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Integrated magnetic field sensor based on magnetoresistive spin valve structures
Sensors and Actuators A 94 (2001) 64±68
Integrated magnetic ®eld sensor based on magnetoresistive spin valve structures J.L. Prietoa,*, N. Rouseb, N.K. Todda, D. Morecrofta, J. Wolfmana,1, J.E. Evettsa, M.G. Blamirea a
Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, UK b Telcon Ltd., Manor Royal, Crawley, West Sussex, RH10 2PY, UK Received 3 April 2001; received in revised form 23 July 2001; accepted 25 July 2001
Abstract A new integrated magnetic sensor, which is based on a micro-patterned spin valve bridge design, is described in this paper. The main advantage of the design is that an adjacent soft magnetic layer guides the magnetic ¯ux, allowing a balanced bridge sensor to be sensitive to the direction of the external magnetic ®eld. This new design avoids some of the additional circuitry and processing steps required in previous spin valve-based ®eld sensors. We report a signi®cant increase in the device sensitivity (15 mV/(VxOe)), even in non-optimised prototype devices compared to other commercially available lake NVE (15 mV/(VOe)). # 2001 Elsevier Science B.V. All rights reserved. Keywords: Magnetic sensor; Spin valve; Magnetoresistance; Current sensor
1. Introduction The recent rapid development of a wide range of magnetoresistive materials and devices has led to the development of microfabricated magnetic ®eld sensors with increasing sensitivity. This activity compliments existing device development based on superconducting quantum interference devices [1] ¯ux gate magnetometers [2] and Hall-effect sensors. Although the microfabrication of digital ®eld sensors based on magnetoresistive structures has reached a high level of sophistication in the speci®c area of magnetic data storage, there is still a major gap in the market for inexpensive microfabricated linear ®eld sensors which might ®nd application in a range of areas including current sensors and non-destructive evaluation. This paper reports the development of an integrated magnetic ®eld sensor based on magnetoresistive materials which is linearised by on-chip coils and ¯ux guides. Recent research on integrated magnetic sensors has focussed on the giant magnetoresistance effect (GMR) and the spin valve (SV) devices [3]. In both cases the *
Corresponding author. Tel.: 44-1223-334-375; fax: 44-1223-334-373. E-mail address: [email protected] (J.L. Prieto). 1 Present address: IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099, USA.
magnetoresistance comes from the spin scattering dependence of the conduction electrons. GMR heterostructure consist of multilayers of magnetic and non-magnetic metals, classically Co/Cu. The non-magnetic interlayer thickness controls the coupling between adjacent ferromagnetic layers so that at low ®elds the magnetic layers couple in an antiferromagnetic con®guration producing a high resistance, while at high ®elds all the layers are ferromagnetically coupled and the resistance is low. In general GMR materials require on a relatively large magnetic ®eld to achieve a reasonable change in resistance and they are independent of the direction of the magnetic ®eld. In order to apply GMR materials in devices which require sensitivity to the direction of the magnetic ®eld, the GMR sensor element has to be biased to a certain point in the resistance versus ®eld (R±H) curve [4]. The construction of a conventional spin valve is of the form AFM/FM1/NM/FM2, where AFM is antiferromagnetic material, FM1 is the ferromagnetic pinned layer, NM is a non magnetic metal and FM2 is the ferromagnetic free layer. The AFM layer induces a uniaxial anisotropy in FM1 by exchange coupling and forces the magnetic moment to lie in one direction until the external ®eld is large enough to unpin the material [5]. SVs offer a good magnetoresistance (10%) and a low switching ®eld because the free layer is decoupled from the pinned layer by an appropriate thickness of NM: the magnetisation reversal process of the free layer is
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 7 0 3 - 8
J.L. Prieto et al. / Sensors and Actuators A 94 (2001) 64±68
a high permeability transition, making it suitable for low magnetic ®eld detection. Although SVs have been applied as digital ®eld sensors in read heads for data storage, they are not suited for analogue ®eld measurements because the range of ®eld sensitivity is limited and the output (Voltage) does not cross zero at zero ®eld. A Wheatstone bridge design which incorporates SVs avoids the offset of the resistance for H 0 was introduced several years ago [6]. To achieve this, the SV resistors in the bridge are annealed with orthogonal easy axes in the pinned and free layers, allowing easy rotation of the free layer and minimising the hysteresis. The sensor is designed to be sensitive to the direction of the external ®eld by ensuring that the pinning directions in alternate branches of the bridge are in opposite directions. This is achieved by a complicated annealing process in which the heating and the local ®eld direction are created by passing a current through a copper track on top of the bridge. This annealing process makes this design unsuitable for volume manufacture. This paper describes the design and fabrication of a new magnetic sensor device, which is based on a Wheatstone bridge with micropatterned SVs, but with several improvements that make it suitable for production and real applications. In our device the magnetic ®eld experienced by the GMR elements in the bridge controlled by the presence of an integrated soft magnetic ¯ux guide. 