Due to mechanically cou-pling between these two composite components, when the Terfecohan films are deformed under applied magnetic fields, the PZT plate which is poled in its thickness
Trang 1Journal of Alloys and Compounds 449 (2008) 214–218
Magnetic sensors based on piezoelectric–magnetostrictive composites
N.H Duc∗, D.T Huong Giang
Laboratory for Nano Magnetic Materials and Devices, Faculty of Physics Engineering and Nanotechnology, College of Technology,
Vietnam National University, Hanoi, E3 Building, 144 Xuan Thuy Road, Cau Giay, Hanoi, Viet Nam
Received 16 November 2005; received in revised form 6 January 2006; accepted 10 January 2006
Available online 16 January 2007
Abstract
Magnetoelectric (ME) composites have been fabricated by sandwiching a lead titanate (PZT) laminate between two magnetostrictive (Tb(Fe0.55Co0.45)1.5) (known as Terfecohan) films Giant ME effect at low fields obtained is associated to large magnetostriction as well as high magnetostrictive susceptibility of the Terfecohan films Magnetoelectric voltage coefficients,αE= (∂E/∂H), as large as 3350 and 9650 V m/kA m
were achieved, respectively, for the as-deposited, and annealed films The coefficientαEwas, however, highly dependent on the direction of the magnetic field with respect to the electrical polarization On the basic of this magnetoelectric composite, a magnetic sensor operating in an ac
magnetic field of 0.1 mT at a resonant frequency of 40 Hz has been prepared The ME voltage response in applied magnetic fields (dVME/dH) as
large as 130 mV/mT was obtained
© 2006 Elsevier B.V All rights reserved
Keywords: Magnetization; Magnetostriction; Piezoelectricity; Magnetoelasticity; Magnetic sensor
1 Introduction
The magnetoelectric (ME) effect is defined as the
dielec-tric polarization of a material in an applied magnetic field
(H) and/or and induced magnetization in an external electric
field (E) The ME effect can be used for many applications,
such as microwave field, smart sensors and signal
process-ing[1–5] Magnetoelectricity is a product property and needs
a biphasic surrounding to exhibit the complex behavior ME
effect can be observed in single phase or composite materials
In the past few years, however, extensive studies have been
conducted in composite materials due to the fact that these
materials yield a large magnitude of the ME voltage coefficient
(αE=∂E/∂H) The composites can be consisted of individual
piezomagnetic and piezoelectric phases or magnetostrictive and
piezoelectric phases Recently, Ryu et al obtained anαE-value as
high as 12,875 V m/kA m[6]in a composite using piezoelectric
PMN–PT (Pb(Mn1/3Nb2/3)O3–PbTiO3) and magnetostrictive
Terfenol-D laminates[7–9]
In this paper, a development for large ME coefficient at low
fields has been realized in ME composites using a lead titanate
∗Corresponding author Tel.: +84 4 7547203; fax: +84 4 7547460.
E-mail address:ducnh@vnu.edu.vn (N.H Duc).
(PZT) laminate and magnetostrictive Tb(Fe0.55Co0.45)1.5 (de-notes as Terfecohan) films[10–12] On the basic of this mag-netoelectric configuration, a novel magnetic sensor has been fabricated
2 Experimental
Terfecohan films with thickness tTFC = 5 m were fabricated on glass micro-scope cover-slips with nominal thickness of 150 m by rf-magnetron sputtering The ME composites were fabricated by bonding a piezoelectric PZT plate
APCC-855 (American Piezoceramics Inc., PA, USA) (thickness tPZT = 200 m) between two Terfecohan films ( Fig 1 (a)) The electrical contacts were made with silver paint and the ME composites were poled Due to mechanically cou-pling between these two composite components, when the Terfecohan films are deformed under applied magnetic fields, the PZT plate (which is poled in its
thickness direction) will undergo deformation Consequently, an electric field E (or voltage VME ) is induced across the thickness of the piezoelectric plate The
ME output voltage VME was measured directly as a response of the composite
to an ac magnetic field hac of 0.4 mT at the resonant frequency around 40 Hz
superimposed on and parallel to a dc bias field H (up to 1 T) (seeFig 1 (b)) by means of an open circuit condition using a differential amplifier based on the INA 121 FET-input Instrument Amplifier [13] The value of theαE coefficient is given byαE= VME/h0tPZT Different magnetic field orientation with respect to
the electrical polarization (i.e ϕ-angle inFig 1 (a) can be controlled by rotating composite plane.
