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

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

3. Outline High sensitivity applications Noise & signal measurement
Description of various types of sensors
Addition of flux concentrators
Summary

4. Applications Health Care Geophysical Astronomical Archeology
Magnetic RAM
Magneto-encephalography
Magneto-
Cardiography
“Biomagnetism using SQUIDs: Status and Perspectives” Sternickel, Braginski, Supercond. Sci. Technol. 19 S160–S171 (2006).
Bio-magnetic tag detection
Frietas, ferreira, Cardoso, Cardoso
J. Phys.: Condens. Mater 19, (2007)
North Caroline Department of Cultural Resources
“Queen Anne’s Revenge” shipwreck site Beufort, NC
Mars Global Explorer (1998)
Health Care
Geophysical
Astronomical
Archeology
Non-destructive evaluation (NDE)
Data storage
JPL - SAC-C mission
Nov. (2000)
SI units

5. SI - Le Système International d’Unitès
“Magnetic field intensity”: H-field  A/m
What do we measure?
B = flux density
= “Magnetic induction” field
r (m)
I (A)
Use m0 = permeability of free space
F = BA A
B = m0 H
B-field  tesla (T)
kg/(As2)
H = ~0.1 A/m
B = 210-7 T
e.g. 1 1 m
Frequency

6. B-field Ranges & Frequencies
1 mT
1 gauss  10-4 T ~ Earth’s B-field
1 mT
1 1 m
Industrial
1 nT
Geophysical
Geophysical
Industrial/NDE
Magneto-
cardiography
1 nT
Magnetic field Range
B-field
Magnetic
Anomaly
Magneto-cardiography
1 pT
Magnetic
Anomaly
1 pT
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)
effects

7. B-fields… Induce voltages Affect scattering of electrons in matter
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
Technologies

8. Magnetometer technologies
Induction
Search Coil
Fluxgate
Giant magneto-impedance
Scattering
Anisotropic magneto-resistive (AMR)
Spintronic – Giant MR, Tunneling MR, Spin Xtor…
Hall Effect
Magneto-optical
Superconducting
SQUIDS
Spin Resonance
Proton, electron
State

9. S S B State measurement  Low noise excitation source -
Voltage, current, light, …
Sense state with detector
Flux feedback is typical
Linearize
Dynamic Range
Complicated
Limits slew rate & bandwidth  Bf S
Bext S B
Noise

10. Noise metrology Magnetically shielded container
(or room)
Low noise preamplifiers
Spectrum analyzer – P = V2/Hz
field noise = power / sensitivity
Units:
Low TCSQUID
magnetometer
(w/gradiometer) 1 10
100
1,000
10,000
100,000
1/f
White
State
Measurement
f T/Hz
Preamp Hz
Benchmarks

11. Benchmark properties:
State variable
V, f, etc
B-field measurement
Vector/scalar
1 Hz
T/Hz
Operating T
Cryogenic/RT
Volume of sense element
cm3 – mm3
Power – form factor
Line/Battery
SQUID

12. Superconducting Quantum Interference Devices
Signal I
Superconductor B B IL IR
Tunnel
Junctions
Left-Right phase shifted by B
PU loop

13. Pickup loops for SQUIDS
One loop: measure BZ
Two opposing loops:
dBZ/dz (1st order gradiometer)
Good noise rejection
Opposing gradiometers:
dBZ2/dz2 (2nd order gradiometer)
High noise rejection N S Z
SQUID specs

14. SQUID Magnetometer State variable Voltage (10’s mV) B-field
Integrated
Systems
State variable
Voltage (10’s mV)
B-field
Vector, gradients
1 Hz
~10 fT/Hz Low TC
Operating T
cryogenic
Volume
~ 1 cm3 coil
Power
Line
Discrete
components
Commercial: 10 – 100’s k$
“The SQUID Handbook,”
Clarke & Braginski, Wiley-VCH 2004
Resonance

15. Real time magneto-cardiography with SQUID magnetometer
Oh, et. al JKPS (2007)
~60 pT
Benchmark

16. Resonance magnetometers
f  Bext
Nuclear spin resonance
Protons
water, methanol, kerosene
Overhauser effect:
He3, Tempone
Electron spin resonance
He4,Alkali metals (Na, K, Rb)
Bext
Proton

17. Proton magnetometer State variable Frequency ~ kHz B-field Scalar
1 Hz
sources
~10 pT/Hz
depolarization
Operating T
-20 => 50 oC
Volume
1 cm3 cell
Power
Battery
Kerosene cell
Toroidal excitation
& pickup
Commercial: 5 k$
Olsen, et. al (1976)
From Ripka, (2001)
e- spin

