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

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

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

4. 4 Applications Health Care Geophysical Astronomical Archeology Non-destructive evaluation (NDE) Data storage North Caroline Department of Cultural Resources “Queen Anne’s Revenge” shipwreck site Beufort, NC Mars Global Explorer (1998) Magnetic RAM SI units 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, 165221 (2007) JPL - SAC-C mission Nov. (2000)

5. 5 SI - Le Système International d’Unitès I (A) r (m) “Magnetic field intensity”: H-field  A/m  B = flux density = “Magnetic induction” field Frequency What do we measure? Use  0 = permeability of free space B =  0 H B-field  tesla (T) kg/(As 2 ) A  = B  A H = ~0.1 A/m B = 2  10 -7 T e.g. 1 A @ 1 m

6. 6 B-field Ranges & Frequencies Adapted from “Magnetic Sensors and Magnetometers”, P. Ripka, Artech, (2001) F.L. Fagaly Magnetics Business & Technology Summer 2002 1 fT 1 nT  110010,000 0.01 0.0001 Frequency (Hz) Magnetic field Range 1 pT Geophysical Industrial Magnetic Anomaly Magneto- cardiography Magneto- encephalography 1 fT 110010,000 0.01 0.0001 Frequency (Hz) B-field 1 pT Geophysical Industrial/NDE Magnetic Anomaly Magneto-cardiography Magneto- encephalography 1 nT 1 gauss  10 -4 T ~ Earth’s B-field 1 mT effects 1  T 1 A @ 1 m

7. 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. 8 Magnetometer technologies Induction  Search Coil  Fluxgate  Giant magneto-impedance Scattering  Anisotropic magneto-resistive (AMR)  Spintronic – Giant MR, Tunneling MR, Spin transistor  Hall Effect  Magneto-optical Superconducting  SQUIDS Spin Resonance  Proton, electron State

9. 9 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 B ext  BfBf B S Noise S

10. 10 Noise metrology Magnetically shielded container (or room) Low noise preamplifiers Spectrum analyzer – P = V 2 /Hz  field noise =  power / sensitivity State Measurement Low T C SQUID magnetometer (w/gradiometer) Hz 0.01 0.1 1 10 100 1 10 100 1,000 10,000 100,000 f T/  Hz Benchmarks 1/f White Units: Preamp

11. 11 Benchmark properties: State variableV, f, etc B-field measurementVector/scalar Noise @ 1 Hz T/  Hz Operating TCryogenic/RT PowerLine/Battery/very low Form Factorm3m3 SQUID

12. 12 Superconducting Quantum Interference Devices ILIL IRIR I Tunnel Junctions B Left-Right phase shifted by B  Signal PU loop

13. 13 Pickup loops for SQUIDS One loop: measure B Z Two opposing loops:  dB Z /dz (1 st order gradiometer)  Good noise rejection Opposing gradiometers:  dB Z 2 /dz 2 (2 nd order gradiometer)  High noise rejection NSNS SQUID specs Z

14. 14 SQUID Magnetometer “The SQUID Handbook,” Clarke & Braginski, Wiley-VCH 2004 State variable Voltage (10’s  V) B-fieldVector, gradients Noise @ 1 Hz ~10 fT/  Hz Low T C Operating Tcryogenic PowerLine Form Factorm3m3 Commercial: 10 – 100’s k$ Resonance Discrete components Integrated Systems

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

16. 16 Resonance magnetometers Nuclear spin resonance  Protons water, methanol, kerosene Overhauser effect:  He 3, Tempone Electron spin resonance  He 4,Alkali metals (Na, K, Rb) B ext f  B ext Proton

17. 17 Proton magnetometer State variableFrequency ~ kHz B-fieldScalar Noise @ 1 Hz sources ~10 pT/  Hz depolarization Operating T-20 => 50 o C PowerBattery Form Factorcm 3 Commercial: 5 k$ Kerosene cell Toroidal excitation & pickup Olsen, et. al (1976) From Ripka, (2001) e - spin

