Giant Magnetoimpedance Effect in Co70Fe5Si15B10 and Co70Fe5Si15Nb2:2Cu0:8B7 RibbonsManh-Huong PHAN, Yong-Seok KIM, Nguyen Xuan CHIEN1, Seong-Cho YU, Heebok LEE2 and Nguyen CHAU1 Departmen
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Giant Magnetoimpedance Effect in Co70Fe5Si15B10 and Co70Fe5Si15Nb2.2Cu0.8B7 Ribbons
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2003 Jpn J Appl Phys 42 5571
(http://iopscience.iop.org/1347-4065/42/9R/5571)
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Trang 2Giant Magnetoimpedance Effect in Co70Fe5Si15B10 and Co70Fe5Si15Nb2:2Cu0:8B7 Ribbons
Manh-Huong PHAN, Yong-Seok KIM, Nguyen Xuan CHIEN1, Seong-Cho YU, Heebok LEE2 and Nguyen CHAU1 Department of Physics, Chungbuk National University, Cheongju 361-763, Korea
1 Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Vietnam
2 Department of Physics Education, Kongju National University, Kongju 314-701, Korea
(Received January 27, 2003; accepted for publication May 22, 2003)
Giant magnetoimpedance (GMI) effect has been observed in Co70Fe5Si15B10 and Co70Fe5Si15Nb2:2Cu0:8B7 melt-spun
amorphous ribbons The magnetoimpedance (MI) of these samples has been studied up to a frequency of 10 MHz and varying
a dc magnetic field (Hdc) within 150 Oe A maximum change of 89% in MI has been observed for Co70Fe5Si15Nb2:2Cu0:8B7
composition around a frequency of 3.1 MHz Substitution of Cu and Nb for B in an initial Co70Fe5Si15B10 composition
forming the Co70Fe5Si15Nb2:2Cu0:8B7composition not only favors the GMI effect but also gives rise to the sensitivity of the
magnetic response (18%/Oe), which is very beneficial for magnetic sensors applications The GMI effect for both samples
annealed at 550 K is further enhanced due to the presence of the ultrasoft magnetic materials, compared to their as-quenched
samples [DOI: 10.1143/JJAP.42.5571]
KEYWORDS: magnetoimpedance effect, magnetic sensing sensors, Co-based amorphous alloys, proper heat treatments
1 Introduction
Recently, a giant magnetoimpedance (GMI) effect,
dis-covered in amorphous magnetic materials, has generated
growing interests among researchers and manufacturers
because of its practical potential for magnetic sensing and
recording applications.1–4)Among amorphous alloys, Co- or
Co–Fe-based amorphous wires and ribbons with vanishing
magnetostriction show a giant magnetoimpedance
change.2–7)The GMI effect is demonstrated to arise from a
combination of a skin effect and strong field dependence of
the circumferential magnetic permeability associated with
circular domain wall movements As an ac current I ¼
I0expðj!tÞ is applied to such materials, their impedance
Z ¼ R þ j!L changes sensitively with changes in the
biasing dc magnetic field, Hdc At low frequencies (kHz),
the ac current just generates a circumferential magnetic field
This time varying magnetic field changes the transverse
component of magnetization When the biasing dc magnetic
field is applied along the wire (or ribbon) axis, the effective
magnetic field on the wire (or ribbon) changes
Consequent-ly, both the transverse component of magnetization and the
transverse permeability are varied thus leading to a large
change in the magneto-inductive effect At high frequencies,
the GMI effect can be interpreted in terms of the dc magnetic
field dependence of impedance as a result of the transverse
magnetization with respect to the ac current direction
flowing through the sample and the skin effect due to this
ac current.1)Because the ac current tends to concentrate near
the surface of a conductor, as frequency increases, the
impedance responds to the current distribution which
depends not only on the shape of the conductor and
frequency but also on the transverse permeability with
respect to the applied current In such a magnetic material,
the transverse permeability ? effects on the magnetic
penetration depth m; m¼ ð=f ?Þ1=2, where f is the
