1. High Sensitivity Magnetic Field Sensor Technology
David P. Pappas
National Institute of Standards & Technology
Boulder, CO

2. Market Analysis - Worldwide
Application
Type
2005 Revenue Worldwide - $947M
Growth rate 9.4%
“World Magnetic Sensor Components and Modules/Sub-systems Markets”
Frost & Sullivan, (2005)

3. Applications Health Care Geophysical Astronomical Archeology
North Caroline Department of Cultural Resources
“Queen Anne’s Revenge” shipwreck site Beufort, NC
Magnetic RAM
Mars Global Explorer (1998)
Health Care
Geophysical
Astronomical
Archeology
Non-destructive testing
Data storage

4. Field Ranges & Frequencies
Industrial
1 nT
1 nT
Geophysical
Geophysical
Industrial/NDE
Magneto-
cardiography
Magneto-
cardiography
Magnetic field Range
Magnetic field Range
1 pT
1 pT
Magnetic
Anomaly
Magnetic
Anomaly
Magneto-
encephalography
Magneto-
encephalography
1 fT
1 fT
0.0001
0.0001
0.01
0.01 1 1
100
100
10,000
10,000
Frequency (Hz)
Frequency (Hz)
F.L. Fagaly
Magnetics Business & Technology
Summer 2002
Adapted from “Magnetic Sensors and Magnetometers”, P. Ripka, Artech, (2001)

5. Magnetic fields… Induce currents in conductors
Change magnetic properties matter
Affect electronic properties of materials
Change phase of currents flowing in superconductors
Create energy level splittings in atoms
=> Use these effects to build a toolbox for field detection in important applications

6. Magnetometer technologies
Induction
Search Coil
Ferromagnetic
Fluxgate
Magneto-Resistive, Impedance, Optical, Electric…
Semiconducting
Hall Effect, spintronics
Superconducting
SQUIDS – DC,RF / Low TC, High TC
Spin Resonance
Proton, electron

7. Important properties State variable B Measurement – Vector or scalar?
Noise 1 Hz)
Operating Temp.
Power
Form Factor
Commercially available?

8. M B State measurement Low noise excitation source -
Voltage, current, light, …
Sense state with detector
Flux feedback is typical
Linearize
Dynamic Range ~ 22 bits
Limits bandwidth & slew rate
Complicated Bf
Bext M B

9. Noise metrology Shielded container/room Low noise preamplifiers
Spectrum analyzer
A-B Cross-correlation reduces amplifier noise
Billingsley (2007)
State
Measurement A B
Stutzke, Russek, Pappas, and Tondra, J. Appl. Phys. 97, 10Q107 (2005)

10. SQUID Magnetometer 2nd Derivative Pickup coil State variable
Voltage (100’s mV)
B-field
Vector
1 Hz
sources
5 fT/Hz
Johnson
Flux trapping
Operating T
cryogenic
Power
Line
Form Factor m3
2nd
Derivative
Pickup coil
“The SQUID Handbook,”
Clarke & Braginski, Wiley-VCH 2004
Commercial: 10 – 100’s k$

11. Noise Spectral distribution
Normalized for 1 Hz bandwidth
Noise integrates for wider bandwidth
Low TC SQUID array
1 pT/Hz
fT/Hz
Synthetic
Gradiometer
Unprocessed
10 fT/Hz Hz
Penne, et al., PHILOSOPHICAL MAGAZINE B, 2000, VOL. 80, NO. 5, 937± 948

12. e.g. magneto-cardiography
Low TC SQUID – 10 1 Hz
High TC SQUID – 1 Hz
Oh, et. al JKPS (2007)
1pT/Hz

13. Harold Weinstock
“Don’t use a SQUID when a simpler technology is available”

14. Resonance magnetometers
Nuclear spin resonance
Protons
water, methanol, kerosene
He3
Molecules - Tempone
Electron spin resonance
He4
Alkali metals – Na, K, Rb
Bext

15. Proton magnetometer State variable Frequency B-field Scalar
1 Hz
sources
~10 PT/Hz
depolarization
Operating T
-20 => 50 oC
Power
Battery
Form Factor
cm3
Commercial: 5 k$

16. Electron spin magnetometer
State variable
Frequency
Photodetector
Noble gas – He4
ge = 28 GHz/T => f ~ 1 MHz
Spin-exchange relaxation free (SERF)
K metal vapor
Low field, high density gas
Optimized for high sensitivity
Chip scale atomic magnetometer (CSAM)
Rb metal vapor
Optimized for low power
Very small form factor
Can also implement SERF RF
Coils
Vapor cell B
l/4 Filter
Laser
Budker, Romalis, Nature
Physics, 3(4), (2007)

17. e--spin magnetometer - He4
Scalar B-field
1 pT/Hz
Cell not heated
Low power
~ 10’s cm3
JPL SAC-C mission
Nov. (2000)

18. e--spin magnetometer Spin-exchange relaxation free
Vector B-field
<1 f T/Hz
cell ~ 180 oC
Line power
~ m3

19. e--spin magnetometer Chip Scale Atomic magnetometer
Scalar B-field
5 pT/Hz
Cell 110 oC
Very low power
~20 mm3
P. D. D. Schwindt, et al.
APL 90, (2007).

