A magnetoelectric Sensor of Threshold DC Magnetic Fields. Abstract

Transcription

1 1 A magnetoelectric Sensor of Threshold DC Magnetic Fields Leonid Y. Fetisov, 1 Vladimir. N. Serov, 1 Dmitri V. Chashin, 1 Sergey A. Makovkin, 1 G. Srinivasan, 2 D. Viehland, 3 and Yuri K. Fetisov 1 1 Moscow Technical University (MIREA), Moscow , Russia 2 Physics Department, Oakland University, Rochester, MI 48309, USA 3 Department of Materials Science and Engineering, Virginia Tech, Blacksburg, VA 24061, USA Abstract A multiferroic magnetic field sensor capable of producing an output for threshold magnetic fields has been fabricated and characterized. The sensor consists of a trilayer composite of piezoelectric X-cut lanthanum gallium tantalate (LGT) and magnetostrictive Metglas placed inside a solenoid and a wide-band amplifier. The composite has two distinct roles in the device; it forms the feedback loop of an oscillator and sets the frequency of sustained oscillations. The sensor generated an output of 2.5 V at the longitudinal acoustic resonance frequency of 87.5 khz for the trilayer for DC magnetic fields H = 0.3 to 50 Oe parallel to the composite plane. The device functions as a threshold magnetic field sensor for this H-interval and the threshold ON and OFF H-values for an ac voltage output could be controlled electronically or with proper choice for the ferromagnetic phase in the composite. * fetisov@mirea.ru; srinivas@oakland.edu

2 2 1. Introduction Sensors of magnetic fields are of interest for a variety of applications in electronics and biomedical imaging [1]. Magnetic sensors have several advantages such as a sufficiently long working distance, high sensitivity, fast response, and response independent of environmental conditions. Modern magnetic sensors are mostly based on the following effects: electromagnetic induction, Hall effect, giant magnetoresistance, and piezoelectric effects [2-5]. In recent years, significant progress has been made in the design of magnetic field sensors based on magnetoelectric (ME) effect in composites of ferromagneticpiezoelectric phases [6]. In such composites the coupling between the magnetic and electric subsystems originate from mechanical strain. When the composite is subjected to the magnetic field to be measured, the strain due to magnetostriction in the ferromagnetic layer is transferred to the ferroelectric layer resulting in a voltage response due to piezoelectric effect [7]. Several composite systems with ferromagnetic metals/alloys, ferrites or manganites for the ferromagnetic phase and lead zirconate titanate (PZT), lead magnesium niobate-lead titanate (PMN-PT) or barium titanate for the ferroelectric phase were reported to show a giant ME effect [7]. The ME sensors demonstrate high sensitivity, for fields as low as 10 6 Oe at room temperature, and have the advantage of simple design, low power consumption, and operation at frequencies up to 10 5 Hz [6]. Further enhancement in the sensitivity could be achieved by utilizing frequency modulation techniques with ME composites operating at bending resonance or longitudinal or thickness acoustic resonance modes [8-10]. We recently reported on a new type of DC magnetic field sensor with an ac output and was made of a ferromagnetic-ferroelectric composite [11]. The sensor capable of measuring a threshold magnetic field consisted of a bilayer of Metglas for the ferromagnetic phase and PZT for the ferroelectric phase and was located inside a solenoid. The

