We demonstrate a simple low-power, magnetic sensor system suitable for high-sensitivity magnetic-field mapping, based on solid-state magnetic tunnel junction devices with minimum detectable fields in a 100 pT range at room temperature. In this paper, we discuss a method that uses multilayer thin films to improve the performance of the soft ferromagnetic layer in magnetoresistive sensor applications, by reducing the coercivity and/or improving the reversibility.We have used it in the design of our new magnetic sensor. This sensor has a sensitivity as high as 750%/mT. The magnetic sensor only dissipates 1 mW of power while operating under an applied voltage of 1 V

Da Silva, F. C. S.

Heindl, Ranko

Liou, Sy-Hwang

Moreland, J.

Pappas, David P.

Russek, Stephen

Shen, J.

Yin, Xiaolu

Yuan, L.

English

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University of Nebraska - Lincoln 
DigitalCommons@University of Nebraska - Lincoln 
Si-Hwang Liou Publications Research Papers in Physics and Astronomy 
10-2011 
Picotesla Magnetic Sensors for Low-Frequency Applications 
Sy-Hwang Liou 
University of Nebraska-Lincoln, sliou@unl.edu 
Xiaolu Yin 
University of Nebraska-Lincoln 
Stephen Russek 
National Institute of Standards and Technology, stephen.russek@nist.gov 
Ranko Heindl 
National Institute of Standards and Technology, heindl@boulder.nist.gov 
F. C. S. Da Silva 
National Institute of Standards and Technology, fabio.dasilva@nist.gov 
See next page for additional authors 
Follow this and additional works at: https://digitalcommons.unl.edu/physicsliou 
 Part of the Physics Commons 
Liou, Sy-Hwang; Yin, Xiaolu; Russek, Stephen; Heindl, Ranko; Da Silva, F. C. S.; Moreland, J.; Pappas, David 
P.; Yuan, L.; and Shen, J., "Picotesla Magnetic Sensors for Low-Frequency Applications" (2011). Si-Hwang 
Liou Publications. 100. 
https://digitalcommons.unl.edu/physicsliou/100 
This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at 
DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Si-Hwang Liou Publications 
by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 
Authors 
Sy-Hwang Liou, Xiaolu Yin, Stephen Russek, Ranko Heindl, F. C. S. Da Silva, J. Moreland, David P. Pappas, 
L. Yuan, and J. Shen 
This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/
physicsliou/100 
3740 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011
Picotesla Magnetic Sensors for Low-Frequency Applications
S. H. Liou , Xiaolu Yin , Stephen E. Russek, Ranko Heindl, F. C. S. Da Silva, John Moreland,
David P. Pappas, L. Yuan, and J. Shen
Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience,University of Nebraska, Lincoln,
NE 68588-0111 USA
National Institute of Standards and Technology, Boulder, CO 80305 USA
Western Digital Corporation, Fremont, CA 94539 USA
We demonstrate a simple low-power, magnetic sensor system suitable for high-sensitivity magnetic-field mapping, based on solid-state
magnetic tunnel junction devices with minimum detectable fields in a 100 pT range at room temperature. In this paper, we discuss a
method that uses multilayer thin films to improve the performance of the soft ferromagnetic layer in magnetoresistive sensor applica-
tions, by reducing the coercivity and/or improving the reversibility. We have used it in the design of our new magnetic sensor. This sensor
has a sensitivity as high as 750%/mT. The magnetic sensor only dissipates 1 mW of power while operating under an applied voltage of
1 V.
Index Terms—Magnetic concentrators, magnetic noise, magnetic sensor, magnetic tunnel junction (MTJ).
I. INTRODUCTION
T HE measurement of magnetic field in the picotesla rangeis important for a wide range of homeland security, indus-
trial, and biomedical applications. Many of these applications
require sensors with field noise of less than 1 nT/Hz , as well
as low-frequency operation, low maintenance, and low power
consumption [1]–[12].
The minimum detectable field (the field noise times the
measurement bandwidth) of magnetic tunnel junction (MTJ)
sensors is limited by thermal Johnson, shot, electronics, and
magnetic noise, in which magnetic noise dominates at low
frequencies and nonreversible (hysteretic) behavior, current
state-of-the-art MTJ devices have not demonstrated the desired
performance.
In this paper, we discuss a method that uses multilayer
thin films to improve the performance of the soft ferromag-
netic layer in magnetoresistive (MR) sensor applications, by
reducing the coercivity and/or improving the reversibility. In
this study, a layer of very soft Fe Ni is added onto the
