DSpace at VNU: Valve behavior of giant magnetoimpedance in field-annealed Co70Fe5Si15Nb2.2Cu0.8B7 amorphous ribbon

Valve behavior of giant magnetoimpedance in field-annealed Co 70 Fe 5 Si 15 Nb 2.2 Cu
0.8 B 7 amorphous ribbon
Manh-Huong Phan, Hua-Xin Peng, Michael R. Wisnom, Seong-Cho Yu, and Nguyen Chau
Citation: Journal of Applied Physics 97, 10M108 (2005); doi: 10.1063/1.1854891
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JOURNAL OF APPLIED PHYSICS 97, 10M108 ͑2005͒

Valve behavior of giant magnetoimpedance in field-annealed
Co70Fe5Si15Nb2.2Cu0.8B7 amorphous ribbon
Manh-Huong Phan,a͒ Hua-Xin Peng, and Michael R. Wisnom
Advanced Composites Group, Department of Aerospace Engineering, Bristol University, University Walk,
Bristol BS8 1TR, United Kingdom

Seong-Cho Yu
Department of Physics, Chungbuk National University, Cheongju, 361-763, Korea

Nguyen Chau
Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam

͑Presented on 9 November 2004; published online 13 May 2005͒
The influence of longitudinal field annealing on the giant magnetoimpedance ͑GMI͒ effect in
Co70Fe5Si15Nb2.2Cu0.8B7 amorphous ribbons has been investigated. It was found that annealing in
the open air at magnetic fields smaller than the anisotropy field along the ribbon gave rise to the
GMI-valve phenomenon, while annealing at magnetic fields higher than the anisotropy field
significantly reduced the GMI effect. The GMI-valve behavior corresponding to the highest field
sensitivity of GMI ͑125% / Oe͒ was observed at a frequency of 0.1 MHz in the ribbon annealed
under an applied field of 2 Oe. This is ideal for developing sensitive and quick-response magnetic
sensors. The GMI-valve behavior observed in the Co-based amorphous ribbon due to field annealing
can be explained by considering the complex permeability spectra in relation to the rotational dc
magnetization. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1854891͔
I. INTRODUCTION

Giant magnetoimpedance ͑GMI͒ sensors have greatly
benefited from the development of amorphous soft magnetic
materials. It has been shown that amorphous Co-based ribbons, wires, and glass-covered microwires are promising
candidates for GMI sensor applications.1–3 The GMI effect
occurs at high frequency range and can be explained by the
classical electrodynamics.1 The maximum GMI is generally
found in an alloy with the lowest value of magnetostriction,
which corresponds to a maximum transverse permeability.1,3
Nonetheless, Sommer and Chien2 pointed out that such a
high permeability of an amorphous alloy does not necessarily lead to a high GMI effect, and the observed large effect
was due to the presence of transverse magnetic anisotropy

induced by the application of an external magnetic field during the annealing process. They also showed that the longitudinal field annealing reduced the transverse component of
the anisotropy and consequently eliminated the GMI effect.2
This is in contrast to the recently observed asymmetric giant
magnetoimpedance ͑AGMI͒ phenomenon, i.e., the so-called
GMI valve, in a longitudinally weak-field-annealed Co-based
amorphous ribbon.4,5 The reason for this discrepancy is that
an annealing field of ϳ2 kOe applied along the ribbon2
might be high enough to entirely suppress the domain wall
motion in the transverse direction and lead to the observed
AGMI effect.4,5
In order to gain some insights into the nature of the GMI
valve or the AGMI phenomena, the aim of the present work
is to experimentally verify the AGMI due to longitudinal
a͒

Author to whom correspondence should be addressed; electronic mail:


0021-8979/2005/97͑10͒/10M108/3/$22.50

weak-field annealing by means of magnetoimpedance and
complex permeability spectra. In a recently published
work,6 we found that the substitution of Cu and Nb for B in
an initial Co70Fe5Si15B10 composition forming the
Co70Fe5Si15Nb2.2Cu0.8B7 composition improved both the
GMI effect and the field sensitivity of the amorphous ribbon.
A pronounced GMI effect is also expected in the annealed
samples. Therefore, we selected the Co70Fe5Si15Nb2.2Cu0.8B7
amorphous ribbons for the present study.
II. EXPERIMENT

