From obtained experimentally dependences of the GMI ratio on magnetic field at different frequencies we estimated the penetration depth and its dependence on applied mag-neticfield and fre
Trang 1Original Article
skin depth in Co-rich magnetic microwires from GMI experiments
Arcady Zhukova,b,c,*, Ahmed Talaata,b, Mihail Ipatova,b, Alexandr Granovskyd,
a Dpto de Fís Mater., UPV/EHU, San Sebastian, 20009, Spain
b Dpto de Física Aplicada, EUPDS, UPV/EHU, 20018, San Sebastian, Spain
c IKERBASQUE, Basque Foundation for Science, 48011, Bilbao, Spain
d Faculty of Physics, Lomonosov Moscow State University, 11991, Moscow, Russian Federation
a r t i c l e i n f o
Article history:
Received 23 July 2016
Received in revised form
8 August 2016
Accepted 8 August 2016
Available online 15 August 2016
Keywords:
Magnetic glass-coated microwires
Amorphous materials
Giant magnetoimpedance effect
Soft magnetic properties
Taylor-Ulitovsky technique
a b s t r a c t
We studied giant magnetoimpedance (GMI) effect in magnetically soft amorphous Co-rich microwires in the extended frequency range From obtained experimentally dependences of the GMI ratio on magnetic field at different frequencies we estimated the penetration depth and its dependence on applied mag-neticfield and frequency
© 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
Studies of amorphous soft magnetic materials attract attention
during last three decades owing to excellent soft magnetic
proper-ties, enhanced mechanical and corrosion properties and fast
prep-aration method [1,2] Amorphous wires present quite peculiar
magnetic properties such as magnetic bistability related to the
single and large Barkhausen jump and Giant Magneto Impedance
effect [3,4] From the viewpoint of applications of soft magnetic
amorphous materials so called“Giant Magneto Impedance effect”
(GMI) recently attracted special attention This GMI effect defined as
the large change of the electrical impedance of a magnetic
conductor when is subjected to an axial dc magnetic field, H[4e6] It
has been recognized that the large sensitivity of the total impedance
of a soft magnetic conductor at low magneticfields and high
fre-quencies of the driven ac current originates from the dependence of
the transverse magnetic permeability upon the dc magnetic field
and skin effect Large GMI effect (up to 600%) has been reported for Co-based amorphous glass-coated microwires with nearly zero magnetostriction value [6] This extremely high magnetic field sensitivity allows to use soft magnetic materials for creation of sensitive and cheap magnetic sensors and magnetometers[6e13] Consequently studies of different magnetic wires have attracted considerable attention of researchers and engineers along many years[4e13] Perfectly cylindrical symmetry is quite favorable for achievement of high MI effect[4e6,14,15]
As reported elsewhere GMI effect exhibited by amorphous wires
is quite sensitive to external stimuli, such as applied stress, perature that enables them for detection of stresses and/or tem-perature variation[13,16]
On the other hand it is well-known, the penetration depth,d, of conductor depends on the current frequency For observation of high MI effect the penetration depth must be smaller than the magnetic wires diameter[4e6] Moreover, the penetration depth can be evaluated from the GMI effect[17]
In this evaluation it was assumed that the changes in the real component of the impedance (in-phase component) are due only
to changes in the effective area where the AC-currentflows as a consequence of the skin effect
* Corresponding author Dpto de Fís Mater., UPV/EHU, San Sebastian, 20009,
Spain.
E-mail address: arkadi.joukov@ehu.es (A Zhukov).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
http://dx.doi.org/10.1016/j.jsamd.2016.08.002
2468-2179/© 2016 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license
Journal of Science: Advanced Materials and Devices 1 (2016) 388e392
Trang 2It should be proposed method of the penetration depth
evalu-ation is only meaningful when the penetrevalu-ation depth,d, is smaller
than the wire radius, r, or what is equivalent RAC> RDC(in fact the
penetration depth should be infinitively large at the frequency
f ¼ 0) As mentioned previously[17]for a simple estimation of the
penetration depth at high enough frequencies proposed approach
is sufficient for a qualitative interpretation the GMI-response of the
magnetic wire-element and possible use of this element in sensors
Recently most attention is paid to magnetic glass-coated
micro-wires with diameter of magnetic metallic nucleus of fewmm[6,14,15]
This tendency of the miniaturization of the magnetic sensors is
demanded by the industries for the most modern applications[6]
As described elsewhere the diameter reduction must be associated
with the increasing of the optimal MI frequency range: a tradeoff
be-tween dimension and frequency is required in order to obtain a
maximum MI effect[18] Therefore recently GMI effect in thin
mag-netic wires at GHz frequency range has been studied[14,15] Recently
