Slawomir TUMANSKI - Warsaw University of Technology www.tumanski.pl

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Slawomir TUMANSKI - Warsaw University of Technology

www.tumanski.pl

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- Introductory general remarks

- Guide on commonly used sensors - New principles and sensors

- Conclusions

www.tumanski.pl www.red.pe.org.pl

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Range of magnetic field sensor

Hx

Hfeedback

I_out

H_x

with feedback without feedback

0.1 1.0

0.1 1.0 10

f [Hz]

MR sensor

fluxgate

PSD [nTrms/ Hz]

10⁰ 10² 10⁴

1 10 100

4.2K SQUID 77K SQUID

fluxgate

MR sensors

inductive

noise [fT/ Hz]

f [Hz]

(6)

Improvement of the resolution

fluxgate oscillator

synchronous detector

f 2f

2f

DC magnetic field

shield

H_x T T

Hx

T T

Hx

a) b)

(7)

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Induction coil sensor (search coil)

0

d dB dH

V n nA nA

dt^ dt



dt

    

air coil can exhibits sensitivity of 0.3 pT/Hz at 20 Hz while coils with ferromagnetic core 2.5 pT/Hz at 1 Hz

m

2

m

V    nAfB

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Induction sensor – advantages and drawbacks Drawbacks:

- Large dimensions

- Varying magnetic field

- Integrating amplifier necessary

- Ouptut signal depends on frequency Advantages:

- Very simple design - Possible high accuracy

- Three components measurements (vector measurement) - Non-invasive measurement (especially in air coil)

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Induction sensor – output circuit

-

+ Vin

Vout

+ -

offset R₁

C R'

V [dB]

0 20 40

-20

1

f/f₀ 10

0.01 0.1

 = 0

1

 > 

 = R/R₀

-

+ Iin

Vout

R1

C

R

0.1 1 10 100 1k 10k f [Hz]

V [dB]

-20

-40

A B

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Induction sensor – special design

a) b)

A

B

dl H V

0

cos

B

A

AB

d d n d

V n A Hdl

dl dt l dt n d

A H

l dt

 



 

    



 

0

n dI

V A

l dt





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Flux-gate sensor: principle of operation

B

H Hx

t

A

A A’

A’

0

0'

Hs

Hm

odd odd + even

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Fluxgate sensor – properties

e1

e2' e2" e2

H_x Hx

e₁

e₂

a) b)

' "

2 2 2

16

2 _x

sin

^s

sin 2 ...

m

e e e n fA H H t

  H 

   

2

2 2 0 2 0

~ 10 1

_x

2

_x

E n fA H n f l H

 N 

 

For typical values n₁ = n₂ = 1000, f = 3 kHz, l = 2  60 mm, A = 3  0.1 mm it is possible to obtain sensitivity 10 V/nT.

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Fluxgate sensors - design

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Magnetoresistive sensor

DR/R

0.01 0.1

1.0 10

0.001 0.01 0.1 1.0 B[T]

MTJ-a AMR

SV

InSb Bi

GMR

CMR MTJ-b

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AMR sensors – principle of operation

sin

2 x

x

R R

 



D   D sin

^x

k y

H H H

 



 is an angle between direction of magnetization and direction of a current.

 

2

x x

x k y

R H

R H H



D D

  

0^o

90^o 45^o

-45^o

Hx/Hk

0.5

0 1

-0.5 -1

0.5 1

DRx/DRxm

1 2

x x

x k x

R H

R H H



 

D D

      

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AMR sensors - design

Barber-pole MR layer

Vout [mV]

H [kA/m]

0 0

-1 1 -2

20 40

-20 -40

Advantages: simple design, sensitivity about 20 uV/A:m or 16 uV/uT, differential pair Drawbacks: small D/ = 2%, component H_y should not exceeding 0.1 H_x,possibility of demagnetization by high magnetic field

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GMR sensors

M₁

M₂

ferro ferro conducting

0 20 40

0

-200 200

H [kA/m]

DR/R [%]

Co/Cu/Co

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Spin valve GMR sensors

M₁ AF

pinned layer free layer

M

R

H

sensor area

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MTJ sensors

1995 2000 2005 year

100 200 300 400

DR/R [%]

AlO barrier

MgO barrier

TMR [%]

1000

500

Hx [kA/m]

8 0 8

5 K

RT

(29)

Hall effect sensors

B

VH VH

0

H x

V w V B

 l



InSb – 80 000, InAs – 33 000, GaAs – 8 500, Si – 1 400 cm²/Vs

InAs or GaAs sensors with sensitivity of about 0.2 mV/mT (sensors of F.W.Bell/Sypris) or InSb – 5 mV/mT (sensors of Asashi Kasei).

integrated Hall sensor HAL 401 of Micronas with dimensions 0.370.17 mm exhibits sensitivity of about 50 mV/mT, range  50 mT,

nonlinearity error less than 0.5% for FS and frequency bandwidth 0 – 10 kHz.

