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Electric/Magnetic

A New Perspective on Magnetic Field Sensing

December 1, 1998 By: Tamara Bratland, Michael J. Caruso, Robert W. Schneider, Nonvolatile Electronics, Inc. , Carl H. Smith, Nonvolatile Electronics


  figure

Figure 1. Conventional sensors detect a physical property directly (A);
magnetic sensors detect changes in magnetic fields and from them derive
information on physical properties (B).

Magnetic sensors differ from most other detectors in that they do not
directly measure the physical property of interest. Devices that monitor
properties such as temperature, pressure, strain, or flow provide an output
that directly reports the desired parameter (see Figure 1). Magnetic sensors,
on the other hand, detect changes, or disturbances, in magnetic fields that
have been created or modified, and from them derive information on properties
such as direction, presence, rotation, angle, or electrical currents. The
output signal of these sensors requires some signal processing for translation
into the desired parameter. Although magnetic detectors are somewhat more
difficult to use, they do provide accurate and reliable data — without physical
contact.

Magnetic sensors can be classified according to low-, medium-, and high-field
sensing range. In this article, devices that detect magnetic fields <1
µG (microgauss) are considered low-field sensors; those with a range of
1 µG to 10 G are Earth's field sensors; and detectors that sense fields
>10 G are referred to as bias magnet field sensors. Table 1 lists magnetic
sensor technologies and their sensing ranges [1].

Table 1. Magnetic Sensor Technology Field Ranges
  Magnetic Sensor

Technology


DETECTABLE FIELD RANGE (gauss)*

10-8
10-4
100
104
108
  SQUID


FIBER-OPTIC


OPTICALLY PUMPED


NUCLEAR PRECESSION


SEARCH-COIL

ANISOTROPIC MAGNETORESISTIVE


FLUX GATE


MAGNETOTRANSISTOR


MAGNETODIODE


MAGETO-OPTICAL SENSOR


GIANT MAGNETORESISTIVE


HALL-EFFECT SENSOR


*Note: 1 gauss = 10-4 tesla = 105 gamma

A magnetic field is a vector quantity with both magnitude and direction.
The scalar sensor measures the field's total magnitude but not its direction.
The omnidirectional sensor measures the magnitude of the component of magnetization
that lies along its sensitive axis. The bidirectional sensor includes direction
in its measurements. The vector magnetic sensor incorporates two or three
bidirectional detectors. Some magnetic sensors have a built-in threshold
and produce an output only when it is surpassed.


Unit Conversion from SI to Gaussian

79.6 A/m = 1 oersted

100 microtesla = 1 gauss

1 gauss = 1 oersted (in free air)

1 gauss = 10-4 tesla = 105 gamma

1 nanotesla = 10 microgauss = 1 gamma

Low-Field Sensors

Low-field sensors tend to be bulky and costly compared to other magnetic
devices. Care must be taken to account for the effects of the Earth's field,
whose daily variations may exceed the sensor's measurement range. The devices
are used for medical applications and military surveillance.

  SQUID. The most sensitive low-field sensor is the superconducting
quantum interference device (SQUID). Developed about 1962, it is based on
Brian J. Josephson's work on the point-contact junction designed to measure
extremely low currents [1]. SQUID magnetometers can detect fields from several
femtotesla up to 9 tesla, a range of more than 15 orders of magnitude. This
is essential in medical applications since the neuromagnetic field of the
human brain is only a few tenths of a femtotesla [2]; Earth's magnetic field,
by way of comparison, is ~50 microtesla, or 0.5 oersted. SQUIDs require
cooling to liquid helium temperature (4 kelvin) at present, but devices
are under development that will operate at higher temperatures.

  Search-Coil. The basic search-coil magnetometer is based on Faraday's
law of induction, which states that the voltage induced in a coil is proportional
to the changing magnetic field in the coil. This induced voltage creates
a current that is proportional to the rate of change of the field. The sensitivity
of the search-coil is dependent on the permeability of the core and the
area and number of turns of the coil. Because search-coils work only when
they are in a varying magnetic field or moving through one, they cannot
detect static or slowly changing fields. Inexpensive and easily manufactured,
the devices are commonly found in the road at traffic control signals.

