8/8/2019 Sensor GMR
1/15
http://ramlisultan.wordpress.com
1
Giant Magnetoresistance Sensor
A. MagnetoresistanceThe basic principle of the magnetoresistance (MR) is the variation of the resistance
(R) of a material or a structure as a function of an external magnetic field (B), as
generally described by the following general equation:
R =f(B) (1)
Magnetoresistance (MR) is defined as a change in the electrical resistance of a substance
in the presence of a magnetic field. The signal response of a device is often characterised
by the percentage MR, as shown by Eqn. (2), where
R is the change in resistance in anapplied field and R is the resistance in the absence of an applied field
%100(%)
=R
RMR
(2)
B. Giant Magnetoresistance (GMR)In 1988, Baibich et al. [1] and Binasch et al. [2] reported for the first time on what
they called Giantmagnetoresistance measured on Fe/Cr thin multilayers. They
demonstrated that the electric current in a magnetic multilayer consisting of a sequence of
thin magnetic layers separated by equally thin non-magnetic metallic layers is strongly
influenced by the relative orientation of the magnetizations of the magnetic layers (about
50% at 4.2 K) . The cause of this giant variation of the resistance is attributed to the
scattering of the electrons at the layers interfaces. This way, any structure with metal-
magnetic interfaces is a candidate to display GMR. Since then, a huge effort has been
carried out in finding structures to enhance this effect (MR levels at room temperature
above 200% are achieved in modern GMR structures).
The giant magnetoresistive effect is a benefit of technological development that
allows different magnetic structures to be obtained at nanometre and even atomic scales
[3]. These ultrathin structures exhibit a wide range of mysterious and surprising
phenomena which do not exist in bulk materials. The typical thin film nanostructures may
be created as epitaxial films (films with ordered crystal structures of good
monocrystalline quality) prepared by using the MBE method (molecular beam epitaxy) or
8/8/2019 Sensor GMR
2/15
http://ramlisultan.wordpress.com
2
polycrystalline films prepared by using the sputtering method. These films are formed as
multilayers, for example a sandwich structure (fig. 1(a)) where a third, very thin
nonmagnetic film (the spacer), is placed between two magnetic films. The border
between the films is called the interface. The artificially grown structure consists of
periodically alternating single-crystal film layers and is called the superlattice. Small
magnetic entities may be introduced into the nonmagnetic matrix and this nanocomposite
system is called the granular system (fig. 1(c)).
Fig. 1. Various types of GMR structures: (a) multilayer; (b) spin valve and (c) granular
films.
B.1. Multilayer GMR Structure
A multilayered structures consist of two or more magnetic layers of a FeCoNi
alloy, as can be permalloy, separated by a very thin non magnetic conductive layer, as
can be Cu [4]. A general scheme is shown in Figure 1(a). With magnetic films of
about 46 nm width and a conductor layer of about 35 nm, magnetic coupling
between layers is slightly small. With this configurations, MR levels of about 4%9%
are achieved, with linear ranges of about 50 Oe. The figures of merit of these devices
can be improved by continuously repeating the basic structure. Successful
applications of multilayered structures in magnetic field sensing include
bioelectronics [5, 6] and angle detectors [7].
B.2. Spin Valve GMR Structure
8/8/2019 Sensor GMR
3/15
http://ramlisultan.wordpress.com
3
An effective method of improving of the sensitivity of GMR sensors was
introduced in 1991 by Dieny and co-workers [8]. They proposed a new type of GMR
sandwich structure termed a spin-valve (SV) sensor. In the spin-valve structure the
antiferromagnetic alignment is obtained not by exchange coupling between two
ferromagnetic films (fig. 2 (a)) but by exchange biasing one of the films (fig. 2 (b)).
This biased layer (called the fixed or pinned layer) is separated by a nonferromagnetic
spacer from the second ferromagnetic layer (free or unpinned layer).
(a) (b)
Fig. 2. The spin-valve structure and results reported by Dieny and co-workers [8] for
NiFe/Cu/NiFe/FeMn structure.
The origin of spin valves are a particular case of multilayered structure. In spin
valves, an additional antiferromagnetic (pinning) layer is added to the top or bottom
part of the structure, as shown in Figure 1(b). In this sort of structures, there is no
need of an external excitation to get the antiparallel alignment. In spite of this, the
pinned direction (easy axis) is usually fixed by raising the temperature above the knee
temperature (at which the antiferromagnetic coupling disappears) and then cooling it
within a fixing magnetic field. Obviously, so obtained devices have a temperature
limitation below the knee temperature. Typical values displayed by spin valves are a
MR of 4%20% with saturation fields of 0.86 kA/m [9].
