Sensors and Actuators A 179 (2012) 78–82 Contents lists available at SciVerse ScienceDirect Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna Geomagnetic sensors based on Metglas/PZT laminates D.T Huong Giang ∗ , P.A Duc, N.T Ngoc, N.H Duc Department of Nano Magnetic Materials and Devices, Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University, Hanoi, E3 Building, 144 Xuan Thuy Road, Cau Giay, Hanoi, Viet Nam a r t i c l e i n f o Article history: Received 21 November 2011 Received in revised form 19 March 2012 Accepted 19 March 2012 Available online 28 March 2012 Keywords: Magnetic sensors Geomagnetic sensors Magnetoelectric effects Multiferroics a b s t r a c t A potential geomagnetic-field sensor is proposed on the basis of an optimal 2D configuration of magnetoelectric Ni-based Metglas/PZT laminates This sensor can perfectly serve to measure both the strength and the orientation of the earth’s magnetic field An incredibly high ME-voltage response of 0.871 V/Oe to the geomagnetic field with a resolution of × 10−4 Oe has been achieved for composite laminates with a size of 15 mm × mm With respect to the field inclination, an angular sensitivity of 3.86 × 10−3 V/degree and an angular resolution of 10−1 degree have been determined This simple and low-cost magnetic-field sensor is promising for applications not only as novel smart compasses and in global positioning devices, but also as magnetic biosensors © 2012 Elsevier B.V All rights reserved Introduction The principle of global positioning is based on the fact that both the strength and the inclination of the geomagnetic field is a welldefined function of the geographic position The weak geomagnetic fields, however, can only be detected with sensing devices of very high sensitivity Beside the traditional types of magnetic sensors on the basis of fluxgate, Hall effect, superconducting quantum interference and giant magnetoresistance spin valves, such a sensor could recently be realized thanks to the magnetoelectric (ME) effect [1–3] This simple, low-cost sensor, furthermore, is featured by functioning at the room temperature The ME effect has been observed in multiferroics and/or ferromagnetic-ferroelectric composites (hereafter denoted as ME materials) In these materials, an electric polarization P in the material shall respond to the applied magnetic field H, whereas a magnetization M will respond to the applied electric field E The polarization process in an ME sample as response to the external applied magnetic field shall creates an electric field of E = ˛E ·H, where ˛E (= dE/dH) denotes the magnetoelectric voltage coefficient As a result, a voltage VME = t·E (= ˛E ·t·H) appears between the surfaces of the sample of the thickness t Large magnetoelectric voltage coefficients offer potential device applications as highly sensitive magnetic-field sensors, microwave filters, transformers, and gyrators [4] ∗ Corresponding author Tel.: +84 3754 9665; fax: +84 3754 7460 E-mail address: giangdth@vnu.edu.vn (D.T.H Giang) 0924-4247/$ – see front matter © 2012 Elsevier B.V All rights reserved doi:10.1016/j.sna.2012.03.030 Regarding the high magnetoelectric voltage coefficients, multiferroic composites on the basis of magnetostrictive ferrites and rare earth-transition intermetallics have been studied intensively since the beginning of this century [2,3,5–10] In particular, operation principle, design and functioning characteristics of these new ME sensors have also been described [2,3] Values of magnetic field response (dVME /dH) as high as 0.06 × 10−3 V/Oe, 56 × 10−3 V/Oe and 13 × 10−3 V/Oe were reported for ME sensors using magnetostrictive Ni0.5 Zn0.5 Fe2 O3 ferrites [5], Terfenol-D laminates [6] and Terfecohan thin films [7], respectively In an approach to use ME sensors for the determination of ac magnetic-field strengths, Fetisov et al [8] have successfully developed a promising sensor with a sensitivity better than 10−3 Oe for milli-Hz frequency magnetic fields Furthermore, strong efforts have been undertaken to enhance the ME effects by altering the shape and the volume ratio of the piezoelectric/magnetostrictive laminates [11] or by improving the lamination process [12] In the case of micro fabrication, Greve et al [13] reported a giant ME coefficient as high as 737 