  Other Low-Field Sensors. Other low-field sensor technologies include
nuclear precession, optically pumped, and fiber-optic magnetometers. These
precision instruments are used in laboratories and medical applications.
For instance, the long-term stability of the nuclear precession magnetometer
can be as low as 50 pT/yr. These magnetometers will not be discussed in
this article.

Earth's Field (Medium-Field) Sensors 
The magnetic range of medium-field sensors lends itself well to using
the Earth's magnetic field to determine compass headings for navigation,
detect anomalies in it for vehicle sensing, and measure the derivative of
the change in field to determine yaw rate.

  Flux Gate. The flux-gate magnetometer, the most widely used sensor for
compass-based navigation systems, was developed about 1928 and later refined
by the military for submarine detection. The devices have also been used
for geophysical prospecting and airborne magnetic field mapping operations.

figure

Figure 2. The most common type of flux-gate magnetometer consists of
two coils wrapped around a common high-permeability ferromagnetic core whose
magnetic induction changes in the presence of an external magnetic field.

The most common type, called the second harmonic device [3-5], incorporates
two coils, a primary and a secondary, wrapped around a common high-permeability
ferromagnetic core. The core's magnetic induction changes in the presence
of an external magnetic field. A drive signal applied to the primary winding
at frequency f (e.g., 10 kHz) causes the core to oscillate between saturation
points. The secondary winding outputs a signal that is coupled through the
core from the primary winding (see Figure 2). This signal is affected by
changes in the core's permeability and appears as an amplitude variation
in the sensing coil's output. The signal can be demodulated with a phase-sensitive
detector and low pass filtered to retrieve the magnetic field value. Another
way of looking at the flux-gate operating principle is to sense the ease
of or resistance to core saturation caused by the change in its magnetic
flux. The difference is due to the external magnetic field.

A well-designed flux-gate magnetometer can sense a signal in the tens
of microgauss range, as well as measure both magnitude and direction of
static magnetic fields. The upper frequency band limit is ~1 kHz due to
the drive frequency limit of ~10 kHz. These devices tend to be bulky and
not so rugged as smaller, more integrated sensor technologies.

  Magnetoinductive. Magnetoinductive magnetometers are relatively new,
with the first patent issued in 1989. This sensor is simply a single winding

figure

Figure 3. A magnetoinductive sensor is simply a single winding coil in
a ferromagnetic core that changes permeability within the Earth's field.

coil on a ferromagnetic core that changes permeability within the Earth's
field. The coil is the inductance element in a L/R relaxation oscillator.
The oscillator's frequency is proportional to the field being measured.
A static DC current is used to bias the coil in a linear region of operation
(see Figure 3). As the sensor is rotated 90º from the applied magnetic
field, the observed frequency shift can be as much as 100%. The oscillator
frequency can be monitored by a microprocessor's capture/compare port to
determine field values. These magnetometers are simple in design, inexpensive,
and have low power requirements. Their temperature range is -20ºC
to 70ºC, and they are repeatable to within 4 mG. Automatic assembly
and axis alignment are difficult due to the sensor's small size and its
physical configuration.

  Anisotropic Magnetoresistive (AMR). William Thompson, later Lord
Kelvin [6], first observed the magnetoresistive effect in ferromagnetic
metals in 1856. His discovery had to wait more than 100 years before thin
film technology could make it into a practical sensor. Magnetoresistive
sensors come in a variety of shapes and forms and are used in high-density
read heads for tape and disk drives, as well as for automotive wheel speed
and crankshaft measurement, compass navigation, vehicle detection, and current
sensing.

AMR sensors are well suited to measuring both linear and angular position
and displacement in the Earth's magnetic field. These devices are made of

figure

Figure 4. Anisotropic magnetoresistive sensors, used to measure linear
and angular position and displacement in the Earth's magnetic field, are
made of a nickel-iron thin film deposited on a silicon wafer and patterned
as a resistive strip.

a nickel-iron (Permalloy) thin film deposited on a silicon wafer and patterned
as a resistive strip [6-8]. The film's properties cause it to change
resistance by 2%-3% in the presence of a magnetic field. In a typical
configuration, four of these resistors are connected in a Wheatstone bridge
(see Figure 4) to permit measurement of both field magnitude and direction
along a single axis. The bandwidth is usually in the 1-5 MHz range.
The reaction of the magnetoresistive effect is very fast and not limited
by coils or oscillating frequencies.