2. Device design In order to avoid the annealing step described in the previous paragraph, the new design of the sensor retains the as-deposited anisotropy orientation of the SV heterostructure. In this design, driving the ¯ux generated by a ¯ux guide or soft adjacent layer (SAL) controls the local direc-
65
tion of the applied ®eld. Using this method, it is possible to swing the free layer of alternate arms of the bridge in different directions with respect to the pinning direction and hence gain an output which depends on the sign of the applied magnetic ®eld. Fig. 1 shows the basic structure of the sensor. The SAL con®guration drives the ¯ux lines of the external magnetic ®eld in a direction perpendicular to the SV segments, which are placed in narrow gaps in the SAL. The approximate dimension of the SAL is 2 mm 80 mm 1 mm and the gap where the resistor is located is 6 80 mm. Each SV is 4 100 mm and the gap between the SAL and the SV is l mm each side. Provided the ¯ux remains within the SAL guide structure, all the branches of the bridge experience an identical magnetic ®eld in the y direction when an external magnetic ®eld is applied in the longitudinal (x) direction. Since the anisotropy directions are set at the wafer level the pinning direction and the free layer easy axis will have the same orientation in all arms of the bridge; our design requires the use of a crossed SV so that the pinning direction and the free layer easy axis are orthogonal. Since the magnetic ®eld in the gap will not be large enough to change the pinning direction, the free layers rotate under the action of the local ®eld and a resistance change will result as a consequence of the different local ®eld directions imposed by the SAL structure. Fig. 1 shows the MR results for the crossed SV in the bridge with the SAL con®guration described above. For a zero external magnetic ®eld, the direction of magnetisation in the pinned layer is perpendicular to the gap of the SAL, and the anisotropy axis in the free layer is parallel to the gap. The magnetisation direction of the free layers (black arrows) move towards being either parallel or antiparallel to the direction of the pinned layers (grey arrows), corresponding either to a minimum or maximum measured resistance. If the external ®eld is applied in
Fig. 1. Schematic configuration of the SAL and the bridge (top left). The top right picture shows a scaled cross-section of one resistor in the middle of the bridge. The bottom picture shows the behaviour of the free (black) and pinned (gray) layer following the flux driven by the SAL. For exact dimensions, see text.
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J.L. Prieto et al. / Sensors and Actuators A 94 (2001) 64±68
the opposite direction, the con®guration and the sign of Vout, will be reversed. To linearise the detector and to provide a large working range this device must be designed with a feedback system so that it can operate at zero ¯ux in a null mode. Integrated coils wound around the ends of the SAL structure provide the feedback. 3. Device fabrication The SV structure used in these experiments, supplied by Nordico Ltd, was patterned from a wafer in which the magnetisation of the pinned layer and the anisotropy axis in the free layer were set to be orthogonal during the deposition process. The composition of the SV is Ta(5 nm)/NiFe(2.5 nm)/CoFe(2 nm)/Cu(2.6 nm)/CoFe(2 nm)/IrMn(10 nm)/Ta(5 nm). The MR of this multilayer is around 8%. The R±H curve for the external ®eld in both x and y directions is displayed in Fig. 2. The fabrication of the sensor requires a four step lithographic process (six if integrated feedback coils are required). To pattern the SV in the bridge from the wafer, we used Ar ion milling; to avoid imbalances in the bridge,
Fig. 2. R±H loop for the spin valve with the anisotropy axis of the free layer perpendicular to the pinning direction in the pinned layer. The curve with less magnetoresistance corresponds to the field applied perpendicular to the pinned axis and the magnetoresistance is larger when the field is parallel (see the inset). The small asymmetry of the curve is due to an offset field in the measuring system.
the sample holder is rotating continuously during the milling process, making the process uniform in the four resistors. The base layer for the Cu feedback coil was deposited by dc magnetron sputtering using a conventional lift-off
Fig. 3. Top: Photograph of the sensor. Bottom: Different lithography steps.
