ModernSensors

Lecture9X.WuAReviewofLecture8AnoverviewofModernIRsensorsQuantum,e.g.MCTThermalMicrobolometerPyroAnanalysisofthermalIRsensorSensitivityMicrobolometerUncooledIRsensorarrayBasicIntent

Thislecturewilldiscussthebasicprinciplesbehinduseofelectromagnetisminsensing.Sincemanyestablishedsensorsrelyonelectromagnetism,thislecturewillcoverabroadsetofdesignsandreviewseveralproducts.

ElectromagnetismandInductanceAreviewofthepropertiesofaninductorasusedinelectroniccircuits.apassivecircuitelementwhichresistschangesincurrent.Theequationwhichgovernsitsbehavioris:assume(asforallofthedifferentialequationsinthiscourse)thatthecurrentandvoltagearebothoscillatingquantities:

Effective

R=-iLAnalogy:inductorv.s.resistorinductancemeasuringcircuits,

exactlyliketheresistancemeasuringcircuitsthemostcommonapproachtoinductancemeasurementisabridge.Howbigisatypicalinductance?acoilwitha1cmdiameter,a1cmlength,with1000loopsofwire:mH

Effectiveresistance@1kHz=60Whatisreallypreferredforaninductor?Highinductance:Thisachievedinawaythatisverymuchlikewhatisdoneincapacitors:materialandareaInaninductor,wecanfillthecoilwithamaterialwhichhasahighermagneticpermittivity.ironhasarelativepermittivityof~300,

permalloy(nickel/iron80/20)~5000TheconceptofinductanceIfwehaveasimplecoil,andtrytosuddenlycauseacurrenttoflowthroughit,theinitialcurrentflowcausesamagneticfieldtobegintoforminthecoil.Sincethismagneticfieldisincreasing,thereisachangeinthefluxthroughthecoil,andanopposingvoltageappears.Eventually,thecurrentrisestoitslimitingvalue,themagneticfieldisstable,andtheopposingvoltagediesaway.MagneticmotiondetectorThemagneticfieldisconfinedtotheregionbetweenthepolefaces,andisessentiallyzeroelsewhere.Themagneticfluxthroughtheloopsissimplytheareaoftheloopthatiswithinthemagneticfieldtimesthevalueofthefield.Avoltageisinducedintheloopwheneveritmoveslaterally.Motionsensor,notapositionsensorMagneticMotionDetectoranditsTransferfunctionsGeophonesThisapproachisthebasisofmanyso-called`movingcoil'detectors,inwhichavoltageisgeneratedwheneveranexternalsignalcausesacoiltomoverelativetoapermanentmagnet.Agoodexampleofacommercialproduct:theGeophone,asmadebyGeoSpaceCorpInthisdevice,asetofcoilsmeasureadifferentialvoltagewheneveraspring-supportedmagnetmovesgenerallyconstructedwithafairlylowfrequencyresonance-about1Hmonlyusedfordetectionofseismicsignalsorotherlow-frequencygroundvibrations,orintheoilexplorationbusinesswithburiedexplosivechargestomapundergroundresourcedeposits.GeophoneSeismometerThesensitivityofageophoneisafunctionoffrequency