The magnetization and magnetostriction were measured using a vibrating sample magnetometer and an optical deflectometer, respectively.
0925-8388/$ – see front matter © 2006 Elsevier B.V All rights reserved.
doi: 10.1016/j.jallcom.2006.01.121
Trang 2Fig 1 Illustration of the Terfecohan/PZT sandwich ME composite (a) and experimental setup for ME-voltage measurement (b).
3 Results and discussion
Presented inFig 2, the magnetic hysteresis loops were
mea-sured in magnetic fields applied parallel and perpendicular to
the plane of the 5m-thick Terfecohan films before and after
annealing at 350◦C It is clearly seen that these films exhibit
an out-of-plane magnetic anisotropy The magnetic anisotropy,
however, is strongly weakened after annealing (seeFig 2(a))
The perpendicular magnetic loops of both samples are almost
similar (Fig 2(b)) It turns out from this figure that the value
of the demagnetization factor equals about 0.8 This small N⊥
-value can be attributed to the strip domain[12] Magnetostriction
response in magnetic fields up to 0.6 T applied in plane,
paral-lel to the long side of the sample (i.e corresponding to paralparal-lel
magnetostriction) for the investigated Terfecohan film/glass
sub-strate/PZT plate composites is shown inFig 3 Due to the lack
of elastic parameters (e.g Young’s modulus, Poisson ratio) of
the PZT material, the absolute value of the magnetostriction is
not determined yet Here, only a scaling of the magnetostriction
is presented The magnetostriction curves imply a magnetiza-tion rotamagnetiza-tion from the out-of-plane into the film-plane direcmagnetiza-tion
In accordance to the magnetic investigation, the improvement
of the low-field magnetostrictive strain in the annealed film was also evidenced from this figure
The ME voltage coefficient induced across the PZT plate
of composites was measured at the resonant frequencies fr= 30 and 42 Hz for the composites using as-deposited and annealed
films, respectively Data of the transversal (i.e in ϕ = 90◦
con-figuration) ME voltage coefficient are shown inFig 4 It turns out from this figure that, theαEcoefficient of the investigated composites increases rapidly at low fields, and reaches a maxi-mum value ofαE= 3350 V m/kA m andαE= 9650 V m/kA m at
μ0H = 0.12 and 0.15 T for the samples using as-deposited and
350◦C-annealed Terfecohan films, respectively The observed
maximalαEvalue is comparable with that reported by Ryu et
al for bulk magnetostrictive laminates [9] In the composite
of annealed magnetostrictive film, theαE is three times higher than that of as-deposited one A faster increase at low fields
Fig 2 Room-temperature normalized parallel (a) and perpendicular (b) magnetic loops for the as-deposited and 350 ◦C-annealed Terfecohan films.
Trang 3216 N.H Duc, D.T.H Giang / Journal of Alloys and Compounds 449 (2008) 214–218
Fig 3 Parallel magnetostrictive strainσ|| of the as-deposited and 350 ◦C-annealed Terfecohan films bonded on the PZT plate Experimental arrangement is shown
in the right.
Fig 4 Transverse ME coefficient voltage as a function of dc magnetic field.
