Giant magnetoresistance in evaporated NiFe/Cu and NiFeCo/Cu multilayersAlexander M. Zeltser and Neil Smith Citation: Journal of Applied Physics 79, 9224 (1996); doi: 10.1063/1.362596 View online: http://dx.doi.org/10.1063/1.362596 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/79/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Giant magnetoresistance of perpendicular magnetic Co/Au multilayers J. Appl. Phys. 80, 5175 (1996); 10.1063/1.363501 Giant magnetoresistance in Co/Cu multilayers fabricated by focused ionbeam direct deposition J. Appl. Phys. 80, 4217 (1996); 10.1063/1.363299 Oscillatory interlayer exchange and giant magnetoresistance in magnetic multilayers AIP Conf. Proc. 378, 466 (1996); 10.1063/1.51138 Giant magnetoresistance in Co/Cu multilayers: Influence of Co thickness at the first antiferromagnetic maximum J. Appl. Phys. 79, 7395 (1996); 10.1063/1.362449 Giant magnetoresistance in Co/Cu multilayers with Co layers of alternating thicknesses: Reduction ofmagnetoresistive hysteresis J. Appl. Phys. 79, 7090 (1996); 10.1063/1.361477

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Giant magnetoresistance in evaporated Ni-Fe/Cu and Ni-Fe-Co/Cumultilayers

Alexander M. Zeltsera) and Neil SmithSan Diego Laboratories, Eastman Kodak Company, San Diego, California 92121

~Received 28 December 1995; accepted for publication 21 February 1996!

The magnetic and transport properties of electron-beam evaporated~Ni83Fe17/Cu!10 and~Ni66Fe16Co18/Cu!10 multilayers were studied as a function of the Cu spacer, magnetic layer and Tabuffer layer thicknesses, as well as annealing conditions. All multilayers exhibited very small giantmagnetoresistance~GMR! effect ~,0.3%! in the as-deposited state, however, after magneticpost-annealing at 300–325 °C, GMR increased up to 4.5%–7%, depending on the multilayer type.In contrast to sputtered Ni-Fe-~Co!/Cu multilayers, GMR showedno oscillatory behavior as afunction of Cu thickness. Similar to that reported in sputtered ‘‘discontinuous’’ Ni-Fe/Agmultilayers, it is believed that Cu diffusion along the Ni-Fe-~Co! grain boundaries createsintra-layermagnetic discontinuities in Ni-Fe-~Co! layers which promoteinter-layer antiferromagneticcoupling. The evaporated Ni-Fe/Cu multilayers exhibited very low remanence, exceptionally lowhysteresis, and quite uniform GMR properties through the thickness of the multilayer. All of thesemakes them potentially attractive for application to future magnetoresistive reproduce heads forvery high areal density magnetic storage systems. ©1996 American Institute of Physics.@S0021-8979~96!03011-3#

I. INTRODUCTION

In spite of higher giant magnetoresistance~GMR! andsensitivity demonstrated in sputtered Ni-Fe-Co/Cu multilay-ers ~MLs!,1,2 standard permalloy/copper~Ni-Fe/Cu! multi-layers still have attracted attention because of materials sim-plicity and technological importance for reproduce headapplications.3–5 Several papers were published on the GMReffect in as-depositedNi-Fe/Cu multilayers grown by ion-beam and magnetron sputtering.6–8 It was found that GMR inthe ion-beam sputtered~Ni-Fe/Cu! multilayers grown on Febuffer layers exhibits strong oscillatory behavior as a func-tion of copper spacer thickness~tCu!.

6 The GMR ratioDR/Rhad peak values of;19, 7.5, and 2.5% fortCu510, 22, and38 Å, respectively, and the saturation field corresponding tothe second oscillatory peak was;50 Oe.6 The oscillations inGMR magnitude as a function oftCuwere strongly correlatedto the ~200! preferred orientation of Ni-Fe and Cu crystal-lites, which, in turn, strongly depended on the thickness ofthe Fe buffer layer~tFe!, and showed maximumDR/R attFe;50 Å.9,10 On the other hand,magnetron sputteredNi-Fe/Cu multilayers grown on Fe, Cu, or Ni-Fe underlayersshowed very different behavior, i.e., weak and monotonicallydecreasing dependence of GMR on buffer layer thickness,weak ~111! texture, and most importantly, only one~first!strong GMR peak at room temperature.7,8 The above resultsindicate that GMR properties of Ni-Fe/Cu multilayersstrongly dependent on the deposition technique and the typeof buffer layer, both of which control film microstructure andinterface roughness. The effect of annealing on GMR prop-erties in Ni-Fe/Cu/Ni-Fe spin valves was studied by Speriosuet al.11,12They showed that in contrast to Fe/Cr, the interfaceroughening induced by post-deposition annealing decreasedGMR, and attributed that to the presence of compositionally