AMR sensors can be bulk manufactured on silicon wafers and mounted in
commercial IC packages, permitting automated assembly with other circuit

figure

Figure 5. Anisotropic magnetoresistive sensors offer high sensitivity, small
size, and immunity to noise.

and systems components. They also offer high sensitivity, small size, and
noise immunity (see Figure 5).

A detailed explanation of AMR technology will be published in the March
1999 issue of Sensors.

Bias Magnetic Field Sensors

Most industrial sensors use permanent magnets as a source of the magnetic
field to be detected. These magnets magnetize, or bias, ferromagnetic objects
close to the sensor, which then detects changes in the total field around
itself. Bias field sensors must detect fields that are typically larger
than the Earth's, but must not be temporarily upset or permanently affected
by a large field. Sensors in this category include reed switches, InSb magnetoresistors,
Hall devices, and GMR sensors. Although some of these sensors, such as magnetoresistors,
are capable of measuring fields up to several teslas, others, such as GMR
devices, can detect fields down to the milligauss region with research extending
their capabilities to the microgauss region.

  Reed Switches. The reed switch can be considered the simplest
magnetic sensor to produce a usable output for industrial control. It consists
of a pair of flexible, ferromagnetic contacts hermetically sealed in an
inert gas filled container, often glass. The magnetic field along the long
axis of the contacts magnetizes the contacts and causes them to attract
each other, closing the circuit. Because there is usually considerable hysteresis
between the closing and releasing fields, the switches are quite immune
to small fluctuations in the field.

Reed switches are maintenance free and highly immune to dirt and contamination.
Rhodium-plated contacts ensure long contact life. Typical capabilities are
0.1-0.2 A switching current and 100-200 V switching voltage. Contact
life is measured at 106-107 operations at 10 mA. Reed switches are available
with normally open, normally closed, and class C contacts. Latching reed
switches are also available. Mercury-wetted reed switches can switch currents
as high as 1 A and have no contact bounce.

Low cost, simplicity, reliability, and zero power consumption make reed
switches popular in many applications. The addition of a separate small
permanent magnet yields a simple proximity switch often used in security
systems to monitor the opening of doors or windows. The magnet, affixed
to the movable part, activates the reed switch when it comes close enough.
The desire to sense almost everything in cars is increasing the number of
reed switch sensing applications in the automotive industry.

Lorentz Force Devices. There are several sensors that use the
Lorentz force, or Hall effect, on charge carriers in a semiconductor. The
Lorentz force equation describes the force FL experienced by a charged particle
with charge q moving with velocity v in a magnetic field B:

   FL = q (v × B)

Since FL, v, and B are vector quantities, they have both magnitude and
direction. The Lorentz force is proportional to the cross product between
the vectors representing velocity and magnetic field; it is therefore perpendicular
to both of them and, for a positively charged carrier, has the direction
of advance of a right-handed screw rotated from the direction of v toward
the direction of B. The acceleration caused by the Lorentz force is always
perpendicular to the velocity of the charged particle; therefore, in the
absence of any other forces, a charge carrier follows a curved path in a
magnetic field.

figure

Figure 6. The Hall effect is illustrated by a semiconductor slab showing
magnetic field, applied voltage, forces on electrons and holes, and paths
of electrons and holes.

The Hall effect is a consequence of the Lorentz force in semiconductor
materials. When a voltage is applied from one end of a slab of semiconductor
material to the other, charge carriers begin to flow. If at the same time
a magnetic field is applied perpendicular to the slab, the current carriers
are deflected to the side by the Lorentz force. Charge builds up along the
side until the resulting electrical field produces a force on the charged
particle sufficient to counteract the Lorentz force. This voltage across
the slab perpendicular to the applied voltage is called the Hall voltage
(see Figure 6).

