**Magnetic Field Sensors**

**Roadmap**

**November 2003**

**[TABLE OF CONTENTS]{.underline}**

  -----------------------------------------------------------------------
  List of Figures....................3

  List of Tables..................\...3

  List of Acronyms............\.....3

  List of Attendees ...............4

  Section 1 -- Introduction........5

  Section 2 -- Devices.........\...7

  Section 3 -- Applications.....18

  Section 4- Materials..........28

  Section 5 -- Outlook.......... .38

  Section 6 -References........40
  -----------------------------------------------------------------------

LIST OF FIGURES

  --------------------------------------------------------------- ----------
  Figure 1. Detectivity of various magnetic field sensors as a    .......7
  function of frequency (see text for a list of references).      

  Figure 2. Detectivity at 10 Hz as a function of sensor size.    .......9

  Figure 3. Field ranges for battlefield magnetic anomaly         ......20
  detection. Courtesy of A. Edelstein, Army Research Laboratory.  

  Figure 4. Medical applications for sensors. From                ......22
  "Biomagnetism," by Malvivuo.                                    

  Figure 5. Magnetic bead array counter (BARC) as implemented by  ......24
  NVE and NRL. The left panel shows the concept of tagging both   
  sensors and beads which target specific proteins. When either   
  the A or B target is present, the field from the bead will be   
  sensed magnetically at the corresponding site. The right panel  
  shows a picture of the actual device. An individual sensor is   
  shown in the zoom, and illustrates the serpentine design of the 
  spin valve GMR sensors. Images courtesy of NVE and NRL.         

  Figure 6. Non-destructive testing of VLSI chips. Left panel:    ......24
  Magnetic field above entire 20x20 cm RAM chip, showing the      
  fields from the current supply lines. Right panel: Current      
  vector calculated by inverting the fields. Images courtesy of   
  NIST, samples courtesy of Intel Corp.                           

  Figure 7. Rendering of magnetic field above a cassette tape.    ......29
  Courtesy of NIST and the FBI Audio Forensics Laboratory.        

  Figure 8. Comparison of a variety of different magnetic         ......29
  materials. Modified from Ref \[1\].                             

  Figure 9. TEM image showing two nanocrystals in an amorphous    ......33
  matrix. \[2\]                                                   

  Figue 10. Schematic of self-powered sensor concept Ref \[3\]    ......34
  --------------------------------------------------------------- ----------

LIST OF TABLES

  -------------------------------------------------- --------------------
  Table 1. Magnetic sensors matix                    ...............16

  Table 2. Applications requirements matrix          ...............26

  Table 3 Materials requirements matrix              ...............36
  -------------------------------------------------- --------------------

LIST OF ACRONYMS

  ------------------ ----------------------------------------------------
  AMR                Anisotropic magnetoresistance

  BARC               Bead array counter

  G                  Gauss

  GMI                Giant magnetoimpedance

  GMR                Giant magnetoresistance

  HDD                Hard disk drive

  MCG                Magnetocardiography

  MEG                Magnetoencepholography

  MO                 Magneto-optic

  MR                 Magnetoresistive

  MRAM               Magnetic random access memory

  Oe                 Oersted

  NDE                Nondestructive evaluation

  Oe                 Oersted

  SFA                Strategic Focus Areas

  T                  Tesla

  TMR                Tunneling magnetoresistance
  ------------------ ----------------------------------------------------

List of Workshop Participants and Contributors

  ----------------------- ----------------------- -----------------------
  Group                   Name                    Organization

  Planning Committee      David P. Pappas         NIST, EEEL

                          Fabio da Silva          NIST, EEEL

  Devices                 Chair - Mark Tondra     NVE

                          Secretary -- Stephen    NIST, EEEL
                          Russek                  

                          Tony Arrott             NIST, MSEL

                          Jack Rife               NRL

                          Purabi Mazumdar         NIST, ATP

                          X. Liu                  Micromagnetics

                          Barry Megdal            SHB

                          Casey Uhlig             NIST, PL

                          Don Porter              NIST, ITL

                          Solomon Woods           Neocera

                          Hong Wan                Honeywell

                          Alan Edelstein          Army Research Labs

                          Roger Koch              IBM

  Applications            Chair - Yacine          Quantum Magnetics
                          Dalichaouch             

                          Secretary - John        NIST, PL
                          Unguris                 

                          Konrad Bussman          NRL

                          Jim Daughton            NVE

                          Alan Edlestein          ARL

                          Charles Krafft          University of Maryland

                          John Matthews           Neocera

                          Ken Neimiller           NSA

                          Lloyd Whitman           NRL

  Materials               Chair - Michael McHenry CMU

                          Secretary - William     NIST

                          Thomas Ambrose          Seagate

                          Harsh Harshavardhan     Neocera

                          Frank Johnson           CMU

                          Chester Lo              Iowa State

                          Bob O'Handley           MIT

                          Martha Pardavi-Horvath  GWU

                          Robert McMichael        NIST, MSEL

                          Mark Stiles             NIST, PL

                          William Egelhoff, Jr.   NIST, MSEL

  Other workshop          William Anderson        NIST, EEEL (Director)
  attendees                                       

                          Lawrence H. Bennett     George Washington
                                                  University

                          Bob Celotta             NIST, Physics Lab

                          Edward Della Torre      George Washington
                                                  University

                          Vince Fratello          Integrated Photonics

                          Barbara Goldstein       NIST, EEEL

                          Can Korman              George Washington
                                                  University

                          Dave Krefft             NSA

                          Steve Licht             Integrated Photonics

                          Lin, XY                 Brown University

                          Bob Shull               NIST, MSEL

                          Cy Tamanaha             Naval Research Library

                          Venky Venkatessan       Neocera

                          Han Wen                 National Institutes of
                                                  Health

                          Dave Wollman            NIST, EEEL

                          Lloyd Whitman           NRL
  ----------------------- ----------------------- -----------------------

Section 1 - Introduction

Magnetic field sensors have had a significant impact over the past fifty
years in many different technological areas, ranging from data storage
to geomagnetic exploration. In 2003, the NIST leadership recognized that
this technology has important applications in its four strategic focus
areas (SFAs). These SFAs include homeland security, health care,
information technology, and nanotechnology. Therefore, a five-year
program was established to develop competence in the general area of
metrology for magnetic field sensors. In particular, low noise; high
sensitivity devices were identified as a key technology. In addition, a
significant thrust of the program was to cultivate interaction with
national industry and academia to support and transfer technology in
this area. Therefore, in the first year of the program a workshop was
held to gather information relevant to this field and develop a roadmap
that outlines where this field is going and where significant progress
can be made.

At the workshop, presentations were given that outlined many economic
benefits, types of devices, applications, and materials research related
to magnetic field sensors. These were discussed in separate breakout
sessions. In the course of the discussion, 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
various sensors with the specific applications.

A number of issues must be considered in choosing the materials for use
in each of the various sensors. For instance one must consider the
targeted range of fields to be measured, the required field sensitivity,
the sensitivity to both environmental and internal noise sources, the
type of environment in which the sensor is used, the power requirements
of the devices, and the read out mechanism for the device structures
employed, to name a few. The performance of a material generally must be
stated with reference to a particular geometry, with the magnetic domain
configuration for the material in that geometry being of paramount
importance to the ultimate sensitivity and noise spectrum.

