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Development of a very high sensitivity magnetic field sensor based on planar Hall effect
Journal Pre-proofs Development of a very high sensitivity magnetic field sensor based on planar Hall effect Arnab Roy, P. Sampathkumar, P.S. Anil Kumar PII: DOI: Reference:
S0263-2241(20)30127-5 https://doi.org/10.1016/j.measurement.2020.107590 MEASUR 107590
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
12 September 2019 22 January 2020 4 February 2020
Please cite this article as: A. Roy, P. Sampathkumar, P.S. Anil Kumar, Development of a very high sensitivity magnetic field sensor based on planar Hall effect, Measurement (2020), doi: https://doi.org/10.1016/ j.measurement.2020.107590
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier Ltd.
Title: Development of a very high sensitivity magnetic field sensor based on planar Hall effect Authors: Arnab Roy1*, P. Sampathkumar1¶ and P.S. Anil Kumar1 1. Department of Physics, Indian Institute of Science, Bangalore 560012, India * Present address: Department of Physics, Bar Ilan University, Ramat Gan 5290002, Israel ¶ Present address: DRDO-BU Center for Life Sciences, Bharathiar University Campus, Coimbatore-641046, India Corresponding Author: Arnab Roy. Email: [email protected] Highlights: UHV-PLD grown Permalloy used as PHE based ‘Hall cross’ magnetic field sensor Sensitivity of 650 Ω/T in linear regime 400% output voltage change in the switching regime with slope of 1200Ω/T Field detection threshold of 5nT Angular sensitivity of 6×10-5 O for geomagnetic field detection. Useful for magnetic anomaly detection Abstract: While planar Hall effect based magnetic field detection is a wellstudied area, the sensing capabilities of ‘Hall cross’ sensors had not improved significantly for more than a decade. Here we report a major improvement in the sensing characteristics of ‘cross’ geometry by using pulsed laser ablation grown Permalloy as sensing material. Our sensor has two modes of operation. In the linear regime, it has a field sensitivity of 650Ω/T with an estimated detection threshold of 5nT in open-loop condition. In switching applications, it shows a 400% output voltage swing in a range of ±0.3mT, a value comparable with modern TMR sensors at room temperature. These values, which are the highest reported for the ‘cross’ geometry, make it applicable in a wide range of scenarios like geomagnetic field and magnetic anomaly detection. The small size of the sensing area also makes it a desirable choice for magnetic microbead based biomolecule detection. Keywords: Planar Hall Effect; High sensitivity low magnetic field sensor; magnetic anomaly sensor; geomagnetic field sensor; Permalloy
1. Introduction
Magnetic field sensors rule our daily life. From computer hard disks and electronic compass in a cellphone, to the myriad of proximity and position sensors required for the normal operation of an ordinary car, magnetic field sensors occupy an indispensable role in the way our civilization operates. Among the magnetic field sensors that can operate at room temperature, search coil magnetometers and fluxgate magnetometers have been able to achieve sensitivities of 20fT and 10pT respectively. However, they are endowed with several shortcomings, the biggest disadvantage being that they are sensitive to the flux linked through a coil, and the signal is, therefore proportional to the area of the coils. In practice, such sensors do not achieve any useful sensitivity unless they are at least a few mm2 in area. Besides, both these types of sensors require fairly complicated driving and readout electronics for closed loop operation, in order to achieve the above-mentioned level of sensitivity. Magnetoresistive sensors and Hall effect sensors are devoid of these difficulties, as the signal is based on a change in the material properties of the sensor element. Their dimensions can be as small as a few µm2 and may possess field sensitivities in the range of 10nT and 100nT respectively [1] Hall effect sensors are commonly constructed out of the III-V semiconductor InSb, as metals do not generate an adequate Hall voltage due to their large carrier densities. The drawback to using semiconductors as a sensing element is that the concentration of charge carriers can vary widely depending on temperature. So, despite being sensitive, they require separate temperature compensating circuitry to achieve accurate readings. Magnetoresistive sensors, on the other hand, are devoid of this problem and have the additional advantage of higher operating frequency, which is typically limited to a few kHz for a Hall-effect sensor. Anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) based sensors constitute an active area of development [2–6] their main applicability is in areas that require high sensitivity and small size. Hall effect sensors have been outperformed by PHE based sensors in this area. Three major areas can be identified where there is considerable potential for application of PHE based sensors. These are (1) Bio-molecule sensors based on magnetic microbead detection. Here, due to small size of the particles to be detected, the total magnetic flux available for detection is very small.