Magnetoresistors. The simplest Lorentz force devices are magnetoresistors
that use semiconductors such as InSb and InAs with high room-temperature
carrier mobility. If a voltage is applied along the length of a thin slab
of semiconductor material, a current will flow and a resistance can be measured.
When a magnetic field is applied perpendicular to the slab, the Lorentz
force will deflect the charge carriers. If the width of the slab is greater
than the length, the charge carriers will cross the slab without a significant
number of them collecting along the sides. The effect of the magnetic field
is to increase the length of their path and, thus, the resistance. An increase
in resistance of several hundred percent is possible in large fields. To
produce sensors with hundreds to thousands of ohms of resistance, long,
narrow semiconductor stripes a few micrometers wide are produced using photolithography.
The required length-to-width ratio is accomplished by forming periodic low-resistance

figure

Figure 7. Magnetoresistors can be built of boules with needle-shaped
low-resistance precipitates of NiSb in a matrix of InSb. The precipitates
serve as the shorting bars.

metal shorting bars across the traces. Each shorting bar produces an equipotential
across the semiconductor stripe. The result is, in effect, a number of small
semiconductor elements with the proper length-to-width ratio connected in
series.

A second method is to use lapped wafers cut from boules that have needle-shaped
low-resistance precipitates of NiSb in a matrix of InSb. These precipitates
serve as the shorting bars [9]. In Figure 7, which shows the effect of these
shorting bars on the current path, notice that the higher the magnetic field,
the longer the current path and the higher the resistance.

Magnetoresistors formed from InSb are relatively insensitive in
low fields; in high fields, however, they exhibit a resistance that changes
approximately as the square of the field. They are sensitive only to that
component of the magnetic field perpendicular to the slab and not to whether
the field is positive or negative. Their large temperature coefficients

figure

Figure 8. Resistance is plotted against field for an InSb magnetoresistor
at temperatures of ­20ºC, 0ºC, 25ºC, 60ºC, 90ºC,
and 120ºC (top to bottom). Resistance is normalized to the resistance
at zero field.

of resistivity are caused by the change in mobility of the charge carriers
with temperature. The sensors are made with either single resistors or pairs
of spaced resistors. The latter are used to measure field gradients and
are usually combined with external resistors to form a Wheatstone bridge.
A permanent magnet is often incorporated in the field gradient sensor to
bias the magnetoresistors up to a more sensitive part of their characteristic
curve. Figure 8 shows a typical InSb sensor's change of resistance with
field and temperature. Different doping levels of the semiconductor material
account for the differences in characteristics and conductivity, e.g., 200
(omegacm) -1 for D material and 550 (omegacm) -1 for L material.

Hall sensors. Hall sensors typically use n-type silicon when cost
is of primary importance and GaAs for higher temperature capability due
to its larger band gap. In addition, InAs, InSb, and other semiconductor
materials are gaining popularity due to their high carrier mobilities that
result in greater sensitivity and frequency response capabilities above
the 10-20 kHz typical of Si Hall sensors. Compatibility of the Hall
sensor material with semiconductor substrates is important since Hall sensors
are often used in integrated devices that include other semiconductor structures.

A Hall sensor uses a geometry similar to that shown in Figure 6; in this
case, however, the length in the direction of the applied voltage is greater
than the width. Charge carriers are deflected to the side and build up until
they create a Hall voltage across the slab with a force equaling the Lorentz
force on the charge carriers. At this point the charge carriers travel the
length in approximately straight lines, and no additional charge builds
up. Since the final charge carrier path is essentially along the applied
electric field, the end-to-end resistance changes little with the magnetic
field. When the Hall voltage is measured between electrodes placed at the

figure

Figure 9. Three Hall effect sensors made of different semiconductor materials
exhibit input resistances that change with temperature.

middle of each side, the resulting differential voltage is proportional
to the magnetic field perpendicular to the slab. It also changes sign when
the sign of the magnetic field changes. The ratio of the Hall voltage to
the input current is called the Hall resistance, and the ratio of the applied
voltage to the input current is called the input resistance.

The Hall resistance and Hall voltage increase linearly with applied field
to several teslas (tens of kilogauss). The temperature dependence of the
voltage and the input resistance is governed by the temperature dependence

figure

Figure 10. The Hall voltage at 0.05 T (500 Oe) of several Hall sensors
made of different semiconductor materials is plotted vs. temperature. The
input voltage is given for each material.

of the carrier mobility and that of the Hall coefficient. Different materials
and different doping levels result in tradeoffs between sensitivity and
temperature dependence (see Figures 9 and 10) [10].