For low noise sensors, the sources of increased noise with respect to
standard thin-film resistors need to be understood and decreased in
magnetic systems. This can potentially be accomplished with improved
device fabrication but will likely need improved materials performance
as well. Nanoscale anisotropy variations in materials will need to be
decreased while controlling the materials' susceptibility. Improved AMR,
GMR, and TMR materials would simplify incorporation of these new
materials into functioning devices. Magnetostrictive and composite
materials hold great promise for low power applications by making
self-powered sensors that require no external sources of power to
function. These materials also allow the manufacture of multifunctional
sensors capable of responding to more than one type of stimulus. For low
field applications the sensitivity of all of these materials will need
to be improved over the next several years.

The data obtained from the workshop and the various breakout sessions
was organized into a single document. This document is comprised of
three sections, Devices, Applications, and Materials, representing the
three breakout session. The sections are organized into a common
hierarchy, including the participants, strategy, a matrix showing the
relevant considerations, and a vision for the future.

[Section 2 - Devices]{.underline}

# I. Participants

  ----------------------------------- -----------------------------------
  Mark Tondra (chair)                 NVE

  Stephen Russek (secretary)          NIST, EEEL

  Tony Arrott                         NIST, MSEL

  Jack Rife                           NRL

  Purabi Mazumdar                     NIST, ATP

  X. Liu                              Micromagnetics

  Barry Megdal                        SHB

  Fabio da Silva                      NIST, EEEL

  Casey Uhlig                         NIST, PL

  Don Porter                          NIST, ITL

  Solomon Woods                       Neocera

  Hong Wan                            Honeywell

  Alan Edelstein                      Army Research Labs

  Roger Koch                          IBM
  ----------------------------------- -----------------------------------

# II. Strategy for Development of Magnetic Sensors

# Magnetic sensor technologies are extremely diverse with many different types of existing and proposed field sensors, each with its own set of advantages and disadvantages.^1-8^ Fig. 1 shows a plot of the magnetic field detectivity for some of the commercially available magnetic sensor technologies versus frequency. While field detectivity is an important parameter in optimizing field sensors, it is not the only and often not the most important metric in optimizing field sensors. Several general considerations were identified as underlying elements in advancing magnetic sensor technology. The first step in planning the advancement of magnetic sensor technology is to determine which areas of magnetic sensor applications will have the greatest impact. This step is critical since the numbers of existing and proposed magnetic sensor technologies are so large that is imperative to target critical applications, understand the application requirements, and target sensor development towards these applications. To pursue this targeted development it is important to clearly identify the current state-of-the-art in different sensor technologies and determine what advancements are theoretically and reasonably possible. 

In parallel with targeted sensor development, it is necessary to
continue to build understanding through basic research. Particularly, it
is essential to understand and control sources of noise and to
understand the fundamental limitations due to both noise and signal.
Research must provide clear pathways for applications developers to
improve signal and reduce noise. Continued work on materials will be
required for improved sensors. In particular, there is considerable room
for improvement in ferromagnetic materials.

Magnetic sensor development must be cognizant of market considerations.
The market drives the acceptable cost of a sensor as well as the total
allowable funds that are available to develop new sensor technologies.
The cost of making sensors includes much more than the cost of
fabricating the transducer. The costs include research and development,
testing, characterization, calibration, etc. For many of the
applications the market will not support the cost of research and
development, therefore, it may be necessary to leverage off of other
technologies, such as magnetic recording sensor technology.

Magnetic sensors can be distinguished by several criteria that determine
their range of applications. The important criteria include:

**Operation temperature:** room temperature or cryogenic. The best field
sensors (SQUIDs) require cryogenic (\< 77 K) operating conditions. The
required cryogenic system and thermal isolation prohibit use of such
sensors in many applications.

**Integrability:** Many applications can be enabled and made less costly
if the sensor can be integrated with conventional semiconductor
technology and into large arrays. Conversely, if this is not the case,
then the system costs are increased and applications are limited to only
a few input channels. The requirement of integrability favors the
development of thin film or semiconductor sensors.

**Type of field sensing:** vector or scalar, field or flux. Most sensors
sense the magnetic field in a certain direction (vector sensitivity). In
many applications this is desired, however, there are cases, such as
geologic imaging, when high sensitivity total field measurements are
required and scalar sensors perform much better than multi-axis vector
sensors. Magnetic field sensors can also be divided into field or flux
sensors. Search coils and SQUIDs are flux sensors, hence the field
sensitivity is proportional to size. This is advantageous for large
systems since the sensitivity can be greatly increased going to larger
input structures. For small scale sensing, field sensors are
advantageous since their sensitivity is, in general, constant as the
sensor size is scaled down.

**Frequency range:** low frequency 0.1 Hz -- 100 Hz or high frequency
(up to 10 GHz). Each application has a particular frequency range of
operation and each type of sensor has a particular frequency range of
operation. Magnetocardiography, for instance requires low frequency
sensors (0.1 to 10 Hz), while characterization of high-speed circuits
and data storage applications requires operation in the GHz range.

**Sensor size:** Different applications require different sensor sizes.
The smallest sensor size (\<1 μm) is required for data storage
applications and imaging fields above integrated circuits. Many
applications, such as magneto cardiogram and battlefield sensors the
size requirements are lenient (1 μm to 1 mm). In general, there is a
trade off between selectivity and sensor size with the detectivity
scaling as d^-2^ to d^-0.5^ for many types of sensors (see Fig. 2).

These general criteria listed above are used in the next section to
assess present and future magnetic sensor technologies.

