Multisegment magnetoresistive sensor based on the GMR effect

SEHSORS ACTUATORS A

PHYSICAL

ELSEVIER

Multisegment magnetoresistive sensor based on the GMR effect P. Lobotka "'*, P. KrivoSfk h, I. Vavra ", M. Duree ", R. Schad", G. Verbanck " Y. Bruynscraede " h

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We ll:Jv~ develoJled u multiple lI1agnclon.~ ... i..t1\,c .. cn~(ll l.l~ an alternative 10 a Hull ...cn..or lor U\C in mappIng tht..llllagnctic field In the vIcinity of u ~uP"ICOlldllCI()r, MagncUJlc'll..tivlly CUfVC:-' 01 32 inu:viull
I. Introduction

Magnetic multilaycrs have attracteu much ilUenl1011 ~incc Ihey di'play a wide variety 01 intClclting phy,ical propcllies. Such multilayer, arc prep'lred by the srquential depo,ilion of thm ferromagnetic matenal (e.g., Fe, Co, Ni, CoFe) and nonmagnetic material (e.g., Cr. Cu. Ag. Au. 1'1). The thiekne;; of the individual ,ublayer, ranges from I to 10 nm. and the number of hilayers ranges from 3 to 100, In 1988 Baihich et a" II J found that the re,btance of an Fe/Cr muliliayer decrea,ed by a faclor 01 two when 11m multilayer was placed tn a parallel magnetic lield Th" phenomenon wns named giant magnetorc"'l~tance (GMR) a., the mugn~torcsbtance valuc'\ obtuined In comlllon magnetIc malerial, did not exceed I%. In an Fe/Cr multilayer Ihe changc in magnetorcslslUl1CC (normalilcd to the satUlation leve!) achievc, more Ih,1I1 100% and can be even higher it the electriC current fiows perpendiculal 10 the multilayer plane UofOrlUnlllely, thb vallie hold, for cryogenic temperatures; at room lempcnlturc the magnetorCsi\lancC decreascs to 16-20%. depen(hng on the material; u,ed. The ,alurallon field above which the res"tanee hecomc, practically coo"anI b aboull T. The Origin ofthe GMR effect" explained as a 'pin-dependenl ,ealterlng of conduct"," clectrons. The delail; can be found in the review a'licle by Dieny [21. * Corrc\pondlng .lUlllOr CornulI"lOll COST SCI.-fct,lrl,ll. CEe. DlJ XIII B/I.SOME I/M.200rucdclo1[{Ol,B-IW9Ilru\\CI",Bl:lgiulIl Tel +12 2. 290 5512 Fa, +32 ~ 296 5925. E'Ill,lll r ]Ohl11",I(g)Jgl2lCC hI:

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Recently. novel nunCI mb have been di~covercd: mnngnne,e oxide, 13 J. The,e nallve layered perovskile ,Iructures exhibil extremcly high magnetore'''lance up 10 10"%. At pre,enl, unfortunately. these materia" cannot be used for accurate magnetic-field mea!o>urements due to the hystere~is of the magnetoresbtance curve' and due to a wong dependence of Ihe rc,iSlance not only on the magnetic field but on the tempeIHture as well. Magnetic-field mapping in the Vicinity of a supercunductor is a very allmctive non-destrucllve method of evaluating a snpereondnclor', quahty. By solving Ihe inverse problem, a di,mbution of current inside the ,uperconduetor i, obtainable. Two methods arc commonly u,ed for IhlS purpo,e: (i) "lagnelo-optieal imagmg (limited 10 ,mall apphed fields of about 3 mT) and (ii) Hall sen,or imaging The lalter method h,,, ,ome drawl.aeks. Fnsl, il i,difficult to place a conventional Hall 'ensor close enough 10 a superconduclor due to sen'or encapsulalion. The typical distance between the ,ample ,urface and Ihe Hall sensor ranges from 0,25 to Imm [4,51. Secondly.lhe aClive areaofconvenlional Hall 'ensor, IS too large, aboul 12 Jlm X 12 Jlm to 50 f,m X 100 Jlm 15-71. which III fact impedes the observation oftilly local magnetiC-field fiucmations caused. e.g.• by cl11ek, mthe BSCCO ,upercondueling layer. Since they me already ,moolhcd at the ,onsor', position Thirdly. tl lake, at lea't 30-'10 mm to ,can the whole ,ample [4]. DUring this relatively long lime the field disll ibulion can change due to relaxation procc:-.~es occurring in the ~upcrconductor.