Integrated Hall sensors. Hall devices are often combined with
semiconductor elements to create integrated sensors. Adding comparators
and output devices to a Hall element, for example, yields unipolar and bipolar
digital switches. Adding an amplifier increases the relatively low voltage
signals from a Hall device to produce ratiometric linear Hall sensors with
an output centered on one-half the supply voltage. Power usage can even
be reduced to extremely low levels by using a low duty cycle [11].

  Giant Magnetoresistive (GMR) Devices. Large magnetic field dependent
changes in resistance are possible in thin film ferromagnet/nonmagnetic
metallic multilayers. The phenomenon was first observed in France in 1988
[12], when changes in resistance with magnetic field of up to 70% were seen.
Compared to the small percent change in resistance observed in anisotropic
magnetoresistance, this phenomenon was truly giant magnetoresistance.

The resistance of two thin ferromagnetic layers separated by a thin nonmagnetic
conducting layer can be altered by changing the moments of the ferromagnetic
layers from parallel to antiparallel, or parallel but in the opposite direction.

figure

Figure 11. In a giant magnetoresistive sensor, the resistance of two
thin ferromagnetic layers separated by a thin nonmagnetic conducting layer
can be altered by changing the moments of the ferromagnetic layers from
parallel to antiparallel.

Layers with parallel magnetic moments will have less scattering at the interfaces,
longer mean free paths, and lower resistance. Layers with antiparallel magnetic
moments will have more scattering at the interfaces, shorter mean free paths,
and higher resistance (see Figure 11). For spin-dependent scattering to
be a significant part of the total resistance, the layers must be thinner
than the mean free path of electrons in the bulk material. For many ferromagnets
the mean free path is tens of nanometers, so the layers themselves must
each be typically <10 nm (100 Å). It is therefore not surprising that GMR was
only recently observed with the development of thin film deposition systems.

Various methods of obtaining antiparallel magnetic alignment in thin
ferromagnet-conductor multilayers have been discussed elsewhere [13-15].
The structures currently used in GMR sensors are unpinned sandwiches and
antiferromagnetic multilayers, although spin valves are of considerable
interest especially for magnetic read heads.

Unpinned sandwich GMR materials consist of two soft magnetic layers
of iron, nickel, and cobalt alloys separated by a layer of a nonmagnetic
conductor such as copper. With magnetic layers 4-6 nm (40-60 Å)
thick separated by a conductor layer typically 3-5 nm thick, there is
relatively little magnetic coupling between the layers. For use in sensors,
the sandwich material is usually patterned into narrow stripes. The magnetic
field caused by a current of a few milliamps per micrometer of stripe width
flowing along the stripe is sufficient to rotate the magnetic layers into
antiparallel or high-resistance alignment. An external field of 3-4
kA/m (35-50 Oe) applied along the length of the stripe is sufficient
to overcome the field from the current and rotate the magnetic moments of
both layers parallel to the external field. A positive or negative external
field parallel to the stripe will also produce the same change in resistance.
An external field applied perpendicular to the stripe will have little effect
due to the demagnetizing fields associated with the extremely narrow dimensions

figure

Figure 12. Voltage is plotted against applied field for a 2 mm wide stripe
of unpinned sandwich GMR material with 1.5 mA current. GMR = 5%.

of these magnetic objects. The value usually associated with the GMR effect
is the percent change in resistance normalized by the saturated or minimum
resistance. Sandwich materials have values of GMR typically 4%-9% and
saturate with 2.4-5 kA/m (30-60 Oe) applied field (see Figure 12).

Antiferromagnetic multilayers consist of multiple repetitions
of alternating conducting magnetic and nonmagnetic layers. Because multilayers
have more interfaces than do sandwiches, the size of the GMR effect is larger.
The thickness of the nonmagnetic layers is less than that for sandwich material
(typically 1.5-2.0 nm), and it is critical. For certain thicknesses
only, the polarized conduction electrons cause antiferromagnetic coupling
between the magnetic layers. Each magnetic layer has its magnetic moment
antiparallel to the moments of the magnetic layers on each side — exactly
the condition needed for maximum spin-dependent scattering. A large external
field can overcome the coupling that causes this alignment, and can align

figure

Figure 13. Resistance is plotted against applied field for a 2 mm wide
stripe of antiferromagnetically coupled multilayer GMR material. GMR = 14%.

the moments so that all the layers are parallel — the low-resistance state.
If the conducting layer is the wrong thickness, the same coupling mechanism
can cause ferromagnetic coupling between the magnetic layers with the result
of no GMR effect.

