Sensors and Actuators A: Physical

Sensors and Actuators A 161 (2010) 266 270 Contents lists available at ScienceDirect Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna Magnetic force memory effect using a magnetostrictive material and a shape memory piezoelectric actuator composite Takeshi Morita, Tomoya Ozaki Department of Human Environmental Studies, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan article info abstract Article history: Received 22 December 2009 Received in revised form 26 February 2010 Accepted 11 March 2010 Available online 18 March 2010 Keywords: Magnetostrictive material Shape memory piezoelectric actuator Composite Imprint electrical field A magnetic force memory effect was realized by applying a pulse voltage to a composite structure comprising a magnetostrictive material and a shape memory piezoelectric actuator. The latter was produced by controlling the electrical imprint field, and the magnetostrictive material attached to the shape memory piezoelectric actuator maintained a certain permeability value. This composite structure was mounted into a magnetic circuit together with a permanent magnet. By applying a pulsed voltage to the actuator, the magnetic force from the permanent magnet was modified due to a change in the permeability. Following pulse operation, two distinct magnetic forces could be maintained without electrical input. 2010 Published by Elsevier B.V. 1. Introduction Recently, we have proposed a shape memory piezoelectric actuator based on control of the imprint electrical field [1 6]. This actuator exhibited a shape memory effect when subjected to a pulsed voltage [1 5]. Usually, conventional piezoelectric actuators require a DC voltage to maintain certain positions and the polarization is not reversed. This is because even if the polarization is reversed, the piezoelectric displacement returns to its previous value. However, if the piezoelectric material has an imprint electrical field, it acquires a shape memory effect, and it can be operated with a pulse-shaped voltage to reverse the polarization. Magnetic actuators, such as stepping motors, voice coil motors and solenoids are widely utilized in practical applications. However, there are difficulties in miniaturizing such actuators due to their complicated coil structure. Furthermore, the magnetic coils are operated under current flow, which results in Joule heating problems, and high response is restricted due to large inductive impedance. It is well known that Joule heating accounts for most of the energy loss in magnetic actuators. In recent years, magnetostrictive materials that exhibit giant magnetostriction (over 1000 ppm) have been produced, such as Terfenol-D (Tb x Dy 1 x Fe 2 ), and various applications of these materials have been investigated [7,8]. A voltage controllable mechanism of magnetic force was proposed by using such a material in Corresponding author. Tel.: +81 4 7136 4613. E-mail address: morita@k.u-tokyo.ac.jp (T. Morita). combination with a piezoelectric material and a permanent magnet [9,10]. In these studies, the shape change induced by the piezoelectric actuator is utilized to change the permeability of a magnetostrictive material. The permeability change results in a change in the magnetic flux density from a permanent magnet. This principle has been applied to realize voltage control of magnetic flux density in the absence of a coil; however, to maintain a certain magnetic flux density, a continuous voltage supply to the piezoelectric actuator is required. We propose a composite structure that contains a shape memory piezoelectric actuator and a magnetostrictive material. The magnetic permeability of the magnetostrictive material is controllable by means of the strain present in the material. By using a magnetostrictive-shape memory piezoelectric actuator composite with a permanent magnet, the magnetic flux density can be controlled by a voltage applied to the shape memory piezoelectric actuator. Thereby, a coil-less structure based on voltage rather than current operation becomes possible. By attaching the shape memory piezoelectric actuator to a magnetostrictive material, the magnetic permeability can be controlled with a pulsed voltage and the memory effect can be maintained in the absence of an applied voltage. In a previous fundamental study, we have already confirmed this permeability memory effect using a similar composite [11]. In this composite, a piezoelectric plate actuator was used as the shape memory actuator; therefore its strain memory was quite small. In the previous experimental setup, it was difficult to evaluate the magnetic memory force because the magnetic circuit was closed. For this reason, the magnetic flux memory effect was examined using a Hall-effect sensor. However, by using the improved magnetic circuit structure, the magnetic force effect 0924-4247/$ see front matter 2010 Published by Elsevier B.V. doi:10.1016/j.sna.2010.03.025