V/cm Oe for (Fe90 Co10 )78 Si12 B10 -AlN thin film composites Recently, an ME sensor using Co-based Metglas/PZT laminates was designed, fabricated and characterized for determining the strengths as well as the orientations of dc- and ac-magnetic fields [3], where an MEvoltage response (dVME /dH) of × 10−3 V/Oe at low dc fields and, in particular, a response (dVME /dhac ) as high as 17 × 10−3 V/Oe at the low ac-ones was reported These findings imply a great potential for self-powered detection of low ac-magnetic fields For an optimal design of ME laminate sensors, modeling approaches have been undertaken by several research groups (e.g [14–20]) While some models [15] have taken into account the effect of the thickness ratio between the piezoelectric and D.T.H Giang et al / Sensors and Actuators A 179 (2012) 78–82 79 Fig Schematic of the bilayer (a) and sandwich (b) Ni-based Metglas/PZT composite configuration Vector Hdc , hac and P shows the applied magnetic fields and the electrical polarization direction, respectively magnetostrictive phases, other finite-element magnetostatic simulations [18–20] have considered the role of the magnetostrictive length on the magnetic field concentration in ME sensors Although more appropriate descriptions of some specific aspects of sensor’s behavior seem still necessary in the models, these results have demonstrated a useful approach to significantly enhance the sensitivity of magnetostrictive/piezoelectric laminates as geomagnetic field sensors In this paper, a potential geomagnetic sensor is presented by optimizing the 2D configuration of the magnetoelectric Ni-based Metglas/PZT laminates At low dc-magnetic fields, a huge MEvoltage response as high as 0.871 V/Oe was obtained for the sensor with size of 15 mm × mm The sensor is promising not only for applications in novel smart compasses and global positioning devices, but also in magnetic biosensors Results and discussion 3.1 Shape and size dependence of the resonant frequency Fig shows the ac-magnetic-field-frequency dependence of the ME coefficient ˛E measured under a fixed bias dc-magnetic field of Oe for the investigated square-shaped (r = 1) bilayer composite laminates of different sizes of mm × mm, 10 mm × 10 mm, 12 mm × 12 mm and 15 mm × 15 mm The results show that with the increasing laminate size, the resonance is shifted toward lower frequencies (fr ), whereas ˛E significantly increases The observed phenomena can be described in term of the vibrating plates, in which one of the natural frequencies (fnm ) of the modes is obtained from the solution to the two-dimensional wave equation in Cartesian coordinates [21]: fnm = Experimental The ME magnetic-field sensor was fabricated by bonding an out-of-plane polarized piezoelectric PZT plate with magnetostrictive laminates For this purpose, the 200-m-thick PZT plate (APCC-855) of American Piezoceramics Inc., PA, USA was used The magnetostrictive laminates were cut from the 18-m-thick Fe76.8 Ni1.2 B13.2 Si8.8 melt-spun ribbons (also called Ni-based Metglas) in different sizes according to the length-to-width ratio (r = L/W), with r = 1, 1.5, 3, 7.5 and 15, and used for the various samples in this work It is worth to note that the Ni-based Metglas ribbon is a soft magnetostrictive material with a magnetostriction coefficient ( ) of about 70 × 10−6 and a magnetostrictive susceptibility ( = d /dH) of 1.5 × 10−6 Oe−1 Thanks to the mechanic coupling between the components, the PZT plate undergoes a forced strain induced by the magnetostrictive layers under the inplane (and/or out-of-plane) applied magnetic field In this case, the ME-voltage VME is induced across the thickness of the piezoelectric plate Fig presents the configuration of such fabricated bilayer Metglas/PZT and sandwich Metglas/PZT/Metglas ME composite laminates In the sample configurations under investigation, a linear electric polarization P is induced by a weak ac magnetic field hac (= ho sin(2 fo t)) oscillating at the resonant frequency in the presence of a dc bias field H and the ME voltage VME is directly measured as a response of the ME composite to the applied magnetic field In the experimental setup, the bias field H was provided by an electromagnet, and the oscillating field with amplitudes of hac = 10−2 Oe was generated by