J.L. Prieto et al. / Sensors and Actuators A 94 (2001) 64±68
67
process. Insulation for this layer and the SV sensor bridge was provided by rf-sputtered SiO2. We have shown previously that sputter-deposited Vitrovac 6025 may be optimised for low coercitivity [7]; this material was used in our devices for the SAL structure; coercivities less than 0.2 Oe can be achieved with this material in an unpatterned ®lm. Slightly higher coercivities are normally obtained in patterned layers. Fig. 3 shows a photograph of the device and the different lithography and deposition steps. The ®nal SiO2 isolation layer and Cu wiring layer (which forms the upper turns of the feedback coil) were deposited by rf- and dc-sputtering, respectively. Chips were wirebonded for measurement. 4. Experimental results Fig. 4 Shows the R±H curve of a single SV element of the bridge when the external magnetic ®eld is applied along the x direction, with and without the SAL layer. There is a clear difference between the graphs: at low ®elds, when the SAL layer is present, the response is similar to when the ®eld is applied in the y direction. This shows that the SAL is correctly guiding the ¯ux around the spin valve. For higher ®elds the magnetisation in the SAL is saturated and the longitudinal external magnetic ®eld acts on the resistor tending to align the free and pinned layer of the SV. The output of the entire sensor, for a 500 mA dc excitation current (each branch of the bridge has a resistance of approximately 1 kO), is shown in Fig. 5. The quasi-linear range covers 5 Oe and it is magni®ed in the inset of the ®gure; the sensitivity is approximately 25 mV/Oe. The curve shows a small coercivity that can be easily removed using an ac technique [8]. As the R±H curve of the spin valve tends to saturation, the magnetisation is mainly controlled by the demagnetising factor of the SAL. As the thickness of the SAL layer
Fig. 5. Response of the sensor to the external magnetic field, showing a sensitivity of 25 mV/Oe for a 0.5 mA current excitation in the bridge.
increases, the demagnetising factor along the two pemendicular axis increases and a larger external magnetic ®eld is required to saturate the SAL. Therefore, increasing the thickness of the SAL layer gives a smoother approach to saturation. The working point for correct feedback operation must lie within the turning points of the response; beyond this point conventional feedback methods would tend to drive the sensor away from zero. The feedback coils deposited around the SAL can produce a ®eld of 0.3 Oe/mA, with a maximum current of 50 mA. This value is limited by the upper part of the coil and the via interconnects, however, it is suf®cient to null small applied magnetic ®elds and demonstrate the correct operation of the device as a null detector. The maximum feedback coil current could be increased using an improved fabrication technology such as a damascene process. In this type of sensor, the sensitivity, the hysteresis and the dynamic range are mainly controlled by the magnetic characteristics of the SAL. The higher the permeability of the SAL, the higher the sensitivity but the smaller the dynamic range. Therefore it is possible to tailor the response of the sensor by changing the magnetic material and the design of the SAL. For a device operated in a null mode clearly the dynamic range is unimportant and it is desirable to have the highest sensitivity possible. In our device this sensitivity is controlled both by the SAL structure and the MR of the SV used; neither are by any means optimised, it is possible to obtain substantially lower coercivities in the thin ®lm 6025 material, and much higher performance dual spin valves are currently under development. 5. Conclusion
Fig. 4. Behaviour of one sensor element in the bridge with the magnetic field applied along the length. The dashed curve corresponds to the behaviour without SAL and the continuous one to that with SAL. This plot clearly shows that the SAL is driven properly the magnetic flux.