T. Morita, T. Ozaki / Sensors and Actuators A 161 (2010) 266 270 267 Fig. 1. Principle of memory effects induced by control of the imprint electrical field. could be directly investigated in this study. In addition, using a multilayered piezoelectric actuator, a large memory effect could be realized. By combining this type of composite with a permanent magnet, it is expected that an innovative magnetic actuator with a magnetic memory effect can be developed for practical use. 2. Principle 2.1. Shape memory piezoelectric actuator We have already demonstrated a shape memory piezoelectric actuator operated by a pulse-shaped voltage [1 5]. This approach is far different from that used in conventional piezoelectric actuators. The purpose is to achieve an asymmetric piezoelectric strain curve by means of an imprint electrical field. Usually, the piezoelectric actuator has a completely symmetric butterfly curve, and has no memory effect even if the polarization is reversed. However, with an imprint electrical field, asymmetric butterfly piezoelectric curves were observed [4,5]. The imprint electrical field is an internal electrical field present in ferroelectric materials, and is a well-known phenomenon in ferroelectric thin films; however its detailed origin has yet to be clarified. The principle of the memory effect is shown in Fig. 1. Ifan imprint electrical field exists, the D E hysteresis characteristics of ferroelectric materials shift along the electrical field axis and become asymmetric. With an asymmetric butterfly curve, the shape memory piezoelectric actuator has two different stable strain values at 0 V, depending on the direction of polarization. This asymmetric feature affects not only the shape memory, but also the permittivity, optical properties, and so on. Because the shape memory piezoelectric actuator has two stable states in the absence of electrical input, it does not require any voltage to maintain its state. To switch the state, a pulsed voltage is used to reverse the polarization. Following this, no electrical energy is consumed to maintain its state. 2.2. Magnetic flux density memory effect Fig. 2. (a) Extension and (b) contraction conditions of the composite model. The permeability of magnetostrictive materials can be controlled by their stress. In the proposed device, the magnetostrictive material is bonded to the shape memory piezoelectric actuator, and mechanical stress is induced by the piezoelectric shape memory. A permanent magnet was used as the source of the magnetic flux to the magnetostrictive material. In this study, the magnetic circuit to convert magnetic flux into

268 T. Morita, T. Ozaki / Sensors and Actuators A 161 (2010) 266 270 Fig. 3. Magnetostrictive-shape memory piezoelectric actuator composite. magnetic force was fabricated as shown in Fig. 2(a) and (b). In this system, the magnetostrictive material can be controlled by a voltage applied to the shape memory piezoelectric actuator. The magnetic flux from the permanent magnet flows along two flux paths in the magnetostrictive material and the outer yoke. When a positive voltage is applied, the magnetostrictive material undergoes an expansion driven by the shape memory piezoelectric actuator, as shown in Fig. 2(a). In this condition, the permeability of the magnetostrictive material becomes large due to the strain present. Therefore, a large amount of the magnetic flux from the permanent magnet is directed to the magnetostrictive material. As a result, the magnetic force to attract the outer yoke becomes small. In contrast, when a negative voltage is applied to the actuator, the magnetostrictive material contracts and the magnetic force becomes large (Fig. 2(b)). Thus, depending on the polarity of the voltage to the actuator, two different conditions can be set, and in the absence of electrical power, two different magnetic forces can be maintained. 