DSpace at VNU: Optimization of Spin-Valve Structure NiFe Cu NiFe IrMn for Planar Hall Effect Based Biochips

6, JUNE 2009Optimization 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 Dan

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= 4 8 10 12 16 26

nm) and pinned layer ( P = 1 2 6 8 9 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= 1 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

ap-plications 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

mag-netic bead detection and biosensor design [2]–[6] The

trans-verse voltage on a planar Hall cross depends on the

orienta-tion of the magnetizaorienta-tion of the ferromagnetic layer with

re-spect to the longitudinal sensing current Thus, a large PHE

is expected to be observed in exchange coupling based

struc-tures because they can ensure a sufficient uniaxial anisotropy

with well-defined single domain state to introduce a

unidirec-tional 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 3 m 3 m when detecting the 2.8 m

mag-netic beads

The present paper deals with studies of the magnetic field

sen-sitivity as a function of the thickness of the individual free

fer-romagnetic (FFM) and the pinned ferfer-romagnetic (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

1 nm This optimum structure is proposed to apply for magnetic bead detections

II EXPERIMENTALPROCEDURE

The thin films with typical spin-valve structure of Ta(5)/NiFe /Cu(1.2)/NiFe /IrMn(15)/Ta(5) (nm) with

layer thickness , nm are fabricated by dc 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

of mTorr During sputtering process, a uniform magnetic field of A/m is applied in plane 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 fields up to 4 kA/m applied along Oy direction and sensing

currents of 1 mA Longitudinal magneto-resistance was mea-sured by means of a collinear four-point probe method, for sam-ples with the size of 2 mm 10 mm, with applied magnetic field and sensing current are along direction Magnetization was measured by using a Lakeshore 7400 vibrating sample magne-tometer (VSM) on films

0018-9464/$25.00 © 2009 IEEE

Fig 1 (a) Top view micrograph of the single 50 m 2 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 8 single sensors in the two middle lines and 1 single

sensor in each edge line).

III RESULTS ANDDISCUSSION

A Fixed PFM Layer Thickness Spin-Valve System

Fig 2(a) presents the magnetization data of spin-valve Ta(5)/

NiFe /Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) (nm) structures with

different free layer thicknesses varying from 4 to 16 nm It

is clearly seen that all the samples exhibit two hysteresis loops

The magnetization accounting from the first loop linearly

in-creases with increasing while that from the second loop is

almost constant These two hysteresis loops are attributed to

the FFM and PFM layers, respectively The FFM layer is

ex-pected to dominate the sensor response at low magnetic fields

The values of the coercivity and exchange coupling

fields determined from the first hysteresis loop [see insert in

Fig 2(a)] are collected and listed in the Table I Note that,

exper-imentally, shows a tendency to increase with while

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

maximal value of 1.6% for nm This maximum lightly

decreases with the increasing of the FFM layer thickness and

equals to 1.2% for nm Further increasing the

(de-magnetizing) magnetic field, the magnetization rotation in PFM

layers starts rather early in the thin FFM layer thickness

sam-ples and later in the thick FFM layer thickness samsam-ples

How-ever, the final parallel configuration of individual layer

magne-Fig 2 Magnetic hysteresis loops (a) GMR data (b) and low field PHE profiles (c) of spin-valve structures with the fixed PFM layer thickness t = 2 nm and FFM layer thickness (t ) varying from 4 to 16 nm.