intermixed regions with high resistivity at the Ni-Fe/Cu in-terfaces, which contribute to spin-independentscattering.

The main goal of this study was to investigate GMRproperties of Ni-Fe/Cu, and Ni-Fe-Co/Cu multilayers grownby electron-beam evaporation. It is well known that the ki-netic energy of arriving atoms in the case of evaporation isusually at least an order of magnitude smaller than insputtering.13 Therefore, evaporated multilayers generallyhave less diffuse~intermixed! interfaces, are more porous,and have smaller grain size. These inherent microstructuraldifferences are expected to cause differences in evolution ofGMR with annealing in evaporated versus sputtered multi-layers. Indeed, as will be shown below, evaporated multilay-ers exhibited drastically different GMR behavior than sput-tered ones, and as we reported earlier, showed certain uniqueadvantages over sputtered multilayers for future generationultrahigh density magnetic read head applications.14

II. EXPERIMENTAL DETAILS

A series of~Ni-Fe/Cu!10 and~Ni-Fe-Co/Cu!10 ten bilayermultilayers were electron-beam evaporated onto 39 thermallyoxidized Si wafers in a field of;180 Oe, with the magneticlayer being deposited first. The~Ni-Fe/Cu!10 multilayerswere also evaporated on Ta underlayers of different thick-ness. The evaporation was done from graphite crucibles us-ing Ni83Fe17, Ni66Fe16Co18 ~at. %!, and Cu charges, all 99.95pure. The deposition conditions were: background pressure,531028 Torr; evaporation rates;2 Å/s, for Ni-Fe, Cu, andTa, and;0.5 Å/s for Ni-Fe-Co; and substrate temperature200 °C. Coated wafers were diced in 1.5930.259 couponsand post-annealed at temperaturesTan from 250 to 350 °Cfor 2 or 4 h in a99.995% purified atmosphere of flowing Ar,with a magnetic field of;150 Oe applied in the same direc-tion as during deposition. The multilayers structure was char-acterized by high and low angleu-2u x-ray diffraction usinga!Electronic mail: alexz@kodak.com

9224 J. Appl. Phys. 79 (12), 15 June 1996 0021-8979/96/79(12)/9224/7/$10.00 © 1996 American Institute of Physics [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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CuKa radiation and cross-sectional transmission electronmicroscopy ~TEM!. The room-temperature magnetic andtransport properties of multilayers were measured using anM-H loop tracer, VSM, and both in-line and square four-point probes. The R-H loops were recorded using mechanicalx-y plotter and later manually digitized. The GMR ratio herewas measured asDR/R[[R(0)2R(H52 kOe!#/R(0). Thefield H50, was defined as thehalf width at the half maximumof thedR/R vsH curve, or field producing 50% reduction ofresistance, viz.,R(0)2R(H50)5DR/2.

III. RESULTS

A. GMR in Ni-Fe/Cu multilayers versus Cu spacerthickness

GMR properties were first investigated as a function ofCu spacer thickness in a series of~Ni-Fe/Cu!10 multilayers inwhich tCu was varied from 10 to 40 Å in increments of;4Å, with the magnetic layer thickness fixed at 27 Å. This isthe range whereDR/R oscillations vstCu were previouslyreported in various sputtered Ni-Fe/Cu multilayers.6–8Cross-sectional TEM of the as-deposited ML at relatively low mag-nifications ~<50 0003! shows that NiFe and Cu layers arecontinuous across lateral dimensions of at least the order ofmicrons. A higher magnification cross-section TEM imageshown in Fig. 1 indicates that both NiFe and Cu layers areuniform through the ML thickness, but exhibit some wavi-ness or roughness which is most apparent near the top sur-face of the ML. However, even the top surface has maximumroughness of less than about 5% of the total ML thickness.The ML grain size is about 150–300 Å. Figure 2 showsrepresentative high-angle and low-angle x-ray diffraction~HAXRD and LAXRD! patterns as a function of annealingtemperature. The~111! and~200! peaks in the HAXRD pat-tern @Fig. 2~a!# due to permalloy and copper are superim-posed because both have fcc crystal structure and small lat-tice mismatch of;2%. Also, the intensity ratioI 200/I 111indicates that both Ni-Fe and Cu layers preferentially growwith the ~111! planes perpendicular to the growth direction,similar to that observed in as-deposited magnetron sputteredNi-Fe/Cu multilayers.7 The small decrease inI 200/I 111 with

