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Vol. 102 (2002) ACTA PHYSICA POLONICA A No . 1 Proceedingsof theIV I SSP MS '01 , J aszo wi ec 200 1 M u lt il ay er Stru ctu re s w it h G ia nt Ma gn et ore sis tan ce F. St obiecki a and T. St obie ck i b; Ê a I nst i t ut e of Mo l ecul ar Physi cs, Pol i sh Aca demy of Sciences Sm oluchowskiego 17, 60-179 Pozna¥, Poland b D epartm ent of El ectronics, Uni versity of Mi ning and Meta llurgy Al . Mi cki ewicza 30, 30-059 Kra k§w, Poland T h e phe nomeno logi cal de scrip tio n of the giant magnetoresistance e˜ect as w ell as the discussion of the requirements w hich must be fulÙlled in gi- ant magnetoresistance thin Ùlm structures are given in the Ùrst part of our review . In the second part the magneti zation reversal and giant magnetore- sistance e˜ect of antif erromagnetical l y coupled multilayers , spin valve and pseudo- spin v alve thin Ùlm structures are explained . For these structures we also discuss the inÛuence of the structure def ects such as surf ace roughness and pinholes on the giant magnetoresistance e˜ect. PACS numb ers: 75.70.Pa, 75.70.{i 1. I n t r o d u ct io n Ma gneto resistance (M R ) is the change in electri cal resistance of a conduc- to r caused by m agneti c Ùeld. The dom inati ng ori gin of thi s e˜ect can be di˜erent dep endentl y on the m ateri al and / or the structure of the sam ple. In nonm agneti c conducto rs the MR is due to the Lorentz force the magneti c Ùeld exerts on m ov- ing electrons. Thi s e˜ect is relatively small, for example in Cu the relati ve in- crease in resistance wi th m agneti c Ùeld is  R =( R H ) = 1:3 È 10 À 3 %/ kOe (at room tem perature, RT) [1]. In m agneti c materi als and parti cul arl y in m agneti c thi n Ùlm structure s the spin polari zati on of electrons generates another, usually larger, contri buti on to the MR e˜ect. In ferromagneti c sam ples the resistance de- p ends on the m utua l ori enta ti on of the magneti zati on and the current di recti ons. Ab out thi rty years ago thi s e˜ect kno wn i n l i tera ture as ani sotro pi c m agneto resis- ta nce (AMR ) wa s intensi vel y i nvesti gated i n thi n ferro m agneti c Ùlms. In p ermal l oy (Ni 80 Fe 20 ) Ùlm s the resistance changes up to 5% were observed in magneti c Ùeld Ê corr esp on din g au t h o r; e-m ail : sto bi eck @uci.ag h .edu. p l (95)
14

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Page 1: M ult il ayer Stru ctu re s w it h G iant Ma gnetore sistan ceprzyrbwn.icm.edu.pl/APP/PDF/102/A102Z107.pdf · M ult il ayer Stru ctu re s w it h G iant Ma gnetore sistan ce ... dependentl

Vol . 102 (2002) ACT A PHY SIC A POLON IC A A No . 1

P ro ceed in g s of t h e I V I SSP MS '01 , J aszo wi ec 200 1

M ult il ay er Stru ctu re s

w it h G ia nt Ma gn et ore sis tan ce

F. St obi eck i a and T. St o bie ck ib; Ê

a I nst i t ut e of Mo lecul ar Physics, Pol ish Aca demy of Sciences

Sm oluchowskiego 17, 60-179 Pozna¥, Polandb D epartm ent of El ectronics, Uni versi ty of Mi ning and Meta l lurgy

Al . Mi cki ewicza 30, 30-059 Kra k§w, Poland

Th e phe nomeno logi cal descrip tio n of the giant magnetoresistance e˜ectas w ell as the discussion of the requirements w hich must be fulÙlled in gi-

ant magnetoresistance thin Ùlm structures are given in the Ùrst part of ourreview . In the second part the magneti zation reversal and giant magnetore-sistance e˜ect of antif erromagnetical l y coupled multilayers , spin valve andpseudo- spin valve thin Ùlm structures are explained . For these structures w e

also discuss the inÛuence of the structure defects such as surf ace roughnessand pinholes on the giant magnetoresistance e˜ect.