The selected Co70Fe5Si15Nb2.2Cu0.8B7 amorphous
samples were annealed at a temperature of 350 °C for 5 h in
open air in order to develop an oxide coating on the ribbon
surface. During the annealing process, an annealing field
͑Ha͒ was applied along the ribbon axis using a solenoid and
its magnitude was varied from 50 mOe to 8 Oe. Magnetic
measurements were carried out using a vibrating sample
magnetometer ͑VSM͒. Magnetoimpedance and complex permeability spectra were measured using a HP4129A impedance analyzer.7 The GMI ratio was obtained from the following relation:
⌬Z/Z͑%͒ = 100 % ϫ ͓Z͑H͒ − Z͑Hmax͔͒/Z͑Hmax͒,

͑1͒

where Hmax is the external magnetic field sufficient to saturate the impedance and is equal to 35 Oe in the present work.
III. RESULTS AND DISCUSSION

GMI profiles were measured as a function of frequency
͑f͒ and the annealing field ͑Ha͒. It was found that, at relatively low frequencies ͑ϳ0.1 MHz͒, the GMI profiles
showed a typical two-peak characteristic for Ha ഛ 100 mOe

97, 10M108-1

© 2005 American Institute of Physics

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10M108-2

Phan et al.

J. Appl. Phys. 97, 10M108 ͑2005͒

FIG. 3. Variation of static permeability from domain wall motion ͑␮Ldw͒ and
from rotational magnetization ͑␮Lrot͒ in the longitudinal direction with the
annealing field ͑Ha͒.

whereas only a single peak appeared under higher annealing
fields with the peak in the antiparallel field region disappearing completely. The GMI for the field-annealed samples also
showed a distinct variation in AGMI features with increasing
frequency. The peak in the antiparallel field appeared again
at frequencies above 1 MHz, and it developed strongly with
increasing frequencies. Similar behaviors were observed by
Kim et al.4 and Jang et al.5 In the present work, we focused
our study on the characterization of the GMI valve behavior
or the AGMI features. Figure 1 shows the GMI profiles measured at a frequency of 0.1 MHz under various annealing
fields. As one can see clearly from Fig. 1, at a frequency of
0.1 MHz, the amplitude of AGMI increases with Ha up to 2
Oe and then decreases under higher annealing fields, while
the shape of the AGMI profile remains almost unchanged. In
addition, the AGMI peak is found to shift slightly toward a
higher value of the external magnetic field ͑Hex͒ as the annealing field is increased. These can be of significant importance in developing sensitive and quick-response magnetic
sensors.3 Figure 2 shows the field sensitivity of GMI as a
function of the annealing field at different frequencies. The

highest field sensitivity of GMI of 125% / Oe was found at
Ha = 2 Oe and at f = 0.1 MHz. This is obviously much higher
than those of giant magnetoresistance ͑GMR͒ materials,8 and
therefore is ideal for developing autobiased linear field sensors. It is worth noting from Fig. 2 that, with increasing

frequency, the field sensitivity of GMI decreases considerably and the maximum sensitivity is found at a higher annealing field.
Furthermore, it has recently been demonstrated by Phan
et al.9 that the permeability spectra can be used as a useful
measure to assess the AGMI phenomena in such a Co-based
amorphous microwire under a dc bias current. We estimated
the static permeability as a function of the annealing field
͑Ha = 500 mOe− 8 Oe͒ using the experimental procedures
similar to that in Ref. 9. The results are given in Fig. 3. It can
be seen that the longitudinal permeability from domain wall
motion ͑␮Ldw͒ decreases with increasing annealing field,
while the longitudinal permeability from magnetization rotation ͑␮Lrot͒ first increases with annealing field up to Ha
= 2 Oe and then deceases under higher annealing fields. To
further scrutinize this feature, we measured the axial hysteresis loops from which both the coercivity ͑HC͒ and the anisotropy field ͑Hk͒ were deduced and plotted as a function of
the annealing field in Fig. 4. It shows that Hk decreases with

FIG. 2. Variation of field sensitivity of GMI with the annealing field for
various frequencies.

FIG. 4. Variation of the coercivity ͑HC͒ and the anisotropy field ͑Hk͒ with
annealing field.

FIG. 1. GMI profiles measured at a frequency of 0.1 MHz for various
annealing fields.