We have developed an integrated magnetic sensor based on a Wheatstone bridge of micropatterned SV resistors with the addition of a SAL structure that controls the principal characteristics of the sensor. This design differs from earlier devices in that it is the local ®eld direction rather than the
68
J.L. Prieto et al. / Sensors and Actuators A 94 (2001) 64±68
local anisotropy of the SV material which is varied around the bridge. This design offers high sensitivity, small size, a relatively easy manufacturing process and potentially low price. The device therefore has potential for applications in a number of ®elds. Acknowledgements This work was supported by the DTI/EPSRC LINK Sensors programme. J.L. Prieto is ®nancially supported the EU Marie Curie Fellowship programme. N.K. Todd is grateful for support from Scienti®c Generics Ltd. References [1] J. Heremans, Solid state magnetic field sensors and applications, J. Phys. D: Appl. Phys. 26 (1993) 1149±1168. [2] F. Prindahl, The fluxgate magnetometer, J. Phys. E: Sci. Instrum. 12 (1979) 241±253. [3] B. Dieny, Giant magnetoresistance in spin-valve multilayers, J. Magn. Magn. Mater. 136 (1994) 335±359. [4] W. Ku, F. Silva, J. Hernando, P.P. Freitas, Integrated giant magnetoresistance bridge sensors with transverse permanent magnet biasing, J. Appl. Phys. 87 (9) (2000) 5353±5355. [5] J. Norgues, I.K. Schuller, Exchange bias, J. Magn. Magn. Mater. 192 (1999) 203±232. [6] J.K. Spong, V.S. Speriosu, R.E. Fontana, Giant magnetoresistive spin valve bridge sensor, IEEE Trans. Magn. 32 (2) (1996) 366± 370. [7] D.B. Jardine, N.D. Mathur, M.G. Blamire, J.E. Evetts, Increased field sensitivity in Co/Cu multilayers with soft adjacent layers, IEEE Trans. Magn. 34 (4) (1998) 1297±1300. [8] J.L. Prieto, C. Aroca, P. SaÂnchez, E. LoÂpez, M.C. SaÂnchez, Current effects in magnetostrictive piezoelectric sensors, J. Magn. Magn. Mater. 174 (1997) 289±294.
Biographies Jose L. Prieto did his first degree in physics in the Complutense University of Madrid (1993). He obtained his PhD degree from Politenica University of Madrid developing a new magnetic sensor technology based in
magnetostrictive and piezoelectric materials (1998). Since January 2000, he is researching for Cambridge University on magnetoresistive materials and applications. Nick Rouse received his BSc in Electrical and Electronic engineering at Portsmouth Polytechnic 1978. He then carried out post-graduate research at Portsmouth Polytechnic in the photoelectromagnetic effect. From 1981, he was engaged in industrial research, design and engineering in the following fields: harmonic radar, buried object detectors, image processing (at Plessey), television image processing hardware (at Quantel) electron microscope imaging (at VG Microscopes) and magnetically based sensors (at Telcon). He is presently employed as Senior Electronic Engineer at Telcon Ltd, Crawley, England. Neil K. Todd is currently writing his PhD thesis on ``Colossal Magnetoresistance Devices''. Prior to his doctoral work, Neil studied for a BA in Natural Sciences, also at Cambridge University. Debbie Morecroft did her undergraduate degree in materials science and metallurgy at Liverpool University. After working for one year in industry for New Transducers Ltd, she started her PhD project on spin valve devices. She is currently in the second year of her PhD. Jerome S. Wolfman studied solid state physics and materials science at Denis Diderot Paris University, France. He received his PhD degree from Caen University (France) for his work on manganites thin films in 1998. After a postdoctoral fellowship at the University of Cambridge (UK) devoted to GMR field sensors, he is now at the IBM Almaden Research Center in California working on magnetic tunnel junctions for read head applications. Jan E. Evetts is Professor of Device Materials and leader of the Device Materials Research Group at the Department of Materials Science, working on a wide range of materials for device applications including magnetic and superconducting oxides. The group has many years of experience in the deposition of thin films and heterostructures involving oxides, nitrides and carbides as well as metallic systems. Research on applied magnetism has included work on metallic glass, soft magnetic films, multilayers for recording media, and for giant magnetoresistance. In addition Professor Evetts edited the ``Concise Encyclopaedia of Magnetic and Superconducting Materials'' for Pergamon in 1992. Mark G. Blamire received his BA and PhD degrees from the University of Cambridge. Since then he has been on the research and academic staff of the Department of Material Science at the University of Cambridge. In 1999 he was promoted to Reader in device materials. His research interests span magnetic and superconducting devices, nanotechnology and thin film growth.
