# Magnetic Sensor Applications Workgroup

# II. Driver Applications

Presentations were given that outlined many applications that are based
on magnetic field sensors. In the course of the workshop, several
specific "driver applications" were identified. These applications were
defined as those that would have a large impact in areas such as
homeland security, health care, and industrial applications. Once these
applications were identified, it was then possible to start evaluating
the specific criteria that they need to comply with. This will then
allow us to match up various sensors with the specific applications.

These criteria included:

1)  Sensitivity -- For the various applications, this parameter varies
    significantly. The units are typically reported in tesla, T, or
    oersted, Oe (1 Oe = 10^-4^ T). In SI units, these should be
    converted to ampere/meter using the relation 1 A/m=1000/4π Oe.
    Intrinsic to this parameter is the dynamic range that is required in
    order to achieve acceptable performance. This may imply the
    necessity of using shielded rooms or dynamic feedback. The main
    problem here is that ultra-high sensitivity devices will tend to
    saturate under ambient conditions.

2)  Frequency range -- Again, there are a wide range of frequency ranges
    that are needed for the various applications. Fortunately, however,
    most applications need a relatively small bandwidth. This leads to a
    natural separation of applications into low and high frequency. In
    general, active feedback can be used in the low frequency range (as
    is done for SQUID magnetometers), while band pass filters can be
    used for high frequency applications (e.g. magneto-resistive sensors
    for hard disk drives).

3)  Operation temperature -- Nearly all of the applications involved
    measurement of magnetic fields in a room temperature environment.
    However, the sensors themselves may require cooling. In this case,
    it is necessary for the cooling apparatus (typically a dewer) to be
    non-magnetic and allow the sensor to be close enough to the source
    of the magnetic field.

4)  Sensor size -- This specification refers to the volume that the
    package or individual sensor can be allowed to encompass. While
    there is a wide range of package sizes, it must also be noted that
    cooling apparatus can significantly increase the required volume.

5)  State-of-the-art sensors -- There has been significant development
    in most of these application areas. One of the main goals of this
    work is to identify areas where new, innovative technologies can
    have a significant impact. This may be either by supplanting the
    existing technology or enhancing the current methods, sensors, and
    associated materials.

6)  Arrays vs. single element and scanning techniques -- Many
    applications require arrays of sensors for field mapping or parallel
    detection methods. The actual spatial dimensions that the field
    needs to be mapped over also help define what sensor will be
    optimal. For example, very large arrays of sensors can be fabricated
    on wafers, thereby allowing for small-scale magnetic field mapping.
    However, for mapping magnetic fields in a battlefield or border
    situation may require arrays of individual sensors that are with
    telemetry.

7)  Nano-scale vs. macro-scale -- This parameter is similar to that of
    (4), however, it refers to the relevant dimensions of the actual
    sensor, independent of the packaging.

8)  Cost -- In general, low cost is important when transferring
    technology. However, some applications may be important enough that
    cost is no object. Examples might be in specific homeland security
    and health care applications.

9)  Power consumption -- This is most important in situations where
    either individual sensors are distributed with limited local sources
    of power, or where there is a very high density of magnetic sensors
    in a small area.

10) Environment -- In contrast to (3), this item includes other aspects
    such as corrosion and high temperature environments. These must be
    considered when comparing various technologies.

# III. Magnetic Sensor Applications Worksheet

Driver applications

1)  Battlefield/unattended ground sensors -- This is an application with
    possibly the most stringent requirements. These include operation
    with high field sensitivity in relatively high ambient fields, low
    frequency, low power consumption, and low cost. In addition, the
    environment that these sensors operate in is highly variable (see
    Fig. X.1). All possible types of terrain, weather conditions, and
    deployment methods must be accommodated. In general, distributed
    arrays of sensors with telemetry to a central station would have the
    most impact. Finally, the reliability of these devices is critical
    because lives and homeland security are at stake. Therefore, this is
    a high-risk, high-payoff
    application.![](media/image1.wmf){width="3.1597222222222223in"
    height="2.1416666666666666in"}

2)  Magneto-cardiography -- It is well known that bio-magnetic signals
    from both the heart (MCG) and the brain (MEG) can be detected with
    ultra-high sensitivity magnetometers. Related applications include
    early detection fetal MCG. Again, these signals require very low
    frequency operation. Fig. X.2 illustrates this, and also compares
    various sensor response curves with the background magnetic noise.
    At present, these signals are typically measured using
    superconducting quantum interference devices (SQUIDs). These devices
    require cryogens, and are therefore limited in how close to the
    subject they can get and how many can be deployed effectively in an
    array. This is not such a critical concern, however, due to the fact
    that the magnetic fields vary relatively slowly over the regions to
    be measured. In general, the cost of these systems is not as
    critical as identifying the specific impact and mode of operation.
    However, it can be seen that relatively modest improvements room
    temperature sensors such as fluxgates or induction coils could allow
    for these techniques to be applied to MCG. In addition, if
    magneto-resistive (MR) sensors are integrated with flux guides, they
    may also have an impact. The goal would be to develop a real-time
    system with at least 1 cm source localization capability. The
    advantages of these devices would be low cost screening systems for
    routine checkups, arrhythmia diagnosis, and advance myocardial
    electrical diagnosis. This is a relatively high payoff, medium risk
    application.![](media/image2.wmf){width="3.0416666666666665in"
    height="3.1847222222222222in"}

