Magnetoelectric effects in layered samples of lead zirconium titanate and nickel films

Article abstract of DOI:10.1016/j.ssc.2008.07.015

The resonance enhancement of magnetoelectric (ME) interactions due to bending and radial modes are reported in structures consisting of nickel films deposited onto lead zirconate titanate (PZT). The ME voltage vs. frequency profiles have been obtained on samples with Ni layer thickness ranging from 1 to 100?μm. The mode frequencies and strength of ME interactions are found to increase linearly with increasing thickness of Ni. Theoretical mode frequencies are in very agreement with measured values.

Full text of DOI:10.1016/j.ssc.2008.07.015

Solid State Communications 148 (2008) 55–58
Contents lists available at ScienceDirect
Solid State Communications
journal homepage: www.elsevier.com/locate/ssc
Magnetoelectric effects in layered samples of lead zirconium titanate and
nickel films
a
a
a
b,∗
D.V. Chashin , Y.K. Fetisov , E.V. Tafintseva , G. Srinivasan
a
Moscow State Institute of Radio Engineering, Electronics and Automation, Russia
Physics Department, Oakland University, Rochester, MI 48309, USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The resonance enhancement of magnetoelectric (ME) interactions due to bending and radial modes are
reported in structures consisting of nickel films deposited onto lead zirconate titanate (PZT). The ME
voltage vs. frequency profiles have been obtained on samples with Ni layer thickness ranging from 1
to 100 µm. The mode frequencies and strength of ME interactions are found to increase linearly with
increasing thickness of Ni. Theoretical mode frequencies are in very agreement with measured values.
Received 26 June 2008
Accepted 10 July 2008 by A.H. MacDonald
Available online 18 July 2008
PACS:
7
7
7
7
7.65.Ly
5.50.Cc
2.80.Tm
2.55.+s
©
2008 Elsevier Ltd. All rights reserved.
Keywords:
A. Magnetic films and multilayers
D. Piezoelectric
D. Magnetostrictive
D. Magnetoelectric
1
. Introduction
Here we report investigations on ME effects in bilayers and
trilayers of nickel electrodeposited onto lead zirconium titanate
−6
Multilayer structures consisting of mechanically coupled fer-
(PZT) disks. Nickel has high magnetostriction, λ ∼ 30 × 10
s
romagnetic and piezoelectric layers show strong magnetoelectric
at relatively low bias fields H ∼ 50–100 Oe, and PZT has high
piezoelectric coupling, the key ingredients for strong ME effects
[4–6]. The electro deposition is of specific interest for fabrication
of Ni layers for ME structures. It allows rapid deposition of layers
(
ME) effects and are promising for use as magnetic field sensors,
solid-state electric power generators and transformers [1]. When
such a structure is subjected to a bias DC magnetic field H and
AC magnetic field h, magnetostriction induced deformations are
transmitted to the piezoelectric layers leading to generation of an
AC voltage U. The strength of the ME effect is characterized by
3
with thickness in the range of bm ∼ 1–10 µm, thus filling a
gap between different thin film deposition techniques and thick
layers of Ni bonded to PZT. Thin film methods such as thermal
evaporation, magnetron spattering, laser ablation, sol-gel are used
for ∼0.01–5 µm thick layers while epoxy bonding is convenient
only for Ni layers of thickness 0.1 mm or more. The electro
deposition is well developed and inexpensive, provides good
mechanical coupling at Ni–PZT interfaces, and allows deposition
of metals of desired thickness on surfaces of complex shape. The
method was used recently to fabricate Ni–PZT–Ni rectangular
and cylindrical structures, which have demonstrated strong ME
interactions [7–9].
the voltage coefficient α
E
= U/(b · h), where b is the structure
thickness. For structures containing layers of high magnetostric-
tion λ (transition metals Fe, Co, Ni, rare-earth alloys, nickel–zinc
and cobalt ferrites) and high piezoelectric coefficient d (lead zir-
conate titanate or lead magnesium niobate - lead titanate), the
ME coefficient at room temperature can reach values up to α
E
∼
1
V/(cm Oe), that is several orders of magnitude higher than for
single-phase multiferroics like Cr2O3 (see review [2] and refer-
ences herein). The ME coefficient increases by 1–2 orders of mag-
nitude, reaching values as high as α
E
∼ 100 V/(Oe cm), when the
AC field frequency coincides with the frequencies of electrome-
chanical resonance (EMR) corresponding to acoustical modes of
the structures [3].
2. Sample fabrication and measurements
Disks of PZT of the composition Pb0.52Zr0.48TiO3 with diameter
of D
=
25 mm and thickness of bp
=
210–220 µm were
used to fabricate ME resonators. Both surfaces of the disks were
metalized with 2 µm thick Ag layers and then they were poled in
an electrical field of 30 kV/cm applied perpendicular to the sample
∗
Corresponding author.
E-mail address: srinivas@oakland.edu (G. Srinivasan).
0
038-1098/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2008.07.015

