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David P. PappasNational Institute of Standards and Technology

Boulder, CO

CNRS Thematic SchoolHigh Sensitivity Magnetometers

Sensors & Applications

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• Health care • Non-invasive medical evaluation

• Magnetic liver & lung “biopsies” • Magneto-cardiography (MCG)• Magneto-encephalography (MEG)• BARC

• Geophysical exploration• Surveying• Locating artifacts

• Non-destructive testing• Bridges• Electronics• Current monitoring

• Data storage technology• Millitary applications

• Mine detection• Naval situations

Applications of high sensitivity magnetic sensors

=> Magnetic sensors have large impact

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F.L. FagalyMagnetics Business & TechnologySummer 2002

1 fT

1 nT

1 100 10,0000.010.0001Frequency (Hz)

Mag

netic

fiel

d R

ange

1 pT

Geophysical

Industrial

MagneticAnomaly

Magneto-cardiography

Magneto-encephalography

High sensitivity magnetometer applicationsfield vs. frequency

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High Sensitivity Magnetic Field Sensors

SuperconductingSQUID

High, low TC

SemiconductorsHall

Magneto-electricEMR - nanostructures

Resonance MZ - ProtonsMX - Electrons

FerromagneticFluxgate

Induction coilMagneto-resistive

(AMR,GMR, TMR)Giant Magneto-Impedence

Magneto-opticMagneto-strictive

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Sensor specifications– Equivalent noise spectral density referred to the input– Sensitivity (V/T, Hz/T, A/T…)– Dynamic range, linearity, slew-rate– Working type (flux locked vs. open loop)– Offet, temperature stability, Resistance against environment

(humidity, vibration)– Hysterisis, Perming– Cross talk between field components – cross field– Bandwidth– Form factor – spatial resolution– Power– Price– Operating temperature (cryogenics)– Multi-sensors

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Oh, et. al JKPS (2007)

SQUID Noise Spectral Density – T/Hz

High TC SQUID Low TC SQUID

.01 0.1 1 10 100 Frequency (Hz)

1 10 100 1000

1 pT/ Hz

10 fT/ Hz

Penna et. al Phil M B (2000)

100fT/ Hz

100 fT/ Hz

Why is noise spectrum important?• Integrated over bandwidth of the sampling!

How to read these graphs:• ultiply by the frequency

~1 pT/ Hz @ 1Hz ~20 fT/ Hz @ 1Hz

7Example – Magneto-cardiography• Beats are not perfectly regular

• Heart signals edges go up to > 500 Hz

• Need bandwidth ~ 1 kHz for real-time and full signal analysis

~ 30x noise in raw LTC data

Oh, et. al JKPS (2007)

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Technique Noise floor (1 Hz)

SQUID 1 femto-Tesla

Optical pumping 500 femto-Tesla

Fluxgates 1 pT / Hz

Magneto-resistive 100 pT/ Hz at 1 Hz

Hall effect 100 nT / Hz at 1 Hz

Types of magnetic sensors

9Superconducting Quantum Interference Device (SQUID)

ILIR

ITot=IL+IR

0

Tot cosI

B

A

AB

ITot

Super-conductor

Tunneljunctions

10Flux-locked loop feedback

Ib

preamp

Integrator

VO

N

S

Shielded box in cryostat

ITot

11SQUID measurement of quantum bits

“Phase Qubit”Superconducting

loop with junction

IQ

Sensor:asymmetricDC SQUID

1 0

•Asymmetric current self biases SQUID• Off • On

•SQUID off - no interaction/dissipation • operate qubit

• SQUID on – Measure qubit

ITot

IL

12Measure quantum coherence

IsIwave

100m

QubitDCSQUID

Bias

qubitin

1 ofy Probabilit

13SQUID magnetometer considerations

Advantages

• High sensitivity with large pickup

• Arbitrary pickup loop geometry

• Magnetically “clean”

• Linear with feedback

Disadvantages

• Lower sensitivity for small loops

• Cryogenic operation

• Bandwidth limited by feedback

• High power

• High cost

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Nuclear Precession Magnetometers

• Proton magnetometer - H2O, Methanol, Kerosene

• Polarize protons in medium with high field• Remove field - measure precession frequency

• Scalar technique• Overhauser effect

• Polarize protons using electron spins• Tempone – electron spin resonance pump

• Lower power• Pumping frequency far from measurement

• He3 – optical pumping of electrons => protons

extB

BextM

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Nuclear magnetometer applications• Stable• Predominant geological survey tool• Medium – high sensitivity ~ 0.1 nT/• Spatial resolution ~ 10’s cm2