3)  Bead Array counters (BARC) -- These devices are direct competition
    with the fluorescence tag technique that us used in many health care
    assay applications. The sensors needed for these must be packed into
    a very high density array, and single bead detection is desirable.
    An example of this technology is shown in Fig. X.2. The field
    sensitivity is modest, and medium frequency operation (\~1 kHz) is
    acceptable. Existing devices are currently limited by the
    micro-fluidics and the chemistry of tagging, however, as these
    issues are worked out, the opportunity for magnetic sensors to make
    a high impact is there. This is a high payoff, relatively low risk
    application.![](media/image3.wmf){width="1.7604166666666667in"
    height="1.4277777777777778in"}![](media/image4.wmf){width="4.375694444444444in"
    height="2.3965277777777776in"}

Other identified applications

1)  Sensors for measuring currents on high tension wires -- Reliability
    of the power grid are dependent on adequate distributed diagnostics.
    Magneto-optic sensors for this application would allow reduced costs
    significantly reduced size (3% of current technologies). These are
    installable without breaking the circuit (as opposed to technologies
    currently in use) and can be safely queried by fiber optic means.
    There are no conductors and the fiber cables can be permanently
    installed to the ground. Such pervasive sensors are an enabling
    technology of an autonomously reconfureable power grid. The
    challenges of this are the harsh environment, with a linear response
    to 100 kV over a temperature range from subzero to 160 C . The
    sensor material (typically ferromagnetic garnets) and readout system
    must be able to deal with this. Improvements in sensitivity at low
    currents enable opportunities in "smart" buildings, optical power
    metering, and shipboard sensors. Coordination with voltage sensors
    can yield a complete power sensor. Moderate field sensors are
    inherently required. The timeline for development will depend on
    when mil-spec qualified prototypes could be available. This
    typically takes 1 -- 2 years. Temperature independent and high
    sensitivity materials can be developed another 1 -- 2 years down the
    line. Qualification by power companies will depend on their own
    agendas, but will be accelerated by military applications.

2)  Bead tracking in-vivo - This application has been identified by
    researchers at NIH as being a very promising avenue of studying
    blood flow in capillaries and organs. This can be used, for example,
    in conjunction with other imaging techniques to understand the
    impact of stroke on cardiac function and the electrical circuits in
    the heart. Pneumo-magnetometry -- Magnetometry is a unique method of
    evaluating ron overload in organs such as the liver and lungs. It is
    non-invasive and real-time, showing the integrated signal from the
    area under
    evaluation.![](media/image5.wmf){width="2.886111111111111in"
    height="2.5618055555555554in"}![](media/image6.wmf){width="2.75in"
    height="2.488888888888889in"}

3)  Non-destructive evaluation (NDE) for micro-electronics -- NDE using
    magnetic field mapping to obtain source current distribution can be
    conducted at two scales -- either at the macro-scale, to evaluate
    the current vectors in a circuit, or at the nanoscale, to evaluate
    the current distribution in individual traces on a die. In an
    example of the first case, Fig. X.3 shows the magnetic field map
    above a RAM chip. The perpendicular component of the field is
    measured. This allows for de-convolution of the currents in the two
    in-plane directions, as shown in the right hand panel. In this
    application, arrays of relatively large (10's of micro-meters) scale
    sensors can be used. This is because the sensors are typically
    scanned at distances at least the same distance from the source,
    i.e. from the back-side of a flip-chip package, circuit board, or
    other electronics package. In the second case, where the current
    distribution in individual traces is of interest, nano-scale sensors
    that come in direct contact are needed. These sensors can be highly
    shielded, and good results have been obtained using off-the-shelf
    components designed for hard disk drive readers.

4)  Magnetic media scanning and characterization -- This application
    includes testing for advanced magnetic data storage media in hard
    disk drives as well as forensic analysis of conventional media from
    cassettes and other ubiquitous magnetic data storage media. In the
    first case, both of-the-shelf and custom read/write assemblies have
    been used to study the efficacy of nano-scale, fine-grained and
    patterned media. In general, this application requires scanning of a
    single, nano-scale sensor. In the second case, arrays of sensors
    that can map out wide swaths of data from audio and video storage
    media are needed. This is demonstrated in the image from a cassette
    tape, shown in Fig. X.Y. The image is approximately 4 mm long and 4
    mm wide, representing 0.1 seconds of sound. The ridge along the left
    side is a ½ width (mono) erase head stop event, and the washboard on
    the top right is a test pattern written with a dual ¼ width (stereo)
    head. The magnetic texture of the tape and the details of the write
    stop even are significant in a forensic evaluation, and this image
    represents details that are orders of magnitude better than can be
    obtained by traditional magnetic powder/fluid techniques. The main
    obstacle is the need to screen many hours of tape in real time with
    high resolution.\
    ![](media/image7.jpeg){width="5.788194444444445in"
    height="4.336805555555555in"}

**IV. Development of applications: vision for the future**

Many uses for advanced sensors were put forward at the meeting. However,
the point was made that only a few specific applications may drive a
given technology to its ultimate potential. Given that consideration,
the three main driver applications were identified. These applications
fit in the general areas of homeland security and health care, and are
therefore good matches with NIST Strategic Focus Areas (SFA's). Other
identified applications also fit well with the Information Technology
SFA, and are compatible with existing funded programs at NIST.

The general approach to this work will therefore be to focus on these
areas of research. We will do this by identifying specific sensor
technologies and materials systems that will enable the driver
technologies.