a Helmholtz coil The output voltage (VME ), induced across the PZT layer of the ME laminate by the ac field (hac ) was measured on a commercial DSP lock-in amplifier (Model 7265 of Signal Recovery), which simultaneously controlled the input current to the Helmholtz coil The value of the ˛E coefficient is derived then from the equation: ˛E = VME /hac ·tPZT v n2 m2 + 2, L W with v as the wave velocity in the PZT, n and m as integer numbers (1, .) Indeed, the experimental results are well fitted with a fundamental frequency f11 (i.e with n = m = 1) (see Fig 3) From this description, the phase velocity turns out to be of 2800 m/s for PZT This finding is consistent with that reported for the piezoelectric bulk material For sandwich Metglas/PZT/Metglas structures, the ME effect can be remarkably increased, while the resonant frequency exhibits no change As regards to the shape effect, in this paper, rectangular composite laminates with different length to width ratios were investigated For the fabrication of the investigated samples, its longitudinal edge (the length L) was kept fixed at 15 mm while its transversal edge (the width W) was varied from to 15 mm, Fig ME coefficient as a function of the ac magnetic field frequency for squareshaped samples 80 D.T.H Giang et al / Sensors and Actuators A 179 (2012) 78–82 Fig The magnetic field dependent of ME coefficient for mm × mm, 12 mm × 12 mm and 15 mm × 15 mm bilayer square-shaped samples Fig Resonant frequency vs (1/W ) + (1/L2 ) for square-shaped bilayer com- posites of different sizes L × W so that a series of rectangular laminate samples was obtained with the respective length/width ratios (r = L/W) varying from 15 to The respective values of the resonant frequency fr , obtained for these samples are shown in Fig for the case the magnetic fields are applied along the length of samples It is interesting that, except the square-shaped sample (with r = 1), all composite laminates exhibit an invariant fr of about 100 kHz, which is nearly 1.5 times lower than that observed for the sample with r = By using the above extracted wave velocity, the measured resonant frequencies of the rectangular composite laminates are well fitted with the fundamental frequencies of the one-dimensional wave equation f10 = v/2L In this case, the resonant frequency is ascribed as mainly governed by the longitudinal length of the sample 3.2 Shape and size dependence of the ME coefficient Fig shows the bias-field dependence of the ME coefficient for the different investigated square-shaped samples measured at the resonant frequencies As can be seen, for all samples the magnetoelectric coefficient exhibits a similar behavior: it initially increases at low applied magnetic fields, reaches a maximum value Fig Resonant frequency vs the ratio of the length to the width r (= L/W) for bilayer ME composites The fitted line f10 = v/2L is shown at a certain magnetic field (denotes as the optimal field for the maximal ME response) and then decreases with further increasing magnetic field It is apparent that the value of ME coefficient is strongly influenced by the sample size: the larger the interfacial area (i.e the sample size), the lower the optimal magnetic field, the higher the ME voltage coefficient and, consequently, the higher the initial slope at low magnetic fields is found for the ˛E (H) curves This observation can be understood in term of the so-called “shear lagging” edge effect [22] Furthermore, a huge ME coefficient of 75.9 V/cm Oe is found at low bias field of only 10 Oe in the composite laminate with r = By using the sandwich Metglas/PZT/Metglas laminate structures, the ME effect can increase up to ˛E = 132.1 V/cm Oe Although this value is about times lower than the highest ME coefficient reported for (Fe90 Co10 )78 Si12 B10 AlN thin film by Greve et al [13], the composite laminates fabricated with a simple and low-cost technology in this work suggest a very promising application in practical sensors With the motivation to further enhance the low-field ME voltage coefficient, rectangular-shape composite laminates with various length/width ratios have been prepared and investigated This motivation is based on the fact that the enhancement of magnetoelectric softness is related to the shape anisotropy Fig shows