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

Original ArticleMagnetoresistive performances in exchange-biased spin valves and a International Training Institute for Materials Science ITIMS, Ha Noi University of Science and Technolo

Original Article

Magnetoresistive performances in exchange-biased spin valves and

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

b 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

a r t i c l e i n f o

Article history:

Received 18 May 2018

Received in revised form

2 August 2018

Accepted 14 September 2018

Available online 19 September 2018

Keywords:

GMR

Giant magnetoresistance

Magnetic sensors

RF sputtering

Spin valve

a b s t r a c t

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 SVfilm structure of (Si/ SiO2)/Ta(50Å)/[NiFe(30 Å)/Co(15 Å)]/Cu(24 Å)/Co80Fe20(25Å)/IrMn(100 Å)/Ta(50 Å) was found sig-nificant, and the SV elements were patterned using the lithographic lift-off method with the active cell dimensions of 2mm
150mm To define a pinning axis, a cool-field anneal was applied at 250C for

30 min in a magneticfield of 2 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 5 V/T was observed with a linear range of ±2 mT To demonstrate the performance of the designed sensor, a measurement of the Earth magneticfield 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/)

1 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

per-formance of GMR is very sensitive to the fabricating conditions and

the specific equipment [19] This can also be found in various

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 elec-trodeposition [32e35], etc However, the challenges of the GMR fabrication still remain owing to the interplay between the mag-netoresistive properties and the specific equipment used Further-more, the magnetoresistive properties are the factors which directly determine the architecture and the features of the mag-netic sensors For example, the exchange biasfield (Heb) expresses the strength of the externalfield making a saturation, where the sensor is inactive with any external magneticfields The intercou-plingfield (Hin) is induced by the coupling between a pinned layer (PL) and a free layer (FL) The Hincan cause a shift of the operating point of the MR curve The Hinis mainly caused by the thickness and the roughness of the spacer layer, and by the strayfield of the PL as

* Corresponding author.

E-mail address: sulv@itims.edu.vn (V.S Luong).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

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

Journal of Science: Advanced Materials and Devices 3 (2018) 399e405

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

trade-off between these factors tofind 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 SVfilms 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

per-formance of the patterned SV was demonstrated via the Earth's

magneticfield measurement The obtained experimental results

of the SV sheetfilms and the prototype sensor are presented and

discussed

2 Experimental

Exchange-biased SVs were prepared by the RF sputtering

tech-nique The vacuum in this procedure was 6
105Pa, and the

working pressure (Argon) was 0.7 Pa The distance from the target to

the substrate was about 8 cm Firstly, a 400 nm layer of SiO2was

sputtered onto a 5 mm
5 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 afield emission

scanning electron microscop of the model JEOL JSM-7600F Thefilm

thickness was measured by an atomic force microscop The

sput-tering 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 2 kOe The heating time was 30 min, and the

cooling time to room temperature (RT) lasted 60 min The SVfilms

were magnetically characterized using a vibration sample

magne-tometer (VSM) The magnetoresistance (MR) was measured by a

conventional four-probe method with a bias current passing in-plane

of the SV

To demonstrate the features of the engineered sensor, a sweeping magneticfield 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 sweepingfield 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 Thefield 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 magneticfield 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 0to

360 The sensor probe wasfixed 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 Na-tional Instruments The software was coded in LabVIEW Wefirst study the magnetoresistive properties of SV sheetfilms for finding out the appropriate parameters of the SV film fabrication The conventional approach of sputtering was used to prepare thefilms based on our previous work[37]and refering to the recent reports

on the SVfilm fabrication technology[40,41]

3 Results and discussion 3.1 Magnetoresistive properties of the SV sheetfilms 3.1.1 Exchange bias (Heb)

Exchange-biased spin valves werefirst 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 so-called 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 magneticfield This free layer was made of soft magnetic materials The magnetic properties are demonstrated by the M-H loop, shown inFig 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 magneticfield 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 magneticfield 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 magneticfields 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

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 Heband 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

mag-netic 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 Hebhas been reported by M Pakala et al

