- Email: [email protected]
Performances and place of magnetometers based on amorphous wires compared to conventional magnetometers
Journal of Magnetism and Magnetic Materials 249 (2002) 393–397
Performances and place of magnetometers based on amorphous wires compared to conventional magnetometers D. Robbes*, C. Dolabdjian, Y. Monfort Groupe de Recherche en Informatique, Image et Instrumentation de Caen, GREYC-ISMRA-CNRS-UMR6072, 6, Bd du mar!echal Juin, F-14050 Caen, Cedex, France
Abstract We discuss and compare performances of various room temperature magnetometers. The work is directed towards the search of those magnetometers having a high sensitivity (>1000 V/T), a very low noise level (>1 pT/OHz at white noise) attainable in a volume typically smaller than 1 cm3. The choice of this set of parameters is related to the useful comparison of room temperature magnetometers versus cryogenic ones, such as Superconducting Quantum Interferometer Devices (SQUIDs). The latter have highly degraded performances when their working operations needs an open unshielded environment as required for example in industrial application (non-destructive evaluation). SQUIDs have also a rather poor spatial resolution, and could be replaced by room temperature sensors in some magnetic imaging systems, which require a high spatial resolution. The paper is ‘‘highlighted’’ in the field of magnetic sensors based on amorphous magnetic wires that were used to carry out wide bandwidth (>100 kHz), very low noise flux gate (EpT/OHz at white noise) and highly sensitive, low noise magnetometers (EpT/OHz at white noise) Colpitts oscillator configuration use by K. Bushida’s. r 2002 Elsevier Science B.V. All rights reserved. PACS: 85.70.Ay Keywords: Magnetometer; Amorphous magnetic wires
1. Introduction The world of magnetic sensors is very wide, due to the magnetic field to be measured, which may range from 10 16 T to more than 10 T and to the increasing number of applications. We have recently reviewed [1] those magnetic sensors exhibiting a high sensitivity (over 1000 V/T), a *Corresponding author. Tel.: +33-231452695; fax: +33231452698. E-mail address: [email protected] (D. Robbes).
very low noise (below 1 nT), a wide dynamic range (over 120 dB), a wide bandwidth, all these properties obtained in a small active volume, less than 1 cm3. Among these, sensors based on amorphous magnetic wires as proposed in Ref. [2], appear very promising if room temperature operation is required, together with a cheep compact design. We report on noise measurements made on such devices. The paper is presented as follows: we first give a compilation of the state of the art of magnetic sensors, then, we report on noise properties of two devices based on
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 5 6 4 - 4
394
D. Robbes et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 393–397
amorphous magnetic wires (AMW), namely a wide bandwidth flux gate and a magneto-inductive device. We then discuss goals to be reached for these sensors, in order to extend the field of their applications.
2. High-sensitivity magnetometers: state of the art Coming from the world of cryogenic sensors, our basic know-how in magnetic sensing is related to very low noise superconducting devices,
Table 1 Comparison of highly sensitive magnetic sensors Sensors
Flux gate Bartington Instr. (Mag 03)a GREYC [3] IBM [4] Hinnrichs et al. [5] LETI [6] Magnetoresistance AMR Philips (KMZ51) [7] Honeywell (HMC 1001)b IMO—GREYC [8] San Diego magnetics (SDM 531)c GMR Non-volatile electronics (AA002-02)d Smith et al. [9] Jardine et al. [10] S.D.T. Non-volatile electronicsd Magnetoinductive Bushida et al. [2] GREYC—EPM Superconducting SQUID HTS SQUID [11] LTS SQUID [11] JFM GREYC [12] GREYC [13] Hall effect Mosser et al. [14] Hybrid Seidel et al. [15]
White noise level (theoretical/measured or minimum) value (pT/OHz)
1=f noise corner (Hz)
Bandwidth (Hz)
Geometrical size (mm)
?/2.5 ?/o3 0.1–0.2/0.5 ?/0.75 ?/100
51 1–100 >10 100 1–10
DC–3 103 DC–> 105 DC–10 DC–103 DC–5 104
f: 0.5 1.8 f: 0.2 1 f: 24 ? f: 1 60 0.4 0.5 1
50–100/50–100 40/40 0.08/0.2 1–10/1–10
10–100 ? >105 100–103
DC–>105 DC–108 >105 DC–106
o1 mm2 o1 mm2 18 3 1.8 1.2
20/o100 6=200=Of 1/?