2DEG (2D electron gas) Hall sensors

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SQUID sensors

V_dc

I_dc

0

ex

V_dc

Idc

  ⁿ0

  n + ½)0

15 0

2.067833667 10

2 h Wb

e

   



If we assume that a ring has a cross section of about 1 cm² (in practice it can be much smaller) this corresponds with flux density 2.067  10^-11 T.

with noises of about 5fT/Hz they enable to detect fT magnetic field (in typical application pT).

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SQUID magnetometers

SQUID

_x

_in

_fb

osc

_osc

0



V

osc

t

V

t

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SQUID sensors design

SQUID

vacuum

liquid helium

SQUID

electronics

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Resonance magnetometers

Protons or electrons are rotating due to spin. Because they have electric charge this rotation results in magnetic moment. If we put such rotating part into external magnetic field the torque causes that particles act as a gyroscope rotating with precession around the direction of external field.

For constant magnetic field the frequency of this rotation depends only on gyromagnetic factor  and value of magnetic field B

0 B

  

We know very exactly the value of gyromagnetic factor equal to

_p = 42.576375 MHz/T for protons and

_e = 28.1481 GHz/T for electrons.

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Free precession NMR magnetometers

Vout

B0 Bx t 1 2

1/f

Vout

N

S

Bx

Bm

water flow

Earth’s magnetic field 50 T corresponds with frequency of only 2130 Hz.

Market available free precession proton magnetometers – model G-856 of

Geometrics has resolution 0.1 nT, range 20 – 90 T, sensor dimensions 9  13 cm.

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NMR magnetometers

absorption



0

N S

RF In

RF Out

mixer sensor

sample

DAQ PC

RF osc pulse

osc

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ESR – optically pumped magnetometers

5²P1/2

5²S1/2

-2 2 -1 1 0

-1 1 0 -1-21 2 E

H

6.8347 109 Hz8.18 108 Hz 3.7725 1014 Hz

1083 nm osc

rf osc He lamp

lens

filter/polarizer

detector

f

f0

helium 4 cell

Market available potassium magnetometer of GEM Systems has resolution 0.1 fT, range 20 – 100 T and accuracy 0.1 nT.

Market available free precession proton magnetometers – model G-856 of Geometrics has resolution 0.1 nT, range 20 – 90 T, sensor

dimensions 9  13 cm.

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New principles – Overhauser magnetometer

Overhauser magnetomweter (dynamic nuclear polarization DNP magnetometer) the cell is filled with mixture of proton rich substance (for example methanol) and free electron rich substance – usually nitroxide free radical. This way both

resonances NMR and ESR can exist in one cell and ESR resonance can be used to polarize NMR cell (instead of large magnetic

field).

Overhauser magnetometer joints advantages of both NMR and ESR methods – simplicity, low power consumption, high

sensitivity and additionally almost continuous operation. Indeed market available Overhauser magnetometer GSM-19 of GEM Systems has resolution 10 fT, accuracy 0.1 nT and speed 5 samples/s.

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New principles – MTJ magnetometers

1995 2000 2005 year

100 200 300 400

DR/R [%]

AlO barrier

MgO barrier

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New (?) principles – GMI sensor

sensor

Hx

Eout

-2 0 2

Hx [kA/m]

DE/E [%]

200 400

C1 C2

R(H)+jL(H)

coil wire

In 2001 Aichi Microintelligent Corparation started with manufacturing of various transducers (magnetic field sensor, compass) based on GMI sensor. As the sensor is used an amorphous wire with diameter 20 m and length 2 mm on which are wound two coils – for bias and feedback. Sensitivity 1V/1 uT.

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New principles - magnetooptical sensors

laser

detectors

interferometer reference

Bx

magnetostrictive line

laser

polarizer

lens Wollaston

prism detectors

fiber link

Faraday/Kerr sensor

B_x

(41)

New principles – MEMS sensor

magnet

N S

torque Bx

balance beam

Bx

a) b)

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Conclusions