1 Magnetoelectric Effects in Ferromagnetic Metal-Piezoelectric
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1 Magnetoelectric Effects in Ferromagnetic Metal-Piezoelectric Oxide Layered Structures U. Laletsin1, N. Paddubnaya1, G. Srinivasan2, M.I. Bichurin3 1Institute of Technical Acoustics, National Academy of Sciences of Belarus, 210717 Vitebsk, Belarus 2Physics Department, Oakland University, Rochester, Michigan, USA 3 Department of Engineering Physics, Novgorod State University, Novgorod, Russia ABSTRACT Frequency dependence of magnetoelectric (ME) coupling is investigated in trilayers of ferromagnetic alloy and piezoelectric lead zirconate titanate (PZT). The ferromagnetic phases studied include permendur, a soft magnet with high magnetostriction, iron, nickel, and cobalt. Low frequency data on ME voltage coefficient versus bias magnetic field indicate strong coupling only for trilayers with permendure or Ni. Measurements of frequency dependence of ME voltage reveal a giant ME coupling at electromechanical resonance. The ME interactions for transverse fields is an order of magnitude stronger than for longitudinal fields. The maximum voltage coefficient of 90 V/cm Oe at resonance is measured for samples with nickel or permendure and is three orders of magnitude higher than low-frequency values. 1. INTRODUCTION parallel to magnetic fields (longitudinal ME effect). The measurements were carried out at low Harshe and co-workers proposed a new frequencies (1.0 kHz) and at electromechanical direction in the development of ME composites, i.e., resonance frequency (~330 kHz). The resonance magnetostrictive-piezoelectric layerd structures [1]. frequency was determined by sample size and The primary advantages of such structures are low composite parameters. leakage currents and a high degree of polarization. The d.c magnetic field dependence of ME The use of ferromagnetic metals helps to accomplish voltage coefficient (MEVC) was measured for a very high degree of polarization compared to oxide transverse and longitudinal fields. We measured an ferromagnets [2]. This work is devoted to increase in MEVC with increasing H, followed by a investigations of ME effects in a trilayer metal- drop at high fields. The magnitude of MEVC was piezoelectric-metal structure. We used lead zirconate found to depend on the thickness and nature of the titanate (PZT) for the piezoelectric phase. The bonding medium. The maximum MEVC was thickness of PZT layers varied from 0.3 to 0.8 mm. measured for samples with PZT thickness of 0.4 mm Permendure, a Ni-Fe alloy, Fe, Ni or Co with a and 0.36 mm. Longitudinal ME effect was an order thickness of 0.18 mm was used as the ferromagnetic of magnitude smaller than transverse ME effect. The phase. Trilayers were made by bonding the PZT and maximum ME voltage coefficient obtained at low the metal disks. frequency varied from 0.2 to 0.65 V/cm Oe, The ME voltage coefficient was determined by depending on the nature of ferromagnetic phase. measuring the electric field generated across the Samples with Co yielded the lowest MEVC and the piezoelectric layer when an a.c. magnetic field and a highest are for trilayers with permendur. The d.c. magnetic field were applied to sample. There maximum ME voltage coefficient obtained at were two variants of the experiment. In the first resonance frequency was equal to 90 V/cm Oe. These configuration, the electrical polarization was results are analyzed in terms of recent theoretical perpendicular to magnetic fields (transverse ME models for ME effects in bilayers [3]. effect). In the second, electrical polarization was 2 2. Experiment thickness of 0.36 mm. For the longitudinal fields, as Trilayer structures of ferromagnetic permendur, H is increased from zero, one observes increase in iron, nickel, or cobalt and piezoelectric PZT were αE,33 to a maximum at Hm = 600 Oe. With further synthesized. Permendur (P) is a soft magnetic alloy increase in H, αE,33 decreases rapidly to a minimum. consisting of 49% iron, 49% cobalt and 2% When H is reversed, αE,33 becomes negative (a 180 vanadium. It is an ideal material for studies on ME deg. phase difference between δH and δE), but the composites due to desirable low resistivity and high field dependence remains the same as for positive H. magnetization (23.4 kG), Curie temperature (1213 For transverse fields, Fig.1 shows a similar variation K), permeability and magnetostriction (70 ppm) [4]. in the ME voltage coefficient with H as for the PZT was chosen due to high ferroelectric Curie longitudinal case but with the following departures. temperature and