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 SV film structure of (Si/
SiO2)/Ta(50 Å)/[NiFe(30 Å)/Co(15 Å)]/Cu(24 Å)/Co80Fe20(25 Å)/IrMn(100 Å)/Ta(50 Å) was found significant, and the SV elements were patterned using the lithographic lift-off method with the active cell
dimensions of 2 mm 150 mm. To define a pinning axis, a cool-field anneal was applied at 250 C for
30 min in a magnetic field 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 magnetic field was carried out. The
engineered SV sensor finds its usefulness in low-field magnetometer and electronic compass
applications.
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ence
Cau Giay, Ha Noi 100000, Viet Nam
tion (UT
a r t i c l e i n f o
This is an open access article under the CC BY license (
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
esides, numerous
icular equipment,
n of GMR metallic
5], the ion beam
ion [31], the elec-
nges of the GMR
etween the mag-
ent used. Further-
directly determine the architecture and the features of the mag-
netic sensors. For example, the exchange bias field (Heb) expresses
the strength of the external field making a saturation, where the
sensor is inactive with any external magnetic fields. The intercou-
pling field (Hin) is induced by the coupling between a pinned layer
(PL) and a free layer (FL). The Hin can cause a shift of the operating
point of theMR curve. TheHin is mainly caused by the thickness and
the roughness of the spacer layer, and by the stray field of the PL as
* Corresponding author.
E-mail address: sulv@itims.edu.vn (V.S. Luong).
Contents lists availab
Journal of Science: Advance
.e l
Journal of Science: Advanced Materials and Devices 3 (2018) 399e405Peer review under responsibility of Vietnam National University, Hanoi.effect have been reported. Nevertheless, extensive studies on the more, the magnetoresistive properties are the factors which1. 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
articles on the GMR spin valve effect [20e24]. B
works have discussed in detail about the part
methods and technical conditions for fabricatio
multilayers, such as the sputtering method [2
deposition [26e30], the chemical vapor deposit
trodeposition [32e35], etc. However, the challe
fabrication still remain owing to the interplay b
netoresistive properties and the specific equipm© 2018 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.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 valvehttps://doi.org/10.1016/j.jsamd.2018.09.004
2468-2179/© 2018 The Authors. Publishing services b
( 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 SV film structure of (Si/
SiO2)/Ta(50 Å)/[NiFe(30 Å)/Co(15 Å)]/Cu(24 Å)/Co80Fe20(25 Å)/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 2 mm 150 mm. To define a pinning axis, a cool-field anneal was applied at 250 C for
30 min in a magnetic field 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 magnetic field was carried out. The
engineered SV sensor finds its usefulness in low-field magnetometer and electronic compass
applications.c Hanoi Community College (HCC), Trung Kinh,
d Hung Yen University of Technology and Educa EHY), Dan Tien, Khoai Chau, Hung Yen 160000, Viet NamHa Noi 100000, Viet Nam
b Institute of Engineering Physics (IEP), Ha Noi University of Science and Technology (HUST), Ha Noi 100000, Viet NamOriginal Article
Magnetoresistive performances in excha
their roles in low-field magnetic sensin
Van Su Luong a, *, Anh Tuan Nguyen a, Quoc Khanh
Anh Tue Nguyen b, Tuan Anh Nguyen c, Van Cuong
a International Training Institute for Materials Science (ITIMS), Ha Noi University of Sci
journal homepage: wwwy Elsevier B.V. on behalf of Vietnamge-biased spin valves and
applications
oang a, Tuyet Nga Nguyen b,
iap d
and Technology (HUST), Dai Co Viet, Hai Ba Trung,
le at ScienceDirect
d Materials and Devices
sevier .com/locate/ jsamdNational University, Hanoi. This is an open access article under the CC BY license
cedwell [36]. Furthermore, for the low magnetic field measurement,
errors should be kept as small as possible. The error arises from the
hysteresis (Hc) of the FL. A large Hc of the FL leads to a big error.
Regarding the responsivity of the sensor, the high MR ratio, e.g. the
large change of the resistance in an external magnetic field pro-
vides a high sensitivity and a large signal-to-noise ratio. The MR
ratio depends onmany factors, such as the texture of the seed layer,
the composition of the ferromagnetic alloy layer, and the critical
thickness of the spacer or each of the functional layers. Eventually,
the cool-field anneal process also strongly affects the MR perfor-
mance owing to the inter-diffusion. In fact, the technical factors
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 to find 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 SV films 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
magnetic field measurement. The obtained experimental results
of the SV sheet films 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 6 105 Pa, 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 SiO2 was
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 a field emission
scanning electron microscop of the model JEOL JSM-7600F. The film
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 SV films
were magnetically characterized using a vibration sample magne-
tometer (VSM). The magnetoresistance (MR) was measured by a
conventional four-probemethodwith a bias current passing in-plane
V.S. Luong et al. / Journal of Science: Advan400of the SV.In the sensor applications, the lithographic lift-off method was
used to pattern the device. The active bar SV was of 2 mm 150 mm
in size. The easy axis was patterned into the short dimension of the
bar, hereby, the cross shape anisotropy between the FL and the PL
contributed to a collapse of the hysteresis of the patterned cells. The
line width of the contact pads was 25 mm, and the bonding pad was
150 mm 150 mm, which was designed by a photo-mask for silver
(Ag) deposition. The chip dimensionwas cut into 1 mm 2 mm. To
construct a sensor, two cut chips were glued to the printed circuit
board (PCB) for wire bonding, while the other side of the PCB was
soldering connected by two identically passive resistances.