III\. Magnetic Sensor and Properties Matrix

> **1. Search coil**
>
> Search coils are a mature, inexpensive technology that have great
> sensitivity. They are based on Faraday's law of induction and provide
> a voltage signal proportional to the time rate of change of flux
> through the coil. Search coils can have either an air core or a soft
> magnetic core. As a flux sensor, search coils can achieve high
> sensitivity only for large coils and their utility decreases on small
> size scales. Further, as a dynamic sensor (with a signal proportional
> to the rate of change of flux) stationary search coils cannot operate
> at DC and their frequency of operation is limited to \> 1 Hz. While
> rotating coils can be used for DC measurements, it involves additional
> mechanical complications.
>
> **2. Hall probes**
>
> Hall probes rely on voltages induced by Lorentz forces on charge
> carriers in semiconductor structures. Hall probes are a mature
> technology, and have good sensitivity over a wide range of device
> sizes (\<1 μm to 1 mm).^9^ The frequency range of operation is
> generally limited to \< 1 kHz due to signal-to-noise problems. Hall
> probes can be implemented in many semiconductor technologies from InAs
> to Si with a general trade-off between performance and material
> complexity. Hall probes have several advantages, including a large
> dynamic range, linear response, and scalability over a wide range of
> dimensions. Disadvantages include temperature dependence of the Hall
> coefficient and relatively small signal. Given the maturity of
> semiconductor technology there appears little room for improving the
> performance of traditional Hall sensors.
>
> **3. Fluxgate**
>
> Fluxgate magnetometers, originally developed in 1928, are based on the
> nonlinear response of a soft magnetic core. Typically, a soft magnetic
> core is driven by an AC signal at a frequency between 100 Hz and 10
> kHz and the magnetic field is determined by monitoring the second
> harmonic signal. Fluxgates provide extremely high sensitivity with the
> convenience of room temperature operation and relatively simple system
> requirements.^10^ Recent advances in understanding and eliminating
> noise in bulk flux gate sensors have improved the sensitivity to \~ 1
> pT/Hz^0.5^ .^11^ Bulk fluxgates, however, are hard to integrate into
> arrays or integrate on-chip with standard semiconductor electronics.
> The complete system tends to be bulky and require considerable power.
> Several attempts have been made to fabricate thin-film versions of
> fluxgates with limited success. Several theoretical models indicate
> that low noise thin-film fluxgates may not be possible.
>
> **4. Superconducting Quantum Interference Device (SQUID)**
>
> SQUIDs are, at present, the most sensitive magnetic field
> sensors.^12,\ 13^ The advent of high-temperature superconductors have
> pushed their operation temperatures up to \~77K and has greatly
> simplified cryogenic requirements.^14^ SQUIDs, as flux sensors, lose
> sensitivity when scaled down. However, given their high sensitivity,
> they can still perform well at dimensions down to a few micrometers.
> When coupled with flux guides, their effective spatial resolution can
> be sub-micrometer. SQUID technology is relatively mature with systems
> approaching their theoretical limits. Further improvements in SQUID
> technology can come from materials development that increases the
> operation temperature.
>
> **5. Anisotropic Magnetoresistance (AMR)**
>
> AMR refers to the change in the resistance of a magnetic material that
> is dependent on the relative orientation of the magnetization and the
> applied current.^15^ AMR is a mature technology that has seen wide
> applications for industrial sensors as well as in recording
> heads.^16,\ 17^ The devices are scalable with device sizes ranging
> from \< 1 μm to 100 μm. The systems can be integrated with
> semiconductor electronics and can be integrated into arrays. The
> sensitivity is limited to the sized of the anisotropic
> magnetoresistance (\< 5%). No theoretical limitations have been
> determined for this effect and it may be possible to find materials
> with larger AMR. AMR systems, which rely on single layer, single
> material systems, in general, have lower 1/f noise than the more
> complicated GMR and TMR systems. They can, therefore, in many
> instances, provide good signal to noise even though the intrinsic
> output is lower.
>
> **6. Giant Magnetoresistance (GMR)**
>
> The GMR effect, discovered in 1988,^18,19^ arises from the change in
> resistance due to the change in relative orientations of adjacent
> magnetic thin-film layers. GMR provides considerably higher resistance
> changes (up to 70%) than obtainable by AMR. The similarity to AMR in
> device configuration and system implementation has led to rapid
> incorporation of GMR sensors for applications in data storage,
> industrial sensors, and magneto-electronic devices such as
> magneto-isolators.^20-24^ One major limitation of GMR sensors is that
> high magnetoresistance has been obtained only in systems that require
> a high saturation field. Hence, devices with high GMR often have the
> same sensitivity as devices with lower GMR and lower saturation
> fields. The major challenge for GMR sensors is to understand and
> eliminate noise sources and bring the 1/f noise down to the same
> levels found in AMR sensors.^25^ Most GMR sensors have detectivity
> greater than 0.1 nT/Hz^0.5^. However, with a flux concentrator,
> detectivities of 10 fT/Hz^0.5^ at 1 Hz and 4.2 K have been
> reported.^26^
>
> **7. Tunneling Magnetoresistance (TMR)**
>
> TMR utilizes the change in tunneling resistance between two adjacent
> ferromagnetic electrodes.^27^ TMR provides high signal levels (up to
> 220%) and has the same scalability and integrability advantages that
> AMR and GMR devices have.^7,\ 8^ The advantage of TMR devices is that
> the larger change in resistance can be obtained in smaller fields and
> the resistance can be engineered over a large range while maintaining
> constant device geometry. Obtaining high-impedance sensors allows for
> low power operation, although these high-impedance devices will be
> Johnson noise limited and may not have as good signal to noise as
> lower impedance lower magnetoresistance devices. One major limitation
> of TMR devices is that the ∆R/R falls off with applied voltage so that
> the maximum signal from a single TMR device is limited to about 50 mV.
> A key issue in improving the performance of TMR devices is
> understanding and eliminating low frequency noise.^28-32^
>
> **8. Extraordinary Magnetoresistance (EMR)**
>
> A new class of magnetic sensors, extraordinary magnetoresistance
> (EMR), has been recently introduced that has many similarities with
> Hall sensors.^33-35^ These types of probes use careful design of
> electron trajectories in high-mobility semiconductors and close
> proximity of high conductivity shunts to manipulate the device
> resistance. This sensor has been scaled down to sub-micrometer
> dimensions and shown to have detectivities of \~ 1nT/Hz^0.5^.
>
> **9. Giant Magneto-Impedance (GMI)**
>
> Giant magneto-impedance is a new technology that, at present, is
> restricted to bulk applications using amorphous wires. GMI relies on
> the inductance change of a soft magnetic sample due to changes in the
> permeability in an applied magnetic field. GMI has many of the same
> advantages and disadvantages as flux-gate sensors. At present this
> technology is similar to fluxgate technology in that the sensor
> systems are large, hard to integrate into arrays and require
> considerable power.
>
> **10. Magneto-optic**
>
> Magneto-optic (MO) sensing is a relatively mature technology. For most
> implementations, MO sensors rely on the same type of magnetization
> rotation that occurs in AMR, GMR and most other sensors. The
> difference is that the detection mechanism is optical. This provides
> several advantages in applications in which electrical connection is
> undesirable, such as sensing magnetic fields from high-voltage
> transmission lines.
>
> **11. Nuclear and electron magnetic resonance**
>
> Nuclear resonance magnetometers are, at present, the most accurate
> magnetometers available. The combination of narrow NMR line widths,
> the ability to precisely measure frequencies and the absolute
> calibration make this sensor useful for metrology. The small
> bandwidth, restricted field range, large size, power consumption, and
> high cost make this type of sensor, at present, not practical in many
> applications.
>
> **12. Optical pumping**
>
> Recent advances in optical pumping have demonstrated field
> sensitivities down to 1 fT. The high resolution is due to the ability
> to precisely measure atomic transitions optically. The optical pumped
> magnetometers are scalar magnetometers suitable for geophysical
> imaging and biomedical applications such as magnetocardiograms. At
> present the optical cells and control hardware are large and bulky and
> emphasis is on developing compact low power systems.
>
> **13. Magnetostrictive / Magnetoelectric**
>
> Magnetoelectric sensors are a new sensor concept in which a
> magnetostrictive sensor is coupled with piezoelectric sensor. The
> applied field rotates the magnetic elements' magnetization and the
> associated strain is detected as a piezoelectric voltage. This
> technology has the advantage of being self-powered and has the
> potential of large output voltages (up 10 V/mT). The noise performance
> for magneto-electric sensors is still under investigation. Fabricating
> magnetoelectric sensors in an integrated format, on a silicon wafer
> with conditioning electronics, remains a challenge and requires a
> micromachined structure to allow magnetostrictive stress to be applied
> to the piezoresistive element.

The information from Section II and Section III is combined in order to
compare the properties directly. This matrix is shown in Table 1. It
shows the identified magnetic sensor technologies and their properties.
The magnetic sensor technologies are listed roughly in order of their
maturity, with the most mature technologies listed first.

