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Low-Field Magnetic Sensing with GMR Sensors
Low-Field Magnetic Sensing with GMR Sensors
Carl H. Smith and Robert W. Schneider
Nonvolatile Electronics, Inc.
11409 Valley View Road
Eden Prairie, MN 55344, USA
Abstract -- Industry continues to reap the benefits of
solid state magnetic field sensing. Every day new
applications are found for solid state magnetic field
sensors due to their small size, low power. And
relatively low cost. The new frontier for these solid
state sensors is very low magnetic fields; the kinds
encountered when looking for geophysical anomalies,
either natural or man-made, various physiological
functions, metal defects, magnetic ink and minute
magnetic particles associated with immunoassay. In
the past, equipment to perform many of these
functions has required a substantial amount of power
and has been quite large. In addition, because of their
size, the equipment has become costly. Leveraging
on substantial government efforts, GMR technology is
being applied to these applications today and the
newer GMR technologies such as Spin Dependent
Tunneling (SDT) will make even more of these
applications possible. As with all of solid state
technology, many of the now difficult measurements
will become more commonplace and provide the
ingredients for more precise measurement, diagnosis,
and control. Several applications will be presented on
the use of individual sensors and arrays being used to
address some of these new areas.
INTRODUCTION
Magnetic field sensing in industry is often utilized for
control and measurement purposes linear and rotary
position sensing, gear tooth sensing, and current
sensing.
1
In these applications, relatively large
magnetic fields are used to avoid confusion with
background magnetic fields such as the Earths
magnetic field, fields from ferromagnetic objects, and
EMI. The fields detected are provided either from
permanent magnets or from currents in coils,
sometimes with soft magnetic cores. The size of these
magnetic fields is usually tens to hundreds of
oersteads (One Oe equals approximately 80 A/m.)
Since magnetic field sources are inherently dipole in
nature, they decrease with the inverse cube of
distance. Therefore, the fields from these sources are
relatively localized.
Despite the increased measurement difficulties
encountered with low fields, magnetic fields of less
than an Oe are gaining increasing attention in industry.
Compassing applications detect the components of the
Earths magnetic field (less than one-half Oe) to
determine direction relative to magnetic North.
Sensitive instruments which measure magnetic fields
or magnetic field gradients can detect the small
magnetic fields at considerable distances from soft
magnetic materials magnetized by the Earths
magnetic fields. These objects include motor vehicles,
buried iron surveying stakes, and lost wrenches.
There are other objects which produce very small
magnetic fields because they are small themselves.
The black ink in many currencies and other negotiable
documents contains small magnetic particles which act
as dipoles. Denomination determination and currency
validation can be based on the magnetic signature of a
bill passed close to a magnetic sensor. The more
sensitive the sensor, the larger the allowable head-to-
sensor gap. Eddy current sensing to detect flaws in
conducting materials or even differing conductivity in
the soil requires high-frequency, low-field sensors.
Solid state magnetic field sensors have an inherent
advantage in size and power when compared to
search coil, flux gate, and more complicated low-field
sensing techniques such as Superconducting Quantum
Interference Detectors (SQUID) and spin resonance
magnetometers. A solid-state magnetic sensor
directly converts the magnetic field into a voltage or
resistance with, at most a dc current supply. The
sensing can be done in an extremely small,
lithographically patterned area further reducing size
and power requirements. The small size of a solid
state element increases the resolution for fields that
change over small distances and allows for packaging
arrays of sensors in a small package. Figure 1 shows
a comparison in cost and power of several low field
sensors all designed with the same minimum field
resolution limited by thermal noise, 10
-8
Oe/
Hz.
Presented at Sensors EXPO-Baltimore May, 1999 2
10 100 1,000 10,000
Cost of sensor system ($)
P
ower
(mW)
10
100
1000
Low-field sensors (10
-8
Oe/
Hz)
SQUID
GMR
Fluxgate
Spin
Resonance
AMR
Figure 1. Comparison of several low-field magnetic
sensor technologies. Power and cost indicated
for each sensor. Size of circle indicates relative
size.
GMR TECHNOLOGY
Recent developments in thin-film magnetic technology
have resulted in films exhibiting a large change in
resistance with magnetic field. This phenomenon is
known as giant magnetoresistance (GMR) to
distinguish it from conventional anisotropic
magnetoresistance (AMR). Whereas AMR resistors
exhibit a change of resistance of less than 3 %,
various GMR materials achieve a 10 to 20 % change
in resistance. GMR films have two or more magnetic
layers separated by a non-magnetic layer. Due to
spin-dependent scattering of the conduction electrons,
the resistance is maximum when the magnetic
moments of the layers are antiparallel and minimum
when they are parallel. Various methods of obtaining
antiparallel magnetic alignment in thin ferromagnet-
conductor multilayers are discussed elsewhere.
2,3,4
A
brief overview is given in the section on GMR sensors
below. The structures currently being used in GMR
sensors are unpinned sandwiches, antiferromagnet
pinned spin valves, and antiferromagnetic multilayers.
Spin valves are gaining considerable interest for use
as magnetic read heads in computer hard disks.
5
Spin dependent tunneling (SDT) structures also exhibit
GMR. In these structures an insulating layer separates
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. 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 several
µm on a side with high resistance
can be fabricated using photolithography allowing very
dense packing of magnetic sensors in small areas.
These recent materials are a topic of considerable
research. Values of GMR of 10 to 30 % have been
regularly observed. The saturation fields depend upon
the composition of the magnetic layers and the
method of achieving parallel and antiparallel
alignment. Values of saturation field range from 0.1 to
10 kA/m (1.25 to 125 Oe) offering the possibility of
extremely sensitive magnetic sensors. The insulating,
tunneling layer provides inherently high resistance
sensors suitable for battery operation.
MAGNETIC FIELDS IN INDUSTRY AND MEDICINE
There are many places in industry and in medicine in
which magnetic fields the size of the Earths magnetic
field and smaller are of interest. The source of these
fields can be magnetized objects, electrical currents,
or the Earths field itself. The low-field aspect of these
applications can be due to the distance to a magnetic
object or the size of the magnetic object itself. All
magnetic sources produce a magnetic dipole field if
the observer is at a distance from the source. Dipole
fields decrease as the inverse cube of the distance
from the source. They are also proportional to the
volume of the source and to the maximum
magnetization at the source. A magnetized cylinder
whose diameter and length are one-half those of a
larger cylinder at any distance will have a magnetic
field 1/8 as strong as the field from the larger cylinder.
In addition, doubling the distance from a magnetized
cylinder will decrease the field to 1/8 the field at the
original position. Distance and miniaturization lead to
low fields.
Objects made from soft magnetic materials are easily
magnetized by relatively small magnetic fields
including the Earths magnetic field. These objects
can be as simple as small iron pipes used as
surveying markers or entire automobiles and trucks.
In one case the object is to locate a b