Thus, sensors relying on flux-linkage are less useful. So are any other types of sensor requiring the magnetic field to be uniform over many mm2. (2) Geomagnetic field sensors: These require high sensitivity and stability in order to detect small changes in the angle of the applied field. (3) Magnetic anomaly sensors. The requirement for this type of sensors is high sensitivity, robustness to environment and simplicity, enabling deployment in large numbers. In this paper we present a PHE based thin film sensor constructed with the above applications in mind. Our PHE ‘cross’ sensors fabricated out of NiFe thin films possess characteristics of simplicity, small size, high sensitivity, high stability and relative high durability under ambient conditions. Field tests are also presented, that test the usefulness of the sensor for these applications.
Anisotropic magnetoresistance and planar Hall effect have been utilized extensively in the past for the construction of low field magnetic sensors. The Planar Hall effect is expressed [7] by the equation
|| ET J 2
sin 2
(1)
Here α is the angle between the magnetization and the current density J, ET is the electric field component in a perpendicular direction to J in the plane of the film, ρ’s are the parallel and perpendicular resistivities. A range of sensor architectures that utilize planar Hall effect can be found in the literature. The simplest of these is a continuous film of anisotropic magnetoresistive material [7,8] such as CoFe or NiFe deposited on a suitable substrate. NiFe continuous film sensors have been demonstrated to show a 44% Hall resistance change and 900 Ω/T sensitivity in our previous work [7]. The major drawback in these sensor designs is the inadequate decoupling between the ordinary resistance and the planar Hall resistance, leading to appreciable quiescent voltages at the operating point. The problem is addressed by fabricating Hall bars out of the sensing material [9]. Various types of studies on the sensing material can be performed using microstructured Hall bars [10–14] such as, the probing of the magnetization reversal mechanism of multilayers and the effects of biasing magnetic fields on planar Hall effect. In the domain of magnetic field sensors, a significant amount of effort has been put on the construction of exchange biased Hall bars, the most common being NiFeIrMn exchange biased system. The purpose of making an exchange biased system in to make the
planar Hall voltage linear in the applied field, as has been described by Schuhl, van Dau[15,16], Ejsing[17,18] and later by Tu and coworkers[19]. The motivation has been to create a sensor suitable for detecting magnetic micro and nano-beads, which can be suitably adapted for the detection of bio molecules. For more than a decade, the sensitivity of this sensor architecture has remained between 38 Ω/T and 330 Ω/T [17,18,20–23] with a detection threshold of around 10 nT [16]. These values were exceeded later by more complicated Hall bridge sensor geometries [3–6]. But the larger area of these designs made them less suitable for certain applications like magnetic microbead detection. So, the simple Hall cross geometry continues to be an active area of research [24]
2. Materials and Methods
2.1 Sample preparation, structural studies and preliminary characterization
Permalloy (Ni80Fe20) of thickness 15nm was deposited on undoped Si (100) substrates using pulsed laser ablation under UHV conditions at a base pressure of 410-10 mbar. Structural characterization of the films was done by X Ray diffraction and microstructure was determined using AFM. XRD showed the presence of Permalloy (111) peak at 43.8O and the absence of all other peaks of Permalloy (Fig.1a). Pole figure measured around the (111) peak showed strong out-of-plane orientational order, with FWHM < 6O (Fig.1b). These indicated a strong degree of (111) orientational order. Under AFM, the samples showed no evidence of droplet formation (Fig.1c), which are a common problem for metal films grown by PLD. The average grain diameter was 160 nm with r.m.s. surface roughness of 1nm (Fig.1d). The films were also characterized by transport and magnetization measurements (Fig.2). Transport data showed a residual resistivity of 15.5 µΩ-cm, one of the smallest values reported for Permalloy [25]. Magnetization measurements indicated uniaxial anisotropy with easy axis along (110) substrate direction (Fig.2 inset). 100µm 2mm Hall bar patterns were generated using optical lithography. The sensor structure was realized by lift-off.