Table 1 -- Magnetic sensors matrix

<table>
<colgroup>
<col style="width: 13%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 10%" />
<col style="width: 8%" />
<col style="width: 8%" />
<col style="width: 9%" />
<col style="width: 11%" />
<col style="width: 12%" />
<col style="width: 6%" />
</colgroup>
<tbody>
<tr class="odd">
<td>Sensor type</td>
<td><p>Sensitivity/</p>
<p>Field range</p></td>
<td><p>Frequency</p>
<p>Range</p></td>
<td><p>Operation</p>
<p>Temperature</p></td>
<td><p>Minimum sensor size/</p>
<p>Scalability</p></td>
<td>Vectorial/ scalar</td>
<td>Cost</td>
<td>Advantages</td>
<td>Disadvantages</td>
<td>Status</td>
</tr>
<tr class="even">
<td>Search coil</td>
<td>30 fT</td>
<td>&gt;1 Hz</td>
<td>RT</td>
<td>1 mm</td>
<td>vector</td>
<td>Moderate</td>
<td>Low cost for the sensitivity</td>
<td>Limited to &gt; 1Hz, sensitive to angular vibrations, Loose
sensitivity as decrease size</td>
<td>Mature</td>
</tr>
<tr class="odd">
<td>Hall probe/EMR</td>
<td>100 nT/ 1nT</td>
<td>&lt; 1kHz</td>
<td>RT</td>
<td>&lt; 1μm</td>
<td>Vector</td>
<td>Moderate</td>
<td>Large range, linear</td>
<td>Temperature dependent</td>
<td>Mature</td>
</tr>
<tr class="even">
<td>Fluxgate</td>
<td>1 pT/Hz)<sup>0.5</sup> @ 1Hz</td>
<td>&lt; 1kHz</td>
<td>RT</td>
<td>Loose S/N scaling down</td>
<td>Vector</td>
<td>Moderate</td>
<td>High sensitivity</td>
<td>Cost, size, energy consumption</td>
<td>Bulk filmv versions possible</td>
</tr>
<tr class="odd">
<td>SQUID</td>
<td>1 fT</td>
<td>&lt;1kHz</td>
<td>&lt;77K</td>
<td><p>&lt; 1μm</p>
<p>(system large)</p></td>
<td>Vector</td>
<td>Expensive</td>
<td>Sensitivity</td>
<td>Need for low temperature</td>
<td>mature</td>
</tr>
<tr class="even">
<td>AMR</td>
<td>50 – 100 pT</td>
<td>0-5 GHz</td>
<td>RT</td>
<td>&lt; 1μm</td>
<td>Vector</td>
<td>Moderate</td>
<td>Lower 1/f noise</td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td>GMR</td>
<td>20 nT</td>
<td>0- 5 GHz</td>
<td>RT</td>
<td>&lt; 1μm</td>
<td>Vector</td>
<td>Cheap</td>
<td>Low cost in large quantities</td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td>TMR</td>
<td>1 nT</td>
<td>0-1 GHz</td>
<td>RT</td>
<td>&lt; 1μm</td>
<td>Vector</td>
<td>Cheap</td>
<td>Large MR, low cost in large quantities</td>
<td>High 1/f noise, hysteretic</td>
<td></td>
</tr>
<tr class="odd">
<td>GMI</td>
<td>100 pT</td>
<td>&lt;500 kHz</td>
<td>RT</td>
<td>1 mm</td>
<td>Vector</td>
<td>Moderate</td>
<td></td>
<td>Cost, size, high power</td>
<td></td>
</tr>
<tr class="even">
<td>Magneto-optic</td>
<td>100 pT</td>
<td>0-5GHz</td>
<td>RT</td>
<td>. 1 mm</td>
<td>Vector</td>
<td>Moderate</td>
<td>No electrical connection</td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td>Resonance</td>
<td></td>
<td>DC</td>
<td>RT</td>
<td></td>
<td>Scalar</td>
<td>Expensive</td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td>Optical pumping</td>
<td>10-1000 fT</td>
<td>&lt; 100 Hz</td>
<td>RT</td>
<td>10 mm</td>
<td>Scalar or vector</td>
<td>Expensive</td>
<td>Insensitive to angular vibrations</td>
<td>Cost, power consumption, loss of sensitivity at higher
frequencies</td>
<td></td>
</tr>
<tr class="odd">
<td><p>Magnetostrictive/</p>
<p>Magneto-electric</p></td>
<td>1 nT</td>
<td></td>
<td>RT</td>
<td>10 μm</td>
<td>Vector</td>
<td>Moderate</td>
<td><p>Low power</p>
<p>Large output</p>
<p>voltage</p></td>
<td>Sensitive to vibrations</td>
<td></td>
</tr>
</tbody>
</table>

IV\. Improvement of magnetic sensors: vision for the future

There are several clear directions for improvement of magnetic sensor
technology to enable new applications. While SQUID sensors provide
adequate low field sensitivity for most applications it is difficult and
expensive to implement SQUID systems due to the cryogenic requirements.
Several approaches exist to make SQUID systems more useable including
implementation of high T~c~ materials and developing smaller integrated
cooling systems.

Fluxgate sensors have shown continual improvement in performance with
the use of new types of magnetic materials and geometries. Continued
improvement is required on miniaturizing flux gate sensors and making
them more integrable with signal processing and control circuits. It
remains a challenge to fabricate a thin film flux gate that has
comparable performance with bulk versions. GMI sensors have similar
requirements to fluxgates requiring improved materials, miniaturization,
and integrability.

While magnetic resonance and optical pumping magnetometers are extremely
accurate, measuring deviations in magnetic fields of less than 1 pT,
there is considerable room for improving these sensors to allow a
greater range of applications. More effort is required to reduce the low
field performance of these sensors, make the systems smaller and more
integrable in multi-sensor systems.

Magnetoresistive sensors (GMR, TMR, CMR) are already operating in the
0.1 nT regime, show great promise in that they can easily be integrated
with silicon processing, can be miniaturized, and can be fabricated
cheaply in large quantities. The challenge remains in increasing the
signal size and understanding and eliminating the low frequency noise
sources. There are good reasons to believe that this type of sensor
technology can be pushed into the 1 pT regime.

New sensor concepts, such as integrated magnetostrictive/piezoelectric
or EMR sensors, must continue to be explored. There is however, a need
for consistent testing and benchmarking of magnetic sensors so that the
relevant merits and weakness of each technology are quantified and
readily accessible.

[Section 3 - Applications]{.underline}

I. Participants

  ----------------------------------- -----------------------------------
  Yacine Dalichaouch -- chair         Quantum Magnetics

  John Unguris -- secretary           NIST, PL

  David Pappas                        NIST, EEEL

  Konrad Bussman                      NRL

  Jim Daughton                        NVE

  Alan Edlestein                      ARL

  Charles Krafft                      University of Maryland

  John Matthews                       Neocera

  Ken Neimiller                       NSA

  Lloyd Whitman                       NRL
  ----------------------------------- -----------------------------------

II\. Strategy for Identification of sensor 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.

1)  **Sensitivity** -- For the various applications, this parameter
    varies significantly. The units are typically reported in tesla, T,
    or gauss,G (1 G = 10^-4^ T). In free space, the conversion from
    gauss to oersted (Oe) is unity. In SI units, the corresponding units
    are ampere/meter, and can be found using the relation 1 Oe =1000/4π
    A/m. 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 many different 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 dewar) 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 linked
    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 were identified in the general discussion leading up
to the breakout sessions. These
included:![](media/image5.wmf){width="2.647222222222222in"
height="1.8in"}

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. 2). 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.