In the plot of resistance vs. applied field for a multilayer GMR material
shown in Figure 13, note the higher GMR value, typically 12%-16%, and
the much higher external field required to saturate the effect, typically
20 kA/m (250 Oe). Multilayer GMR materials have better linearity and lower
hysteresis than typical sandwich GMR material.

Spin valves, or antiferromagnetically pinned spin valves, are
similar to the unpinned spin valves or sandwich materials described above.
An additional layer of an antiferromagnetic material is provided on the
top or the bottom. The antiferromagnetic material such as FeMn or NiO couples
to the adjacent magnetic layer and pins it in a fixed direction; the other
magnetic layer is free to rotate. These materials do not require the field
from a current to achieve antiparallel alignment or a strong antiferromagnetic
exchange coupling to adjacent layers. The direction of the pinning layer
is usually fixed by elevating the temperature of the GMR structure above
the blocking temperature. Above this temperature, the antiferromagnet is
no longer coupled to the adjacent magnetic layer. The structure is then
cooled in a strong magnetic field that fixes the direction of the moment
of the pinned layer. Because the spin valve material looses its orientation
if heated above its blocking temperature, spin valve sensors must operate
below that temperature. Since the change in magnetization in the free layer
is due to rotation rather than domain wall motion, hysteresis is reduced.
Values for GMR are 4%-20% and saturation fields are 0.8-6 kA/m (10-80
Oe).

Spin valves are receiving considerable interest from the research community
due to their potential for use in magnetic read heads for high-density data
storage applications [16]. IBM has announced the introduction of a 16.8
GB hard drive with a spin valve read head. Bridge sensor designs using spin
valve materials have also been described in the literature [17] and rotational
position sensors in a product bulletin [18].

Spin-dependent tunneling (SDT) structures are very similar to
those shown in Figure 11. The difference is that an extremely thin insulating
layer is substituted for the conductive interlayer separating the two magnetic
layers. The conduction is due to quantum tunneling through the insulator.
The size of the tunneling current between the two magnetic layers is modulated
by the direction between the magnetization vectors in the two layers [19].
The conduction path must be perpendicular to the plane of the GMR material
since there is such a large difference between the conductivity of the tunneling
path and that of any path in the plane. Extremely small SDT devices measuring
several micrometers on a side with high resistance can be fabricated using
photolithography, which allows very dense packing of magnetic sensors in
small areas. Although these recent materials are very much a topic of current
research, values of GMR of 10%-25% have been observed. The saturation
fields depend on the composition of the magnetic layers and the method of
achieving parallel and antiparallel alignment. Values of saturation field
range from 0.1 kA/m to 10 kA/m (1-100 Oe), offering the possibility
of extremely sensitive magnetic sensors with very high resistance that promise
to be suitable for battery operation.

Colossal magnetoresistance. Scientists, to surpass the term giant,
have proceeded on to colossal magnetoresistive materials (CMR). Under certain
conditions these mixed oxides undergo a semiconductor-to-metallic transition
with the application of a magnetic field of a few teslas (tens of kilogauss).
The size of the resistance ratios, measured at 103%-108%, have generated
considerable excitement even though they initially required high fields
and liquid nitrogen temperatures. Academic researchers have recently developed
CMR materials that work at room temperature and have fabricated Wheatstone
bridge topography sensors out of these materials [20]. Although still a
long way from commercial applications, these CMR materials bear watching.

  GMR Circuit Techniques. The best use of GMR materials for magnetic
field sensors has so far been in Wheatstone bridge configurations, although
simple GMR resistors and GMR half bridges can also be fabricated. A sensitive
bridge can be made from four photolithographically patterned GMR resistors,
two of which are active elements. These resistors can be as narrow as 2
µm, allowing a serpentine 10 omegak resistor to be patterned in an area as small
as 100 µm2. The vary narrow width also makes the resistors sensitive only
to the magnetic field component along their long dimension. Small magnetic
shields are plated over two of the four equal resistors in a Wheatstone
bridge, protecting them from the applied field and allowing them to act
as reference resistors. Since they are fabricated from the same material,
they have the same temperature coefficient as the active resistors. The
two remaining GMR resistors are both exposed to the external field. The
bridge output is therefore twice the output from a bridge with only one
active resistor. The bridge output for a 10% change in these resistors is
~5% of the voltage applied to the bridge.

figure

Figure 14. GMR resistors can be configured as a Wheatstone bridge sensor.
In the flux concentrators shown here, D1 is the length of the gap between
the flux concentrators and D2 is the length of one flux concentrator.

