- Email: [email protected]
Sensors based on soft magnetic materials Panel discussion
Journal of Magnetism and Magnetic Materials 215}216 (2000) 795}799
Sensors based on soft magnetic materials Panel discussion夽 Pavel Ripka , GaH bor VeH rtesy * Czech Technical University, Faculty of Electrical Engineering, Technicka 2, 166 27 Praha 6, Czech Republic Hungarian Academy of Sciences, Research Institute for Technical Physics and Materials Science, P.O. Box 49, H-1525 Budapest, Hungary
The economic realities of productivity, quality and reliability for the industrial societies of the 21th century are placing major demands on existing manufacturing technologies. To meet both present and anticipated requirements, new and improved methods are needed. To be e!ective, the measurement, electronics and control components, and sub-systems, in particular sensors and sensor systems, have to be developed parallely as a part of computer-controlled manufacturing systems. Soft magnetic materials serve as a basic material for a wide scale of di!erent sensors. The aim of the present panel discussion was to give a survey of the latest technology and of the most important features of the sensors, which are based on soft magnetic materials, and to compare systematically their parameters. Magnetic sensors which are not based on soft magnetic materials (such as SQUIDs, NMR and semiconductor sensors) as well as reading heads were not covered by this panel The attention was concentrated mainly on device parameters and on applications, not on the theoretical background. P. Ripka gave the general introduction. He emphasized the keywords of the panel discussion: application, speci"cations and requirements. He had some general remarks: Sensors containing core, made of magnetic material seem to be always vectorial. It is possible to measure 1, 2, or even 3 components of the magnetic "eld using a single core. We are going to speak about sensors, which do measure "elds from DC; AC magnetic sensors
* Corresponding author. Tel.:#36}1-169-5165; fax:#36-13959284. E-mail address: [email protected] (G VeH rtesy). 夽 List of panelists: P. Ripka (discussion leader), Czech Technical University; L. Panina, Russian Academy of Sciences; M. Yamaguchi, Tohoku University; H. Hauser, Vienna University of Technology; D. Mapps, Plymouth University; J.M. Barandiaran, Universidad del Pais Vasco; L. Lanotte, Universita di Napoli `Federico IIa.
(such as induction coils) are not a subject of this discussion. We restricted our topic mainly to magnetic "eld sensors; although the broader meaning of the term &magnetic sensor' includes devices which, using magnetic principle, measure non-magnetic variables such as position and speed, force and torque, temperature or others. Invited researchers gave a brief overview of di!erent types of sensors. Each contribution had a similar structure. The general questions connected with each sensor type were: E Which is the application "eld of the given sensor? E Are the sensors already being produced or are they just laboratory prototypes? E What are their parameters? Besides the parameters well known for other sensors and instruments, magnetic sensors have peculiarities given by non-linear materials and complex e!ects used. The most important parameters and properties of magnetic "eld sensors can be grouped into the following categories: E Full-scale range, linearity, hysteresis, temperature coe$cient of sensitivity E Bias stability, o!set, o!set temperature coe$cient, long-term stability E Perming (if the sensor is subjected to a strong magnetic "eld, the o!set can be changed) E Noise E Resistance against environment (temperature, humidity, vibrations, radiation) E Resistance against perpendicular "eld and "eld gradient E Bandwidth E Power consumption, size E Reliability E Cost The possibilities of sensor miniaturization are of increasing importance. We often hear questions about the
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 9 1 - 2
796
P. Ripka, G. Ve& rtesy / Journal of Magnetism and Magnetic Materials 215}216 (2000) 795}799
Table 1 Application types
Application
Requirements
Precise (high-amplitude resolution)
Location of ferromagnetic objects Navigation Non-contact current meters
High linearity Low cross"eld e!ect Stable o!set
Low-noise
Biomagnetic Space research
Low-noise
Industrial
Position, speed Car industry Magnetic marking
Low-cost resistance against environment
Miniature (high spatial resolution)
Marking Surface mapping
Micropower
Fast
Pulse "elds, reading
High bandwidth
Table 2 Fluxgate sensor parameters
Top parameter
Standard
Range Linearity error Temperature coe!. of sensitivity O!set temperature coe$cient Perming Noise Long-term stability of the o!set Bandwidth Operating temperature range Power consumption Size Cross"eld error
10 mT 10 ppm 10 ppm/3C (0.05 nT/3C (1 nT o!set change after 10 mT shock 5 pT rms (0.05}10 Hz) 2 nT/yr 10 kHz !60}#2003C 1 mW 2 mm (1 nT for 50 lT "eld
200 lT 100 ppm 50 ppm/3C 0.2 nT/3C (5 nT 100 pT rms 5nT/8 hours 20 Hz !20}#703C 100 mW 30 mm 5 nT