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. 4 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 a significant 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/image6.wmf){width="3.0416666666666665in"
    height="3.1847222222222222in"}

![](media/image7.wmf){width="1.7604166666666667in"
height="1.4277777777777778in"}![](media/image8.png){width="4.030555555555556in"
height="2.196527777777778in"}

3)  **Bead Array counters (BARC)** -- These devices represent direct
    competition for the fluorescence tag technique that is used in
    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. 5. 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.

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 reconfigurable
    power grid. The challenges of this are the harsh environment, with a
    linear response up to 100 kV/m 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. Pneumomagnetometry is a unique method of evaluating an
    iron 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.

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. 6 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 (tens of micrometers) 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.![](media/image9.wmf){width="1.76875in"
    height="1.5666666666666667in"}![](media/image10.wmf){width="1.7013888888888888in"
    height="1.5409722222222222in"}

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. 7. 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/image11.jpeg){width="2.4055555555555554in"
    height="1.8027777777777778in"}

After the breakout session, the applications and various requirements
discussed in sections II and III were compiled. This data is presented
in Table 2. From this table, it can be seen that there is a wide
disparity in the requirements for the various driver applications

Table 2 -- Applications requirements matrix

<table>
<colgroup>
<col style="width: 16%" />
<col style="width: 11%" />
<col style="width: 7%" />
<col style="width: 7%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 7%" />
<col style="width: 9%" />
<col style="width: 6%" />
<col style="width: 7%" />
<col style="width: 8%" />
</colgroup>
<tbody>
<tr class="odd">
<td><strong>Application</strong></td>
<td><p><strong>Sensitivity</strong></p>
<p><strong>/field range</strong></p></td>
<td><strong>Freq. Range</strong></td>
<td><strong>Oper. temp</strong></td>
<td><strong>Sensor size</strong></td>
<td><strong>Current sensor</strong></td>
<td><strong>Array vs. scan</strong></td>
<td><p><strong>Nano</strong></p>
<p><strong>vs. macro-scale</strong></p></td>
<td><strong>Cost</strong></td>
<td><p><strong>Power</strong></p>
<p><strong>Issues</strong></p></td>
<td><strong>Envir-onment</strong></td>
</tr>
<tr class="even">
<td><strong>BARC</strong></td>
<td>0.01 mT</td>
<td><p>DC-1</p>
<p>kHz</p></td>
<td>20 – 80 C</td>
<td>100x100 μm “area”</td>
<td>GMR, Hall, coils</td>
<td>Array</td>
<td></td>
<td><p>$1/</p>
<p>chip</p></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td><strong>Magneto-Cardiogram</strong></td>
<td>5 – 100 pT</td>
<td>0.1 – 40 Hz</td>
<td>4 – 77 K</td>
<td>10s μm2</td>
<td>SQUID</td>
<td>Single and arrays</td>
<td></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td><strong>Magneto-Encephalogram</strong></td>
<td>10’s fT</td>
<td><p>10</p>
<p>mHz- 1 kHz</p></td>
<td>4K</td>
<td>10 μm2</td>
<td>SQUID</td>
<td>Array</td>
<td></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td><strong>Forensic</strong></td>
<td>0.1 mT</td>
<td>Dc- 60 kHz</td>
<td>RT</td>
<td>1x1 cm</td>
<td>Garnets</td>
<td>Imag-ing</td>
<td></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td><strong>VLSI NDE Package vs die</strong></td>
<td>PT</td>
<td><p>DC – 100</p>
<p>MHz</p></td>
<td>77K- RT</td>
<td>1- 10 μm</td>
<td>SQUID/MR</td>
<td>Both</td>
<td><p>1- 10 μm, package</p>
<p>10 nm, device</p></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td><strong>Geophysical</strong></td>
<td>PT – 10 nT</td>
<td>0 – 300 kHz</td>
<td>RT</td>
<td>1x1 cm</td>
<td>Induction coils, MR</td>
<td>Both</td>
<td>Nano</td>
<td></td>
<td></td>
<td><p>RT-250C (bore</p>
<p>hole)</p></td>
</tr>
<tr class="even">
<td><strong>MRAM</strong></td>
<td>10 mT</td>
<td>10 – 500 mHz</td>
<td>-25 – 100C</td>
<td>100 nm</td>
<td>TMR</td>
<td>Array</td>
<td>Nano</td>
<td colspan="2">&lt;DRAM</td>
<td></td>
</tr>
<tr class="odd">
<td><strong>Data sensors (read heads)</strong></td>
<td>1 mT</td>
<td>500 mHz</td>
<td>60 C</td>
<td>500nm</td>
<td>GMR</td>
<td>Scan-ning</td>
<td>Nano</td>
<td>&lt;$2</td>
<td></td>
<td></td>
</tr>
<tr class="even">
<td><strong>Filtration monitors</strong></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr class="odd">
<td><strong>Battlefield</strong></td>
<td>1pT – 1nT</td>
<td>0.01 – 1 kHz</td>
<td>-20 – 40 C</td>
<td>3x3x3</td>
<td>AMR, FG, coils, OPM</td>
<td>Both</td>
<td></td>
<td>&lt;$1</td>
<td>&lt;50 mW</td>
<td></td>
</tr>
<tr class="even">
<td><strong>Naval situations</strong></td>
<td>1pT – 1nT</td>
<td>1 Hz- 5 kHz</td>
<td>5 – 40 C</td>
<td>3x3x3</td>
<td>Fluxgate, AMR</td>
<td>Both</td>
<td>Macro</td>
<td>&lt;$10</td>
<td>&lt;10 mW</td>
<td>Water</td>
</tr>
<tr class="odd">
<td><strong>Field sensing communications</strong></td>
<td></td>
<td>GHz</td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
</tr>
</tbody>
</table>

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: battlefield, magneto
cardiography, and bead arrays. 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.

**[Materials for magnetic field sensors]{.underline}**

I. Participants

  ----------------------------------- -----------------------------------
  Michael McHenry (chair)             CMU

  William Rippard (secretary)         NIST

  Thomas Ambrose                      Seagate

  Harsh Harshavardhan                 Neocera

  Frank Johnson                       CMU

  Chester Lo                          Iowa State

  Bob O'Handley                       MIT

  Martha Pardavi-Horvath              GWU

  Robert McMichael                    NIST, MSEL

  Mark Stiles                         NIST, PL

  William Egelhoff, Jr.               NIST, MSEL
  ----------------------------------- -----------------------------------

II\. Materials identification strategy

As discussed above there are a great number of potential uses for new
magnetic field sensing devices. The applications for these devices are
widespread, with the specific requirements of each device being
particular to its application. A number of issues must be considered in
choosing the materials for use in each of the various sensors. For
instance one must consider the targeted range of fields to be measured,
the required field sensitivity, the sensitivity to both environmental
and internal noise sources, the type of environment in which the sensor
is used, the power requirements of the devices, and the read out
mechanism for the device structures employed, to name a few. The
performance of a material generally must be stated with reference to a
particular geometry with the magnetic domain configuration for the
material in that geometry being of paramount importance to the ultimate
sensitivity and noise spectrum.

