DSpace at VNU: Sensitivity Dependence of the Planar Hall Effect Sensor on the Free Layer of the Spin-Valve Structure

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 6, JUNE 2009

Sensitivity Dependence of the Planar Hall Effect Sensor on the
Free Layer of the Spin-Valve Structure
T. Q. Hung1 , S. J. Oh1 , B. D. Tu2 , N. H. Duc2 , L. V. Phong1 , S. AnandaKumar1 , J.-R. Jeong1 , and C. G. Kim1
Department of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, Korea
Department of Nano Magnetic Materials and Devices, Faculty of Physics Engineering and Nanotechnology,
College of Technology, Vietnam National University, Hanoi, Vietnam
Planar Hall effect (PHE) sensors with the junction size of 50 m 50 m were fabricated successfully by using spin-valve thin films
Ta(5)/NiFe( )/Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) (nm) with = 4 8 10 12 16. The magnetic field sensitivity of the PHE sensors increases with increasing thickness of ferromagnetic (FM) free layer. The sensitivity of about 95.5 m
/(kA/m) can be obtained when
the thickness of the FM-free layer increases up to 16 nm. The enhancement of sensitivity is explained by the shunt current from other
layers. The PHE profiles are well described in terms of the Stoner–Wohlfarth energy model. The detection of magnetic micro-beads label
Dynabeads® M-280 is demonstrated and the results revealed that the sensor is feasible for high-resolution biosensor applications.
Index Terms—Biosensor applications, high field sensitivity, micro-beads detection, planar Hall effect.

I. INTRODUCTION
AGNETORESISTIVE biosensors have attracted a lot
of attention [1] because of their numerous advantages
such as an easy-to-use, highly portable sensing platform with
high sensitivity and faster read out technique [2]. Among the
various kinds of magnetoresistive biosensors, the planar Hall
effect (PHE) sensor has vast potential used in nano-Tesla field
range detection sensors and biosensors due to its extremely high
signal-to-noise ratio, high linearity at low field range, and high
field sensitivity [3].
PHE, known as anisotropic magnetoresistance (AMR), is induced from spin-orbit coupling and spin polarization of the materials. Alternatively, NiFe permalloy was chosen to develop the
high field sensitivity PHE sensor. Dau et al. [4] found that the
PHE sensor using single NiFe layer was able to reduce thermal

drift known as main noise source by at least four orders of magnitude so it can detect the nano-Tesla field range. Furthermore,
in the exchange bias system, exchange coupling induced from
the interface between ferromagnetic (FM) and antiferromagnetic (AFM) layers can enhance the single domain state of NiFe
layer, constrain the magnetization in coherent rotation, and prevent Barkhausen noise associated with magnetization reversal
and thermal stability [5]. Ejsing et al. developed the PHE sensor
based on bilayer structure NiFe/IrMn/NiFe for erroneous detection of the small stray field of micro- and nano-bead coated
biomolecules with the advantage of ultra high signal-to-noise
ratio [6]. The spin-valve structure not only has the same advantages of the bilayer structure but also has the dynamic range due
to the magnetic field created by the FM-pinned layer acting on
the FM-free layer with closed fringes. Therefore, the spin-valve
thin films are better candidate for development of high field
sensitivity sensor at small field range. Earlier, we reported our
work on PHE sensors based on the spin-valve structure NiFe(6)/
Cu(3)/NiFe(3)/IrMn(10) (nm) for biochip applications [7]. The

M

Manuscript received October 09, 2008. Current version published May 20,
2009. Corresponding author: C. G. Kim (e-mail: ).
Color versions of one or more of the figures in this paper are available online
at .
Digital Object Identifier 10.1109/TMAG.2009.2018578

sensor sensitivity of about 31.4 m /(kA/m) was observed in the
A/m.
linear region of the PHE profile at the field range of
It has been well applied for single 2.8 m diameter Dynabeads®
M-280 detection by using the sensor size of 3 m 3 m.
However, it is revealed from the context that the field sensitivity of the PHE sensor using the spin-valve structure is still
relatively small. Therefore, a novel PHE sensor, which shows