canbealteredbyfeedingbacktheconventionaloutput.Thiswillnotimprovetheresolution,butwillamplifylowfrequencysignals.However,theresolutioncanbeimprovedbymeasuringproofmassposition.ExistingGeophones/SeismometersWhatisaconventionalgeophone?Howdoesitwork?(Mechanicalsensitivity)Howdoesitwork?(Electricalsensitivity)TotalSensitivityResolutionConventionalGeophoneResolutionHowtoimprovebothsensitivityandresolution?Changealow-costgeophoneintoacapacitiveoneCapacitanceElectricalSystemControlSystemDesignControllerDesignConceptsPredictedandMeasuredPerformanceAComparisonofPerformanceExperimentalSetupatStanfordProximitySensorApairofcoilsarewiredinabridgecircuitandbiasedwithanacsignal.Ifaconductingobjectispositionedneartheendofthedevice,itisclosertothesensecoilthanthereferencecoil.Thepresenceofaconductorcauseanadditionalopposingvoltageinthecoil,andtheeffectistoincreasetheinductanceofthecoils.Sincethesensecoilisclosertothesheet,itsinductanceisincreasedmore.SensingcoilReferencecoilProximitySensing:bridgecircuitTheeffectofthesensecoilchangingmorethanthereferencecoilTheamplitudeofthisdifferenceisproportionaltotheinductancedifference-whichisrelatedtothedistancetothemetalsheetinaverycomplicatedway.Theactualrelationbetweendistanceandinductancechangeistoocomplicatedtoderiveingeneral,sinceitreliesonthegeometryofthesituation,sothisapproachisnotgenerallyusedforaccuratepositionsensing.CommercialInductiveProximitySensorsApplicationsAutomationRobotics……CommercialinductivesensorInthissystem,theamountofmagneticfieldfromonecoilthatisdirectedtowardspartofasecondcoilisdependentonthepositionofaferromagneticelement.Thetwohalvesofthepick-upcoil(wiredtoVout)arewoundinoppositedirections.Iftheferromagnetwerenotpresent,thefluxineachhalfofthepick-upcoilwouldbeequalandopposite,andVoutwouldbezero.Whentheferromagnetispositionedinthemiddle,thereisalsoacompletecancellationoftheflux.However,whenevertheferromagnetisdisplaced,thefluxbalanceischanged,andtheneteffectisthattherewillbeavoltageacrossthepickupcoilwhoseamplitudeisproportionaltothedisplacementoftheferromagnetfromitscenterposition.CaseStudy:GiantmagnetoresistanceGMRdiscoveredin1988independentlybyBaibich

etal.inParisandBinasch

etal.in

Jülich.Itisthephenomenonwheretheresistanceofcertainmaterialsdropsdramaticallyasamagneticfieldisapplied.ItisdescribedasGiantsinceitisamuchlargereffectthanhadeverbeenpreviouslyseeninmetals.Ithasgeneratedinterestfrombothphysicists&deviceengineers,asthereisbothnewphysicstobeinvestigatedandhugetechnologicalapplicationsinmagneticrecordingandsensors.

Outline IntroductionScienceofGMRAnisotropicmagnetoresistanceGiantmagnetoresistanceDiscoveryofGMRFert’sandGrünberg’soriginalpapersFurtherresearchbyIBMApplicationofGMRGMR-basedspinvalvesinharddrivesImpactofGMRonthestoragemediaindustryGMR–whyisituseful?DiscoveryandapplicationoftheGMRphenomenonisresponsiblefortheubiquitousavailabilityofeconomical,highdensityinformationstorageinoursociety.Compact160GBMp3playersand1TBharddrives,nowwidelyavailable,owetheirexistencetoGMRandsubsequentrelatedadvances.ScienceofGMR

AnisotropicMagnetoresistanceAnisotropicMagnetoresistance–Reportedin1857byBritishphysicistLordKelvin.Whenacurrentispassedthroughamagneticconductor,resistancechangesbasedontherelativeanglebetweenthecurrentandtheconductor’smagnetization.Resistanceincreaseswhencurrentisperpendiculartomagnetizationanddecreaseswhencurrentisparalleltomagnetization.Cause:electronspin-orbitcouplingUsedasthebasisofharddrivereadingbeforeGMRwasdiscovered.ScienceofGMR

GiantmagnetoresistanceSystem:athinlayerofnonmagneticmaterialsandwichedbetweentwolayersofmagneticmaterial.Right:aFe-Cr-FetrilayerusedinGrünberg’soriginalexperiment.[3]ScienceofGMR:MottModelTheelectricalconductivityinmetalscanbedescribedintermsoftwolargelyindependentconductingchannels,correspondingtotheup-spinanddown-spinelectrons,andelectricalconductionoccursinparallelforthetwochannels.Inferromagneticmetalsthescatteringratesoftheup-spinanddown-spinelectronsaredifferent.(Wewillassumethatthescatteringisstrongforelectronswithspinantiparalleltothemagnetizationdirectionandweakforelectronswithspinparalleltothemagnetizationdirection.)ScienceofGMR

GiantmagnetoresistanceParallelmagnetizationUp-spinelectronsexperiencesmallresistance,down-spinelectronsexperiencelargeresistance.Totalresistanceis

ScienceofGMR

GiantmagnetoresistanceAntiparallelmagnetizationBothelectronspinsexperiencesmallresistanceinonelayerandlargeresistanceintheother.Totalresistanceis