we modified the experimental facility that allowed us to extend the
frequency range and measure GMI effect at GHZ frequencies[14,15]
and reported that in soft magnetic amorphous microwires the GMI
ratio above 100% can be observed even at GHz frequencies band[14,15]
Consequently we present our recent studies on the penetration
depth of the alternating currentflowing through the magnetically
soft conductor caused by the applied static magneticfield in thin
amorphous wires
2 Material and methods
We studied various Co-rich (Co67Fe3.85Ni1.45B11.5Si14.5Mo1.7,
Co67.71Fe4.28Ni1.57Si11.24B12.4Mo1.25C1.55and Co69.2Fe4.1B11.8Si13.8C1.1)
with different metallic nucleus diameter, d, and total microwire
diameter, D, produced by the modified Taylor-Ulitovsky method
described elsewhere[14,15]
We have measured the magneticfield dependence of
imped-ance, Z, and GMI ratio,DZ/Z, for as-prepared samples and after heat
treatments
We used a specially designed micro-strip sample holder
described elsewhere[14,15] The sample holder was placed inside a
sufficiently long solenoid that creates a homogeneous magnetic
field, H The sample impedance, Z, was measured using a vector
network analyzer from reflection coefficient S11
The magneto impedance ratio,DZ/Z, has been defined as:
DZ=Z ¼ ½ZðHÞ ZðHmaxÞ$100=ZðHmaxÞ; (1)
An axial DC-field with a maximum value Hmaxup to 8 kA/m was
supplied by magnetizing coils
The frequency range for the diagonal impedance component has
been measured from 1 MHz up to 7 GHz
As described above the diameter reduction must be associated
with the increasing of the optimal MI frequency range: a tradeoff
between dimension and frequency is required in order to obtain a
maximum MI effect[18] Consequently we measured GMI effect at
different frequencies
Hysteresis loops have been measured by fluxmetric method
previously used by us for similar studies[19] We represent the
normalized magnetization, M/M0versus magneticfield, H, where M
is the magnetic moment at given magnetic field and M0 is the
magnetic moment of the sample at the maximum magneticfield
amplitude, Hm The sample length was 10 cm
3 Experimental results and discussion
All Co-rich microwires present linear hysteresis loops As an
example the hysteresis loop of as-prepared Co Fe Ni B
Si14.5Mo1.7 microwires is shown Fig 1a This microwire also presents rather high GMI effect in as-prepared state (Fig 1b)
As discussed elsewhere [4e6,14,15]the magnetic field depen-dence of GMI ratio is affected b y the magnetic anisotropy When the samples present transversal magnetic anisotropy the double peaks magnetic field DZ/Z (H) dependence takes place Conse-quently the double peaks magnetic field DZ/Z (H) dependence observed for Co67Fe3.85Ni1.45B11.5Si14.5Mo1.7 microwire must be associated to the transversal magnetic anisotropy observed in
Fig 1a
Maximum GMI ratio,DZ/Zmaxz 320% can be observed varying the frequency (Fig 1b) A frequency dependence of DZ/Zmax pre-sents the frequency range where the highest DZ/Zmax can be observed (Fig 1c) For the metallic nucleus diameter, d z 10.5mm the optimum frequency for the GMI effect is about 200 MHz (see
Fig 1b)
Fig 1 Hysteresis loop (a), GMI ratio (b) and frequency dependence ofDZ/Z max (c) measured in Co Fe Ni B Si Mo microwire with d z 10.5mm.
A Zhukov et al / Journal of Science: Advanced Materials and Devices 1 (2016) 388e392
Trang 3The origin of the maximum on DZ/Zmax (f) dependences has
been discussed elsewhere[14,20,21] There are few reasons for the
optimum frequency for theDZ/Zmax
i) It might be understood from the following: as can be seen
from Fig 1b the magneticfield of the GMI peaks, Hm,
in-creases with frequency, f Therefore if the frequency is high
enough and the maximum applied DC magneticfield, Hmaxis
fixed (in our case Hmax¼ 25000 A/m), Hmbecomes
compa-rable with Hmax Consequently for a given value of maximum
applied magnetic field, Hmax, the optimum frequency for
highest GMI ratio takes place
ii) On the other hand, Hmvalue is associated with the magnetic
anisotropyfield and therefore one can expect different Z/Zmax
(f) dependences for microwires with differentr-ratios
iii) As recently discussed the decreasing of the GMI effect at high
frequencies can be explained considering that the magnetic
structure and the anisotropy can be different inside the
microwire and near the surface This difference can be
attributed to the existence of the interfacial layer between the
metallic nucleus and glass coating recently reported for
glass-coated microwires[20] Indeed if the chemical composition of
the interfacial layer is considerably different from the metallic
nucleus than the effective magnetic anisotropy field and
magnetic permeability closer to the surface can be different
from those in the inner part of metallic nucleus[20]
It is worht mentioning that the penetration depth, d, of
conductor decreases increasing the AC current frequency Therefore
the impedance values at H ¼ 0 increase with increasing of the AC
current frequency
As mentioned above, fromDZ/Z(H) dependences it is possible to
