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Magnetostrictive-piezoelectric magnetic sensor with current excitation
Journal of Magnetism and Magnetic Materials 215}216 (2000) 756}758
Magnetostrictive-piezoelectric magnetic sensor with current excitation J.L. Prieto *, C. Aroca , E. Lopez, M.C. Sanchez, P. Sanchez Dpt. Fn& sica Aplicada. E.T.S.I. Telecomunicacion, Universitaria Polite& cnica, 28040 Madrid, Spain Dpt. Fis de Materiales, Fac. Fisica, Universitaria Complutense, 28040 Madrid, Spain
Abstract A new working con"guration for magnetostrictive-piezoelectric magnetic sensors is presented. In this con"guration, the excitation is caused using an electrical current #owing through the ferromagnetic sample and the induced signal is sensed in the piezoelectric support as an electrical voltage. This new idea allows a magnetic "eld detection without any coil and opens a possibility for a future miniaturisation of the sensor. 2000 Elsevier Science B.V. All rights reserved. Keywords: Magnetic sensor; Magnetostrictive; Piezoelectric; Miniaturisation
Magnetostrictive-piezoelectric sensors are very interesting because of their good characteristics such as their high sensitivity, great frequency response, easy construction, low cost, etc. [1}5]. Moreover, the original idea and purpose of these kind of sensors was to reach a magnetic sensor with minimum size and the possibility of an integration [6]. This reduction in size is seriously limited if a coil is required for detection. Usually, a magnetostrictive-piezoelectric sensor is excited by an alternating electrical signal applied to the electrodes of the piezo and its vibration is transmitted through a viscous #uid to the highly magnetostrictive ferromagnetic sample [1]. This vibration produces anisotropy changes that depend linearly on the external magnetic "eld up to the anisotropy "eld [3]. The induced signal is recovered with a secondary coil that limits its minimum size in a miniaturisation process. In this paper we discuss the alternative con"guration in which the excitation is caused with a current #owing through the ferromagnetic sample and the detection is made with the piezoelectric, avoiding any coil for magnetic "eld detection.
* Corresponding author. Tel.:#34-91-3367241; fax: #34-913367271. E-mail address: jlprieto@"s.upm.es (J.L. Prieto).
The sensor, like the traditional magnetostrictivepiezoelectric sensor, is made by interfacing a highly magnetostrictive ferromagnetic material with a piezoelectric support through a viscous #uid. However, in this operating mode, the excitation is caused by using a current #owing through the ferromagnetic sample, and the piezoelectric support detects the size changes of the ferromagnetic material. A current #owing through a ferromagnetic material generates a magnetic "eld transversal to the sample axis. To induce size changes with this current, the induced magnetic "eld must rotate the magnetization from the longitudinal axis to the transversal one. The ferromagnetic sample must be annealed to erase any mechanical tension or residual anisotropy, so the only anisotropy present in the sample is the weak shape anisotropy and the current that goes through the sample makes the magnetisation rotate in a free and continuous way (Fig. 1). The ferromagnetic sample is a highly magnetostrictive amorphous ribbon, Metglas 2605-SC, annealed 30 min above its Curie temperature. This ribbon is attached with a viscous #uid to a piezoelectric support APC-855. The size of the system is 50;3;0.5 mm (Fig. 2). If an alternating current of frequency f #ows through the amorphous ribbon, this material su!ers size changes at 2f frequency, because they are independent of the current direction.
0304-8853/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 2 7 9 - 1
J.L. Prieto et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 756}758
757
Fig. 1. Size changes in a magnetostrictive ferromagnetic sample, free of any anisotropy, due to the magnetic "eld generated by an electrical current.
Fig. 3. Response of the sensor at zero "eld around a frequency half of the longitudinal resonance frequency of the piezoelectric support.
Fig. 2. Picture of the magnetostrictive-piezoelectric sensor with current excitation.
To obtain maximum sensitivity with this sensor, the exciting current must #ow at a half-frequency of the piezoelectric longitudinal resonance. In the absence of an external magnetic "eld, the 2f signal detected in the piezoelectric is completely due to the exciting current amplitude. Fig. 3 shows the response of the sensor exposed to a zero external magnetic "eld versus frequency around the half-frequency of the piezoelectric longitudinal resonance. When the sensor is exposed to a non-null external magnetic "eld (sensor axis oriented), the response in the piezoelectric decreases with respect to the signal obtained without "eld because, in this situation, the external magnetic "eld makes the magnetization rotation di$cult. This decrease in the piezoelectric response is proportional in a range to the external magnetic "eld amplitude but independent of its direction. To obtain a response dependent on magnetic "eld direction a DC magnetic "eld must be applied; its amplitude depends on the exciting current amplitude, then could be about the dynamic range amplitude of the sensor for every current. This DC "eld can be "xed with a permanent magnet attached to the sensor.
