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

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)

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

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)

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

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

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

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

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)

SQUID Magnetometer 2nd Derivative Pickup coil State variable Voltage (100’s mV) B-field Vector Noise @ 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$

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

e.g. magneto-cardiography Low TC SQUID – 10 fT/Hz @ 1 Hz High TC SQUID – 1pT/Hz @ 1 Hz Oh, et. al JKPS (2007) .01 0.1 1 10 100 1 10 100 1000 1pT/Hz

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

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

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

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), 227-234 (2007)

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

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

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, 081102 (2007).

Ferromagnetic magnetometers Fluxgate Magneto-resistive AMR – Anisotropic MR GMR – Giant MR TMR – Tunneling MR Giant Magneto-impedance 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 ~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$

Advances in Fluxgate technology Circumferential Magnetization Micro-fluxgates Apply current in core Single domain rotation 100 f T/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)

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).

Specs: MR sensors Commercial: ~ $ State variable Resistance B-field AMR Honeywell Philips State variable Resistance B-field Vector Noise @ 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, 102504 (2007).

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

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

Disruptive technologies? Hybrid superconductor/GMR Superconducting flux concentrator State variable GMR B-field Vector Noise @ <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)

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 19 165221 (2007)

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

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, 278-289 (2002)

Disruptive technology? Magneto-electric H State variable Piezo voltage B-field Vector Noise @ <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, 102901 (2005)

Giant Magneto-impedance Iac H Magnetic amorphous wire State variable Z @ ~ 1 MHz B-field Vector Noise @ 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 125-133 (2004) Commercial: ~100 $

Tried and True - Hall Effect State variable Voltage B-field Vector Noise @ 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 $

Spin Transistors

Magneto-optic

Search Coils

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

Comparison of MR & Fluxgates