integration compatibility, relation between the core size and sensitivity and about the suitability of the used technologies for the mass production. The &application types' of magnetic "eld sensors can be assorted in the following way, where the main requirements are also indicated (see Table 1): Fluxgate sensors are historically the "rst type of highresolution solid-state magnetic "eld sensors. They are still widely used in applications, although they were far beaten in production volume by magnetoresistors, especially AMR. The parameters of #uxgates should serve as a reference for newer types of sensors. However, any comparison should be made carefully, taking into consideration also the sensor application type. It is also tricky to compare the properties of the fresh new sensors with a traditional device which was constantly being improved for decades. Table 2 summarizes &top' values achieved for specialized types of #uxgates (but certainly not all of them simultaneously) and &standard' values achieved in highperformance devices. The disadvantage of #uxgate sensors is that they are usually o!ered only as a part of
magnetometer; high-performance #uxgates are only rarely available as components. The recent improvements and developments in #uxgate sensors are covered in a separate paper. L. Panina gave an overview about giant magnetoimpedance (GMI) sensors. Giant magnetoimpedance e!ect is a kind of high-frequency analogy of giant magnetic resitance (GMR), because it is basically a change in the AC impedance under the application of a magnetic "eld. GMI is essentially a high-frequency e!ect: a kind of side e!ect of the skin e!ect. The sensitivity is very high compared to GMR. The typical material of this type of sensor is amorphous wire with well-de"ned circumferential anisotropy and corresponding circular domain structure. In this case the sensing "eld is applied along the wire The sensitivity is almost zero if the "eld is perpendicular to the wire axis. It is possible to create a directional sensor using three wires perpendicular to each other. If a ribbon or a thin "lm is used instead of a wire, two-dimensional sensor using a single core can be created. GMI sensors are already used by industry. The response of the sensor strongly depends on excitation. Symmetrical
P. Ripka, G. Ve& rtesy / Journal of Magnetism and Magnetic Materials 215}216 (2000) 795}799
characteristics can be obtained almost without hysteresis. It is also possible to create asymmetrical characteristics that would be important to get a good linearity. The linearity can be improved and a stable operation point can be achieved by using negative feedback. There is no change in the sensor parameters up to 803C temperature. Several modi"cations of the GMI sensor were developed. It is possible to detect very low, highly localized magnetic "eld with 40 lm resolution. The length of the sensor based on amorphous wire can be reduced down to 3 mm without losing sensitivity. There are many prospective applications of this type of sensors, e.g. in medical electronics, in automobiles, in textile industry. The sensor can be integrated if a thin "lm is applied as a sensing material. M. Yamaguchi talked about thin xlm sensors. This type of sensors is mostly associated with thin "lm type of the previously discussed GMI sensors. The skin e!ect is utilized in a ferromagnetic body. The impedance of the element is proportional to the surface current density. With the application of magnetic "eld the permeability is changed. Very high sensitivity can be achieved by this type of sensor: the highest possible magnetic "eld resolution is 10\ T (if thermal #uctuation dominates the resolution). There are many forgoing works with thin "lm type sensors, e.g. Eddy Current Testing (ECT), accelerometers, magnetic "eld sensors. One of the main e!orts is to develop thin "lm micromagnetic devices with good characteristics. In the case of these GMI-type sensors the size can be easily reduced below 1 mm by applying photolithography technique. By reducing the length, the sensitivity is lost because of the demagnetizing e!ect. Because of this the width of the elements should also be reduced. However, to keep the sensitivity with the miniaturization it is necessary to control the domain con"guration. A DC bias is necessary to get high sensitivity. Without using any windings, the bias can be introduced by an integrated thin magnetically hard "lm; SmCo with in-plane anisotropy is utilized for this purpose. The driving frequency can be increased up to 300 MHz. 1 nT sensitivity has already been demonstrated. These sensors can be applied in intelligent transportation systems, in "eld compensation in big CRT monitors, in 3D position systems, etc. The initial cost can be rather high, but the running cost is very low, which is one of the main advantages of thin "lm sensors. Other advantages are the high sensitivity, small size and the possibility of integration. H. Hauser reviewed magnetooptical sensors. Magnetooptical sensors are a relatively new "eld of research and development. Magnetooptical devices are mainly used for switching, polarization and amplitude modulation of light. The only industrially available magnetooptical sensor is magnetooptical current transformer measuring magnetic "eld of strong currents by the Faraday rotation in diamagnetic "bers. Our group is developing a new generation of magnetooptical sensors for
797