frequency and is the electrical resistivity It is expected
that with increasing frequency, the impedance in case of the
skin effect a=m 1 is proportional to ðf ?Þ1=2 Because
the external dc magnetic field is a hard axis field with respect
to the circumferential anisotropy, the magnetic field applied
along the ribbon axis will suppress transverse magnetization processes by domain wall movements at low frequencies or the motion of localized magnetic moments at high frequen-cies In other words, the transverse permeability decreases rapidly as the external dc magnetic field is applied, which in turn causes a giant magnetoimpedance change, i.e., the GMI effect In a series of recent works,7–10)moreover, it has been demonstrated that proper heat treatments on amorphous ribbons led to a nanocrystalline structure with ultra-high permeability (>106) and thus further enhanced the GMI effect Under these cases, the high permeability values have been believed to be associated with a transverse magnetic domain structure perpendicular to the long axis of the sample, in which the reversible magnetization rotation is a dominant mechanism for changes in the magnetic state.9,10)
In this article, we report on the GMI effect in the
Co70Fe5Si15B10 and Co70Fe5Si15Nb2:2Cu0:8B7 melt-spun amorphous ribbons The GMI effect further enhanced in the nanocrystalline samples due to thermal treatment is also reported
2 Experimental The Co70Fe5Si15B10 and Co70Fe5Si15Nb2:2Cu0:8B7 rib-bons with 4 mm in width and 20 mm in thickness were prepared by rapid quenching from the melt The samples were annealed in a conventional furnace at 550 K for one hour, in vacuum Amorphous and nanocrystalline states were confirmed by X-ray diffractometry (XRD) The hysteresis loops were performed using a vibrating sample magneto-meter (VSM) The resistivity measurements for both samples were carried out with the four-probe method The
5m and
5m for Co70Fe5Si15B10 and Co70Fe5Si15
-Nb2:2Cu0:8B7compositions, respectively The GMI measure-ments were carried out along the ribbon axis with the longitudinal applied magnetic field The samples were cut out about 15 mm in length for all the GMI measurements A schematic diagram of the GMI measurement system has been described in details elsewhere.3)A computer-controlled
RF signal generator with its power amplifier was connected
to the sample in a series with resistors for monitoring the driving ac current An ac current and a voltage across the
Corresponding author E-mail address: scyu@chungbuk.ac.kr
Jpn J Appl Phys Vol 42 (2003) pp 5571–5574
Part 1, No 9A, September 2003
#2003 The Japan Society of Applied Physics
5571
Trang 3sample were measured by digital multimeters (DMM) with
RF/V probes for calculating the impedance The external dc
field applied by a solenoid can be swept through the entire
cycle equally divided by 800 intervals from 150 to 150 Oe
The frequency of the ac current was varied from 1 to
10 MHz while its magnitude was kept constantly at 10 mA
3 Results and Discussions
The magnetoimpedance ratio (MIR) can be defined as
MIRð%Þ ¼ 1 jZðHÞ=ZðHmaxÞj, where Hmax is the external
magnetic field sufficient for saturating the
magnetoimpe-dance and equals to 150 Oe for the present work Similarly,
the longitudinal permeability ratio (LPR) measured as a
function of the external magnetic field can be also defined as
LPRð%Þ ¼ 1 jðHÞ=ðHmaxÞj Regarded to the GMI
ef-fect, recent studies4,7,9–11)have proposed that change in the
magnetoimpedance is closely related to that of the
longi-tudinal permeability It means that evaluations for the GMI
effect in amorphous soft magnetic alloys can be realized
either by the MIR measurements or the LPR measurements
As shown in Fig 1, the MIR curves, measured at
frequencies up to f ¼ 4:1 MHz, for both Co70Fe5Si15B10
and Co70Fe5Si15Nb2:2Cu0:8B7 compositions have a single
