Sensor GMR

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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

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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

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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]:

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( )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

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(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.

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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

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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]

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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.

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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.

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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

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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:

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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.

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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.

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[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

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