Integrated magnetic ®eld sensor based on magnetoresistive spin valve structures J.L. Prietoa,*, N. Rouseb, N.K. Todda, D. Morecrofta, J. Wolfmana,1, J.E. Evettsa, M.G. Blamirea a
Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, UK b Telcon Ltd., Manor Royal, Crawley, West Sussex, RH10 2PY, UK Received 3 April 2001; received in revised form 23 July 2001; accepted 25 July 2001
Abstract A new integrated magnetic sensor, which is based on a micro-patterned spin valve bridge design, is described in this paper. The main advantage of the design is that an adjacent soft magnetic layer guides the magnetic ¯ux, allowing a balanced bridge sensor to be sensitive to the direction of the external magnetic ®eld. This new design avoids some of the additional circuitry and processing steps required in previous spin valve-based ®eld sensors. We report a signi®cant increase in the device sensitivity (15 mV/(VxOe)), even in non-optimised prototype devices compared to other commercially available lake NVE (15 mV/(VOe)). # 2001 Elsevier Science B.V. All rights reserved. Keywords: Magnetic sensor; Spin valve; Magnetoresistance; Current sensor
1. Introduction The recent rapid development of a wide range of magnetoresistive materials and devices has led to the development of microfabricated magnetic ®eld sensors with increasing sensitivity. This activity compliments existing device development based on superconducting quantum interference devices [1] ¯ux gate magnetometers [2] and Hall-effect sensors. Although the microfabrication of digital ®eld sensors based on magnetoresistive structures has reached a high level of sophistication in the speci®c area of magnetic data storage, there is still a major gap in the market for inexpensive microfabricated linear ®eld sensors which might ®nd application in a range of areas including current sensors and non-destructive evaluation. This paper reports the development of an integrated magnetic ®eld sensor based on magnetoresistive materials which is linearised by on-chip coils and ¯ux guides. Recent research on integrated magnetic sensors has focussed on the giant magnetoresistance effect (GMR) and the spin valve (SV) devices [3]. In both cases the *
Corresponding author. Tel.: 44-1223-334-375; fax: 44-1223-334-373. E-mail address: [email protected] (J.L. Prieto). 1 Present address: IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099, USA.
magnetoresistance comes from the spin scattering dependence of the conduction electrons. GMR heterostructure consist of multilayers of magnetic and non-magnetic metals, classically Co/Cu. The non-magnetic interlayer thickness controls the coupling between adjacent ferromagnetic layers so that at low ®elds the magnetic layers couple in an antiferromagnetic con®guration producing a high resistance, while at high ®elds all the layers are ferromagnetically coupled and the resistance is low. In general GMR materials require on a relatively large magnetic ®eld to achieve a reasonable change in resistance and they are independent of the direction of the magnetic ®eld. In order to apply GMR materials in devices which require sensitivity to the direction of the magnetic ®eld, the GMR sensor element has to be biased to a certain point in the resistance versus ®eld (R±H) curve [4]. The construction of a conventional spin valve is of the form AFM/FM1/NM/FM2, where AFM is antiferromagnetic material, FM1 is the ferromagnetic pinned layer, NM is a non magnetic metal and FM2 is the ferromagnetic free layer. The AFM layer induces a uniaxial anisotropy in FM1 by exchange coupling and forces the magnetic moment to lie in one direction until the external ®eld is large enough to unpin the material [5]. SVs offer a good magnetoresistance (10%) and a low switching ®eld because the free layer is decoupled from the pinned layer by an appropriate thickness of NM: the magnetisation reversal process of the free layer is
0924-4247/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 ( 0 1 ) 0 0 7 0 3 - 8
J.L. Prieto et al. / Sensors and Actuators A 94 (2001) 64±68
a high permeability transition, making it suitable for low magnetic ®eld detection. Although SVs have been applied as digital ®eld sensors in read heads for data storage, they are not suited for analogue ®eld measurements because the range of ®eld sensitivity is limited and the output (Voltage) does not cross zero at zero ®eld. A Wheatstone bridge design which incorporates SVs avoids the offset of the resistance for H 0 was introduced several years ago [6]. To achieve this, the SV resistors in the bridge are annealed with orthogonal easy axes in the pinned and free layers, allowing easy rotation of the free layer and minimising the hysteresis. The sensor is designed to be sensitive to the direction of the external ®eld by ensuring that the pinning directions in alternate branches of the bridge are in opposite directions. This is achieved by a complicated annealing process in which the heating and the local ®eld direction are created by passing a current through a copper track on top of the bridge. This annealing process makes this design unsuitable for volume manufacture. This paper describes the design and fabrication of a new magnetic sensor device, which is based on a Wheatstone bridge with micropatterned SVs, but with several improvements that make it suitable for production and real applications. In our device the magnetic ®eld experienced by the GMR elements in the bridge controlled by the presence of an integrated soft magnetic ¯ux guide. 