FIG. 1. Representative bright-field cross-section TEM image of the as-deposited~27 Å Ni-Fe/28 Å Cu!10 multilayer.

FIG. 2. ~a! High-angle and~b! low-angle x-ray diffraction patterns vs annealing temperature from a~27 Å Ni-Fe/37 Å Cu!10 multilayer. The first- andsecond-order Bragg superlattice reflections due to composition modulation are indicated by arrows in~b!.

9225J. Appl. Phys., Vol. 79, No. 12, 15 June 1996 A. M. Zeltser and N. Smith [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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annealing temperature indicates that~111! preferred orienta-tion slightly increases. The very weak ‘‘extra’’ peak or sat-ellite ~2u'42.4°! near the main~111! peak corresponds tothe first-order (n521) reflection from~111! planes due tothe Ni-Fe/Cu superlattice structure. The LAXRD pattern@Fig. 2~b!# of ten-bilayer multilayers showed well-definedseries of Kiessig interference fringes indicative of the totalfilm thickness,T510(tNiFe1tCu! as well as clearly visiblefirst-order Bragg superlattice reflection~2u;1.5°! due tocomposition modulation, which coincides with them510thKiessig fringe. The presence of a relatively weak first-orderBragg reflection, and a very weak second-order reflection isbelieved to result from a very small difference in atomicscattering factors of Cu and Ni-Fe, but may also be attribut-able to incipient intermixing of the Cu/Ni-Fe interfaces, evenin the as-deposited state. From VSM measurements takenperpendicularto the film plane, a demagnetization limited,tNiFe-independent value of 4pMs;7 kG ~in the as-depositedstate! was determined. The value oftNiFe'27 Å was calcu-lated by combining the VSM data with the in-plane M-Hloop measurements. After that, values for thetCu were deter-mined from the LAXRD patterns by fitting to the modifiedBragg’s law (2um)

25(mlx/T)218d, where um is Bragg

angle corresponding tom-th Kiessig fringe, 12d is index ofrefraction andlx'1.54 Å ~wavelength of the CuKa radia-tion!. The plots of (2um)

2 vs m2, were well fit by straightlines, whose slope (lx/T)

2 determined the multilayers periodT/105tNiFe1tCu.

We found that evaporated Ni-Fe/Cu multilayers exhibitunique features not observed in sputtered ones. Firstly, in theas-deposited state these multilayers exhibit very small GMReffect,DR/R,0.3% for all Cu spacer thicknesses. Secondly,and most importantly, it was found that GMR in these mul-tilayers could be drastically increased by annealing. This be-havior is summarized in Fig. 3 which shows graphs ofDR/RandH50 as a function oftCu after annealing between 275 and350 °C for 2 h. Multilayers withtCu.15 Å show a sharpincrease inDR/R to a maximum of 4.0%–4.5% after anneal-ing at Tan5300–325 °C with subsequent deterioration atTan5350 °C. Thirdly, bothDR/R andH50 did not show os-cillatory behavior as a function oftCu between 10 and 40 Å,and both quantities were approximately constant in a broadrange oftCu;20–40 Å after annealing atTan5300–325 °C.

H50, which is a measure of the interlayer coupling field,dropped precipitously after annealing at 300 °C~whentCu.20 Å! to a minimum of;13 Oe. ThenH50 increased byabout a factor of 2 after annealing at 325 °C, and then againroughly doubled after annealing at 350 °C, indicating thatH50 is much more sensitive to annealing temperature thanDR/R.