18. Electron spin magnetometers
Photodetector
Vapor cell
Bext RF
Coils
l/4 Filter
Laser
Budker, Romalis, Nature
Physics, 3(4), (2007)
He4

19. He4 e--spin magnetometer
State variable
Frequency ~ MHz
B-field
vector/scalar
1 Hz
1 pT/Hz
Operating T
ambient
Volume
~10 cm3 cell
Power
battery
JPL - SAC-C mission
Nov. (2000)
Smith, et al (1991)
from Ripka (2001)
CSAM

20. e--spin magnetometer Chip scale atomic magnetometer
State variable
Frequency
B-field
Scalar
1 Hz
5 pT/Hz
Operating T
110 oC
Volume
20 mm3
Power
Small battery
Rb metal vapor
Optimized for low power
Very small form factor
4.5 mm
P. D. D. Schwindt, et al.
APL 90, (2007).
SERF

21. e--spin magnetometer Spin-exchange relaxation free
K metal vapor
Low field, high density of atoms
Line narrowing effect
All-optical excitation & pickup
=>Optimized for high sensitivity
State variable
Frequency
B-field
Vector < 100 nT
1 Hz
< 1 fT/Hz
Operating T
180 oC
Volume
~ 3 cm3
Power
line
Kominis, et. al, Nature 422, 596 (2003)
Solid state

22. Solid state magnetometers
Inductive
Fluxgate
Giant magneto-impedance
Magneto-resistive
AMR – Anisotropic MR
Spintronic
GMR – Giant MR
TMR – Tunneling MR
Disruptive technologies
Hybrid superconductor/solid state
Magneto-striction

23. 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
<10 pT/Hz
Magnetic
Johnson
Perming
Operating T RT
Volume
1 cm3
Power
battery M M H
Bext
Hmod(f)
Commercial: ~1 k$
“Magnetic Sensors and
Magnetometers” P. Ripka, Artech, 2001
FG innov.

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

25. Giant Magneto-impedance (GMI)
Iac
Magnetic amorphous wire
CoFeSiB M
frequency
external field 
Enhanced skin effect in magnetic wire
“Giant magneto-impedance and its applications”
Tannous C., Gieraltowski, Jour Mat. Sci: Mater. in
Electronics, V15(3) pp (2004)
GMI spec.

26. GMI specifications Commercial: ~100 $ State variable Z @ MHz B-field
Vector
1 Hz
sources
~3 nT/Hz
1/f mag noise
Temp fluct. M
Johnson
Perming
Operating T RT
Volume
0.01 mm3 wire
Power
Batter
Commercial: ~100 $ MR

27. Magneto-resistive (MR) sensors
AMR - Anisotropic MR
Single ferromagnetic film NiFe
2% change in resistance
Spintronic:
GMR – Metal spacer
60% DR/Rmin Co/Cu/Co
Parkin, APL (1991)
TMR – Insulator spacer
472% DR/Rmin at R.T.
CoFeB/MgO/CoFeB
Hayakawa, APL (2006) I
“Thin Film Magneto-resistive Sensors
S. Tumanski, IOP (2001). M FM * * NM FM *
hdd

28. MR Sensors – spatial resolutionV+ V- I-
16 mm x 256 I+
256 element AMR linear array
Thermally balanced bridges
High speed magnetic tape imaging – forensics, archival
NDE imaging
Current flow in
VLSI RAM w/short
2 cm
-Iy
+Ix
-Ix
+Iy
4 mm
Cassette Tape – forensic analysis
4 mm
45 mm
erase head
stop event
write head
BARC.
da Silva, et al., subm. RSI (2007)

29. MR biomolecular recognition
Nano-scale magnetic labels that attach to target
GMR arrays with probe
Probes attach to targets
 Need very sensitive magnetic detector arrays
 Goal: high accuracy chemical assays
“BARC”
Bead
Array
Counter
Device Applications Using SDT, Tondra et. al
Springer Lecture Notes in Physics, V593, (2002)
MR sens.

30. MR as low field sensors Commercial: ~ $ State variable Resistance
AMR
Large area films
State variable
Resistance
B-field
Vector
1 Hz
sources
~200 pT/Hz
1/f mag noise
Temp fluct. M
Johnson/Shot
Perming
Operating T RT
Volume
0.001 mm3 film
Power
battery
2 Unshielded sensors
GMR
Small
sensors
Flux
concentrators
2 shielded
TMR
Commercial: ~ $
“Low frequency picotesla field detection…”
Chavez, et. al, APL 91, (2007).
F.C.

