2378 IEEE TRANSACTIONS ON MAGNETICS, VOL 45, NO 6, JUNE 2009 Optimization of Spin-Valve Structure NiFe/Cu/NiFe/IrMn for Planar Hall Effect Based Biochips Bui Dinh Tu1 , Le Viet Cuong2 , Tran Quang Hung3 , Do Thi Huong Giang1 , Tran Mau Danh1 , Nguyen Huu Duc1;2 , and CheolGi Kim3 Department of Nano Magnetic Materials and Devices, Faculty of Physics Engineering, College of Technology, Vietnam National University, Hanoi, Vietnam Laboratory for Micro-Nano Technology, College of Technology, Vietnam National University, Hanoi, Vietnam Department of Materials Science and Engineering, Chungnam National University, Yuseong, Daejeon 305-764, Korea This paper deals with the planar Hall effect (PHE) of Ta(5)/NiFe( F )/Cu(1.2)/NiFe( P )/IrMn(15)/Ta(5) (nm) spin-valve structures Experimental investigations are performed for 50 m 50 m junctions with various thicknesses of free layer ( F = 10 12 16 26 nm) and pinned layer ( P = 12 nm) The results show that the thicker free layers, the higher PHE signal is observed In addition, the thicker pinned layers lower PHE signal The highest PHE sensitivity of 196 V/(kA/m) is obtained in the spin-valve configuration with F = 26 nm and P = nm The results are discussed in terms of the spin twist as well as to the coherent rotation of the magnetization in the individual ferromagnetic layers This optimization is rather promising for the spintronic biochip developments Index Terms—Biosensors, Hall effect, magnetization reversal, magnetoresistance, magnetoresistive devices I INTRODUCTION T HE discovery of giant magnetoresistance (GMR) effect in metallic multilayer has made a strong impact on the development of computer memory technologies [1], [3] Recently, this effect has been well developed for biochip applications due to its large resistance change in small magnetic field range [4]–[10] The GMR effect is related to the switching of magnetic domain It has low signal-to-noise ratio (SNR), leading to a high error in detections of the small stray field The planar Hall effect (PHE), however, is related to the rotation process of magnetic domain and is originated as the anisotropic magnetoresistance This effect exhibits a nano-Tesla sensitivity and rather high SNR, so it has received great attention for magnetic bead detection and biosensor design [2]–[6] The transverse voltage on a planar Hall cross depends on the orientation of the magnetization of the ferromagnetic layer with respect to the longitudinal sensing current Thus, a large PHE is expected to be observed in exchange coupling based structures because they can ensure a sufficient uniaxial anisotropy with well-defined single domain state to introduce a unidirectional anisotropy For this purpose, Ejsing et al [6], [7] have reported a single PHE sensor of NiFe/IrMn/NiFe Furthermore, a PHE magnetic bead array counter microchip integrated 24 of single sensors based on a simple NiFe/IrMn bilayer structure has been successfully prepared [8] Recently, Thanh et al [9] have found that the sensor signal can be further improved by using spin-valve structure of NiFe(6)/Cu(3.5)/NiFe(3)/IrMn(10) (nm) with the size of m m when detecting the 2.8 m magnetic beads The present paper deals with studies of the magnetic field sensitivity as a function of the thickness of the individual free ferromagnetic (FFM) and the pinned ferromagnetic (PFM) layers Manuscript received October 09, 2008 Current version published May 20, 2009 Corresponding author: N H Duc (e-mail: ducnh@vnu.edu.vn) 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.2009.2018580 in the pattern 50 m 50 m PHE sensors based on NiFe/Cu/ NiFe/IrMn spin-valve structures The highest PHE sensitivity of 196 V/(kA/m) was obtained in the spin-valve configuration with FFM layer thickness of 26 nm and PFM layer thickness of nm This optimum structure is proposed to apply for magnetic bead detections II EXPERIMENTAL PROCEDURE The thin films with typical spin-valve structure of /Cu(1.2)/NiFe /IrMn(15)/Ta(5) (nm) with Ta(5)/NiFe , nm and PFM FFM layer thickness , nm are fabricated by dc layer thickness magnetron sputtering system with the base pressure less than mTorr The spin-valve structures were sputtered on SiO wafer at room temperature with Argon working pressure mTorr During sputtering process, a uniform of A/m is applied in plane of magnetic field of the films, parallel to the Ox direction This magnetic field induces a magnetic anisotropy in the FFM and PFM layers and then aligns the pinning direction of the antiferromagnetic (AFM) Ir-Mn layer The PHE sensors were structured by using photolithography technique into four-electrode bars with the patterned size of 50 m 50 m [Fig 1(a)] The sensors were passivated by sputtering a 150 nm thick Si N layer to protect against the fluid used during the experimentation The bead array counter (BARC) microchip was fabricated by integrating 10 single sensor patterns as shown in Fig 1(b) The PHE characteristics of sensors were measured at room temperature by using a nanovoltmeter in the external magnetic up to kA/m applied along Oy direction and sensing fields currents of mA Longitudinal magneto-resistance was measured by means of a collinear four-point probe method, for samples with the size of mm 10 mm, with applied magnetic field direction Magnetization was and sensing current are along measured by using a Lakeshore 7400 vibrating sample magnetometer (VSM) on films 0018-9464/$25.00 © 2009 IEEE TU et al.: OPTIMIZATION OF SPIN-VALVE STRUCTURE NiFe/Cu/NiFe/IrMn 2379 Fig (a) Top view micrograph of the single 50 m 50 m PHR cross The pinning direction H as well as the direction of the bias field H and sensing current I are indicated (b) The bead array counter microchip including 10 of single PHE sensors (with single sensors in the two middle lines and single sensor in each edge line) III RESULTS AND DISCUSSION A Fixed PFM Layer Thickness Spin-Valve System Fig 2(a) presents the magnetization data of spin-valve Ta(5)/ /Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) (nm) structures with NiFe varying from to 16 nm It different free layer thicknesses is clearly seen that all the samples exhibit two hysteresis loops The magnetization accounting from the first loop linearly inwhile that from the second loop is creases with increasing almost constant These two hysteresis loops are attributed to the FFM and PFM layers, respectively The FFM layer is expected to dominate the sensor response at low magnetic fields and exchange coupling The values of the coercivity fields determined from the first hysteresis loop [see insert in Fig 2(a)] are collected and listed in the Table I Note that, expershows a tendency to increase with while imentally, seems to exhibit a maximum at nm Illustrated in Fig 2(b) are the GMR data of the samples under investigation It can be seen from this figure that the reversal of the magnetization in the FFM layer brings the GMR ratio to its nm This maximum lightly maximal value of 1.6% for decreases with the increasing of the FFM layer thickness and nm Further increasing the (deequals to 1.2% for magnetizing) magnetic field, the magnetization rotation in PFM layers starts rather early in the thin FFM layer thickness samples and later in the thick FFM layer thickness samples However, the final parallel configuration of individual layer magne- Fig Magnetic hysteresis loops (a) GMR data (b) and low field PHE profiles nm and (c) of spin-valve structures with the fixed PFM layer thickness t FFM layer thickness t varying from to 16 nm ( ) =2 tizations seems to be completed at the same magnetic field of kA/m for all samples This finding is comparable with the magnetization data mentioned in Fig 2(a) These GMR results are consistent with those reported in [11] , as Shown in Fig 2(c) are the PHE voltage profiles, a function of the applied field First, the PHE voltage initially develops rather fast at low fields reaching a maximal value at A/m and finally decreases with further increasing in the magnetic fields It is interesting to note that the magnetic 2380 IEEE TRANSACTIONS ON MAGNETICS, VOL 45, NO 6, JUNE 2009 TABLE I VALUES SENSOR SENSITIVITY (S ), COERCIVE (H ), ANISOTROPY (H ), ) AT GMR EXCHANGE COUPLING (H ) FIELDS, AND MAXIMAL FIELD (H PEAK FOR SPIN-VALVE SYSTEM WITH DIFFERENT FREE LAYER THICKNESSES field, at which GMR reaches the maximum , is systematically close to the sum of (see Table I) For this fixed PFM layer spin-valve system, the maximal value of the PHE voltage increases with increasing FFM layer thickness It innm creases from the value of 15 V for the sample with to the value of 48 V for nm Consequently, the sensor , see below) is enhanced from the value sensitivity ( of 21.4 V/(kA/m) to 95.5 V/(kA/m), respectively (Table I) It is well known that when the magnetization vector makes an angle with an easy axis along the direction (and/or with ), the transverse induced PHE voltage (or ) parallel direction is given as follows: with (1) where with and are the resistivity measured with the current parallel and perpendicular to the is the free ferromagnetic layer magnetization, respectively; thickness curves are fitted well by using the Typically, these single domain model with the magnetic energy per unit of the magnetic layer In this case, the Stoner-Wohlfarth energy can be expressed as [12] (2) and are the angles between the magnetization Here, the of the free and pinned layers and the easy axis direction, respecis the effective anisotropy constant, tively; is the saturation magnetization of the free layer, and is the interlayer coupling constant that can be extracted from the relation with the exchange coupling field between two FM layers by the formula If the exchange bias field between PFM and AFM layers is strong enough, the angle between magnetization and the easy axis direction of the PFM layer will be fixed at low applied magequals to zero This can be applied for the netic fields, i.e., present case, where the magnetization reversal of the free and pinned layers occurred separately [see in Fig 2(a)] , the PHE voltage exhibits a linear For small angles, characteristics as well as high sensitivity at low fields ( A/m) In this case, the sensitivity of sensor is given as (3) Applying this theoretical approach to experimental data, we can determine the values for and fields as well as the Fig Hysteresis loops (a) GMR data (b) and low field PHE profiles (c) measured in spin-valve structures with the fixed free layer thickness x = 10 nm and different pinned layer thickness (t) from to 12 nm sensor sensitivity The obtained results of is also sumobtained marized in Table I Note that the values of and from the fits of the PHE data are in excellent agreement with those derived from experimental data The calculated values , however, are systematically larger than determined of from the magnetization measurements The increasing of the sensitivity in these sensor junctions is usually explained simply by the shunting current in the spin valve thin films The more FFM layer is thick, the more shunting current from other layers is small Other explanations will be extended below TU et al.: OPTIMIZATION OF SPIN-VALVE STRUCTURE NiFe/Cu/NiFe/IrMn 2381 TABLE II VALUES SENSOR SENSITIVITY (S ), COERCIVE (H ), ANISOTROPY (H ), ) AT GMR EXCHANGE COUPLING (H ) FIELDS AND MAXIMAL FIELD (H PEAK FOR SPIN-VALVE SYSTEM WITH DIFFERENT FREE LAYER THICKNESSES B Fixed FFM Layer Thickness Spin-Valve System Fig 3(a) presents the magnetization data of Ta(5)/NiFe(10)/ /IrMn(15)/Ta(5) (nm) spin-valve structures Cu(1.2)/NiFe varying from to with different PFM layer thickness 12 nm Here, all samples exhibit the two hysteresis loops too However, contrary to the fixed PFM layer thickness system, the magnetization accounting from the first loop is almost constant while that from the second loop increases with increasing and exchange coupling The values of the coercive fields determined from the first hysteresis loop are collected lightly varies around and listed in the Table II Note that the value as small as 80 kA/m, whereas strongly increases with increasing Typical magnetoresistive characteristics of spin-valve structures are presented in Fig 3(b) The magnetic field interval for the existence of the antiparallel configuration between FFM and PFM layer magnetizations decreases with increasing the PFM thickness The final parallel configuration is completed at the , and kA/m for magnetic field of , and nm, respectively This behavior is consistent with the magnetization data reported in Fig 3(a) The maximal GMR ratio, however, increases from 0.85% to 2.84% when increases from to 12 nm Shown in Fig 3(c) are the PHE voltage profiles as a function of the applied fields For this fixed free layer spin-valve system, it is clear that with increasing , the maximal value ) of the PHE voltage decreases In addition, this peak (at shifts to higher magnetic fields and once again the relation beand the sum of is found (see Table II) tween