Magnetic Field Sensing

Submitted toDr.A.Kasi.VishwanathAssociate ProfessorCenter for Nanoscience & Technology Zaahir Salam

Submitted by

Course: Nanomagnetic Materials and Devices-NAST 736

Contents

• What are Sensors?

• Detectable Phenomenon

• Physical Principles – How Do Sensors Work?

• Need for Sensors

• Choosing a Sensor

• Market analysis and World wide Revenue

• General Applications

• Types of Sensors

What are Sensors?

• American National Standards Institute (ANSI) Definition

– A device which provides a usable output in response to aspecified measurand.

• A sensor acquires a physical parameter and converts it into asignal suitable for processing (e.g. optical, electrical,mechanical)

Sensor

Input Signal Output Signal

Detectable Phenomenon

Stimulus Quantity

Acoustic Wave (amplitude, phase, polarization), Spectrum, Wave

Velocity

Biological & Chemical Fluid Concentrations (Gas or Liquid)

Electric Charge, Voltage, Current, Electric Field (amplitude,

phase,

polarization), Conductivity, Permittivity

Magnetic Magnetic Field (amplitude, phase, polarization), Flux,

Permeability

Optical Refractive Index, Reflectivity, Absorption

Thermal Temperature, Flux, Specific Heat, Thermal Conductivity

Mechanical Position, Velocity, Acceleration, Force, Strain, Stress,

Pressure, Torque

Physical Principles• Amperes’s Law

– A current carrying conductor in a magnetic field experiences a force (e.g. galvanometer)

• Curie-Weiss Law– There is a transition temperature at which ferromagnetic materials exhibit

paramagnetic behavior

• 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, chemicalplants, industrial plants and countless other applications.

• Without the use of sensors, there would be no automation !!

– Imagine having to manually fill water bottles.

Choosing a Sensor

8

Market analysis - magnetic sensors• 2005 Revenue Worldwide - $947M

• Growth rate 9.4%

TypeApplicationHT SQUID,

$0.38M

LT SQUID,

$5.3M

Magnetometer

$5.5M

Compass,

$4.8M

Position

sensor,

$3.4M

GMR,

$40.2M

AMR

$121.6M

Hall

element,

$94.7M

Hall IC,

$671.2M

“World Magnetic Sensor Components and Modules/Sub-systems Markets”Frost & Sullivan, (2005)

Medical

$24M

Other

$11M

Aerospace

Defense

$37M

Industrial,

$156M

Auto

$338M

Computer,

$380M

Research,

$0.8M

NDE $0.1M

Worldwide Revenue Forecast for Magnetic Sensors in Industrial and Medical Applications

Applications

• Health Care

• Geophysical

• Astronomical

• Archeology

• Non-destructive evaluation (NDE)

• Data storage

Bio-magnetic tag detection

Frietas, ferreira, Cardoso, CardosoJ. Phys.: Condens. Mater 19, 165221 (2007)

Mars Global Explorer (1998)

Magneto-encephalography

Magneto-Cardiography

“Biomagnetism using SQUIDs: Status andPerspectives” Sternickel, Braginski, Supercond.Sci. Technol. 19 S160–S171 (2006).

North Caroline Department of Cultural Resources “Queen Anne’s Revenge”

shipwreck site Beufort, NC

Magnetic RAM

Introduction• 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 sensorsencompass many aspects of physics and electronics.

• There are many ways to sense magnetic fields, most of them based on theintimate connection between magnetic and electric phenomena.

Fig. 1. Estimate of sensitivity of different magnetic sensors. The symbols and GMN are used toindicate 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 fieldand 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 magnetometry.

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 thatthey are affected by rotational vibrations.

Vector Magnetometers

Search-Coil Magnetometer

• The principle of working Faraday’s law of induction.

• The search coil (also known as Inductive Sensor) is a sensor whichmeasures 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 inmagnetic 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 dependson the permeability of the core material, the area of the coil,the number of turns, and the rate of change of the magneticflux through the coil.

• The frequency response of the sensor may be limited by theratio of the coil’s inductance to its resistance, whichdetermines the time it takes the induced current to dissipatewhen the external magnetic field is removed. The higher theinductance, the more slowly the current dissipates, and thelower the resistance, the more quickly it dissipates.

• Detect fields as weak as 20 fT , and there is no upper limit totheir 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’sinductance 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 pieceof ordnance such as a land mine, naval mine, depth charge, orshell when the fuse's magnetic equilibrium is upset by amagnetic object such as a tank or a submarine.

Fig 2(a), a balanced inductive bridgewhere an inductance change in oneleg of the bridge produces an out-of-balance voltage in the circuit.

Fig. 2(b), incorporates a resonantcircuit where a change in inductanceresults in a change in the circuit’sresonant frequency.

Called eddy-killed oscillator, sinceconductive materials near the activecoil will have eddy currents induced,which will produce a mutualinductance change in the circuit.Ferrite cores are often used in thisapproach because they can bedesigned with the coil to offer atemperature insensitive impedance.

Fig. 2(c) uses a single coil in the sensorand the remainder of the electronicsis connected remotely.

Fluxgate Magnetometer

• The fluxgate magnetometer consists of a ferromagnetic material woundwith two coils, a drive and a sense coil.