20. Ferromagnetic magnetometers
Fluxgate
Magneto-resistive
AMR – Anisotropic MR
GMR – Giant MR
TMR – Tunneling MR
Giant Magneto-impedance
Magneto-striction

21. Fluxgate Commercial: ~1 k$ Bext State variable Inductive 2f B-field
Drive(f)
Pickup(2f)
Bext
State variable
Inductive 2f
B-field
Vector
1 Hz
sources
~5 pT/Hz
Magnetic
Johnson
Perming
Operating T
RT, 30 ppm/oC
Power
Battery
Form Factor
~20 mm core M M H
Bext
Hmod(f)
“Magnetic Sensors and
Magnetometers” P. Ripka, Artech, 2001
Commercial: ~1 k$

22. Advances in Fluxgate technology
Circumferential Magnetization
Micro-fluxgates
Apply current in core
Single domain rotation
100 f 1 Hz
Planar fabrication
80 pT 1 Hz I M
Kawahito S., IEEE J. Solid State
Circuits 34(12), 1843 (1999)
Koch, Rosen, APL 78(13) 1897 (2001)

23. Magneto-resistive (MR) sensors
AMR - Anisotropic MR
Single ferromagnetic film
2% change in resistance
GMR – Cu spacer
Up to 60% change
TMR – Insulator spacer
Insulator => 400% change I FM M FM * * NM FM *
“Thin Film Magneto-resistive Sensors
S. Tumanski, IOP (2001).

24. Specs: MR sensors Commercial: ~ $ State variable Resistance B-field
AMR
Honeywell
Philips
State variable
Resistance
B-field
Vector
1 Hz
sources
~100 pT/Hz
1/f mag noise
Temp fluct. M
Johnson/Shot
Perming
Operating T
RT, 60 ppm/oC
Power
Low
Form Factor
mm2
2 Unshielded sensors
GMR
NVE
2 shielded
Flux
guides
TMR
Commercial: ~ $
“Low frequency picotesla field detection…”
Chavez, et. al, APL 91, (2007).

25. M Bext The joy of flux guides: For small gap: Bg ~ 10 Bext Bext Bound
Current
Bint~10Bext
Bext
Bg~Bint
Bext M
For small gap:
Bg ~ 10 Bext
“soft” ferromagnet

26. M Bext Advanced flux guides Bext Need: Soft ferromagnet High M = cH
“a” concentrated
“b” shielded a b
Bext M
Need:
Soft ferromagnet
High M = cH
No hysterisis
=> Gain up to ~50

27. Disruptive technologies? Hybrid superconductor/GMR
Superconducting
flux concentrator
State variable
GMR
B-field
Vector
<1 Hz
32 fT/Hz
Operating T
cryogenic
Power
low
Form Factor
Loop ~ 3x3 mm
Field Gain
YBCO ~ 100
Nb ~ 500
“…An Alternative to SQUIDs”
Pannetier, et. al, IEEE Trans SuperCond 15(2), 892 (2005)

28. MR sensors - high spatial resolution High Frequency
Data storage
HDD read head 100 Gb/in2
0.2 mm wide, 1 GHz BW
~ 100 pT/Hz from SNR
MgO MTJ
10 pT/Hz
Frietas, Ferreira, Cardoso, Cardoso
J. Phys.: Condens Mater (2007)

29. MR Sensors – scalability & imaging
256 element AMR linear array
Thermally balanced bridges
Image magnetic tapes real time – forensics, archival
NDE imaging V+ V- I-
16 mm I+
Cassette Tape – forensic analysis
Ferrofluid image

30. MR biomolecular recognition
MR arrays with probe DNA
Magnetic labels that attach to target DNA
Labels => Probes, read out MR
“BARC”
Bead
Array
Counter
Device Applications Using Spin Dependent Tunneling and Nanostructured Materials
M. Tondra, D. Wang, Z. Qian, Springer Lecture Notes in Physics, V593, (2002)

31. Disruptive technology?
Magneto-electric H
State variable
Piezo voltage
B-field
Vector
<1 Hz
sources
1 nT/Hz
pyro/static
Operating T
-40 to 150C
Power
Form Factor
10’s mm3
Terfenol-D
Magnetostrictive +
piezo-electric
multilayer
Disruptive technology?
No power required
Two terminal device
High impedance output
Dong, Zhai, Xin, Li, Viehland
APL V86, (2005)

32. Giant Magneto-impedance
Iac H
Magnetic amorphous wire
State variable
~ 1 MHz
B-field
Vector
1 Hz
sources
~100 pT/Hz
1/f mag noise
Temp fluct. M
Johnson
Perming
Operating T RT
Power
Very low
Form Factor
mm2 M
CoFeSiB
frequency 
external field 
“Giant magneto-impedance and its applications”
Tannous C., Gieraltowski, Jour Mat. Sci: Mater. in
Electronics, V15(3) pp (2004)
Commercial: ~100 $

33. Tried and True - Hall Effect
State variable
Voltage
B-field
Vector
1 Hz
sources
~100 pT/Hz
e- mobility
Offsets
1/f noise
Operating T RT
Power
Very low
Form Factor
0.1 mm2 - Si
Commercial: ~0.1 $

34. Spin Transistors

35. Magneto-optic

36. Search Coils

37. Acknowledgements Steve Russek Fabio da Silva Bill Egelhoff
John Unguris
Mike Donahue
John Kitching
Fabio da Silva
Sean Halloran
Lu Yuan

38. Comparison of MR & Fluxgates