2. Device design In order to avoid the annealing step described in the previous paragraph, the new design of the sensor retains the as-deposited anisotropy orientation of the SV heterostructure. In this design, driving the ¯ux generated by a ¯ux guide or soft adjacent layer (SAL) controls the local direc-
65
tion of the applied ®eld. Using this method, it is possible to swing the free layer of alternate arms of the bridge in different directions with respect to the pinning direction and hence gain an output which depends on the sign of the applied magnetic ®eld. Fig. 1 shows the basic structure of the sensor. The SAL con®guration drives the ¯ux lines of the external magnetic ®eld in a direction perpendicular to the SV segments, which are placed in narrow gaps in the SAL. The approximate dimension of the SAL is 2 mm 80 mm 1 mm and the gap where the resistor is located is 6 80 mm. Each SV is 4 100 mm and the gap between the SAL and the SV is l mm each side. Provided the ¯ux remains within the SAL guide structure, all the branches of the bridge experience an identical magnetic ®eld in the y direction when an external magnetic ®eld is applied in the longitudinal (x) direction. Since the anisotropy directions are set at the wafer level the pinning direction and the free layer easy axis will have the same orientation in all arms of the bridge; our design requires the use of a crossed SV so that the pinning direction and the free layer easy axis are orthogonal. Since the magnetic ®eld in the gap will not be large enough to change the pinning direction, the free layers rotate under the action of the local ®eld and a resistance change will result as a consequence of the different local ®eld directions imposed by the SAL structure. Fig. 1 shows the MR results for the crossed SV in the bridge with the SAL con®guration described above. For a zero external magnetic ®eld, the direction of magnetisation in the pinned layer is perpendicular to the gap of the SAL, and the anisotropy axis in the free layer is parallel to the gap. The magnetisation direction of the free layers (black arrows) move towards being either parallel or antiparallel to the direction of the pinned layers (grey arrows), corresponding either to a minimum or maximum measured resistance. If the external ®eld is applied in
Fig. 1. Schematic configuration of the SAL and the bridge (top left). The top right picture shows a scaled cross-section of one resistor in the middle of the bridge. The bottom picture shows the behaviour of the free (black) and pinned (gray) layer following the flux driven by the SAL. For exact dimensions, see text.
66
J.L. Prieto et al. / Sensors and Actuators A 94 (2001) 64±68
the opposite direction, the con®guration and the sign of Vout, will be reversed. To linearise the detector and to provide a large working range this device must be designed with a feedback system so that it can operate at zero ¯ux in a null mode. Integrated coils wound around the ends of the SAL structure provide the feedback. 3. Device fabrication The SV structure used in these experiments, supplied by Nordico Ltd, was patterned from a wafer in which the magnetisation of the pinned layer and the anisotropy axis in the free layer were set to be orthogonal during the deposition process. The composition of the SV is Ta(5 nm)/NiFe(2.5 nm)/CoFe(2 nm)/Cu(2.6 nm)/CoFe(2 nm)/IrMn(10 nm)/Ta(5 nm). The MR of this multilayer is around 8%. The R±H curve for the external ®eld in both x and y directions is displayed in Fig. 2. The fabrication of the sensor requires a four step lithographic process (six if integrated feedback coils are required). To pattern the SV in the bridge from the wafer, we used Ar ion milling; to avoid imbalances in the bridge,
Fig. 2. R±H loop for the spin valve with the anisotropy axis of the free layer perpendicular to the pinning direction in the pinned layer. The curve with less magnetoresistance corresponds to the field applied perpendicular to the pinned axis and the magnetoresistance is larger when the field is parallel (see the inset). The small asymmetry of the curve is due to an offset field in the measuring system.
the sample holder is rotating continuously during the milling process, making the process uniform in the four resistors. The base layer for the Cu feedback coil was deposited by dc magnetron sputtering using a conventional lift-off
Fig. 3. Top: Photograph of the sensor. Bottom: Different lithography steps.