measurement of magnetic "eld and position of light. These sensors are based on domain wall motion in soft magnetic garnets and in orthoferrites. The speci"cations are only preliminary, because only a laboratory prototype exists. In these sensors the position of domain walls is detected by means of the Faraday e!ect, using polarized light. A periodic magnetic "eld with a frequency up to several MHz can be measured using a simple ruler. To make a good sensor, a material with extraordinary properties is needed. One of these materials is the orthoferrite. In these materials the domain wall velocity is very high (up to 20 km/s), Faraday rotation is also very high (29003/cm), and the transparency is also extremely large. The only disadvantage of this material is the high birefringence; a complex sample preparation is therefore necessary. The modulation depth of the light transmitted through the two-domain structure is about 15%/lT, so at least 1 nT resolution of the sensor can be achieved, depending on the bandwidth. The presently existing 100 MHz bandwith can be extended up to 1 GHz. The output signal of the sensor is usually a time duration, and the domain walls are excited by an AC "eld. One of the parameters which are dependent on temperature is the signal/noise ratio. The maximum "eld to be measured is between 1 and 10 kA/m, and the size of the plate is 10;10 mm. D. Mapps summarized the state of art of magnetoresisitive (MR) sensors. This sensor technology has been known for quite a long time. MR sensors are widely used, especially in magnetic recording. What has happened in the last few years is, that materials have been signi"cantly improved. This means that MR sensors are being used in many more applications than just a few years ago. The sensitivity was improved by reducing the thickness of the sensing material. Even 150 As thick "lms can produce a very smooth magnetoresistance vs. applied "eld curve, producing a small noise. It is possible to get 97 dB signal-to-noise ratio which is quite enormous for such a sensor. The thickness is very important because in very thin "lms NeeH l walls exist instead of Bloch walls. Due to the much larger thickness of NeeH l walls, these walls pass imperfections with very low change of wall energy, resulting in very small noise compared to Bloch walls. This shows the important role of domain walls in the proper operation of MR devices which are not in a singledomain state. Another improvement which was achieved is to regularize the microstructure of the "lm material using new underlayer materials (e.g. platinum), thus improving the signal-to-noise ratio. Sophisticated electronic techniques were also used to improve the quality of the signal. The hysteresis can be removed by applying high-frequency bias "eld. A very sensitive detector can be produced from a pair of AMR sensors in a switch}bias mode where each sensor is switched alternately to magnetise in the opposite direction to the other at kHz frequencies. The resulting di!erential output can be
798
P. Ripka, G. Ve& rtesy / Journal of Magnetism and Magnetic Materials 215}216 (2000) 795}799
detected using a phase-sensitive detector. The equipment for producing GMR sensors is quite complicated, which makes GMR sensors expensive. A drawback is that MR sensors are temperature sensitive and if you do not optimise the material they can be noisy. However, they have a multitude of uses in magnetic "eld detection. J.M. Barandiaran gave an overview about magnetoelastic sensors. Most of these sensors can be separated into two categories: static and dynamic. The topic of his talk was the static sensors; dynamic sensors are a subject of a separate paper. The main characteristic of magnetoelastic sensor is the magnetoelastic coupling coe$cient, which measures the conversion of magnetic to elastic energy or vice versa. In the 1960s there were already some static sensors based on the stress e!ect on the hysteresis loop of iron silicon; most of the present static magnetoelastic sensors work on the same basis. The direct application of a static magnetoelastic sensor is measurement of stress or strain. It is clear that magnetoelastic strain sensors are much better than strain gauges; their sensitivity is thousand times higher. However, the application of magnetoelastic sensors is more complicated than strain gauges because they need AC current supply. They have another interesting and important application possibility, which is the non- contact sensing element. They have given a good performance in rotating shaft measurement of the stresses. Torque measurement has been one of the most widespread applications of magnetoelastic sensors. Another interesting use of these sensors can be in medical applications e.g. sensors of breathing and blood pressure, which were presented during the conference, but these devices are not on the market yet. The sensitivity and wide dynamical range of these sensors are among the best features . For instance, the static magnetoelastic sensors have an equivalent &gauge factor' of several thousands, as compared with resistive semiconductor strain gauges, which have a limit of about 200, and metallic strain gauges that are only around 2. The temperature dependence is very small compared with the other sensors. Typical "gures of temperature coe$cient and maximum operating temperature are metallic strain gauges (0.05% deg\ and 803C), semiconductor strain gauges ( 0.1% deg\ and 1003C), magnetoelastic sensors based on amorphous ribbons ( 0.02% deg\ and 2003C). One of the most important problems is that these sensors will never be miniaturized because of the need of coils to drive the sensor and pick up the signal. The dynamical magnetoelastic sensors are much better placed for widespread applications at this moment as they work at resonance, and remote sensing is possible with distances of about 1 m from the coils to the stripes. They are already present on the market (e.g. for security purposes or for recognizing objects in supermarkets). Dr. Hasegawa in his talk gave an example of magnetoelastic security tags being currently used in the States. These have better performance than the old tags