[48] The Hebdependence 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

pub-lished papers have confirmed that good MR performance is

ob-tained 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 Heband

MR ratio, in this work, x¼ 20% was the iron content chosen for the

SVfilm fabrication

Furthermore, the Hebalso strongly depends on the AFM layer

thickness.Fig 1(c) shows the effects of the IrMn layer thickness,

which was varied from 50Å to 300 Å on the MR behaviors and the

Hebvalue A lower Hebin 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 Hebhas reached a maximum at 100Å and it drops as this film

becomes thicker (so, at thicknesses>100 Å) As a result, both the

Hebstrength and the MR ratio reached a maximum at tIrMn¼ 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

byDR/R0inFig 2(a), where R0is the base resistance of the SV in the

zero externalfield andDR is calculated using the resistances in the

parallel and anti-parallel magnetization states of the PL and the FL

In the smallfield 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 magneticfield sensing

and measurements as well as in the data storage applications

because it can detect an extremely weak magneticfield, e.g., the

magneticfield induced by a data bit memory, the biological

mag-neticfields, and the Earth's magnetic field etc The resistive

sensi-tivity could be defined by the slope of theDR/R versusDH 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 perfor-mance 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 tTa¼ 50 Å At the thickness above

50 Å, the MR ratio decreases with the further increasing Ta thick-ness 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 SVfilms[58] Moreover, with the present of the (111) texture, the Hebis 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 SVfilms 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 magneto-resistance oscillating with the varying spacer thickness Therefore, this investigation aimed tofind 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

Hinbecome stronger leading to the shift of the working point This

is mainly caused by the roughness of spacer layers, the pinholes, and the interlayer couplingfield 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 tCu¼ 30 Å, the MR decreases with an increasing spacer layer thickness The suppres-sion 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

Fig 1 (a) Illustration of a M-H loop of a typical SV, (b) H eb and MR versus Fe contents of CoFe alloy of SVs (Si/SiO 2 )/Ta(150 Å)/Co(45 Å)/Cu(30 Å)/Co 100-x Fe x (25 Å)/IrMn(250 Å)/ Ta(50 Å), and (c) MR and H eb as a function of IrMn thickness of SVs Si/SiO 2 )/Ta(50 Å)/[NiFe(30 Å)/Co(15 Å)]/Cu(24 Å)/Co 80 Fe 20 (25 Å)/IrMn(t IrMn )/Ta(50 Å).

V.S Luong et al / Journal of Science: Advanced Materials and Devices 3 (2018) 399e405 401

weaker in a thicker spacer layer Finally, by the trade-off between

the MR and Hin, tCu¼ 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

of the SV is certainly determined by the N
eel temperature (TN) of

the AFM layer[27,63,64] A high N
eel temperature of the AFM layer

will ensure a high thermal stability of SV based on it Since TNof the

IrMn alloy is about 427C, so the Tbof the SV can be in a range of

250Ce300C[53] At a higher annealing temperature above the

Tb, the MR ratio and Hebvalue significantly decrease because of the

inter-diffusion A sudden decrease in the MR ratio from 8.5% down

to below 3% at 350C is shown inFig 2(d) An appropriate

cool-field annealling condition has been found to be 250 C in a

vac-uum of ~104Pa and under an applied magneticfield of 2 kOe (or

stronger) for an annealling time of 30 min followed by 1 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 magneticfield

sensor, as illustrated inFig 3(a) Accordingly, large extended

hys-teresis 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 inFig 3(a), a large Hcgives 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

cool-field anneal process

Research has shown that the introduction of an additional

per-malloy layer to the SV structure could help reduce or even eliminate

the coercivity Hc The effects of the combination of a permalloy

layer with Co90Fe10alloys 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 Hcof 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 5 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 SVfilm structure of (Si/SiO2)/Ta(50Å)/[NiFe(30 Å)/Co(15 Å)]/Cu(24 Å)/Co80Fe20(25Å)/ IrMn(100Å)/Ta(50 Å) was chosen as an appropriate one It was treated with a cool-field anneal process presented in previous sections The SVfilms 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) sen-sors[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