PAC S numb ers: 75.70.Pa, 75.70. {i

1. I n t rod uct io n

Ma gneto resistance (M R ) is the change in electri cal resistance of a conduc-to r caused by m agneti c Ùeld. The dom inati ng ori gin of thi s e ect can be di ˜erentdependentl y on the m ateri al and/ or the structure of the sam ple. In nonm agneti cconducto rs the MR is due to the Lorentz force the magneti c Ùeld exerts on m ov-ing electrons. Thi s e˜ect is relati vel y smal l, for exam ple in Cu the relati ve in-crease in resistance wi th m agneti c Ùeld is  R =( R H ) = 1 : 3 È 1 0 À 3 %/ kOe (atroom tem perature, RT) [1]. In m agneti c materi als and parti cul arl y in m agneti cthi n Ùlm structure s the spin polari zati on of electrons generates another, usual lylarger, contri buti on to the MR e˜ect. In ferromagneti c sam ples the resistance de-pends on the m utua l ori enta ti on of the magneti zati on and the current di recti ons.Ab out thi rty years ago thi s e˜ect kno wn in l i tera ture as anisotro pic m agneto resis-ta nce (AMR ) wa s intensi vel y investigated in thi n ferrom agneti c Ùlms. In permal loy(Ni 8 0 Fe2 0 ) Ùlm s the resistance changes up to 5% were observed in magneti c Ùeld

Ê corr esp on din g au t h o r; e-m ail : sto bi eck @uci.ag h .edu. p l

(95)

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96 F . St obiecki , T . Stobiecki

H ç 1 0 Oe [2{ 5]. Such considerabl e resistance changes at low m agneti c Ùeld (hi ghÙeld sensiti vi ty) m ake AMR e˜ect attra cti ve for various appl icati ons (see for ex-am ple [6, 7]). From all of them , the m agneto resisti ve read heads for magneti c harddi scs, intro duced on the mark et by I BM in 1990, seem to be parti cul arl y im por-ta nt. Thus AMR e˜ect was the precursor for appl icati on of new MR e˜ects such asgiant magnetoresistance (GMR ), tunnel m agneto resistance (TMR ) [8] and colossalm agneto resistance (CMR ) [9].

In thi s lecture we concentra te on the giant m agnetoresistance e˜ect in currentin plane (CIP) conÙgura ti on onl y. Parti cul arl y, in Sec. 2 the ori gin of GMR e˜ectis expl ained, in Sec. 3 di ˜erent GMR thi n Ùlm structures are described.

2. G i ant m agn et or esis t ance e˜ ect | ph enom enolo gical d escr i pt ion

The GMR e˜ect (Â R =R ¤ 7 0 % at 4.2 K) was discovered in Fe/ Cr m ul-ti layers (MLs) [10, 11]. Two years earl ier [12, 13] i t wa s demonstra ted tha t forsuch MLs due to exchange interl ayer coupl ing (see Sec. 3.1) between ferrom ag-neti c layers (F e in thi s case) across a m etal l ic spacer layer (Cr) the anti para l lelm agneti zati on conÙgurati on (anti ferrom agneti c interl ayer exchange coupl ing) be-tween neighbouri ng Fe layers can be obta ined for a given thi ckness of Cr. It shouldbe noti ced tha t GMR e˜ect described in Ref. [10, 11] was observed onl y for Fe/ CrMLs wi th anti f erromagneti c coupl ing. For such MLs the resistance drops as them agneti zati on conÙgurati on in neighbouri ng Fe layers goes from anti paral lel al ign-m ent at magneti c Ùeld H = 0 , to para l lel one at H = H S (H S i s the satura ti onÙeld, i .e. H necessary to order the magneti zati on of ferromagneti c layers in Ùelddi recti on). The comm on expl anati on of the GMR e˜ect (see e.g. [14]) is the di f-ferent spin-dependent scatteri ng pro babi l ity f or spin-up and spin-down electrons,i .e., di ˜erent resisti vi ty for ei ther spins conÙgurati on (£

"and £

#). Due to the Paul i

exclusion pri ncipl e, electrons can be scattered from impuri ties or defects into thequantum states in the vi cini ty of the Ferm i level (E F ). Thus the scatteri ng prob-abi l i ty is proporti onal to the numb er of states avai lable for scatteri ng at E F , i .e.,to the density of states (£ / D ( E F ) ). In the tra nsiti on meta l ferro magnet, theup and down spin bands are spli t and the density of states at the Ferm i levelfor electrons wi th spin up and down is di ˜erent (D