peak at zero external magnetic field At the frequency of
3.1 MHz, the maximum MIR magnitude is 55% and 89%
for Co70Fe5Si15B10 and Co70Fe5Si15Nb2:2Cu0:8B7
composi-tions, respectively A higher MIR value for
Co70Fe5Si15Nb2:2Cu0:8B7 composition, measured at f ¼ 3:1 MHz, can be attributed to the presence of its special domain structure containing transverse domains formed by a magnetomechanical coupling between internal stress and magnetostriction.9–11)In the present case, partial substitution
of Cu and Nb for B in the initial Co70Fe5Si15B10 compo-sition seems to favor the formation of a transverse domains structure, because of the presence of Cu and Nb allowing the formation of well-differentiated microstructures.13)Thereby, higher transverse permeability value for Co70Fe5Si15
-Nb2:2Cu0:8B7than for Co70Fe5Si15B10can be expected This results in a larger MIR value for Co70Fe5Si15Nb2:2Cu0:8B7 composition In addition, the obtained maximum MIR values for Co70Fe5Si15Nb2:2Cu0:8B7 composition are higher than those for Co70Fe5Si15B10 composition at all measured frequencies, indicative of a favorably formed transverse domain structure in Co70Fe5Si15Nb2:2Cu0:8B7 alloy rather than in Co70Fe5Si15B10alloy Another reason that also led to the difference in MIR values of the above two compositions
is the difference in their electrical resistivity As reported in ref 1, the higher the electrical resistivity of amorphous alloy, the lower the obtained MIR value is In the present
Co70Fe5Si15B10 composition, and is higher than ¼
5m for Co70Fe5Si15Nb2:2Cu0:8B7 composition Because of that, the higher MIR values obtained for
Co70Fe5Si15Nb2:2Cu0:8B7 composition can be understanda-ble This is consistent with the results that had been reported
in Ref 5 It is clear that, from Fig 1(b), there is a small full width at haft maximum (FWHM) in MIR curves for
Co70Fe5Si15Nb2:2Cu0:8B7 composition, which indicates high sensitivity of the MIR to magnetic field The
Co70Fe5Si15Nb2:2Cu0:8B7, GMI sensitive material, has high sensitivity of 17–18%/Oe for a current driving frequency of
f ¼ 3:1 MHz It is interesting to note that the high sensitivity of the magnetic response almost remains at high frequencies, meanwhile it had not been often found in the other materials.1,3,11–16) Thus the Co70Fe5Si15Nb2:2Cu0:8B7
material is, at present, a very promising candidate for magnetic sensors applications Compared to Co70Fe5Si15
-Nb2:2Cu0:8B7composition, the larger FWHM of MIR curves for Co70Fe5Si15B10 composition, corresponding to the magnetic response sensitivity of 1%/Oe at f ¼ 3:1 MHz, was found Moreover, this broadening was also clarified by the hysteresis loops observations; the local magnetic anisotropy along the ribbon axis in the Co70Fe5Si15B10
composition was much larger than that in the
Co70Fe5Si15Nb2:2Cu0:8B7 composition It is the local aniso-tropy that considerably reduced the transverse magnetization associated with the transverse magnetic permeability and thus led to the broadening in the MIR curves and the smaller value of MIR for Co70Fe5Si15B10 composition A similar behavior was also observed by the LPR curves (see Fig 2), where the FWHM of the LPR curves were much larger for
Co70Fe5Si15B10 composition than for Co70Fe5Si15
-Nb2:2Cu0:8B7 composition In order to further interpret the broadening of the LPR curves, a model for the transverse biased permeability in thick ferromagnetic films can be adopted.17) According to this model, it is the eddy current damping and the ripple field H incorporating with the
0
10
20
30
40
50
60
-150 -100 -50 0 50 100 150
0
15
30
45
60
75
90
2.1 MHz 3.1 MHz 4.1 MHz
(b)
Field (Oe)
1.1 MHz 2.1 MHz 3.1 MHz 4.1 MHz
Fig 1 The MIR curves measured as a function of the external magnetic
field at various frequencies (1.1–4.1 MHz) for the as-quenched
amor-phous samples: (a) Co 70 Fe 5 Si 15 B 10 composition and (b)
Co Fe Si Nb Cu B composition.