J.L. Prieto et al. / Sensors and Actuators A 94 (2001) 64±68
67
process. Insulation for this layer and the SV sensor bridge was provided by rf-sputtered SiO2. We have shown previously that sputter-deposited Vitrovac 6025 may be optimised for low coercitivity [7]; this material was used in our devices for the SAL structure; coercivities less than 0.2 Oe can be achieved with this material in an unpatterned ®lm. Slightly higher coercivities are normally obtained in patterned layers. Fig. 3 shows a photograph of the device and the different lithography and deposition steps. The ®nal SiO2 isolation layer and Cu wiring layer (which forms the upper turns of the feedback coil) were deposited by rf- and dc-sputtering, respectively. Chips were wirebonded for measurement. 4. Experimental results Fig. 4 Shows the R±H curve of a single SV element of the bridge when the external magnetic ®eld is applied along the x direction, with and without the SAL layer. There is a clear difference between the graphs: at low ®elds, when the SAL layer is present, the response is similar to when the ®eld is applied in the y direction. This shows that the SAL is correctly guiding the ¯ux around the spin valve. For higher ®elds the magnetisation in the SAL is saturated and the longitudinal external magnetic ®eld acts on the resistor tending to align the free and pinned layer of the SV. The output of the entire sensor, for a 500 mA dc excitation current (each branch of the bridge has a resistance of approximately 1 kO), is shown in Fig. 5. The quasi-linear range covers 5 Oe and it is magni®ed in the inset of the ®gure; the sensitivity is approximately 25 mV/Oe. The curve shows a small coercivity that can be easily removed using an ac technique [8]. As the R±H curve of the spin valve tends to saturation, the magnetisation is mainly controlled by the demagnetising factor of the SAL. As the thickness of the SAL layer
Fig. 5. Response of the sensor to the external magnetic field, showing a sensitivity of 25 mV/Oe for a 0.5 mA current excitation in the bridge.
increases, the demagnetising factor along the two pemendicular axis increases and a larger external magnetic ®eld is required to saturate the SAL. Therefore, increasing the thickness of the SAL layer gives a smoother approach to saturation. The working point for correct feedback operation must lie within the turning points of the response; beyond this point conventional feedback methods would tend to drive the sensor away from zero. The feedback coils deposited around the SAL can produce a ®eld of 0.3 Oe/mA, with a maximum current of 50 mA. This value is limited by the upper part of the coil and the via interconnects, however, it is suf®cient to null small applied magnetic ®elds and demonstrate the correct operation of the device as a null detector. The maximum feedback coil current could be increased using an improved fabrication technology such as a damascene process. In this type of sensor, the sensitivity, the hysteresis and the dynamic range are mainly controlled by the magnetic characteristics of the SAL. The higher the permeability of the SAL, the higher the sensitivity but the smaller the dynamic range. Therefore it is possible to tailor the response of the sensor by changing the magnetic material and the design of the SAL. For a device operated in a null mode clearly the dynamic range is unimportant and it is desirable to have the highest sensitivity possible. In our device this sensitivity is controlled both by the SAL structure and the MR of the SV used; neither are by any means optimised, it is possible to obtain substantially lower coercivities in the thin ®lm 6025 material, and much higher performance dual spin valves are currently under development. 5. Conclusion
Fig. 4. Behaviour of one sensor element in the bridge with the magnetic field applied along the length. The dashed curve corresponds to the behaviour without SAL and the continuous one to that with SAL. This plot clearly shows that the SAL is driven properly the magnetic flux.