It exploits magnetic induction together with the fact that all ferromagneticmaterial becomes saturated at high fields. This saturation can be seen in thehysteresis loops shown on the right side of Fig. 4.

• When a sufficiently large sinusoidal current is applied to thedrive coil, the core reaches its saturation magnetization onceeach half-cycle.

• As the core is driven into saturation, the reluctance of thecore to the external magnetic field being measured increases,thus making it less attractive for any additional magnetic fieldto pass through the core.

• This change is detected by the sense coil. When the corecomes out of saturation by reducing the current in the drivecoil, 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 themagnetic lines of flux to cut the sense coil. The voltage outputfrom the sense coil consists of even-numbered harmonics ofthe 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 inducedelectromotive force (emf) for a given value of the magnetic field. Forminimum power consumption, the core material should have lowcoercivity and saturation values.

But they consume roughly five times morepower than proximity sensors.

Most of these achieve lower powerconsumption by operating the sensor on aminor hysteresis loop, thus not driving thecore from saturation to saturation.

Superconductor Magnetometers• SQUID sensorsThe most sensitive of all instruments for measuring a magnetic field atlow frequencies ( 1 Hz) is the superconducting quantum interferencedevice (SQUID) illustrated in Fig. 6.

It is based on the remarkable interactions of electric currents andmagnetic fields observed when certain materials are cooled below asuperconducting transition temperature. At this temperature, thematerials become superconductors and they lose all resistance to the flowof electricity.

• For a large number of applications extremely small magnetic signals have tobe detected and accurately measured.– Sensitivities of magnetic sensors:

Hall probes ~ mTFlux gate sensors ~ nTSQUIDs ~ fT

• SQUIDs allow to detect and characterize the magnetic signals which are sosmall as to be virtually immeasurable by any other sensors.

• How sensitive? Allows to measure magnetic fields produced by the nervecurrents 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’sconstant.

• SQUIDs are the most sensitive detectorsof extremely small changes in magnetic flux.

Fluxes can be created by currents – therefore the most sensitive current sensorsas well

SQUIDs - basic facts

• SQUIDs combine the physical phenomenaof flux quantization in superconductingloops and Josephson tunneling.

• The Josephson effect refers to the ability oftwo weakly coupled superconductors tosustain at zero voltage a supercurrentassociated with transport of Cooper pairs,whose magnitude depends on the phasedifference between the twosuperconductors.

• The maximum current which a Josephsonweak link can support without developingany voltage across it is known as its criticalcurrent Ic. When the current passedthrough a Josephson weak link exceeds Ic, avoltage appears across it

• If a closed loop made of superconductormagnetic field cannot enter the loop(“ideal diamagnetism”). But if there is aweak link flux enters the loop in quanta!Flux quantum

215

0 1007.22

mTe

h

Applications of SQUIDS

Magnetoencephalograph Magnetocardiography Rock magnetometry

imaging currents in semiconductor packages

Biosensors

Hall Sensor

• Utilizes the Lorentz force on charge carriers

• 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 sensitivityand temperature dependence.

Applications of Hall Field Sensors

Response to South or North Polarity Motor-Tachometer applicationwhere each rotation of themotor shaft is to be detected

When ring magnet rotates w/motor, South Pole passes thesensing face of the Hall sensorafter each revolution.

SensorActuated when the South Poleapproaches sensorDeactuated when South Polemoves away from sensor

Single digital pulse produced foreach revolution.

Gear Tooth Sensing

• Sensor detects change in flux level • Translates it into a change in the sensor output (high to low)

• Sense movement of ferrous metal targets (magnetically biased)

application continued…..

AMR – anisotropic magnetoresistance

Operation of AMR

Typical application of AMR Sensors

• Cylinder position sensing in pneumatic cylinders

• Elevator sensor

• Lid sensor for laptop computers

• Position sensor for materials handling equipment (lift trucks)

• Blood analyzer

• Magnetic encoders

GMR is achieved by using a four layer structure that consists of two thinferromagnets separated by a conductor. The fourth layer is anantiferromagnet that is used to pin (inhibit the rotation) themagnetization of one of the ferromagnetic layers. The ferromagnet layerthat is being pinned is between the conductor and the antiferromagnet.The pinned ferromagnet is called the hard ferromagnet and the unpinnedferromagnet is called the soft ferromagnet. This structure is called a spinvalve.

• The difference in resistivity between the case when themagnetizations are parallel to when they are antiparallel canbe 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 functionof the field, it is necessary that the soft ferromagnetic have itseasy axis of magnetization in zero field perpendicular to themagnetization of the pinned ferromagnet.

• The zero field orientation of the two magnetizations isdepicted in Fig. 11(a). The resistance is measured either in theplane of the ferromagnetics or perpendicular to this plane.

• This multilayer geometry increases the percentage resistancechange because it increases the probability of spin flipscattering by increasing the number of interfaces where spinflip scattering occurs.

• Total field magnetometers have the important advantage ofinsensitivity to rotational vibrations. Total fieldmagnetometers use the fact that the splitting between someelectron or nuclear spin energy levels is proportional to themagnitude of the magnetic field over a field range sufficientfor magnetometry.

Total Field Magnetometers

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.

Nuclear-Precession Magnetometer