high-sensitivity, is highly desirable from both fundamental and
application point of views. There were several reports disclosing
the enhancement of the sensitivity of a sensor based on spinvalve structure such as changing the applied field direction [8],
[9] and developing the spin-valve structure with the uniaxial
field normal to the unidirectional field [10], [11]. For the first
case, the PHE sensor has maximum sensitivity when the applied
field direction is parallel to the easy axis of the thin film; unfortunately, a hysteresis of the PHE profile was observed. For
the second case, when there is a tilt angle between the uniaxial and unidirectional fields, the coherent rotation of FM-free
layer in the applied fields no longer exists. These deteriorate
the signal-to-noise ratio of the sensors as it gives possibility for
bio-applications.
To obtain high sensitivity sensors while avoiding the above
disadvantages, we optimized the thicknesses of the other layers,
increased FM-free layer in the spin-valve structure and studied
the role of PHE in these thin films systematically. The experimental results revealed that the sensitivity of the sensors increases due to the increased thickness of the FM-free layer. The
sensitivity of about 95.5 m /(kA/m) can be obtained as the
thickness of FM-free layer increases up to 16 nm.
II. EXPERIMENTAL PROCEDURE
A. Sensors Fabrication
The cross-junction sensors with the junction size of
50 m 50 m were prepared on SiO substrate using lift off
method. Firstly the cross junctions sized 50 m 50 m were
stenciled out on the photoresist coated on silicon dioxide wafer,
The spin-valve thin films Ta(5)/NiFe /Cu(1.2)-/NiFe(2)/
(nm), were deposited on
IrMn(15)/Ta(5),

0018-9464/$25.00 © 2009 IEEE

HUNG et al.: SENSITIVITY DEPENDENCE OF THE PLANAR HALL EFFECT SENSOR

Fig. 1. Top view micrograph of the single 50
junction.

m 2 50 m

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PHE sensor

these stenciled photoresist layer by using magnetron sputtering
system. The base pressure of the system is less than
Torr
and the Ar working pressure is 3 mTorr. During the deposition,
a uniform magnetic field of 16 kA/m was applied in the film
plane to induce magnetic anisotropy of the FM pinned layers
and to define the unidirectional field of the thin films. To
connect the external electronic circuitry with sensor junction,
the 90 nm Au electrodes were prepared. Finally the sensor
junctions were passivated with 120 nm SiO layer on top of
the sensor junctions and electrodes to protect them from the
corrosion and fluid environment during the magnetic bead drop
experiments.

Fig. 2. Experimental results and calculated results (solid lines) of the PHE
voltage profiles (solid lines) of the sensor junction 50 m 50 m using spinvalve structure Ta(5)/NiFe(x)/Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) (nm).

2

TABLE I
THE PARAMETERS: INTERLAYER COUPLING (H ), EFFECTIVE ANISOTROPY
CONSTANT (K ), SATURATE MAGNETORESISTANCE (M ), MAXIMUM
PHE VOLTAGE (V ), AND FIELD SENSITIVITY (S ) OF THE SENSORS USING
SPIN-VALVE THIN FILMS WITH DIFFERENT FREE LAYER THICKNESSES (x)

B. Sensor Characterization
Fig. 1 shows the SEM image of the passivated single sensor
junction 50 m 50 m. The terminals - represent the current line and - represent the voltage line. The unidirectional
anisotropy field,
, and/or the uniaxial anisotropy field of the
thin film is aligned parallel to the long terminals - . The PHE
profiles were measured by the electrodes bar - with a sensing
current of 1 mA applied through the terminals - and under
the external magnetic fields ranging from 4 kA/m to 4 kA/m
applied perpendicular to the direction of the current line. The
induced output voltages of cross-junctions were measured by
means of a Keithley 2182A Nanovoltmeter with a sensitivity
of 10 nV. All these sensor characterizations were carried out at
room temperature. To detect the magnetic beads, we performed
the magnetic drop and wash experiments on the sensor junction with 1 l solution 0.1% of the Dynabeads® M-280 by using
the micro pipette-lite SL-10 under an applied magnetic field of
550 A/m and a sensing current of 1 mA.
III. RESULTS AND DISCUSSION
Fig. 2 shows the PHE profiles of the sensor junctions with
various free layer thicknesses are characterized as a function of
external magnetic fields in the range from 4 kA/m to 4 kA/m.
, show linearly response
These PHE voltage profiles,
at small fields, reach the maximum voltage at about their inter, and finally decrease with a further