For linear applications, and without excitation, pinned (easy axis) and free layers
are arranged in a crossed axis configuration (at 900). The response this structure is
given by [10]:
8/8/2019 Sensor GMR
4/15
http://ramlisultan.wordpress.com
4
( )fps
h
iWR
R
RR
= cos
2
1
(3)
where (R=R) is the maximum MR level (5%20%), Rs is the sensor sheet
resistance (1520 /m), L is the length of the element, W is its width, h is the
thickness, i is the sensor current, and p and f are the angle of the magnetization
angle of pinned and free layers, respectively. Assuming uniform magnetization for
the free and pinned layers, for a linearized output, p = /2 and f = 0.
The term spin-valve followed from the term magnetic valve introduced by
Slonczewski (1989) [11] for the tunnelling effect in ferromagnetic layers. Spin-valve
means that the magnetizations of the layers act as a sort of valve for conductionelectrons.
B.3. Granular GMR Structure
The granular GMR structure consists of nanometre-size granules of magnetic
material dispersed in a nonmagnetic host material [12, 13, 14, 15]. This structure is
usually prepared by codeposition of magnetic and nonmagnetic metals. Because of
the immiscibility of some transition metals with noble metals it is possible to obtain a
mixture of a nonmagnetic matrix with precipitated magnetic entities. It is also
possible to obtain the granular structure straight away by sputtering deposition from
the composite material target [16]. Directly after the sputtering process a large
saturation field is necessary to exhibit the full value of magnetoresistance about
4000 kA/m. But after annealing, when the granular structure stabilizes it is possible to
obtain magnetoresistance for a saturation field smaller than 1000 A/m.
Another method of manufacturing the granular structure was proposed by Parkin
and co-workers [17]. The granular structure was obtained by appropriate molecular
beam epitaxial growth of Co/Cu and Co/Ag films. It was possible to prepare the
granular structure by slow co-evaporation under UHV conditions and at moderate
(below 400C) substrate temperature. Such a process led to spontaneous phase
separation. Depending on the substrate temperature the next step of annealing may
change the magnetoresistance in different ways. Even without annealing a large
magnetoresistance ratio was obtained, exceeding 70% at 4.2 K for the CoAg alloy. In
8/8/2019 Sensor GMR
5/15
http://ramlisultan.wordpress.com
5
(111)-oriented Co-Ag film Co particles with diameter of 2.5 nm separated from each
other by a distance of 8 nm were created.
Fig. 3 presents the typical transfer characteristics R/R =f(H) determined for the
CoFe/Ag composition. Characteristics R/R = f(H) are similar to the transfer
characteristics describing the GMR effect in multilayers with antiferromagnetic
coupling. The granular structure may be interpreted as a special form of nanometric
entities with the resistance changed by spin-dependent scattering. After applying an
external magnetic field the particles are aligned parallel to each other. Thus the GMR
effect in multilayer structures and in granular alloys has a common origin.
Fig.3. An example of the transfer characteristics of granular structure
(Co70Fe30)36/Ag64 after annealing at 300C at room temperature [18].
The important phenomenon utilized in many GMR structures is the exchange
magnetic coupling between the magnetic films in the sandwich system. In 1986 Grnberg
and co-workers [19] discovered that for some thickness of nonmagnetic spacer (0.9 nm in
their case) antiferromagnetic exchange coupling occured in the sandwich multilayer
structure of the magnetic films (FeCrFe). Antiferromagnetic coupling means that two
magnetic films are magnetized antiparallel to each other (fig. 4). In 1990, Parkin et al
[20] discovered that magnetoresistance oscillated with some period when the thickness of
the interlayer was varied. These oscillations (fig. 4) indicate that the character of coupling
changes with the change of thickness of the spacer as a periodic sequence from
magnetizations of the films aligned parallel to antiparallel.
8/8/2019 Sensor GMR
6/15
http://ramlisultan.wordpress.com
6
Fig. 4. Evidence of oscillatory exchange coupling in thin film multilayers
Another more promising GMR structure was proposed by Slonczewski [11] and
investigated by Moodera [21] and Miyzaki and Tezuka [22]. In this structure the
magnetic layers are separated by a thin insulator barrier. It was expected that by using
this magnetic tunnel junction (MTJ) sensors with magnetoresistivity of about 1020% for
very low magnetic fields (about 10100 A/m) could be obtained. Typical MR levels of
MTJ are above 40%, with Al2O3 as isolating layer [23]. More recently, MR levels about
200% have been reported for MgO based structures [24]. Saturation fields are in the order
of 1-100 Oe.