based on the second harmonic detection and made up of soft magnetic stripes of permalloy or other materials with very low magnetostriction. L. Lanotte stressed some applications of dynamic magnetostriction and in particular recent applications which are based on magnetoelastic wave devices. A new type of sensor, based on a new type of amorphous alloy was shown, which can be used to measure the strain. The device works without bias magnetic "eld. It is also possible to measure the change in local magnetic "eld. It can be applied for measuring vibrations without contact. The sensitive parameter of the sensor is the amplitude of resonant magnetoelastic waves. It is possible to measure the deformation of another sample due to application of magnetic "eld using magnetostrictive materials. The direction is important because the displacement on the vibration can be measured only in the direction which is parallel to the ribbon core. The size is about 30 mm long with 5 mm diameter. The cost is low. It is only a prototype, there is no industrial production. However, the technical requirements are not di$cult, so industrial application is possible. In principle it can operate in a large temperature range. G. VeH rtesy presented a new type of magnetic "eld sensors (Fluxset) for measuring DC and AC (up to 100 kHz frequency) low-level magnetic "elds with high accuracy. Its principle of operation is close to the pulseposition-type #uxgate magnetometers. The particular advantage of these magnetometers is an output signal that can be simply converted into binary code. The measurement of a small magnetic "eld is reduced to a high-accuracy time measurement through the displacement of the magnetization curve produced by the "eld. The probes are suitable for axial measurement of the magnetic "eld. The transverse sensitivity is negligible. The available maximum resolution of the sensor is below 0.1 nT. The linearity is better than 1%. The temperature stability is extremely good, the measuring head can operate (without measurable change of the parameters) in the !200}#2003C temperature range. The time stability is also good, as was proved by a long-term measurement (using simultaneously a reference observatory magnetometer) of the variations of the Earth's magnetic "eld. The length of the 1 mm diameter probe can be varied between 5 and 20 mm; a longer probe ensures higher sensitivity. The spatial resolution is better than 0.1 mm. One of the main advantages of this sensor is the simple construction and low price. It can be applied in the same "elds like conventional #uxgates. Its parameters make it very suitable for application in eddy current testing, combining the eddy current method with magnetic "eld measurement. The summaries of the panelists were followed by discussion. It was inquired whether there is any theoretical limit for the detectable minimum magnetic xeld in the case of
P. Ripka, G. Ve& rtesy / Journal of Magnetism and Magnetic Materials 215}216 (2000) 795}799
di!erent types of sensors. Dr. Yamaguchi answered that in the case of thin "lm-type GMI sensor, if the limit is given by thermal stability and if a Co-based amorphous "lm is used, this theoretical limit is about 10\. T. P. Ripka commented that in case of #uxgate sensors, the real detection limit is much larger than the theoretical value estimated from models for Barkhausen noise. The e!ective "eld noise highly depends on the sensor size and geometry: long station magnetometers may have 10 pT resolution. But it should be clearly stated that in most cases (especially when measuring in the presence of the Earth's "eld) the sensor noise is not critical. The performance is usually limited by other factors like linearity, perming, cross"eld response and temperature stability. Another question was raised about the disadvantage of GMI sensor compared to GMR sensors. Dr. Panina answered: GMI is a kind of high-frequency sensor. There are problems with impedance mismatching and with re#ected signals, so electronic circuits should be developed to e!ectively operate at high frequency. Some types of self-oscillation circuits are being used, but their oscillation frequency is limited. The problem is that the
799
oscillation frequency of these devices is not stable (it is too sensitive to the environment), which will result in unstable sensor parameters. Better results were achieved with hi-speed CMOS circuits, which generate pulse excitation, but their performance is still limited. Another disadvantage of GMI is: Because the origin of this e!ect is a kind of skin e!ect, if we want to reduce the size further, there is a limiting size reduction, and we lose by this way sensitivity and response speed. If the excitation frequency is 1 GHz, a "eld greater than 100 MHz cannot be detected. For modern computer technology even faster operation is needed. Dr. Yamaguchi: Another possible disadvantage of GMI, especially for thin "lm sensor is that this kind of sensor needs a DC bias to get high sensitivity. In the case of GMR exchange bias can be utilized, because the active "lm thickness is very small. However, the main disadvantage of GMR is its complicated manufacturing process. The narrow time slot devoted for the panel had expired; most of the topics were only raised, but the fruitful discussions between the participants continued in the informal part of the conference, which is out of the scope of the present record.