5572 Jpn J Appl Phys Vol 42 (2003) Pt 1, No 9A M.-H P et al.
Trang 4anisotropy HKthat give rise to the peak of permeability in an
external field as well as the broadening of the LPR curves at
high frequencies With the above indications, we can
conclude that the evaluation for the GMI effect in
amor-phous soft magnetic alloys either through the MIR
measure-ments or the LPR measuremeasure-ments is valid
As can be seen from Fig 3, frequency dependences of the
maximum MIR for both compositions show that at first the
MIR increased as frequency increased up to f ¼ 3:1 MHz
and then reduced at higher frequencies It should be noted
that, at frequencies below 1.1 MHz (the ribbon thickness,
a < ), the maximum MIR value was not very large because
of the contribution of the induced magneto-inductive voltage
in the case of skin effect, the higher MI values are reported
Beyond f ¼ 3:1 MHz, the maximum MIR decreased with
increasing frequency The reason here is that in the
frequency region (3.1 MHz f ), domain wall
displace-ments were strongly damped owing to Eddy currents and
thus contributed less to the transverse permeability, which in
turn caused a small magnetoimpedance change and hence,
the small value of MIR
In order to investigate annealing effects on the GMI
profiles, we have carried out the LPR measurements for
550 K-annealed Co70Fe5Si15B10 and Co70Fe5Si15Nb2:2
-Cu0:8B7 ribbons Prior to the LPR measurements, from
XRD patterns it is confirmed that the annealed ribbons were
in a nanocrystalline phase It indicates that soft magnetic properties of the as-quenched amorphous samples seemed to
be further improved by annealing at 550 K As shown in Fig 4, the LPR magnitude and the FWHM of LPR curves for the 550 K-annealed samples did change much, as compared
to the as-quenched samples (see Fig 2) It is worthy that
0
2
4
6
8
10
12
14
-150 -100 -50 0 50 100 150 0
20
40
60
80
100
120
140
(b)
(a)
1.1 MHz 2.1 MHz 3.1 MHz 4.1 MHz 5.1 MHz 6.1 MHz
H (Oe)
1.1 MHz 2.1 MHz 3.1 MHz 4.1 MHz 5.1 MHz 6.1 MHz
Fig 2 The LPR curves measured as a function of the external
magnetic field at various frequencies (1.1–6.1 MHz) for the as-quenched
amorphous samples: (a) Co70Fe5Si15B10 composition and (b)
Co70Fe5Si15Nb2:2Cu0:8B7composition.
0 15 30 45 60 75 90 105
Frequency (MHz)
Co70Fe5Si15B10
Co70Fe5Si15Nb2.2Cu0.8B7
Fig 3 The maximum of MIR versus frequency for Co 70 Fe 5 Si 15 B 10 and
Co 70 Fe 5 Si 15 Nb 2:2 Cu 0:8 B 7 as-quenched amorphous samples.
0 10 20 30 40 50 60 70
-150 -100 -50 0 50 100 150 0
50 100 150
200
(b)
(a)
1.1 MHz 2.1 MHz 3.1 MHz 4.1 MHz 5.1 MHz 6.1 MHz
H (Oe)
1.1 MHz 2.1 MHz 3.1 MHz 4.1 MHz 5.1 MHz 6.1 MHz
Fig 4 The LPR curves measured as a function of the external magnetic field at various frequencies (1.1–6.1 MHz) for the 550 K annealed samples: (a) Co 70 Fe 5 Si 15 B 10 composition and (b)
Co Fe Si Nb Cu B composition.
