Magnetic Field Sensing
Course: Nanomagnetic Materials and DevicesNAST 736
Center for Nanoscience & Technology
What are Sensors?
Physical Principles – How Do Sensors Work?
Need for Sensors
Choosing a Sensor
Market analysis and World wide Revenue
Types of Sensors
What are Sensors?
• American National Standards Institute (ANSI) Definition
– A device which provides a usable output in response to a
• A sensor acquires a physical parameter and converts it into a
signal suitable for processing (e.g. optical, electrical,
Biological & Chemical
Wave (amplitude, phase, polarization), Spectrum, Wave
Fluid Concentrations (Gas or Liquid)
Charge, Voltage, Current, Electric Field (amplitude,
polarization), Conductivity, Permittivity
Magnetic Field (amplitude, phase, polarization), Flux,
Refractive Index, Reflectivity, Absorption
Temperature, Flux, Specific Heat, Thermal Conductivity
Position, Velocity, Acceleration, Force, Strain, Stress,
• Amperes’s Law
– A current carrying conductor in a magnetic field experiences a force
• Curie-Weiss Law
– There is a transition temperature at which ferromagnetic materials exhibit
• Faraday’s Law of Induction
– A coil resist a change in magnetic field by generating an opposing
voltage/current (e.g. transformer)
• Photoconductive Effect
– When light strikes certain semiconductor materials, the resistance of the
material decreases (e.g. photoresistor)
Need for Sensors
• Sensors are omnipresent. They embedded in our bodies,
automobiles, airplanes, cellular telephones, radios, chemical
plants, industrial plants and countless other applications.
• Without the use of sensors, there would be no automation !!
– Imagine having to manually fill water bottles.
Market analysis - magnetic sensors
• 2005 Revenue Worldwide - $947M
• Growth rate 9.4%
“World Magnetic Sensor Components and Modules/Sub-systems Markets”
Frost & Sullivan, (2005)
Worldwide Revenue Forecast for Magnetic Sensors in Industrial
and Medical Applications
• Data storage
Bio-magnetic tag detection
Frietas, ferreira, Cardoso, Cardoso
J. Phys.: Condens. Mater 19, 165221 (2007)
Mars Global Explorer (1998)
“Biomagnetism using SQUIDs: Status and
Perspectives” Sternickel, Braginski, Supercond.
Sci. Technol. 19 S160–S171 (2006).
North Caroline Department of Cultural
Resources “Queen Anne’s Revenge”
shipwreck site Beufort, NC
• Magnetic sensors can be classified according to whether they measure the
total magnetic field or the vector components of the magnetic field.
• The techniques used to produce both types of magnetic sensors
encompass many aspects of physics and electronics.
• There are many ways to sense magnetic fields, most of them based on the
intimate connection between magnetic and electric phenomena.
Fig. 1. Estimate of sensitivity of different magnetic sensors. The symbols and GMN are used to
indicate the strength of the Earth’s magnetic field and geomagnetic noise, respectively.
The symbols E and GMN are used to indicate the strength of the Earth’s magnetic field
and geomagnetic noise, respectively.
Types of Magnetic Sensors
• Vector Magnetometers.
• Total Field Magnetometers.
– insensitivity to rotational vibrations.
– splitting between some electron or nuclear spin
energy levels is proportional to the magnitude of
the magnetic field over a field range sufficient for
Measures both the magnitude and the direction.
First, nearly all vector magnetometers suffer from noise,
especially 1/f noise (Geomagnetic Noise).
Solution- MEMS flux concentrator
which will shift the operating frequency above
the range where noise dominates.
Another major problem with vector magnetometers is that
they are affected by rotational vibrations.
• The principle of working Faraday’s law of induction.
• The search coil (also known as Inductive Sensor) is a sensor which
measures the variation of the magnetic flux.
• It is just coils wound around a core of high magnetic permeability.
• They measure alternating magnetic field and so can resolve changes in
magnetic fields quickly, many times per second.