"( E F ) 6= D

#( E F )) (Fi g: 1) . For

m ajori ty electrons wi th spin para l lel to the local m agneti zati on (spi n up) usual lythe density of sta tes at the Fermi level is lower as for m inori ty electro ns wi th spinori ented anti para l lel (spi n down) D " ( E F ) > D # (E F ) and, consequentl y, £ " < £ # .Co nsidering low probabi l i ty of electron scatteri ng wi th spin reorienta ti on we canassume tha t the to ta l current can be vi ewed as the one consisti ng of two chan-nels in para llel (Mo tt ' s two- current m odel [15]), one for electro ns wi th spi n upand the other one wi th spin down. Taki ng into account two- current m odel andthe di ˜erence in resistivi ty (£ " < £ # ) we can expl ain the GMR e˜ect in layeredstructures consisti ng of ferrom agneti c (F) layers separated by non- ferrom agneti cm etal l ic layer (S).

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M ult i layer Str uctures wi th Gi ant Ma gnetoresi stance 97

Fig. 1. Schematic spin- spli t density of states (D O S) for 3d transition metal (F e, C o

and N i) and Cu representing the spacer and magnetic layer, respectively . T he relative

p osition s of the bands for spin- up and spin- dow n electrons give rise to spin- dep endent

scattering.

For para l lel m agneti zati on conÙgurati on ( " " ) of adj acent F layers (Fi g. 2a)there are di ˜erent scatteri ng probabi l i ti es for either spin di recti ons. The electronswi th spi n up (low scatteri ng pro babi l i ty) form low resistivi ty channel and the to ta lresisti vi ty £ "" = £ " £ # =( £ " + £ # ) i s low. For anti para llel arrangement (Fi g. 2b) ofm agneti zati on in successive F layer there are sim ilar scatteri ng events for bothtyp es of electrons. W hat is the low resistivi ty electron species in a layer becomesthe high resistivi ty electron species in the next. Thus in the channel for electronswi th spin up as wel l as wi th spin down the resistivi ty is expressed as ( £

"+ £

#) =2

and the to ta l resisti vi ty £"#

= (£"

+ £#

)=4 . Consi dering the equati ons whi ch de-term ine resisti vi ti es for both m agneti zati on conÙgurati on we can express the re-sistance changes observed duri ng magneti zati on reori enta ti on from anti para l lel topara l lel as

GMR = Â R =R = ( R "# À R "" ) =R ""

= [ (£#

À £"

) = ( £#

+ £"

) ] 2 = [ ( ˜ À 1 ) = ( ˜ + 1 )] 2 : (1)

In the above equati on the param eter ˜ = £ # =£ " describes the di ˜erence in thescatteri ng pro cessesoccurri ng in the volume of f erromagneti c layers f or both spin

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98 F. Stobiecki , T . Stobiecki

Fig. 2. (a) Schematic explanati on of GM R in a magnetic multilayers ; arrow s in the

layers indicate the magnetizatio n direction; "" ferromagnetic and " # antif erromagnetic

state; stars represent electron scattering at the interf aces. (b) T he low resistivi ty state

is for electrons w ith spin parallel to the lo cal magnetizati on, w hen the high resistivi ty

is for electrons w ith antiparall el spin to the magnetizati on.

ori enta ti ons. However, due to hi gher concentra ti on of scatteri ng centres (i m pu-ri ti es and defects) at the interf ace of thi n Ùlm layered structures , the scatteri ngof electrons in interf ace region gives usual ly the main contri buti on to the GMRe˜ect. Al tho ugh spi n dependent scatteri ng in the volum e and/ or at the inter-faces expl ains the microscopic ori gin of the GMR e˜ect, in m acroscopi c sampl estwo suppl ementary requi rem ents must be fulÙlled. Fi rstl y, there must exist a cer-ta in non-coll inear conÙgura ti on of m agneti zati on in adjacent ferrom agneti c layerswhi ch can be changed by an externa l m agneti c Ùeld.

The resistance changes in F1 =S=F 2 structure s vs. the angle ' between m ag-neti zati on di recti ons of F1 and F 2 layers are expressed as [16]:

R ( ' ) = R 0 + Â R (1 À cos ' ) =2 : (2)

Thus the GMR e˜ect is m axi m al for structures in whi ch the changes of ' wi thm agneti c Ùeld are from 180£ to 0£ (f rom anti para l lel to para l lel conÙgurati on).