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]

Shown in Fig 2(c) are the PHE voltage profiles, , as

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

V ALUES S ENSOR S ENSITIVITY (S), C OERCIVE (H ), A NISOTROPY (H ),

E XCHANGE C OUPLING (H ) F IELDS , AND M AXIMAL F IELD (H ) AT GMR

P EAK FOR S PIN -V ALVE S YSTEM W ITH D IFFERENT F REE L AYER T HICKNESSES

field, at which GMR reaches the maximum , is

systemat-ically 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

in-creases from the value of 15 V for the sample with nm

to the value of 48 V for nm Consequently, the sensor

sensitivity ( , see below) is enhanced from the value

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

with direction is given as follows:

(1)

resis-tivity measured with the current parallel and perpendicular to the

magnetization, respectively; is the free ferromagnetic layer

thickness

Typically, these curves are fitted well by using the

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) Here, the and are the angles between the magnetization

of the free and pinned layers and the easy axis direction,

respec-tively; is the effective anisotropy constant,

is the saturation magnetization of the free layer, and is the

interlayer coupling constant that can be extracted from the

re-lation with the exchange coupling field between two FM layers

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

mag-netic fields, i.e., equals to zero This can be applied for the

present case, where the magnetization reversal of the free and

pinned layers occurred separately [see in Fig 2(a)]

For small angles, , the PHE voltage exhibits a linear

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 3 Hysteresis loops (a) GMR data (b) and low field PHE profiles (c) mea-sured in spin-valve structures with the fixed free layer thickness x = 10 nm and different pinned layer thickness (t) from 2 to 12 nm.

sensor sensitivity The obtained results of is also sum-marized in Table I Note that the values of and obtained from the fits of the PHE data are in excellent agreement with those derived from experimental data The calculated values

of , however, are systematically larger than determined 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

TABLE II

V ALUES S ENSOR S ENSITIVITY (S), C OERCIVE (H ), A NISOTROPY (H ),

E XCHANGE C OUPLING (H ) F IELDS AND M AXIMAL F IELD (H ) AT GMR

P EAK FOR S PIN -V ALVE S YSTEM W ITH D IFFERENT F REE L AYER T HICKNESSES

B Fixed FFM Layer Thickness Spin-Valve System

Fig 3(a) presents the magnetization data of Ta(5)/NiFe(10)/

Cu(1.2)/NiFe /IrMn(15)/Ta(5) (nm) spin-valve structures

with different PFM layer thickness varying from 1 to

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

The values of the coercive and exchange coupling

fields determined from the first hysteresis loop are collected

and listed in the Table II Note that lightly varies around

the value as small as 80 kA/m, whereas strongly increases

with increasing

Typical magnetoresistive characteristics of spin-valve

struc-tures 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 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 1 to 12 nm

Shown in Fig 3(c) are the PHE voltage profiles as a

func-tion 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

be-tween and the sum of is found (see Table II)

Consequently, the sensor sensitivity is strongly reduced from

the value of 110.6 to 42.7 V/(kA/m) when increases from

1 to 12 nm The values of anisotropy , exchange coupling

fields and sensor sensitivity derived from the

theoret-ical fits show an excellent consistence with experimental results

While a rather large difference between and (calculation)

is observed for 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

tures with thin PFM and thick FFM layers In spin-valve

struc-tures, 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

Fig 4 Hysteresis loops (a), GMR data (b) and low field PHE profiles (c) mea-sured in Ta(5)/NiFe(26)/Cu(1.2)/NiFe(1)/IrMn(15)/Ta(5) (nm) spin-valve struc-ture, i.e., with t = 26 nm and t = 1 nm.

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 in-terface 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 elim-inated in the structure with thin pinned layer nm Prac-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

estab-lished 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)

spin-valve structure, i.e., with nm and nm Its

magnetization and PHE data are presented in Fig 4 Although

the magnetization reversal is mainly contributed to the first

magnetic hysteresis loop [Fig 4(a)], the rotation of the

magneti-zation 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

its maximal value of about 62 V at A/m and this

spin-valve configuration shows a sensor sensitivity as large as

196 V/(kA/m) Additionally, the values of and fields

are as small as 160 and 330 A/m, respectively

IV CONCLUSION

The influence of the individual free and pinned layer

thick-ness on the sensitivity of PHE sensor based on the spin-valve

structure of NiFe /Cu(1.2)/NiFe /IrMn(15) (nm) with

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

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