3 3 bilayer-solenoid assembly formed the feedback loop of an oscillator containing a wideband amplifier and served the dual role of determining the frequency of oscillations. When operated under the bending mode frequency of ~ 2.3 khz for the bilayer, the conditions for sustained oscillations were satisfied over a DC field interval H 1 to H 2 so that the sensor produced an ac output for H 1 <H < H 2 and was essentially a threshold magnetic field sensor [11]. The fields H 1 and H 2 are essentially threshold switch-on and switch- OFF fields for the sensor to produce an ac output voltage. Here we discuss results of our studies on a similar, but potentially a remote DC magnetic field sensor that operates at ~ 88 khz at the longitudinal acoustic mode of a trilayer of ferromagnetic Metglas and piezoelectric lanthanum gallium tantalate (LGT). There are several advantages with the use of single crystal X-cut LGT, including an order of magnitude or higher ME response than for PZT and high Q for mechanical resonance [12]. The ME coupling strength for the sensor element is directly proportional to d/ where d is the piezoelectric coupling coefficient and is the permittvity. Although d for X-cut LGT is much small compared to PZT, the ratio d/ is quite high [12]. Other advantages in the use of LGT are high Q (~ 50000) for mechanical resonance and absence of pyroelectric noise. We used a symmetric trilayer of Metglas-LGT-Metglas in this work that resulted in strong ME response, high sensitivity to DC field, and an ac voltage output desirable for use as threshold field sensors. We also show that the threshold field ON and OFF fields H 1 and H 2 can be tuned electronically. 2. Experiment The block-diagram of the sensor is shown in Fig. 1 and consists of a trilayer of X- cut LGT and Metglas located inside a solenoid, a wide-band amplifier with a gain coefficient K 1, phase-shifter with a transmission coefficient K 2, and an output wide-band ampli-

4 4 fier with a voltage gain coefficient K 3. All the elements are connected in series in a closed loop. The solenoid is powered by a voltage u 1 provided by a voltage divider formed by the resistors R and R 0 at the output of the amplifier and results in an ac magnetic field. The trilayer in turn produces an output ME voltage u 2 across LGT and the transmission coefficient for the ME element K 0 = u 2 /u 1. The input for the precision instrumentation amplifier INA326 is u 2. As discussed later, the ME element operates at the longitudinal acoustic resonance of ~ 87 khz. Within the frequency band of khz the amplifier had a gain of K 1 =100, input impedance of 1 GΩ, output impedance of several Ω, and output saturation level of ~2 V. The RC-phase-shifter had a gain coefficient K 2 = 0.33 and a phase shift of 90 0 at 86.7 khz. The output operation amplifier OPA2314 provided currents up to 20 ma for the solenoid and had K 3 = 2 and output saturation level of 2.5 V. The ME resonator is made of an X-cut single crystal langatate La 3 Ga 5.5 Ta 0.5 O 14 (LGT) platelet and two ferromagnetic layers of amorphous metal (Metglas 2605S3A) bonded to LGT on each side with a fast dry epoxy. The LGT layer was 24.7 mm x 4 mm x 0.5 mm in dimensions. It had 2 μm thick Ag electrodes on the outer surfaces. The Metglas layers were 24.7 mm x 4 mm x 0.03 mm in size and had saturation magnetostriction of λ S 20 ppm in saturation field H S 100 Oe. The trilayer was rigidly fixed at its center allowing longitudinal oscillations with the resonance frequency of f 0 = 87.6 khz. The ME composite was placed inside a solenoid with 200 turns of 0.2 mm diameter wire, resistance R 0 = Ω, and inductance L = 550 µh. The coil created ac magnetic fields up to h = 10 Oe for I = 20 ma. The resonator was placed between a pair of Helmhotz coils that provided dc magnetic fields H up to 100 Oe along the long axis of the trilayer. The variable resistor R was used in the voltage divider to control the voltage u 1 ap-

5 5 plied to the coil of the ME resonator: u 1 = K 4 u 3, where the voltage divider coefficient K 4 = R 0 /(R 0 +R) varies from 1 for R = 0 up to for R = 60 kω. 3. Results and Discussion The principle of operation of the DC magnetic field sensor is as follows. A voltage u 1 cos(2πft), where u 1 is the amplitude and f is the frequency, is applied to the solenoid which creates ac magnetic field along the long axis of the composite. The DC field H is also applied along the same direction. The ac magnetostriction in the Metglas is transmitted to LGT and results in a voltage response with an amplitude u 2 and frequency f. The amplitude of u 2 is proportional to the input u 1 and shows a maximum when f coincides with the frequency of the longitudinal acoustic oscillations. The transmission coefficient of the resonator K 0 (H) = u 2 (H)/u 1 depends on the DC magnetic field H and it may be varied by varying H. Sustained oscillations appear in the loop under the fulfillment of the amplitude balance and phase matching conditions [13] where K i and i are the transmission coefficient and the phase shift for the i th element of the block-diagram, respectively, and n is the integer. For this sensor the frequency of oscillation will be equal to the frequency of longitudinal resonance in the composite, while the amplitude of u 3 is limited by the output amplifier saturation level. When the voltage divider coefficient K 4 is varied, Eq. (1) will be fulfilled for various values of the transducer transmission coefficient K 0. Thus, the oscillator may be used as a threshold dc magnetic field sensor with tuned switching levels as discussed next.