The basis of linear magnetic tunnel junctions is analogous to that of linear spin valve.
When configured in a crossed axis configuration, linear ranges suitable for sensor
applications can be achieved [10]. Nevertheless, the usage of linear MTJ is still in its
initial stage and is demanding additional research efforts.
C. GMR Sensors DesignGMR sensors are rather complex systems and therefore various parameters influence
their performances. The composition of the material or the thickness of the layers are
8/8/2019 Sensor GMR
7/15
http://ramlisultan.wordpress.com
7
specific for different GMR structures and have been analysed in previous chapters.
Common to all structures are the technology dependent problems of the quality of
deposited structures. The technological conditions affecting spin-valve performances as
an example will be analysed in more detail because this structure is the most promising
candidate for sensors or reading heads.
A good quality sensor should exhibit following features [25]:
a large value of magnetoresistivity ratio (R/R)max, = (Rmax -Rmin)/Rmax,
large sensitivity S = (R/R)max/Hs,
small hysteresis usually described by coercive fieldHc,
small anisotropy fieldHkof the unpinned layer (sensitivity depends onHk), large exchange biasing fieldHex,
small changes of parameters with temperature,
good repeatability and reliability
GMR sensors have greater output than conventional anisotropic magnetoresistive (AMR)
sensors or Hall effect sensors, and are able to operate at fields well above the range of
AMR sensors. In addition, high fields will not flip GMR sensors or reverse their output
as is possible with AMR sensors. GMR sensors have significant advantages over Hall
Effect and AMR sensors as shown in Table 1.
Table. 1. Benefits of Magnetic Sensors [26]
8/8/2019 Sensor GMR
8/15
http://ramlisultan.wordpress.com
8
The design and development of a GMR based sensor depends on considerations
coming from different involved fields in order to get functional devices. Moreover, the
specific design will be necessarily linked to the particular application for which it is
designed. Every part of the sensor design will have bigger or lesser repercussions on the
final device performance. For example, both the linear range and the thermal
characteristics of the sensors will be functions of the characteristics of the sensing
structure as well as the final encapsulation. A more detailed knowledge of these
parameters is absolutely necessary prior to the starting of the sensor development.
Even though a unique resistance can be used as sensing element, a Wheatstone bridge
setup is always a good recommendation as the starting step in the design of resistivesensors. Table 2 displays a summary of possible bridge configurations with calculated
output voltage. As easily observed, a full bridge configuration is the best choice in terms
of signal level and linearity. Nevertheless, and in the case of GMR, often it is impossible
to take advantage of such setup. Because of the orientation dependence in the fabrication
process of GMR structures, a half bridge configuration with two active resistances and
two shielded ones is used (see Table 1, center) [27]. If a two steps deposition is assumed,
a full bridge can be obtained.
8/8/2019 Sensor GMR
9/15
http://ramlisultan.wordpress.com
9
Table. 2. Bridge configurations
In any case, a GMR sensor is a magnetic field sensor. These sensors can be used for
detecting a magnetic field or a disturbance in the earth magnetic field produced by a
magnetic issue. This way, the design of the sensing system is, in the most of the cases, ad
hoc.
Nordling, et.al [28] has developed the integrated sensor is a passive circuit consisting
of four GMRs, deposited in a serpentine pattern and wired together as a Wheatstone
bridge. As shown in Fig. 5, the two interdigitated GMRs serve as sense resistors (RS1 and
RS2) and the two spatially separated GMRs act as reference resistors (RR1 andRR2). Each
strip is 2 m wide, separated from its neighbor by 2 m, and has a total length of over 11
mm. The edges of the sense and reference GMRs are separated by 30 m to isolate theinnate magnetic field of each resistor, which is a function of sourced current through the
leads, from each other. The sensing GMRs are coated with a thin film of silicon nitride to
planarize the active surface and protect it from sample contact.