Additional Permalloy structures are plated onto the substrate to act
as flux concentrators. The active resistors, placed in the gap between two
flux concentrators (see Figure 14), experience a field that is larger than
the applied field by approximately the ratio of the gap between the flux
concentrators, D1, to the length of one of the flux concentrators, D2. In
some sensors the flux concentrators are also used as shields by placing
two resistors beneath them, as is shown for R2 and R3. The sensitivity of
a GMR bridge sensor can be adjusted in design by changing the lengths of
the flux concentrators and the size of the gap between them. Thus, a GMR
material that saturates at ~300 Oe can be used to build different sensors
that saturate at 15 Oe, 50 Oe, and 100 Oe. For sensors with even more sensitivity,
external coils and feedback can be used to produce resolution in the 100
mA/m or millioersted range.

Smart sensors with sensing elements and associated electronics such as
amplification and signal conditioning on the same die are the latest trend.
GMR materials are sputtered onto wafers and can therefore be directly integrated
with semiconductor processes. The small sensing elements fit well with the
other semiconductor structures and are applied after most of the semiconductor
fabrication operations are complete. Because of the topography introduced
by the many layers of polysilicon, metal, and oxides over the transistors,
areas must be reserved with no underlying transistors or connections. These
areas will have the GMR resistors. The GMR materials are actually deposited
over the entire wafer, but the etched sensor elements remain only on these
reserved, smooth areas on the wafers [21].

Among the functions built into an integrated sensor are regulated voltage
or current supplies to the sensor elements; threshold detection to provide
a switched output when a preset field is reached; amplifiers; logic functions,
including divide-by-2 circuits; and various options for outputs. With these
elements, a 2-wire sensor can be designed that has two current levels — low
when the field is below a threshold and high when the field is above the
threshold.

Onboard sensor electronics can increase signal levels to significant
voltages with the least pickup of interference. It is always best to amplify

figure

Figure 15. The schematic and logic output characteristic of a commercial
integrated digital GMR sensor is illustrated.

low-level signals close to where they are generated. Converting analog signals
to digital (switched) outputs within the sensor is another way to minimize
electronic noise. The use of comparators and digital outputs makes the nonlinearity
in the output of sandwich GMR materials of less concern. Even the hysteresis
in such materials can be useful, since some hysteresis is usually built
into comparators to avoid multiple triggering of the output due to noise.
The circuit diagram and output characteristics for a commercial digital
GMR sensor are illustrated in Figure 15.

GMR materials have been successfully integrated with both BiCMOS and
bipolar semiconductor underlayers. The wafers are processed with all but
the final layer of connections complete. GMR material is deposited on the
surface and patterned. The next step is the application of a passivation
layer through which windows are cut to permit contact to both the upper
metal layer in the semiconductor wafer and to the GMR resistors. The final
layer of metal is then deposited and patterned to interconnect the GMR sensor
elements and to connect them to the semiconductor underlayers. This layer
also forms the pads to which wires will be bonded during packaging. A final
passivation layer is deposited, magnetic shields and flux concentrators
are plated and patterned, and windows are etched through to the pads.

GMR Sensor Applications

  Proximity Detection. A magnetic field sensor can directly detect
a magnetic field from a permanent magnet, an electromagnet, or a current.
Ferrous object presence sensing often entails the use of a biasing magnet
that magnetizes a ferromagnetic object such as a gear tooth. The sensor
then detects the combined magnetic fields from the object and the magnet.
To keep its direct influence on the target to a minimum, the magnet is usually
mounted on top of the sensor with its magnetic axis perpendicular to the
sensitive axis of the sensor. Centering the biasing magnet such that there

figure

Figure 16. In a GMR configuration designed for proximity detection, the
biasing magnet and sensor in an 8-pin package are shown in side view with
and without the presence of a ferrous object.

is little or no field in the sensitive direction of the sensor permits the
use of a reasonably large magnet. Occasionally, a spacer is used between
the sensor and the magnet to reduce the field at the sensor and thus the
criticality of magnet placement (see Figure 16).

