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GMR and SDT Sensors and Arrays for Low-Field Magnetic Applicat GMR and SDT Sensors and Arrays for Low-Field Magnetic Applications
Carl H. Smith and Robert W. Schneider
Nonvolatile Electronics, Inc.
11409 Valley View Road
Eden Prairie, MN 55344, USA
Abstract -- Because of their small size, low power,
and relatively low cost, solid state sensors that detect
magnetic fields lower than Earths have found
applications in all major industries. As these devices
become more sensitive, they are being used to
determine precise orientation and to detect natural
and man-made geophysical anomalies, various
physiological functions, metal defects, magnetic inks,
and minute particles associated with immunoassay.
New applications are being discovered daily as the
current technology involves limitations due to size,
weight, power consumption, and cost. As these
applications develop, there is an emerging
requirement to provide a matrix of low-field magnetic
sensors for magnetic field images of the subject
material or, as in the case of bioasssays, to handle
multiple variables simultaneously. Application areas
being investigated include currency detection (for
reading the whole bill), eddy-current mapping for
defect detection, geophysical anomalies, and
bioassays. Matrices are currently being designed with
several hundred magnetic pixels using both GMR
(giant magnetoresistive) and SDT (spin dependent
tunneling) materials. Several applications using these
new magnetic arrays are presented.
INTRODUCTION
Sensing of magnetic fields is often utilized in industry
for control and measurement.
1
In these industrial
applications, relatively large magnetic fields are used
to minimize the effect of background magnetic fields
such as the Earths magnetic field and fields from
adjacent ferromagnetic objects. Despite the
increased difficulties encountered with measuring 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 that measure magnetic fields or magnetic
field gradients can detect small magnetic fields at
considerable distances from soft magnetic materials
magnetized by the Earths magnetic fields. These
objects include vehicles, buried surveying stakes, and
lost wrenches. The black ink in many currencies and
other negotiable documents contains small magnetic
particles that act as dipoles. Currency validation
including country and denomination can be based on
the magnetic signature of a bill passed close to a
magnetic sensor. Greater sensitivity allows larger bill-
to-sensor gaps with less potential jamming of the bill
path. Eddy current sensing to detect flaws in
conducting materials or even differing conductivity in
the soil requires high-frequency, low-field sensors.
New concepts in bioassay require sensors that will
detect the presence of micrometer-sized
superparamagnetic particles.
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 graphical comparison in cost and
power of several low field sensors all designed with
the same minimum field resolution, 10
-8
Oe/ Hz,
limited by thermal noise.
Figure 1. Comparison of low-field magnetic sensors
designed with the same low field limit, 10
-8
Oe,
using several magnetic sensor technologies. The
size of circle indicates relative size. GMR and SDT Sensors and Arrays for Low-Field Magnetic Applications
Carl H. Smith and Robert W. Schneider
Page 2 of 12
Nonvolatile Electronics, Inc., 11409 Valley View Road, Eden Prairie, MN 55344, USA
Phone: 952-829-9217 FAX: 952-996-1600 Email: lowfield@nve.com
Increasingly, low-field applications require more
information than the magnetization at a single point or
along a single line as the sample passes the sensor.
In these cases arrays of detectors must be used.
Arrays can be used to build up an image of the
magnetic fields in 1, 2, or 3 orthogonal directions over
an extended area. An image can be generated by
passing a linear array of sensors over the object to be
imaged such as currency. In contrast, the information
from a two dimensional array of sensors tens of cm
on a side can be used to build an image of a buried
object without moving the array.
GMR sensors are ideal for array applications because
of their very small size and low power requirements.
Although presently available packaged GMR low-field
sensors can only be placed on approximately 6 mm
centers, bare GMR die can be mounted on substrates
with less than 1 mm spacing and wire bonded to pads
on the substrate. The multiple sensors can be
sequentially addressed and the output multiplexed
with on-board electronics to minimize the number of
connections to the sensor array. GMR sensors or
sensor dice can be packaged along three orthogonal
axes to give miniature 3-axis magnetic sensors. For
extremely high spatial resolution arrays, GMR sensors
with multiple sensing elements on each die can be
fabricated. Sensing elements can be spaced with
less than 10 µm on centers.
SDT/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 % and
greater 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
Spin dependent tunneling (SDT) structures are a
recent addition to the materials exhibition GMR. In
SDT structures an insulating layer separates two
magnetic layers. Conduction is allowed by quantum
tunneling through the insulator. The size of the
tunneling current between the two magnetic layers is
modulated by the angle between the magnetization
vectors in the two layers. Figure 2 shows the layers
and the structure of an SDT resistor manufactured
using thin-film deposition and photolithography.
Figure 2. The layers and structure of an SDT resistor.
Changes of resistance with magnetic field of 10 to 40
% have been observed. The field required for
maximum change in resistance depend upon the
composition of the magnetic layers and the method of
achieving antiparallel alignment. Values of saturation
field range from 0.1 to 10 kA/m (1.25 to 125 Oe)
offering at the low end, the possibility of extremely
sensitive magnetic sensors. Figure 3 compares the
output for various magnetoresistive sensors. Note the
significantly larger sensitivity for the bipolar SDT
sensor compared to the bipolar AMR sensor and the
unipolar GMR sensors. GMR and SDT Sensors and Arrays for Low-Field Magnetic Applications
Carl H. Smith and Robert W. Schneider
Page 3 of 12
Nonvolatile Electronics, Inc., 11409 Valley View Road, Eden Prairie, MN 55344, USA
Phone: 952-829-9217 FAX: 952-996-1600 Email: lowfield@nve.com
Figure 3. Comparison of sensors constructed from
various magnetoresistive materials. The inset box
shows response over a larger range of fields.
The insulating, tunneling layer provides inherently
high resistance sensors suitable for battery operation.
Extremely small SDT devices several
µ
m on a side
with high resistance can be fabricated using
photolithography allowing very dense packing of
magnetic sensors in small areas.
LOW 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 magneti