• Medium - high operation fields• Medium power ~ W

– Proton ~ 10 pT/ Hz @ 1Hz– Overhauser ~ 100 fT

16Electron spin MZ optical magnetometers – He4

C.P. light

Bext(z)

He4

23S1

RF depopulates 1,-1 => 0, creates more absorbtion

1

0

-1

extB splitting

=> 2signal on resonance

I (%)

RF frequency

100

50

I

17MZ magnetometer properties

• Higher frequency that proton nuclear resonance ~1000

18MX magnetometers – optical pumpingAlkali metals– Na23,K39, Rb85,87

Heated cell

Optical

Microwave hf

Ene

rgy

D1RF coils

Bext

Recent advances:

Spin exchange, relaxation-free K magnetometer

Chip scale atomic magnetometer (CSAM)

90o

19SERF MX magnetometers - K• Optimal K-atom concentration (180 C)

• Very low field operation (<10 G)

=> No precession between K-K collisions

• Intrinsically vector capability • Unshielded version:

• Lockin feedback on helmholtz - xzy• ~ 1 pT/ Hz above 5 Hz

I. M. Savukov, M. V. Romalis. PRA (2005). S. J. Seltzer and M. V. Romalis, APL (2004).

1 fT/ Hz

20NIST CSAM Package

4.5

mm

1.7 mm

Volume: 19 mm3

Power Consumption: 198 mW

1 10 100 10000

1000

2000

3000

Me

asu

red

fie

lda

mp

litu

de

[p

T RM

S]

Frequency [Hz]P. D. D. Schwindt, et al. Appl. Phys. Lett. 90, 081102 (2007).

Anticipated improvements to < 100 fT/Hz, 20 mW

0.1 1 10 100 1000

4

6

8

101214

Mag

net

ic fi

eld

noi

se

[pT R

MS

/ Hz1/

2 ]

Frequency [Hz]

5.9 pT / Hz1/2

over 1-10 Hz

21Fluxgate

Principle of Operation:• Soft magnetic core M=• Modulate magnetization

Flux “gated” when core saturated• B-H curve shifts with applied field

=> Asymmetric Vout

• Readout & linearize with feedback

vout

M

H

Bext

Bext

Size ~ 10’s cm3

Hmod

22Billingsley “best” fluxgate specifications

• Noise: 3.0 picoTesla Rms/vHz @ 1Hz• Range: ± 65 Tesla standard Accuracy: ± .02 % of Full Scale• Zero offset: +/- 5 nanoTesla• Susceptibility To Perming: < ± 5 nanoTesla Shift with ± 5 Gauss applied • Axial Alignment: Orthogonality better than ± 0.1° (0.02 ° special)• Digital Output Resolution: 28 bits • Conversion speed: 25 microseconds per sample• Linearity: ± .001% of Full Scale• Scale Factor Temperature Shift: .002 % / ° Celsius typical• Power: 16 to 34 VDC @ 750 milliWatts Field Measurement •.Weight ~ 909 grams PVC housing• Size w/Underwater Housing 7.8cm Diameter x 30.5 cm Length (PVC ) • Price: ~1 k$

23Typical fluxgate specifications

Billingsley (2007)

S~ 10 pT/Hz @ 1 Hz

Dynamic range – 65 TField resolution

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• Noise sources– Thermal fluctuation of magnetization– Incoherent rotation of magnetization during switching– 1/f magnetization jumps of two-state systems– Electronics noise

Recent Advances Radial magnetization => smoother rotationMicro-fluxgate fabrication

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GMI

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Magnetoelectric

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AMR

28

GMR

29TMR

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Hybrid GMR/Superconductor

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Anisotropic magneto-resistive sensor

Honeywell HMC 1001/1002

Frequency (Hz)

No

ise

leve

l (pT

/H

z)

10

104

103

102

10-1 1 10 102 103

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2000.00 4000.00 6000.00 8000.00 10000.00 12000.00 14000.00 16000.00

2000.00

4000.00

6000.00

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10000.00

12000.00

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Measured magnetic field - BzCalculated currents

Intel flip-chip RAM a with short

33Magneto-Optic

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• 2nd derivative unshielded pickup coil• Ambient laboratory environment• < 1 Monolayer Fe resolution at 10 cm• ~ 100 pT/Hz @ 8 Hz noise floor