the ME coefficient ˛E as a function of the dc-magnetic field strength for sandwich composite laminates of different sizes as 15 mm × 15 mm, 15 mm × mm and 15 mm × mm, corresponding to the respective length/width ratios of r = 1, and 15 The measurements were carried out with the magnetic fields applied Fig The ME coefficient as a function of bias magnetic field for rectangular-shaped sandwich Metglas/PZT/Metglas composites of different sizes 15 mm × 15 mm, 15 mm × mm and 15 mm × mm D.T.H Giang et al / Sensors and Actuators A 179 (2012) 78–82 81 Fig The output ME voltage as a function of bias magnetic field for sensor prototype The fitted curve is included Fig ME coefficient measured at Hdc = Oe (open square) and maximum ME coefficient (close circle) as a function of the length to the width ratio (L/W) along the length of the sample The results show that the maximal ME coefficient is significantly unchanged in samples with high r values (remaining significantly at 131 V/cm Oe, obtained for the sample with r = 15, see Fig 7) The optimal magnetic field for the maximal ME response, however, strongly decreases from 21 Oe in the sample with r = to Oe in the sample with r = 15 Consequently, the much higher initial slope at low-magnetic fields of the ˛E (H) curves is observed This important behavior is illustrated in Fig with the data measured in an applied field of Oe The highest ME coefficient of 62.61 V/cm Oe has been found in the sample with length to width ratio r = 15 As will be presented in the next section, for practical geomagnetic sensor applications, the optimal size of 15 mm × mm (i.e r = 15) could be chosen for sensor prototypes 3.3 Geomagnetic sensor prototype Fig shows photographs of the ME composite laminates and a geomagnetic sensor prototype fabricated using an ME composite laminate with optimal rectangular size of 15 mm × mm A solenoid coil is wrapped around the ME composite laminate to generate the ac-magnetic field at the resonant frequency The effective field is by this way aligned in-plane and along the length of the ribbons (i.e perpendicular to the electrical polarization of the PZT plate) For testing the sensor operation in the range of the geomagnetic field strength, a Helmholtz coil supplied by a Keithley 230 current source was used to generate the bias magnetic fields in the range up to 1.5 Oe with the accuracy of 10−5 Oe Shown in Fig is the ME voltage response to the external magnetic field As can be seen, a linear variation of the MEvoltage with the external magnetic field has been found in the field range up to 1.0 Oe From this result, the sensor sensitivity could be derived as high as 0.871 V/Oe In a more detailed analysis, the field resolution of × 10−4 Oe has been estimated Surprisingly, the present ME-based sensor exhibits a sensitivity, which is two orders of magnitude higher than that previously reported for similar magnetic-sensor devices and is comparable with that of available commercial geomagnetic sensors [23] This configuration presents a good combination of the excellent magnetic softness of Ni-based Metglas ribbons and the effects of the shape anisotropy This sensor enables to detect not only the geomagnetic fields, but also the magnetic fields of magnetic micro- and nano-beads in biochip applications Regarding the application of the proposed sensor in determining the orientation of the Earth’s magnetic field, another experimental setup is illustrated in Fig 10(a) Shown in Fig 10(b) is the sensor output voltage as a function of the ϕ-angle between the sensor axis, i.e the axis in the plane along the length of the laminate The zero angle (ϕ = 0◦ ) is defined when the sensor is in such a position that its axis is aligned parallel to the Earth’s North magnetic Pole It is clearly seen from this figure that by rotating the sensor in horizontal plane from ϕ = 0–360◦ , the sensor signal varies periodically with ϕ, reaching a maximum value of 356 mV in the parallel Fig Sensor construction: Ni-based Metglas/PZT 15 mm × mm laminates (a) and sensor prototype where the coil generating an ac field directly wraps around the ME laminate (b) 82 D.T.H Giang et al / Sensors and Actuators A 179 (2012) 78–82 Fig 10 Experimental setup for measuring orientation of the Earth’s magnetic field (a) and the output signal as