2.2 Measurement The whole device, including the contact pads were fabricated out of NiFe (Fig.3a inset). Contact to Cu electrical leads was achieved through Ag paint. The active area of the sensor was where the voltage leads contacted the Hall bar, and thus had dimensions of 100µm×100µm Planar Hall effect measurements were carried out at room temperature in the magnetic field of a Helmholtz coil (±12 mT) driven by a digitally controlled current source. Magnetic field was applied in the plane of the sample. A constant current of 100µA – 1mA was passed through the sample using a Keithley 6221 and the AMR and PHE were measured using a nanovoltmeter (Keithley 2182A). Currents higher than ~5mA were fund to degrade the sensor characteristics irreversibly. Angular dependence of the AMR/PHE curves was studied using a homemade automated sample rotation system. Two figures of merit of the planar Hall device were calculated using the following formulae: 1) Percentage change =
2) Hall sensitivity S H
VP. H . ( sat ) VP. H . ( sw) 100 VP. H . ( sw)
RH H
(2)
(3) T
VP.H.(sat) and VP.H.(sw) are the planar Hall voltages measured at magnetic saturation and switching fields respectively. A typical planar Hall voltage field hysteresis curve is shown in Fig.3a. The Permalloy was saturated magnetically at fields above ~2 mT, where the ‘forward’ and ‘backward’ field sweeps overlapped. The sharp inverted peaks indicate the coercive field of the material, where the greatest fraction of the magnetic domains have their magnetization directions rotated away from the direction of magnetic saturation. The switching represents a collapse of the domain state, which is an irreversible process with hysteretic behavior w.r.t. applied field. However, near zero applied field, there exists a region of reversible magnetization, which can be utilized to construct a linear magnetic field sensor within appropriate limits.
Fig.3c shows the value of the planar Hall voltage for a typical device at magnetic saturation as a function of angle of the applied field, showing the expected agreement with Equation 1. The PHE percentage was calculated according to the formula (2). α = 5O and α = 55O corresponded to the
highest positive (100%) and negative (-415%) changes in the Hall resistance respectively. A noticeable feature is the observable asymmetry in the angular dependence of the PHE percentage w.r.t. the angle (Fig.3c). This was due the hysteretic nature of the magnetization reversal. The Hall sensitivity, defined in Equation (3) was estimated to be 1200Ω/T near the switching fields and 650Ω/T in the reversible region near zero applied field. The values compared favorably with those obtained for planar Hall effect bridge sensors [3–6] having complex (4-28 arms) geometry and an active area up to 0.4mm2. Thus, compared to existing designs our sensors have the advantage of architectural simplicity and small size (0.01mm2), both of which are useful from the point of view of application, particularly in the area of magnetic microbead detection.
We proceeded to estimate the field detection threshold through noise analysis. A time series of the Hall voltage output was collected at 10 samples/sec using a Keithley 2182A Nanovoltmeter (Fig.4). The threshold for measurement was estimated from the r.m.s value of the time series after mean subtraction and linear drift correction. A value of 88.4 nV was obtained as the r.m.s. noise level corresponded to a field measurement noise of 136 nT. This was the noise figure applicable to long time scales of the order of 100s (our sampling duration). However, actual detection events are expected to occur at much shorter time scales, of the order of 1 second. To estimate the noise in those time scales, an FFT power spectral density was calculated, which gave a noise level of 20nT/√Hz below 0.1Hz, and 5nT/√Hz around 1 Hz. Due to the reason stated above, a more realistic estimate of the detection threshold would be 5 nT, rather than the integrated value of 136 nT over a timescale that is unlikely to be explored in actual usage. This was an improvement over the values reported in earlier literature for sensors with the ‘cross’ geometry based on PHE [16–20,22,23,26].
2.3 Field tests Field tests were aimed at verifying the effectiveness of the sensor for the areas of application stated above. The sensor was subjected to 2 types of field tests. Two of the potential areas of applicability are geomagnetic field sensing and magnetic anomaly detection.
2.3.1 Geomagnetic field detection Geomagnetic field was detected by rotating the sensor in a horizontal plane and measuring the Hall voltage as a function of angle (Fig.5). Before the measurement, all likely sources of DC magnetic field were removed from the vicinity. From this, the value of the Earth’s magnetic field at our location (Bangalore, India) was estimated to be 0.046 mT. One notable aspect of both these plots was that the angular period was 360O instead of 180O in the previous case. This was due the fact that the sensors were operating in the linear region of their characteristics near H = 0, and their response was fully reversible in that regime. 180O period is observed only when the moment aligns with the field direction after saturation. An important parameter considered for evaluating a geomagnetic sensor is the angular resolution in Earth’s magnetic field. Assuming an angular dependence of the output voltage of the form V V 0 sin( 0 ) with θ as the angle between the Earth’s field and the current direction, the angular resolution can be estimated as
1 V IS H 2V0 cos( 0 )
The value obtained this way was 6× 10-5 O, which gives the lower bound to the error if the sensor is used for direction determination. For this estimate, a noise level of 5nT/√Hz over a bandwidth of 1 Hz was assumed. The value compares favorably with 10-5 O angular sensitivity reported in the literature [27] for PZT based magnetoelectric field sensors.