Sensors based on soft magnetic materials Panel discussion夽 Pavel Ripka , GaH bor VeH rtesy * Czech Technical University, Faculty of Electrical Engineering, Technicka 2, 166 27 Praha 6, Czech Republic Hungarian Academy of Sciences, Research Institute for Technical Physics and Materials Science, P.O. Box 49, H-1525 Budapest, Hungary
The economic realities of productivity, quality and reliability for the industrial societies of the 21th century are placing major demands on existing manufacturing technologies. To meet both present and anticipated requirements, new and improved methods are needed. To be e!ective, the measurement, electronics and control components, and sub-systems, in particular sensors and sensor systems, have to be developed parallely as a part of computer-controlled manufacturing systems. Soft magnetic materials serve as a basic material for a wide scale of di!erent sensors. The aim of the present panel discussion was to give a survey of the latest technology and of the most important features of the sensors, which are based on soft magnetic materials, and to compare systematically their parameters. Magnetic sensors which are not based on soft magnetic materials (such as SQUIDs, NMR and semiconductor sensors) as well as reading heads were not covered by this panel The attention was concentrated mainly on device parameters and on applications, not on the theoretical background. P. Ripka gave the general introduction. He emphasized the keywords of the panel discussion: application, speci"cations and requirements. He had some general remarks: Sensors containing core, made of magnetic material seem to be always vectorial. It is possible to measure 1, 2, or even 3 components of the magnetic "eld using a single core. We are going to speak about sensors, which do measure "elds from DC; AC magnetic sensors
* Corresponding author. Tel.:#36}1-169-5165; fax:#36-13959284. E-mail address: [email protected] (G VeH rtesy). 夽 List of panelists: P. Ripka (discussion leader), Czech Technical University; L. Panina, Russian Academy of Sciences; M. Yamaguchi, Tohoku University; H. Hauser, Vienna University of Technology; D. Mapps, Plymouth University; J.M. Barandiaran, Universidad del Pais Vasco; L. Lanotte, Universita di Napoli `Federico IIa.
(such as induction coils) are not a subject of this discussion. We restricted our topic mainly to magnetic "eld sensors; although the broader meaning of the term &magnetic sensor' includes devices which, using magnetic principle, measure non-magnetic variables such as position and speed, force and torque, temperature or others. Invited researchers gave a brief overview of di!erent types of sensors. Each contribution had a similar structure. The general questions connected with each sensor type were: E Which is the application "eld of the given sensor? E Are the sensors already being produced or are they just laboratory prototypes? E What are their parameters? Besides the parameters well known for other sensors and instruments, magnetic sensors have peculiarities given by non-linear materials and complex e!ects used. The most important parameters and properties of magnetic "eld sensors can be grouped into the following categories: E Full-scale range, linearity, hysteresis, temperature coe$cient of sensitivity E Bias stability, o!set, o!set temperature coe$cient, long-term stability E Perming (if the sensor is subjected to a strong magnetic "eld, the o!set can be changed) E Noise E Resistance against environment (temperature, humidity, vibrations, radiation) E Resistance against perpendicular "eld and "eld gradient E Bandwidth E Power consumption, size E Reliability E Cost The possibilities of sensor miniaturization are of increasing importance. We often hear questions about the
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 9 1 - 2
796
P. Ripka, G. Ve& rtesy / Journal of Magnetism and Magnetic Materials 215}216 (2000) 795}799
Table 1 Application types
Application
Requirements
Precise (high-amplitude resolution)
Location of ferromagnetic objects Navigation Non-contact current meters
High linearity Low cross"eld e!ect Stable o!set
Low-noise
Biomagnetic Space research
Low-noise
Industrial
Position, speed Car industry Magnetic marking
Low-cost resistance against environment
Miniature (high spatial resolution)
Marking Surface mapping
Micropower
Fast
Pulse "elds, reading
High bandwidth
Table 2 Fluxgate sensor parameters
Top parameter
Standard
Range Linearity error Temperature coe!. of sensitivity O!set temperature coe$cient Perming Noise Long-term stability of the o!set Bandwidth Operating temperature range Power consumption Size Cross"eld error
10 mT 10 ppm 10 ppm/3C (0.05 nT/3C (1 nT o!set change after 10 mT shock 5 pT rms (0.05}10 Hz) 2 nT/yr 10 kHz !60}#2003C 1 mW 2 mm (1 nT for 50 lT "eld
200 lT 100 ppm 50 ppm/3C 0.2 nT/3C (5 nT 100 pT rms 5nT/8 hours 20 Hz !20}#703C 100 mW 30 mm 5 nT
integration compatibility, relation between the core size and sensitivity and about the suitability of the used technologies for the mass production. The &application types' of magnetic "eld sensors can be assorted in the following way, where the main requirements are also indicated (see Table 1): Fluxgate sensors are historically the "rst type of highresolution solid-state magnetic "eld sensors. They are still widely used in applications, although they were far beaten in production volume by magnetoresistors, especially AMR. The parameters of #uxgates should serve as a reference for newer types of sensors. However, any comparison should be made carefully, taking into consideration also the sensor application type. It is also tricky to compare the properties of the fresh new sensors with a traditional device which was constantly being improved for decades. Table 2 summarizes &top' values achieved for specialized types of #uxgates (but certainly not all of them simultaneously) and &standard' values achieved in highperformance devices. The disadvantage of #uxgate sensors is that they are usually o!ered only as a part of
