2374 IEEE TRANSACTIONS ON MAGNETICS, VOL 45, NO 6, JUNE 2009 Sensitivity Dependence of the Planar Hall Effect Sensor on the Free Layer of the Spin-Valve Structure T Q Hung1 , S J Oh1 , B D Tu2 , N H Duc2 , L V Phong1 , S AnandaKumar1 , J.-R Jeong1 , and C G Kim1 Department of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, Korea Department of Nano Magnetic Materials and Devices, Faculty of Physics Engineering and Nanotechnology, College of Technology, Vietnam National University, Hanoi, Vietnam Planar Hall effect (PHE) sensors with the junction size of 50 m 50 m were fabricated successfully by using spin-valve thin films Ta(5)/NiFe( )/Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) (nm) with = 10 12 16 The magnetic field sensitivity of the PHE sensors increases with increasing thickness of ferromagnetic (FM) free layer The sensitivity of about 95.5 m /(kA/m) can be obtained when the thickness of the FM-free layer increases up to 16 nm The enhancement of sensitivity is explained by the shunt current from other layers The PHE profiles are well described in terms of the Stoner–Wohlfarth energy model The detection of magnetic micro-beads label Dynabeads® M-280 is demonstrated and the results revealed that the sensor is feasible for high-resolution biosensor applications Index Terms—Biosensor applications, high field sensitivity, micro-beads detection, planar Hall effect I INTRODUCTION AGNETORESISTIVE biosensors have attracted a lot of attention [1] because of their numerous advantages such as an easy-to-use, highly portable sensing platform with high sensitivity and faster read out technique [2] Among the various kinds of magnetoresistive biosensors, the planar Hall effect (PHE) sensor has vast potential used in nano-Tesla field range detection sensors and biosensors due to its extremely high signal-to-noise ratio, high linearity at low field range, and high field sensitivity [3] PHE, known as anisotropic magnetoresistance (AMR), is induced from spin-orbit coupling and spin polarization of the materials Alternatively, NiFe permalloy was chosen to develop the high field sensitivity PHE sensor Dau et al [4] found that the PHE sensor using single NiFe layer was able to reduce thermal drift known as main noise source by at least four orders of magnitude so it can detect the nano-Tesla field range Furthermore, in the exchange bias system, exchange coupling induced from the interface between ferromagnetic (FM) and antiferromagnetic (AFM) layers can enhance the single domain state of NiFe layer, constrain the magnetization in coherent rotation, and prevent Barkhausen noise associated with magnetization reversal and thermal stability [5] Ejsing et al developed the PHE sensor based on bilayer structure NiFe/IrMn/NiFe for erroneous detection of the small stray field of micro- and nano-bead coated biomolecules with the advantage of ultra high signal-to-noise ratio [6] The spin-valve structure not only has the same advantages of the bilayer structure but also has the dynamic range due to the magnetic field created by the FM-pinned layer acting on the FM-free layer with closed fringes Therefore, the spin-valve thin films are better candidate for development of high field sensitivity sensor at small field range Earlier, we reported our work on PHE sensors based on the spin-valve structure NiFe(6)/ Cu(3)/NiFe(3)/IrMn(10) (nm) for biochip applications [7] The M Manuscript received October 09, 2008 Current version published May 20, 2009 Corresponding author: C G Kim (e-mail: cgkim@cnu.ac.kr) Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org Digital Object Identifier 10.1109/TMAG.2009.2018578 sensor sensitivity of about 31.4 m /(kA/m) was observed in the A/m linear region of the PHE profile at the field range of It has been well applied for single 2.8 m diameter Dynabeads® M-280 detection by using the sensor size of m m However, it is revealed from the context that the field sensitivity of the PHE sensor using the spin-valve structure is still relatively small Therefore, a novel PHE sensor, which shows high-sensitivity, is highly desirable from both fundamental and application point of views There were several reports disclosing the enhancement of the sensitivity of a sensor based on spinvalve structure such as changing the applied field direction [8], [9] and developing the spin-valve structure with the uniaxial field normal to the unidirectional field [10], [11] For the first case, the PHE sensor has maximum sensitivity when the applied field direction is parallel to the easy axis of the thin film; unfortunately, a hysteresis of the PHE profile was observed For the second case, when there is a tilt angle between the uniaxial and unidirectional fields, the coherent rotation of FM-free layer in the applied fields no longer exists These deteriorate the signal-to-noise ratio of the sensors as it gives possibility for bio-applications To obtain high sensitivity sensors while avoiding the above disadvantages, we optimized the thicknesses of the other layers, increased FM-free layer in the spin-valve structure and studied the role of PHE in these thin films systematically The experimental results revealed that the sensitivity of the sensors increases due to the increased thickness of the FM-free layer The sensitivity of about 95.5 m /(kA/m) can be obtained as the thickness of FM-free layer increases up to 16 nm II EXPERIMENTAL PROCEDURE A Sensors Fabrication The cross-junction sensors with the junction size of 50 m 50 m were prepared on SiO substrate using lift off method Firstly the cross junctions sized 50 m 50 m were stenciled out on the photoresist coated on silicon dioxide wafer, The spin-valve thin films Ta(5)/NiFe /Cu(1.2)-/NiFe(2)/ (nm), were deposited on IrMn(15)/Ta(5), 0018-9464/$25.00 © 2009 IEEE HUNG et al.: SENSITIVITY DEPENDENCE