Potential soft magnetic materials include traditional crystalline and
amorphous ferromagnetic transition metals and alloys, high resistivity
oxides (ferrites) as well as more recently developed materials such as
nanocrystalline alloys, nanocomposities, and magnetic oxides, each of
which can be incorporated into a multilayered structure. A number of
different materials properties may be exploited in sensor application
including magnetoresistance, giant magnetoimpedence, magnetocaloric,
magneto-optical, and magnetostrictive effects. Each of these effects
will also have its own advantages and disadvantages for a particular
field sensing application and device structure, and each presently has
its own obstacles to be overcome for full integration into new field
sensing technologies. Below we will give a general overview of the
various materials systems with regards to both device structures and
their present limitations.

The materials will be distinguished by two broad features that determine
their potential integration into the different types of sensors and
applications discussed above. These features include:

**Magnetic Properties**: The magnetic properties of materials can be
partitioned into intrinsic and extrinsic properties. The intrinsic
properties include saturation induction, Curie temperature,
magnetocrystalline anisotropy, and are derived from the composition of a
particular phase. Some extrinsic magnetic properties, like coercivity,
induced anisotropy, permeability, depend on microstructure, while
others, such as shape and thickness, are engineered. Many materials
systems are innately well suited for measuring magnetic fields over a
specific range of applied fields. This depends on the anisotropies of
the material system, coercivities, and permeabilities that can be
engineered. Further limitations on integration into devices include the
functional temperature range, operating environment, susceptibility, and
tunability of these properties. Figure 8 shows a comparison of two
figures of merit, permeability and saturation induction, of a number of
the different magnetic materials discussed below. For many applications
the frequency dependent magnetic loss and the temperature dependence of
properties are also a consideration.\
![](media/image12.png){width="2.8826388888888888in"
height="2.386111111111111in"}

**Field Response Mechanisms**: Different materials systems can respond
to the application of a magnetic field in a number of different ways
that determine the read out mechanism appropriate for each system. These
include electrical responses, caloric response, optical responses, and
magnetostrictive responses. Together the combined magnetic response
mechanisms, read out schemes, and field sensitivity will determine the
applicability of individual materials systems to the different field
sensing devices. Perhaps the most difficult task in realizing the
materials required for new sensors is that efficacy of a particular
material system in meeting the requirements for a specific set of
devices can not be determined from simple magnetometry measurements
alone. Instead each must be characterized and optimized in order to meet
the specific requirements of active devices, which not only measure
magnetic fields but also magnetic noise and noise associated with the
device readout. A systems based approach to sensor development is thus
preferred.

III Specific Materials for Sensors

1.  **Semiconductors**

> Conventional semiconductor materials have long been used to make Hall
> sensors. The materials range from simple to compound semiconductors,
> with the latter typically giving improved performance. These materials
> are fairly mature and seemingly unlikely to provide dramatically
> improved magnetic sensors. More recently, magnetic semiconductors have
> been used to produce novel field sensors. These materials are
> relatively unexplored and may provide promise in new sensor
> technologies. Presently most of these materials are limited to low
> temperature operation.

2.  **Superconductors**

> SQUIDs are presently the most sensitive field sensors. Both
> conventional and high Tc materials have been demonstrated. This is a
> mature technology with the clear path towards increasing the utility
> of these sensors based on increased critical temperatures. As the
> sensitivity of the superconductor based sensors are close to their
> theoretical limits, it seems unlikely that other materials
> improvements are needed.

3.  **Transition Metals and Alloys (Bulk)**

> Transition metals and alloys have been used as field sensors since the
> advent of the compass. Modern devices typically rely on the
> anisotropic magnetoresistance, AMR, effect as the field sensing
> mechanism and are still being used as the sensing mechanism in tape
> read heads. The drawback of these systems is that the AMR, typically a
> few percent, is a relatively small effect. Presently the theoretical
> limit on the size of the AMR effect in materials is not understood.
> Hence while being a relatively mature technology there may be a great
> potential for improvement for these materials. However, since there
> has already been a great deal work done in investigating the AMR
> effect in both simple and complex transition metal alloys, it seems
> unlikely that great improvements will be made without the study of
> novel alloyed materials. These materials are also presently used as
> flux concentrators. Improvements in both susceptibility and saturation
> magnetization would increase the functionality of these materials as
> flux concentrators.

4.  **Single Crystal Metals**

> Single crystal metals have number of properties that make them
> attractive for use in magnetic sensors. They can have very high
> susceptibilities, theoretically they can have very high spin
> polarization at the Fermi level, and they should lack both the
> electronic and magnetic noise associated with defect states. Their
> major drawback is the difficulty associated with growing single
> crystal materials. A small density of defects or slight material
> contamination can remove the advantages of single crystal materials.
> Without a large technology need to drive mass production, they are
> likely to remain prohibitively expensive for incorporation into sensor
> devices.

5.  **Oxides**

The most commonly studied materials are ferromagnetic garnets, ferrites,
and perovskite structures which can be both conducting and insulating.
Depending on the particular class of materials under consideration,
these types of materials may be queried by either optical or electrical
methods. They are comparatively less sensitive than other materials or
have high sensitivity over a small range of temperatures. However, they
have great potential for incorporation into field sensors for niche
markets in which these are not limitations. Because of their high
resistivities, these are the materials of choice for high frequency
applications. At lower frequencies their relatively small inductions are
a drawback.

6.  **Multilayers**

In practice most electronic based sensors will incorporate multilayered
structures for improving the sensitivity of the device readout. Layered
materials can also be used to create multifunctional sensors and
manipulate the readout mechanism of the device. Multilayered materials
can also result in new interactions between magnetic layers such as
interlayer exchange coupling. However, undesired magnetostatic
interactions between the layers can also be problematic. Multilayered
structures can also be used to effectively magnetically bias a material
without the need for an externally applied field. Novel laminating
schemes can potentially enable the engineering of an array of magnetic
sensing devices. Lamination also provides the means for taking advantage
of the high moments of metals and the high resistivity of intervening
oxide layers.

**7. Nanocrystalline (Nanocomposite)
Alloys**![](media/image13.wmf){width="2.0375in"
height="1.7659722222222223in"}

> These alloys consist of small grains embedded in a host material
> (often an amorphous metal) system (see Fig. 9). They have a number of
> appealing aspects for their incorporation into ultra-low field
> sensors. These materials can be made to have large inductions and very
> large susceptibilities as well as tunable anisotropies while retaining
> very low losses. This makes them particularly attractive as the core
> material in flux gate sensors and as flux concentrators. The choice of
> materials, their magnetorestriction coefficients and geometries to
> promote favorable domain configurations is essential to many of these
> applications where a giant magnetoimpedance (GMI) effect is of
> interest. These materials can also potentially be incorporated into
> electrical sensors through the GMR or TMR effects, although electrical
> resistance noise associated with the relatively high resistivity of
> these materials may be problematic. These materials are relatively new
> and have a large phase space of materials combinations that remains to
> be explored. Development of novel nanocrystalline alloys has the
> potential to greatly impact magnetic sensors over the next several
> years.
>
> **8. Multifunctional Nanocomposites**

Another class of materials is that of multifunctional nanocomposites.
These materials typically consist of a magnetically sensitive material
embedded in a host material matrix. Because the material composition is
controlled on a length scale similar to magnetic length scales the
resulting materials can have properties more complicated than just the
average properties of the constituent components. Composite materials
can have a wide array of response mechanisms to magnetic fields
including magnetoelectrical, magnetooptical, magnetocaloric, and
magnetorestrictive responses. Furthermore, these composite materials
hold great promise in creating dual function sensors. By appropriate
choice of the materials used in creating a composite the sensors can be
made to be sensitive to multiple types of stimuli, creating hybrid
sensors. Each of these materials has advantages and drawbacks. In
general, the largest present impediment to their use in widespread
applications is that they presently lack sensitivity or ability to be
used in a wide range of environments.