magnetometer; high-performance #uxgates are only rarely available as components. The recent improvements and developments in #uxgate sensors are covered in a separate paper. L. Panina gave an overview about giant magnetoimpedance (GMI) sensors. Giant magnetoimpedance e!ect is a kind of high-frequency analogy of giant magnetic resitance (GMR), because it is basically a change in the AC impedance under the application of a magnetic "eld. GMI is essentially a high-frequency e!ect: a kind of side e!ect of the skin e!ect. The sensitivity is very high compared to GMR. The typical material of this type of sensor is amorphous wire with well-de"ned circumferential anisotropy and corresponding circular domain structure. In this case the sensing "eld is applied along the wire The sensitivity is almost zero if the "eld is perpendicular to the wire axis. It is possible to create a directional sensor using three wires perpendicular to each other. If a ribbon or a thin "lm is used instead of a wire, two-dimensional sensor using a single core can be created. GMI sensors are already used by industry. The response of the sensor strongly depends on excitation. Symmetrical
P. Ripka, G. Ve& rtesy / Journal of Magnetism and Magnetic Materials 215}216 (2000) 795}799
characteristics can be obtained almost without hysteresis. It is also possible to create asymmetrical characteristics that would be important to get a good linearity. The linearity can be improved and a stable operation point can be achieved by using negative feedback. There is no change in the sensor parameters up to 803C temperature. Several modi"cations of the GMI sensor were developed. It is possible to detect very low, highly localized magnetic "eld with 40 lm resolution. The length of the sensor based on amorphous wire can be reduced down to 3 mm without losing sensitivity. There are many prospective applications of this type of sensors, e.g. in medical electronics, in automobiles, in textile industry. The sensor can be integrated if a thin "lm is applied as a sensing material. M. Yamaguchi talked about thin xlm sensors. This type of sensors is mostly associated with thin "lm type of the previously discussed GMI sensors. The skin e!ect is utilized in a ferromagnetic body. The impedance of the element is proportional to the surface current density. With the application of magnetic "eld the permeability is changed. Very high sensitivity can be achieved by this type of sensor: the highest possible magnetic "eld resolution is 10\ T (if thermal #uctuation dominates the resolution). There are many forgoing works with thin "lm type sensors, e.g. Eddy Current Testing (ECT), accelerometers, magnetic "eld sensors. One of the main e!orts is to develop thin "lm micromagnetic devices with good characteristics. In the case of these GMI-type sensors the size can be easily reduced below 1 mm by applying photolithography technique. By reducing the length, the sensitivity is lost because of the demagnetizing e!ect. Because of this the width of the elements should also be reduced. However, to keep the sensitivity with the miniaturization it is necessary to control the domain con"guration. A DC bias is necessary to get high sensitivity. Without using any windings, the bias can be introduced by an integrated thin magnetically hard "lm; SmCo with in-plane anisotropy is utilized for this purpose. The driving frequency can be increased up to 300 MHz. 1 nT sensitivity has already been demonstrated. These sensors can be applied in intelligent transportation systems, in "eld compensation in big CRT monitors, in 3D position systems, etc. The initial cost can be rather high, but the running cost is very low, which is one of the main advantages of thin "lm sensors. Other advantages are the high sensitivity, small size and the possibility of integration. H. Hauser reviewed magnetooptical sensors. Magnetooptical sensors are a relatively new "eld of research and development. Magnetooptical devices are mainly used for switching, polarization and amplitude modulation of light. The only industrially available magnetooptical sensor is magnetooptical current transformer measuring magnetic "eld of strong currents by the Faraday rotation in diamagnetic "bers. Our group is developing a new generation of magnetooptical sensors for
797