2.3.2 Magnetic anomaly detection A change in the Earth’s magnetic field at a given location is considered to be a magnetic anomaly. Apart from geological causes, which involve extremely slow processes, these can only be caused by intrusion of man-made objects in a given area. The detection of magnetic anomalies has very important military as well as civilian applications[1]. All metallic objects in a magnetic field carries a magnetic moment, either due to permanent magnetic polarization of its ferromagnetic components, or
due to a paramagnetic/diamagnetic induced moment. Though the magnetic field drops off rather sharply away from the object, the field from a sufficiently large object (eg. a military target) remains detectable several hundred feet from the source. Detecting such objects magnetically has the advantage of not being a line-of-sight method, making it immune to obstructions and weather conditions [7]. Our device was tested in ambient weather to detect passing traffic. To do so, a continuous time series of the voltage output was measured at the remnant state of the sensor, and the data was scanned for anomalies beyond the noise level. Fig.6 shows the magnetic signal from a moving car of mass ~900 Kg at a distance of ~5 m. The magnitude of the field anomaly was measured to be ~5 µT, which was well above the detection threshold.
3. Summary and Conclusions
A planar Hall effect sensor for low magnetic fields has been constructed using a single layer of Permalloy was grown by pulsed laser deposition involving only one step of optical lithography. The PHE characteristics enable the sensor to be used in both linear and switching applications. The significant improvement over previous studies probably arises from the use of pulsed laser ablation over sputtering, as commonly used in previous studies. This created a strongly (111) oriented film with large (>100nm) grains with strong inter-grain coupling. The sensing characteristics of the material has been shown earlier to be stable up to 110OC. The main outcomes of our experiment are as follows: (i) The simple Hall ‘cross’ sensor architecture gave a Hall sensitivity of 1200 Ω/T at switching and 650 Ω/T in the linear regime, which is the highest reported for any PHE based system of this geometry. The value compares favorably with contemporary 2DEG Hall sensors at room temperature [28]. (ii) From noise analysis, we estimated a field detection threshold of ~5nT at 1Hz, without the use of lock-in techniques in open-loop conditions, which makes it possible to use very simple readout electronics in a linear application. The sensitivity is expected to increase much further with closed-loop operation using a local field coil. (iii) Field tests have proved its usefulness as a geomagnetic sensor (estimated angular resolution of 0.06mdeg) as well as an effective magnetic
anomaly detector under street conditions (iv) A large relative change in the planar Hall voltage at magnetization reversal (>400%) also justifies its usefulness in nonlinear (switching) applications. The main area of application of this sensor is expected to be one which requires small size and high field sensitivity at the same time. As discussed earlier, the field of detection of magnetic microbeads for bio-medical use, this device has a large potential. The simplicity in the construction readout mechanism also makes it ideal for inexpensive area-wide deployment, particularly as a magnetic anomaly detector.
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Figure 1. (a) Symmetricθ-2θ XDR pattern for the continuous film. Radiation: Cu. Apart from substrate peaks, Ni-Fe (111) peak is seen at 43.8O. (b) Pole figure of the 43.8O peak. FWHM ~5.5O. Angular span of the figure is 0O to 9O (c) Tapping mode AFM topography of 1.6µm×1.6µm area of the sensing material (7nm thickness) after lift-off. (d) Histogram of grain diameters. Data was obtained by analyzing the ‘phase’ image of a 3µm×3µm area
Figure 2. Resistivity vs temperature curve for a typical device. Residual resistivity was 15.48 µΩ-cm. Inset: Magnetic hysteresis curves along easy and hard axes at 300K.
Figure 3. (a) Typical planar Hall hysteresis curve for a 100µm Permalloy Hall bar sensor. Arrows indicate the direction of sweep Inset: Optical image of the samples along with schematic of the planar Hall effect measurement configuration (b) Schematic diagram of the setup used for angle-dependent measurements (c) Saturated planar Hall voltage and switching ratio for NiFe Hall bar as a function of angle of the applied field.
Figure 4. Output voltage noise converted to magnetic field for the sensor as a function of frequency. The data is derived from DC time series voltage measurements (Inset)
Figure 5. Geomagnetic field detection using the sensor by performing in-plane rotation. Offset subtracted planar Hall signal.
