], #6 behaviour has been already modelled [7,83 as a function of the anisotropy angle, 0, respect to the ribbon axis. This model predicts that for purely perpendicular anisotropy (0 = 90 °) the
* Corresponding author. Tel.: + 34-94-601-2553; fax: + 3494-485-81-39; e-mail: [email protected].
transverse susceptibility is zero only if there is no domain wall movement in the sample. However, large domain wall motion can lead to significant deviations from this zero value. In this work we have tested these features using the low-magnetostriction alloy Co66Fe4MozSi16B12 (Vitrovac ® 6025, 2~ < 0.2 ppm). Several strips of this sample were cut and after thermal stress relaxation at 400°C for 30 min, annealed at 200°C for 2 h under an applied static field of 4.5 KG. The field was applied in the plane of the sample and making an angle 0 with the longitudinal ribbon axis that was varied from 90 ° to 0 °. The impedance was measured in an axial applied field (that varies between _ 1000A/m) using a fully automatized experimental set-up that incorporates a Hewlett-Packard 4194A impedance analyser. This apparatus allows separation of the real and imaginary components of the impedance. The MI measurements were performed at frequencies up to 10 MHz, with a constant amplitude ac voltage of 0.4 V. Short circuiting of the sample contacts provides direct subtraction of the residual circuit impedance. In Fig. 1 the R ( f H = 0) behaviour for some of the studied samples is shown. The technical specifications of the studied sample give for the resistivity and the maximum permeability values of p = 1351a-Ocm and ~t = 100 000, respectively, which lead to a skin depth of 0.5 ~tm < 5 < 15 p.m for 104 Hz < f < 10 7 Hz. Clearly,
0304-8853/99/$ - see front matter ~c~ 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 85 3(9 8 ) 0 0 7 0 8 - 2
J. Guti~rrez et al. /'Journal of Magnetism and Magnetic' Materials 196-197 (1999) 169-170
170
65
14
"
(a)
t: 10
5.5
8
5
6
45
g
4
i1::
55
0 0.5
1
15
2
25
3
3.5
5
f(MHz) 4.5
Fig. 1. R(f, H = 0) dependencies measured for some of the sampies studied in this work. the low-frequency condition 6 > 1/2 is not satisfied in all cases, since the thickness of the sample is ~ 25 ~tm. F r o m Fig. 1 also, it can be seen that there is a good linear behaviour from 310 K H z up. At this frequency the skin depth penetration is ~2.7 gm, so that the current flows very near the surfaces of the sample. As a consequence we will assume that for higher frequency values we perform measurements at high-frequency regime. This means a linear behaviour with frequency of the R(oo)/RDc ratio, as it has been checked using linear regression analysis. This supposition of high frequency range allows the impedance to be expressed as Z = R + (~oL)i [9], where R~(wL)~RDc
(') ~
~/¢t,p
and, consequently, I ~ oc (R/RDc) 2. In fact, at a frequency of 1 M H z measured reactances and resistances are of the same order of magnitude and of similar value. In Fig. 2a, the measured R(H) behaviour at 1 M H z for the sample annealed at 20' is shown. This behaviour is similar for samples annealed at an angle 0 < 40 °. In Fig. 2b, the corresponding curve for the sample annealed at 8 0 is shown. This second type of curve appears for the M I in all for samples annealed at 50' < 0 < 90 °. F r o m this two types of curves we deduce that the effect of the annealings varies strongly for 0 below or above 4 5 . The existance of these two different kinds of behaviours hints for different magnetization processes in different samples. To confirm this, we have also measured the M(H) loops for all samples annealed at the different easy axis angles. Two set of loops, with applied fields up to 75 A/m and also to 1440 A/m, were measured. The annealing process was effective in the sense that the hysteresis of all the samples is almost zero. So, only magnetic m o m e n t rotation should be the dominant mechanism of magnetization. At low field, where most of the interesting features in the G M I data appear, all samples except the 90 ° annealed one are demagnetizing factor limited when measuring M(H) behaviour.
4
35 -1000
-500
0
500
1000
H(A/m)
Fig. 2. R(H) dependencies measured at l MHz for the samples annealed at an angle: (a) 2 0 and (b) 80, between the ribbon axis and the applied field. On the other hand, the measured normalized values of tLq, are in all cases higher than 0.4, in clear contradiction with theory predictions. In consequence, domain walt movements play an important role at low applied magnetic fields that still has to be investigated. F r o m measured M(H) curves and G M I data it is not clear yet which magnetization processes occur at low fields giving rise to the different magnetoimpedance behaviours. This is also a direct consequence of the preliminary nature of the data. Other factors as the demagnetizing fields inside the magnetic domains will have to be taken into account in order to clarify this observed behaviour. J. Guti6rrez is thankful for the financial support received from the Convenio British Council/Gobierno Vasco 1997//98.