Fig 2 (a) Illustration of an MR curve, (b) MR as a function of the seed layer thickness of SVs (Si/SiO 2 )/Ta(t Ta )/Co(45 Å)/Cu(30 Å)/Co 80 Fe 20 (25 Å)/IrMn(250 Å)/Ta(50 Å), (c) the MR curves of SVs with the increasing spacer thickness of (Si/SiO 2 )/Ta(50 Å)/[NiFe(30 Å)/Co(15 Å)]/Cu(t Cu )/Co 80 Fe 20 (25 Å)/IrMn(100 Å)/Ta(50 Å), and (d) MR as a function of cool-field annealling temperature of SVs (Si/SiO 2 )/Ta(50 Å)/[NiFe(30 Å)/Co(15 Å)]/Cu(24 Å)/Co 80 Fe 20 (25 Å)/IrMn(100 Å)/Ta(50 Å).

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 4shows 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 4 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 inFig 5(a) Since

the free layer is coupled by the strayfield of the pinned layer, the operation point (OP) is shifted by 4 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 magneticfield

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 5 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 5 V/T sensitivity inFig 5(a)

To demonstrate the features of the sensor in the low magnetic field measurement, the device was used to detect the Earth mag-neticfield By rotating the sensor in a complete circle of 360with

the interval of 10, the obtained response of the sensor was a si-nusoidal 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 35mT, which was the horizontal component of the Earth magneticfield, as it is shown

in Fig 5(b) As it is known, the strength of the Earth magnetic field is about 50mT The measurement error was determined by taking the subtraction between afitting cosine and the measured data The maximum error was about 0.04 mV, which corresponds

to approximately 8mT magneticfield amplitude The error is caused

by the hysteresis of the SV bridge, by the distortion of the Earth's magneticfield in the measurement laboratory, and also by the extra contribution of the AMR behavior in the total MR response of the

Fig 3 (a) The error of field measurement caused by hysteresis, and (b) the impact of the composite NiFe/Co layer of SVs (Si/SiO 2 )/Ta(150 Å)/Co(45 Å)/Cu(30 Å)/Co 80 Fe 20 (25 Å)/ IrMn(300 Å)/Ta(50 Å) and (Si/SiO 2 )/Ta(50 Å)/[NiFe(30 Å)/Co(15 Å)]/Cu(30 Å)/Co 80 Fe 20 (25 Å)/IrMn(250 Å)/Ta(50 Å).

Fig 4 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

photo-graph and the zoom-in photophoto-graph of the SV sensor.

Fig 5 (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.

V.S Luong et al / Journal of Science: Advanced Materials and Devices 3 (2018) 399e405 403

4 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

sup-pression of the hysteresis (5 Oe) that affected directly the

sensi-tivity of the SV The optimal thickness of the AFM (Ir25Mn75) layer

was found to be 100Å to maximize Heb(500 Oe) The Hebwas also

improved by the optimal composition of the CoFe layer The

selected SV structure was Si/SiO2)/Ta(50Å)/[NiFe(30 Å)/Co(15 Å)]/

Cu(24Å)/Co80Fe20(25Å)/IrMn(100 Å)/Ta(50 Å) To establish a bias

direction, an appropriate cool-field anneal procedure was applied

with 250C for 30 min in a vacuum of ~104Pa followed by 1 h

cooling to RT in an applied magneticfield of 2 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

half-bridge construction were characterized A sensitivity of 5 V/T

was observed with the magnetic bias technique Finally, the Earth

magneticfield measurement was carried out using the fabricated

sensor The experimental results suggest that the selected SV

structure is suitable for weak magnetic field measurements and

navigation applications

Conflicts of interest

The authors declare that there is no conflict of interest regarding

the publication of this article

Acknowledgments

This research is funded by Vietnam National Foundation for

Science and Technology Development (NAFOSTED) under the grant

number 103.02-2016.03

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