Figure 6. Magnetic anomaly caused by a moving vehicle: ~900kg at ~5m. Offset subtracted planar Hall signal.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0263-2241(20)30127-5 https://doi.org/10.1016/j.measurement.2020.107590 MEASUR 107590
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
12 September 2019 22 January 2020 4 February 2020
Please cite this article as: A. Roy, P. Sampathkumar, P.S. Anil Kumar, Development of a very high sensitivity magnetic field sensor based on planar Hall effect, Measurement (2020), doi: https://doi.org/10.1016/ j.measurement.2020.107590
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier Ltd.
Title: Development of a very high sensitivity magnetic field sensor based on planar Hall effect Authors: Arnab Roy1*, P. Sampathkumar1¶ and P.S. Anil Kumar1 1. Department of Physics, Indian Institute of Science, Bangalore 560012, India * Present address: Department of Physics, Bar Ilan University, Ramat Gan 5290002, Israel ¶ Present address: DRDO-BU Center for Life Sciences, Bharathiar University Campus, Coimbatore-641046, India Corresponding Author: Arnab Roy. Email: [email protected] Highlights: UHV-PLD grown Permalloy used as PHE based ‘Hall cross’ magnetic field sensor Sensitivity of 650 Ω/T in linear regime 400% output voltage change in the switching regime with slope of 1200Ω/T Field detection threshold of 5nT Angular sensitivity of 6×10-5 O for geomagnetic field detection. Useful for magnetic anomaly detection Abstract: While planar Hall effect based magnetic field detection is a wellstudied area, the sensing capabilities of ‘Hall cross’ sensors had not improved significantly for more than a decade. Here we report a major improvement in the sensing characteristics of ‘cross’ geometry by using pulsed laser ablation grown Permalloy as sensing material. Our sensor has two modes of operation. In the linear regime, it has a field sensitivity of 650Ω/T with an estimated detection threshold of 5nT in open-loop condition. In switching applications, it shows a 400% output voltage swing in a range of ±0.3mT, a value comparable with modern TMR sensors at room temperature. These values, which are the highest reported for the ‘cross’ geometry, make it applicable in a wide range of scenarios like geomagnetic field and magnetic anomaly detection. The small size of the sensing area also makes it a desirable choice for magnetic microbead based biomolecule detection. Keywords: Planar Hall Effect; High sensitivity low magnetic field sensor; magnetic anomaly sensor; geomagnetic field sensor; Permalloy
1. Introduction
Magnetic field sensors rule our daily life. From computer hard disks and electronic compass in a cellphone, to the myriad of proximity and position sensors required for the normal operation of an ordinary car, magnetic field sensors occupy an indispensable role in the way our civilization operates. Among the magnetic field sensors that can operate at room temperature, search coil magnetometers and fluxgate magnetometers have been able to achieve sensitivities of 20fT and 10pT respectively. However, they are endowed with several shortcomings, the biggest disadvantage being that they are sensitive to the flux linked through a coil, and the signal is, therefore proportional to the area of the coils. In practice, such sensors do not achieve any useful sensitivity unless they are at least a few mm2 in area. Besides, both these types of sensors require fairly complicated driving and readout electronics for closed loop operation, in order to achieve the above-mentioned level of sensitivity. Magnetoresistive sensors and Hall effect sensors are devoid of these difficulties, as the signal is based on a change in the material properties of the sensor element. Their dimensions can be as small as a few µm2 and may possess field sensitivities in the range of 10nT and 100nT respectively [1] Hall effect sensors are commonly constructed out of the III-V semiconductor InSb, as metals do not generate an adequate Hall voltage due to their large carrier densities. The drawback to using semiconductors as a sensing element is that the concentration of charge carriers can vary widely depending on temperature. So, despite being sensitive, they require separate temperature compensating circuitry to achieve accurate readings. Magnetoresistive sensors, on the other hand, are devoid of this problem and have the additional advantage of higher operating frequency, which is typically limited to a few kHz for a Hall-effect sensor. Anisotropic magnetoresistance (AMR) and planar Hall effect (PHE) based sensors constitute an active area of development [2–6] their main applicability is in areas that require high sensitivity and small size. Hall effect sensors have been outperformed by PHE based sensors in this area. Three major areas can be identified where there is considerable potential for application of PHE based sensors. These are (1) Bio-molecule sensors based on magnetic microbead detection. Here, due to small size of the particles to be detected, the total magnetic flux available for detection is very small.