The in-planeR-H andM -H loops measured parallel andtransverse to original easy axis showed virtually hysteresisfree, magnetically isotropic behavior for all multilayers withtCu>24 Å annealed above 275 °C. Figure 4 shows typicalR-H loops, which illustrate the above described behaviorand plotted asdR(H)/R[[R(H)-R(H52000 Oe!#/R(0).Representative easy axisM -H loops in Fig. 5 indicate thatthe character of coupling between Ni-Fe layers is changingfrom ferromagnetic ~FM! in the as-deposited state(Mr /Ms;0.9), to almost purely antiferromagnetic~AF! afterannealing atTan>300 °C (Mr /Ms,0.2). Not surprisingly,this transition from FM to AF coupling occurred concomi-tantly with a dramatic increase in GMR@cf. Fig. 3~a!#. Theas-deposited~FM coupled! multilayers showed uniaxial an-isotropy, Hk;3 Oe, while the multilayers annealed atTan.275 °C~AF coupled! were essentially isotropic, consis-tent with theR-H loops. It can be also seen~see Fig. 5! that

FIG. 3. ~a! DR/R and ~b! H50 as a function of Cu spacer thickness and annealing temperature for a series of~27 Å Ni-Fe/t Cu!10 multilayers.

FIG. 4. Hard axisR-H loops from a~27 Å Ni-Fe/32 Å Cu!10 multilayerillustrating evolution of GMR with annealing temperature. Note that 325and 350 °C loops were shifted vertically for clarity.

9226 J. Appl. Phys., Vol. 79, No. 12, 15 June 1996 A. M. Zeltser and N. Smith [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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the as-deposited multilayers essentially saturated at;5 Oe,while multilayers annealed atTan>300 °C did not com-pletely saturate even at 200 Oe. Comparison of the annealedR-H andM -H loops ~cf. Figs. 4 and 5! indicate that mag-netization is approaching saturation considerably faster thanthe resistivity. This latter behavior is expected, becauseM (H)}cos~u/2!, while [R(0)2R(H)]}cosu, whereu(H)is the angle between the magnetization vectors of adjacentNi-Fe layers, andu→0 at largeH. The origin of very slowapproach to saturation of bothR(H) andM (H) loops will bediscussed further in Sec. IV. The decrease in the magneticmoment with the increase in annealing temperature observedin Fig. 5 likely indicates interdiffusion between Ni-Fe andCu layers.11,12

Somewhat similar GMR behavior have been reported byHylton et al. in sputtered ‘‘discontinuous’’ ~Ni-Fe/Ag!multilayers.15 However, in contrast to the Ni-Fe/Ag multilay-ers, which showed a decrease in resistance upon annealing,the sheet resistivity of Ni-Fe/Cu multilayers monotonicallyincreased with annealing temperature, Fig. 6. This, coupled

with decrease in magnetic moment after annealing, indicatesthat lattice interdiffusion plays more important role in theCu/Ni-Fe system than in Ag/Ni-Fe, which is consistent withgreater equilibrium solubility of Cu vs Ag in the Ni-Fematrix.16 Also, Fig. 6 suggests that the onset of appreciablelattice interdiffusion occurs only atTan.250 °C, consistentwith M -H andR-H curves. Additionally, it can be seen thatGMR vs Tan is governed by the magnetoresistivity,Dr5r(0)2r(H), rather than mean resistivityr~0! vs Tan.

It is believed that initially~at relatively lowTan<325 °C!both grain-boundary and lattice interdiffusion between Cuand Ni-Fe, lead to the formation of a discontinuous, quasi-granular multilayer structure with thin Cu-rich nonmagneticgaps between Ni-Fe platelet-shaped grains, similar to thatobserved in Ni-Fe/Ag.15 As was shown by Slonczewski,17

formation of thin nonmagneticintra-layer gaps betweenNi-Fe grains is sufficient to cause a magnetostatically in-duced antiferromagneticinter-layer coupling, which can ex-plain initial increase of GMR with annealing up to;300–325 °C. A subsequent deterioration of GMR upon furtherannealing above;325 °C may be attributed to the increasein thickness of compositionally intermixed, spin-independentscattering regions at the Ni-Fe/Cu interfaces due to an in-crease of interdiffusion with the increase in annealingtemperature.11,12 The increase inH50 upon annealing atTan.300 °C may be related to the combination of increasedCu-rich gap thickness and decreased Ni-Fe platelet size,therefore increasing the strength of the AF coupling betweenthe Ni-Fe layers.17