OF THE PLANAR HALL EFFECT SENSOR Fig Top view micrograph of the single 50 junction m 50 m 2375 PHE sensor these stenciled photoresist layer by using magnetron sputtering system The base pressure of the system is less than Torr and the Ar working pressure is mTorr During the deposition, a uniform magnetic field of 16 kA/m was applied in the film plane to induce magnetic anisotropy of the FM pinned layers and to define the unidirectional field of the thin films To connect the external electronic circuitry with sensor junction, the 90 nm Au electrodes were prepared Finally the sensor junctions were passivated with 120 nm SiO layer on top of the sensor junctions and electrodes to protect them from the corrosion and fluid environment during the magnetic bead drop experiments Fig Experimental results and calculated results (solid lines) of the PHE voltage profiles (solid lines) of the sensor junction 50 m 50 m using spinvalve structure Ta(5)/NiFe(x)/Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) (nm) TABLE I THE PARAMETERS: INTERLAYER COUPLING (H ), EFFECTIVE ANISOTROPY CONSTANT (K ), SATURATE MAGNETORESISTANCE (M ), MAXIMUM PHE VOLTAGE (V ), AND FIELD SENSITIVITY (S ) OF THE SENSORS USING SPIN-VALVE THIN FILMS WITH DIFFERENT FREE LAYER THICKNESSES (x) B Sensor Characterization Fig shows the SEM image of the passivated single sensor junction 50 m 50 m The terminals - represent the current line and - represent the voltage line The unidirectional anisotropy field, , and/or the uniaxial anisotropy field of the thin film is aligned parallel to the long terminals - The PHE profiles were measured by the electrodes bar - with a sensing current of mA applied through the terminals - and under the external magnetic fields ranging from kA/m to kA/m applied perpendicular to the direction of the current line The induced output voltages of cross-junctions were measured by means of a Keithley 2182A Nanovoltmeter with a sensitivity of 10 nV All these sensor characterizations were carried out at room temperature To detect the magnetic beads, we performed the magnetic drop and wash experiments on the sensor junction with l solution 0.1% of the Dynabeads® M-280 by using the micro pipette-lite SL-10 under an applied magnetic field of 550 A/m and a sensing current of mA III RESULTS AND DISCUSSION Fig shows the PHE profiles of the sensor junctions with various free layer thicknesses are characterized as a function of external magnetic fields in the range from kA/m to kA/m , show linearly response These PHE voltage profiles, at small fields, reach the maximum voltage at about their inter, and finally decrease with a further layer coupling field, increase in the magnetic fields Particularly, the field sensitivity of the sensor is increased due to the increase in the free layer thickness of the sensor material The magnetoresistance and magnetization results in the spin-valve structure Ta(5)/NiFe /Cu(1.2)/NiFe(2)/IrMn(15)-/ Ta(5) (nm) at low applied magnetic fields confirm that the magnetization of FM-free layer can easily be rotated in the presence of the external magnetic fields Whereas the FM pinned layer remains in the exchange bias field direction due to the exchange interaction between the AFM and FM pinned layers When the applied field overcomes the unidirectional field induced from exchange coupling (almost over 16 kA/m for all samples), the FM pinned layer starts rotating towards the applied field direction By further increasing the kA/m, magnetization direction magnetic field up to of the FM-free and FM pinned layers will be aligned in parallel configuration for all samples These behaviors satisfy the early reported results in [12] The uniaxial fields induced from the , is obtained from the interlayer coupling of the thin films, MR and magnetization profiles and are listed in Table I The details of this work will be published elsewhere [13] Therefore, at the small magnetic fields, the PHE effect known as AMR effect is almost contributed from the FM-free layer, and the Stoner-Wohlfarth energy term of the FM-free layer can then be simply expressed as [10] (1) 2376 IEEE TRANSACTIONS ON MAGNETICS, VOL 45, NO 6, JUNE 2009 where , and are the thickness, uniaxial anisotropy constant, saturation magnetization of the thin film and interlayer coupling energy, respectively is the angle between the external magnetic field and easy axis of thin film and is the angle between the unidirectional field and the easy axis of the FM-free and layer In our experimental conditions For the FM-free layer, the PHE output voltage is described by Ohm’s law and could be given by [14] (2) where and , and are the current applied to the sensor junction, thickness, transverse and longitudinal resistivity, and angle between the magnetization direction of the , of the thin film, FM-free layer and unidirectional field, respectively The angle can be calculated from the minimum energy condition of the above Stoner-Wohlfarth equation at each value of applied magnetic fields Hence, the PHE voltage profile can then be calculated from (2) The calculated results presented as solid mA; lines in Fig are obtained for the best fit with , and are listed in Table I, and Since the PHE voltage is contributed from the FM-free layer, magnitude in (2) is the only curthe current term in the rent passing through this layer The quantitative analysis of this current is a complicated work because the applied current distributed in each layer is different and very sensitive to the interface of the thin films [15] When increase the thickness of FM-free layer of the thin film, consequently the current passing through this free layer is also increased, the enhancement of the PHE output voltage is achieved We performed the magnetic bead detection using PHE sensor with highest sensitivity to demonstrate the feasibility