5
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D.V. Chashin et al. / Solid State Communications 148 (2008) 55–58
Fig. 1. Schematic diagram showing: (a) Ni–PZT bilayer and (b) Ni–PZT–Ni trilayers, and (c) picture of the resonator.
plane. Electrodeposition of Ni was carried out using a solution
of NiSO4 ∗ 7H2O–240 g/l, NiCl2 ∗ 6H2O–30 g/l, H3BO3–30 g/l.
During the process the solution temperature was maintained at
◦
∼
55–60 C. Nickel was deposited on one side at a time with
the other side covered with a thin layer of insulating plastic. In
order to fabricate resonators with Ni layer thickness from 1 µm
to 100 µm, the current density was varied in the range of j =
2
1
00–400 A/m and deposition time ranged from 5 min to 4 h.
The rate of Ni deposition was ∼1.5 µm/min for a current density
2
4
00 A/m . After deposition, Ni layer thickness was determined
from deposited mass and density of Ni. Well bonded Ni layers of
thickness bm ∼ 100 µm could be deposited. Layers of higher
thickness did not bond to PZT because of internal strains arising
from difference in thermal expansion coefficients.
Bilayers (Ni–PZT) and trilayers (Ni–PZT–Ni) as in Fig. 1 with
bm = 5–57 µm were selected for detailed characterization. The
samples were placed in a DC bias magnetic field H = 0–1.2 kOe. A
pair of Helmholtz coils was used to generate an AC magnetic field
h(t) = h0 cos(2πft), with h0 = 0–10 Oe and f = 0.1–150 kHz,
parallel to H. The resonator was oriented so both the fields H and h
were directed parallel or perpendicular to its plane. The ac voltage
U(t) generated across the structure due to ME interactions was
measured as a function of H and f with a voltmeter with input
resistance >10 M and a digital oscilloscope.
Fig. 2. Measured frequency dependence of ME voltage U generated by a Ni–PZT
bilayer with Ni layer thickness 57 µm and for tangential bias field H = 110 Oe and
AC modulation field h0 = 2.5 Oe. The inset shows the profile of the low-frequency
bending resonance in an expanded scale.
3. Results
Fig. 2 shows typical frequency dependence of U(f ) measured
across a tangentially magnetized Ni–PZT bilayer with bm = 57 µm.
The data are for H
=
110 Oe and h0
=
2.5 Oe. It is seen
that the ME voltage U ∼ 1–2 mV in the whole frequency band
and resonance enhancements at specific frequencies. As it will
be shown later, the frequencies f1 and f2 correspond to bending
oscillation modes of the disk, while the frequency f3 correspond
to the radial acoustic mode. The profile for the low-frequency
bending mode on an expanded scale is also shown in Fig. 2. The
frequency response also contains a set of weak resonances which
arise probably due to nonuniformity of the structure and will not
be considered further. Similar data as in Fig. 2 were obtained for
samples with Ni layer thicknesses bm = 5–57 µm. For all cases
U(f1) is a factor of 2 or more higher than U(f3) and the voltages
increased nearly linearly with Ni layer thickness. The quality factor
for resonances was Q1 ∼ 30–120 and Q3 ∼ 60–160 and increased
linearly with Ni layer thickness. For magnetic fields perpendicular
to the disk plane, U vs. f had a similar shape as in Fig. 2 but the
maximum in U occurred at a higher bias field H, on the order of
Fig. 3. Measured frequency dependences of the voltage U(f) generated by a Ni–PZT
(
curve 1) and a Ni–PZT–Ni (curve 2) resonators with Ni layer thickness of 10 µm for
tangential fields H = 70 Oe and h0 = 2.5 Oe.
as evidenced by the absence of any maximum at f1. The voltage
U(f3) generated at radial modes in the trilayer is twice the value
for the bilayer that scales linearly with the effective thickness of
Ni in the two samples. In addition, a local maximum in the U(f )
dependence is observed for both samples at ∼1 kHz.
4
00 Oe, due to demagnetization.
Next we compare the features of U vs. f in bilayers and trilayers.
Representative data are shown in Fig. 3 for samples with PZT
thickness bp = 218 µm, Ni thickness bm = 10 µm, and for
tangential fields H = 72 Oe and h0 = 2.5 Oe. It is seen that bending
oscillations are not excited in a symmetrical Ni–PZT–Ni structure
Fig. 4 shows measured tangential magnetic field H dependence
of the ME voltage U(f1)in bilayers with PZT thickness bp = 220 µm