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Keeping in mind all the drawbacks of maglletlc-lield mappmg by Hall ,ell,or,. we tried to fabricate allotber ~cn~or bH~cd

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2. Experimcnt:t1 III Fig. I the maglletoresi,tivity cUlves measured atthlee ditferellt temperatures for the multilayer IFe(20 A) ICr( 12 A) I X 10 arc shown. A, can be seell, the sellsitivities at 4.2 K and at 77 K nre practically the same. This fact encouraged u, to use the Fe/Cr multilayer for sen,or fabrication. since the bigh-7; superconductors arc commonly tested at liquidnitrogen temperature. In Fig. 2 the depende""e of the mag· netores;;llInee on the lielddireetion is illustrated It is obvious that for measurement at lower lields the parallel onentation

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is more favourable than the perpendicular one due to a higher sensor sensitIvity. Our design was focused on ti,e fabrication of a multiple magnctoresistive sensor, which should allow the measurement of the magnetic field in 32 pOInts at once. Thus, if the width of a BSCCOI Ag ,upereonducting tape does not exceed the length of the GMR sensor, it is not necessary to move the ,ensor transversally during the mappmg (Fig. 3). We used an [Fe(30 A)/Cr(12 A)] X50 multilayer prepared by molecular beam epitaxy (MBE) on a MgO( 100) substrate [8]. It was patterned by photo:lthographic techniques into a 10 fun wide and 3 mOl long 'tripe with 33 voltage contacts dIviding the 'tripe into 32 'eetions, each 01 them 80 fLm long. Afterpalleming, the substrate edge near the sensitive stripe was ground to miOlmize the distance between the stripe and the object to be measured (a supereonducting tap~). CarefUlly performed grinding allows this dimension to be decreased to below 10 fLm. For the calibratIon of the sensor we mea,ured the magne· loresistan"e of each stripe section by a d.c. four-probe method. The resistance was measured at both current polarities to avoid the thermo-e.m.f. The sensor was placed ;n a copper magnet giving a maximull1 field of about 0.3 T. The magnetic field applied was parallel to the ;:-axis as shown in FIg. 3. The measurements were performed at 77 K.

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3. Results and discussioll Fig. 4 !ripe segments versus the magnetic field, As can be seen, in a small applied field, the sensitivity oflhissensor is about 10% T- f, and inr;reases al higher lield, to the maXlmul11 sensitivilY of about 30% T- l • In the external field of 0.1 T the resolution in resistance is 3 X 10-'. This mean< thallhe correspondlllg lesolution of the magnetic field is about O.OlmT (0.10). This value is comparable with convenlional Hall sensOl'. SlIlce the typical value of the magnellc field in the vicinity of a supercunductor is about 2 mT [51, we conclude thatlhis scnsor b at least a good alternative to the Hall sensor for mapping purposes. The origin of the section resi
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S,llce a decisive dimen,ion of the OMR sen,or is the tolUl thickness of the multilayer, the a(.ll':e area of one section or this prototypc multiple sensor is 80 fUll (length) X0,21 J1111 (tolllilhickness), a value much sl11aller than thai or a Hall sensor (e.g.. 12 111m X 12 111m). We want to point outthallhe ael,ve area or the OMR sensor can be easily reduced: a feasible value is 3 J1Il1XO.04 J1m. It is worlh noting that the saturation Iield H, of the multilayer can be lailored simply by varying lhe thiekncss of the magnetic sublayers (If, -l/lrel. In Ref. 18] the aUlhors achieved a saturation lield H, = 13 T in the multilayer I Fe (0,45 nnt)/Cr( 1.2nm) I x50. 4. Conclllsinlls We have discussed a ll1ultisegment magnetoresistive scnsor based on an Fe/Cr multilaycl exhibiting a giantmagneloreslStance effect. Thl< sen,or was designed for the purpose of magnelic-lield l11apping in the viemily ofasuperconductor. In our oplllion, it has ~OIllC advantages in cOlI'IlJanson with common Hall ~cnsors:

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Rel'erences II J M N B.liblch. J M Brolo. A Fen. r: Nguyen Van DJu. F. PClroft. P. ElIel1nc. G Creu7ct. A Fricdcnclt JIl(J J ChMcl.". Giant mngnctorC\I,t.mcc of (00 I ) Fc/ (00 I )Cr nlLlgncllc \UpCrl"ltIC(''', P/n·s. RI'I' Lt'II , 61 (1988) 2472-2475. 121 B, Dlcny. GmnllllJgnctorc\I\IJ.lIcc III \pm-val ...c multllayer\, J M(IK" Mugu Maler, 136 (199../.) 335-354 [31 B. R"...eJu. A. M.ugn.m :lnu V Clign.lcrt. Spe<..!.Lcul.lr gmnl lll.lgnclO· rC"l\t,IllCC effect-. mlhc polycry\[lllhne j,CWV1>klle PrSrCJMnO.J SoJlt1 Slclfe Clli'm" 1/7 ()()95l 424--416, [-I] P D Gr.lllt.M W,Denfolf. W Xmg.P Brown.S Gu\'orko.... J C Irwin, B Hemnch. H Zhou. A A, Fife and A R. Cragg. Dclcfl111n.lt!on of currenl .mu nu.x dll,(nbutlOn In \quare" 01 thm-hlm hlrh-lcmpel,llure \upcrconuucror\. Pllnrc (l C, 229 ( 199-1) 289-300, 151 p, Kolll1lJIl. M, Polak. 1. P,lc!. S Dueht:l. L D.lmchk. F HJ.n1c and G PIl:\ch. MJgllCIlc li.-Ill t!l\tnbU[lOn .,hove ,I ~upcrcOllullctlng VB.ICUO \,unplc\ u\ .m mUle.llion of \,lI11p1c IOhornogcncltlc\. Su(,er£'fl/ld. Sci Tl'c!mol.. 7 (199-1) 67-71 161 M, LJhllOcn. J P,I.l\I, J. S,lrJ..anlcml. Z HJn .lOd T. Frc1tolt. Homogenelly \tully 01 [li·2123/ Ag monohlJmcniary IUPC1> U\lOg ~IJII \Cl1\or mJgnc[omctry. Plml(a C. 2-14 (1995) 115-121,

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lIiogrnphics I'e/<'I'!.oho/I.I/ (boll1 in 1950) b a ,el1lOI selenlbl althe Instilute uf Elecli ical Engineering 01 the Slovak Academy of Sciences (lEE SAS) in Ilmli,la"I, Slovakia. He graduated at tbe Technical Univer,ity in 13H1lblava in 1973. and ,n 1980 received hb Ph.D. in electronics and vacuum techniques at the SAS. His main ,dentilic interests include electncaltHln,pori in thin metallic IiIm~ .... lIpcrcol1dllcting phcnomena in Illultilaycn;, (I11t1magnctic ~en'\or~ ba~ed 011 multilaycr.... 1'1//'0/ Krif'O.lIl (born in 1970) received hb M,S. degree m electronic> In 1994 at the Slovak T~chnical Univer,i1y. [lralblava He b currently prepal'lng his Ph.D. on magnetic

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It'o Wpm (born in 1949) i, a senior ,cienti,t allhe lEE

SAS. I3ralislava. In 1972 he received Ihe tv; ~. degree in phy,ical engmeering althe Slova~ TeehnieallJniver,ity. He obtained hb Ph.D. in elecllOnics and vaenum phy,b althe SAS. Hb main intere,ls me lhin-film leehnology and 'Iruclural analy,b of thin films by t",nsmi"ion election miero,copy. In li.e ""tlen yems his aelivities have been concentrated on ,uperconducllng and ferromagnetic multilayers Meit;o/)I/ree (born in 1974) ba,tudenlolthe Faculty of Eleclrical Engineering and Informalion Technology al the

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lIail/l't set",J! (born in 1961 i graduHled allhe Univer,ily of Hannover in 1987 (,olid ,talc physic,). where he also received hb Ph.D, in 1991. For the next Iwo year, he was wOlking al the KU Leuven, Belgium. mainly on magnetic and lranspnrt propeJlies of magnelic superlallice,. Now he I"" a po'ldoetoral po"lion al Ihe KU Nijmegen, The Nelherlands. coordinating a I3RITE-EURAM projeci on magnelooptical nem-field STM. Hb main ,cienlilie inlelest, arc t"m'porl and magnelie propel tic, oimelalilc thin liiJm and multilayel~.
Geer/ Vet/}{IIIc/. obtained hi, masler's degree in physic" fromthe Katholieke Umversileil Leuven in 1992.le,em·ching strueluml and magnetic properties of FelCr supellallice,. He j, currently prepming hb PhD. m phy,ies in Ihe Laboratnry 1111 Solid Stale Phy,ks and Magnel"m in Leuven. ,tudying milgl1ctorc~blallcc effects in cpililxialthill films and ~upcr· laHices. U11l1 ~~lpcr<:ol1dlll'ting prnpcrtic,., 01 superconductor! ferromagnelmnltilayers.

YI'W' IJlllyl1seltU't./e is a prolc~!olsor and direclor or the LabOlalory lor Solid Slate Physics and Magneli"" al the Katholiekc Univel"lell Leuven, Belgium. The laboralory groups ,ome 40 scienlb". active IJ1many fields 01 eonden,edmalleI' relemeh, includll1g low-dimenSIOnal 'yslems, 'UI face phy,ies, superlatuces, 'llpcreondJlelivity and magnelbm. He is a member of the Royal Ilclgian Academy of Sciences. Lileratore and Arl'. member of the Council of the Belgian Nuclear Cenler and the EUlOpean Synchrolron Radialion Facilny. edJlmlal board member of Physico/lledell' IJ and a fellow of the American Physical SocIety. He has published more Ihan 300 papers.