Co Fe B transition-metal layers that show a moderate
improvement of the soft-magnetic performance. We demon-
strate a simple low-power, magnetic sensor system suitable for
high-sensitivity magnetic field mapping, based on solid-state
MTJ devices with minimum detectable fields in the picotesla
range at room temperature. The key features of this design
are: 1) decreasing the noise by the use of a 64-element MTJ
bridge, 2) reducing the magnetic hysteresis by using a cou-
pled soft magnetic layer, and 3) increasing the signal by using
an external flux-to-field magnetic flux concentrators (MFCs).
This sensor has a sensitivity as high as 750%/mT. The mag-
netic sensor only dissipates 1 mW of power, while operating
under a applied voltage of 1 V.
Manuscript received February 22, 2011; revised May 14, 2011, May 15, 2011;
accepted May 15, 2011. Date of current version September 23, 2011. Corre-
sponding author: S.-H. Liou (e-mail: sliou@unl.edu).
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.2011.2157997
II. EXPERIMENTAL DETAILS
We have used two types of MTJ layer structures for this
study. The MTJ layer structure for the ferromagnetic–ferro-
magnetic coupling study (MTJ-A) is 1.5 nm Ta/15 nm
Ru/3 nm Co Fe /10 nm Ir Mn /2.2 nm Co Fe /0.85 nm
Ru/2.8 nm Co Fe B /2 nm MgO/4 nm Co Fe B /10 nm
Ta/7 nm Ru. The MTJ layer structure for the sensor
(MTJ-B) is 1.5 nm Ta/25 nm Ru/7 nm Ir Mn /2.2 nm
Co Fe /0.85 nm Ru/2.8 nm Co Fe B /2 nm MgO/1.5 nm
Co Fe B /3.3 nm Ni Fe /1 nm Ta/15 nm Ru.
The junctions were patterned into ellipses with a size of
10 m 15 m (area 117.8 m , eccentricity 0.74) and a
circle 30 m 30 m (area 706.9 m ) by photolithography
and argon milling. The MTJ sensor bridge was annealed at
350 C under a 1 T magnetic field for 30 min and a vacuum
of Pa. The pinning direction of the reference
layer is along the short axis of the ellipse. It has been reported
early that the magnetic sensor noise will increase the SNR
by , where N is the number of MTJ elements either in
series or parallel [13]. So, we have designed our magnetic
sensor with a 64-element bridge for reducing the noise. The
design of the magnetic sensor with a 64-element bridge is the
same as that in [14]. The noise measurement system is in a
shielded environment to avoid picking up unwanted external
magnetic-field fluctuations. The magnetic sensor used in this
study was made using an asymmetric bridge (formed by taking
two dies and rotating them so that the pinned directions were
opposite to each other). The sensor voltage and preamplifier
bias are supplied by batteries to minimize noise. The MFC
was made using a Conetic alloy, which was annealed under
hydrogen environment at 1150 C for 20 h with the cooling
rate of about 1 C/min. The gap between the concentrators
was typically 1 mm. All the measurements were taken under
ambient conditions.
III. RESULTS AND DISCUSIONS
By using a coupled soft magnetic layer, we are able to reduce
the magnetic hysteresis loss in the free layer and the MTJ junc-
0018-9464/$26.00 © 2011 IEEE
LIOU et al.: PICOTESLA MAGNETIC SENSORS FOR LOW-FREQUENCY APPLICATIONS 3741
Fig. 1. MR loops of the MTJ-A with the junction area of 5  m   10  m,
which have the same pinned layer structures. (a) Dashed line represents that the
MTJ-A has 4 nm Co Fe B as a free layer. (b) Solid line represents that the
MTJ-A has 4 nm Co Fe B and additional 3.3 nm Ni Fe as a free layer.
tions could have a better reversibility and linearity around zero-
magnetic field, as well as smaller coercivity [15]. We prepare
two different free-layer structures from the MTJ-A wafer. First,
we remove the 7 nm Ru/10 nm Ta layers by argon milling on
the half of the wafer and then deposited 3.3 nm Ni Fe /10 nm
Ta/7 nm Ru on it. So, all the MTJ junctions are made from the
same wafer (i.e., MTJ-A wafer).
The MR loops with or without additional Ni Fe as a free
layer are shown in Fig. 1. The tunneling magneto resistance
(TMR) drops from 132% to 115%, which may be due to the
reduction of the thickness of Co Fe B free layer duing the
milling process. The magnetic hysteresis loss in the free layer
is reduced by about 42% of the coercivity that is from 4.26
to 2.46 mT. However, the MTJ-A wafer (both samples with or
without Ni Fe ) has a large exchange bias, which is not de-
sired for sensor applications. We would prefer to design a mag-
netic sensor without using an external magnetic bias.
From the aforementioned results, we prepared the MTJ-B that
reduced the thickness of Co Fe B to 1.5 nm and additional
3.3 nm Ni Fe as a free layer. We anneal these samples using