Biasing magnets are customarily used only if the ferrous object is nearby.
Because the field from a dipole magnet falls off at the reciprocal of the
distance cubed, it is difficult to magnetize an object several meters away
with the field from a sensor-sized permanent magnet. In vehicle detection
and certain other applications, the Earth's field acts as a biasing magnet
and creates a magnetic signature from the parts of the vehicle that are
magnetized by the Earth's field. Vehicles can thus be counted and classified
as they pass over sensors in the road. Small, low-power GMR sensors and
their associated electronics, memory, and battery can be packaged in a low-profile
aluminum housing the size of a hand [22].

Currency detection is another application in which the biasing magnet
is not mounted on the sensor. The particles in the ink on many countries'
currency have ferromagnetic properties. Bills are passed over a permanent
magnet array and magnetized along their direction of travel. A magnetic
sensor located several inches away with its sensitive axis parallel to the
direction of travel can detect the remnant field of the ink particles. The
purpose of the biasing magnet in this case is to achieve a controlled orientation
of the magnetic moments of the ink particles, resulting in a maximum and
recognizable magnetic signature. Reversing the magnetizing field can actually invert the signature.

  Displacement Sensing. GMR bridge sensors can provide position
information from small displacements associated with actuating components

figure

Figure 17. GMR sensors are shown as positioned to measure displacement
relative to a permanent dipole magnet. The sensitive axes are indicated
and the component of field along the sensitive axes for the two sensors
is plotted.

in machinery, proximity detectors, and linear position transducers. Due
to the nonlinear characteristic of dipole magnetic fields produced by permanent
magnets, the range of linear output may be limited. In Figure 17, showing
the position and motion of two sensors with differing sensitive axis directions
relative to a cylindrical permanent magnet, the sensors' sensitive axes
are indicated by the two double-headed arrows. The rate of change of the
component of the magnetic field along the sensitive axis for each sensor
is shown superimposed on the line of motion. Note that the field for the
lower sensor changes direction and is negative in the center and positive
at both ends.

  Rotational Reference Detection. GMR sensors offer a rugged, low-cost
solution to rotational reference detection. High sensitivity and DC operation
afford the GMR bridge sensor an advantage over inductive sensors, which
tend to have very low outputs at low frequencies and can generate large
noise signals when subjected to high-frequency vibrations. Because GMR sensors
are field sensors, they do not measure the induced signal from the time
rate of change of fields as is the case with variable reluctance sensors.
The output from a GMR bridge sensor will have a minimum when the sensor
is centered over a tooth or a gap and a maximum when a tooth approaches
or recedes. The bridge sensor shown in Figure 16 is in position for angular
position sensing.

  Current Sensing. Current in a wire creates a magnetic field that
surrounds the wire or a trace on a PCB. The field decreases as the reciprocal
of distance from the wire; GMR bridge sensors can be used to detect this
field and thus either DC or AC currents. Bipolar AC current will be rectified

figure

Figure 18. A GMR bridge sensor is oriented as shown to detect the magnetic
field created by a current-carrying wire.

by the sensor's omnipolar sensitivity unless some method is used to bias
the sensor away from zero. Unipolar and pulsed currents can be measured
with good reproduction of fast rise time components due to the sensor's
excellent high-frequency response. Since the films are extremely thin, response
to frequencies up to 100 MHz is possible. Figure 18 shows a GMR bridge sensor
positioned to detect current in a wire. Placing a wire immediately over
or under the sensor will produce a field of ~0.080 A/m (1 mOe) per milliamp
of current. The sensor can also be mounted immediately over a current-carrying
trace on a PCB. High currents may require more separation between the sensor
and the wire to keep the field within the sensor's range. Low currents may
best be detected when the current is being carried by a trace on the chip
immediately over the GMR resistors.

Summary

Magnetic field detection has vastly expanded as industry has used a variety
of magnetic sensors to detect the presence, strength, or direction of magnetic
fields from the Earth, permanent magnets, magnetized soft magnets, and the
magnetic fields associated with current. These sensors are used as proximity
sensors, speed and distance measuring devices, navigation compasses, and
current sensors. They can measure these properties without actual contact
to the medium being measured and have become the eyes of many control systems.

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