**9. Rare-earth alloys**

Rare earth alloyed materials is a less mature class of materials but one
with novel material attributes. These are typically magneto-restrictive
and magneto-optical materials with some showing shape memory attributes.
These materials can be engineered to have either high or low
anisotropies making them making them attractive as both magnets as well
as sensors. By combining them in multilayered structures with, for
instance, piezoelectric materials, self-powered sensors can be made
(Fig.10). These materials would benefit from increased sensitivity and
increased operational temperature range. They can also be used in GMI
device structures. Comparatively little is known about the noise output
in these materials.

10. **Nanofabricated Materials**

> In principle, all of the discussed materials systems are
> nanofabricated in that their compositions and thicknesses are
> typically controlled at the nanoscale. However, in order to create
> both single devices and arrays, their physical geometries must also be
> controlled at the nanoscale as well. With fine enough control,
> structured materials can be induced to behave as single-domain
> particles, likely improving control and repeatability of device
> performance. Nanopatterning of materials also allows some control over
> the coercivity of the materials and can lead to the observation of new
> effects such as spin-transfer effects. Manufacturing technology
> presently allows device geometries to be controlled at this level for
> transition metal alloys, semiconductors, oxides, and some other
> materials. Fabrication techniques for newer materials such as
> amorphous and rare-earth alloys are less developed.

11. **Amorphous alloys**

> The materials properties of amorphous alloys such as anisotropies,
> magnetization, and coercivity can be tuned over a wide range, similar
> to transition metal alloys. They also offer the advantage of being
> able to create very smooth magnetic films and the absence of grain
> boundaries that can act as nucleation and pinning sites for magnetic
> domain walls. Hence they are a potential avenue to pursue for use in
> ultra-low noise sensors and the realization of nanostructured elements
> that behave as single-domain structures. They can also provide
> interesting electronic properties. For instance, B doped materials are
> presently in use in prototype MRAM configurations not only exhibit
> better switching behavior but also larger TMR signals. Novel materials
> and doping structures may prove very useful in future sensor devices.
> In addition to being incorporated into GMR and TMR devices they are
> also used in GMI sensors.

The data from sections II and III are compiled in Table 3 to show the
considerations needed for the various materials. This table shows a
matrix of materials that can be used in magnetic fields sensors along
with descriptions of their properties, potential applications and
drawbacks of the materials.

Table 3 -- Materials requirements matrix

![](media/image16.wmf){width="8.00763888888889in" height="5.7125in"}

IV\) Improvements and Goals

To reach the goals outlined in the previous sections new materials will
need to be engineered and integrated into device structures. For low
noise sensors, the sources of increased noise with respect to standard
thin-film resistors need to be understood and decreased in magnetic
systems. This can potentially be accomplished with improved device
fabrication but will likely need improved materials performance as well.
Nanoscale anisotropy variations in materials will need to be decreased
while controlling the materials susceptibility. Improved AMR, GMR, and
TMR materials would simplify incorporation of these new materials into
functioning devices. Magnetostrictive and composite materials hold great
promise for low power applications by making so called self-powered
sensors that require no external sources of power to function. These
materials also allow the manufacture of multifunctional sensors capable
of responding to more than one type of stimulus. For low field
applications the sensitivity of all of these materials will need to be
improved over the next several years.

**Section 5 -- Outlook**

It was found that there are many different devices, applications, and
materials relevant to the general field of magnetic field sensors. In
fact, there is often a wide disparity in the characteristics,
requirements, and properties of the sensors, applications, and
materials, respectively. This presents significant difficulty in
defining one unique direction to pursue at NIST.

However, this problem is mitigated to some extent by specifically
considering NIST's four strategic focus areas, Homeland Security, Health
Care, Information Technology, and Nanotechnology. In the case of the
first area, Homeland Security, low power, high sensitivity, room
temperature specifications are called for in the applications matrix
shown in Table 2. Comparing this to the sensor matrix on Table 1, it can
be seen there is not yet a specific sensor that meets all of these
specifications. Therefore, this is one direction that can immediately be
identified as important to pursue. Some of the low power sensors from
Table 1 include the magnetoresistive sensors. From Section 4, and
refering to the materials matrix, Table 3, it is clear that this
technology would require very high susceptibility materials coupled with
high MR ratio, pointing to the TMR devices. However, it is clear that
significant technical problems must be overcome in order for this
technology to be viable in these applications. These include, ost
significantly, the noise and dynamic range issues.

Similarly the Health Care SFA also calls out for high sensitivity;
however, the requirements for power and room temperature operation are
relaxed. Therefore, a less aggressive approach using mature
technologies, i.e., SQUID and fluxgate sensors, may be called for.

When considering the Information Technology SFA, the first application
that comes to mind is data storage, specifically the area of hard disk
drive (HDD) read heads. At present, work by industry in this area
probably represents the most intensive, coordinated research ever
conducted in the area of magnetic field sensors. In the HDD area, the
requirements for sensors are intricately coupled to the specific
product. While general characteristics, such as are listed in Table 1,
can be given, the exact specifications for a given device must be
determined by the industry. In addition, with data densities pushing
past 100 Gbit/in^2^, and still increasing, it is extremely hard to
compete with the rapid progress made by teams at large private labs.
That being said, however, it still remains incumbent on NIST to keep up
to date in this technology because it is likely that many of the
developments there can be transferred over to other areas. This holds
particularly true in the area of magneto-resistive sensors, where
significant advances are being reported on a monthly basis. In order to
keep up with this rapid pace, it is critical that NIST researchers
maintain a high profile at international conferences and workshops.

Nanotechology, the fourth NIST SFA, appears as an underlying theme
throughout the magnetic sensor development. This is illustrated in the
materials matrix, Table 3, where nearly every entry can be correlated
with nanoscale properties. This is true not only for the
nanocrystalline, nanocomposite, and nanofabricated materials, but also
for nanometer scale grains in amorphous alloys, spacers in multilayers,
clusters in magnetic semiconductors, and defects in single crystal
materials.

In addition, however, nanoengineering is particularly important when
designing sensors. For example, the dimensions for read heads in Hdds
are pushing the limits of optical lithography, and it can be expected
that the actual magnetic elements may soon be defined using e-beam
lithography. Because the magnetic properties are intricately linked to
the shape of the sample through the magnetic dipole energy, it is very
important that defects at the nanometer level are removed. In fact, due
to the long range nature of the dipole interaction, this consideration
of nm control holds for sensors over a wide range of size scales.

In summary, we have identified research into high sensitivity, low
power, room temperature magnetic field sensors as having impact in all
four SFA's. However, the Roadmap clearly shows that there are many
combinations of devices, applications and materials research that need
be investigated in order to meet all industrial demands. This is truly a
rich field of research.