Thus, sensors relying on flux-linkage are less useful. So are any other types of sensor requiring the magnetic field to be uniform over many mm2. (2) Geomagnetic field sensors: These require high sensitivity and stability in order to detect small changes in the angle of the applied field. (3) Magnetic anomaly sensors. The requirement for this type of sensors is high sensitivity, robustness to environment and simplicity, enabling deployment in large numbers. In this paper we present a PHE based thin film sensor constructed with the above applications in mind. Our PHE ‘cross’ sensors fabricated out of NiFe thin films possess characteristics of simplicity, small size, high sensitivity, high stability and relative high durability under ambient conditions. Field tests are also presented, that test the usefulness of the sensor for these applications.
Anisotropic magnetoresistance and planar Hall effect have been utilized extensively in the past for the construction of low field magnetic sensors. The Planar Hall effect is expressed [7] by the equation
|| ET J 2
sin 2
(1)
Here α is the angle between the magnetization and the current density J, ET is the electric field component in a perpendicular direction to J in the plane of the film, ρ’s are the parallel and perpendicular resistivities. A range of sensor architectures that utilize planar Hall effect can be found in the literature. The simplest of these is a continuous film of anisotropic magnetoresistive material [7,8] such as CoFe or NiFe deposited on a suitable substrate. NiFe continuous film sensors have been demonstrated to show a 44% Hall resistance change and 900 Ω/T sensitivity in our previous work [7]. The major drawback in these sensor designs is the inadequate decoupling between the ordinary resistance and the planar Hall resistance, leading to appreciable quiescent voltages at the operating point. The problem is addressed by fabricating Hall bars out of the sensing material [9]. Various types of studies on the sensing material can be performed using microstructured Hall bars [10–14] such as, the probing of the magnetization reversal mechanism of multilayers and the effects of biasing magnetic fields on planar Hall effect. In the domain of magnetic field sensors, a significant amount of effort has been put on the construction of exchange biased Hall bars, the most common being NiFeIrMn exchange biased system. The purpose of making an exchange biased system in to make the
planar Hall voltage linear in the applied field, as has been described by Schuhl, van Dau[15,16], Ejsing[17,18] and later by Tu and coworkers[19]. The motivation has been to create a sensor suitable for detecting magnetic micro and nano-beads, which can be suitably adapted for the detection of bio molecules. For more than a decade, the sensitivity of this sensor architecture has remained between 38 Ω/T and 330 Ω/T [17,18,20–23] with a detection threshold of around 10 nT [16]. These values were exceeded later by more complicated Hall bridge sensor geometries [3–6]. But the larger area of these designs made them less suitable for certain applications like magnetic microbead detection. So, the simple Hall cross geometry continues to be an active area of research [24]
2. Materials and Methods
2.1 Sample preparation, structural studies and preliminary characterization
Permalloy (Ni80Fe20) of thickness 15nm was deposited on undoped Si (100) substrates using pulsed laser ablation under UHV conditions at a base pressure of 410-10 mbar. Structural characterization of the films was done by X Ray diffraction and microstructure was determined using AFM. XRD showed the presence of Permalloy (111) peak at 43.8O and the absence of all other peaks of Permalloy (Fig.1a). Pole figure measured around the (111) peak showed strong out-of-plane orientational order, with FWHM < 6O (Fig.1b). These indicated a strong degree of (111) orientational order. Under AFM, the samples showed no evidence of droplet formation (Fig.1c), which are a common problem for metal films grown by PLD. The average grain diameter was 160 nm with r.m.s. surface roughness of 1nm (Fig.1d). The films were also characterized by transport and magnetization measurements (Fig.2). Transport data showed a residual resistivity of 15.5 µΩ-cm, one of the smallest values reported for Permalloy [25]. Magnetization measurements indicated uniaxial anisotropy with easy axis along (110) substrate direction (Fig.2 inset). 100µm 2mm Hall bar patterns were generated using optical lithography. The sensor structure was realized by lift-off.