8/8/2019 Sensor GMR
10/15
http://ramlisultan.wordpress.com
10
Fig. 5. (a) Electronic schematic of the Wheatstone bridge. (b) Photomicrograph of the
GMR sense pad used in this publication showing the reference GMR traces (top and
bottom) and the GMR sense pad (center). (c) Schematic illustration of the Wheatstone
bridge GMR sensor shown in (b) depicting the circuit as well as the direction of theexternal magnetic field and the motion of the GMR relative to the sample.
In the design of thin film structures it is useful to operate at the resistivity determined
for the condition w =L (w width,L length):
tRsq
=
(4)
This resistance ohms per square is the same for any square irrespective of size. It
depends only on the thickness of the film. The dependence of the output signal on the
aspect ratioL/w may be described by the following expression [29]:
(5)
For the multilayer (exchange coupling) sensor expression can be rewritten as:
(6)
where:I bias current,J current density,Hs saturation field,H1 coupling
8/8/2019 Sensor GMR
11/15
http://ramlisultan.wordpress.com
11
field,Hk anisotropy field,Hd demagnetizing field.
In many applications resolution of the sensor is also very important. This resolution is
limited by the noise. Nor and co-workers (1998) estimated that signal-to-noise ratio is
proportional to the square root of the device area.
(7)
Where c is constants.
D. Applications for Currents Sensing
The first commercial GMR sensors were introduced in 1995 [30]. The rapid evolution
of GMR sensors technology has opened a wide and promising range of applications.
Apart from electrical measurement related systems, GMR based sensors are nowadays
being utilized in different fields as engineering, physics, biology, space, biomedical, etc.
Conventional methods of current measurements utilize the resistor shunt technique.
However, this technique exhibits several serious drawbacks. The most important
disadvantage is the lack of a galvanic separation between the source (very often of high
voltage) and measuring equipment. Another disadvantage of the shunt is the necessity of
breaking the conductor, which is not always possible, for example in printed circuitboards (PCB) or high voltage circuits. Another conventional method of current
measurement is the use of measuring transformers. This method introduces additional
errors (for example hysteresis) and is typically limited to large AC currents. Therefore the
shunts or the measuring transformers are often substituted by Hall-sensor current
transducers.
Due to better sensitivity, linearity and temperature behaviours the permalloy
magnetoresistive current transducers can be substituted for similar Hall-sensor
transducers. Moreover thin film permalloy sensors are very convenient to prepare as
integrated current transducers.
The simplest way to convert the current into the output signal of the sensor is to
measure the magnetic field around the current conducting layer (Fig. 6(a)), above the
current conducting track (Fig. 6(b)) or inside the coil (Fig.6(c)). The magnetic field
around the current conducting layer (Fig.7) can be calculated from the expression:
8/8/2019 Sensor GMR
12/15
http://ramlisultan.wordpress.com
12
x
IH x
2=
(4)
Fig. 6. The detection of the electrical current: (a) around the current conducting wire; (b)above the printed circuit board; (c) as a field produced by a coil.
Fig. 7. The magnetic field around the current conducting wire
The magnetic field above the printed circuit board (Fig.5(b)) can be calculated from the
expression:
'
2
' )(
4
1)( dr
R
RrJrH
s
=
rr
RRrrR
r
rr
r
==;
'
(4)
The Fig 7. below illustrates the sensor package orientation for detecting the field from
a current-carrying wire. This application allows for current measurement without
breaking or interfering with the circuit of interest. The wire can be located above or
below the chip, as long as it is oriented perpendicular to the sensitive axis.
8/8/2019 Sensor GMR
13/15
http://ramlisultan.wordpress.com
13
Fig.7 GMR sensor package orientation for detecting the field from a current-carrying
wire [31].
Fig. 8 presents the experimentally determined transfer characteristic of the GMR AC004
sensor placed above the track with dimensions 1.5 mm width and 0.05 mm thickness
[25].
Fig. 8. The recommended application of the AC004 sensor for current measurements:
arrangement of the sensor and typical transfer characteristics [25].
References:
[1]. Baibich, M.N.; Broto, J.M.; Fert, A.; Vandau, F.N.; Petroff, F.; Eitenne, P.; Creuzet,
G.; Friederich, A.; Chazelas, J. Giant magnetoresistance of (001)Fe/(001)Cr
magnetic superlattices. Phys. Rev. Lett. 1988, 61, 2472-2475.
[2]. Binasch, G.; Grunberg, P.; Saurenbach, F.; Zinn, W. Enhanced magnetoresistance
in layered magnetic-structures with antiferromagnetic interlayer exchange. Phys.
Rev. B. 1989, 39, 4828-4830.