layer coupling field,
increase in the magnetic fields. Particularly, the field sensitivity
of the sensor is increased due to the increase in the free layer
thickness of the sensor material.

The magnetoresistance and magnetization results in the
spin-valve structure Ta(5)/NiFe /Cu(1.2)/NiFe(2)/IrMn(15)-/
Ta(5) (nm)
at low applied magnetic fields
confirm that the magnetization of FM-free layer can easily be
rotated in the presence of the external magnetic fields. Whereas
the FM pinned layer remains in the exchange bias field direction due to the exchange interaction between the AFM and FM
pinned layers. When the applied field overcomes the unidirectional field induced from exchange coupling (almost over
16 kA/m for all samples), the FM pinned layer starts rotating
towards the applied field direction. By further increasing the
kA/m, magnetization direction
magnetic field up to
of the FM-free and FM pinned layers will be aligned in parallel
configuration for all samples. These behaviors satisfy the early
reported results in [12]. The uniaxial fields induced from the
, is obtained from the
interlayer coupling of the thin films,
MR and magnetization profiles and are listed in Table I. The
details of this work will be published elsewhere [13].
Therefore, at the small magnetic fields, the PHE effect known
as AMR effect is almost contributed from the FM-free layer, and
the Stoner-Wohlfarth energy term of the FM-free layer can then
be simply expressed as [10]

(1)

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 6, JUNE 2009

where
, and are the thickness, uniaxial anisotropy
constant, saturation magnetization of the thin film and interlayer
coupling energy, respectively. is the angle between the external magnetic field and easy axis of thin film and is the angle
between the unidirectional field and the easy axis of the FM-free
and
.
layer. In our experimental conditions
For the FM-free layer, the PHE output voltage is described by
Ohm’s law and could be given by [14]

(2)
where
and
, and are the current applied to the
sensor junction, thickness, transverse and longitudinal resistivity, and angle between the magnetization direction of the
, of the thin film,
FM-free layer and unidirectional field,
respectively.
The angle can be calculated from the minimum energy condition of the above Stoner-Wohlfarth equation at each value of
applied magnetic fields. Hence, the PHE voltage profile can then
be calculated from (2). The calculated results presented as solid
mA;
lines in Fig. 2 are obtained for the best fit with

, and
are listed in Table I,
and
.
Since the PHE voltage is contributed from the FM-free layer,
magnitude in (2) is the only curthe current term in the
rent passing through this layer. The quantitative analysis of this
current is a complicated work because the applied current distributed in each layer is different and very sensitive to the interface of the thin films. [15] When increase the thickness of
FM-free layer of the thin film, consequently the current passing
through this free layer is also increased, the enhancement of the
PHE output voltage is achieved.
We performed the magnetic bead detection using PHE sensor
with highest sensitivity to demonstrate the feasibility of digital
bead detection for bio applications. The diluted 0.1% magnetic
bead solution streptavidin coated Dynabeads® M-280 is used for
bead drop and wash experiments on the sensor surface.
The real-time profile measurements of the PHE voltage for
magnetic beads detection is carried out in the optimum conditions, that is, in an applied magnetic field of 557 A/m and with a
sensing current of 1 mA. The results are illustrated in Fig. 3 for
three consecutive cycles, where the lower state represents the
signal change in sensor output voltage after dropping the magnetic bead solution on the sensor surface and the higher state
represents the sensor output voltage after washing the magnetic
bead from the sensor surface. The total output signal annuls in
three consecutive cycles were found to be about 7.1 V, 16 V
and 21.8 V for the first step and 11.3 V and 16.7 V in the
second step of the second and third cycles, respectively. It is
clearly shown from the figure that for the first cycle, the signal
changed by one-step and the signal was further changed into two
steps in the second and third cycles. This two step-type profile
is due to the aggregation of the magnetic beads on the sensor

surface. The aggregation of the magnetic beads occurs at the
drying stage. That is, after dropping the bead solution on the
sensor surface, it needs some time to dry. The first step changes
of the signals are assumed to be due to the viscous flow motion