and a higher maximal value ofαE observed in this composite
can be attributed to the improvement of the
magnetostric-tive properties in annealed films Indeed, we have previously
reported for the 1m-thick Terfecohan films that the
magne-tostrictive susceptibility χ λ= 1.8× 10−2T−1 obtained in the
350◦C-annealed film is much larger than that of the as-deposited
film (χ λ= 0.23× 10−2T−1)[10–12] In more detail, however, it
is interesting to note that the variation ofαEis not fully described
by the magnetostrictive strain susceptibility (χ σ=∂σ/∂μ0H)
(see Fig 5) One observes a position shift of αE-maximum with respect to that of χ σ-maximum For the composite of as-deposited magnetostrictive films,αE reaches its maximum earlier thanχ σ The opposite shift was observed in the compos-ite of annealed magnetostrictive films This implies thatαEdoes
not depend simply on the magnetostrictive response (i.e on χ σ)
as expected, but it is also governed by the magnetoelastic energy
(i.e on λ), which is transferred from the magnetostrictive films
into the piezoelectric plate
The ME voltage coefficient αE measured under different magnetic field orientations with respect to electrical
polariza-tion (i.e with different ϕ-angle) is shown in Fig 6 for the composite of annealed Terfecohan films The highest ME was obtained for the magnetic field applied perpendicular to the electrical polarizationϕ = 90◦.αEdecreases with decreasing
ϕ-angle and exhibits a rather complex variation in both magnitude and sign Finally, a complete change in its sign with respect
toϕ = 90◦ is found at ϕ = 0◦ The different sign of ME
volt-age coefficient is related to the different strain in PZT plate
Fig 5 A comparison of the variation ofα and the magnetostrictive strain susceptibilityχ σin composites with as-deposited and annealed Terfecohan films.
Trang 4Fig 6 ME voltage coefficient measured at different directions between
elec-trical polarization and fields of composite using 350 ◦C-annealed Terfecohan
film.
When magnetic field applied parallel to the electrical
polariza-tion, the Terfecohan films will be lengthened in the polarization
direction In this case, the strain in PZT plate is compressed
that generates a negative output ME voltage Conversely, the
magnetic field applied perpendicular to the electrical
polariza-tion will cause a tensile in the PZT plate leading to a positive
voltage In general, for the angles 0◦<ϕ < 90◦, the external
magnetic field can be expressed as a sum of two components
perpendicular and parallel to the electrical polarization
direc-tion: H = H⊥+ H||= H cos ϕ + H sin ϕ Then, the strain acting
a long the electrical polarization direction and then the sign
of ME voltage will depend on the competition of these two
magnetic field components The action of H|| contributes to
a positive ME voltage, while that of H⊥ causes a negative
one
As shown inFig 7is the magnetic sensor structure fabricated
based on the piezoelectric–magnetostrictive composite and its
prototype A solenoid coil is wrapped around the composite for
generating a small alternative magnetic field perpendicular to
the electrical polarization (ϕ = 90◦) In order to make this sensor
in use, an electronic apparatus is designed It can supply an
alternative current at the frequency of 42 Hz In this case, an
ac magnetic field of about 0.1 mT can be generated A charge
amplifier part of this electronic apparatus can detect the ME
voltage
Fig 8 Field dependence of ME voltage of the magnetic sensor prototype.
Fig 8 shows the field dependence of the output ME volt-age It is seen that the output signal behavior is similar with the above mention presented inFig 4 It is worth to note that at low
fields, an extremely high ME voltage response (dVME/d(μ0H))
of about 130 mV/mT was obtained It corresponds to a sensor sensitivity of 10−3mT According to the field direction
depen-dence of the ME voltage reported inFig 6, it is able to extend the function of the fabricated sensor for both field magnitude and orientation detector Indeed, a preliminary sinus dependence of
ME voltage is found for VME(ϕ) This result will be published
elsewhere
4 Conclusions
The novel magnetoelectric composites were prepared by sandwiching a lead titanate laminate between two magnetostric-tive Terfecohan films The magnetoelectric effect depends not only on the magnetostriction response of the magnetostrictive layers, but also on their magnetoelastic energy By using the films combining large magnetostrictive susceptibility and large mag-netostriction we can develop the large magnetoelectric voltage response in low magnetic fields On the basic of this compos-ite configuration, sensors with the sensitivity of about 10−3mT
were manufactured It is rather sensitive to detect microtesla magnetic fields
Fig 7 Sensor construction: piezoelectric–magnetostrictive composite (a), sensor structure (b) and sensor prototype (c).
Trang 5218 N.H Duc, D.T.H Giang / Journal of Alloys and Compounds 449 (2008) 214–218
Acknowledgements
This work was supported by the State Program for
Nanoscience and Nanotechnology of Vietnam under the Project
811.204 and by the College of Technology, VNU under project
CN.05.03
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