>103 ? ?
106 ? ?
0.44 3.37 0.75 1.5 1.4 0.3
1–10/?
?
?
1.65 2.14
? ?/5
? 1–100
106 >105
f: 0.1 10 f: 0.04 10
1–103 o1
DC–107 DC–107
10 10 77
30/30 0.1/0.6
o1 10–103
DC– DC–
0.6 0.6 10 10
9 103/9 103
>105
DC–>105
0.08 0.08
? /1.5 103
1–10
200
10 10
10 2/o4 10 10 3/10 3
2
f : frequency. a Bartington Instrument Ltd., 10 Thorney Leys Business Park, Oxford OX8 7GE, UK. b Honeywell Solid State Electronics Center, 12001 State Highway 55, Plymouth MN 55441-4799 (http://www.ssec.honeywell.com/ magnetic/). c San Diego Magnetics, 3985A Sorrento Valley Blvd San Diego, CA 92121-1402 (http://www.sdmagnetics.com/). d Non-Volatile Electronics Corporation, 11409 Valley View Road, Eden Prairie, MN 55344-3617 (http://www.nve.com).
D. Robbes et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 393–397
3. Noise results on AMW devices Among the devices referenced above, flux-gate sensors (FGS) have an important place, because they are very popular in industry. They are built using an easily saturable soft magnetic core, an excitation coil and a balanced pick-up coil. The driving AC current at a few tens kHz is mixed with the low-frequency applied field through the core and the readout signal contains a large second harmonic component, which is lock-in detected. The noise of the FGS has two main sources: that associated to the Johnson noise of the windings
1000 Field Sensitivity [pT/√Hz]
especially the Superconducting QUantum Interference Devices (SQUIDs). More than 10 years ago, as the critical temperature of superconducting materials reached more than 90 K, the question of 77 K SQUIDs was quickly opened and solved, but the commercial applications remained limited and led researchers to work with SQUIDs in open (unshielded) environment. However, in such conditions the intrinsic SQUID performances are no longer available, and choice criteria of magnetic sensors for high-sensitivity application in industry had to be analyzed over the set of high sensitivity sensors. Table 1 is a compilation of a previous review [1]. Data that are gathered give the white noise level (theoretical and/or measured), the ‘‘1=f ’’ noise corner that is very important for low-frequency applications, the bandwidth and the geometrical active size. Reviewed sensors are flux gates, magnetoresistive devices (anisotropic magnetoresistive devices—AMR, giant magnetoresistive devices and spin valves—GMR, spin dependent tunneling devices—SDT), magneto inductive devices, superconducting devices, Hall devices. In conclusion to this review, performances such that noise floor down to 1 pT/OHz with active areas of the order of 2 mm2 have been demonstrated at 300 K (see table footnote c of Table 1), which starts to be competitive with high TC SQUIDs used in unshielded environment, because it is difficult, even using gradiometer configuration and post-signal processing, to reduce the noise level much below 1 pT/OHz at low frequency.
395
100 (b)
10 (a)
1
0.1 1
10 100 1000 Frequency [Hz]
10000
Fig. 1. Noise spectra: (a) commercial low-noise flux gate, and (b) our device.