similar to that of the MTJ-A, that is to anneal the samples at
350 C under 1 T magnetic field along the direction of short
axis of the ellipse. These samples have smaller exchange bias,
as well as lower coercive force than that of MTJ-A. However,
the TMR ratio of these samples is only about 45%. This may
relate to the fact that the crystal orientation of the Co Fe B
layer is influenced by the crystal orientation of the Ni Fe
layer. The slope of the TMR loop at the zero-magnetic field is
small. In order to improve the slope at the zero-magnetic field,
the second annealing process was introduced in Fig. 2. The sam-
ples were heated to 350 C under 0.1 T magnetic field that is
along the long axis of the ellipse. The second annealing process
helps the magnetization in free layer keep as a magnetic dipole,
which could rotate smoothly with decreasing the magnetic field.
The slope of the TMR loop at zero-magnetic field (red line in
Fig. 2) is increased by about eight times, the sensitivity changes
from 0.744%/mT to 5.89%/mT. However, the second annealing
Fig. 2. TMR ratio versus applied magnetic field with different annealing treat-
ments. (a) Loops with dots are the samples that annealed at 350 C under 1 T
magnetic field in the direction along the short axis of the ellipse. (b) TMR loops
with triangle dots are the samples that annealed at 350 C under 1 T magnetic
field along the short axis and, then, annealed at 350 C under 0.1 T along the
long axis of the ellipse. The slopes of the TMR loops at zero-magnetic field were
marked in red lines to illustrate the changing sensitivity by the annealing.
Fig. 3. Sensor output voltage (under applied voltage of 1 V) versus field curve
of the MTJ sensor with or without MFCs. It demonstrates that the signal of an
MTJ bridge with MFCs increases by a factor of 77.
process reduces the TMR ratio to about 35% because the mag-
netization of the pinned layer is also aligned toward the long
axis of the ellipse.
We have made a magnetic sensor with an asymmetric bridge
(formed by taking two dies and rotating them so that the pinned
directions were opposite to each other). The sensor voltage
output with 1 V bias is shown in Fig. 3. There is quite large
magnetic hysteresis in the sensor without MFC in the magnetic
field range of mT. The sensing field range is reduced and
the sensitivity is increased in the sensor with MFC. The signal
of the magnetic sensor bridge with MFCs increases by a factor
of 77. The magnetic hysteresis of the sensor reduced also by
a factor of 77. In Fig. 4, we show that the sensitivity of this
magnetic sensor as high as 750%/mT has been achieved. The
magnetic sensor was operated under the applied voltage of
1 V. The output voltage of the magnetic sensor response to
increasing and decreasing dc magnetic field is quite linear in
the field range of nT.
3742 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011
Fig. 4. Output voltage of the magnetic sensor response to increasing and de-
creasing dc magnetic field range of   nT. The magnetic sensor was operated
under the applied voltage of 1 V. The sensitivity of this sensor is 750%/mT.
Fig. 5. Noise versus frequency of the MTJ junction of 30  m  30  m mea-
sured under the applied voltage at 1 and 4 V at zero-magnetic fields.
We measured the noise of the MTJ bridge in the frequency
range from 1 to Hz with 0 T biasing fields and under
the applied voltages of 1 and 4 V, as shown in Fig. 5. The mag-
netic field noise was calculated by dividing the voltage noise
spectrum measured at the bridge outputs by the bridge sensi-
tivity dV/dB taken from Fig. 4. At an applied voltage of 1 V
across the bridge and zero applied magnetic field, the sensitivity
is 7500 V/T. The noise level at 1 Hz is V/Hz giving
a field noise of V/Hz pT/Hz .
At 1 kHz, the noise level is V/Hz , and the field
noise is 2.6 pT/Hz .
The noise spectrum was fitted with the calculated thermal
noise for the measured value of resistance. Hooge-like param-
eter is about m , which is about the same order of
magnitude as that reported by Stearrett et al. [16].
Fig. 6 shows a real-time magnetic response curve of the mag-
netic sensor that was measured under an ac variation of mag-
netic field at 1 Hz and amplitude of 250 pT. The applied voltage
on the sensor bridge is at 1 V and under ambient condition. The
magnetic sensor was placed inside a coil and the signal was col-
lected with a low-noise amplifier (gain was set at 100) and an
A/D convertor. The magnitude of the sensor voltage output is
Fig. 6. Magnetic response curve: magnetic sensor output versus time under a