6 6 Figure 2(a) shows measured amplitude-frequency and phase-frequency responses for the ME resonator. The figure shows u 2 vs f for u 1 = 0.25 V. The data shows a resonance peak in the vicinity of the frequency f 0 = 86.7 khz with quality factor Q = 1080, which corresponds to excitation of longitudinal acoustic modes in the composite. The resonance frequency f 0 is in good agreement with the estimated value of 90 khz [14]. At resonance the phase of u 2 lags by 90 0 from the phase of voltage applied to the coil. The phase shift induced by the solenoid itself is insignificant. In order to compensate this phase shift and fulfill the phase matching condition, the RC-phase-shifter shown in Fig. 1 was used. Figure 2(b) shows dependence of u 2 generated by the composite at the resonance as a function of u 1 applied to the coil. It is seen that the dependence is linear in the whole range. The voltage transmission coefficient of the resonator K 0 = u 2 /u Figure 2(c) shows dependence of the voltage u 2 at f 0 on DC magnetic field H for the voltage applied to the coil u 1 = 1.27 V. With an increase in H, the voltage u 2 increases first, reaches the maximum value of ~ 4.6 V for bias field H m 7.5 Oe (which corresponds to the maximum in the piezomagnetic coefficient of Metglas layer q = λ/ H, where λ(h) is the magnetostriction), and after that it gradually drops to zero with further increase in H. As it follows from the shape of the curve in Fig. 2(c) one can expect sustained oscillations over the field interval H 1 H H 2, where H 1 is the switch on field and H 2 is the switch off field. Figure 3 shows the output signal waveform u 3 (t) measured at the amplifier output for the DC magnetic field interval H 1 to H 2 for which oscillations are present. The results are for the divider transmission coefficient K at R = 40 k. One can see from Fig. 3(a) that output signal is non-harmonic. The distortion of the signal is due to the saturation of the amplifier output at the level of u V. The dependence of u 3 on H was measured when the field was increased at the rate H/ t = 2.5 Oe/s and the oscilla-

7 7 tions were observed in the range H 1 H H 2 (H 1 1 Oe and H 2 31 Oe for R = 40 kω), where the amplitude balance condition Eq.(1) was fulfilled. The output u 3 shows a sharp increase at the switch-on field H 1 and a decrease at the switch-off fields H 2 and the two fields are well defined. One can control the magnetic field region over which the oscillations occur by varying the input voltage u 1 by varying R-value in the voltage divider. Figure 4 shows the dependence of values of the lower and upper threshold fields, H 1 and H 2, on R. It is seen that with increasing R, the field range (H 2 -H 1 ) decreases with increasing R. It has also been observed that presence of oscillations depended on the direction of dc field H. Upon reversing the field direction the oscillations were absent for any value of H. It was due to phase change in the signal generated by LGT and resulting violation of the amplitude balance condition in Eq.(1). In order to restore the oscillations after H reversal it was necessary to introduce additional phase shift in the loop. Power consumption of the sensor in the oscillation regime was mw, depending on R-value. Finally, we compare the characteristics of the present device with similar sensors. These sensors are based on (i) a piezoelectric bimorph that produces a voltage at bending resonance at ~ 80 Hz when subjected to an Ampere force produced by a static magnetic field and an ac excitation current [15] and (ii) a bilayer of PZT and Metglas. The PZT/Metglas sensor operated at the bending resonance frequency of ~2.3 khz, Q = 100 and a voltage gain K 0 = for the ME element [11]. In this work the use of ME element with a symmetric 3-layer structure of LGT-Metglas and operation under longitudinal acoustic mode at 87.6 khz resulted in a much higher gain K 0 ~ 4 and Q ~1080. The sensor shows a sharp switch on and switch off processes compared to the sensor based on PZT-Metglas. Thus the device demonstrated in this work may be useful as a threshold magnetic field sensor with an ac voltage output for which the threshold fields