ScienceofGMR

GiantMagnetoresistanceDifferenceinresistanceisgivenby:DiscoveryofGMR

FertandGrünbergDiscoveredbyindependentlybyProfessorAlbertFertofUniversitéParis-SudinFranceandProfessorPeterGrünbergofForschungszentruminJülich,Germany.BothgroupssubmittedpaperstoPhysicalReviewinthesummerof1988.DiscoveryofGMR

Fert60-bilayeredFe-Crstructureat4.2KNearly50%dropinresistanceobserved!![5]DiscoveryofGMR

GrünbergFe-Cr-Fetrilayeratroomtemperature1.5%dropinresistancereported[3]DiscoveryofGMR

IBMStuartParkinofIBMattemptedtoreproducetheeffectusingthesputteringtechniqueFertandGrünbergusedmolecularbeamepitaxy,amoreprecisebutslowerandmoreexpensivemethod.Parkin’sgroupsucceeded,observingGMRinthefirstmultilayersample’sproduced.Parkin’sgroupbeganexperimentingwithvarioussamplecompositionsandlayerthicknessestobetterunderstandGMRandhowtointegrateitintomagneticstorage.GMRinpractice

SpinValve[7].[8]GMR’seffectonharddriveindustryFirstGMRharddrivedeployed:16.8GB,IBM,1997Currentlargestharddrive:1TBbyHitachi,20074TB,2012[11][12]CaseStudy:GiantmagnetoresistanceGMRhasbeenthesubjectofahugeinternationalresearcheffortduetothenumeroustechnologicalapplications.Thelargestisinthedatastorageindustry:IBMTheeffectismostusuallyseeninmagneticmultilayeredstructures,wheretwomagneticlayersarecloselyseparatedbyathinspacerlayerafewnmthick.CaseStudy:GMRItisanalogoustoapolarisationexperiment,wherealignedpolarisersallowlighttopassthrough,butcrossedpolarisersdonot.Thefirstmagneticlayerallowselectronsinonlyonespinstatetopassthrougheasily-ifthesecondmagneticlayerisalignedthenthatspinchannelcaneasilypassthroughthestructure,andtheresistanceislow.Ifthesecondmagneticlayerismisalignedthenneitherspinchannelcangetthroughthestructureeasilyandtheelectricalresistanceishigh.GMR:SpinValveAspinvalveisingeneralasampleconsistingessentiallyofaGMR

trilayer:Onelayerisverymagneticallysoft-meaningitisverysensitivetosmallfields.Theotherismademagnetically'hard'byvariousschemes-meaningitisinsensitivetofieldsofmoderatesize.Asthesoft'free'layermovesaboutduetoappliedfields,theresistanceofthewholestructurewillvary.Thecentralpartofthesampleconsistsoftwomagneticlayers(inourcaseusuallypermalloywithathincoveringofCo),separatedbyaCuspcerlayer.Onemagneticlayerispinnedorexchangebiasedbyanantiferromagneticmaterial-usingFeMnandIrMn,

CaseStudy:GiantMagnetoresistanceBrain-computerinterfaceBytheendofthisyear,SanFrancisco–basedEmotiv'ssensor-ladenEPOCheadsetenablegamerstousetheirownbrainactivitytointeractwiththevirtualworldswheretheyplay.headset's14strategicallyplacedheadsensorsareattheendsofwhatlooklikestretched,plasticfingersthatdetectpatternsproducedbythebrain'selectricalactivity.detectsbrainactivitynoninvasivelyusingelectroencephalography(EEG),ameasureofbrainwaves,viaexternalsensorsalongthescalpthatpickuptheelectricalbustleinvariouspartsofthefurrowedsurfaceofthebrain'scortex,aregionthathandleshigherorderthoughts.Theseneuralsignalsarethennarroweddownandinterpretedin30possiblewaysasreal-timeintentions,emotionsorfacialexpressionsthatarereflectedinvirtualworldcharactersandactions

EEGandMEGElectroencephalography(EEG)isthemeasurementofelectrical

activityproducedbythebrain

asrecordedfromelectrodes

placedonthescalp.MEGSensorArrayFielddistributionby510coilsatdistinctlocations.Coilsconfiguredinto306MEGchannels.Thecoilconfigurationoptimallycombinesthefocals