variation of magnetic field at 1 Hz and amplitude of 250 pT.
about 2 V. It is not a perfect sinusoid curve. The distortion
may due to the thermal fluctuation of the magnetic domain in
the free layer of the magnetic sensor. However, one can see that
the peak-to-peak output voltage is much higher than the noise
level of about 100 pT.
IV. SUMMARY
We show a method that uses multilayer thin films to improve
the performance of the soft ferromagnetic layer in MR sensor
applications, by reducing the coercivity and/or improving the
reversibility.
We present a simple low-power, magnetic sensor system
that combines a 64-element MTJ bridge and a set of low-noise
MFCs. This sensor has a sensitivity as high as 750%/mT. The
magnetic sensor only dissipates 1 mW of power while operating
under an applied voltage of 1 V.
ACKNOWLEDGMENT
This work was supported by the Department of Defense
(Army Research Office) under Awards W911NF-08-1-01311,
W911NF-09-2-0039, and W911NF-10-2-0099, by the National
Science Foundation MRSEC under Grant DMR-0820521, and
by the Strategic Environmental Research and Development
Program (SERDP) under Grant MM-1716.
REFERENCES
[1] W. F. Egelhoff, Jr., V. E. Höink, J. W. Lau, W. F. Shen, B. D. Schrag,
and G. Xiao, “Magnetic tunnel junctions with large tunneling magne-
toresistance and small saturation fields,” J. Appl. Phys., vol. 107, p.
09C705, 2010.
[2] J. M. Almeida, P. Wisniowski, and P. P. Freitas, “Field detection in
single and double barrier MgO magnetic tunnel junction sensors,” J.
Appl. Phys., vol. 103, p. 07E922, 2008.
[3] P. P. Freitas, R. Ferreira, S. Cardoso, and F. Cardoso, “Magnetoresis-
tive sensors,” J. Phys., Condens. Matter, vol. 19, p. 165221, 2007.
[4] X. Liu, D. Mazumdar, W. Shen, B. D. Schrag, and G. Xiao, “Thermal
stability of magnetic tunneling junctions with MgO barriers for high
temperature spintronics,” Appl. Phys. Lett., vol. 89, p. 023504, 2006.
[5] J. Scola, H. Polovy, C. Fermon, M. Pannetier-Lecoeur, G. Feng, K.
Fahy, and J. M. D. Coey, “Noise in MgO barrier magnetic tunnel
junctions with CoFeB electrodes: Influence of annealing temperature,”
Appl. Phys. Lett., vol. 90, p. 252501, 2007.
[6] R. C. Chaves, P. P. F. Ocker, and W. Maass, “Low frequency picotesla
field detection using hybrid MgO based tunnel sensors,” Appl. Phys.
Lett., vol. 91, p. 102504-0, 2007.
LIOU et al.: PICOTESLA MAGNETIC SENSORS FOR LOW-FREQUENCY APPLICATIONS 3743
[7] P. W. T. Pong, B. Schrag, A. J. Shapiro, R. D. McMichael, and W. F.
Egelhoff, Jr., “Hysteresis loop collapse for linear response in magnetic-
tunnel-junction sensors,” J. Appl. Phys., vol. 105, p. 07E723, 2009.
[8] N. A. Stutzke, S. E. Russek, D. P. Pappas, and M. Tondra, “Low-fre-
quency noise measurements on commercial magnetoresistive magnetic
field sensors,” J. Appl. Phys., vol. 97, p. 10Q107, 2005.
[9] S. H. Liou, R. Zhang, S. E. Russek, L. Yuan, S. T. Halloran, and D. P.
Pappas, “Dependence of noise in magnetic tunnel junction sensors on
annealing field and temperature,” J. Appl. Phys., vol. 103, p. 07E920,
2008.
[10] S. Ikeda, J. Hayakawa, Y. Ashizawa, Y. M. Lee, K. Miura, H.
Hasegawa, M. Tsunoda, F. Matsukura, and H. Ohno, “Tunnel mag-
netoresistance of 604% at 300 K by suppression of Ta diffusion in
CoFeB/MgO/CoFeB pseudo-spin-valves annealed at high tempera-
ture,” Appl. Phys. Lett., vol. 93, p. 082508, 2008.
[11] P. M. Drljacˇa, F. Vincent, P.-A. Besse, and R. S. Popovic´, “Design of
planar magnetic concentrators for high sensitivity hall devices,” Sens.
Actuators A, vol. 97–98, pp. 10–14, 2002.
[12] F. G. Aliev, R. Guerrero, D. Herranz, R. Villar, F. Greullet, C. Tiusan,
and M. Hehn, “Very low 1/  noise at room temperature in fully epi-
taxial Fe/MgO/Fe magnetic tunnel junctions,” Appl. Phys. Lett., vol.
91, p. 232504-0, 2007.
[13] M. Tondra, J. M. Daughton, D. Wang, R. S. Beech, A. Fink, and J. A.
Taylor, “Picotesla field sensor design using spin-dependent tunneling
devices,” Appl. Phys. Lett., vol. 83, p. 6688, 1998.
[14] S. H. Liou, D. Sellmyer, S. E. Russek, R. Heindl, F. C. S. Da Silva,
J. Moreland, D. P. Pappas, L. Yuan, and J. Shen, “Magnetic noise in
a low-power picotesla magnetoresistive sensor,” Proc. IEEE 8th Ann.
Conf. Sens., p. 1848, 2009.
[15] R. Zhang, R. Skomski, X. Yin, S. H. Liou, and D. Sellmyer, “Re-
versibility and coercivity of Fe-alloy-Fe:SiO multilayers,” J. Appl.
Phys., vol. 107, p. 09E710-0, 2010.
[16] R. Stearrett, W. G. Wang, L. R. Shah, A. Gokce, J. Q. Xiao, and E. R.
Nowak, “Evolution of barrier-resistance noise in CoFeB/MgO/CoFeB
tunnel junctions during annealing,” J. Appl. Phys., vol. 107, p. 064502,
2010.