Photograph of the search coil magnetometers used
on the THEMIS and Cluster/Staff mission
• The signal detected by a search-coil magnetometer depends
on the permeability of the core material, the area of the coil,
the number of turns, and the rate of change of the magnetic
flux through the coil.
• The frequency response of the sensor may be limited by the
ratio of the coil’s inductance to its resistance, which
determines the time it takes the induced current to dissipate
when the external magnetic field is removed. The higher the
inductance, the more slowly the current dissipates, and the
lower the resistance, the more quickly it dissipates.
• Detect fields as weak as 20 fT , and there is no upper limit to
their sensitivity range.
• Their useful frequency range is typically from 1 Hz to 1 MHz,
the upper limit being that set by the ratio of the coil’s
inductance to its resistance.
• They require between 1 and 10 mW of power.
In addition to this passive use, one can also operate a search coil in an
active mode to construct a proximity sensor.
A proximity sensor is a sensor able to detect the presence of
nearby objects without any physical contact.
A proximity sensor often emits an electromagnetic field or a beam
of electromagnetic radiation (infrared, for instance), and looks for
changes in the field or return signal.
Magnetic proximity fuze
It is a type of proximity fuze that initiates a detonator in a piece
of ordnance such as a land mine, naval mine, depth charge, or
shell when the fuse's magnetic equilibrium is upset by a
magnetic object such as a tank or a submarine.
Fig 2(a), a balanced inductive bridge
where an inductance change in one
leg of the bridge produces an out-ofbalance voltage in the circuit.
Fig. 2(b), incorporates a resonant
circuit where a change in inductance
results in a change in the circuit’s
Called eddy-killed oscillator, since
conductive materials near the active
coil will have eddy currents induced,
which will produce a mutual
inductance change in the circuit.
Ferrite cores are often used in this
approach because they can be
designed with the coil to offer a
temperature insensitive impedance.
Fig. 2(c) uses a single coil in the sensor
and the remainder of the electronics
is connected remotely.
• The fluxgate magnetometer consists of a ferromagnetic material wound
with two coils, a drive and a sense coil.
It exploits magnetic induction together with the fact that all ferromagnetic
material becomes saturated at high fields. This saturation can be seen in the
hysteresis loops shown on the right side of Fig. 4.
• When a sufficiently large sinusoidal current is applied to the
drive coil, the core reaches its saturation magnetization once
• As the core is driven into saturation, the reluctance of the
core to the external magnetic field being measured increases,
thus making it less attractive for any additional magnetic field
to pass through the core.
• This change is detected by the sense coil. When the core
comes out of saturation by reducing the current in the drive
coil, the external magnetic field is again attracted to the core,
which is again detected by the sense second coil.
• Thus, alternate attraction and lack of attraction causes the
magnetic lines of flux to cut the sense coil. The voltage output
from the sense coil consists of even-numbered harmonics of
the excitation frequency.
• The sensitivity of this sensor depends on the shape of the hysteresis curve.
For maximum sensitivity, the magnetic field magnetic induction (B-H)
curve should be square, because this produces the highest induced
electromotive force (emf) for a given value of the magnetic field. For
minimum power consumption, the core material should have low
coercivity and saturation values.
But they consume roughly five times more
power than proximity sensors.
Most of these achieve lower power
consumption by operating the sensor on a
minor hysteresis loop, thus not driving the
core from saturation to saturation.
• SQUID sensors
The most sensitive of all instruments for measuring a magnetic field at
low frequencies ( 1 Hz) is the superconducting quantum interference
device (SQUID) illustrated in Fig. 6.
It is based on the remarkable interactions of electric currents and
magnetic fields observed when certain materials are cooled below a
superconducting transition temperature. At this temperature, the
materials become superconductors and they lose all resistance to the flow
• For a large number of applications extremely small magnetic signals have to
be detected and accurately measured.
– Sensitivities of magnetic sensors:
Hall probes ~ mT
Flux gate sensors ~ nT
SQUIDs ~ fT
• SQUIDs allow to detect and characterize the magnetic signals which are so
small as to be virtually immeasurable by any other sensors.