We have developed an integrated magnetic sensor based on a Wheatstone bridge of micropatterned SV resistors with the addition of a SAL structure that controls the principal characteristics of the sensor. This design differs from earlier devices in that it is the local ®eld direction rather than the
68
J.L. Prieto et al. / Sensors and Actuators A 94 (2001) 64±68
local anisotropy of the SV material which is varied around the bridge. This design offers high sensitivity, small size, a relatively easy manufacturing process and potentially low price. The device therefore has potential for applications in a number of ®elds. Acknowledgements This work was supported by the DTI/EPSRC LINK Sensors programme. J.L. Prieto is ®nancially supported the EU Marie Curie Fellowship programme. N.K. Todd is grateful for support from Scienti®c Generics Ltd. References [1] J. Heremans, Solid state magnetic field sensors and applications, J. Phys. D: Appl. Phys. 26 (1993) 1149±1168. [2] F. Prindahl, The fluxgate magnetometer, J. Phys. E: Sci. Instrum. 12 (1979) 241±253. [3] B. Dieny, Giant magnetoresistance in spin-valve multilayers, J. Magn. Magn. Mater. 136 (1994) 335±359. [4] W. Ku, F. Silva, J. Hernando, P.P. Freitas, Integrated giant magnetoresistance bridge sensors with transverse permanent magnet biasing, J. Appl. Phys. 87 (9) (2000) 5353±5355. [5] J. Norgues, I.K. Schuller, Exchange bias, J. Magn. Magn. Mater. 192 (1999) 203±232. [6] J.K. Spong, V.S. Speriosu, R.E. Fontana, Giant magnetoresistive spin valve bridge sensor, IEEE Trans. Magn. 32 (2) (1996) 366± 370. [7] D.B. Jardine, N.D. Mathur, M.G. Blamire, J.E. Evetts, Increased field sensitivity in Co/Cu multilayers with soft adjacent layers, IEEE Trans. Magn. 34 (4) (1998) 1297±1300. [8] J.L. Prieto, C. Aroca, P. SaÂnchez, E. LoÂpez, M.C. SaÂnchez, Current effects in magnetostrictive piezoelectric sensors, J. Magn. Magn. Mater. 174 (1997) 289±294.
Biographies Jose L. Prieto did his first degree in physics in the Complutense University of Madrid (1993). He obtained his PhD degree from Politenica University of Madrid developing a new magnetic sensor technology based in
magnetostrictive and piezoelectric materials (1998). Since January 2000, he is researching for Cambridge University on magnetoresistive materials and applications. Nick Rouse received his BSc in Electrical and Electronic engineering at Portsmouth Polytechnic 1978. He then carried out post-graduate research at Portsmouth Polytechnic in the photoelectromagnetic effect. From 1981, he was engaged in industrial research, design and engineering in the following fields: harmonic radar, buried object detectors, image processing (at Plessey), television image processing hardware (at Quantel) electron microscope imaging (at VG Microscopes) and magnetically based sensors (at Telcon). He is presently employed as Senior Electronic Engineer at Telcon Ltd, Crawley, England. Neil K. Todd is currently writing his PhD thesis on ``Colossal Magnetoresistance Devices''. Prior to his doctoral work, Neil studied for a BA in Natural Sciences, also at Cambridge University. Debbie Morecroft did her undergraduate degree in materials science and metallurgy at Liverpool University. After working for one year in industry for New Transducers Ltd, she started her PhD project on spin valve devices. She is currently in the second year of her PhD. Jerome S. Wolfman studied solid state physics and materials science at Denis Diderot Paris University, France. He received his PhD degree from Caen University (France) for his work on manganites thin films in 1998. After a postdoctoral fellowship at the University of Cambridge (UK) devoted to GMR field sensors, he is now at the IBM Almaden Research Center in California working on magnetic tunnel junctions for read head applications. Jan E. Evetts is Professor of Device Materials and leader of the Device Materials Research Group at the Department of Materials Science, working on a wide range of materials for device applications including magnetic and superconducting oxides. The group has many years of experience in the deposition of thin films and heterostructures involving oxides, nitrides and carbides as well as metallic systems. Research on applied magnetism has included work on metallic glass, soft magnetic films, multilayers for recording media, and for giant magnetoresistance. In addition Professor Evetts edited the ``Concise Encyclopaedia of Magnetic and Superconducting Materials'' for Pergamon in 1992. Mark G. Blamire received his BA and PhD degrees from the University of Cambridge. Since then he has been on the research and academic staff of the Department of Material Science at the University of Cambridge. In 1999 he was promoted to Reader in device materials. His research interests span magnetic and superconducting devices, nanotechnology and thin film growth.