measurement of magnetic "eld and position of light. These sensors are based on domain wall motion in soft magnetic garnets and in orthoferrites. The speci"cations are only preliminary, because only a laboratory prototype exists. In these sensors the position of domain walls is detected by means of the Faraday e!ect, using polarized light. A periodic magnetic "eld with a frequency up to several MHz can be measured using a simple ruler. To make a good sensor, a material with extraordinary properties is needed. One of these materials is the orthoferrite. In these materials the domain wall velocity is very high (up to 20 km/s), Faraday rotation is also very high (29003/cm), and the transparency is also extremely large. The only disadvantage of this material is the high birefringence; a complex sample preparation is therefore necessary. The modulation depth of the light transmitted through the two-domain structure is about 15%/lT, so at least 1 nT resolution of the sensor can be achieved, depending on the bandwidth. The presently existing 100 MHz bandwith can be extended up to 1 GHz. The output signal of the sensor is usually a time duration, and the domain walls are excited by an AC "eld. One of the parameters which are dependent on temperature is the signal/noise ratio. The maximum "eld to be measured is between 1 and 10 kA/m, and the size of the plate is 10;10 mm. D. Mapps summarized the state of art of magnetoresisitive (MR) sensors. This sensor technology has been known for quite a long time. MR sensors are widely used, especially in magnetic recording. What has happened in the last few years is, that materials have been signi"cantly improved. This means that MR sensors are being used in many more applications than just a few years ago. The sensitivity was improved by reducing the thickness of the sensing material. Even 150 As thick "lms can produce a very smooth magnetoresistance vs. applied "eld curve, producing a small noise. It is possible to get 97 dB signal-to-noise ratio which is quite enormous for such a sensor. The thickness is very important because in very thin "lms NeeH l walls exist instead of Bloch walls. Due to the much larger thickness of NeeH l walls, these walls pass imperfections with very low change of wall energy, resulting in very small noise compared to Bloch walls. This shows the important role of domain walls in the proper operation of MR devices which are not in a singledomain state. Another improvement which was achieved is to regularize the microstructure of the "lm material using new underlayer materials (e.g. platinum), thus improving the signal-to-noise ratio. Sophisticated electronic techniques were also used to improve the quality of the signal. The hysteresis can be removed by applying high-frequency bias "eld. A very sensitive detector can be produced from a pair of AMR sensors in a switch}bias mode where each sensor is switched alternately to magnetise in the opposite direction to the other at kHz frequencies. The resulting di!erential output can be
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P. Ripka, G. Ve& rtesy / Journal of Magnetism and Magnetic Materials 215}216 (2000) 795}799
detected using a phase-sensitive detector. The equipment for producing GMR sensors is quite complicated, which makes GMR sensors expensive. A drawback is that MR sensors are temperature sensitive and if you do not optimise the material they can be noisy. However, they have a multitude of uses in magnetic "eld detection. J.M. Barandiaran gave an overview about magnetoelastic sensors. Most of these sensors can be separated into two categories: static and dynamic. The topic of his talk was the static sensors; dynamic sensors are a subject of a separate paper. The main characteristic of magnetoelastic sensor is the magnetoelastic coupling coe$cient, which measures the conversion of magnetic to elastic energy or vice versa. In the 1960s there were already some static sensors based on the stress e!ect on the hysteresis loop of iron silicon; most of the present static magnetoelastic sensors work on the same basis. The direct application of a static magnetoelastic sensor is measurement of stress or strain. It is clear that magnetoelastic strain sensors are much better than strain gauges; their sensitivity is thousand times higher. However, the application of magnetoelastic sensors is more complicated than strain gauges because they need AC current supply. They have another interesting and important application possibility, which is the non- contact sensing element. They have given a good performance in rotating shaft measurement of the stresses. Torque measurement has been one of the most widespread applications of magnetoelastic sensors. Another interesting use of these sensors can be in medical applications e.g. sensors of breathing and blood pressure, which were presented during the conference, but these devices are not on the market yet. The sensitivity and wide dynamical range of these sensors are among the best features . For instance, the static magnetoelastic sensors have an equivalent &gauge factor' of several thousands, as compared with resistive semiconductor strain gauges, which have a limit of about 200, and metallic strain gauges that are only around 2. The temperature dependence is very small compared with the other sensors. Typical "gures of temperature coe$cient and maximum operating temperature are metallic strain gauges (0.05% deg\ and 803C), semiconductor strain gauges ( 0.1% deg\ and 1003C), magnetoelastic sensors based on amorphous ribbons ( 0.02% deg\ and 2003C). One of the most important problems is that these sensors will never be miniaturized because of the need of coils to drive the sensor and pick up the signal. The dynamical magnetoelastic sensors are much better placed for widespread applications at this moment as they work at resonance, and remote sensing is possible with distances of about 1 m from the coils to the stripes. They are already present on the market (e.g. for security purposes or for recognizing objects in supermarkets). Dr. Hasegawa in his talk gave an example of magnetoelastic security tags being currently used in the States. These have better performance than the old tags
