journal radial direction

					                                                                                                                          J. Am. Ceram. Soc., 84 [12] 2905–908 (2001)
                           Effect of the Magnetostrictive Layer on Magnetoelectric Properties in
                                        Lead Zirconate Titanate/Terfenol-D Laminate Composites

                                                      Jungho Ryu,*,† Shashank Priya, Alfredo Vazquez Carazo, and Kenji Uchino**
                       International Center for Actuators and Transducers, Materials Research Laboratory, The Pennsylvania State University,
                                                                                                      University Park, Pennsylvania 16802

                                                                                                                                         Hyoun-Ee Kim*
                                                  School of Materials Science and Engineering, Seoul National University, Seoul, 151-742 Korea

Magnetoelectric laminate composites of piezoelectric/magne-                           were used as a piezoelectric material and magnetostrictive mate-
tostrictive materials were prepared by stacking and bonding                           rial, respectively. The composites were manufactured by sand-
together a PZT disk and two layers of Terfenol-D disks with                           wiching and bonding a PZT disk between two layers of Terfenol-D
different directions of magnetostriction. These composites                            disks. These composites demonstrated a superior magnetoelectric
were studied to investigate (i) dependence on the magnetos-                           voltage coefficient (dE/dH      4.68 V/cm Oe) in comparison with
triction direction of the Terfenol-D disk and (ii) dependence on                      previously reported magnetoelectric composites.2–5 In previous
the direction of the applied ac magnetic field. Three different                       work, we pointed out that the most important factors for obtaining
types of assemblies were prepared by using two types of disks:                        a high magnetoelectric property were the high piezoelectric volt-
one with magnetostriction along the radial direction, the other                       age coefficient (gij) and thickness ratio of the PZT layer over the
with magnetostriction along the thickness direction. The max-                         Terfenol-D layer.
imum magnetoelectric voltage coefficient (dE/dH) of 5.90                                 In addition to their high dE/dH, the laminated magnetoelectric
V/cm Oe was obtained for a design where the composite was                             composites have a very simple structure and a relatively simple
made by two Terfenol-D layers with a radial magnetostriction                          fabrication method, i.e., bonding each disk. The laminated mag-
direction.                                                                            netoelectric composites can be easily applied to practical applica-
                                                                                      tions, such as magnetic field sensing devices, leak detectors for
                                                                                      microwave ovens, and current measurement of high-power electric
                               I.      Introduction                                   transmission systems.
                                                                                         In this study, we investigate the directional dependence of
           last three decades, many particulate1–3 and in-situ-
   grown magnetoelectric composite materials have been devel-
oped by using piezoelectric materials and magnetostrictive ferrite
                                                                                      magnetostriction of the Terfenol-D disk and of ac magnetic field
                                                                                      on the magnetoelectric response of the PZT/Terfenol-D laminate
                                                                                      composites. This helps in optimizing the performance of laminate
materials to overcome the problems of single-phase magnetoelec-                       magnetoelectric composites.
tric materials, i.e., low magnetoelectric response and requirement
of low temperature.6,7 These composite materials used the product
property of piezoelectric and magnetostrictive effects of each
phase.7,8 This can be explained as follows: the magnetostrictive                                        II.   Experimental Procedure
phase is strained when a magnetic field is applied to the composite,
                                                                                         PZT (APC– 840, American Piezoceramics, Inc., PA) is used as
and this strain induces stress on the piezoelectric phase, generating
                                                                                      a piezoelectric material for our composites. This PZT material has
an electric field in the piezoelectric phase. Thus, the product
                                                                                      a high piezoelectric voltage coefficient (g33       26.5 mV m/N).
property in such composites is the magnetoelectric effect, i.e., an
                                                                                      Regarding the magnetostrictive material, TbDyFe2 (Terfenol-D)
applied magnetic field induces an electric field. These particulate
                                                                                      giant magnetostrictive alloy (Etrema Products, Inc., Ames, IA) is
or in situ magnetoelectric composites made of piezoelectric mate-
                                                                                      one of the most ideal magnetostrictive materials for these com-
rials and magnetostrictive ferrite materials show higher magneto-
                                                                                      posites because of its high magnetostriction and coupling con-
electric properties compared with single-phase magnetoelectric
                                                                                      stants.10 To investigate the dependence of the magnetostriction
materials, such as Cr2O3.1–5 However, these composites still have
                                                                                      direction, we used two types of Terfenol-D disks: one with
some problems in reproducibility and reliability, such as control of
                                                                                      principal magnetostriction along the radial direction, the other with
the connectivity and chemical reaction between two phases, low
                                                                                      magnetostriction along the thickness direction.
electric resistivity, and an insufficient magnetoelectric voltage
                                                                                         PZT/Terfenol-D magnetoelectric composite samples were pre-
coefficient for practical applications.3
                                                                                      pared by stacking and bonding together the PZT and Terfenol-D
    Recently, laminated magnetoelectric composites made by using
                                                                                      disks with silver epoxy. Detailed fabrication procedures have been
piezoelectric and magnetostrictive materials have been studied by
                                                                                      described in a previous paper.9 The dE/dH was determined by
our group.9 Lead zirconate titanate (PZT) and Terfenol-D disk
                                                                                      measuring the electric charge generated across the sample when an
                                                                                      ac magnetic field and dc bias were applied. The magnetoelectric
                                                                                      property was measured in terms of the variation of the coefficient
                                                                                      dE/dH as a function of dc magnetic bias field. An electromagnet
    S.-I. Hirano—contributing editor                                                  (Model GMW 5403 Magnet, Power and Buckley, Inc., New
                                                                                      Zealand) was used for generating the bias field up to 0.45 T (4.5
                                                                                      kOe). The ac magnetic field of 1 kHz was applied using Helmholtz
                                                                                      coils superimposed on the dc bias field, both parallel to the sample
    Manuscript No. 188019. Received January 16, 2001; approved July 9, 2001.          thickness axis. To investigate the dependence of the applied ac
    *Member, American Ceramic Society.
    **Fellow, American Ceramic Society.                                               magnetic field direction, samarium– cobalt permanent magnets
      Visiting Researcher from Seoul National University, Seoul, Korea.               were used instead of the electric magnet for applying the dc
2906                                        Journal of the American Ceramic Society—Ryu et al.                                 Vol. 84, No. 12