and that associated to the magnetic core, a part of which has an excess ‘‘1=f ’’ component. A theory was proposed by Koch et al. [16] to estimate the magnetic white noise of flux gates. We have built a small FGS using two pairs of five AMW pieces (E2 cm long and 40 mm diam.). A driving coil was wound on each pair (25 turns of copper wire, 60 mm diam.), and the two pieces were gathered by the wiring of the pick-up coil. Two Noise spectra are reported in Fig. 1. They were recorded simultaneously, one record is that of a commercial flux gate (Bartington Company) with a cutoff frequency around 1 kHz. The other record is that of our device, placed in the vicinity of the reference one. The numerous lines above 1 kHz of our FGS are linked to the reference FGS and its pumping signal; they disappear if it is turned off. The bandwidth of our AMW device is made much higher: its small size leads to a much higher pumping frequency, around 1 MHz. The white noise level of the device is not well measured because it is limited by the noise of the detecting electronic. The second AMW device that we have built and measured is a replication of that proposed earlier by K. Bushida et al. [2]. It makes good use of a single short AMW element that acts as the feedback impedance of a modified Colpitts oscillator. A preliminary study was made on the impedance of a wire between the frequency range 1 and 500 MHz. The wire was 1 cm long and 40 mm
D. Robbes et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 393–397
396
in diameter. It was placed at the end of a 50 O coaxial line, and we used an HP 4195A network analyzer to return the reflecting coefficient at this terminal. The DC field was applied using calibrated Helmoltz coils. The largest sensitivity was found above 100 MHz, mainly on the real part of the impedance, with a maximum value of the field to impedance transfer coefficient of 0.5 O/mT which means an (R dR=dH) coefficient of about 100%/Oe. Using these values and a SPICE modelization, we then designed a Bushida-Colpitts oscillator at 300 MHz. The highest field to voltage coefficient at the oscillator output was found to be at about 20000 V/T. Such a high value is needed in order to ensure that the noise of the post-amplifier is neglectable: if a noise level of 2 nV/OHz is adopted for this amplifier, then its contribution referred to the input would be 0.1 pT/OHz. Unfortunately, the K. Bushida schematics uses a diode detection between the oscillator output and the low noise amplifier input. The noise of this detecting unit is at least that of its load resistor. The latter was chosen as small as possible, down to 10 kO, fixing the minimum noise at the input of the voltage amplifier to be around 15 nV/OHz, which means a minimum equivalent field noise just below 1 pT/OHz. The noise spectrum measured at the amplifier output is shown in Fig. 2 (no field feedback use). The cut-off above 20 kHz is that of the detecting unit. The noise level obtained between 1 and 20 kHz is around 5 pT/OHz, a value
Field Sensitivity [pT/√Hz]
10000
1000
100
10
1 10
100 1000 10000 100000 Frequency [Hz]
Fig. 2. Noise spectra of an AMW device Bushida-Colpitts configuration. The floor level between 1 and 20 kHz is around 5 pT/OHz.
we consider very promising because this AMW element had received any annealing treatment. The field-to-voltage transfer was only around 3300 V/T much less than the maximum predicted value. We also want to point out that although the Colpitts strategy is very simple and efficient, the effective noise floor is difficult to predict because there is no simple theory describing the amplitude noise of such oscillators.
4. Goals As we pointed out in Section 2 of the paper, SQUID systems are not fully used in some applications, which needs open environment. A typical application is non-destructive evaluation of aircrafts. It needs a good level of resolution, both noise and spatial, at low frequency, in order to detect and localize deep (a few mm) defects in metallic assembly by rivets. The magnetic environment is then very noisy, and high TC SQUIDs were successfully tested [17]. Because we know the conditions required for SQUIDs in this open application, we easily deduce that for other highsensitivity magnetometers. The needed noise level has to be set down to about 0.1 pT/OHz above 1 Hz, in a bandwidth exceeding 100 kHz—1 MHz would be better—and the dynamic range of the device also needs to reach at least 140 dB (7 orders of magnitude). These high dynamic range and bandwidth are required in order to use advanced signal processing for noise rejection in the open environment. Another important application is that of biomagnetism in the noisy environment of hospitals. Very numerous investigations were made using SQUID systems [18]. A system using a magnetic sensor with the above specifications would give magnetic signals from the heart with a signal-to-noise ratio of about 50–100 in a 100 Hz bandwidth. This signal-to-noise ratio is sufficient to perform real-time signal processing and useful diagnostic by cardiologists. The two devices we have tested are not so far from these required conditions, and it appears interesting to better evaluate both theoretically and experimentally what their fundamental noise limits are. Besides these, the problem
D. Robbes et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 393–397
of ‘‘1=f ’’ noise reduction techniques is open, as it is since many years for SQUIDs.
5. Conclusion With regards to Table 1, we have measured the noise of two different magnetic sensors based on the use of amorphous magnetic wires. The measured white noise levels fall in the range of 1–10 pT/OHz, a promising value, that is limited by the detecting electronic noise at least in the FGS case. Some work has to be done with respect to the low-frequency noise, but noise reduction techniques as developed earlier for SQUIDs systems may exist. The goal of 0.1 pT/OHz at 1 Hz then appears realistic. This level of noise would really open interesting applications in non-destructive testing, magnetocardiography [11] and biomagnetism, especially if the spatial resolution of these very thin wires is much better than that of conventional flux gates.