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10M108-3

J. Appl. Phys. 97, 10M108 ͑2005͒

Phan et al.

increasing Ha, whereas HC is independent of the annealing
field.
Now, let us discuss the realistic influence of the longitudinal field annealing on the GMI effect in the Co-based
amorphous ribbon. First, it has been reported that the Cobased amorphous ribbons annealed in vacuum did not show
AGMI,4 but an asymmetry in GMI was observed in the
samples annealed in open air in the present work. Accordingly, it is proposed that the AGMI phenomenon or the socalled GMI valve observed in the weak-field-annealed Cobased amorphous ribbon can be attributed to the
crystallization of the surface underlayer, which becomes depleted in B and Si due to the surface oxidation.5 Second, it is
known that this type of heat treatment produces asymmetric
hysteresis loops in amorphous ribbons due to the interaction
between the inner amorphous phase and the magnetically
harder crystalline phase on the sample outer layer.10 When
the crystallization took place under a weak magnetic field,
the crystallites were magnetically ordered, resulting in an
effectively unidirectional surface anisotropy. It is the influence of the unidirectional surface anisotropy on domain wall
motion in the transverse direction that in turn causes the
AGMI observed in Fig. 1.
Because the magnitude of the unidirectional surface anisotropy depends on that of the annealing field, the AGMI
and its field sensitivity in the longitudinally weak-fieldannealed Co-based amorphous ribbon also depend on the
magnitude and direction of the annealing field ͑Ha͒ as well
as the measuring field ͑Hex͒. The permeability from domain
wall motion in the transverse direction ͑␮Tdw͒ should increase
with the annealing field up to Ha = 2 Oe since ␮Tdw is proportional to the permeability from magnetization rotation in the
longitudinal direction ␮Lrot ͑Fig. 3͒. That is why the highest
field sensitivity of GMI was observed at Ha = 2 Oe ͑Fig. 1͒.
The reduction of AGMI under higher annealing fields ͑Figs.
1 and 2͒ is a direct consequence of the decrease of the longitudinal permeability from the wall motion and magnetization rotation processes ͑Fig. 3͒. This is also attributed to the

decrease of the magnetic anisotropy in the transverse direction as the annealing field is increased ͑Fig. 4͒. These are
consistent with what was reported in Ref. 2. However, in the
work reported in Ref. 2, the ribbons were field annealed in an
inert atmosphere of Ar with a large magnetic field, Ha
= 2 kOe. This annealing field induced a considerable unidirectional magnetic anisotropy that entirely suppressed domain wall motion in the transverse direction, and therefore
eliminated the GMI effect. It should be further noted that,
due to the decrease of asymmetry in GMI at high frequencies
where the rotational contribution to GMI is dominant, the
field sensitivity of GMI decreases as the measuring frequency is increased ͑Fig. 2͒.
It is pointed out that annealing at small magnetic fields
͑Ha ഛ Hk͒ along the ribbon in open air introduced a peculiar
domain structure.4,5 This enhanced the transverse permeability, and a larger asymmetry in GMI was consequently observed. Annealing at magnetic fields slightly higher than the

anisotropy field ͑Ha Ͼ Hk͒ induces the unidirectional magnetic anisotropy along the ribbon and this anisotropy can be
large enough to hinder domain wall motion in the transverse
direction, thereby significantly reducing the GMI effect. Annealing at much higher magnetic fields ͑Ha ӷ Hk͒ can eliminate the GMI effect as reported in Ref. 2. This finding is
important in understanding theoretically the exchange-biased
asymmetry ͑i.e., the GMI-valve behavior in Co-based amorphous ribbons annealed in a weak field in the open air͒ by
means of the quasistatic model, in which the unidirectional
exchange anisotropy can be replaced by an effective dc bias
field. This warrants further study.
IV. CONCLUSIONS

The influence of field annealing on the GMI effect in
Co70Fe5Si15Nb2.2Cu0.8B7 amorphous ribbons was investigated systematically. Annealing at small magnetic fields
͑Ha ഛ Hk͒ along the ribbon in the open air resulted in AGMI
phenomena—the so-called GMI valve, while annealing at
higher magnetic fields ͑Ha Ͼ Hk͒ significantly reduced the
GMI effect. The optimum GMI-valve behavior, which corresponds to the highest field sensitivity of GMI of 125% / Oe,
was observed at f = 0.1 MHz in the ribbon annealed under a

field of Ha = 2 Oe. This is ideal for developing sensitive and
quick-response magnetic sensors. AGMI and its field sensitivity were induced by the field annealing due to the influence of the dc bias field on domain wall motion in the transverse direction. The reduction in AGMI under higher
annealing fields ͑Ha Ͼ Hk͒ was caused by the decrease of the
transverse permeability from domain wall motion and magnetization rotation processes. This can be attributed to the
decrease of the transverse magnetic anisotropy as the annealing field is increased.
ACKNOWLEDGMENTS

The authors would like to thank Professor Cheol Gi Kim
for helpful comments. This work was partially supported by
the Korea Science and Engineering Foundation through the
Research Center for Advanced Magnetic Materials at Chungnam National University.
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