2.2 Measurement The whole device, including the contact pads were fabricated out of NiFe (Fig.3a inset). Contact to Cu electrical leads was achieved through Ag paint. The active area of the sensor was where the voltage leads contacted the Hall bar, and thus had dimensions of 100µm×100µm Planar Hall effect measurements were carried out at room temperature in the magnetic field of a Helmholtz coil (±12 mT) driven by a digitally controlled current source. Magnetic field was applied in the plane of the sample. A constant current of 100µA – 1mA was passed through the sample using a Keithley 6221 and the AMR and PHE were measured using a nanovoltmeter (Keithley 2182A). Currents higher than ~5mA were fund to degrade the sensor characteristics irreversibly. Angular dependence of the AMR/PHE curves was studied using a homemade automated sample rotation system. Two figures of merit of the planar Hall device were calculated using the following formulae: 1) Percentage change =
2) Hall sensitivity S H
VP. H . ( sat ) VP. H . ( sw) 100 VP. H . ( sw)
RH H
(2)
(3) T
VP.H.(sat) and VP.H.(sw) are the planar Hall voltages measured at magnetic saturation and switching fields respectively. A typical planar Hall voltage field hysteresis curve is shown in Fig.3a. The Permalloy was saturated magnetically at fields above ~2 mT, where the ‘forward’ and ‘backward’ field sweeps overlapped. The sharp inverted peaks indicate the coercive field of the material, where the greatest fraction of the magnetic domains have their magnetization directions rotated away from the direction of magnetic saturation. The switching represents a collapse of the domain state, which is an irreversible process with hysteretic behavior w.r.t. applied field. However, near zero applied field, there exists a region of reversible magnetization, which can be utilized to construct a linear magnetic field sensor within appropriate limits.
Fig.3c shows the value of the planar Hall voltage for a typical device at magnetic saturation as a function of angle of the applied field, showing the expected agreement with Equation 1. The PHE percentage was calculated according to the formula (2). α = 5O and α = 55O corresponded to the
highest positive (100%) and negative (-415%) changes in the Hall resistance respectively. A noticeable feature is the observable asymmetry in the angular dependence of the PHE percentage w.r.t. the angle (Fig.3c). This was due the hysteretic nature of the magnetization reversal. The Hall sensitivity, defined in Equation (3) was estimated to be 1200Ω/T near the switching fields and 650Ω/T in the reversible region near zero applied field. The values compared favorably with those obtained for planar Hall effect bridge sensors [3–6] having complex (4-28 arms) geometry and an active area up to 0.4mm2. Thus, compared to existing designs our sensors have the advantage of architectural simplicity and small size (0.01mm2), both of which are useful from the point of view of application, particularly in the area of magnetic microbead detection.
We proceeded to estimate the field detection threshold through noise analysis. A time series of the Hall voltage output was collected at 10 samples/sec using a Keithley 2182A Nanovoltmeter (Fig.4). The threshold for measurement was estimated from the r.m.s value of the time series after mean subtraction and linear drift correction. A value of 88.4 nV was obtained as the r.m.s. noise level corresponded to a field measurement noise of 136 nT. This was the noise figure applicable to long time scales of the order of 100s (our sampling duration). However, actual detection events are expected to occur at much shorter time scales, of the order of 1 second. To estimate the noise in those time scales, an FFT power spectral density was calculated, which gave a noise level of 20nT/√Hz below 0.1Hz, and 5nT/√Hz around 1 Hz. Due to the reason stated above, a more realistic estimate of the detection threshold would be 5 nT, rather than the integrated value of 136 nT over a timescale that is unlikely to be explored in actual usage. This was an improvement over the values reported in earlier literature for sensors with the ‘cross’ geometry based on PHE [16–20,22,23,26].
2.3 Field tests Field tests were aimed at verifying the effectiveness of the sensor for the areas of application stated above. The sensor was subjected to 2 types of field tests. Two of the potential areas of applicability are geomagnetic field sensing and magnetic anomaly detection.
2.3.1 Geomagnetic field detection Geomagnetic field was detected by rotating the sensor in a horizontal plane and measuring the Hall voltage as a function of angle (Fig.5). Before the measurement, all likely sources of DC magnetic field were removed from the vicinity. From this, the value of the Earth’s magnetic field at our location (Bangalore, India) was estimated to be 0.046 mT. One notable aspect of both these plots was that the angular period was 360O instead of 180O in the previous case. This was due the fact that the sensors were operating in the linear region of their characteristics near H = 0, and their response was fully reversible in that regime. 180O period is observed only when the moment aligns with the field direction after saturation. An important parameter considered for evaluating a geomagnetic sensor is the angular resolution in Earth’s magnetic field. Assuming an angular dependence of the output voltage of the form V V 0 sin( 0 ) with θ as the angle between the Earth’s field and the current direction, the angular resolution can be estimated as
1 V IS H 2V0 cos( 0 )
The value obtained this way was 6× 10-5 O, which gives the lower bound to the error if the sensor is used for direction determination. For this estimate, a noise level of 5nT/√Hz over a bandwidth of 1 Hz was assumed. The value compares favorably with 10-5 O angular sensitivity reported in the literature [27] for PZT based magnetoelectric field sensors.