References [1] L.V. Panina, K. Mohri, K. Bushida, M. Noda, J. Appl. Phys. 76 (1994) 6198. [2] J.D. Jackson, Classical Electrodynamics, Wiley, New York, 1975. [3] RS. Beach, A.E. Berkowitz, Appl. Phys. Lett. 64 (1994) 3652. [4] A.K. Agarwala, L. Berger, J. Appl. Phys. 57 {1985) 3505. [5] P.T. Squire, J. Magn. Magn. Mater 87 (1990} 299. [6] P.T. Squire, J. Magn. Magn. Mater 140-144 (1995) 1829. [7] D. Atkinson, P.T. Squire, IEEE Trans. Magn. 33 (1997) 3364. [8] D, Atkinson, P.T. Squire, J. Appl. Phys. 83 I1998) 6569. [9] A. Sommerfeld, Electrodynamics, Academic Press, New York, 1952.
~
Journalof
,~
and
magnetism magnetic materials
Journal of Magnetism and Magnetic Materials 196-197 (1999) 169-170
Domain wall motion influence on the magnetoimpedance effect J. Guti6rrez a'*, D. Atkinson b, P.T. Squire b, J.M.
Barandiar/m
a
aDepartamento de Electricidad y Electr6nica, Universidad del Pais Vaseo/EHU, Apartado 644, E-48080 Bilbao, Spain bDepartment of Physics, University o f Bath, Bath BA2 7A Y, UK
Abstract
The influence of domain wall movements in the giant magnetoimpedance measured in low magnetostrictive alloys has been studied. Samples were field annealed under a static field that was applied making an angle 0, varied from 90 ° to 0 °, with the longitudinal ribbon axis. Measurements were performed up to 10 MHz. Obtained results and theoretical predictions are compared and discussed. ~) 1999 Elsevier Science B.V. All rights reserved. Keywords: Magnetoimpedance; Domain wall motion; Soft ferromagnetic alloys
One of the most interesting phenomena observed in amorphous ribbons of soft ferromagnetic alloys is the frequency dependence of the giant magnetoimpedance (GMI) presented by low magnetostrictive alloys, usually Co-rich, CoFeSiB compositions [1]. In order to understand the G M I effect, it is necessary to consider the circumferential permeability, /~, of the sample in the form of wire. In ribbons, it corresponds to the transverse susceptibility, but the same symbol will be used throughout. Its square root is proportional to the impedance through the expression [2]. • pL ( 1 - i)L Z = (l -- t) ~-[~ 2lc x/zltP¢°#*'
where 6 is the skin depth penetration, p is the resistivity of the sample, and l and L are the ribbon width and length, respectively. This transverse susceptibility depends on the initial domain structure [3,4] and the magnetization process in the sample. Following previous phenomenological models [5,6], #6 behaviour has been already modelled [7,83 as a function of the anisotropy angle, 0, respect to the ribbon axis. This model predicts that for purely perpendicular anisotropy (0 = 90 °) the
* Corresponding author. Tel.: + 34-94-601-2553; fax: + 3494-485-81-39; e-mail: [email protected].