8 8 H 1 and H 2 can be varied electronically. The sensor could be easily miniaturized by MEMS processing for operation at a much higher frequency [16]. The threshold field sensor discussed in this work differ from the traditional magnetic sensors that are characterized by parameters such as the sensitivity S = U/H, lowest field that can be detected H min and the noise level N. The threshold sensors are characterized by the following parameters: switch ON and switch OFF threshold fields H 1 and H 2, uncertainty in the threshold fields, and switching time. As discussed earlier, H 1 and H 2- values can be controlled electronically using a voltage divider shown in Fig.1. The uncertainty in threshold field values are related to hysteresis in the magnetostriction versus field H and in our case the switch on field H 1 is defined with an accuracy of ~0.2 Oe. It is the coercive force H c of the Metglas because it has hysteresis in vs H. Figure 3b shows U vs H for up-sweep. For down-sweep the curve was shifted down by ~0.2 Oe. The switching time is determined by parameters of amplifiers and the parameters of the ME resonator. All of the amplifiers in Fig.1 are wide-band components and have very fast response. So, ME resonator essentially determines the switch on/off response of the sensor. The 3-dB width of the resonance is about f=f res /Q ~ 86 Hz. So the shortest switching time T ~ 1/ f ~ 12 ms and this estimate is consistent with the data in Fig. 3b. As seen in the data of Fig.3 the ON state the output of the sensor is 2.5 V and in the "off" state the output signal was about ~ 2 mv and is the noise level" of the sensor. The overall performance of the sensor discussed in this work could be enhanced with proper choice of ferromagnetic and ferroelectric materials. It might also be possible achieve desired device characteristics and figures of merit with the use functionally graded ferromagnetic and ferroelectric layers. Recent studies on electrostatic interaction mediated modification of ferroelectric properties of PZT are significant in this regard [17,18].

9 9 4. Conclusion In conclusion, a threshold magnetic field sensor with an ac voltage output has been fabricated and characterized. The sensor contains a broadband amplifier, an RCphase shifter, a resistive voltage divider, and a composite ME resonator in the feedback loop. Under applied dc magnetic field, when amplitude balance and phase matching conditions are satisfied, the circuit generates sinusoidal voltage output at the longitudinal acoustic resonance frequency of the resonator 87.5 khz with amplitude of up to 2.5 V. The switch on field H 1 and switch off filed H 2 for sustained oscillations and RF output can be controlled by varying the R-value in the voltage divider. These characteristic fields can also be varied over a wide range by choosing appropriate ferromagnetic layers with specific values of magnetostriction and external fields required for magnetic saturation. The resonance frequency which determines the frequency of output can be varied from a few khz [7] up to hundreds of MHz [12] by using bending, in-plane, or thickness modes of acoustic oscillations and by varying the dimensions of the ME resonators. Acknowledgments The research at MIREA was supported by grants from the President of Russian Federation МК and the Russian Foundation for Basic Research, grant The efforts at Oakland University was supported by grants from the National Science Foundation (ECCS ) and DARPA-MATRIX program and research at Virginia Tech was supported by a grant from the DARPA-MATRIX program.

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12 12 Caption for Figures Fig. 1. Block-diagram of the threshold dc magnetic field sensor. Fig.2:Measured characteristics of the ME resonator: (a) output voltage u 2 and phase φ vs frequency f responses for u 1 = 0.25 V and H = 8 Oe; (b) output voltage u 2 vs input voltage u 1 at resonance frequency and H = 7.5 Oe; (c) output voltage u 2 vs dc magnetic field H at resonance frequency and u 1 = 1.27 V. Fig. 3 (a) Characteristic waveform of generated voltage u 3 (t) and (b) dependence of generated voltage u 3 on dc magnetic field H. Fig. 4 Field interval H 1 to H 2 for sustained oscillations versus resistance R in the feedback loop.

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