Picotesla Magnetic Sensors for Low-Frequency Applications

DigitalCommons@University of Nebraska

University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Si-Hwang Liou Publications Research Papers in Physics and Astronomy 10-2011 Picotesla Magnetic Sensors for Low-Frequency Applications Sy-Hwang Liou University of Nebraska-Lincoln, sliou@unl.edu Xiaolu Yin University of Nebraska-Lincoln Stephen Russek National Institute of Standards and Technology, stephen.russek@nist.gov Ranko Heindl National Institute of Standards and Technology, heindl@boulder.nist.gov F. C. S. Da Silva National Institute of Standards and Technology, fabio.dasilva@nist.gov See next page for additional authors Follow this and additional works at: https://digitalcommons.unl.edu/physicsliou  Part of the Physics Commons Liou, Sy-Hwang; Yin, Xiaolu; Russek, Stephen; Heindl, Ranko; Da Silva, F. C. S.; Moreland, J.; Pappas, David P.; Yuan, L.; and Shen, J., "Picotesla Magnetic Sensors for Low-Frequency Applications" (2011). Si-Hwang Liou Publications. 100. https://digitalcommons.unl.edu/physicsliou/100 This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Si-Hwang Liou Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Authors Sy-Hwang Liou, Xiaolu Yin, Stephen Russek, Ranko Heindl, F. C. S. Da Silva, J. Moreland, David P. Pappas, L. Yuan, and J. Shen This article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/physicsliou/100 3740 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011Picotesla Magnetic Sensors for Low-Frequency ApplicationsS. H. Liou , Xiaolu Yin , Stephen E. Russek, Ranko Heindl, F. C. S. Da Silva, John Moreland,David P. Pappas, L. Yuan, and J. ShenDepartment of Physics and Astronomy and Nebraska Center for Materials and Nanoscience,University of Nebraska, Lincoln,NE 68588-0111 USANational Institute of Standards and Technology, Boulder, CO 80305 USAWestern Digital Corporation, Fremont, CA 94539 USAWe demonstrate a simple low-power, magnetic sensor system suitable for high-sensitivity magnetic-field mapping, based on solid-statemagnetic tunnel junction devices with minimum detectable fields in a 100 pT range at room temperature. In this paper, we discuss amethod that uses multilayer thin films to improve the performance of the soft ferromagnetic layer in magnetoresistive sensor applica-tions, by reducing the coercivity and/or improving the reversibility. We have used it in the design of our new magnetic sensor. This sensorhas a sensitivity as high as 750%/mT. The magnetic sensor only dissipates 1 mW of power while operating under an applied voltage of1 V.Index Terms—Magnetic concentrators, magnetic noise, magnetic sensor, magnetic tunnel junction (MTJ).I. INTRODUCTIONT HE measurement of magnetic field in the picotesla rangeis important for a wide range of homeland security, indus-trial, and biomedical applications. Many of these applicationsrequire sensors with field noise of less than 1 nT/Hz , as wellas low-frequency operation, low maintenance, and low powerconsumption [1]–[12].The minimum detectable field (the field noise times themeasurement bandwidth) of magnetic tunnel junction (MTJ)sensors is limited by thermal Johnson, shot, electronics, andmagnetic noise, in which magnetic noise dominates at lowfrequencies and nonreversible (hysteretic) behavior, currentstate-of-the-art MTJ devices have not demonstrated the desiredperformance.In this paper, we discuss a method that uses multilayerthin films to improve the performance of the soft ferromag-netic layer in magnetoresistive (MR) sensor applications, byreducing the coercivity and/or improving the reversibility. Inthis study, a layer of very soft Fe Ni is added onto theCo Fe B transition-metal layers that show a moderateimprovement of the soft-magnetic performance. We demon-strate a simple low-power, magnetic sensor system suitable forhigh-sensitivity magnetic field mapping, based on solid-stateMTJ devices with minimum detectable fields in the picoteslarange at room temperature. The key features of this designare: 1) decreasing the noise by the use of a 64-element MTJbridge, 2) reducing the magnetic hysteresis by using a cou-pled soft magnetic layer, and 3) increasing the signal by usingan external flux-to-field magnetic flux concentrators (MFCs).This sensor has a sensitivity as high as 750%/mT. The mag-netic sensor only dissipates 1 mW of power, while operatingunder a applied voltage of 1 V.Manuscript received February 22, 2011; revised May 14, 2011, May 15, 2011;accepted May 15, 2011. Date of current version September 23, 2011. Corre-sponding author: S.