• How sensitive? Allows to measure magnetic fields produced by the nerve
currents associated with the physiological activity of the human heart
(magneto cardiogram – MCG) or the human brain (magnetoencephalogram –
MEG); these signals have a typical strength ~ pT.
• Best of the SQUID sensors have energy sensitivity approaching Planck’s
• SQUIDs are the most sensitive detectors
of extremely small changes in magnetic flux.
Fluxes can be created by currents – therefore the most sensitive current sensors
SQUIDs - basic facts
SQUIDs combine the physical phenomena
of flux quantization in superconducting
loops and Josephson tunneling.
The Josephson effect refers to the ability of
two weakly coupled superconductors to
sustain at zero voltage a supercurrent
associated with transport of Cooper pairs,
whose magnitude depends on the phase
The maximum current which a Josephson
weak link can support without developing
any voltage across it is known as its critical
current Ic. When the current passed
through a Josephson weak link exceeds Ic, a
voltage appears across it
If a closed loop made of superconductor
magnetic field cannot enter the loop
(“ideal diamagnetism”). But if there is a
weak link flux enters the loop in quanta!
2.07 1015T m 2
Applications of SQUIDS
in semiconductor packages
Utilizes the Lorentz force on charge
predominantly use n-type silicon when
cost is of primary importance and
GaAs for higher temperature capability
due to its larger band gap.
Temperature Dependence of Hall Resistance and Hall Voltage
Different materials and different doping levels result in tradeoffs between sensitivity
and temperature dependence.
Applications of Hall Field Sensors
Response to South or North Polarity
where each rotation of the
motor shaft is to be detected
When ring magnet rotates w/
motor, South Pole passes the
sensing face of the Hall sensor
after each revolution.
Actuated when the South Pole
Deactuated when South Pole
moves away from sensor
Single digital pulse produced for
Gear Tooth Sensing
• Sense movement of
ferrous metal targets
detects change in
• Translates it into a
change in the sensor
output (high to low)
Typical application of AMR Sensors
Cylinder position sensing in pneumatic cylinders
Lid sensor for laptop computers
Position sensor for materials handling equipment (lift
• Blood analyzer
• Magnetic encoders
GMR is achieved by using a four layer structure that consists of two thin
ferromagnets separated by a conductor. The fourth layer is an
antiferromagnet that is used to pin (inhibit the rotation) the
magnetization of one of the ferromagnetic layers. The ferromagnet layer
that is being pinned is between the conductor and the antiferromagnet.
The pinned ferromagnet is called the hard ferromagnet and the unpinned
ferromagnet is called the soft ferromagnet. This structure is called a spin
• The difference in resistivity between the case when the
magnetizations are parallel to when they are antiparallel can
be as large as 12.8% at room temperature.
• To optimize the effect the layers must be a very thin, i.e.
about a nanometer thick.
• For the low field response of the sensor to be a linear function
of the field, it is necessary that the soft ferromagnetic have its
easy axis of magnetization in zero field perpendicular to the
magnetization of the pinned ferromagnet.
• The zero field orientation of the two magnetizations is
depicted in Fig. 11(a). The resistance is measured either in the
plane of the ferromagnetics or perpendicular to this plane.
• This multilayer geometry increases the percentage resistance
change because it increases the probability of spin flip
scattering by increasing the number of interfaces where spin
flip scattering occurs.
Total Field Magnetometers
• Total field magnetometers have the important advantage of
magnetometers use the fact that the splitting between some
electron or nuclear spin energy levels is proportional to the
magnitude of the magnetic field over a field range sufficient
Optically Pumped Magnetometer:
Based on the Zeeman effect. In 1896, the
Dutch physicist Zeeman showed that some
of the characteristic spectral lines of atoms
are split when the atoms are placed in a
magnetic field; one spectral line becomes a
group of lines with slightly different
wavelengths. The splitting is particularly
pronounced in alkali elements such as
cesium and rubidium.