Section 6. References

> ^1.^ J.E. Lenz., \"A Review of Magnetic Sensors,\" Proceedings of the
> IEEE, **78**, No. 6:973-989 (1990).
>
> ^2.^ Ciureanu and S. Middelhoek, *Thin Film Resistive Sensors*, New
> York: Institute of Physics Publishing, (1992).
>
> ^3.^ J.M. Janicke, *The Magnetic Measurement Handbook*, New Jersey:
> Magnetic Research Press., (1994).
>
> ^4.^ *Magnetic Sensors International Data Book.* Siemens
> Aktiengesellschaft, München, Germany, (1995).
>
> ^5.^ Pavel Ripka, Magnetic Sensors and Magnetometers (Artech House
> Remote Sensing Library) (2001).
>
> ^6.^ Sensors, Volume 5, Magnetic Sensors, Richard Boll (Editor),
> Kenneth J. Overshott (Editor), ISBN: 3-527-26771-9, December (1996).
>
> ^7.^ J.M. Daughton, **"**GMR and SDT sensor applications," IEEE Trans.
> Magn. **36** , 2773 - 2778,  (2000).
>
> ^8.^ J. M. Daughton, **"**Spin-dependent sensors," Proceedings of the
> IEEE **91**, 681 - 686, (2003 ).
>
> ^9**.**^R. S. Popovic : *Hall Effect Devices*, Adam Hilger (Bristol,
> New York, now IOP Institute of Physics Publishing) (1991).
>
> ^10.^ P. Ripka. \"Review of Fluxgate Sensors,\" Sensors and Actuators
> A, **33**:129-141 (1996).
>
> ^11.^ R. H. Koch and J. R. Rozen "Low-noise flux-gate magnetic-field
> sensors using ring- and rod-core geometries," Appl. Phys. Lett.
> **78**, 1897 (2001).
>
> ^12.^ H. Weinstock, *SQUID Sensors: Fundamentals, Fabrication and
> Applications* (Kluwer Academic, Dordrecht, Netherlands, (1996).
>
> ^13.^ J. Gallop, "SQUIDs: some limits to measurement," Supercond. Sci.
> Technol. 16, 1575 (2003).
>
> ^14.^ H. J. Barthelmess *et al*., "Low-noise biomagnetic measurements
> with a multichannel dc-SQUID system at 77 K," IEEE Trans. Appl.
> Superconduct.**11**, 657 (2001).
>
> ^15.^ T. I. McGuire and R. I. Potter, "Anisotropic Magnetoresistance
> in Ferromagnetic 3d Alloys," IEEE Trans. Magn. 11, 1010-1038, (1975).
>
> ^16.^ D. A. Thompson, L. T. Romankiw, and A. F. Mayadas, "Thin Film
> Magnetoresistors in Memory, Storage, and Related Applications," IEEE
> Trans. Magn. 11, 1039-1050, (1975).
>
> ^17.^ B.B. Pant, \"Magnetoresistive Sensors,\" Scientific Honeyweller,
> **8**, No. 1:29-34. (1987).
>
> ^18.^M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F.
> Petroff, P. Etienne, G. Cruezet, A. Friederich, and J. Chazelas, Phys.
> Rev. Lett. **61**, 2472 (1988).
>
> ^19.^G. Binasch, P. Grünberg, F. Saurenbach, and W. Zinn, Phys. Rev. B
> **39**, 4828 (1989).
>
> ^20.^ J. Daughton and Y. Chen \"GMR Materials for Low Field
> Applications,\" *IEEE Trans Magn,* **29**:2705-2710 (1993).
>
> ^21.^ J. Daughton et al. 1994. \"Magnetic Field Sensors Using GMR
> Multilayer,\" *IEEE Trans* *Magn*, Vol. 30:4608-4610
>
> ^22.^ J.K. Spong et al., \"Giant Magnetoresistive Spin Valve Bridge
> Sensor,\" IEEE Trans Magn, Vol. 32:366-371 (1996).
>
> ^23.^ Neil Smith, Alexander M. Zeltser, Danny L. Yang, and Peter
> Koeppe, „Very High Sensitivity GMR Spin-Valve Magnetometer, IEEE
> Trans. Magn. **33**, 3385 (1997).\
> ^24.^ M. J. Sablik, R. S. Beech, and M. Tondra, "Designing a giant
> magnetoresistance field sensor via inferences from a giant
> magnetoresistance hysteresis model",J. Appl. Phys. **87**, 5347
> (2000).
>
> ^25.^ H. T. Hardner, S. S. P. Parkin, M. B. Weissman, M. B. Salamon,
> and E. Kita , "1/f noise in giant magnetoresistive materials," J.
> Appl. Phys. **75**, 6531 (1994).
>
> ^26.^ Myriam Pannetier, Claude Fermon, Gerald Le Goff,, Juha Simola,
> Emma Kerr, "Femtotesla Magnetic Field Measurement with
> Magnetoresistive Sensors," Science **304**,1648 (2004).
>
> ^27.^ J. Moodera and L. Kinder.
> \"Ferromagnetic-insulator-ferromagnetic tunneling: Spin-dependent
> tunneling and large magnetoresistance in trilayer junctions,\" J Appl
> Phys **79**, No. 8:4724-4729 (1996).
>
> ^28.^Reed, D.S.; Nordman, C.; Daughton, J.M., ** "**Low frequency
> noise in magnetic tunnel junctions," IEEE Trans. Magn **37** , 2028 --
> 2030, (2001).
>
> ^29.^Wan Kyu Park, Jagadeesh S. Moodera, John Taylor, Mark Tondra,
> James M. Daughton, Andy Thomas, and Hubert Brückl, "Noise properties
> of magnetic and nonmagnetic tunnel junctions," J. Appl. Phys. **93**,
> 7020 (2003).
>
> ^30.^Peter K. George, Y. Wu, R. M. White, Ed Murdock, and Mark Tondra,
> "Shot noise in low-resistance magnetic tunnel junctions,**"** Appl.
> Phys. Lett. **80**, 682 (2002).
>
> ^31.^Xiaoyong Liu and Gang Xiao, "Thermal annealing effects on
> low-frequency noise and transfer behavior in magnetic tunnel junction
> sensors," J. Appl. Phys. **94**, 6218 (2003).
>
> ^32.^L. Jiang, E. R. Nowak, P. E. Scott, J. Johnson, J. M. Slaughter,
> J. J. Sun, and R. W. Dave, "Low-frequency magnetic and resistance
> noise in magnetic tunnel junctions," Phys. Rev. B **69**, 054407
> (2004).
>
> ^33.^ S. A. Solin, Tineke Thio, D. R. Hines, and J. J. Heremans
> Enhanced Room-Temperature Geometric Magnetoresistance in Inhomogeneous
> Narrow-Gap Semiconductors," *Science* **,** 1530-1532, (2000).
>
> ^34.^ S. A. Solin, D. R. Hines, A. C. H. Rowe, J. S. Tsai, and Yu A.
> Pashkin, "Nanoscopic magnetic field sensor based on extraordinary
> magnetoresistance," J. Vac. Sci. Technol. B **21**, 3002 (2003).
>
> ^35.^ C. H. Möller, O. Kronenwerth, Ch. Heyn, and D. Grundler
> "Low-noise magnetic-flux sensors based on the extraordinary
> magnetoresistance effect" Appl. Phys. Lett. **84**, 3343 (2004).
>
> ^36^ A. Makino *et al.*, T. Mat. Trans. JIM, **32** 551 (1995).
>
> ^37^ M. E. McHenry and M. A. Willard, Prog. Mat. Sci., **44** 291
> (1999).
>
> ^38^ Y.-Q. Li and R. C. O'Handley, J. Appl. Sens. Tech. **17**, 10
> (2000).