B. GMR in Ni-Fe/Cu multilayers versus Ni-Fe layerthickness

The GMR properties of~Ni-Fe/Cu!10 multilayers asfunction of Ni-Fe layer thickness were also investigated. Aseries of multilayers withtNiFe varied from 24 to 38 Å inincrements of;3.5 Å were evaporated. The average 4pMs

for these multilayers monotonically increased from;6 to;8 kG with the increase intNiFe. Based on earlier observa-tions of the independence of GMR properties with Cu spacerthickness between 20 and 40 Å,tCu was nominally fixed at30 Å. The actual thicknesses of Cu~measured as described

FIG. 5. Easy axisM -H loops from a~27 Å Ni-Fe/32 Å Cu!10 multilayerillustrating the change in character of Ni-Fe interlayer coupling, from FM toAF, with annealing temperature. THeM -H loop for the as-depositedmultilayer is expanded 103 along theH-axis.

FIG. 6. Sheet resistance and magnetoresistance of representative~27 Å Ni-Fe/t Cu!10 multilayers vs annealing temperature.

FIG. 7. DR/R andH50 as a function oftNiFe and annealing temperature fora series of~t Ni-Fe/28 Å Cu!10 multilayers.

9227J. Appl. Phys., Vol. 79, No. 12, 15 June 1996 A. M. Zeltser and N. Smith [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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earlier! in these multilayers turned out to be;28 Å. Bothlow- and high-angle XRD spectra of multilayers as a func-tion of tNiFe and Tan showed very similar trends to thoseobserved in multilayers vstCu. The evolution ofDR/R andH50 as a function oftNiFe is summarized in Fig. 7. BothDR/R andH50 vs tNiFe showed generally similar behavior tothat as a function oftCu ~cf. Fig. 3!, however, higherDR/Rwere achieved after annealing at 325 °C and relative plateauof DR/R appeared to be confined to a narrower range oftNiFe. A further increase inTan to 335 °C caused a noticeabledeterioration ofDR/R and increase inH50. It was found that

multilayers withtNiFe.;30 Å exhibited a small increase inhysteresis, as can be seen from the separation between peaksof R-H loops in Fig. 8.

C. GMR in Ni-Fe/Cu multilayers versus Ta underlayerthickness

Next to be investigated was the effect of Ta underlayerthickness on GMR properties of Ni-Fe/Cu multilayers. Thiswas motivated in part by reports that GMR in sputtered Ni-Fe/Cu multilayers could be strongly affected by the texturewhich in turn is strongly dependent on the buffer layer typeand thickness.9,10Based on previous observations thetCu andtNiFe were selected at 30 and 35 Å, respectively. A series ofthree ten-bilayer Ni-Fe/Cu multilayers were grown on thenominally 30, 50, or 70 Å thick Ta underlayers, which wereevaporated in the same pump-down, but prior to multilayerdeposition. Independent of Ta thickness, all multilayers hadvery similar HAXRD patterns, which did not reveal any sig-nificant changes upon annealing. Fig. 9 shows a representa-tive HAXRD pattern, which exhibited strong a~111! Braggpeak (n50) due to the average Ni-Fe and Cu structure, aswell as composition modulation satellites (n561). Com-parison with HAXRD pattern shown in Fig. 2~a! indicatesthat Ni-Fe/Cu multilayers grown on Ta have much stronger~111! texture and than those grown directly on SiO2. Sincetotal thickness,T5[( tNiFe1tCu!3101tTa#, of these multilay-ers includes two types of nonmagnetic layers~Cu and Ta!,the bilayer wavelength,L5tNiFe1tCu, was computed usingthe formula 2~sinu02sinu21!5lx/L, where lx is theCuKa radiation wavelength andun is the Bragg angle of thepeaks shown in Fig. 9.18 The calculatedL559 Å, fromwhich the actual thicknesses of Ni-Fe~tNiFe534 Å! and Cu~tCu525 Å! were determined as described earlier. OptimumGMR properties in all Ta/~Ni-Fe/Cu! multilayers wereachieved after annealing at 325 °C, as in multilayers of simi-lar geometry~34 Å Ni-Fe/28 Å Cu! grown on SiO2. Theeffect of the Ta underlayer thickness on GMR properties isdemonstrated in Fig. 10. Increase of Ta thickness increasedDR/R from 4.6% to 5.6% leavingH50 essentially unaffected.

