We present evidence for a new type of giant magnetoresistance in (111) cobalt-copper superlattices with atomically smooth interfaces. We propose that the lowered dimensionality of the structure leads to an enhancement of the scattering of conduction electrons from paramagnetic interfaces obeying a Langevin-like saturation at very high fields, well beyond the switching field of the Co layers. The findings help to explain similarities in magnetotransport behavior with recently reported granular systems as well as differences with antiferromagnetically coupled multilayers

Barlett, D.

Clarke, R.

Glick, D.

Lauhon, L.

Mandrekar, T.

Tsui, F.

Uher, C.

Carolina Digital Repository

PHYSICAL REVIEW B VOLUME 49, NUMBER 2 1 JANUARY 1994-IILangevin-like giant magnetoresistance in Co-Cu superlatticesD. Barlett, F. Tsui, D. Glick, L. Lauhon, T. Mandrekar, C. Uher, and Roy ClarkeHarrison Randall Laboratory ofPhysics, Uniuersity ofMichigan,500 East University Street, Ann Arbor, Michigan 48109-1120(Received 10 August 1993)We present evidence for a new type of giant magnetoresistance in (111) cobalt-copper superlatticeswith atomically smooth interfaces. We propose that the lowered dimensionality of the structure leads toan enhancement of the scattering of conduction electrons from paramagnetic interfaces obeying aLangevin-like saturation at very high fields, well beyond the switching field of the Co layers. Thefindings help to explain similarities in magnetotransport behavior with recently reported granular sys-tems as well as differences with antiferromagnetically coupled multilayers.Several groups' have reported giant magnetoresis-tance (GMR) effects in Co-Cu(111} superlattices grownby molecular-beam epitaxy (MBE) that are comparable tothose observed in antiferromagnetically coupled Fe-Crmultilayers. Whether the magnetoresistance (MR)behavior observed in Co-Cu(111) samples also originatesfrom antiferromagnetic (AFM) coupling is somewhat un-clear at this point because their magnetization seems tobe predominantly ferromagnetic in character with only asmall fraction of the sample showing indications of AFMcoupling. ' In another recent experiment AFM cou-pling was not observed in Co-Cu(111) multilayers grownon Cu single-crystal substrates.Sample defects have been invoked as a possible ex-planation of why AFM coupling may be masked in theCo-Cu(111) system. For example, it has been suggestedthat stacking faults and pinholes may lead to ferromag-netic bridging across neighboring layers. Well-controlledsample growth and detailed atomic-scale characterizationare therefore crucial to understanding the magneticbehavior of these materials.In this paper we present MR and magnetization resultson (111) single-crystal Co-Cu superlattices, prepared byMBE techniques with atomically smooth interfaces. Weobserve the appearance of a new type of GMR, onewhich is not dependent on AFM coupling and is opera-tive up to high magnetic fields. By careful control of theinterfacial quality, and consequently the uniformity ofthe layering, we probe the role of the interfaces. In thelimit of atomically smooth interfaces, our results suggestthat the lowered dimensionality of the interfaces dom-inates the behavior rather than sample defects.The samples in this research were grown by MBE onGe-buffered (110) GaAs substrates. Buffer layers of 15 A(110) bcc Co, followed by 20 A (111)Au, were depositedon the Ge to initiate layer-by-layer superlattice growth inthe (111) orientation. The subsequent superlattice sam-ples typically consist of 30 bilayers. The pressure duringsuperlattice growth was (4X10 ' mbar, and the sub-strate temperature was held at 150 C. Co was depositedfrom an electron-beam hearth at rates between 0.15 and0.25 A/sec, and Cu from a Knudsen cell at a rate of 0.33A/sec. Details of the growth are described in a previouspublication. The growth was monitored in situ byreflection high-energy electron diffraction (RHEED) us-ing a charge-coupled device imaging and analysis sys-tem. ' X-ray scattering performed after growthconfirmed that the layer stacking was fcc in the (111)orientation. 9A crucial aspect of the interface characterization in-volved spin-echo NMR measurements of the local cobaltenvironment. " Only two characteristic NMR peakswere observed, one of which corresponds to bulk fcc Cowith 12 Co neighbors and the other to interfacial Co hav-ing 3 Cu neighbors. The measured spectra therefore pro-vide definitive evidence for atomically abrupt interfacesand pure fcc stacking in these samples. These resultswill be described in a subsequent publication. 