The second requi rem ent concerns the relati on between the thi ckness of in-di vi dual layers and mean free path (MFP) (typi cal ly 10 nm for m etal lic Ùlm s) ofspi n-up and spin-down electrons. The GMR wi l l exist only i f electrons can sampl em ore tha n one ferrom agneti c layer. Thus wi th increasing thi ckness of S layer ( t S )in F/ S/ F structures the GMR e˜ect decreases and for t S Ñ MPF disappears. Thi se˜ect, cal led in the l i tera ture as shunti ng e˜ect, expl ains also the decrease in GMRwi th increasing thi ckness of f erromagneti c layers ( t F ) in whi ch the interf ace scat-teri ng is m ore e£ ci ent tha n in bul k. However, both for F and S layers the m inimal

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Mul t i layer Str uctures wi th Gi ant Magnet oresi stance 99

thi ckness assuring suita ble GMR is also l imi ted. The lay er by layer (F rank{ vander Merve) growth m ode of thi n Ùlm s is real ized onl y for well deÙned depositi oncondi ti ons. Volm er{ Weber or Stra nski À Kra stanow growth m ode is observed m orefrequentl y. In these cases the conti nuous layer (f ul l coveri ng of the deposited area)is form ed f or nominal layer thi ckness essentia lly higher tha n one monolayer. Thedi sconti nui ties of S layer in F/ S/ F structures , i .e., the exi stence of m agneti c bri dgesbetween ferrom agneti c layers can destro y anti para l lel alignm ent of m agneti zati onand thus GMR e˜ect (see e.g. [17{ 20]). The granul ar structure of F layers forsmal l clusters ta ke one superparam agneti c behavi our [21]. Theref ore the opti malthi ckness range both for S and F layers in layered GMR structure s is stronglyl im ited (neglecting non- typi cal structures 0 : 4 ç t F ç 3 nm , 0 : 8 ç t S ç 3 nm ). Ofcourse for such smal l thi cknessesthe inÛuenceof structura l defects (di ˜use and/ orrough interf ace, disconti nui t y of spacer layer) on magneti c and m agneto resistancepro perti es can be very strong.

3. Th in Ùl m st r uct ures wi t h G M R

D ue to possible appl icati on as magneti c Ùeld detecto rs and parti cul arl y asm agneti c read heads, a large numb er of thi n Ùlm structures have been investigatedduri ng last years. It wa s dem onstra ted tha t the GMR e˜ect is present not only inm agneti c mul ti lay ers but also in a num ber of other arti Ùcial structures in whi chthe m agneto resistance resul ts from orderi ng of m agneti zati on conÙgurati ons dueto the m agneti c Ùeld. In thi s section we shal l di scuss basic system s as: periodicm ulti layers, spin valves and pseudo-spin valves in whi ch GMR e˜ect is observed.

3. 1. Per iodic mul ti layer s wi th ant i fer romagnet ic exchange coupl ing

As wa s mentio ned above the periodi c mul til ayers F/ (S/ F) N (where N i srepeti ti on num ber of bi layer S/ F) were histori cal ly Ùrst system in whi ch GMR wasobserved. Anti para llel al ignment of m agneti zati on in neighbouri ng ferrom agneti csubl ayers is due to interl ayer exchange coupl ing (IEC) vi a conducti on electronsof non- ferrom agneti c spacer S. Ma ny revi ew arti cles have been wri tten on thi ssubj ect where theo ry and Ùrst exp eriments were discussed (see e.g. [14, 22{ 28]).Theref ore in thi s paper we conÙne ourselves to m entio ni ng onl y the behavi our ofIEC whi ch are im porta nt to expl ain GMR in periodic MLs:

| The oscil lati on of the coupl ing energy J (f erromagneti c J > 0 and " " , anti -ferrom agneti c coupl ing J < 0 and " # ) and decay of the ampl i tude oscil lati onswi th increase in the thi cknessspacer ( t S ). The perpendi cular conÙgurati on ofm agneti zati ons in the special condi ti ons were also observed as a resul t of IEC.

| The oscil lati on period of IEC depends on band and crysta l structure of them ateri al of the spacer. For polycrysta l l ine MLs the typi cal values of theperiod are equal to several latti ce constants.