FIG. 8. The effect of Ni-Fe layer thickness on hard axisR-H loops of ~tNi-Fe/28 Å Cu!10 multilayers. Note thattNiFe527 and 30 Å loops wereshifted vertically for clarity.

FIG. 9. HAXRD patterns vsTan from a ~34 Å Ni-Fe/25 Å Cu!10 multilayersgrown on Ta underlayer. The small satellite reflections (n561) flankingthe ~111! Bragg peak (n50) are due to the multilayer superlattice structure.

FIG. 10. The effect of Ta underlayer thickness on hard axisR-H loops of~34 Å Ni-Fe/25 Å Cu!10 multilayers. The minor loop taken near the maxi-mum slope of theR-H loop of thetTa570 Å multilayer is also shown. Notethat loops from multilayers grown on Ta underlayers were shifted verticallyfor clarity.

9228 J. Appl. Phys., Vol. 79, No. 12, 15 June 1996 A. M. Zeltser and N. Smith [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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However, it can be seen that ML grown on Ta underlayersexhibited slightly higher hysteresis than that grown on SiO2.Unlike the multilayers grown on SiO2, which always exhib-ited deterioration ofDR/R after annealing atTan.325 °C,DR/R in Ta/~Ni-Fe/Cu! multilayers increased to almost 6%after annealing at 350 °C, however, this annealing alsocaused a concomitant increase inH50.

D. GMR in Ni-Fe-Co/Cu multilayers

In the final phase of this work the effect of Co additionon GMR properties of multilayers was investigated. The mo-tivation was the higherDR/R reported in sputtered Ni-Fe-Co/Cu multilayers.1,2 ~Ni-Fe-Co/Cu!10 multilayers withtNiFeCo525 Å and tCu529, 35, and 39 Å were evaporatedfrom Ni66Fe16Co18 ~at. %! charge. The magnetic layer com-position, as determined by particle-induced x-ray emission,was Ni67Fe15Co18 ~at. %!, which is very close to that of zeromagnetostriction.1 The actual Ni-Fe-Co thickness was deter-mined based on VSM measured 4pMs ~'8.7 kG! of themultilayers in the as-deposited state. The GMR behavior ofNi-Fe-Co/Cu multilayers as function of Cu spacer thicknessand annealing temperature was generally similar to that ofNi-Fe/Cu multilayers of similar geometry. In particular, bothDR/R andH50 varied very little ~;10%! with tCu and thebest combination ofDR/R andH50 was achieved after an-nealing 300 °C for 2 h, Fig. 11. However, in contrast toNi-Fe/Cu ML, the magnitude of bothDR/R and H50 washigher, viz. 6%–6.5% and;25 Oe, respectively~cf. Figs. 11and 4!. Moreover, unlike Ni-Fe/Cu multilayers of similargeometry, which were essentially hysteresis free, Ni-Fe-Co/Cu multilayers exhibited significant hysteresis in theirR-H loops, which increased with thickness of Cu spacer.Finally, theR-H loops from all Ni-Fe-Co/Cu multilayers al-ways exhibited a clear anisotropy, so thatH50 ~hard!2H50~easy!;10 Oe, which is a rough measure of the effectiveanisotropy fieldHk in these multilayers.14

IV. DISCUSSION AND CONCLUSION

One physical property of the present electron-beamevaporated multilayers that is not yet fully understood is the

exceptionally slow approach to saturation of both theM (H)andR(H) curves. This has some negative consequence froma device standpoint, because it has the effect of reducing thenet available resistivity change over field excursions inwhich such films show a quasilinear magnetoresistive re-sponse. If one adopts the more practical definition ofDReff[R(H50)2R(H52H50) @such thatDReff5DR in theidealized case of straight-sided, ‘‘triangular’’R(H) curves#,the evaporated multilayers studied so far have shown only amodestDReff/R&3%–4%. One suggestion19 for the physi-cal origin of the ‘‘long tails’’ of theM (H) curves issuper-paramagneticbehavior of the Ni-Fe platelet-shaped grainsformed by Cu diffusion during the annealing process as wasdescribed earlier. This could also explain the lack of anisot-ropy and very low hysteresis observed in theM (H) loops ofNi-Fe/Cu multilayers, which do indeed bear a resemblance~see Fig. 5! to that of the Langevin functionL(H/H0), whereL(x)5coth(x)21/x, H05kT/(MsV), andV is the plateletvolume.20 From Fig. 5, one estimates anH0;10 Oe, whichin turn predicts a platelet volumeV5tmd