'Cross-sectional high-resolution-transmission-electron-microscopy (HRTEM) experiments were also carried outfor the samples studied here. The results reveal that thesuperlattice interfaces are atomically flat with lateralcrystal coherence length of several hundred A, as illus-trated by the micrograph shown in Fig. 1. Owing to thelack of contrast between Co and Cu atoms, the HRTEMimages were deliberately defocused in order to show in-terference fringes due to the superlattice bilayers, asshown in the horizontal bands in Fig. 1. The flatness andcontinuity of these bands do not support the view thatthe growth of (111}-oriented samples is particularly proneto pinhole formation.We now focus on the GMR and magnetization data.The MR measurements were made using the standard dcfour-point probe technique, with the field applied parallelto the current (longitudinal MR). The magnetization wasmeasured in a commercial superconducting quantum in-terference device magnetometer. The measured satura-tion moments are within S%%uo of the bulk value for Co(1440 emu/cm ). Both the magnetization and MR mea-surements were made for fields applied in the growthplane along the [110]and [112]directions of the superlat-tice. ' Figure 2 compares the MR for a [Co(7.5 mono-layer (ML))/Cu(5. 5 ML)]&6 superlattice with its magneti-zation for fields applied along the [112] direction. Im-mediately it is apparent that the magnetization saturationfield is more than 100 times smaller than that of the MR.In fact the MR is sti11 changing significantly at thehighest field we can achieve in our cryostat (5 T). Some0163-1829/94/49(2)/1521(3)/$06. 00 49 1521 1994 The American Physical Society(~ ~1522 D. BARLETT et al. 49authors have ascribed this high-field behavior of the MRto very strong AFM coupling. ' In what follows wewill provide an alternative explanation which resolves thediscrepancy between the different saturation behaviors ofthe MR and the bulk magnetization.The field dependence of the MR shown in Fig. 2(a) canbe described accurately by a Langevin-like saturationfunction, 1 —/3[cotha —1/a], where a=NabOB/k& T, Pand X are fitting parameters, and po is the Bohr magne-ton. The dashed line in Fig. 2(a) is a fit to the experimen-tal data using the Langevin function. This specific fielddependence suggests that scattering from an assemblageof paramagnetic spins, most likely at the interfaces be-tween Co and Cu layers, is responsible for the GMR inour samples. We note the possibility that the Cu conduc-tion electron band could itself become partially polarizedin proximity to the Co layers. ' Interfacial "loose" spinshave also been proposed in order to explain the origin ofbiquadratic coupling. 'It is interesting to point out here that the character ofthe paramagnetism is not that of isolated spins; rather,we find that there are substantial correlations, akin to asuperparamagnetic layer. We envision the paramagneticspin arrangement as forming small patches of correlatedspins of size &N. The values of N returned by theLangevin fit at ambient temperature are roughly 500, fal-ling approximately linearly towards unity at T =0,presumably as more and more of the interface spins lockinto the (saturated) bulk Co magnetization, as shown inFig. 3. The enhanced exchange interactions between theinterfacial spins and the ferromagnetic Co spins at lowtemperatures perhaps give rise to the observed decreaseof N. The value of P, corrected for electron-phononscattering, is practically temperature independent show-ing that the temperature dependence is controlled by cou-pling to the bulk ferromagnetic moment. We note inpassing that these findings make contact with recent ob-servations of GMR in annealed granular Co-Cu films. 'In these systems the field dependence of the GMR is alsoLan gevin-like.The field-dependent MR results discussed here point toa new mechanism for GMR that depends on the magne-tism of the interfaces, not the ferromagnetic spins in theCo layers, since the Co layers are already fully saturatedat low fields. GMR based on AFM coupling ' is notpresent in the results discussed here, since it requiresone-to-one correlation between MR and magnetization.In the sputtered samples, which typically have somewhatdiffuse interfaces, it describes the MR rather well. Conse-quently, the MR curve for sputtered samples exhibitsclear saturation that correlates with the bulk magnetiza-tion. '1.00la:~::+a+ '"j "::+'~i "%s' Ti'~':» ' Ii "w ~,3( I LÃ ': "!IF::xiii;.:-Q I5Ã ."Q y'~, , , zrA I 11~~.. ufii ay::gp;' '+ami .:"3 r(::. II FA ~~' ' " ':.