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100 F. Stobiecki , T . Stobiecki

| The coupl ing energy (J ) of IEC depends also on the typ e of m ateri als ofthe ferrom agneti c and spacer layers. The stro ngest coupl ing is observed forsuch ki nd of m ateri als whi ch are characteri zed by high contra st in the spindependent reÛectivi ty . Thi s condi ti on is f ulÙlled i f m ateri al of ferrom agneti cand spacer layer is selected from the sam e or neighbouri ng colum n of theperiodi c system . The im perfecti ons of the interf ace structure between ferro-m agneti c and spacer layer (e.g. roughness and/ or not sharp concentra ti ongradi ent) and crysta l structure of the spacer (e.g. am orpho us) decreases theJ value.

| For MLs wi th negl igible ani sotro py and thi ckness of the spacer ( t S ) assur-ing anti ferromagneti c coupl ing, the coupl ing energy (J ) is determ ined bysatura ti on Ùeld H S (Ùeld necessary to order the m agneti zati on vecto rs, ofeach indi vi dual ferrom agneti c layers, in the Ùeld di recti on) by the rela ti onJ A F = À M S H S t F =4 (where M S i s the satura ti on magneti zati on and t F i sthe thi ckness of ferromagneti c layers, respect ively).

As we have m enti oned in Sec. 2 the m ain requi rements for GMR e˜ect are thechanges of mutua l magneti zati on conÙgurati ons under externa l m agneti c Ùeld. In

Fig. 3. Magnetizati on hysteresis loop of N i 8 3 Fe (2 nm) /C u( ) multilayers for di˜erent

thic knesses of C u spacer. T he couplin g energy of the Ùrst and second antif erromagnetic

maximum are 4 6 10 J m and 0 6 10 J m , resp ectively .

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Mul ti layer Structures wi t h Gi ant Magnet oresistance 101

Fig. 4. Magnetoresisti vi ty ratio ( Â R =R ) as a function of external magnetic Ùeld of

N i 83 Fe1 7 (2 nm)/C u( t ) for di˜erent thickness of Cu spacer. GMR e˜ect was observed

only for A F I, A F II and mixed state FF/A F.

periodi c MLs typ e of F/ (S/ F) N thi s requi rement is ful Ùlled for anti ferromagneti ccoupl ing (" # ). However, the coupl ing energy J does not inÛuence the  R =R butdeterm ines the H S value.

Since the discovery of GMR in Fe/ Cr MLs [10] thi s phenom enon wa s alsofound in di ˜erent periodic MLs, where ferrom agneti c sublayers are 3 d metals (F e,Co , Ni and thei r al loys) and predo minantly as spacer are used: Cu, Ag, Au andR u. The outsta ndi ng values of GMR (GMR = ( R m ax À R m in ) =R ) are for Fe/ Cr(220% at T = 1 : 5 K) [29] and Co/ Cu (80% at RT) [30]. From the appl icati onpoint of vi ew not only high value of GMR is importa nt but also the shape ofthe dependence of R (H ) and value of H whi ch determ ine the Ùeld sensiti vi ty ofGMR e˜ect. For m agneti cally isotro pic F layers coupl ed anti ferromagneti cal ly inF/ (S/ F) N , the M ( H ) dependence is l inear and corresp ondi ng GMR (H ) (deÙnedby GMR (H ) = 1 0 0 % [ R ( H ) À R ( H )] =R ( H )) i s parabol ic accordi ng to the rela ti onGMR (H ) / [ M ( H ) =M ] for H < H .

In real MLs m ul ti layer system s apart from anti ferrom agneti c orderi ng alsoperpendicul ar m agneti zati on al ignm ent appears whi ch gives ri se to convexi t y ofM ( H ) dependence (Fi g. 3) and l inear (tri angle shape) in GMR (H ) (Fi g. 4). Re-versa l m agneti zati on pro cess in MLs of Ni Fe (2 nm )/ Cu( t ) in the form of M ( H )

and GMR (H ) curves are presented in Fi g. 3 for sampl es wi th di ˜erent thi cknessof Cu spacer.