magnetic bias. In this case, the applied dc magnetic bias to the
composite was 0.075 T (750 Oe).
   A signal generator (Model DS340, Stanford Research Systems,
Sunnyvale, CA) was used to drive the Helmotz coils and generate
the ac magnetic field. The electric charge generated from the
piezoelectric layer was measured through a charge amplifier
(Model 5010B Dual Mode Amplifier, Kistler Instrument Co.,
Amherst, NY). This amplifier was designed for converting a
charge signal from the piezoelectric transducer into a proportional
output voltage. The output voltage from the amplifier was mea-
sured with an oscilloscope (Model 54645A, Hewlett–Packard Co.,
Palo Alto, CA). The measured voltage represented the electric
charge from the piezoelectric PZT layer under a short-circuit
condition. The output voltage was obtained from the charge and
the capacitance of the PZT layer of the composite using V Q/C
(1 kHz). The output voltage divided by the thickness of the PZT
layer and the ac magnetic field gave the dE/dH of the samples.

                  III.   Results and Discussion                          Fig. 2. The parameter dE/dH as function of applied dc magnetic bias with
                                                                         different assembly.
(1) Magnetostriction Direction Dependence
   To explore the direction dependence of magnetostriction, three
types of laminate composites were prepared using two types of               Figure 2 illustrates the variation of the dE/dH as a function of
Terfenol-D disks. These are as follows:                                  the dc magnetic bias for three different composites. The frequency
   (1) Composite with two Terfenol-D disks, which have mag-              of the ac magnetic field was 1 kHz for the measurement. The
netostriction along the thickness direction (denoted as Comp. T-T).      dE/dH values of all composites increased with increasing dc bias
   (2) Composite with one Terfenol-D disk with thickness mag-            and saturated at 4 kOe. The Comp. R-R showed the most
netostriction direction and the other Terfenol-D disk with radial        superior magnetoelectric properties, and the maximum dE/dH was
magnetostriction direction (denoted as Comp. T-R).                       5.90 V/cm Oe under a magnetic bias 4.2 kOe dc. In the
   (3) Composite with two disks, which have magnetostriction             Terfenol-D disks, the magnitude of strain in the principal magne-
along the radial direction (denoted as Comp. R-R).                       tostriction direction was higher than in the other directions.10
   Figure 1 shows the schematic illustrations of each composite          According to the results reported in our previous work,9 the
structure (Fig. 1(a)) and a picture of the sample (Fig. 1(b)). The       piezoelectric voltage coefficient (gij), strain in the Terfenol-D
dielectric polarization direction of the PZT disk and the applied        along the radial direction, and the thickness ratio between the PZT
magnetic field direction were in the thickness direction for all the     and Terfenol-D disks are three important factors for producing a
composites. All composites were fabricated with the same thick-          superior magnetoelectric response in this type of magnetoelectric
ness (total 2.5 mm; two disks of 1 mm thick Terfenol-D and one           laminated composite. The highest dE/dH in Comp. R-R originates
disk of 0.5 mm thick PZT) and diameter (12.7 mm), which were             from high magnetostriction along the radial direction.
optimized in a previous study.9
                                                                         (2) Magnetic Field Direction Dependence
                                                                            In applications, such as magnetic field sensing devices, the
                                                                         dependence of the magnetoelectric response on the magnetic field
                                                                         direction is an important factor. To examine the magnetic field
                                                                         direction dependence, we measured the dE/dH by changing the
                                                                         applied magnetic field direction. The configuration applied for
                                                                         determining the direction dependence of the magnetic field is
                                                                         schematically represented in Fig. 3. The dc magnetic bias was

