Journal 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 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(Fe0.55 Co0.45 )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 coefficient αE was, 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 Introduction The magnetoelectric (ME) effect is defined as the dielectric 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 processing [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 an αE -value as high as 12,875 V m/kA m [6] in a composite using piezoelectric PMN–PT (Pb(Mn1/3 Nb2/3 )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 7547203; fax: +84 7547460 E-mail address: ducnh@vnu.edu.vn (N.H Duc) 0925-8388/$ – see front matter © 2006 Elsevier B.V All rights reserved doi:10.1016/j.jallcom.2006.01.121 (PZT) laminate and magnetostrictive Tb(Fe0.55 Co0.45 )1.5 (denotes as Terfecohan) films [10–12] On the basic of this magnetoelectric configuration, a novel magnetic sensor has been fabricated Experimental Terfecohan films with thickness tTFC = m were fabricated on glass microscope 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 coupling 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 T) (see Fig 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 the αE coefficient is given by αE = VME /h0 tPZT Different magnetic field orientation with respect to the electrical polarization (i.e ϕ-angle in Fig 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 N.H Duc, D.T.H Giang / Journal of Alloys and Compounds 449 (2008) 214–218 215 Fig Illustration of the Terfecohan/PZT sandwich ME composite (a) and experimental setup for ME-voltage measurement (b) Results and discussion Presented in Fig 2, the magnetic hysteresis loops were measured in magnetic fields applied parallel and perpendicular to the plane of the m-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 (see Fig 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, parallel to the long side of the sample (i.e corresponding to parallel magnetostriction) for the investigated Terfecohan film/glass substrate/PZT plate composites is shown in Fig 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 magnetization rotation from the out-of-plane into the film-plane direction 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◦ configuration) ME voltage coefficient are shown in Fig It turns out from this figure that, the αE coefficient of the investigated composites increases rapidly at low fields, and reaches a maximum value of αE = 3350 V m/kA m and αE = 9650 V m/kA m at μ0 H = 0.12 and 0.15 T for the samples using as-deposited and 350 ◦ C-annealed Terfecohan films, respectively The observed maximal αE value is comparable with that reported by Ryu et al for bulk magnetostrictive laminates [9] In the composite of annealed magnetostrictive film, the αE is three times higher than that of as-deposited one A faster increase at low fields Fig Room-temperature normalized parallel (a) and perpendicular (b) magnetic loops for the as-deposited and 350 ◦ C-annealed Terfecohan films 216 N.H Duc, D.T.H Giang / Journal of Alloys and Compounds 449 (2008) 214–218 Fig Parallel magnetostrictive strain σ || of the as-deposited and 350 ◦ C-annealed Terfecohan films bonded on the PZT plate Experimental arrangement is shown in the right Fig Transverse ME coefficient voltage as a function of dc magnetic field and a higher maximal value of αE observed in this composite can be attributed to the improvement of the magnetostrictive properties in annealed films Indeed, we have previously reported for the m-thick Terfecohan films that the magnetostrictive susceptibility χλ = 1.8 × 10−2 T−1 obtained in the 350 ◦ C-annealed film is much larger than that of the as-deposited film (χλ = 0.23 × 10−2 T−1 ) [10–12] In more detail, however, it is interesting to note that the variation of αE is not fully described by the magnetostrictive strain susceptibility (χσ = ∂σ/∂μ0 H) (see Fig 5) One observes a position shift of αE -maximum with respect to that of χσ -maximum For the composite of as-deposited magnetostrictive films, αE reaches its maximum earlier than χσ The opposite shift was observed in the composite of annealed magnetostrictive films This implies that αE does not depend simply on the magnetostrictive response (i.e on χσ ) 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 polarization (i.e with different ϕ-angle) is shown in Fig for the composite of annealed Terfecohan films The highest ME was obtained for the magnetic field applied perpendicular to the electrical polarization ϕ = 90◦ αE decreases with decreasing ϕangle and exhibits a rather complex variation in both magnitude and sign Finally, a complete change in its sign with respect to ϕ = 90◦ is found at ϕ = 0◦ The different sign of ME voltage coefficient is related to the different strain in PZT plate Fig A comparison of the variation of αE and the magnetostrictive strain susceptibility χσ in composites with as-deposited and annealed Terfecohan films N.H Duc, D.T.H Giang / Journal of Alloys and Compounds 449 (2008) 214–218 Fig ME voltage coefficient measured at different directions between electrical polarization and fields of composite using 350 ◦ C-annealed Terfecohan film When magnetic field applied parallel to the electrical polarization, 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 polarization 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 direction: 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 in Fig is 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 217 Fig Field dependence of ME voltage of the magnetic sensor prototype Fig shows the field dependence of the output ME voltage It is seen that the output signal behavior is similar with the above mention presented in Fig It is worth to note that at low fields, an extremely high ME voltage response (dVME /d(μ0 H)) of about 130 mV/mT was obtained It corresponds to a sensor sensitivity of 10−3 mT According to the field direction dependence of the ME voltage reported in Fig 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 Conclusions The novel magnetoelectric composites were prepared by sandwiching a lead titanate laminate between two magnetostrictive 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 magnetostriction we can develop the large magnetoelectric voltage response in low magnetic fields On the basic of this composite configuration, sensors with the sensitivity of about 10−3 mT were manufactured It is rather sensitive to detect microtesla magnetic fields Fig Sensor construction: piezoelectric–magnetostrictive composite (a), sensor structure (b) and sensor prototype (c) 218 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 References [1] [2] [3] [4] T.H O’Dell, Electron Power 11 (1965) 266 L.P.M Bracke, R.G van Vliet, Int J Electron 51 (1981) 255 B.J Linch, H.R Gallantree, GEC J Res (1990) 13 M.I Bichurin, V.M Petrov, R.V Petrov, Y.U.V Kiliba, F.I Bukashev, A.Y.U Smirnov, D.N Eliseev, Ferroelectrics 280 (2002) 199 [5] Y Fetisov, A Bush, K Kamentsev, A Ostashchenko, G Srinivasan, Sensors, 2004, Proc IEEE (2004) 1106 [6] The CGS unit for ME voltage coefficient: mV/cm Oe = (1/0.8) V m/kA m [7] J Ryu, A.V Carazo, K Uchino, H.-E Kim, J Electroceram (2001) 17 [8] J Ryu, S Priya, K Uchino, H.-E Kim, J Electroceram (2002) 107 [9] J Ryu, S Priya, K Uchino, H Kim, D Viehland, J Kor Ceram Soc (2002) 813 [10] N.H Duc, J Magn Magn Mater 242–245 (2002) 1411 [11] N.H Duc, P.E Brommer, in: K.H.J Buschow (Ed.), Handbook of Magnetic Materials, vol 14, North-Holland, Amsterdam, 2002, p 89 (Chapter 2) [12] T.M Danh, N.H Duc, H.N Thanh, J Teillet, J Appl Phys 87 (2000) 7208 [13] A.V Arazo, PhD Thesis, Universidad Polit`ecnica de Catalunya, Spain, 2000 ... two magnetic field components The action of H|| contributes to a positive ME voltage, while that of H⊥ causes a negative one As shown in Fig is the magnetic sensor structure fabricated based on. .. 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 polarization... polarization direction: 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