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**: Magnetic properties can be partitioned into
intrinsic and extrinsic properties. The intrinsic properties include
saturation induction, Curie temperature, magnetocrystalline anisotropy,
etc. which are derived from the composition of a particular phase.
Extrinsic magnetic properties, like coercivity, induced anisotropy,
permeability, etc. depend on microstructure, geometry, etc which are
engineered. Many materials systems are innately suited to measure
magnetic fields over a particularly range of applied field amplitudes,
depending on the anisotropies of the material system and coercivities
and permeabilities that can be engineered for a particular system.
Further limitations on integration into device applications will include
functional temperature range, operating environment, susceptibility, and
tunability of these properties. Figure X.1 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.

**Field Response Mechanisms**: Different materials systems can respond
to the application of a magnetic field in a number of different ways,
which 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.![](media/image1.png){width="2.8826388888888888in"
height="2.386111111111111in"}

Table III 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..

1.  **Semiconductors**

> Conventional semiconducting 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 anomalous
> magnetoresitance, 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.![](media/image2.wmf){width="2.0375in"
height="1.7659722222222223in"}

**7. Nanocrystalline (Nanocomposite) Alloys**

> Nanocrytalline alloys consist of small nanocrystalline grains embedded
> in a host material (often an amorphous metal) system (see Fig. X.2).
> 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 magnetostriction
> coefficients and geometries to promote favorable domain configurations
> are 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 its own 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 magnetooptical 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.X.2). 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.

**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.

Materials Matrix

![](media/image5.wmf){width="8.00763888888889in" height="5.7125in"}

Table III. Summary of materials.

\[1\] A. Makino *et al.*, T. Mat. Trans. JIM, **32** 551 (1995).

\[2\] M. E. McHenry and M. A. Willard, Prog. Mat. Sci., **44** 291
(1999).

\[3\] Y.-Q. Li and R. C. O'Handley, J. Appl. Sens. Tech. **17**, 10
(2000).