Fig. 4. Response curves of the sensor for di!erent excitation currents at a "xed frequency of 14.50 Hz. The increase of the sensitivity and the dynamic range with current excitation can be noticed.
The sensitivity of this sensor increases with the exciting current amplitude (Fig. 4) and the amplitude in this prototype is mainly limited by the current heating process (Joule) in the ferromagnetic sample that a!ects the viscosity of the interface. Fig. 4 points out a good sensitivity (0.5 lT/lV for 90 mA of exciting current) and a good dynamic range (more than 400 lT for 90 mA of
758
J.L. Prieto et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 756}758
exciting current) that also increases with the exciting current amplitude. The great advantage of this sensor is the fact that it is not necessary to use a coil for a magnetic "eld detection. The absence of coils points to the possibility of miniaturisation or even integration. Another point of interest is the 2f detection that minimises the noise from the excitation. This second-harmonic frequency is not possible in conventional magnetostrictive-piezoelectric sensors. The sensitivity is really good, but it can be easily increased taking into account the following points:
made of piezoelectric, ferromagnetic, insulator and "nally a conductor, a current #owing through the conductor can fully magnetise the ferromagnetic (actually, the current magnetises mainly the surfaces * see Fig. 1), avoiding at the same time heating problems in the ferromagnetic material. This work was partially supported by MAT97-1015C02 and MAT98-0824-C02 CICYT projects.
References E The mechanical coupling between the ferromagnetic and piezoelectric materials must be improved. Now we are working on the direct growth of the ferromagnetic material on the piezoelectric support by sputtering. E The size of the ferromagnetic material compared to the piezoelectric support must increase to a comparable size (or even greater) to obtain a greater e!ect. In this sensor we have used commercial materials and sizes are not optimised. E Alternative con"gurations can improve characteristics. For instance, in a sensor built with a multilayer
[1] M.D. Mermelstein, IEEE Trans. Magn. 28 (1992) 36. [2] B.J. Lynch, H.R. Gallantree, GEC J. Res. 8 (1990) 13. [3] M.D. Mermelstein, A. Dandridge, Appl. Phys. Lett. 51 (1987) 545. [4] J.L. Prieto, C. Aroca, P. SaH nchez, E. LoH pez, M.C. SaH nchez, J. Appl. Phys. 79 (1996). [5] J.L. Prieto, C. Aroca, E. LoH pez, M.C. SaH nchez, P. SaH nchez, IEEE Trans. Magn. 34 (1998) 3913. [6] A. Pantinakis, D.A. Jackson, Electron. Lett. 22 (1986) 737.
Magnetostrictive-piezoelectric magnetic sensor with current excitation J.L. Prieto *, C. Aroca , E. Lopez, M.C. Sanchez, P. Sanchez Dpt. Fn& sica Aplicada. E.T.S.I. Telecomunicacion, Universitaria Polite& cnica, 28040 Madrid, Spain Dpt. Fis de Materiales, Fac. Fisica, Universitaria Complutense, 28040 Madrid, Spain
Abstract A new working con"guration for magnetostrictive-piezoelectric magnetic sensors is presented. In this con"guration, the excitation is caused using an electrical current #owing through the ferromagnetic sample and the induced signal is sensed in the piezoelectric support as an electrical voltage. This new idea allows a magnetic "eld detection without any coil and opens a possibility for a future miniaturisation of the sensor. 2000 Elsevier Science B.V. All rights reserved. Keywords: Magnetic sensor; Magnetostrictive; Piezoelectric; Miniaturisation
Magnetostrictive-piezoelectric sensors are very interesting because of their good characteristics such as their high sensitivity, great frequency response, easy construction, low cost, etc. [1}5]. Moreover, the original idea and purpose of these kind of sensors was to reach a magnetic sensor with minimum size and the possibility of an integration [6]. This reduction in size is seriously limited if a coil is required for detection. Usually, a magnetostrictive-piezoelectric sensor is excited by an alternating electrical signal applied to the electrodes of the piezo and its vibration is transmitted through a viscous #uid to the highly magnetostrictive ferromagnetic sample [1]. This vibration produces anisotropy changes that depend linearly on the external magnetic "eld up to the anisotropy "eld [3]. The induced signal is recovered with a secondary coil that limits its minimum size in a miniaturisation process. In this paper we discuss the alternative con"guration in which the excitation is caused with a current #owing through the ferromagnetic sample and the detection is made with the piezoelectric, avoiding any coil for magnetic "eld detection.
* Corresponding author. Tel.:#34-91-3367241; fax: #34-913367271. E-mail address: jlprieto@"s.upm.es (J.L. Prieto).