Acknowledgements We acknowledge P. Ciureanu from EPM for having provide the magnetic wire.
References [1] D. Robbes, C. Dolabdjian, S. Saez, Y. Monfort, G. Kaiser, P. Ciureanu, IEEE Trans. Appl. Superconductors 11 (1) (2001) 629. [2] K. Bushida, K. Mohri, T. Uchiyama, IEEE Trans. Magn. 31 (6) (1995) 3134.
397
[3] C. Dolabdjian, Groupe de Recherche en Informatique, Image et Instrumentation de CAEN CNRS—UMR 6072, 6, Bd du Mar!echal Juin F-14050 CAEN, Cedex, unpublished result. [4] R. Koch, J. Rozen, Appl. Phys. Lett. 78 (13) (2001) 1897. [5] C. Hinnrichs, C. Pels, M. Schilling, J. Appl. Phys. 87 (9) (2000) 7085. [6] J.L. L!eger, B. Guilhamat, F. Alcouffe, R. Cucher, J.-P. Bongiruad, P. Le Thiec, Proceedings of the Second International Conference on Marine Electromagnetics, 1999, pp. 121–124. [7] G. Kaiser, M. Thurk, . P. Seidel, F. Dettmann, U. Loreit, Cryogenics 38 (1998) 187. [8] C. Dolabdjian, S. Saez, D. Robbes, C. Bettner, U. Loreit, F. Dettmann, G. Kaiser, A. Binneberg, Inst. Phys. Conference Series No. 167, 1999, p. 441. [9] N. Smith, A. Zeltser, D. Yang, P. Koeppe, IEEE. Trans. Magn. 33 (5) (1997) 3385. [10] D. Jardine, N. Mathur, M. Blamire, J. Evetts, IEEE Trans. Magn. 34 (4) (1998) 1299. [11] H. Weinstock (Ed.), SQUID Sensors: Fundamentals, Fabrication and Applications, NATO Advanced Scientific Institute Series E, C. Heiden, 1995, pp. 517–567. [12] C. Dolabdjian, P. Poupard, V. Martin, C. Gunther, J.F. Hamet, D. Robbes, Rev. Sci. Instrum. 67 (12) (1996) 4171. [13] E. Sassier, Y. Monfort, C. Gunther, D. Robbes, Inst. Phys. Ser. (EUCAS’99) 167 (1999) 165. [14] V. Mosser, F. Kobbi, S. Contreras, J.M. Mercy, O. Callen, J.L. Robert, S. Aboulhouda, J. Chevrier, D. Adam, Proceedings of the Ninth International Conference on Solid-State Sensors and Actuators, Transducers’97, Chicago, USA, June 1997. . [15] P. Seidel, S. Wunderlich, F. Schmidl, L. Dorrer, S. Linzen, F. Schmidt, F. Schrey, C. Steigmeier, K. Peiselt, S. Muller, . . A. Forster, S. Loche, S. Gudochnikov, IEEE Trans. Magn. 11 (1) (2001) 1179. [16] R. Koch, J. Deak, G. Grinstein, Appl. Phys. Lett. 75 (24) (1999) 3862. [17] M. Kreutzburck, K. Allweins, T. Ruhl, . M. Muck, . C. Heiden, H. Krause, R. Hohmann, IEEE Trans. Appl. Superconductors 11 (1) (2001) 1032. [18] H. Koch, IEEE Trans. Appl. Superconductors 11 (1) (2001) 49.