-H. Liou (e-mail: sliou@unl.edu).Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TMAG.2011.2157997II. EXPERIMENTAL DETAILSWe have used two types of MTJ layer structures for thisstudy. The MTJ layer structure for the ferromagnetic–ferro-magnetic coupling study (MTJ-A) is 1.5 nm Ta/15 nmRu/3 nm Co Fe /10 nm Ir Mn /2.2 nm Co Fe /0.85 nmRu/2.8 nm Co Fe B /2 nm MgO/4 nm Co Fe B /10 nmTa/7 nm Ru. The MTJ layer structure for the sensor(MTJ-B) is 1.5 nm Ta/25 nm Ru/7 nm Ir Mn /2.2 nmCo Fe /0.85 nm Ru/2.8 nm Co Fe B /2 nm MgO/1.5 nmCo Fe B /3.3 nm Ni Fe /1 nm Ta/15 nm Ru.The junctions were patterned into ellipses with a size of10 m 15 m (area 117.8 m , eccentricity 0.74) and acircle 30 m 30 m (area 706.9 m ) by photolithographyand argon milling. The MTJ sensor bridge was annealed at350 C under a 1 T magnetic field for 30 min and a vacuumof Pa. The pinning direction of the referencelayer is along the short axis of the ellipse. It has been reportedearly that the magnetic sensor noise will increase the SNRby , where N is the number of MTJ elements either inseries or parallel [13]. So, we have designed our magneticsensor with a 64-element bridge for reducing the noise. Thedesign of the magnetic sensor with a 64-element bridge is thesame as that in [14]. The noise measurement system is in ashielded environment to avoid picking up unwanted externalmagnetic-field fluctuations. The magnetic sensor used in thisstudy was made using an asymmetric bridge (formed by takingtwo dies and rotating them so that the pinned directions wereopposite to each other). The sensor voltage and preamplifierbias are supplied by batteries to minimize noise. The MFCwas made using a Conetic alloy, which was annealed underhydrogen environment at 1150 C for 20 h with the coolingrate of about 1 C/min. The gap between the concentratorswas typically 1 mm. All the measurements were taken underambient conditions.III. RESULTS AND DISCUSIONSBy using a coupled soft magnetic layer, we are able to reducethe magnetic hysteresis loss in the free layer and the MTJ junc-0018-9464/$26.00 © 2011 IEEELIOU et al.: PICOTESLA MAGNETIC SENSORS FOR LOW-FREQUENCY APPLICATIONS 3741Fig. 1. MR loops of the MTJ-A with the junction area of 5  m   10  m,which have the same pinned layer structures. (a) Dashed line represents that theMTJ-A has 4 nm Co Fe B as a free layer. (b) Solid line represents that theMTJ-A has 4 nm Co Fe B and additional 3.3 nm Ni Fe as a free layer.tions could have a better reversibility and linearity around zero-magnetic field, as well as smaller coercivity [15]. We preparetwo different free-layer structures from the MTJ-A wafer. First,we remove the 7 nm Ru/10 nm Ta layers by argon milling onthe half of the wafer and then deposited 3.3 nm Ni Fe /10 nmTa/7 nm Ru on it. So, all the MTJ junctions are made from thesame wafer (i.e., MTJ-A wafer).The MR loops with or without additional Ni Fe as a freelayer are shown in Fig. 1. The tunneling magneto resistance(TMR) drops from 132% to 115%, which may be due to thereduction of the thickness of Co Fe B free layer duing themilling process. The magnetic hysteresis loss in the free layeris reduced by about 42% of the coercivity that is from 4.26to 2.46 mT. However, the MTJ-A wafer (both samples with orwithout Ni Fe ) has a large exchange bias, which is not de-sired for sensor applications. We would prefer to design a mag-netic sensor without using an external magnetic bias.From the aforementioned results, we prepared the MTJ-B thatreduced the thickness of Co Fe B to 1.5 nm and additional3.3 nm Ni Fe as a free layer. We anneal these samples usingsimilar to that of the MTJ-A, that is to anneal the samples at350 C under 1 T magnetic field along the direction of shortaxis of the ellipse. These samples have smaller exchange bias,as well as lower coercive force than that of MTJ-A. However,the TMR ratio of these samples is only about 45%. This mayrelate to the fact that the crystal orientation of the Co Fe Blayer is influenced by the crystal orientation of the Ni Felayer. The slope of the TMR loop at the zero-magnetic field issmall. In order to improve the slope at the zero-magnetic field,the second annealing process was introduced in Fig. 2. The sam-ples were heated to 350 C under 0.1 T magnetic field that isalong the long axis of the ellipse. The second annealing processhelps the magnetization in free layer keep as a magnetic dipole,which could rotate smoothly with decreasing the magnetic field.The slope of the TMR loop at zero-magnetic field (red line inFig. 