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

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)

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

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, 165221 (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

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 A @ 1 m Frequency

B-field Ranges & Frequencies 1 mT 1 gauss  10-4 T ~ Earth’s B-field 1 mT 1 A @ 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

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

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

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

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 0.01 0.1 1 10 100 Preamp Hz Benchmarks

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

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

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

SQUID Magnetometer State variable Voltage (10’s mV) B-field Integrated Systems State variable Voltage (10’s mV) B-field Vector, gradients Noise @ 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

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

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

Proton magnetometer State variable Frequency ~ kHz B-field Scalar Noise @ 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

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

He4 e--spin magnetometer State variable Frequency ~ MHz B-field vector/scalar Noise @ 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

e--spin magnetometer Chip scale atomic magnetometer State variable Frequency B-field Scalar Noise @ 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, 081102 (2007). SERF

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 Noise @ 1 Hz < 1 fT/Hz Operating T 180 oC Volume ~ 3 cm3 Power line Kominis, et. al, Nature 422, 596 (2003) Solid state

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

Fluxgate Commercial: ~1 k$ Bext State variable Inductive 2f B-field Drive(f) Pickup(2f) Bext State variable Inductive 2f B-field Vector Noise @ 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.

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

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 125-133 (2004) GMI spec.

GMI specifications Commercial: ~100 $ State variable Z @ MHz B-field Vector Noise @ 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

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

MR Sensors – spatial resolution V+ 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)

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, 278-289 (2002) MR sens.

MR as low field sensors Commercial: ~ $ State variable Resistance AMR Large area films State variable Resistance B-field Vector Noise @ 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, 102504 (2007). F.C.

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

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

Integrated Hall sensors with flux concentrators Internal flux concentrators Hall element Noise @ 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, 12-15 MAY, 2002 Hall spec.

Hall Effect specifications V State variable Voltage B-field Vector Noise @ 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

Disruptive technologies? Superconducting flux concentrator Hybrid S.C./GMR State variable GMR voltage B-field Vector Noise @ 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

Magneto-electric H Magnetostrictive + piezo-electric multilayer State variable Piezo voltage B-field Vector Noise @ 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, 102901 (2005). MO

Magneto-optic Magnetometer head State variable Light intensity B-field Mirror-coated iron garnet B Fiber- optic State variable Light intensity B-field Vector Noise @ 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

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)

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

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

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

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.

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