D.V. Chashin et al. / Solid State Communications 148 (2008) 55–58
57
Fig. 4. Measured H dependences of ME voltage U generated by Ni–PZT bilayers for
a series of Ni thickness at bending resonance frequency f1 for h0 = 2.5 Oe. The
thickness of the Ni- layer is indicated in the figure.
Fig. 5. Resonance frequencies of Ni–PZT bilayers with a diameter of 25 mm and
PZT layer thickness of 220 µm as a function of the Ni layer thickness. The lines are
calculated values.
and a series for Ni thickness. As H is increased, the increase in
U linear until a maximum is reached at a certain Hm, and then
gradually decreases as the field is increased. Such a variation
essentially tracks the slope of λ vs. H since the ME voltage U
is proportional to piezomagnetic coefficient q = ∂λ/∂H which
reaches the maximum just at Hm [6]. As Ni thickness is increased
from 5 to 57 µm, Hm increases from 70 to 110 Oe and the
maximum ME voltage increases from 25 to 67 mV. Saturation
of magnetostriction at high fields leads a near-zero value for U.
For bilayers and trilayers magnetized perpendicular to the plane,
the H-dependences of U(f1) were of similar to the data in Fig. 4,
the maximum at H ∼ 400 Oe due to demagnetization associated
with Ni.
where Ym, ρm, bm and Yp, ρp, bp are the Young’s modules, densities,
and thicknesses of magnetic and piezoelectric layers, respectively.
Resonance frequencies for the bending and radial modes
were calculated using Eqs. (1)–(3) and known values of material
10
2
parameters (for Ni: Ym = 21.5 × 10 N/m2 and ρ
m
= 8.8 ×
3
3
3
10
1
1
0
kg/m , for PZT: Yp = 7.0 × 10 N/m and ρ
p
= 7.7 ×
3
0 kg/m , Poisson coefficient ν ≈ 0.35 for both materials) and are
compared with the data in Fig. 5 for a bilayer. There is very good
agreement between estimated values and the data. Frequencies f1
and f2 of the bending modes and f3 of the radial mode increase with
increasing Ni thickness. Finally, we estimate the ME coefficient
4. Resonance frequencies and strength of ME interactions
α
E
= U/h0(bm + bp) for tangentially magnetized resonators using
the data of Figs. 2 and 4. For a sample with bp = 220 µm and
As it was mentioned earlier, dependence of the ME voltage U on
bm = 57 µm, we obtain off-resonance α ∼ 40–50 mV/(Oe cm), at
E
f shown in Fig. 2 is due to excitation of bending and radial modes
in the resonator. Fig. 5 shows the measured variation in the mode
frequencies f1, f2 and f3 with the thickness of Ni in bilayers. Next
we estimate the mode frequencies for comparison with data. The
frequencies of bending oscillations for a uniform disk are given
by [10]
the first bending mode αE1 = 970 mV/(Oe cm), and for the radial
mode αE3 = 530 mV/(Oe cm).
5. Conclusion
s
Magnetoelectric disk resonators have been fabricated by
electrodeposition of Ni layers of thickness of 1–100 µm on PZT
disks. In a bilayer, a substantial enhancement in the ME coefficient
is observed for bending oscillations at 2–3 kHz and for the
radial modes at 90–100 kHz. Such bending modes are absent in
Ni–PZT–Ni trilayers. Estimated mode frequencies are in excellent
agreement with the data. It is shown that the mode frequencies
and strength of ME interactions can be controlled by controlling
the thickness of Ni. Since the ME coefficients at resonance are as
high as αE1 ∼ 1 V/(Oe cm), the samples are potentially useful as
magnetic field sensors and transducers.
b
Y
f = αns
,
(1)
2
2
πR2 12ρ(1 − ν )
where αns is the coefficient corresponding to the mode with n
nodal diameters and s nodal circles, b is the disk thickness, R is
the disk radius, ρ is the density, Y is Young’s module, and ν is the
Poisson coefficient. The coefficients are α01 = 9.076 and α02
=
3
8.52 for modes with one node circle and with two node circles,
respectively.
The frequencies of radial oscillations for a disk with a thickness
much smaller than its diameter (b ꢀ R) are given by [3]
s
β
Y
f =
,
(2)
Acknowledgements
2
2
πR ρ(1 − ν )
where for the fundamental mode β
consisting of mechanically coupled magnetic and piezoelectric
layers, the effective density and effective Young’s module are given
by the expressions:
=
2.088. For a bilayer
The authors gratefully acknowledge help by V.O. Valdner in
resonator fabrication. The work at MIREA was supported by the
Ministry of Science and Education of Russian Federation (grants
#
1351 and #3701) and Russian Fund for Basic Research. The work
ρ
mbm + ρpbp
Ymbm + Ypbp
bm + bp
at Oakland University was supported by grants from the National
Science Foundation.
ρ =
i
Y =
,
(3)
bm + bp

5
8
D.V. Chashin et al. / Solid State Communications 148 (2008) 55–58
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