The sensor, like the traditional magnetostrictivepiezoelectric sensor, is made by interfacing a highly magnetostrictive ferromagnetic material with a piezoelectric support through a viscous #uid. However, in this operating mode, the excitation is caused by using a current #owing through the ferromagnetic sample, and the piezoelectric support detects the size changes of the ferromagnetic material. A current #owing through a ferromagnetic material generates a magnetic "eld transversal to the sample axis. To induce size changes with this current, the induced magnetic "eld must rotate the magnetization from the longitudinal axis to the transversal one. The ferromagnetic sample must be annealed to erase any mechanical tension or residual anisotropy, so the only anisotropy present in the sample is the weak shape anisotropy and the current that goes through the sample makes the magnetisation rotate in a free and continuous way (Fig. 1). The ferromagnetic sample is a highly magnetostrictive amorphous ribbon, Metglas 2605-SC, annealed 30 min above its Curie temperature. This ribbon is attached with a viscous #uid to a piezoelectric support APC-855. The size of the system is 50;3;0.5 mm (Fig. 2). If an alternating current of frequency f #ows through the amorphous ribbon, this material su!ers size changes at 2f frequency, because they are independent of the current direction.
0304-8853/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 2 7 9 - 1
J.L. Prieto et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 756}758
757
Fig. 1. Size changes in a magnetostrictive ferromagnetic sample, free of any anisotropy, due to the magnetic "eld generated by an electrical current.
Fig. 3. Response of the sensor at zero "eld around a frequency half of the longitudinal resonance frequency of the piezoelectric support.
Fig. 2. Picture of the magnetostrictive-piezoelectric sensor with current excitation.
To obtain maximum sensitivity with this sensor, the exciting current must #ow at a half-frequency of the piezoelectric longitudinal resonance. In the absence of an external magnetic "eld, the 2f signal detected in the piezoelectric is completely due to the exciting current amplitude. Fig. 3 shows the response of the sensor exposed to a zero external magnetic "eld versus frequency around the half-frequency of the piezoelectric longitudinal resonance. When the sensor is exposed to a non-null external magnetic "eld (sensor axis oriented), the response in the piezoelectric decreases with respect to the signal obtained without "eld because, in this situation, the external magnetic "eld makes the magnetization rotation di$cult. This decrease in the piezoelectric response is proportional in a range to the external magnetic "eld amplitude but independent of its direction. To obtain a response dependent on magnetic "eld direction a DC magnetic "eld must be applied; its amplitude depends on the exciting current amplitude, then could be about the dynamic range amplitude of the sensor for every current. This DC "eld can be "xed with a permanent magnet attached to the sensor.
Fig. 4. Response curves of the sensor for di!erent excitation currents at a "xed frequency of 14.50 Hz. The increase of the sensitivity and the dynamic range with current excitation can be noticed.
The sensitivity of this sensor increases with the exciting current amplitude (Fig. 4) and the amplitude in this prototype is mainly limited by the current heating process (Joule) in the ferromagnetic sample that a!ects the viscosity of the interface. Fig. 4 points out a good sensitivity (0.5 lT/lV for 90 mA of exciting current) and a good dynamic range (more than 400 lT for 90 mA of
758
J.L. Prieto et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 756}758
exciting current) that also increases with the exciting current amplitude. The great advantage of this sensor is the fact that it is not necessary to use a coil for a magnetic "eld detection. The absence of coils points to the possibility of miniaturisation or even integration. Another point of interest is the 2f detection that minimises the noise from the excitation. This second-harmonic frequency is not possible in conventional magnetostrictive-piezoelectric sensors. The sensitivity is really good, but it can be easily increased taking into account the following points:
made of piezoelectric, ferromagnetic, insulator and "nally a conductor, a current #owing through the conductor can fully magnetise the ferromagnetic (actually, the current magnetises mainly the surfaces * see Fig. 1), avoiding at the same time heating problems in the ferromagnetic material. This work was partially supported by MAT97-1015C02 and MAT98-0824-C02 CICYT projects.
References E The mechanical coupling between the ferromagnetic and piezoelectric materials must be improved. Now we are working on the direct growth of the ferromagnetic material on the piezoelectric support by sputtering. E The size of the ferromagnetic material compared to the piezoelectric support must increase to a comparable size (or even greater) to obtain a greater e!ect. In this sensor we have used commercial materials and sizes are not optimised. E Alternative con"gurations can improve characteristics. For instance, in a sensor built with a multilayer
[1] M.D. Mermelstein, IEEE Trans. Magn. 28 (1992) 36. [2] B.J. Lynch, H.R. Gallantree, GEC J. Res. 8 (1990) 13. [3] M.D. Mermelstein, A. Dandridge, Appl. Phys. Lett. 51 (1987) 545. [4] J.L. Prieto, C. Aroca, P. SaH nchez, E. LoH pez, M.C. SaH nchez, J. Appl. Phys. 79 (1996). [5] J.L. Prieto, C. Aroca, E. LoH pez, M.C. SaH nchez, P. SaH nchez, IEEE Trans. Magn. 34 (1998) 3913. [6] A. Pantinakis, D.A. Jackson, Electron. Lett. 22 (1986) 737.