~ wa~':.'~~ i''~ L"4at. .@~~~~K':iiitilr+I':: '!~ i' ti::%4 ':::::~ ' 5g k': ~ sm ~'~ ~ ~%-'::RE~„:'.:;::::.~~: ~n ' ~ '~','~~, ,~:h,.g r~.1 1~~q, .am gs~] +~r''~'~~~ . z~&~ 'j;~~', : ~ ~'.~ q il:~ ~ sizxo::.'.8 I ~~!'' 'gg~38%g W::~~«~Q'~~&~ I IFNa Nkl I, wE! " ' -:~:,:3am-~:;~.. ~:~~as'~e. m r:~Q. -'K". . ':' % % i3 F~~, M: " ' "WV WM'. ' =--~~':;~'5R,'IK,~ smf 'Ks l~'."-'.3 '&s " .; ':: '''' -]@:;~r'im- ~~i~ rP tm m ~a~me im ~~:::ms ~~:::~~~'-:&P~~" & ~ i':.I~ ':& &'. RGA g ~ mi riPYKMSIF"egg:p: '&'i P I I '~ 4R -'4 ~ %R ~jj,gila;M~ "~' ".' '' +~ ii L I' ll& ag ~ s.i' ii M.:::"s:"-:":e m"~.'~:"'.~ )i~~ i~i a' — '" ' '~ s i Iaj:.i+ ~~ & ~ ~:.jffIQ ~.' ~~~«",,''9, i y::xiii ill.-' "'. "~mm e~~~ sr%~ i ggi ~ ia'.: a~ ~, i sr-..::g Lsts~iiiliy '( ~ -.:~:-',".a Igl-"':,~z. :~~~~~~:.~i: ' ':: ~: a II h3 E l) Kyk m ~ ~@SR 55~~r ' i'." i aw a.~0.950.90—2.5(b)//I/P0.85I0.0(T)I2.5 5.0~JI—0.05 0.00 0.050.99—0.05 0.00 0.05FIG. 1. Typical cross-sectional HRTEM micrograph along[110]azimuth of [Co(7 ML)/Cu(3 ML)]40. The variation in con-trast along the growth direction, which is obtained by defocus-ing the image, corresponds to interference fringes due to the su-perlattice periodicity, as indicated by the arrows. Note that thedefocused image shown here still exhibits atomic resolution in-dicating high crystal coherence. The horizontal bar corre-sponds to 50 A. Inset: HRTEM image over a larger area. The0horizontal bar corresponds to 100 A. The straight horizontalcontrast bands indicate flat superlattice layers.—5.0 —2.5I0.0I) (T)2.5 5.0FIG. 2. (a) Ambient temperature magneto resistance,R(B)/R (0), vs field (Tesla) for a [Co(7.5 ML)/Cu(5. 5 ML)]z6superlattice. The dashed line is a theoretical fit to the Langevinfunction described in the text, where N =480 and P =0. 194. In-set: low-field MR vs field dependence. (b) Magnetization curvefor same sample. M„,=1400 emu/cm . Inset: Low-field mag-netization curve. T =300 K.49 LANGEVIN-LIKE GIANT MAGNETORESISTANCE IN Co-Cu. . . 15230 9(00(D0.80.0I2.5B (T)5.0In light of the above discussion, what are the conse-quences for interlayer coupling? Due to enhancedscattering by the interfacial paramagnetic layers, the ex-change interactions between the neighboring magneticCo layers are significantly weakened. Our results also re-FIG. 3. Field dependence of MR at two different tempera-tures for [Co(7.5 ML)/Cu(5. 5 ML)]30 The solid lines are fits tothe Langevin function described in the text. Inset: Number ofcorrelated spins as a function of temperature.veal that interfacial roughness, as indicated by modulatedRHEED patterns, gives rise to a less prevalent high-fieldLangevin-like MR. One can envisage regions of the sam-ple which have smooth terraces together with regionswhere islands have nucleated (e.g., Stranski-Krastanovgrowth). This may explain the observation of mixed cou-pling in recent polarized neutron scattering measure-ments on (l l l) Co-Cu superlattices grown on sapphire.Magnetic neutron scattering on our samples has so far re-vealed no evidence for coupling, consistent with themechanism described above. We believe the presence ofAFM coupling and the absence of high-field MR in sput-tered films are the result of atomically rough interfaces.The observed GMR in such samples arises from antifer-romagneticaly coupled layers.In summary we have demonstrated what we believe is adifferent type of high-field MR mechanism which resultsfrom scattering of conduction electrons from paramag-netic Co-Cu interfaces. Our observations cail for a morethorough treatment of the band offsets at the interfaces,including s-d hybridization, and for a better understand-ing of the spin-polarized interfacial electronic states. Wehope that our results will stimulate additional theoreticalwork in this area.We acknowledge useful discussions with R. Merlin, A.Ehrlich, D. M. Edwards, J. Mathon, and W. Pratt. Wealso acknowledge the HRTEM assistance provided by B.Demczyk and J. Mansfield of the University of MichiganElectron Microbeam Analysis Laboratory. Funding forthis work was provided by U S. 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Langevin-like giant magnetoresistance in Co-Cu superlattices

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Langevin-like giant magnetoresistance in Co-Cu superlattices

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