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% Type Application “World Magnetic Sensor Components and Modules/Sub-systems Markets” Frost & Sullivan, (2005)

3 Outline High sensitivity applications & signal measurements Description of various types of sensors used New technologies Comparison and analysis of sensor metrics

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, (2007) JPL - SAC-C mission Nov. (2000)

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 ) H = ~0.1 A/m B = 2  T e.g. 1 1 m A  = B  A

6 B-field Ranges & Frequencies Adapted from “Magnetic Sensors and Magnetometers”, P. Ripka, Artech, (2001) 1 fT 1 nT  , Frequency (Hz) Magnetic field Range 1 pT Geophysical Industrial Magnetic Anomaly Magneto- cardiography Magneto- encephalography 1 fT , Frequency (Hz) B-field 1 pT Geophysical Industrial/NDE Magnetic Anomaly Magneto-cardiography Magneto- encephalography 1 nT 1 gauss  T ~ Earth’s B-field 1 mT effects 1  T 1 1 m

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 State measurement Low noise excitation source -  Voltage, current, light, … Detector volume -  = A  h Sense state Flux feedback is typical  Linearize  Dynamic Range  Complicated  Limits slew rate & bandwidth B ext  BfBf B S Noise S A h

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 ,000 10, ,000 f T/  Hz Benchmarks 1/f White Units Preamp

11 Benchmark properties: SQUID State variableV, f, etc B-field measurementVector/scalar B noise - 1 Hz T/  Hz Detector volume (  ) cm 3 -mm 3 Operating temperature, TCryogenic/RT/heated Power – form factorLine/Battery

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

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 SQUID Magnetometer “The SQUID Handbook,” Clarke & Braginski, Wiley-VCH 2004 Commercial: 10 – 100’s k$ MKG Discrete components Integrated Systems State variable Voltage (10’s  V) B-fieldVector, gradients B 1 Hz ~10 fT/  Hz  - Volume ~ 1 cm 3 coil Operating Tcryogenic PowerLine

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

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

18 Electron spin magnetometers Vapor cell Photodetector B ext RF Coils /4 Filter Laser Budker, Romalis, Nature Physics, 3(4), (2007) He4 f~ MHz for e -

19 He 4 e - -spin magnetometer JPL - SAC-C mission Nov. (2000) Smith, et al (1991) from Ripka (2001) CSAM State variableFrequency B-fieldvector/scalar B 1 Hz 1 pT/  Hz  - Volume ~10 cm 3 cell Operating Tambient Powerbattery

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

21 e - -spin magnetometer Spin-exchange relaxation free 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 State variableFrequency B-fieldVector B 1 Hz 0.5 fT/  Hz  - Volume ~ 3 cm 3 Operating T180 o C Powerline

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  Magneto-electric  Spin Transistors

23 Fluxgate M H B ext H mod (f) M State variableInductive 2f B-fieldVector B 1 Hz Sources <10 pT/  Hz Thermal magnetic Johnson Perming  - Volume ~1 cm 3 Operating TRT Powerbattery “Magnetic Sensors and Magnetometers” P. Ripka, Artech, 2001 Commercial: ~1 k$ Pickup(2f) Drive(f) FG innov. B ext

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

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

26 GMI specifications Commercial: ~100 $ MR State MHz B-fieldVector B 1 Hz sources ~3 nT/  Hz 1/f mag noise Temp fluct. M Johnson Perming  - Volume 0.01 mm 3 (wire) Operating TRT PowerBatter

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 trilayer w/NM spacer 60%  R/R min Co/Cu/Co “Spin Valve”  TMR – Insulator spacer 472%  R/R min at R.T. CoFeB/MgO/CoFeB Hayakawa, APL (2006) I M FM NM FM * * * * Forensics

28 MR Sensors – spatial resolution 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+ BARC.

29 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, (2002) “BARC” Bead Array Counter MR sens.

30 MR as low field sensors AMR Large area films GMR Small sensors Commercial: ~ $ 2 Unshielded sensors 2 shielded Flux concentrators TMR “Low frequency picotesla field detection…” Chavez, et. al, APL 91, (2007). F.C. State variableResistance B-fieldVector B 1 Hz sources ~200 pT/  Hz 1/f mag noise Temp fluct. M Johnson/Shot Perming  - Volume mm 3 film Operating TRT Powerbattery

31 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.