Fig. 1. (a) Schematic illustrations for three different PZT/Terfenol-D   Fig. 3. Magnetic field directions to monitor dE/dH dependence with
composites and (b) their photograph. (Ruler is in inches.)               applied ac magnetic field direction.
December 2001                      Effect of Magnetostrictive Layer on Magnetoelectricity in Pb(Zr,Ti)O3 / Terfenol-D Composites                         2907

applied in the thickness direction during all the experiments and
fixed at 750 Oe. By rotating the sample in the Helmholtz coils, the
direction of the applied ac magnetic field could be changed. The
angle of the applied ac magnetic field (1 kHz) indicated the
difference in angle between the ac magnetic field and dc magnetic
bias (thickness direction of the sample).
   The dependence of the dE/dH of the composites on the applied
ac magnetic field direction is shown in Fig. 4. The dependence of
the ac magnetic field direction exhibited similar behavior for all
the composites, with a moderate increase to 25o– 45o. Above that,
the dE/dH decreased with increasing ac magnetic field direction.
   As discussed in the previous section, magnetoelectricity is the
combined effect of magnetostriction and piezoelectricity. The
magnitude of the dE/dH is dependent on the strain produced by the
magnetostrictive material. Thus, the estimation of strain gives an
idea regarding the magnitude of the dE/dH.
   Because strain is a second-rank tensor, we need to consider six
components: three extensional strains and three shear strains. Let
the coordinate axes be defined as shown in Fig. 5(a). The strain
components can be transformed as follows on rotation of the
coordinate axes by an angle, :
    x ij          a ik a jl x kl                                           (1)
           i, j

    where alm is the direction cosine between the l and m directions,
xij the strain component after rotation, and xij the initial strain
component. In the case of rotation along the 2-axis, these strains
are given by
    x1     cos2 x1                  2 sin    cos x5     sin2 x3            (2)
    x2     x2
    x3     sin2 x1                 2 sin    cos x5     cos2 x3
   For these laminate composites, the effective strain to generate
electric charge from the PZT layer is not along the 3-axis but is
areal strain along the 1- and 2-axes. Areal strain ( A/A) is given by
            x1          x2                                                 (3)
   where A is the area of the material. In the above equation, the                  Fig. 5. Definition of coordination axes for (a) theoretical calculation and
strain components are written in the reduced notation. After                        (b) areal strain as function of applied magnetic field direction from Eqs.
performing the rotation operation with respect to the 2-axis, areal                 (4), (5), and (6).
strain is given by
            x1          x2                                                             Hence, if the strain components are known, areal strain can be
                                                                                    determined as a function of orientation. For simplicity, choosing a
                cos2 x1              2 sin    cos x5     sin2 x3   x2      (4)      polycrystalline ferromagnetic material with three axes as the
                                                                                    direction of magnetic polarization (magnetic bias) and assuming
                                                                                    that there is no externally applied stress, strain is given by11

                                                                                          x1          0      0      d 31
                                                                                          x2          0      0      d 31
                                                                                          x3          0      0      d 33
                                                                                                                           H2                              (5)
                                                                                          x4          0      d 15   0
                                                                                          x5          d 15   0      0
                                                                                          x6          0      0      0

                                                                                       where dij are the magnetostrictive constants, sometimes referred
                                                                                    to as piezomagnetic constants, and Hi the components of the
                                                                                    applied field in the ith direction. The components along the 1- and
                                                                                    3-axes of the magnetic field can be written in terms of the applied
                                                                                    magnetic field (Happ):

                                                                                                 3      3
                                                                                        H app    H1     H3

                                                                                        H1      H app sin                                                  (6)
Fig. 4. The parameter dE/dH as function of applied ac magnetic field
direction.                                                                              H3      H app cos
2908                                      Journal of the American Ceramic Society—Ryu et al.                                           Vol. 84, No. 12

   The magnitude of the constants d31          5.3    10 8, d33       showed a moderate increase up to 25°– 45° tilting. Above this
11      10 8, and d15        28      10 8 m/A are reported for        angle, the dE/dH decreased significantly. This behavior seems to
Terfenol-D in Ref. 12. Substituting the values of the strain          originate from the contribution of relatively large shear mode
components from Eq. (5) into Eq. (4), and taking into account         strains.
Eq. (6), the relation for areal strain is obtained in terms of the
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