2) is increased by about eight times, the sensitivity changesfrom 0.744%/mT to 5.89%/mT. However, the second annealingFig. 2. TMR ratio versus applied magnetic field with different annealing treat-ments. (a) Loops with dots are the samples that annealed at 350 C under 1 Tmagnetic field in the direction along the short axis of the ellipse. (b) TMR loopswith triangle dots are the samples that annealed at 350 C under 1 T magneticfield along the short axis and, then, annealed at 350 C under 0.1 T along thelong axis of the ellipse. The slopes of the TMR loops at zero-magnetic field weremarked in red lines to illustrate the changing sensitivity by the annealing.Fig. 3. Sensor output voltage (under applied voltage of 1 V) versus field curveof the MTJ sensor with or without MFCs. It demonstrates that the signal of anMTJ bridge with MFCs increases by a factor of 77.process reduces the TMR ratio to about 35% because the mag-netization of the pinned layer is also aligned toward the longaxis of the ellipse.We have made a magnetic sensor with an asymmetric bridge(formed by taking two dies and rotating them so that the pinneddirections were opposite to each other). The sensor voltageoutput with 1 V bias is shown in Fig. 3. There is quite largemagnetic hysteresis in the sensor without MFC in the magneticfield range of mT. The sensing field range is reduced andthe sensitivity is increased in the sensor with MFC. The signalof the magnetic sensor bridge with MFCs increases by a factorof 77. The magnetic hysteresis of the sensor reduced also bya factor of 77. In Fig. 4, we show that the sensitivity of thismagnetic sensor as high as 750%/mT has been achieved. Themagnetic sensor was operated under the applied voltage of1 V. The output voltage of the magnetic sensor response toincreasing and decreasing dc magnetic field is quite linear inthe field range of nT.3742 IEEE TRANSACTIONS ON MAGNETICS, VOL. 47, NO. 10, OCTOBER 2011Fig. 4. Output voltage of the magnetic sensor response to increasing and de-creasing dc magnetic field range of   nT. The magnetic sensor was operatedunder the applied voltage of 1 V. The sensitivity of this sensor is 750%/mT.Fig. 5. Noise versus frequency of the MTJ junction of 30  m  30  m mea-sured under the applied voltage at 1 and 4 V at zero-magnetic fields.We measured the noise of the MTJ bridge in the frequencyrange from 1 to Hz with 0 T biasing fields and underthe applied voltages of 1 and 4 V, as shown in Fig. 5. The mag-netic field noise was calculated by dividing the voltage noisespectrum measured at the bridge outputs by the bridge sensi-tivity dV/dB taken from Fig. 4. At an applied voltage of 1 Vacross the bridge and zero applied magnetic field, the sensitivityis 7500 V/T. The noise level at 1 Hz is V/Hz givinga field noise of V/Hz pT/Hz .At 1 kHz, the noise level is V/Hz , and the fieldnoise is 2.6 pT/Hz .The noise spectrum was fitted with the calculated thermalnoise for the measured value of resistance. Hooge-like param-eter is about m , which is about the same order ofmagnitude as that reported by Stearrett et al. [16].Fig. 6 shows a real-time magnetic response curve of the mag-netic sensor that was measured under an ac variation of mag-netic field at 1 Hz and amplitude of 250 pT. The applied voltageon the sensor bridge is at 1 V and under ambient condition. Themagnetic sensor was placed inside a coil and the signal was col-lected with a low-noise amplifier (gain was set at 100) and anA/D convertor. The magnitude of the sensor voltage output isFig. 