18. 18 Electron spin magnetometers Vapor cell Photodetector B ext RF Coils /4 Filter Laser Budker, Romalis, Nature Physics, 3(4), 227-234 (2007) He4

19. 19 He 4 e - -spin magnetometer JPL - SAC-C mission Nov. (2000) State variableFrequency ~ MHz B-fieldvector/scalar Noise @ 1 Hz 1 pT/  Hz Operating Tambient PowerBattery Form Factorcm 3 Smith, et al (1991) from Ripka (2001) CSAM

20. 20 e - -spin magnetometer Chip scale atomic magnetometer P. D. D. Schwindt, et al. APL 90, 081102 (2007). State variableFrequency B-fieldScalar Noise @ 1 Hz 5 pT/  Hz Operating T110 o C PowerUltra-low Form Factor20 mm 3 Rb metal vapor Optimized for low power Very small form factor 4.5 mm SERF

21. 21 e - -spin magnetometer Spin-exchange relaxation free State variableFrequency B-fieldVector < 100 nT Noise @ 1 Hz < 1 fT/  Hz Operating T180 o C Powerline Form Factorm3m3 Kominis, et. al, Nature 422, 596 (2003) K metal vapor Low field, high density of atoms Line narrowing effect All-optical excitation & pickup =>Optimized for high sensitivity Solid state

22. 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. 23 Fluxgate M H B ext H mod (f) M State variableInductive 2f B-fieldVector Noise @ 1 Hz sources <10 pT/  Hz Magnetic Johnson Perming Operating TRT PowerBattery Form Factor~20 mm core “Magnetic Sensors and Magnetometers” P. Ripka, Artech, 2001 Commercial: ~1 k$ Pickup(2f) Drive(f) FG innov.

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

25. 25 Giant Magneto-impedance (GMI) “Giant magneto-impedance and its applications” Tannous C., Gieraltowski, Jour Mat. Sci: Mater. in Electronics, V15(3) pp 125-133 (2004) Magnetic amorphous wire I ac M CoFeSiB frequency external field   Enhanced skin effect in magnetic wire GMI spec.

26. 26 GMI specifications State variableZ @ MHz B-fieldVector Noise @ 1 Hz sources ~3 nT/  Hz 1/f mag noise Temp fluct. M Johnson Perming Operating TRT Powerbattery Form Factormm 2 Commercial: ~100 $ MR

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

28. 28 MR sensors - high spatial resolution High Frequency Data storage  HDD read head 100 Gb/in 2  1 GHz BW  < 100 pT/  Hz from SNR MgO MTJ Frietas, Ferreira, Cardoso, Cardoso J. Phys.: Condens Mater 19 165221 (2007) 100 pT/  Hz ~150 nm MR NDE ~100 nm

29. 29 MR Sensors – non-destructive evaluation 256 element AMR linear array Thermally balanced bridges High speed magnetic tape imaging – forensics, archival NDE imaging Cassette Tape – forensic analysis 4 mm 45 mm erase head stop event write head stop event da Silva, et al., subm. RSI (2007) Current flow in VLSI RAM w/short 2 cm -I y +I x -I x +I y 4 mm V+ V- I- 16  m x 256 I+

30. 30 MR biomolecular recognition 1.Nano-scale magnetic labels that attach to target 2.GMR arrays with probe 3.Probes attach to targets  Need very sensitive magnetic detector arrays  Goal: high accuracy chemical assays Device Applications Using SDT, Tondra et. al Springer Lecture Notes in Physics, V593, 278-289 (2002) “BARC” Bead Array Counter MR sens.

31. 31 MR as low field sensors State variableResistance B-fieldVector Noise @ 1 Hz sources ~200 pT/  Hz 1/f mag noise Temp fluct. M Johnson/Shot Perming Operating TRT PowerLow Form Factor m2m2 AMR Honeywell Philips GMR NVE Commercial: ~ $ 2 Unshielded sensors 2 shielded Flux concentrators TMR “Low frequency picotesla field detection…” Chavez, et. al, APL 91, 102504 (2007). F.C.