Performances and place of magnetometers based on amorphous wires compared to conventional magnetometers D. Robbes*, C. Dolabdjian, Y. Monfort Groupe de Recherche en Informatique, Image et Instrumentation de Caen, GREYC-ISMRA-CNRS-UMR6072, 6, Bd du mar!echal Juin, F-14050 Caen, Cedex, France
Abstract We discuss and compare performances of various room temperature magnetometers. The work is directed towards the search of those magnetometers having a high sensitivity (>1000 V/T), a very low noise level (>1 pT/OHz at white noise) attainable in a volume typically smaller than 1 cm3. The choice of this set of parameters is related to the useful comparison of room temperature magnetometers versus cryogenic ones, such as Superconducting Quantum Interferometer Devices (SQUIDs). The latter have highly degraded performances when their working operations needs an open unshielded environment as required for example in industrial application (non-destructive evaluation). SQUIDs have also a rather poor spatial resolution, and could be replaced by room temperature sensors in some magnetic imaging systems, which require a high spatial resolution. The paper is ‘‘highlighted’’ in the field of magnetic sensors based on amorphous magnetic wires that were used to carry out wide bandwidth (>100 kHz), very low noise flux gate (EpT/OHz at white noise) and highly sensitive, low noise magnetometers (EpT/OHz at white noise) Colpitts oscillator configuration use by K. Bushida’s. r 2002 Elsevier Science B.V. All rights reserved. PACS: 85.70.Ay Keywords: Magnetometer; Amorphous magnetic wires
1. Introduction The world of magnetic sensors is very wide, due to the magnetic field to be measured, which may range from 10 16 T to more than 10 T and to the increasing number of applications. We have recently reviewed [1] those magnetic sensors exhibiting a high sensitivity (over 1000 V/T), a *Corresponding author. Tel.: +33-231452695; fax: +33231452698. E-mail address: [email protected] (D. Robbes).
very low noise (below 1 nT), a wide dynamic range (over 120 dB), a wide bandwidth, all these properties obtained in a small active volume, less than 1 cm3. Among these, sensors based on amorphous magnetic wires as proposed in Ref. [2], appear very promising if room temperature operation is required, together with a cheep compact design. We report on noise measurements made on such devices. The paper is presented as follows: we first give a compilation of the state of the art of magnetic sensors, then, we report on noise properties of two devices based on
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 5 6 4 - 4
394
D. Robbes et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 393–397
amorphous magnetic wires (AMW), namely a wide bandwidth flux gate and a magneto-inductive device. We then discuss goals to be reached for these sensors, in order to extend the field of their applications.
2. High-sensitivity magnetometers: state of the art Coming from the world of cryogenic sensors, our basic know-how in magnetic sensing is related to very low noise superconducting devices,
Table 1 Comparison of highly sensitive magnetic sensors Sensors
Flux gate Bartington Instr. (Mag 03)a GREYC [3] IBM [4] Hinnrichs et al. [5] LETI [6] Magnetoresistance AMR Philips (KMZ51) [7] Honeywell (HMC 1001)b IMO—GREYC [8] San Diego magnetics (SDM 531)c GMR Non-volatile electronics (AA002-02)d Smith et al. [9] Jardine et al. [10] S.D.T. Non-volatile electronicsd Magnetoinductive Bushida et al. [2] GREYC—EPM Superconducting SQUID HTS SQUID [11] LTS SQUID [11] JFM GREYC [12] GREYC [13] Hall effect Mosser et al. [14] Hybrid Seidel et al. [15]
White noise level (theoretical/measured or minimum) value (pT/OHz)
1=f noise corner (Hz)
Bandwidth (Hz)
Geometrical size (mm)
?/2.5 ?/o3 0.1–0.2/0.5 ?/0.75 ?/100
51 1–100 >10 100 1–10
DC–3 103 DC–> 105 DC–10 DC–103 DC–5 104
f: 0.5 1.8 f: 0.2 1 f: 24 ? f: 1 60 0.4 0.5 1
50–100/50–100 40/40 0.08/0.2 1–10/1–10
10–100 ? >105 100–103
DC–>105 DC–108 >105 DC–106
o1 mm2 o1 mm2 18 3 1.8 1.2
20/o100 6=200=Of 1/?
>103 ? ?
106 ? ?
0.44 3.37 0.75 1.5 1.4 0.3
1–10/?
?
?