6. Magnetic response curve: magnetic sensor output versus time under avariation of magnetic field at 1 Hz and amplitude of 250 pT.about 2 V. It is not a perfect sinusoid curve. The distortionmay due to the thermal fluctuation of the magnetic domain inthe free layer of the magnetic sensor. However, one can see thatthe peak-to-peak output voltage is much higher than the noiselevel of about 100 pT.IV. SUMMARYWe show a method that uses multilayer thin films to improvethe performance of the soft ferromagnetic layer in MR sensorapplications, by reducing the coercivity and/or improving thereversibility.We present a simple low-power, magnetic sensor systemthat combines a 64-element MTJ bridge and a set of low-noiseMFCs. This sensor has a sensitivity as high as 750%/mT. Themagnetic sensor only dissipates 1 mW of power while operatingunder an applied voltage of 1 V.ACKNOWLEDGMENTThis work was supported by the Department of Defense(Army Research Office) under Awards W911NF-08-1-01311,W911NF-09-2-0039, and W911NF-10-2-0099, by the NationalScience Foundation MRSEC under Grant DMR-0820521, andby the Strategic Environmental Research and DevelopmentProgram (SERDP) under Grant MM-1716.REFERENCES[1] W. F. Egelhoff, Jr., V. E. Höink, J. W. Lau, W. F. Shen, B. D. Schrag,and G. Xiao, “Magnetic tunnel junctions with large tunneling magne-toresistance and small saturation fields,” J. Appl. Phys., vol. 107, p.09C705, 2010.[2] J. M. Almeida, P. Wisniowski, and P. P. Freitas, “Field detection insingle and double barrier MgO magnetic tunnel junction sensors,” J.Appl. Phys., vol. 103, p. 07E922, 2008.[3] P. P. Freitas, R. Ferreira, S. Cardoso, and F. Cardoso, “Magnetoresis-tive sensors,” J. Phys., Condens. Matter, vol. 19, p. 165221, 2007.[4] X. Liu, D. Mazumdar, W. Shen, B. D. Schrag, and G. Xiao, “Thermalstability of magnetic tunneling junctions with MgO barriers for hightemperature spintronics,” Appl. Phys. Lett., vol. 89, p. 023504, 2006.[5] J. Scola, H. Polovy, C. Fermon, M. Pannetier-Lecoeur, G. Feng, K.Fahy, and J. M. D. Coey, “Noise in MgO barrier magnetic tunneljunctions with CoFeB electrodes: Influence of annealing temperature,”Appl. Phys. Lett., vol. 90, p. 252501, 2007.[6] R. C. Chaves, P. P. F. Ocker, and W. Maass, “Low frequency picoteslafield detection using hybrid MgO based tunnel sensors,” Appl. Phys.Lett., vol. 91, p. 102504-0, 2007.LIOU et al.: PICOTESLA MAGNETIC SENSORS FOR LOW-FREQUENCY APPLICATIONS 3743[7] P. W. T. Pong, B. Schrag, A. J. Shapiro, R. D. McMichael, and W. F.Egelhoff, Jr., “Hysteresis loop collapse for linear response in magnetic-tunnel-junction sensors,” J. Appl. Phys., vol. 105, p. 07E723, 2009.[8] N. A. Stutzke, S. E. Russek, D. P. Pappas, and M. Tondra, “Low-fre-quency noise measurements on commercial magnetoresistive magneticfield sensors,” J. Appl. Phys., vol. 97, p. 10Q107, 2005.[9] S. H. Liou, R. Zhang, S. E. Russek, L. Yuan, S. T. Halloran, and D. P.Pappas, “Dependence of noise in magnetic tunnel junction sensors onannealing field and temperature,” J. Appl. Phys., vol. 103, p. 07E920,2008.[10] S. Ikeda, J. Hayakawa, Y. Ashizawa, Y. M. Lee, K. Miura, H.Hasegawa, M. Tsunoda, F. Matsukura, and H. Ohno, “Tunnel mag-netoresistance of 604% at 300 K by suppression of Ta diffusion inCoFeB/MgO/CoFeB pseudo-spin-valves annealed at high tempera-ture,” Appl. Phys. Lett., vol. 93, p. 082508, 2008.[11] P. M. Drljacˇa, F. Vincent, P.-A. Besse, and R. S. Popovic´, “Design ofplanar magnetic concentrators for high sensitivity hall devices,” Sens.Actuators A, vol. 97–98, pp. 10–14, 2002.[12] F. G. Aliev, R. Guerrero, D. Herranz, R. Villar, F. Greullet, C. Tiusan,and M. Hehn, “Very low 1/  noise at room temperature in fully epi-taxial Fe/MgO/Fe magnetic tunnel junctions,” Appl. Phys. Lett., vol.91, p. 232504-0, 2007.[13] M. Tondra, J. M. Daughton, D. Wang, R. S. Beech, A. Fink, and J. A.Taylor, “Picotesla field sensor design using spin-dependent tunnelingdevices,” Appl. Phys. Lett., vol. 83, p. 6688, 1998.[14] S. H. Liou, D. Sellmyer, S. E. Russek, R. Heindl, F. C. S. Da Silva,J. Moreland, D. P. Pappas, L. Yuan, and J. Shen, “Magnetic noise ina low-power picotesla magnetoresistive sensor,” Proc. IEEE 8th Ann.Conf. Sens., p. 1848, 2009.[15] R. Zhang, R. Skomski, X. Yin, S. H. Liou, and D. Sellmyer, “Re-versibility and coercivity of Fe-alloy-Fe:SiO multilayers,” J. Appl.Phys., vol. 107, p. 09E710-0, 2010.[16] R. Stearrett, W. G. Wang, L. R. Shah, A. Gokce, J. Q. Xiao, and E. R.Nowak, “Evolution of barrier-resistance noise in CoFeB/MgO/CoFeBtunnel junctions during annealing,” J. Appl. Phys., vol. 107, p. 064502,2010.

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Picotesla Magnetic Sensors for Low-Frequency Applications

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