Jpn J Appl Phys Vol 42 (2003) Pt 1, No 9A M.-H P et al 5573
Trang 5after annealing, changes in the LPR magnitude and the
FWHM of LPR curves for Co70Fe5Si15B10 composition
drastically occurred, which is connected to a considerable
reduction in the local magnetic anisotropy and
magneto-striction.12) Moreover, smaller value of the magnetic
anisotropy for the 550 K-annealed Co70Fe5Si15B10 sample
than its as-quenched amorphous samples was also verified
by the hysteresis loops measurements More briefly, it is the
improved soft magnetic properties of the Co70Fe5Si15B10
sample through heat treatment that led to an increase in the
MIR and reductions in the FWHM of MIR curves for this
sample In Fig 5, the maximum value of LPR versus
frequency for the as-quenched and 550 K-annealed samples
are presented It is obvious that in a region of frequencies
(1.1–6.1 MHz), LPRmax(%) values calculated for the 550
K-annealed samples were higher than those for their
as-quenched samples We should recall that the higher the
longitudinal permeability value, the larger the obtained MIR
value is.9,10) Thus, the 550 K-annealed Co70Fe5Si15Nb2:2
-Cu0:8B7 sample here is expected to exhibit the higher value
of MIR Additionally, LPRmax(%) changes as a decreasing
function of the measured frequency can be explained as
follow: in the frequency region (1.1–6.1 MHz), the
longi-tudinal permeability resulting from rotational magnetization
decreased with increasing frequency, thus leading to the
corresponding reduction of LPRmax(%) It should be
empha-sized that the GMI effect is further enhanced in both
Co70Fe5Si15B10 and Co70Fe5Si15Nb2:2Cu0:8B7 melt-spun
amorphous ribbons by annealing at 550 K
4 Conclusions The GMI effect in Co70Fe5Si15B10 and
Co70Fe5Si15Nb2:2Cu0:8B7 melt-spun amorphous ribbons have been studied A maximum change of 89% in MI has been observed for the Co70Fe5Si15Nb2:2Cu0:8B7composition around f ¼ 3:1 MHz Substitution of Cu and Nb for B in the initial Co70Fe5Si15B10 composition forming the
Co70Fe5Si15Nb2:2Cu0:8B7 composition not only favors the GMI effect but also provides higher sensitivity of the magnetic response (17–18%/Oe at f ¼ 3:1 MHz), which is very beneficial for magnetic sensors applications The GMI effect for both samples annealed at 550 K is further enhanced
by the presence of the ultra-soft magnetic materials, as compared to the as-quenched samples The difference in the electrical resistivity and the existence of specific domain structures in the amorphous alloys could be originated from their different MIR values
Acknowledgements One of the authors (M H Phan) would like to thank Professor Sunk Kun Oh for helpful discussions Research at Korea was supported by the Korea Research Foundation Grant (KRF-2001-005-D20010)
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0
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80
120
160
200
Frequency (MHz)
Co70Fe5Si15B10 (as-quenched)
Co70Fe5Si15Nb2.2Cu0.8B7 (as-quenched)
Co70Fe5Si15B10 (annealed at 550 K)
Co70Fe5Si15Nb2.2Cu0.8B7 (annealed at 550 K)
Fig 5 The maximum of LPR versus frequency for the as-quenched and
550 K-annealed samples.
5574 Jpn J Appl Phys Vol 42 (2003) Pt 1, No 9A M.-H P et al.
... annealed ribbons werein a nanocrystalline phase It indicates that soft magnetic properties of the as-quenched amorphous samples seemed to
be further improved by annealing at 550... Co70Fe5Si15B10
sample through heat treatment that led to an increase in the
MIR and reductions in the FWHM of MIR curves for this
sample In Fig 5, the maximum value of LPR versus... magnetic materials, as compared to the as-quenched samples The difference in the electrical resistivity and the existence of specific domain structures in the amorphous alloys could be originated from