a function of ϕ-angle between the sensor’s longitudinal axis and the Earth’s North magnetic Pole (b) alignments (i.e ϕ = and 180◦ ) and vanishing in the perpendicular alignments (i.e ϕ = 90 and 270◦ ) of the sensor axis with respect from the North-South direction of the Earth’s magnetic field This finding suggests that the fabricated sensor can be used for detecting both the strength and the orientation of the geomagnetic field Concluding remarks The composite laminate configuration combining high performance Ni-based Metglas ribbons and piezoelectric PZT plates has brought by an optimal giant magnetoelectric effect with a significant ME coefficient in the low magnetic field range A potential geomagnetic-field sensor is prepared on the basis of the optimal laminate configuration The sensor can detect precisely not only the strength, but also the orientation of the Earth’s magnetic field A high sensibility of 0.871 V/Oe and a resolution in the order of 10−4 Oe without amplification make this configuration a potential sensor for applications in novel smart compasses and global positioning devices Acknowledgements This work was supported by Vietnam National University, Hanoi under the granted Research Project QG 09.29, by the NAFOSTED of Vietnam under the Research Project Number 103.02.86.09 and by the National Program for Space Technology of Vietnam The authors thank Assoc Prof Dr N.T Hien from the VNU University of Engineering and Technology for critical reading of the manuscript References [1] J Zhai, S Dong, Z Xing, J Li, D Viehland, Geomagnetic sensor based on giant magnetoelectric effect, Appl Phys Lett 91 (2007) 123513 [2] N.H Duc, D.T Huong Giang, Magnetic sensors based on piezoelectric–magnetostrictive composites, J Alloys Compd 449 (2008) 214 [3] D.T Huong 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Dover Publications, New York, 1985 [22] C.M Chang, G.P Carman, Modeling shear lag and demagnetization effects in magneto-electric laminate composites, Phys Rev B 76 (2007) 134116 [23] M Johnson, Magnetoelectronics, Elsevier, Amsterdam, 2004 Biographies D.T Huong Giang received her PhD degree in Physics from the Rouen University, France in 2005 In 2006, she joined the Faculty of Engineering Physics and Nanotechnology at VNU University of Engineering and Technology, Vietnam National University, Hanoi, where she is currently an assistant professor Her research interests include magnetostrictive, magnetoresistance, magnetoelectric and multiferroics materials, sensors and microsystems P.A Duc received his BSc and MSc degree in Physics from Hanoi National University in 2004 and 2007, respectively He is currently working on his PhD dissertation in the area of magnetoelectric composites and applications N.T Ngoc has studied at the University of Engineering and Technology, Vietnam National University, Hanoi and is finishing her master in Nanotechnology She is developing 3D-sensor for geomagnetic applications N.H Duc joined the Cryogenic Laboratory, University of Hanoi as researcher after his graduation from the same group in 1980 He obtained his doctor degree in the same group in 1988 He has then received the French Habilitation in Physics at the Joseph Fourier University of Grenoble in 1997 and became a full professor of the College of Technology (now VNU University of Engineering and Technology), Vietnam National University, Hanoi in 2004 His extended research includes various aspects of magnetism, such as: 4f–3d exchange interactions; giant magnetovolume, magnetostrictive, magnetoresistive and magnetocaloric effects; magnetic phase transition; magnetic nanostructures; multiferroics; MERAM and biochips ... piezoelectric plate Fig presents the configuration of such fabricated bilayer Metglas/ PZT and sandwich Metglas/ PZT /Metglas ME composite laminates In the sample configurations under investigation, a linear... magnetostrictive/piezoelectric laminates as geomagnetic field sensors In this paper, a potential geomagnetic sensor is presented by optimizing the 2D configuration of the magnetoelectric Ni -based Metglas/ PZT laminates At. .. comparable with that of available commercial geomagnetic sensors [23] This configuration presents a good combination of the excellent magnetic softness of Ni -based Metglas ribbons and the effects