32 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 

33 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, MAY, 2002 Hall spec. 1Hz (nT/  Hz) White noise (> 100 Hz) (nT/  Hz) BICMOS Hall no flux concentrator CMOS - internal flux concentrators30030 CMOS - both internal and external flux concentrators 303 Internal flux concentrators Hall element

34 V I Hall Effect specifications State variableVoltage B-fieldVector B 1 Hz 300 nT/  Hz 30 nT/  Hz w/FC’s  - Volume mm 3 Operating TRT Powerbattery Commercial: ~$ 0.1  1 Applications Keyboard switches Brushless DC motors Tachometers Flowmeters  InAs thin film Disruptive - hybrid B

35 Disruptive technologies? Superconducting flux concentrator State variableGMR voltage B-fieldVector B 1 Hz 32 fT/  Hz  - Volume 0.1 cm 3 Operating Tcryogenic Powerline 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

36 Magneto-electric Disruptive No power required Two terminal device High impedance output State variablePiezo voltage B-fieldVector B 1 Hz sources 1 nT/  Hz pyro/static  - Volume 1 mm 3 Operating T -40 to 150  C Power0 Dong, et. al APL V86, (2005). Hristoforou, Sensors and Actuators A132 (2006). Magnetostrictive + piezo-electric multilayer Terfenol-D H PMN-PT V ME MO

37 Magneto-optic State variableLight intensity B-fieldVector B 1 Hz 1.4 pT/  Hz  - Volume 1 cm 3 Operating Tambient Powerline 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

38 More spintronic sensors… Extraordinary MR (EMR)  Hall effect with metal impurity Based on 10 6 MR in van der Pauw disks  Non-magnetic materials   Mesoscopic devices   R/R ~ 35% in field Replacement for GMR in hdds?  220% TMR in 2004 … Au InSb B Semiconductor Au impurity “Magnetic Field Nanosensors, Solin, Scientific American V291, 71 (2004) V I CMR

39 Colossal Magneto-resistance Manganite materials  La 1-x M x MnO 3 Phase transition – Jahn-Teller distortion  Low T – Ferromagnetic  High T – Paramagnetic semiconductor Large B-dependent resistance change at critical temperature (~260 K) Uses at room temperature:  Contact-less potentiometers  Bolometers Barriers to commercialization  High fields  Single crystal materials  High growth temperatures Haghiri-Gosnet, Renard, J. Phys. D: Appl. Phys. 36 (2003) R127–R150 Transistors

40 Spin transistors  Tunnel junction based devices van Dijken, et. al, APL V83(5) 951 (2003) Spin dependent hot e- transmission  Cu  Tunnel Barrier  Spin Valve  Schottky barrier 3400% magneto-conductance at 77 K Relatively low currents (10  A), a lot of noise sources  Spin-FET Quantum wire channel between 2 half metal contacts Many devices in parallel (10 11 ) can give ~fT/  Hz noise level   Wan, Cahay, Bandyopadhyay, JAP 102, (2007) Compilation

41 SensorBB n ( pT/  1 Hz) VolumePower SQUIDv.011 cm 3 Line Protons11 cm 3 battery e - He 4 /CSAM/SERF s/s/v1 / 5 /.001mm 3 – cm 3 Battery-line Fluxgatev101 cm 3 battery GMIv mm 3 battery MRv mm 3 battery Hybrid GMR/SCv cm 3 line Hallv30, mm 3 battery MEv10001 mm 3 0 Magneto-opticv1 cm 3 line Compilation Trend: Noise decreases as Volume increases Bn vs. V

42 B noise vs. Volume SERF He4 SQUID Proton F.G MO Hybrid CSAM ME GMI Hall MR Magn. Sensors

43 Noise vs. volume in magnetic sensors  Fluxgate magnetometers Increase volume (  ) & decrease loss (  )  AMR – make up for low  R/R by: Large arrays of elements (volume) good magnetic properties (reduce   Flux concentrators Increase volume Softer, low hysterisis to reduce  E resolution “Fundamental limits of fluxgate magnetometers…” Koch et al, APL V75, 3862 (1999)

44 Compare sensors based on volumetric energy resolution Energy resolution  Noise Power  Volume D. Robbes / Sensors and Actuators A 129 (2006) 86–93 Conclusions  S.C. F.M Device Energy Resolution e(J/Hz) SQUID w/pickup1 x SERF3 x Hybrid GMR/SC4 x GMI6 x AMR7 x CSAM2 x He44 x Fluxgate3 x GMR w/feedback4 x Hall5 x Magnetoelectric5 x TMR w/FC1 x

45 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 Noise floor decreases with volume Can look at intrinsic energy resolution of sensor Also need to evaluate high sensitivity against many other parameters:  Spatial resolution  bandwidth  dynamic range  cost, … Acknowledgement. Pick the right tool for the job!

46 Acknowledgements Steve Russek Bill Egelhoff John Unguris Mike Donahue John Kitching Fabio da Silva Sean Halloran Lu Yuan Jeff Kline