3. Experimental setup 3.1. Fabrication process of the magnetostrictive-shape memory piezoelectric composite A multilayered lead zirconate titanate (PZT) actuator (NEC Tokin Co. Ltd., AE0505D16F) was used in the experiments. Its dimensions were 5 mm 5mm 20 mm, and the thickness of each PZT layer was 0.1 mm. The total number of layers was 130. To produce the memory effect, an imprint electrical field was applied to the actuator by means of a 400 DC voltage to the driving electrode at 110 C for 3 h in an electric oven (Yamato Co. Ltd., DKN302). The shape memory piezoelectric actuator and the Terfenol-D magnetostrictive material (Etrema Products Inc., Tb 0.3 Dy 0.7 Fe 1.92, 1mm 5mm 15 mm) were bonded using an epoxy-based adhesive to form a composite structure (Fig. 3). A strain gauge was attached to the actuator to measure the piezoelectric strain in the longitudinal direction. The output signal was calibrated by comparing the capacitive sensor output voltage before the experiments. 3.2. Magnetic circuit to convert magnetic flux into magnetic force Fig. 4 shows the magnetic circuit, which comprised a permanent magnet (Nd B Fe, 0.24 T), a silicon steel yoke, and the above composite. In this photo, the reverse side of the composite is up, so that the strain gauge is seen rather than the magnetostrictive plate. The magnetic force was measured using a load cell (Kyowa, LTS-1kA). Although it is difficult to see in the image, a small gap of 0.75 mm exists between the outer yoke and the magnetic yoke, and a 1320 mn offset force existed. The operating voltage came from a function generator (NF Co. Ltd., WF1946) through a voltage amplifier (NF Co. Ltd., 4010). 4. Experiments and results 4.1. Relationship between piezoelectric strain and magnetic force Fig. 4. Experimental setup. In Fig. 5 (left), the piezoelectric displacement is shown to be symmetric at the initial condition; therefore it has no memory effect. On the other hand, with an imprint electrical field, the butterfly curve was deformed and the actuator obtained the memory effect as shown in Fig. 5 (right). The applied voltage was a 1 Hz triangular waveform with an amplitude of 65 V. These parameters were decided after some trials to obtain large memory displacement. When the imprint electrical field treatment was weak, the memory gap was small. A detailed explanation is reported elsewhere [1]. The imprint electrical field treatment is similar to a conventional poling treatment; however, these conditions are much more severe than a normal poling treatment. Therefore, as shown later, the imprint electrical field continued to exist even after polarization reversal. The shape memory piezoelectric actuator was attached to the magnetostrictive material, and this composite was then inserted into the experimental setup as explained above. The piezoelectric

T. Morita, T. Ozaki / Sensors and Actuators A 161 (2010) 266 270 269 Fig. 5. Relationship between piezoelectric strain and applied voltage before (left) and after imprint electrical field treatment (right). 4.2. Magnetic force memory effect by pulse voltage operation Fig. 6. Relationship between piezoelectric strain and variation in Force with applied voltage. The magnetic force memory was controlled using the pulse voltage. Alternating positive and negative pulses of 65 V (pulse width: 100 ms) were applied at 0.4 Hz to the actuator. The magnetic force memory results are shown in Fig. 7. The shape memory composite exhibited two distinct stable values depending on the polarization direction. When the pulse voltage was applied, the shape memory piezoelectric actuator was subjected to a strain of 228 ppm (Fig. 7), which corresponds to that shown in Fig. 6. The magnetic flux density was controlled using the change in the permeability of the magnetostrictive material. The memory value of the magnetic force was 9.3 mn, as shown in Fig. 7. This value is almost the same as that of the asymmetric butterfly curve at 0 V, as shown in Fig. 6. From these results, the expected memory effect was confirmed, and the magnetic force memory maintained a stable value at 0 V. butterfly curve and the change in magnetic force are shown in Fig. 6. The piezoelectric strain