2 with d;500 Ådiameter, takingtm;30 Å as the magnetic layer thickness.Although this platelet diameter is comparable with grain sizeobtained from cross-sectional TEM, the superparamagneticargument ignores the large magnetostatic interaction fields;2Mstm/d;100 Oe@H0 that would be generated betweenneighboring platelets withuncorrelated, thermally randommagnetizations. Further, the long tails ofM (H) andR(H)are equally present in the obviously hysteretic, and henceferromagneticNi-Fe-Co layers of Fig. 11. An alternative ex-planation is that of a broad distribution in the magnitude ofthe local demagnetizing fields, which result from random-ness in the size and shape of the platelets, and which will onaverage be directed in opposition to the net magnetizationvector of the multilayer film. The long tails of theM (H) andR(H) curves then reflect that fraction of magnetic plateletsthat experience the largest local demagnetizing fields.

This hypothesis is consistent with preliminary spin-polarized neutron reflectivity~SPNR! studies,21 which indi-cate that a small portion of the Ni-Fe layer moments in an-nealed multilayers remain magnetically disordered throughthe multilayer thickness in applied fields ranging from 3 Oeto 6.3 kOe. These SPNR measurements also indicate that thelarge portion of the Ni-Fe moments thar are antiferromag-netically coupled across the intervening Cu show magneticordering over very large in-plane domains of the order of100 mm. These are much larger than the micron-sized do-mains observed by SPNR and Lorentz microscopy in sput-tered Ni-Fe/Ag multilayer systems.22,23 This is consistentwith the complete lack of Barkhausen-like jumps observed inthe R(H) response of small~,100 mm2!, patterned deviceelements made from Ni-Fe/Cu,14 in contrast to the noticeableBarkhausen noise observed in similarly sized test elementsof Ni-Fe/Ag.23 The large apparent domain size in the Ni-Fe/Cu is further evidence that these multilayers do not existin some form of superparamagnetic state at room tempera-ture.

In conclusion, the evaporated Ni-Fe/Cu and Ni-Fe-Co/Cu GMR multilayers described above have several prop-erties in common that are beneficial for applications, but

FIG. 11. The effect of Cu spacer thickness on hard axisR-H loops of~25 ÅNi-Fe-Co/t Cu!10 multilayers. Also shown is the minor loop taken near themaximum slope of theR-H loop of the tCu529 Å multilayer. Note thattCu535 and 39 Å loops were shifted vertically for clarity.

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which are not often found in sputtered multilayers. Theseinclude enhanced temperature stability, as well as Cu~andmagnetic! layer thickness insensitivity of theDR/R ratio. Inaddition, the Ni-Fe/Cu multilayers exhibited very low rema-nence and exceptionally low hysteresis, both of which shouldsubstantially enhance the magnetic linearity and stability ofsuch films when used in the micron-sized geometries ex-pected of future MR reproduce heads for very high storagedensity magnetic disc-drive systems. It has also been shownexperimentally that the GMR properties of the evaporatedmultilayers are quite uniform through the thickness of themultilayer film, which makes these films particularly attrac-tive for application to a novel, ultrahigh recording densityGMR head design described in detail elsewhere.14

ACKNOWLEDGMENTS

The authors would like to thank K. Pentek, M. Zhou, J.Klemm, and E. Haftek for their assistance in this work. Theyalso wish to thank J. Borchers~NIST, Gaithersburg, MD! forpreliminary SPNR analysis of the Ni-Fe/Cu multilayers andL. Toth ~Research Institute of Technical Physics, Budapest,Hungary! for TEM data.

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9230 J. Appl. Phys., Vol. 79, No. 12, 15 June 1996 A. M. Zeltser and N. Smith [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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