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102 F. Stobiecki , T . Stobiecki

The presented dependencies allow us to dra w the fol lowi ng conclusi ons:

| Sampl es wi th the thi ckness of Cu spacer t C u = 1 nm and 2 nm show an-ti ferrom agneti c coupl ing (AF I and AF I I, respecti vel y (Fi g. 3)), whi ch isindi cated by M R ¤ 0 (anti para ll el orderi ng of magneti zati ons in neighbour-ing F layers) and GMR e˜ect is observed (Fi g. 4).

| The satura ti on Ùeld (H S ) for MLs wi th t C u = 1 nm (Fi g. 3 (AF I )) is about8 ti m eshi gher tha n for t C u = 2 nm (Fi g. 3 (AF I I)). It is due to the decreasein IEC wi th increase in the spacer thi ckness.

| The value of GMR for MLs wi th t C u = 2 nm (Fi g. 4 (AF I I)) is smal ler tha nfor t C u = 1 nm (Fi g. 4 (AF I)) due to the shunti ng e˜ect (see expl anati on inSec. 2).

| The Ùeld sensiti vi ty of GMR (S = GMR =H S ) for MLs wi th t C u = 2 nm ishi gher tha n for t Cu = 1 nm because the changes of H S are bi gger tha n thechanges of GMR .

| Ferrom agneti cally coupl ed Ùlm s do not show GMR e˜ect because m agne-ti zati ons of adj acent layers are always (i ndependentl y of value of externa lm agneti c Ùeld) ori ented in para l lel.

| Parti al ly ferro- and anti ferrom agneti cal ly coupl ed Ùlm s (f or whi ch 0 < M <

M ) show tha t GMR is pro porti onal to anti ferromagneti cal ly coupled frac-ti on (F ) of MLs accordi ng to the relati on: GMR / F = 1 À M =M .

Presented M ( H ) and GMR (H ) curves (Fi g. 3 and 4) and the pro perti es discussedabove are typi cal also of other F/ (S/ F) MLs but in num erical values di ˜erencesof GMR , IEC energy and oscil lati on period are possibl e. It shoul d be stated tha tNi Fe/ Cu MLs achi eve for t = 2 nm hi gh sensiti vi ty S = 0 : 6 % / Oe [21, 31{ 33], al -tho ugh smal l GMR values (due to smal l AF coupl ing energy) are observed. Hi ghervalues were obta ined for Ni Fe/ Au MLs [34].

3. 2. Spin val ves

The mul til ayers typ eof F =S=F / AF is a spin valve(SV) structure [16, 35, 36]and in its sim plest form consists of a \ free" ferromagneti cal ly soft layer (F ) sep-arated by a non- ferrom agneti c m etal l ic spacer layer (S) from the second \ pi nned"ferrom agneti c layer (F ), whi ch has its m agneti zati on pinned by a bi asing intera c-ti on wi th an anti ferrom agneti c layer. The spacer layer is thi ck enough to m inimi zethe magneti c coupl ing between F and F ferrom agneti c layers.

The exchange biased ani sotro py e˜ect wa s for the Ùrst ti m e observed insurf ace oxi dized Co parti cl es i .e. CoO/ Co system (Co O is an anti ferromagnetwi th NÇeel tem perature T = 293 K [37]). By means of thi s e˜ect in the systemAF(Co O)/ F(Co ) it is possibl e to shift hysteresi s loop of Co along the Ùeld axi swi th respect to H = 0 . The shift Ùeld H is the uni di recti onal exchange bi ased

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Mul ti layer Structures wi t h Gi ant Magnet oresistance 103

Ùeld expressed by H E B = E EB =( M F t F ) , where M F and t F are satura ti on magneti -zati on and the thi ckness of ferromagneti c layer, respecti vel y, E EB i s the exchangeenergy between AF and F lay ers. The operati on of SV can be understo od from them agneti zati on M ( H ) (Fi g. 5a) and m agneto resistance  R ( H ) curves (Fi g. 5b). Ifthe exchange coupl ing between F1 and F 2 i s neglected the \ free" layer rem agne-ti zes in Ùeld H ¤ 0 wherea s the hysteresi s loop of the \ pinned" layer remagneti zesat H EB . The di recti on of H EB Ùeld is determ ined by the externa l m agneti c Ùelddi recti on, duri ng Ùlm growth or duri ng tem perature decrease from T > T B to T

(where T i s the bl ocki ng tem perature of frozen spins). For j H j < j H j the m ag-neti zati on vectors of F1 and F2 are ori ented anti para l lel, theref ore in thi s rangeof magneti c Ùeld a m aximum of resistance is observed (R > R ) ). It shoul d benoti ced tha t E and H decrease wi th tem perature increasing and vani sh atT Ñ T . Theref ore proper selection of anti ferrom agneti c m ateri als wi th regard totem perature stabi l i ty of spi n v alve is very im porta nt.