1.65 2.14
? ?/5
? 1–100
106 >105
f: 0.1 10 f: 0.04 10
1–103 o1
DC–107 DC–107
10 10 77
30/30 0.1/0.6
o1 10–103
DC– DC–
0.6 0.6 10 10
9 103/9 103
>105
DC–>105
0.08 0.08
? /1.5 103
1–10
200
10 10
10 2/o4 10 10 3/10 3
2
f : frequency. a Bartington Instrument Ltd., 10 Thorney Leys Business Park, Oxford OX8 7GE, UK. b Honeywell Solid State Electronics Center, 12001 State Highway 55, Plymouth MN 55441-4799 (http://www.ssec.honeywell.com/ magnetic/). c San Diego Magnetics, 3985A Sorrento Valley Blvd San Diego, CA 92121-1402 (http://www.sdmagnetics.com/). d Non-Volatile Electronics Corporation, 11409 Valley View Road, Eden Prairie, MN 55344-3617 (http://www.nve.com).
D. Robbes et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 393–397
3. Noise results on AMW devices Among the devices referenced above, flux-gate sensors (FGS) have an important place, because they are very popular in industry. They are built using an easily saturable soft magnetic core, an excitation coil and a balanced pick-up coil. The driving AC current at a few tens kHz is mixed with the low-frequency applied field through the core and the readout signal contains a large second harmonic component, which is lock-in detected. The noise of the FGS has two main sources: that associated to the Johnson noise of the windings
1000 Field Sensitivity [pT/√Hz]
especially the Superconducting QUantum Interference Devices (SQUIDs). More than 10 years ago, as the critical temperature of superconducting materials reached more than 90 K, the question of 77 K SQUIDs was quickly opened and solved, but the commercial applications remained limited and led researchers to work with SQUIDs in open (unshielded) environment. However, in such conditions the intrinsic SQUID performances are no longer available, and choice criteria of magnetic sensors for high-sensitivity application in industry had to be analyzed over the set of high sensitivity sensors. Table 1 is a compilation of a previous review [1]. Data that are gathered give the white noise level (theoretical and/or measured), the ‘‘1=f ’’ noise corner that is very important for low-frequency applications, the bandwidth and the geometrical active size. Reviewed sensors are flux gates, magnetoresistive devices (anisotropic magnetoresistive devices—AMR, giant magnetoresistive devices and spin valves—GMR, spin dependent tunneling devices—SDT), magneto inductive devices, superconducting devices, Hall devices. In conclusion to this review, performances such that noise floor down to 1 pT/OHz with active areas of the order of 2 mm2 have been demonstrated at 300 K (see table footnote c of Table 1), which starts to be competitive with high TC SQUIDs used in unshielded environment, because it is difficult, even using gradiometer configuration and post-signal processing, to reduce the noise level much below 1 pT/OHz at low frequency.
395
100 (b)
10 (a)
1
0.1 1
10 100 1000 Frequency [Hz]
10000
Fig. 1. Noise spectra: (a) commercial low-noise flux gate, and (b) our device.
and that associated to the magnetic core, a part of which has an excess ‘‘1=f ’’ component. A theory was proposed by Koch et al. [16] to estimate the magnetic white noise of flux gates. We have built a small FGS using two pairs of five AMW pieces (E2 cm long and 40 mm diam.). A driving coil was wound on each pair (25 turns of copper wire, 60 mm diam.), and the two pieces were gathered by the wiring of the pick-up coil. Two Noise spectra are reported in Fig. 1. They were recorded simultaneously, one record is that of a commercial flux gate (Bartington Company) with a cutoff frequency around 1 kHz. The other record is that of our device, placed in the vicinity of the reference one. The numerous lines above 1 kHz of our FGS are linked to the reference FGS and its pumping signal; they disappear if it is turned off. The bandwidth of our AMW device is made much higher: its small size leads to a much higher pumping frequency, around 1 MHz. The white noise level of the device is not well measured because it is limited by the noise of the detecting electronic. The second AMW device that we have built and measured is a replication of that proposed earlier by K. Bushida et al. [2]. It makes good use of a single short AMW element that acts as the feedback impedance of a modified Colpitts oscillator. A preliminary study was made on the impedance of a wire between the frequency range 1 and 500 MHz. The wire was 1 cm long and 40 mm
D. Robbes et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 393–397
396