transverse susceptibility is zero only if there is no domain wall movement in the sample. However, large domain wall motion can lead to significant deviations from this zero value. In this work we have tested these features using the low-magnetostriction alloy Co66Fe4MozSi16B12 (Vitrovac ® 6025, 2~ < 0.2 ppm). Several strips of this sample were cut and after thermal stress relaxation at 400°C for 30 min, annealed at 200°C for 2 h under an applied static field of 4.5 KG. The field was applied in the plane of the sample and making an angle 0 with the longitudinal ribbon axis that was varied from 90 ° to 0 °. The impedance was measured in an axial applied field (that varies between _ 1000A/m) using a fully automatized experimental set-up that incorporates a Hewlett-Packard 4194A impedance analyser. This apparatus allows separation of the real and imaginary components of the impedance. The MI measurements were performed at frequencies up to 10 MHz, with a constant amplitude ac voltage of 0.4 V. Short circuiting of the sample contacts provides direct subtraction of the residual circuit impedance. In Fig. 1 the R ( f H = 0) behaviour for some of the studied samples is shown. The technical specifications of the studied sample give for the resistivity and the maximum permeability values of p = 1351a-Ocm and ~t = 100 000, respectively, which lead to a skin depth of 0.5 ~tm < 5 < 15 p.m for 104 Hz < f < 10 7 Hz. Clearly,
0304-8853/99/$ - see front matter ~c~ 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 85 3(9 8 ) 0 0 7 0 8 - 2
J. Guti~rrez et al. /'Journal of Magnetism and Magnetic' Materials 196-197 (1999) 169-170
170
65
14
"
(a)
t: 10
5.5
8
5
6
45
g
4
i1::
55
0 0.5
1
15
2
25
3
3.5
5
f(MHz) 4.5
Fig. 1. R(f, H = 0) dependencies measured for some of the sampies studied in this work. the low-frequency condition 6 > 1/2 is not satisfied in all cases, since the thickness of the sample is ~ 25 ~tm. F r o m Fig. 1 also, it can be seen that there is a good linear behaviour from 310 K H z up. At this frequency the skin depth penetration is ~2.7 gm, so that the current flows very near the surfaces of the sample. As a consequence we will assume that for higher frequency values we perform measurements at high-frequency regime. This means a linear behaviour with frequency of the R(oo)/RDc ratio, as it has been checked using linear regression analysis. This supposition of high frequency range allows the impedance to be expressed as Z = R + (~oL)i [9], where R~(wL)~RDc
(') ~
~/¢t,p
and, consequently, I ~ oc (R/RDc) 2. In fact, at a frequency of 1 M H z measured reactances and resistances are of the same order of magnitude and of similar value. In Fig. 2a, the measured R(H) behaviour at 1 M H z for the sample annealed at 20' is shown. This behaviour is similar for samples annealed at an angle 0 < 40 °. In Fig. 2b, the corresponding curve for the sample annealed at 8 0 is shown. This second type of curve appears for the M I in all for samples annealed at 50' < 0 < 90 °. F r o m this two types of curves we deduce that the effect of the annealings varies strongly for 0 below or above 4 5 . The existance of these two different kinds of behaviours hints for different magnetization processes in different samples. To confirm this, we have also measured the M(H) loops for all samples annealed at the different easy axis angles. Two set of loops, with applied fields up to 75 A/m and also to 1440 A/m, were measured. The annealing process was effective in the sense that the hysteresis of all the samples is almost zero. So, only magnetic m o m e n t rotation should be the dominant mechanism of magnetization. At low field, where most of the interesting features in the G M I data appear, all samples except the 90 ° annealed one are demagnetizing factor limited when measuring M(H) behaviour.
4
35 -1000
-500
0
500
1000
H(A/m)
Fig. 2. R(H) dependencies measured at l MHz for the samples annealed at an angle: (a) 2 0 and (b) 80, between the ribbon axis and the applied field. On the other hand, the measured normalized values of tLq, are in all cases higher than 0.4, in clear contradiction with theory predictions. In consequence, domain walt movements play an important role at low applied magnetic fields that still has to be investigated. F r o m measured M(H) curves and G M I data it is not clear yet which magnetization processes occur at low fields giving rise to the different magnetoimpedance behaviours. This is also a direct consequence of the preliminary nature of the data. Other factors as the demagnetizing fields inside the magnetic domains will have to be taken into account in order to clarify this observed behaviour. J. Guti6rrez is thankful for the financial support received from the Convenio British Council/Gobierno Vasco 1997//98.
References [1] L.V. Panina, K. Mohri, K. Bushida, M. Noda, J. Appl. Phys. 76 (1994) 6198. [2] J.D. Jackson, Classical Electrodynamics, Wiley, New York, 1975. [3] RS. Beach, A.E. Berkowitz, Appl. Phys. Lett. 64 (1994) 3652. [4] A.K. Agarwala, L. Berger, J. Appl. Phys. 57 {1985) 3505. [5] P.T. Squire, J. Magn. Magn. Mater 87 (1990} 299. [6] P.T. Squire, J. Magn. Magn. Mater 140-144 (1995) 1829. [7] D. Atkinson, P.T. Squire, IEEE Trans. Magn. 33 (1997) 3364. [8] D, Atkinson, P.T. Squire, J. Appl. Phys. 83 I1998) 6569. [9] A. Sommerfeld, Electrodynamics, Academic Press, New York, 1952.