exhibited an asymmetric curve, and the magnetic force had two distinct values at 0 V. The memory value of the strain was confirmed as 233 ppm, and that for the magnetic force was 9.2 mn. 5. Conclusion A composite structure comprising a shape memory piezoelectric actuator and a magnetostrictive material was fabricated to produce a magnetic permeability memory effect. Using this system, a memory effect was achieved in the magnetic force under the influence of a pulsed voltage. In contrast to conventional magnetic devices, the proposed device allows pulsed voltage operation, overcoming the Joule heating problems. In addition, the simple coil-less structure offers advantages for miniaturization. Optimization of this magnetic circuit is ongoing, with the aim to achieve a larger magnetic force memory for practical applications. Acknowledgements The authors would like to acknowledge Dr. Ueno and Prof. Higuchi (the University of Tokyo) for fruitful discussions. This research was partially supported by the Ministry of Education, Culture, Sports, Science, and Technology through a Grant-in-Aid for Scientific Research on Priority Areas, no. 438, Next Generation Actuators Leading Breakthroughs, and also by the Murata Science Foundation, the Futaba Electrons Memorial Foundation and Japan Chemical Innovation Institute. References Fig. 7. Change in the piezoelectric strain and the magnetic force driven by a pulsed voltage. [1] T. Morita, Y. Kadota, H. Hosaka, Shape memory piezoelectric actuator, Appl. Phys. Lett. 90 (2007) 082909. [2] Y. Kadota, H. Hosaka, T. Morita, Utilization of the permittivity memory effect for position detection of a shape memory piezoelectric actuator, Jpn. J. Appl. Phys. 47 (2008) 217 219. [3] T. Morita, Y. Kadota, H. Hosaka, Fundamental study about shape memory piezoelectric actuator, in: IEEE ISAF 2007 (The 16th International Symposium on the Application of Ferroelectrics), 2007, pp. 721 724.

270 T. Morita, T. Ozaki / Sensors and Actuators A 161 (2010) 266 270 [4] Y. Kadota, H. Hosaka, T. Morita, Shape memory piezoelectric actuator by controlling imprint electrical field, J. Ferroelectr. 368 (2008) 185 193. [5] Y. Kadota, H. Hosaka, T. Morita, Shape memory piezoelectric actuator by controlling imprint electrical field, in: 11th European Meeting on Ferroelectricity, Abstracts, 2007, p. 105. [6] T. Ohashi, H. Hosaka, T. Morita, Refractive index memory effect of ferroelectric materials induced by electrical imprint field, Jpn. J. Appl. Phys. 47 (2008) 3985 3987. [7] A. Bayrashev, W.P. Robbins, B. Ziaie, Low frequency wireless powering of microsystems using piezoelectric-magnetostrictive laminate composites, Sens. Actuat. 114 (2004) 244 249. [8] J. Ryu, A. Vazquez Carazo, K. Uchino, H.-E. Kim, Magnetoelectric properties in piezoelectric and magnetostrictive laminate composite, Jpn. J. Appl. Phys. 40 (2001) 4948 4951. [9] T. Ueno, T. Higuchi, Novel composite of magnetostrictive material and piezoelectric actuator for coil-free magnetic force control, Sens. Actuat. 129 (1 2) (2006) 251 255. [10] T. Ueno, C.S. Keat, T. Higuchi, Linear step motor based on magnetic force control using composite of magnetostrictive and piezoelectric materials, IEEE Trans. Magn. 43 (1) (2007) 11 14. [11] T. Ozaki, H. Hosaka, T. Morita, Magnetic flux memory effect using a magnetostrictive material-shape memory piezoelectric, Sens. Actuat. 154 (2009) 69 72. Biographies Takeshi Morita was born in 1970. He received BEng, MEng and Dr Eng degrees in precision machinery engineering from the University of Tokyo in 1994, 1996 and 1999, respectively. After being a postdoctoral researcher at RIKEN (the Institute of Physical and Chemical Research) and at EPFL (Swiss federal institute of technology), he became a research associate at Tohoku University in 2002. Since June 2005, he has been an associate professor at The University of Tokyo. His research interests are a hydrothermal method for ferroelectric thin films, piezoelectric actuators and its control systems. Tomoya Ozaki was born in 1985. After receiving the BE (2007) degree from the University of Tokushima, he engaged in the research on the magnetic memory devices using the shape memory piezoelectric actuator. For this research, he received the ME (2009) degrees from the University of Tokyo. Now, he belongs to Sony Corp., and working on the optical disc devices.