Some representa ti ve values of E ; T , and T are given in T able. AFoxi des (e.g. Ni O) due to hi gh resistivi ty in opposite to m etal l ic AF al loy (e.g.FeMn) do not show the electri cal shunti ng of F1 / S/ F 2 segm ent of SV. The cru-ci al point for GMR enhancement is the spacer thi ckness. As i t wa s mentio nedin Sec. 2 to o thi ck spacer layer decreases GMR , however to o thi n gives ri se toincrease in the pro babi l it y of creati on of pi nholes between F 1 and F2 [17, 20]and also increases the inÛuence of magneto stati c intera cti ons due to interf aceroughness [38{ 41]. W hen interl ayer exchange energy (J ) increases wi th respectto the energy exchange biased (E ), j = J =E , then the di ˜erence of re-m agneti zing Ùelds of \ f ree" (F 1 ) and \ pi nned" (F 2 ) layers gradual ly vanishes. Itta kes place at j > j (where j = 0:25 for \ free" and \ pi nned" layers f romthe same m ateri als and equal thi ckness). In thi s case the magneti zati on rever-

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104 F. Stobiecki , T . Stobiecki

T ABLE

T ypical antif erromagnetic materials in the spin valve struc-ture. E E B | exchange biased energy, T N | N Çeel temp erature,T B | spin blo cking temp erature [27]. \p oly- ann" means poly-crystalli ne af ter annealing .

A F materials E EB [mJ /m 2 ] T N [K ] T B [K ]

Fe Mn (poly- ann. ) 0.05{0. 47 490 423

N i Mn (p oly- ann. ) 0.16{0. 47 1070 770

Pt Mn (p oly- ann .) 480 400

Ir Mn (p oly- ann. ) 0.19 690 538

N iO 0.05{0. 29 535 453

C oO 0.14{0. 48 293

sal of \ free" layer is sim ulta neously accom pani ed wi th magneti zati on pro cess of\ pi nned" lay er and the anti para l lel al ignm ent of m agneti zati ons of adjacent layersis impossible. As a consequence wi th increasing , for GMR decreases.In order to com pensate the inÛuence of ferrom agneti c coupl ing the thi ckness ofspacer layer shoul d be opti mized. The highest GMR sensiti vi ty values were ob-ta ined in the range of assuring weak anti ferrom agneti c interl ayer exchangecoupl ing. For Cu as spacer layer the opti m al thi ckness is 20 ¡A to 22 ¡A [39]. Itis clear tha t also the state of the crysta l structure of Cu is inÛuenced by de-positi on metho ds and condi ti ons [42] and consequentl y the structure im perfec-ti ons modi fy the e˜ecti ve coupl ing between F and F layers. Fi gure 6 presents

and GMR ( ) curves for two identi cal (wi th respect to appl ied m ateri al andthi ckness of parti cul ar layers) spin valve structures deposited by sputteri ng tech-ni que: Si(100)/ SiO/ Ta 52 ¡A Co 44 ¡A Cu 22 ¡A Co 44 ¡A FeMn 85 ¡AT a 52 ¡A Cu 5 ¡A (where segments: Si(100)/ SiO/ T a ¡A) are substra te andbu˜er layers, FeMn (85 ¡A) is an anti ferrom agnet and T a 52 ¡A Cu 5 ¡A are cap-pi ng layers whi ch adjust the conta ct to electrodes and pro tect for oxi datio n). TheSV 1 is characteri zed by stro nger ferrom agneti c coupl ing tha n SV 2 ( ),hence di ˜erence in swi tchi ng Ùelds of free and pi nned layers is smal ler for SV 1tha n for SV 2 and in consequence GMR is larger for SV 2 tha n for SV 1 (m oredeta i ls can be found in paper [41]).

The spin valvesare characteri zed by very hi gh GMR sensiti vi ty up to 17%/ Oe[43], theref ore are used as read element of heads for high densi ty hard disc dri ve(HD D ) since 1999. The GMR { SV system provi des m any advanta ges over AMR --heads: excellent signal (as high as 3.6 mV ) and enhanced therm al stabi l i ty [44].