estimate the penetration depth at different frequencies using the
model previously described in ref.[17]considering that the changes
in the real component of the impedance are related to changes in
the effective area where the AC-currentflows as a consequence of
the skin-effect In this model the penetration depth,d, as a function
of the ratio RDC/RAC(RDCis the DC-resistance of the wire, and RACis
the real component of the impedance), can be expressed as:
d¼ rh1 ð1 RDC=RACÞ1=2i; (2)
where r is the wire radius
Consequently we measured DZ/Z(H) dependences for various
Co-rich microwires at elevated frequencies and tried to estimate
thed(H) dependences
Thus for all studied samples (Co67Fe3.85Ni1.45B11.5Si14.5Mo1.7,
Co67.71Fe4.28Ni1.57Si11.24B12.4Mo1.25C1.55 and Co69.2Fe4.1B11.8
-Si13.8C1.1)) a considerable GMI effect is observed for GHz frequencies
(seeFigs 2e4)
Obtainedd(H) dependences demonstrate that at high frequencies
the minimum penetration depth of studied microwires depends on
the metallic nucleus diameter, d, and on the composition From
obtainedd(H) dependences we evaluatedd-values and dependence
of minimumd-values on frequency (seeFigs 2c, 3c and 4c)
For Co67Fe3.85Ni1.45B11.5Si14.5Mo1.7microwire (d¼ 17.4mm)dmin
is about 2.2mm (Fig 3c)
For the case of Co67.71Fe4.28Ni1.57Si11.24B12.4Mo1.25C1.55microwire
the minimum calculated penetration depth is below 0.5mm for high
frequencies (Fig 4b)
As can be appreciated from Figs 2ce4c minimum d-values,
dmin, decrease with increasing the frequency and for Co67.71Fe4.28
-Ni1.57Si11.24B12.4Mo1.25C1.55microwire at frequency, f, above 1 GHz
d ,z0.33mm (Fig 4c)
Consequently obtained minimum penetration depth for studied Co-rich microwires can be few times smaller than the microwires diameter
Used method of the penetration depth estimation is meaningful when the penetration depth,d, is smaller than the wire radius, r, or what is equivalent RAC> RDC(in fact the penetration depth should be
infinitively large at the frequency f ¼ 0), i.e at high enough fre-quencies On the other hand at high enough frequencies the mag-netic permeability decreases and therefore GMI effect in its classical interpretation must be negligible As discussed elsewhere the magnetic permeability at GHz frequencies, the magnetization rotation is strongly influenced by the gyromagnetic effect[14,15,19]
On the other hand, the penetration depth,d, of conductor de-pends on the current frequency Therefore proposed method for
Fig 2 Z(H) dependence (a), calculatedd(H) dependences (b) anddmin (f) dependence estimated for different frequencies from eq (2) for Co 69.2 Fe 4.1 B 11.8 Si 13.8 C 1.1 microwire (d ¼ 25.6mm, D ¼ 30.2mm).
A Zhukov et al / Journal of Science: Advanced Materials and Devices 1 (2016) 388e392
Trang 4evaluation of the penetration depth in which it is considered that
the changes in the real component of the impedance (in-phase
component) are due only to changes in the effective area where the
AC-currentflows as a consequence of the skin effect must be still
valid for high frequency regime
It is worth mentioning that recently we reported on observation
of the interfacial layer in Fe and Co-rich glass-coated microwires
[20]with thickness of about 0.5mm Consequently considering low
penetration depth evaluated for the case of some of studied
microwires we can assume that the interface layer can affect the
features of the GMI effect at GHz frequencies The chemical
composition and hence the magnetization and magnetostriction
coefficient in the interface layer therefore can be different from the
inner part of the magnetic nucleus Therefore the interface layer
can affect the features of the GMI effect at GHz frequencies
4 Conclusions
We measured the GMI effect and frequency dependence of the GMI ratio and evaluated frequency dependence in Co-rich micro-wires From obtained experimentally dependences of GMI ratio on magnetic field and different frequencies we estimated the skin depth and its dependence on applied magneticfield and frequency
We discussed origin of the optimum frequency for studied micro-wires considering the difference of the magnetic structure and the magnetic anisotropy inside the microwire and near the surface
Acknowledgments This work was supported by Spanish MINECO under MAT2013-47231-C2-1-P and the Russian Science Foundation under the
16-19-Fig 3 Z(H) dependence (a), calculatedd(H) dependences (b) anddmin (f) dependence
estimated for different frequencies from eq (2) of Co 67 Fe 3.85 Ni 1.45 B 11.5 Si 14.5 Mo 1.7
microwires (d ¼ 17.4mm).
Fig 4 Z(H) dependence (a), calculatedd(H) dependences (b) anddmin (f) dependence estimated for different frequencies from eq (2) of Co 67.71 Fe 4.28 Ni
1.57-Si 11.24 B 12.4 Mo 1.25 C 1.55 microwires (d ¼ 10mm, D ¼ 13.8mm).
A Zhukov et al / Journal of Science: Advanced Materials and Devices 1 (2016) 388e392
Trang 510490 grant Technical and human support provided by SGIker
(UPV/EHU, MICINN, GV/EJ, ERDF and ESF) is gratefully acknowledged
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