1 david p. pappas national institute of standards and technology boulder, co cnrs thematic school...
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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
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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
8000.00
10000.00
12000.00
14000.00
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
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