2.3.2 Magnetic anomaly detection A change in the Earth’s magnetic field at a given location is considered to be a magnetic anomaly. Apart from geological causes, which involve extremely slow processes, these can only be caused by intrusion of man-made objects in a given area. The detection of magnetic anomalies has very important military as well as civilian applications[1]. All metallic objects in a magnetic field carries a magnetic moment, either due to permanent magnetic polarization of its ferromagnetic components, or
due to a paramagnetic/diamagnetic induced moment. Though the magnetic field drops off rather sharply away from the object, the field from a sufficiently large object (eg. a military target) remains detectable several hundred feet from the source. Detecting such objects magnetically has the advantage of not being a line-of-sight method, making it immune to obstructions and weather conditions [7]. Our device was tested in ambient weather to detect passing traffic. To do so, a continuous time series of the voltage output was measured at the remnant state of the sensor, and the data was scanned for anomalies beyond the noise level. Fig.6 shows the magnetic signal from a moving car of mass ~900 Kg at a distance of ~5 m. The magnitude of the field anomaly was measured to be ~5 µT, which was well above the detection threshold.
3. Summary and Conclusions
A planar Hall effect sensor for low magnetic fields has been constructed using a single layer of Permalloy was grown by pulsed laser deposition involving only one step of optical lithography. The PHE characteristics enable the sensor to be used in both linear and switching applications. The significant improvement over previous studies probably arises from the use of pulsed laser ablation over sputtering, as commonly used in previous studies. This created a strongly (111) oriented film with large (>100nm) grains with strong inter-grain coupling. The sensing characteristics of the material has been shown earlier to be stable up to 110OC. The main outcomes of our experiment are as follows: (i) The simple Hall ‘cross’ sensor architecture gave a Hall sensitivity of 1200 Ω/T at switching and 650 Ω/T in the linear regime, which is the highest reported for any PHE based system of this geometry. The value compares favorably with contemporary 2DEG Hall sensors at room temperature [28]. (ii) From noise analysis, we estimated a field detection threshold of ~5nT at 1Hz, without the use of lock-in techniques in open-loop conditions, which makes it possible to use very simple readout electronics in a linear application. The sensitivity is expected to increase much further with closed-loop operation using a local field coil. (iii) Field tests have proved its usefulness as a geomagnetic sensor (estimated angular resolution of 0.06mdeg) as well as an effective magnetic
anomaly detector under street conditions (iv) A large relative change in the planar Hall voltage at magnetization reversal (>400%) also justifies its usefulness in nonlinear (switching) applications. The main area of application of this sensor is expected to be one which requires small size and high field sensitivity at the same time. As discussed earlier, the field of detection of magnetic microbeads for bio-medical use, this device has a large potential. The simplicity in the construction readout mechanism also makes it ideal for inexpensive area-wide deployment, particularly as a magnetic anomaly detector.
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Figure 1. (a) Symmetricθ-2θ XDR pattern for the continuous film. Radiation: Cu. Apart from substrate peaks, Ni-Fe (111) peak is seen at 43.8O. (b) Pole figure of the 43.8O peak. FWHM ~5.5O. Angular span of the figure is 0O to 9O (c) Tapping mode AFM topography of 1.6µm×1.6µm area of the sensing material (7nm thickness) after lift-off. (d) Histogram of grain diameters. Data was obtained by analyzing the ‘phase’ image of a 3µm×3µm area
Figure 2. Resistivity vs temperature curve for a typical device. Residual resistivity was 15.48 µΩ-cm. Inset: Magnetic hysteresis curves along easy and hard axes at 300K.
Figure 3. (a) Typical planar Hall hysteresis curve for a 100µm Permalloy Hall bar sensor. Arrows indicate the direction of sweep Inset: Optical image of the samples along with schematic of the planar Hall effect measurement configuration (b) Schematic diagram of the setup used for angle-dependent measurements (c) Saturated planar Hall voltage and switching ratio for NiFe Hall bar as a function of angle of the applied field.
Figure 4. Output voltage noise converted to magnetic field for the sensor as a function of frequency. The data is derived from DC time series voltage measurements (Inset)
Figure 5. Geomagnetic field detection using the sensor by performing in-plane rotation. Offset subtracted planar Hall signal.
Figure 6. Magnetic anomaly caused by a moving vehicle: ~900kg at ~5m. Offset subtracted planar Hall signal.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: