Magnetoresistive performances in exchange biased spin valves and their roles in low field magnetic sensing applications

Journal of Science: Advanced Materials and Devices (2018) 399e405 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Magnetoresistive performances in exchange-biased spin valves and their roles in low-field magnetic sensing applications Van Su Luong a, *, Anh Tuan Nguyen a, Quoc Khanh Hoang a, Tuyet Nga Nguyen b, Anh Tue Nguyen b, Tuan Anh Nguyen c, Van Cuong Giap d a International Training Institute for Materials Science (ITIMS), Ha Noi University of Science and Technology (HUST), Dai Co Viet, Hai Ba Trung, Ha Noi 100000, Viet Nam Institute of Engineering Physics (IEP), Ha Noi University of Science and Technology (HUST), Ha Noi 100000, Viet Nam c Hanoi Community College (HCC), Trung Kinh, Cau Giay, Ha Noi 100000, Viet Nam d Hung Yen University of Technology and Education (UTEHY), Dan Tien, Khoai Chau, Hung Yen 160000, Viet Nam b a r t i c l e i n f o a b s t r a c t Article history: Received 18 May 2018 Received in revised form August 2018 Accepted 14 September 2018 Available online 19 September 2018 The magnetoresistive properties of pinned spin valves (SV) and their roles in low-field sensing applications were characterized The magnetoresistive parameters were extracted, including the exchange bias (Heb) field as a function of the iron content in the CoFe layer and the antiferromagnetic (AFM) thickness, the magnetoresistance (MR) ratio versus the spacer thickness, the coercivity (Hc) as a function of the seed layer, and the composite layer [NiFe/Co] used These parameters are crucial in determining the features of the magnetic sensors Eventually, the selected SV film structure of (Si/ SiO2)/Ta(50 Å)/[NiFe(30 Å)/Co(15 Å)]/Cu(24 Å)/Co80Fe20(25 Å)/IrMn(100 Å)/Ta(50 Å) was found significant, and the SV elements were patterned using the lithographic lift-off method with the active cell dimensions of mm  150 mm To define a pinning axis, a cool-field anneal was applied at 250  C for 30 in a magnetic field of kOe A Wheatstone half bridge was engineered using two SV elements and two external resistors The operation point of the sensor was well tuned using a tiny permanent magnet A sensitivity of V/T was observed with a linear range of ±2 mT To demonstrate the performance of the designed sensor, a measurement of the Earth magnetic field was carried out The engineered SV sensor finds its usefulness in low-field magnetometer and electronic compass applications © 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: GMR Giant magnetoresistance Magnetic sensors RF sputtering Spin valve Introduction Giant magnetoresistance (GMR) devices are widely used in various applications [1e4] The most important application of GMR is in the data storage area [5e10] GMR based sensors have also found wide applications in the automotive markets, navigation, aerospace [11e13], and electronic compass devices [14,15] In the last two decades, numerous comprehensive studies on the GMR effect have been reported Nevertheless, extensive studies on the fabrication techniques of GMR are still useful [5,16e18] The performance of GMR is very sensitive to the fabricating conditions and the specific equipment [19] This can also be found in various * Corresponding author E-mail address: sulv@itims.edu.vn (V.S Luong) Peer review under responsibility of Vietnam National University, Hanoi articles on the GMR spin valve effect [20e24] Besides, numerous works have discussed in detail about the particular equipment, methods and technical conditions for fabrication of GMR metallic multilayers, such as the sputtering method [25], the ion beam deposition [26e30], the chemical vapor deposition [31], the electrodeposition [32e35], etc However, the challenges of the GMR fabrication still remain owing to the interplay between the magnetoresistive properties and the specific equipment used Furthermore, the magnetoresistive properties are the factors which directly determine the architecture and the features of the magnetic sensors For example, the exchange bias field (Heb) expresses the strength of the external field making a saturation, where the sensor is inactive with any external magnetic fields The intercoupling field (Hin) is induced by the coupling between a pinned layer (PL) and a free layer (FL) The Hin can cause a shift of the operating point of the MR curve The Hin is mainly caused by the thickness and the roughness of the spacer layer, and by the stray field of the PL as https://doi.org/10.1016/j.jsamd.2018.09.004 2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 400 V.S Luong et al / Journal of Science: Advanced Materials and Devices (2018) 399e405 well [36] Furthermore, for the low magnetic field measurement, errors should be kept as small as possible The error arises from the hysteresis (Hc) of the FL A large Hc of the FL leads to a big error Regarding the responsivity of the sensor, the high MR ratio, e.g the large change of the resistance in an external magnetic field provides a high sensitivity and a large signal-to-noise ratio The MR ratio depends on many factors, such as the texture of the seed layer, the composition of the ferromagnetic alloy layer, and the critical thickness of the spacer or each of the functional layers Eventually, the cool-field anneal process also strongly affects the MR performance owing to the inter-diffusion In fact, the technical factors interact complicatedly together on the optimization of the MR performance so that during the fabrication of the spin valve (SV) films for sensor developments, technicians must decide the tradeoff between these factors to find out the appropriate parameters for the SV fabrication This case study, therefore, is emphasized on the technical factors that affect the magnetoresistive properties of the SVs fabricated by the RF sputtering deposition towards low magnetic field sensing applications, including Heb, Hc, and MR performance, etc For an ideal comprehensive investigation, the variables should be changed one by one, and the interplay between all the above parameters should be considered This, however, would take a lot of time and, therefore not be feasible We, thus, refer to the knowledge reported on the SV performances to apply in our works Here, we focused on some important factors, such as Heb, Hc, and the MR ratio We changed the fabrication variables in the range based on our previous reports to optimize them separately one by one [37] For these investigations, the SV structure of (Si/SiO2)/Ta(tTa)/[NiFe/Co]/Cu(tCu)/Co100-xFex/IrMn(tIrMn)/Ta was chosen In the sensor applications, the SV films were patterned into micron scale cells and used to form a half-bridge sensor The magnetic properties of the patterned SVs also strongly depend on the micro-scale size of the components [38,39] Finally, the performance of the patterned SV was demonstrated via the Earth's magnetic field measurement The obtained experimental results of the SV sheet films and the prototype sensor are presented and discussed Experimental Exchange-biased SVs were prepared by the RF sputtering technique The vacuum in this procedure was  10À5 Pa, and the working pressure (Argon) was 0.7 Pa The distance from the target to the substrate was about cm Firstly, a 400 nm layer of SiO2 was sputtered onto a mm  mm oxidation silicone substrate The antiferromagnetic (AFM) layer was then sputtered using an alloy target of Ir25Mn75, while the ferromagnetic (FM) material layer of the CoeFe alloy was formed by co-sputtering from two pure Co and Fe targets The composition of Co100-xFex was tuned by fixing the sputtering power of the Co target while adjusting the sputtering rate of Fe The content of the as-prepared CoFe layer was characterized by the X-ray energy dispersive spectroscopy attached to a field emission scanning electron microscop of the model JEOL JSM-7600F The film thickness was measured by an atomic force microscop The sputtering rate was determined by the thickness of each deposited functional layer versus the deposition time The average deposition rate was about 30÷50 Å/minute A cool-field annealling procedure was used to define the bias direction as the easy axis using a static magnetic field of kOe The heating time was 30 min, and the cooling time to room temperature (RT) lasted 60 The SV films were magnetically characterized using a vibration sample magnetometer (VSM) The magnetoresistance (MR) was measured by a conventional four-probe method with a bias current passing in-plane of the SV In the sensor applications, the lithographic lift-off method was used to pattern the device The active bar SV was of mm  150 mm in size The easy axis was patterned into the short dimension of the bar, hereby, the cross shape anisotropy between the FL and the PL contributed to a collapse of the hysteresis of the patterned cells The line width of the contact pads was 25 mm, and the bonding pad was 150 mm  150 mm, which was designed by a photo-mask for silver (Ag) deposition The chip dimension was cut into mm  mm To construct a sensor, two cut chips were glued to the printed circuit board (PCB) for wire bonding, while the other side of the PCB was soldering connected by two identically passive resistances To demonstrate the features of the engineered sensor, a sweeping magnetic field generated by a Helmholtz coil with the amplitude of ±10 mT was applied to the sensor The sensitivity of the sensor was determined by the slope of the sensor's output versus the sweeping field curve In addition, since the intercoupling field will cause a shift of this curve, so the magnetic bias technique was applied using a tiny permanent magnet The field strength of the permanent magnet for biasing the operation point was controlled by adjusting the distance between the sensor and the magnet Finally, an Earth's magnetic field measurement was carried out for verifying the features of the engineered sensor A manual rotation frame was set up allowing the rotation to take from 0 to 360 The sensor probe was fixed in the center of the rotation frame and rotated with an interval of 10 Each point of the rotation, the output of the sensor was recorded by a data acquisition (DAQ), which was a multifunctional module of MyDAQ provided by National Instruments The software was coded in LabVIEW We first study the magnetoresistive properties of SV sheet films for finding out the appropriate parameters of the SV film fabrication The conventional approach of sputtering was used to prepare the films based on our previous work [37] and refering to the recent reports on the SV film fabrication technology [40,41] Results and discussion 3.1 Magnetoresistive properties of the SV sheet films 3.1.1 Exchange bias (Heb) Exchange-biased spin valves were first introduced by Dieny et al in 1990 [17,20,42], based on a simple sandwich structure of a GMR layer with an additional AFM layer, which is in contact with one FM layer of the sandwich structure The result of the AFM-FM contact is an interfacial exchange interaction, which is the socalled exchange biasing effect This structure is a simple exchange-biased spin valve structure The other (free) FM layer in the sandwich structure was unpinned and can rotate freely under a weak external magnetic field This free layer was made of soft magnetic materials The magnetic properties are demonstrated by the M-H loop, shown in Fig 1(a) The interesting feature of a spin valve structure is that the M-H loop is asymmetric caused by the exchange biasing effect As previous reports show that the GMR in multilayers is symmetric, so a strong magnetic field is needed to reverse the magnetization direction and, thus, a static magnetic field bias or modulation technique is required for the low field magnetic sensing and measurements Suppose that the SV is exposed to a strong magnetic field that larger than the Heb (AFM saturated), and the magnetization of both the PL and FL is parallel, the SV is, thus, insensitive to the external magnetic fields Therefore, the working range of an SV sensor is only active within the reversed magnetization state of the FL and must be smaller than Heb [17,20,42e44] In sensor applications, Heb should be as large as possible [45] In this work, the reference layer wof CoFe was used due to its high magnetic moment and high interfacial coupling with the AFM layer Among the ferromagnetic V.S Luong et al / Journal of Science: Advanced Materials and Devices (2018) 399e405 401 Fig (a) Illustration of a M-H loop of a typical SV, (b) Heb and MR versus Fe contents of CoFe alloy of SVs (Si/SiO2)/Ta(150 Å)/Co(45 Å)/Cu(30 Å)/Co100-xFex(25 Å)/IrMn(250 Å)/ Ta(50 Å), and (c) MR and Heb as a function of IrMn thickness of SVs Si/SiO2)/Ta(50 Å)/[NiFe(30 Å)/Co(15 Å)]/Cu(24 Å)/Co80Fe20(25 Å)/IrMn(tIrMn)/Ta(50 Å) materials, Co and its alloy with iron play a significant role in the performance of SV [18] Fig 1(b) shows the obtained experimental results of Heb and the MR ratio as a function of the Fe content in the CoFe alloy The Heb increases with the increasing iron content and reaches a maximum at 40%, inducing a close-to-maximum magnetic moment of the CoFe alloy leading to a large exchange coupling energy in the IrMn/ CoFe The MR is reduced monotonically with the increasing Fe content because the high Fe concentration induces a lower magnetic moment in the CoeFe alloy leading to the weakening of the interfacial exchange coupling in the IrMn/CoFe [46,47] The effects of the microstructure including grain size and texture of the AFM (IrMn-111) layer on the Heb has been reported by M Pakala et al [48] The Heb dependence on the Fe content has also been revealed that an Fe content of 30% induces a high exchange anisotropy [49], while other work claimed that a high exchange bias could be reached at 45% of Fe [46] On the other hand, a series of the published papers have confirmed that good MR performance is obtained with 10% of Iron [50e53] It is also revealed there that the Heb and the MR performance are very sensitive to the specific fabrication equipment Because of the trade-off between Heb and MR ratio, in this work, x ¼ 20% was the iron content chosen for the SV film fabrication Furthermore, the Heb also strongly depends on the AFM layer thickness Fig 1(c) shows the effects of the IrMn layer thickness, which was varied from 50 Å to 300 Å on the MR behaviors and the Heb value A lower Heb in the thicker AFM layer is caused by the suppression of the (111) texture [54], while the MR is reduced significantly in a thicker AFM layer owing to the shunting current [55,56] When the thickness of the IrMn layer has further increased, the Heb has reached a maximum at 100 Å and it drops as this film becomes thicker (so, at thicknesses >100 Å) As a result, both the Heb strength and the MR ratio reached a maximum at tIrMn ¼ 100 Å, and this was chosen for the further investigations 3.1.2 Magnetoresistance performance (DR/R0) The change of the resistance in the spin valve depends on the relative magnetization angles between the PL and the FL, as defined by DR/R0 in Fig 2(a), where R0 is the base resistance of the SV in the zero external field and DR is calculated using the resistances in the parallel and anti-parallel magnetization states of the PL and the FL In the small field region close to H ¼ 0, the magnetizing direction of the FL is reversed, and that is actually the working region of SV This is also a crucial advantage of the SV in low magnetic field sensing and measurements as well as in the data storage applications because it can detect an extremely weak magnetic field, e.g., the magnetic field induced by a data bit memory, the biological magnetic fields, and the Earth's magnetic field etc The resistive sensitivity could be defined by the slope of the DR/R0 versus DH curve within the reverse magnetization of the FL Beside the dependences of the MR ratio on the iron content in the CoFe alloy layer and on the thickness of the AFM layer, as mentioned above, the MR performance is also sensitive to the initial texture of the buffer layer Therefore, the result of the cool-field anneal process, and especially the thickness of the spacer layer are the main factors affecting the MR performance of SVs Fig 2(b) shows the effect of the seed layer (Ta) on the MR ratio The maximum MR is observed at tTa ¼ 50 Å At the thickness above 50 Å, the MR ratio decreases with the further increasing Ta thickness owing to the stable bcc-phase of Ta in the thicker layer, where the (111) texture disappears Currently, a seed or buffer layer is considered a standard part of the SV structures The benefit of the seed layer is to induce a (111) texture in the SV structure [44,57] The (111) texture has been reported to boost an enhancement of the MR in SV films [58] Moreover, with the present of the (111) texture, the Heb is also enhanced [48] Therefore, the disappearance of the (111) texture due to the lack of a seed layer leads to a decrease in Heb and in the working temperature of SV films as well [54] In fact, the magnetoresistive properties of SVs are very sensitive to the seed layer used The effect of the various buffer layers in a typical SV structure with a strong (111) texture has also been reported by Ryoichi et al [59] Finally, a Ta layer thickness of 50 Å was used for the seed layer in all spin SVs studies in this work The crucial role of the non-magnetic spacer layer in the SV is that it provides the coupling mechanism between the two FM layers The coupling sign of magnetizations of the FM layers can be controlled by tuning the spacer thickness leading to the magnetoresistance oscillating with the varying spacer thickness Therefore, this investigation aimed to find out a critical spacer thickness that provides the maximum MR ratio with an appropriate Hin The dependence of the MR ratio on the non-magnetic layer thickness of the SVs has also been extensively studied [20,38,44] Fig 2(c) shows the dependence of the MR on the non-magnetic Cu layer thickness, tCu At tCu < 24 Å, the MR ratio increases, but the Hin become stronger leading to the shift of the working point This is mainly caused by the roughness of spacer layers, the pinholes, and the interlayer coupling field of the PL [36] In the low magnetic field sensing applications, the bias point (or operation point) of the SV should be as close and symmetrical as possible around the zero field (H ¼ 0) At the thicknesses above 24 Å, e.g tCu ¼ 30 Å, the MR decreases with an increasing spacer layer thickness The suppression of the MR could be explained for two reasons Firstly, the probability of the bulk scattering is proportional to the thickness of the Cu conductive layer This scattering is not dominating against the electrons passing the FM layers, so the MR is reduced Secondly, because the high shunting current of the thicker spacer layer also contributes to reduce the MR ratio [45,60,61], while Hin is improved, it revealed that the coupling between the PL and the FL is 402 V.S Luong et al / Journal of Science: Advanced Materials and Devices (2018) 399e405 Fig (a) Illustration of an MR curve, (b) MR as a function of the seed layer thickness of SVs (Si/SiO2)/Ta(tTa)/Co(45 Å)/Cu(30 Å)/Co80Fe20(25 Å)/IrMn(250 Å)/Ta(50 Å), (c) the MR curves of SVs with the increasing spacer thickness of (Si/SiO2)/Ta(50 Å)/[NiFe(30 Å)/Co(15 Å)]/Cu(tCu)/Co80Fe20(25 Å)/IrMn(100 Å)/Ta(50 Å), and (d) MR as a function of cool-field annealling temperature of SVs (Si/SiO2)/Ta(50 Å)/[NiFe(30 Å)/Co(15 Å)]/Cu(24 Å)/Co80Fe20(25 Å)/IrMn(100 Å)/Ta(50 Å) weaker in a thicker spacer layer Finally, by the trade-off between the MR and Hin, tCu ¼ 24 Å was chosen for SV film fabrications As we know, a cool-field anneal process is an indispensable step in the fabrication procedure to induce a pining direction (the sensing axis) in the pinned SV, which can be applied for the low magnetic field measurements [62] The blocking temperature Tb el temperature (TN) of of the SV is certainly determined by the Ne el temperature of the AFM layer the AFM layer [27,63,64] A high Ne will ensure a high thermal stability of SV based on it Since TN of the IrMn alloy is about 427  C, so the Tb of the SV can be in a range of 250  Ce300  C [53] At a higher annealing temperature above the Tb, the MR ratio and Heb value significantly decrease because of the inter-diffusion A sudden decrease in the MR ratio from 8.5% down to below 3% at 350  C is shown in Fig 2(d) An appropriate coolfield annealling condition has been found to be 250  C in a vacuum of ~10À4 Pa and under an applied magnetic field of kOe (or stronger) for an annealling time of 30 followed by h time for self-cooling down to room temperature (RT) 3.1.3 The coercivity of the free layer (Hc) The hysteresis (Hc) of the FL is a crucial parameter which can give information drawing on the accuracy of a low magnetic field sensor, as illustrated in Fig 3(a) Accordingly, large extended hysteresis loop shall lead to a big error in analog measurements In the ideal case, the FL should be reversed freely and be free of any hysteresis Since the uniaxial anisotropy certainly exists in all ferromagnetic materials, the presence of a certain value of Hc is unavoidable As seen in Fig 3(a), a large Hc gives rise to a big error, therefore, for any sensor the material with as small as possible Hc must be chosen [40] However, it should be noted that a large Hc value would also be caused by the cooling process after the coolfield anneal process Research has shown that the introduction of an additional permalloy layer to the SV structure could help reduce or even eliminate the coercivity Hc The effects of the combination of a permalloy layer with Co90Fe10 alloys on the magnetoresistance performance of a SV structure have been studied and reported by H Kanai et al [51] The FL synthesized of NiFe and Co in form of a layer in the SV with the (111) texture can lead to a collapse of the hysteresis of the FL [53] In addition, the MR has also been found enhanced by the spin-dependent scattering at the magnetic interfaces of (NiFe/Co/ Cu) [18] In our experiments, the composite layer was realized as a free layer with two different compositions of [NiFe/CoFe] and [NiFe/ Co] However, we found that the surface of the [NiFe/Co] layer was smoother than that in the [NiFe/CoFe] junction and that allows to improve the surficial quality of the neighboring spacer layer (Cu layer) It was also reported that the roughness of the top Co layer can be modified by the morphological surface of the bottom layer in a sandwich stack of Co/Ru/Co [65] Therefore, the surficial quality of the composite layer in this work was expected to strongly depend on the topography of the bottom layer Fig 3(b) shows the effect of the composite [NiFe/Co] layer to the Hc of the FL As it is clear to see, the Hc of the SV without the NiFe layer was larger than 35 Oe, whereas Hc decreases dramatically to the below Oe with the introduction of the composite layer [NiFe/Co] In addition, the MR ratio also slightly increases from 5.8% to 6.3% because the defect scattering in SVs becomes weaker 3.2 Design and features of the engineered SV magnetic sensor For the magnetic sensing application, the SV film structure of (Si/SiO2)/Ta(50 Å)/[NiFe(30 Å)/Co(15 Å)]/Cu(24 Å)/Co80Fe20(25 Å)/ IrMn(100 Å)/Ta(50 Å) was chosen as an appropriate one It was treated with a cool-field anneal process presented in previous sections The SV films were patterned into micron scale using the lithographic lift-off method and pinned SV sensors were fabricated following the procedure described previously In practical magnetic field sensing and measurement applications, it is effective to arrange the sensors in an Wheatston bridge structure In several previous reports, the Wheatstone bridge configuration has been applied successfully for anisotropy magnetoresistance (AMR) sensors [66], pinned SV sensors [67,68], and tunnel magnetoresistance (TMR) sensors [69,70] Their performances were demonstrated to be suitable for biomedical applications [71] As a special feature, the V.S Luong et al / Journal of Science: Advanced Materials and Devices (2018) 399e405 403 Fig (a) The error of field measurement caused by hysteresis, and (b) the impact of the composite NiFe/Co layer of SVs (Si/SiO2)/Ta(150 Å)/Co(45 Å)/Cu(30 Å)/Co80Fe20(25 Å)/ IrMn(300 Å)/Ta(50 Å) and (Si/SiO2)/Ta(50 Å)/[NiFe(30 Å)/Co(15 Å)]/Cu(30 Å)/Co80Fe20(25 Å)/IrMn(250 Å)/Ta(50 Å) Fig Design of a spin valve half-bridge probe on a PCB The left-hand-side inset: the half-bridge schematic, and the right-hand-side inset: the half-bridge probe photograph and the zoom-in photograph of the SV sensor Wheatstone bridge provides a free offset output and improves the thermal effect In this work, as-fabricated SV sensors were arranged in a half-bridge configuration for further studies Fig shows the design of a prototype spin valve bridge sensor, and the schematic of the half-bridge is shown in the left-hand-side inset while the photograph of the SV sensor is displayed in the right-hand-side inset A zoom-in photograph of the SV cells in the sensor construction is also included in the Fig The sensing element of SV1 and SV2 were attached on the PCB by adhesive varnish, while two identically passive resistances were soldered directly on the other side of the PCB After the wire bonding step, the two active SV cells were covered with varnish for protection The transfer curve (VeB) of the sensor is shown in Fig 5(a) Since the free layer is coupled by the stray field of the pinned layer, the operation point (OP) is shifted by mT (Hin), as it is shown by the curve with a sensitivity of 8.5 V/T Therefore, in the weak magnetic field measurement, the OP of the sensor must be controlled into a symmetric position In this design, we used a static magnetic field to bias the OP by locating a tiny permanent magnet nearby the SV cells The strength of the bias field was tuned by adjusting the position and the distance between SV cells and the permanent magnet As the result, a sensitivity of V/T and a linear range of ±2 mT of the biased sensor was reached Although the sensitivity was slightly reduced in comparison to that of the unbiased sensor, the OP could be located to the center of the sensor response, as it is illustrated by the green curve with a V/T sensitivity in Fig 5(a) To demonstrate the features of the sensor in the low magnetic field measurement, the device was used to detect the Earth magnetic field By rotating the sensor in a complete circle of 360 with the interval of 10 , the obtained response of the sensor was a sinusoidal signal It revealed that the sensor responded linearly as a vector sensor in a certain sensing direction The amplitude of the measured magnetic field is approximately 35 mT, which was the horizontal component of the Earth magnetic field, as it is shown in Fig 5(b) As it is known, the strength of the Earth magnetic field is about 50 mT The measurement error was determined by taking the subtraction between a fitting cosine and the measured data The maximum error was about 0.04 mV, which corresponds to approximately mT magnetic field amplitude The error is caused by the hysteresis of the SV bridge, by the distortion of the Earth's magnetic field in the measurement laboratory, and also by the extra contribution of the AMR behavior in the total MR response of the Fig (a) VeB curves of the half-bridge, including an unbiased bridge (8.5 V/T) and a magnetic biased bridge (5 V/T), and (b) the Earth magnetic field measurement 404 V.S Luong et al / Journal of Science: Advanced Materials and Devices (2018) 399e405 SV A dc offset of 5.7 mV is equivalent to 1.14 mT due to the unidentical resistances of the SV cells and the contribution of the dc offset of the operational amplifier The offset drift was attributed as due to the warming up of the circuit of the two external resistances, and the thermal stability of the SV cells Fortunately, these drawbacks could be well solved by the modification of the uniaxial anisotropy configurations of the SV films [53,72], the microfabricated sensing cell in series to improve the detectivity [73], the flux amplification using integrated flux concentrators [74,75], or the modulation techniques [68,76] Conclusion A spin valve magnetic sensor was successfully engineered in this work The magnetoresistive performances of SV films were studied Tantalum was used in the seed layer to induce the (111) texture The composite [NiFe/Co] layer contributed to the suppression of the hysteresis (5 Oe) that affected directly the sensitivity of the SV The optimal thickness of the AFM (Ir25Mn75) layer was found to be 100 Å to maximize Heb (500 Oe) The Heb was also improved by the optimal composition of the CoFe layer The selected SV structure was Si/SiO2)/Ta(50 Å)/[NiFe(30 Å)/Co(15 Å)]/ Cu(24 Å)/Co80Fe20(25 Å)/IrMn(100 Å)/Ta(50 Å) To establish a bias direction, an appropriate cool-field anneal procedure was applied with 250  C for 30 in a vacuum of ~10À4 Pa followed by h cooling to RT in an applied magnetic field of kOe The SV films were successfully patterned and engineered using the lithographic lift-off method to fabricate SV sensors for magnetic sensing applications The features of the engineered SV sensor in a halfbridge construction were characterized A sensitivity of V/T was observed with the magnetic bias technique Finally, the Earth magnetic field measurement was carried out using the fabricated sensor The experimental results suggest that the selected SV structure is 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