To demonstrate the features of the engineered sensor, a
sweeping magnetic field 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 sweeping field 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. The field 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 magnetic fieldmeasurement was carried
out for verifying the features of the engineered sensor. A manual
rotation frame was set up allowing the rotation to take from 0 to
360. The sensor probe was fixed 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. We first
study the magnetoresistive properties of SV sheet films for finding
out the appropriate parameters of the SV film fabrication. The
conventional approach of sputtering was used to prepare the films
based on our previous work [37] and refering to the recent reports
on the SV film fabrication technology [40,41].
3. Results and discussion
3.1. Magnetoresistive properties of the SV sheet films
3.1.1. Exchange bias (Heb)
Exchange-biased spin valves were first 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 magnetic field. This free layer was made of soft
magnetic materials. The magnetic properties are demonstrated by
the M-H loop, shown in Fig. 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 magnetic field 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 magnetic field 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
magnetic fields. 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
Materials and Devices 3 (2018) 399e405interfacial coupling with the AFM layer. Among the ferromagnetic
of C
NiFe
ncedmaterials, 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 Heb and 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 Heb has been reported by M. Pakala et al.
[48]. The Heb dependence 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 Heb and
MR ratio, in this work, x ¼ 20% was the iron content chosen for the
SV film fabrication.
Furthermore, the Heb also strongly depends on the AFM layer
thickness. Fig. 1(c) shows the effects of the IrMn layer thickness,
which was varied from 50 Å to 300 Å on theMR behaviors and the
Heb value. A lower Heb in 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 Heb has reached a maximum at 100 Å and it drops as this film
Fig. 1. (a) Illustration of a M-H loop of a typical SV, (b) Heb and MR versus Fe contents
Ta(50 Å), and (c) MR and Heb as a function of IrMn thickness of SVs Si/SiO2)/Ta(50 Å)/[
V.S. Luong et al. / Journal of Science: Advabecomes thicker (so, at thicknesses >100 Å). As a result, both the
Heb strength and theMR ratio reached a maximum at tIrMn ¼ 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
by DR/R0 in Fig. 2(a), where R0 is the base resistance of the SV in the
zero external field and DR is calculated using the resistances in the
parallel and anti-parallel magnetization states of the PL and the FL.
In the small field region close to H¼ 0, the magnetizing direction of
the FL is reversed, and that is actually theworking region of SV. This
is also a crucial advantage of the SV in low magnetic field sensing
and measurements as well as in the data storage applications
because it can detect an extremely weak magnetic field, e.g., the
magnetic field induced by a data bit memory, the biological mag-
netic fields, and the Earth's magnetic field etc. The resistive sensi-
tivity could be defined by the slope of the DR/R0 versus DH curvewithin the reversemagnetization of the FL. Beside the dependences
of theMR 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 maximumMR is observed at tTa ¼ 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 SV films [58]. Moreover, with the present of the (111) texture,
the Heb is also enhanced [48]. Therefore, the disappearance of the
(111) texture due to the lack of a seed layer leads to a decrease inHeb
and in the working temperature of SV films 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 to find out a critical spacer thickness that
provides the maximum MR ratio with an appropriate Hin. The
oFe alloy of SVs (Si/SiO2)/Ta(150 Å)/Co(45 Å)/Cu(30 Å)/Co100-xFex(25 Å)/IrMn(250 Å)/
(30 Å)/Co(15 Å)]/Cu(24 Å)/Co80Fe20(25 Å)/IrMn(tIrMn)/Ta(50 Å).
Materials and Devices 3 (2018) 399e405 401dependence of theMR 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, theMR ratio increases, but the
Hin become stronger leading to the shift of the working point. This
is mainly caused by the roughness of spacer layers, the pinholes,
and the interlayer coupling field of the PL [36]. In the lowmagnetic
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. tCu ¼ 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 theMR 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
f SV
15 Å
e20(
cedweaker in a thicker spacer layer. Finally, by the trade-off between
the MR and Hin, tCu ¼ 24 Å was chosen for SV film fabrications.
As we know, a cool-field anneal process is an indispensable step
in the fabrication proce