Consequently, the sensor sensitivity is strongly reduced from increases from the value of 110.6 to 42.7 V/(kA/m) when to 12 nm The values of anisotropy , exchange coupling fields and sensor sensitivity derived from the theoretical fits show an excellent consistence with experimental results and (calculation) While a rather large difference between is observed for nm Fig Hysteresis loops (a), GMR data (b) and low field PHE profiles (c) measured in Ta(5)/NiFe(26)/Cu(1.2)/NiFe(1)/IrMn(15)/Ta(5) (nm) spin-valve structure, i.e., with t = 26 nm and t = nm C Optimal Spin-Valve Structure for PHE Sensor Sensitivity It was provided from above mentioned investigations that the large PHE sensor sensitivity can be reached in spin-valve structures with thin PFM and thick FFM layers In spin-valve structures, the PHE is strongly contributed from the FFM layer By increasing the thickness of this layer and optimizing the thicknesses of other layers, the shunting current can be reduced through remain layers, leading to the observed higher sensitivity of our PHE sensors On the other hand, the high PHE sensitivity may also be related to the spin twist as well as to the coherent rotation of the magnetization in the individual ferromagnetic layers This can be understood as follows In the PFM layer, the well-aligned spin part is usually formed near PFM/AFM interface Further increasing the pinned layer thickness will lead to an enlarging of the twist structure where the magnetization is pinned in different directions from the easy axis (i.e., [13] In this context, the twisted part can be assumed to be elimnm Pracinated in the structure with thin pinned layer tically, the maximal PHE voltage and the highest sensitivity of 2382 IEEE TRANSACTIONS ON MAGNETICS, VOL 45, NO 6, JUNE 2009 sensor were observed in this configuration For the FFM layers, the magnetic influence and then the twist part can be established near NM/FFM interface only The thick free layers thus dominate the collinear ferromagnetic part and enhance the PHE voltage Combining these two optimal tendencies, we prepared the Ta(5)/NiFe(26)/Cu(1.2)/NiFe(1)/IrMn(15)/Ta(5) (nm) spinnm and nm Its valve structure, i.e., with magnetization and PHE data are presented in Fig Although the magnetization reversal is mainly contributed to the first magnetic hysteresis loop [Fig 4(a)], the rotation of the magnetization in the PFM layer to re-establish the parallel configuration is well evidenced in the magnetoresistance [Fig 4(b)] Here, the most interesting result is that the PHE voltage reaches A/m and this its maximal value of about 62 V at spin-valve configuration shows a sensor sensitivity as large as and fields 196 V/(kA/m) Additionally, the values of are as small as 160 and 330 A/m, respectively IV CONCLUSION The influence of the individual free and pinned layer thickness on the sensitivity of PHE sensor based on the spin-valve /Cu(1.2)/NiFe /IrMn(15) (nm) with structure of NiFe size of 50 m 50 m has been studied The results show that the thicker free ferromagnetic layers enhance the PHE signal, whereas the thicker pinned ferromagnetic layers lower the PHE one For a good combination, the highest PHE sensitivity of 196 V/(kA/m) is obtained in the spin-valve configuration with nm and nm The results are discussed in terms of the spin twist as well as to the coherent rotation of the magnetization in the individual ferromagnetic layers This optimization is rather promising for the spintronic biochip developments ACKNOWLEDGMENT This work was supported by Vietnam National University, Hanoi under Grant QG.TD 07.10 REFERENCES [1] M Johnson, Magnetoelectronics Amsterdam, The Netherlands: Elsevier, 2004 [2] S Maekawa, Concepts in Spin Electronics Oxford, U.K.: Oxford Science Publications, 2006 [3] C Chappert, A Fert, and F N Van Dau, “The emergence of spin electronics in data storage,” Nature Mater., vol 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RESULTS AND DISCUSSION A Fixed PFM Layer Thickness Spin-Valve System Fig 2(a) presents the magnetization data of spin-valve Ta(5)/ /Cu( 1.2) /NiFe( 2) /IrMn( 15)/Ta(5) (nm) structures with NiFe varying... is thick, the more shunting current from other layers is small Other explanations will be extended below TU et al.: OPTIMIZATION OF SPIN-VALVE STRUCTURE NiFe/ Cu /NiFe/ IrMn 2381 TABLE II VALUES