W il lekens et al. [45] proposed in the classical SV repl acing the segmentF / AF by three layers segm ent F S F where F and F are in the state of stro nganti ferromagneti c interl ayer exchange coupl ing. Such typ e of SV (F S F S F )is cal led as AF- biased spin valve segment F S F is kno wn in l i tera ture as syn-

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Mul ti layer Structures wi t h Gi ant Magnet oresistance 105

Fig. 6. Magnetizati on (a) and magnetoresisti vi ty (b) hysteresis loops of tw o spin

valve structures SV 1 and SV 2. SV 1 | C u1 spacer sputtered at 2.4 kW RF, J F =

0 : 033 mJ = m2 , E EB = 0: 12 mJ =m , 16 mT . SV 2 | C u spacer sput-

tered at 1. 5 kW DC , 0 012 mJ m , 0 11 mJ m , 14 mT .

Si(100)/Si O/T a 52 ¡A C o 44 ¡A Cu 22 ¡A C o 44 ¡A FeMn 85¡A T a 52 ¡A

C u 5 ¡A .

theti c anti ferromagnet (SAF)) and is used as very preci se angle positi on sensor(e.g. the steering wheel in cars or the program selector of washing machines) [46].

3.3. Pseudo-spin valves

Other soluti on f or obta ining the tra nsi ti on, under externa l m agneti c Ùeld,from para llel to anti para l lel conÙgurati on of m agneti zati ons in two ferrom agneti clayers are structures of the typ e F S F in whi ch F and F are ferrom ag-neti c layers of di ˜erent coerci vi ty (f or exam ple Ni Fe Co (soft), CoFe (hard

)) separated by non- ferrom agneti c layer (f or exampl e Cu) [47{ 49]. D ueto thi ck Cu spacer ( ¡A) the exchange coupl ing between soft (F ) and hard(F ) is neglected. Such structure is kno wn as pseudo-spi n valve (PSV). The typi -cal m agneti zati on and m agnetoresisti vi ty hysteresi s loops are presented in Fi g. 7.From the appl icati on point of vi ew the large di ˜erence of and is desirable(Fi g. 7a). In PSV l ike in SV the ferrom agneti c coupl ing (m agneto stati c or causedby pinholes) reduces the di ˜erence between coerci vi ty Ùelds of both ferrom agneti clayers and GMR values.

D ue to speciÙcbehavi our of dependence (Fi g. 7b), correl ated wi thm agneti zati on reversa l process, PSV can be used as cells of magneto resisti ve ran-dom accessm emories (M- R AM) [6, 7, 50], due to the di ˜erence between resistance

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106 F. Stobiecki , T . Stobiecki

Fig. 7. (a) Magnetization and (b) magnetoresistiv ity hysteresis loop of pseudo-spi n

valve consisting of N i 8 0 Fe20 (2. 8 nm)/C o(2. 1 nm)/C u(2 nm)/C o(3 nm). M 1 ; M 2 denote

the magnetizati on of hard (C o) and soft (N i80 Fe20 ) layer, respectively .

values of ferrom agneti c (l ow resistivi ty) and anti ferrom agneti c (hi gh resisti vi ty)al ignm ent of m agneti zati on vecto rs.

The m ain adv anta ges of M- R AM over the currentl y used dyna mic R AM' sare: sim ple memory cells constructi on and preparati on, non-volati le, unl imi tedread and wri te endurance, high speed operati on.

4. Su m m ar y

The main requi rem ent to obta in GMR in layered structures is reori enta ti onof mutua l m agneti zati on di recti ons in neighbouri ng ferrom agneti c layers inducedby magneti c Ùeld. Thi s requi rement can be fulÙlled in di ˜erent arti Ùcial structuresconsisting of two (or more) ferrom agneti c layers separated by non- ferrom agneti clayers (see Sec. 3). Thi s enables the real izati on of elements wi th attra cti ve m agne-to resisti ve characteri sti cs f or parti cular appl icati ons. Ho wever, considerabl e GMRvalues can be obta ined onl y for thi cknessesof indi vi dual layers smal ler tha n 3 nm .For such thi n Ùlm structures , m agneti c interl ayer coupl ing as wel l as GMR ef-fect are stro ngly dependent on the roughness of interf aces and other structura lim perfections such as pinholes or mixed interf aces.

Ac kn owl ed gm ent

The autho rs are grateful to the Sta te Co mm ittee for Scienti Ùc Research forÙnanci al supp ort under grants: 8T1 1B 05418 and PB Z/ KBN- 013/ T0 8/ 23.

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