of digital bead detection for bio applications The diluted 0.1% magnetic bead solution streptavidin coated Dynabeads® M-280 is used for bead drop and wash experiments on the sensor surface The real-time profile measurements of the PHE voltage for magnetic beads detection is carried out in the optimum conditions, that is, in an applied magnetic field of 557 A/m and with a sensing current of mA The results are illustrated in Fig for three consecutive cycles, where the lower state represents the signal change in sensor output voltage after dropping the magnetic bead solution on the sensor surface and the higher state represents the sensor output voltage after washing the magnetic bead from the sensor surface The total output signal annuls in three consecutive cycles were found to be about 7.1 V, 16 V and 21.8 V for the first step and 11.3 V and 16.7 V in the second step of the second and third cycles, respectively It is clearly shown from the figure that for the first cycle, the signal changed by one-step and the signal was further changed into two steps in the second and third cycles This two step-type profile is due to the aggregation of the magnetic beads on the sensor surface The aggregation of the magnetic beads occurs at the drying stage That is, after dropping the bead solution on the sensor surface, it needs some time to dry The first step changes of the signals are assumed to be due to the viscous flow motion Fig Real-time profile of the highest field sensitivity PHE sensor under an applied magnetic field of 550 A/m and with the sensing current of mA for stabilization as well as the Brownian motion of the beads When the solution dries, the beads rearrange During this time, some beads aggregate and become clusters on the sensor surface This lessens the total stray field on the sensor surface and hence, the second step changes in the second and third cycles were observed in the real-time profile For further understanding the micro-bead detection using PHE sensor, it is noted that the magnetization of the magnetic sphere is purely a dipole at the center of the sphere with a at a distance identified by [3] magnetic field (3) and are the value and vector in the direction of where magnetization of the bead, respectively and are the bead radius and distance from the center of the bead to the observation point, respectively The stray field of a single bead on the sensor surface could be crudely calculated by [16] (4) This stray field is in the opposite direction to the applied field, thus it reduces the effective field on the sensor surface Under experiment conditions, the stray field of beads on the sensor surface reduced the sensor output signal as follows: (5) is the effective field on the sensor surface, is the where sensor sensitivity, is the number of magnetic beads on the sensor surface, is the volume of magnetic bead, is the mass magnetic susceptibility of magnetic beads, for Dynabeads® M-280 [17] HUNG et al.: SENSITIVITY DEPENDENCE OF THE PLANAR HALL EFFECT SENSOR By substituting the value and m (the distance including the radius of Dynabeads® M-280 and the thickness of passivated SiO and Ta layers) into (4), the stray field of single bead is estimated to be 17.5 A/m under the applied field of 550 A/m Theoretically, with the sensor sensitivity m /(kA/m) and the sensing current mA, the number of bead separately placed on the sensor surface can be calculated in the first step of the three cycles by using (5), which are estimated to be about 4, 10, and 13 beads, respectively These estimated results strengthen our explanation It is clearly shown in the first cycle, the number of beads on the sensor surface is estimated to be small, and the distance among beads on the sensor junction is far enough to avoid the effect from the rearrangement of beads during the drying stage In the second and third cycles, the number of magnetic beads on the sensor junction are larger; they easily aggregate to become clusters under applied magnetic field due to short bead-bead distance IV CONCLUSION We enhanced the field sensitivity of PHE sensors by increasing the free layer in the spin-valve structure Ta(5)/NiFe / Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) (nm) The maximum sensitivity of the fabricated sensors of about 95.5 m /(kA/m) can be obtained as the thickness of the free layer increases up to 16 nm The detecting Dynabeads® M-280 results with the highest sensitivity PHE sensor reveals that our sensor is very sensitive in identifying the existence of magnetic beads; different number of magnetic beads give different changes in the real-time profile Moreover, the decrease in stray field occurred due to the bead-bead interaction at the drying stage, which can be recognized by a two step-type of the real-time profile ACKNOWLEDGMENT This work was supported by KOSEF under project M10803001427-08M0300-42710, the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea The work of J.-R Jeong was supported by the Korea Research Foundation (KRF-2008-331-D00234) The work of N H Duc was supported by Vietnam National University, Hanoi under project QG.TD 07.10 2377 REFERENCES [1] S Tumanski, Thin Film 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Particularly, the field sensitivity of the sensor is increased due to the increase in the free layer thickness of the sensor material The magnetoresistance and magnetization results in the spin-valve structure. .. conditions, the stray field of beads on the sensor surface reduced the sensor output signal as follows: (5) is the effective field on the sensor surface, is the where sensor sensitivity, is the. .. the signal was further changed into two steps in the second and third cycles This two step-type profile is due to the aggregation of the magnetic beads on the sensor surface The aggregation of