32. 32 The joy of flux concentrators B ext a Need: Soft ferromagnet High M =  No hysterisis  Gain up to ~50  No increase in noise  Increase in form factor 2 cm Noise TMR with F.C.

33. 33 Spectral noise measurements Hall FC without flux concentrator with external flux concentrator Stutzke, Russek, Pappas, and Tondra, J. Appl. Phys. 97, 10Q107 (2005) Yuan, Halloran, da Silva, Pappas, J. Appl. Phys. submitted (2007) 1 p 1 n 1 

34. 34 Integrated Hall sensors with flux concentrators “Bridging the gap between AMR, GMR and Hall Magnetic sensors Popovic, et. al, PROC. 23rd MIEL, V1, NIŠ, YUGOSLAVIA, 12-15 MAY, 2002 Hall spec. Noise @ 1Hz (nT/  Hz) White noise (f> 100 Hz) (nT/  Hz) Hall with no flux concentrators * 300200 Only internal flux concentrators30030 With both internal and external flux concentrators 303 Internal flux concentrators Hall element

35. 35 Hall Effect specifications State variableVoltage B-fieldVector Noise @ 1 Hz 300 nT/  Hz 30 nT/  Hz w/FC’s Operating TRT PowerVery low Form Factor0.1 mm 2 - Si Commercial: ~$ 0.1  1 Applications Keyboard switches Brushless DC motors Tachometers Flowmeters  InAs thin film I V Disruptive - hybrid B

36. 36 Disruptive technologies? Superconducting flux concentrator State variableGMR voltage B-fieldVector Noise @ 1 Hz 32 fT/  Hz Operating Tcryogenic Power Line (cryogenics) Form FactorLoop ~ 3x3 mm Field Gain YBCO ~ 100 Nb ~ 500 Hybrid S.C./GMR “…An Alternative to SQUIDs” Pannetier, et. al, IEEE Trans SuperCond 15(2), 892 (2005) ME

37. 37 Magneto-electric Disruptive No power required Two terminal device High impedance output State variablePiezo voltage B-fieldVector Noise @ 1 Hz sources 1 nT/  Hz pyro/static Operating T -40 to 150  C Power0 Form Factor10’s mm 3 Dong, Zhai, Xin, Li, Viehland APL V86, 102901 (2005). Magnetostrictive + piezo-electric multilayer Terfenol-D H PMN-PT V ME MO

38. 38 Magneto-optic State variableLight intensity B-fieldVector Noise @ 1 kHz 1.4 pT/  Hz Operating Tambient Powerlow Form Factor10’s cm 3 Mirror-coated iron garnet Magnetometer head Ferrite Flux Concentrators Light polarization changes in garnet Rotation  B-field (Faraday effect) Sensed with interferometer Fiber- optic Deeter, et. al Electronics Letters, V29(11), p 993 (1993). Youber, Pinassaud, Sensors and Actuators A129, 126 (2006). Disruptive Light not affected by B Remote sensors High speed Imaging capability (light) NDE Spintronic B

39. 39 Other spintronic sensors Extraordinary MR (EMR)  Hall effect with metal impurity Based on 10 6 MR in van der Pauw disks  Non-magnetic materials   Mesoscopic device   R/R ~ 35% in field GMR replacement in hdds?  MgO TMR in 2004 hit 220% … Au InSb B Semiconductor Au impurity “Magnetic Field Nanosensors, Solin, Scientific American V291, 71 (2004) 300 nm V I

40. 40 Other disruptive technologies Magneto-strictive delay lines More spintronics: CMR  Nano-wire  Spin-FET APL (2007) concl

41. 41 SensorB Noise @ 1 Hz pt/  Hz Power Length Scale SQUIDv.01linem Protons1batterycm e - He 4 /CSAM/SERFs/s/v1 / 5 /.001battery/linecm /mm / m Fluxgatev10battery10 cm GMIv3000battery10 cm MRv200 small battery mm Hallv30,000battery 100  m Magneto-electricv10000 Magneto-opticvlinecm/  m Compilation

42. 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 Magnetic sensor technology is important Ackn.

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