31. The joy of flux concentrators
Bext a
Need:
Soft ferromagnet
High M = cH
No hysterisis
Gain up to ~50
No increase in noise
Increase in form factor
TMR with F.C.
2 cm
Noise

32. Spectral noise measurements
without flux
concentrator
1 n
with external
flux concentrator
1 p
Stutzke, Russek, Pappas, and Tondra, J. Appl. Phys. 97, 10Q107 (2005)
Yuan, Halloran, da Silva, Pappas, J. Appl. Phys. submitted (2007)
Hall FC

33. Integrated Hall sensors with flux concentrators
Internal flux
concentrators
Hall element
1Hz
(nT/Hz)
White noise (f> 100 Hz)
Hall with no flux concentrators*
300
200
Only internal flux concentrators 30
With both internal and
external flux concentrators 3
“Bridging the gap between AMR, GMR and Hall Magnetic sensors
Popovic, et. al, PROC. 23rd MIEL, V1, NIŠ, YUGOSLAVIA, MAY, 2002
Hall spec.

34. Hall Effect specificationsV
State variable
Voltage
B-field
Vector
1 Hz
300 nT/Hz
30 nT/Hz w/FC’s
Operating T RT
Form Factor
0.001 mm3 chip
Power
battery B I
InAs thin film
Applications
Keyboard switches
Brushless DC motors
Tachometers
Flowmeters 
Commercial: ~$ 0.1  1
Disruptive - hybrid

35. Disruptive technologies? Superconducting flux concentrator
Hybrid S.C./GMR
State variable
GMR voltage
B-field
Vector
1 Hz
32 fT/Hz
Operating T
cryogenic
Volume
0.1 cm3
Power
Battery
Field Gain
YBCO ~ 100
Nb ~ 500
“…An Alternative to SQUIDs”
Pannetier, et. al, IEEE Trans SuperCond 15(2), 892 (2005) ME

36. Magneto-electric H Magnetostrictive + piezo-electric multilayer
State variable
Piezo voltage
B-field
Vector
1 Hz
sources
1 nT/Hz
pyro/static
Operating T
-40 to 150C
Volume
1 mm3
Power H
PMN-PT
VME
Terfenol-D
Disruptive
No power required
Two terminal device
High impedance output
Dong, Zhai, Xin, Li, Viehland
APL V86, (2005). MO

37. Magneto-optic Magnetometer head State variable Light intensity B-field
Mirror-coated iron garnet B
Fiber-
optic
State variable
Light intensity
B-field
Vector
1 kHz
1.4 pT/Hz
Operating T
ambient
Volume
1 cm3
Power
line
Ferrite Flux Concentrators
Light polarization changes in garnet
Rotation  B-field (Faraday effect)
Sensed with interferometer
Disruptive
Light not affected by B
Remote sensors
High speed
Imaging capability (light)
NDE
Deeter, et. al Electronics Letters, V29(11), p 993 (1993).
Youber, Pinassaud, Sensors and Actuators A129, 126 (2006).
Spintronic

38. More spintronic sensors
Extraordinary MR (EMR)
Hall effect with metal impurity
Based on 106 MR in van der Pauw disks
Non-magnetic materials
Mesoscopic device
DR/R ~ 35% in field
Replacement for GMR in hdds?
220% TMR in 2004 … B
Semiconductor
Au impurity Au V I
InSb
300 nm
“Magnetic Field Nanosensors, Solin, Scientific American V291, 71 (2004)

39. Other disruptive technologies
Magneto-strictive delay lines
Spintronics:
CMR
Spin-FET – tunnel junctions
Spin-FET – nanowires - APL (2007)
concl

40. Compilation Sensor B pt/Hz @ 1 Hz Volume Power
SQUID v
.01
1 cm3
Line
Proton s 1
battery
e- He4/CSAM/SERF
s/s/v
1 / 5 / .001
mm3 – cm3
Battery-line
Fluxgate 10
GMI
3000
0.01 mm3 MR
200
0.001 mm3
Hybrid GMR/SC
.032
.1 cm3
Hall
30,000 ME
1000
1 mm3
Magneto-optic
line
Trend: Noise increases as Volume decreases

41. Trend of volume vs. noise
Examples:
AMR
Large arrays of elements, good magnetic properties make up for lower noise
Flux concentrators
Increase volume  reduced noise floor
Fluxgate magnetometers - Sn T ''/
Koch – increase volume and decrease loss in material
“Fundamental limits…” Koch et al, APL V75, 3862 (1999)
Compare sensors based on volumetric energy resolution
Energy resolution  Noise Power  Active magnetic volume
D. Robbes / Sensors and Actuators A 129 (2006) 86–93
SQUIDS & SERF technique lead, hybrd GMR/S.C. coming up

42. Conclusions High sensitivity magnetometers research very active
Many advances to be made in conventional devices
Potentially disruptive technologies
Move to smaller, lower power, nano-fabrication
Evalute high sensitivity against many other parameteters:
Spatial resolution, power, vector..
Ackn.

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