based on the second harmonic detection and made up of soft magnetic stripes of permalloy or other materials with very low magnetostriction. L. Lanotte stressed some applications of dynamic magnetostriction and in particular recent applications which are based on magnetoelastic wave devices. A new type of sensor, based on a new type of amorphous alloy was shown, which can be used to measure the strain. The device works without bias magnetic "eld. It is also possible to measure the change in local magnetic "eld. It can be applied for measuring vibrations without contact. The sensitive parameter of the sensor is the amplitude of resonant magnetoelastic waves. It is possible to measure the deformation of another sample due to application of magnetic "eld using magnetostrictive materials. The direction is important because the displacement on the vibration can be measured only in the direction which is parallel to the ribbon core. The size is about 30 mm long with 5 mm diameter. The cost is low. It is only a prototype, there is no industrial production. However, the technical requirements are not di$cult, so industrial application is possible. In principle it can operate in a large temperature range. G. VeH rtesy presented a new type of magnetic "eld sensors (Fluxset) for measuring DC and AC (up to 100 kHz frequency) low-level magnetic "elds with high accuracy. Its principle of operation is close to the pulseposition-type #uxgate magnetometers. The particular advantage of these magnetometers is an output signal that can be simply converted into binary code. The measurement of a small magnetic "eld is reduced to a high-accuracy time measurement through the displacement of the magnetization curve produced by the "eld. The probes are suitable for axial measurement of the magnetic "eld. The transverse sensitivity is negligible. The available maximum resolution of the sensor is below 0.1 nT. The linearity is better than 1%. The temperature stability is extremely good, the measuring head can operate (without measurable change of the parameters) in the !200}#2003C temperature range. The time stability is also good, as was proved by a long-term measurement (using simultaneously a reference observatory magnetometer) of the variations of the Earth's magnetic "eld. The length of the 1 mm diameter probe can be varied between 5 and 20 mm; a longer probe ensures higher sensitivity. The spatial resolution is better than 0.1 mm. One of the main advantages of this sensor is the simple construction and low price. It can be applied in the same "elds like conventional #uxgates. Its parameters make it very suitable for application in eddy current testing, combining the eddy current method with magnetic "eld measurement. The summaries of the panelists were followed by discussion. It was inquired whether there is any theoretical limit for the detectable minimum magnetic xeld in the case of
P. Ripka, G. Ve& rtesy / Journal of Magnetism and Magnetic Materials 215}216 (2000) 795}799
di!erent types of sensors. Dr. Yamaguchi answered that in the case of thin "lm-type GMI sensor, if the limit is given by thermal stability and if a Co-based amorphous "lm is used, this theoretical limit is about 10\. T. P. Ripka commented that in case of #uxgate sensors, the real detection limit is much larger than the theoretical value estimated from models for Barkhausen noise. The e!ective "eld noise highly depends on the sensor size and geometry: long station magnetometers may have 10 pT resolution. But it should be clearly stated that in most cases (especially when measuring in the presence of the Earth's "eld) the sensor noise is not critical. The performance is usually limited by other factors like linearity, perming, cross"eld response and temperature stability. Another question was raised about the disadvantage of GMI sensor compared to GMR sensors. Dr. Panina answered: GMI is a kind of high-frequency sensor. There are problems with impedance mismatching and with re#ected signals, so electronic circuits should be developed to e!ectively operate at high frequency. Some types of self-oscillation circuits are being used, but their oscillation frequency is limited. The problem is that the
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oscillation frequency of these devices is not stable (it is too sensitive to the environment), which will result in unstable sensor parameters. Better results were achieved with hi-speed CMOS circuits, which generate pulse excitation, but their performance is still limited. Another disadvantage of GMI is: Because the origin of this e!ect is a kind of skin e!ect, if we want to reduce the size further, there is a limiting size reduction, and we lose by this way sensitivity and response speed. If the excitation frequency is 1 GHz, a "eld greater than 100 MHz cannot be detected. For modern computer technology even faster operation is needed. Dr. Yamaguchi: Another possible disadvantage of GMI, especially for thin "lm sensor is that this kind of sensor needs a DC bias to get high sensitivity. In the case of GMR exchange bias can be utilized, because the active "lm thickness is very small. However, the main disadvantage of GMR is its complicated manufacturing process. The narrow time slot devoted for the panel had expired; most of the topics were only raised, but the fruitful discussions between the participants continued in the informal part of the conference, which is out of the scope of the present record.