in diameter. It was placed at the end of a 50 O coaxial line, and we used an HP 4195A network analyzer to return the reflecting coefficient at this terminal. The DC field was applied using calibrated Helmoltz coils. The largest sensitivity was found above 100 MHz, mainly on the real part of the impedance, with a maximum value of the field to impedance transfer coefficient of 0.5 O/mT which means an (R dR=dH) coefficient of about 100%/Oe. Using these values and a SPICE modelization, we then designed a Bushida-Colpitts oscillator at 300 MHz. The highest field to voltage coefficient at the oscillator output was found to be at about 20000 V/T. Such a high value is needed in order to ensure that the noise of the post-amplifier is neglectable: if a noise level of 2 nV/OHz is adopted for this amplifier, then its contribution referred to the input would be 0.1 pT/OHz. Unfortunately, the K. Bushida schematics uses a diode detection between the oscillator output and the low noise amplifier input. The noise of this detecting unit is at least that of its load resistor. The latter was chosen as small as possible, down to 10 kO, fixing the minimum noise at the input of the voltage amplifier to be around 15 nV/OHz, which means a minimum equivalent field noise just below 1 pT/OHz. The noise spectrum measured at the amplifier output is shown in Fig. 2 (no field feedback use). The cut-off above 20 kHz is that of the detecting unit. The noise level obtained between 1 and 20 kHz is around 5 pT/OHz, a value
Field Sensitivity [pT/√Hz]
10000
1000
100
10
1 10
100 1000 10000 100000 Frequency [Hz]
Fig. 2. Noise spectra of an AMW device Bushida-Colpitts configuration. The floor level between 1 and 20 kHz is around 5 pT/OHz.
we consider very promising because this AMW element had received any annealing treatment. The field-to-voltage transfer was only around 3300 V/T much less than the maximum predicted value. We also want to point out that although the Colpitts strategy is very simple and efficient, the effective noise floor is difficult to predict because there is no simple theory describing the amplitude noise of such oscillators.
4. Goals As we pointed out in Section 2 of the paper, SQUID systems are not fully used in some applications, which needs open environment. A typical application is non-destructive evaluation of aircrafts. It needs a good level of resolution, both noise and spatial, at low frequency, in order to detect and localize deep (a few mm) defects in metallic assembly by rivets. The magnetic environment is then very noisy, and high TC SQUIDs were successfully tested [17]. Because we know the conditions required for SQUIDs in this open application, we easily deduce that for other highsensitivity magnetometers. The needed noise level has to be set down to about 0.1 pT/OHz above 1 Hz, in a bandwidth exceeding 100 kHz—1 MHz would be better—and the dynamic range of the device also needs to reach at least 140 dB (7 orders of magnitude). These high dynamic range and bandwidth are required in order to use advanced signal processing for noise rejection in the open environment. Another important application is that of biomagnetism in the noisy environment of hospitals. Very numerous investigations were made using SQUID systems [18]. A system using a magnetic sensor with the above specifications would give magnetic signals from the heart with a signal-to-noise ratio of about 50–100 in a 100 Hz bandwidth. This signal-to-noise ratio is sufficient to perform real-time signal processing and useful diagnostic by cardiologists. The two devices we have tested are not so far from these required conditions, and it appears interesting to better evaluate both theoretically and experimentally what their fundamental noise limits are. Besides these, the problem
D. Robbes et al. / Journal of Magnetism and Magnetic Materials 249 (2002) 393–397
of ‘‘1=f ’’ noise reduction techniques is open, as it is since many years for SQUIDs.
5. Conclusion With regards to Table 1, we have measured the noise of two different magnetic sensors based on the use of amorphous magnetic wires. The measured white noise levels fall in the range of 1–10 pT/OHz, a promising value, that is limited by the detecting electronic noise at least in the FGS case. Some work has to be done with respect to the low-frequency noise, but noise reduction techniques as developed earlier for SQUIDs systems may exist. The goal of 0.1 pT/OHz at 1 Hz then appears realistic. This level of noise would really open interesting applications in non-destructive testing, magnetocardiography [11] and biomagnetism, especially if the spatial resolution of these very thin wires is much better than that of conventional flux gates.
Acknowledgements We acknowledge P. Ciureanu from EPM for having provide the magnetic wire.
References [1] D. Robbes, C. Dolabdjian, S. Saez, Y. Monfort, G. Kaiser, P. Ciureanu, IEEE Trans. Appl. Superconductors 11 (1) (2001) 629. [2] K. Bushida, K. Mohri, T. Uchiyama, IEEE Trans. Magn. 31 (6) (1995) 3134.
397
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