piezoelectric coupling constant. (i) The maximum in αE,31 occurs at a much smaller PZT, 9 mm in diameter, was first poled and then Hm of 150 Oe compared to the longitudinal case. (ii) bonded to metals with 0.01-0.03 mm thick layer of an The peak value of α is an order of magnitude epoxy. Trilayers were made with the central PZT E,31 higher than for α layer bonded to outer metal layers. For P-PZT-P, the E,33. thickness of metal layer was 0.18 mm for all the 800 Permendure-PZT-Permendur samples studied. PZT thickness varied from 0.2 mm f = 1 kHz 600 to 0.8 mm. For samples with transition metals, the ) e Transverse thickness of O m 400 c / metals and PZT was 0.4 mm. V (m 200 t For ME characterization, we measured the n e i electric field produced by an alternating magnetic c 0 ffi Longitudinal field applied to the biased composite. The samples e Co -200 were placed in a shielded 3-terminal holder and e g a t placed between the pole pieces of an electromagnet l -400 o that was used to apply the bias magnetic field H. The V E -600 required ac magnetic field of δH=0.01-1 Oe at 10 Hz M - 1 MHz parallel to H was generated with a pair of -800 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 Helmholtz coils. The induced electric field δE perpendicular to the sample plane was estimated from H (kOe) Fig.1: Variations in longitudinal and transverse the voltage δV measured with an oscilloscope. The magnetoelectric (ME) voltage coefficients with the ME voltage coefficient is estimated from α = δE/δH E bias magnetic field H for a trilayer composite of = δV/t δH where t is the thickness of PZT. The Permendur-PZT-Permendur. measurements were done for two different field orientations. With the sample plane represented by Similar measurements were made on trilayers of (1,2), the transverse coefficient αE,31 was measured Fe-PZT-Fe, Co-PZT-Co and Ni-PZT-Ni and the data for the magnetic fields H and δH along direction-1 for transverse MEVC are shown in Fig.2. From the (parallel to the sample plane) and perpendicular to δE data, one infers the presence of strong ME coupling. (direction-3). The longitudinal coefficient αE,33 is The coupling strength, however, varies with the measured for all the fields perpendicular to the nature of ferromagnetic metal, with the weakest sample plane. Magnetoelectric characterization was coupling for Co containing samples and the strongest carried out at room temperature as a function of for trilayers with Ni. One also observes unique frequency of the ac magnetic field and bias magnetic characteristics such as weak ME coupling for low field H. fields and zero-crossing at high fields for Fe-PZT-Fe. Next we compare αE values in Fig.1 and 2 with 3. Results and Discussion results for similar composites. Systems of interest in Figure 1 shows representative data on the H the past were bulk samples of ferrites with barium dependence of the longitudinal and transverse ME titanate or PZT. The αE in Fig.1 are a lot higher than voltage coefficients. The measurements were done at values reported for bulk composites of cobalt ferrite 1 kHz on a P-PZT-P trilayer sample with a PZT 3 (CFO) or nickel ferrite (NFO) with PZT or BaTiO3 [1]. Layered composites studied so far include ferrite- 70 PZT [2,3] lanthanum manganite-PZT [5] and Permendur-PZT-Permendur 60 terfenol-PZT [6]. For comparison, the highest value ) Oe for αE,31 is 60 mV/cm Oe in bilayers of lanthanum m c 50 / V manganites-PZT, 400 mV/cm Oe for NFO–PZT and ( t Transverse 4680 mV/cm Oe for Terfenol-PZT. Thus the results en 40 ci fi f in Fig.1 provide clear evidence for one of the highest e o 30 ME coupling reported for any composites. C e ag t l 20 o ) 500 100 V Ni-PZT-Ni E Oe Longitudinal M 10 m c / Fe-PZT-Fe V m ( t 250 50 0 n e 310 320 330 340 350 360 i c i Co-PZT-Co f f (kHz) ef o C 0 0 e Fig.3: Frequency dependence of transverse and ag t l longitudinal ME voltage coefficients for the P-PZT-P o V E trilayer. The bias field H was set for maximum ME M -250 -50 e s coupling (Fig.1). The resonance frequency r e sv corresponds to the electromechanical (EMR) for PZT n a r T -500 -100 in the composite. -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 H (kOe) We carried out measurements of resonance ME effect in trilayers of Fe-PZT-Fe, Co-PZT-Co and Ni- Fig.2: Similar data as in Fig.1 for Fe-PZT-Fe, Co- PZT-Ni and the results for transverse MEVC are PZT-Co and Ni-PZT-Ni. shown in Fig.4. A sharp resonance with a high Q is evident for all three samples and the resonance value We also performed studies on the frequency of αE,31 follows the trend observed at low dependence of the ME coupling.