Fig. 3. Real-time profile of the highest field sensitivity PHE sensor under an
applied magnetic field of 550 A/m and with the sensing current of 1 mA.

for stabilization as well as the Brownian motion of the beads.
When the solution dries, the beads rearrange. During this time,
some beads aggregate and become clusters on the sensor surface. This lessens the total stray field on the sensor surface and
hence, the second step changes in the second and third cycles
were observed in the real-time profile.
For further understanding the micro-bead detection using
PHE sensor, it is noted that the magnetization of the magnetic
sphere is purely a dipole at the center of the sphere with a
at a distance identified by [3]
magnetic field

(3)
and
are the value and vector in the direction of
where
magnetization of the bead, respectively. and are the bead
radius and distance from the center of the bead to the observation
point, respectively.
The stray field of a single bead on the sensor surface could be
crudely calculated by [16]

(4)

This stray field is in the opposite direction to the applied field,
thus it reduces the effective field on the sensor surface. Under
experiment conditions, the stray field of beads on the sensor
surface reduced the sensor output signal as follows:

(5)
is the effective field on the sensor surface, is the
where
sensor sensitivity,
is the number of magnetic beads on the
sensor surface, is the volume of magnetic bead, is the mass
magnetic susceptibility of magnetic beads, for Dynabeads®
M-280
[17].


HUNG et al.: SENSITIVITY DEPENDENCE OF THE PLANAR HALL EFFECT SENSOR

By substituting the value
and
m (the
distance including the radius of Dynabeads® M-280 and the
thickness of passivated SiO and Ta layers) into (4), the stray
field of single bead is estimated to be 17.5 A/m under the applied field of 550 A/m. Theoretically, with the sensor sensitivity
m /(kA/m) and the sensing current
mA, the
number of bead separately placed on the sensor surface can be
calculated in the first step of the three cycles by using (5), which
are estimated to be about 4, 10, and 13 beads, respectively.
These estimated results strengthen our explanation. It is

clearly shown in the first cycle, the number of beads on the
sensor surface is estimated to be small, and the distance among
beads on the sensor junction is far enough to avoid the effect
from the rearrangement of beads during the drying stage. In
the second and third cycles, the number of magnetic beads on
the sensor junction are larger; they easily aggregate to become
clusters under applied magnetic field due to short bead-bead
distance.
IV. CONCLUSION
We enhanced the field sensitivity of PHE sensors by increasing the free layer in the spin-valve structure Ta(5)/NiFe /
Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) (nm). The maximum sensitivity of the fabricated sensors of about 95.5 m /(kA/m) can
be obtained as the thickness of the free layer increases up
to 16 nm. The detecting Dynabeads® M-280 results with the
highest sensitivity PHE sensor reveals that our sensor is very
sensitive in identifying the existence of magnetic beads; different number of magnetic beads give different changes in the
real-time profile. Moreover, the decrease in stray field occurred
due to the bead-bead interaction at the drying stage, which can
be recognized by a two step-type of the real-time profile.
ACKNOWLEDGMENT
This work was supported by KOSEF under project
M10803001427-08M0300-42710, the Fundamental R&D
Program for Core Technology of Materials funded by the
Ministry of Knowledge Economy, Republic of Korea. The
work of J.-R. Jeong was supported by the Korea Research
Foundation (KRF-2008-331-D00234). The work of N. H. Duc
was supported by Vietnam National University, Hanoi under
project QG.TD 07.10.

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[17] [Online]. Available: />112.05D06D%20602.10%20Dynabeads%20M280%20Streptavidin%
20(rev%20012).pdf