[3]. Himpsel F J. Electronic states in magnetic nanostructrures.IBM J. Res. Dev. 1998.
42 3342.
8/8/2019 Sensor GMR
14/15
http://ramlisultan.wordpress.com
14
[4]. Ranchal, R.; Torija, M.; Lopez, E.; Sanchez, M.C.; Aroca, C.; Sanchez, P. The
influence of anisotropy on the magnetoresistance of permalloycopperpermalloythin films.Nanotechnology. 2002, 13, 392-397.
[5]. Mujika, M.; Arana, S.; Castano, E.; Tijero, M.; Vilares, R.; Ruano-Lopez, J.M.;Cruz, A.; Sainz, L.; Berganza, J. Microsystem for the immunomagnetic detection
ofEscherichia coli O157 : H7. Phys. Status Sol. A 2008, 205, 1478-1483.
[6]. Mujika, M.; Arana, S.; Castano, E.; Tijero, M.; Vilares, R.; Ruano-Lopez, J.M.;
Cruz, A.; Sainz, L.; Berganza, J. Magnetoresistive immunosensor for the detection
of Escherichia coli O157:H7 including a microfluidic network. Biosens.
Bioelectron. 2009, 24, 1253-1258.
[7]. Lopez-Martn, A.J.; Carlosena, A. Performance tradeoffs of three novel GMR
contactless angle detectors.IEEE Sens. J. 2009, 9, 191-198.[8]. Dieny, B.; Speriosu, V.S.; Metin, S.; Parkin, S.S.P.; Gurney, B.A.; Baumgart, P.;
Wilhoit, D.R. Magnetotransport properties of magnetically soft spin-valve
structures.J. Appl. Phys. 1991, 69, 4774-4779.[9]. Hartmann, U. Magnetic Multilayers and Giant Magnetoresistance: Fundamentals
and Industrial Applications; Springer: Berlin, Germany, 1999.[10]. Freitas, P.P.; Ferreira, R.; Cardoso, S.; Cardoso, F.; Magnetoresistive sensors. J.
Phys.-Condens. Matter. 2007, 19, doi:10.1088/0953-8984/19/16/165221.
[11]. Slonczewski J C. Conductance and exchange coupling of two ferromagnets
separated by a tunneling barrier. 1989. Phys. Rev. B39 69957002[12]. Xiao J Q . Giant magnetoresistance in nonmultilayer magnetic systems Phys. Rev.
Lett. 1992.68 37493752.
[13]. Berkowitz J A E. Giant magnetoresistance in heterogenous alloys. Phys. Rev.Lett.1992. 68 37453748.
[14]. Xiao J Q. Giant magnetoresistive properties in granular transition metals. IEEETrans. Magn.1993.29 26882693.
[15]. Dieny B. Structural, magnetic and transport properties of NiFexAg1-x
heterogeneous alloys. J. Magn. Magn. Mat.1994.130 197215.[16]. Wang J Q and Xiao G. Transition-metal granular solids: microstructure, magnetic
properties and giant magnetoresistance. Phys. Rev. 1994.B49 39823986
[17]. Parkin S S P. Giant magnetoresistance in as-grown epitaxial films of phaseseparated Co-Cu and Co-AgEurophys. Lett. 1993.22 455462.
[18]. Teixeira S R. Giant magnetoresistance in sputtered (Co70Fe30)x Ag1-x
heterogeneous alloys,J. Phys. Cond. Matter.1994.6 55455560
[19]. Grnberg P. Layered magnetic structures: evidence for antiferromagnetic coupling
of Fe layers across Cr interlayer. Phys Rev. Lett. 1986.57 24422445.[20]. Parkin S S P. Oscillations in exchange coupling and magnetoresistance in metallic
superlattice structures: Co/Ru, Co/Cr and Fe/Cr. Phys. Rev. Lett.1990. 64.2304
2307.
[21]. Moodera J S. Large magnetoresistance at room temperature in ferromagnetic thin
film tunnel junctions. Phys. Rev. Lett. 1995. 74 32733276.
[22]. Miyazaki T and Tezuka N. Spin polarized tunneling in ferromagnet/
insulator/ferromagnet junctions.J. Magn. Magn. Mat. 1995. 151 403410.
[23]. Michael, Z., Martin, J.T., Eds. Spin Electronics; Lecture Notes in Physics;Springer: Berlin, Germany, 2001.
8/8/2019 Sensor GMR
15/15