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 ex
Trang 1Sensitivity 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 in-creases with increasing thickness of ferromagnetic (FM) free layer The sensitivity of about 95.5 m
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
M 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
in-duced from spin-orbit coupling and spin polarization of the
ma-terials 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
mag-nitude 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
antiferromag-netic (AFM) layers can enhance the single domain state of NiFe
layer, constrain the magnetization in coherent rotation, and
pre-vent 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
de-tection 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
advan-tages 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
Manuscript received October 09, 2008 Current version published May 20,
2009 Corresponding author: C G Kim (e-mail: cgkim@cnu.ac.kr).
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.2009.2018578
sensor sensitivity of about 31.4 m /(kA/m) was observed in the linear region of the PHE profile at the field range of A/m
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 sensi-tivity 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 spin-valve 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; un-fortunately, a hysteresis of the PHE profile was observed For the second case, when there is a tilt angle between the uni-axial 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 exper-imental results revealed that the sensitivity of the sensors in-creases 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 EXPERIMENTALPROCEDURE
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)/
0018-9464/$25.00 © 2009 IEEE
Trang 2Fig 1 Top view micrograph of the single 50 m 2 50 m PHE sensor
junction.
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
B Sensor Characterization
Fig 1 shows the SEM image of the passivated single sensor
junction 50 m 50 m The terminals - represent the
cur-rent 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
junc-tion with 1 l solujunc-tion 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 ANDDISCUSSION
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
These PHE voltage profiles, , show linearly response
at small fields, reach the maximum voltage at about their
inter-layer coupling field, , and finally decrease with a further
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
Fig 2 Experimental results and calculated results (solid lines) of the PHE voltage profiles (solid lines) of the sensor junction 50 m 2 50 m using spin-valve structure Ta(5)/NiFe (x)/Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) (nm).
TABLE I
T HE P ARAMETERS : I NTERLAYER C OUPLING (H ), E FFECTIVE A NISOTROPY
C ONSTANT (K ), S ATURATE M AGNETORESISTANCE (M ), M AXIMUM
PHE V OLTAGE (V ), AND F IELD S ENSITIVITY (S) OF THE S ENSORS U SING
S PIN -V ALVE T HIN F ILMS W ITH D IFFERENT F REE L AYER T HICKNESSES (x)
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 direc-tion due to the exchange interacdirec-tion between the AFM and FM pinned layers When the applied field overcomes the unidi-rectional 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 magnetic field up to kA/m, magnetization direction
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 interlayer coupling of the thin films, , is obtained from the
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)
Trang 3where , and are the thickness, uniaxial anisotropy
constant, saturation magnetization of the thin film and interlayer
coupling energy, respectively is the angle between the
ex-ternal magnetic field and easy axis of thin film and is the angle
between the unidirectional field and the easy axis of the FM-free
layer In our experimental conditions and
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
resis-tivity, and angle between the magnetization direction of the
FM-free layer and unidirectional field, , of the thin film,
respectively
The angle can be calculated from the minimum energy
con-dition 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
lines in Fig 2 are obtained for the best fit with mA;
, and are listed in Table I, and
Since the PHE voltage is contributed from the FM-free layer,
the current term in the magnitude in (2) is the only
cur-rent passing through this layer The quantitative analysis of this
current is a complicated work because the applied current
dis-tributed in each layer is different and very sensitive to the
in-terface 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
condi-tions, 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
mag-netic 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 sur-face 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 magnetic field at a distance identified by [3]
(3)
where and are the value and vector in the direction of 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)
where is the effective field on the sensor surface, is the 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®
Trang 4By 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
ap-plied 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
in-creasing the free layer in the spin-valve structure Ta(5)/NiFe /
Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) (nm) The maximum
sensi-tivity 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;
dif-ferent number of magnetic beads give difdif-ferent 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: http://tools.invitrogen.com/content/sfs/manuals/ 112.05D06D%20602.10%20Dynabeads%20M280%20Streptavidin% 20(rev%20012).pdf