2l42 IEEE TRANSACTIONS ON MAGNETICS, VOL. 28. NO. 5, SEPTEMBER 1992

Fe/Cu/Fe and ColCdCo Multilayers on Cu(ll1): The Absence of Oscillatory Antiferromagnetic Coupling

W. F. Egelhoff, Jr. and M. T. Kef Surface and Microanalysis Science Division, National Institute of Standards and Technology

Gaithersburg, MD 20899

Abstract-We have used the magneto-optical Kerr effect to search for evidence of the oscillatory antiferromagnetic (AF) coupling associated with the recently discovered "Giant Magnetoresistance Effect" (GMR) in Fe/Cu/Fe and Co/Cu/co multilayers. The GMR effect was reported in samples grown by magnetron sputtering methods on Si wafers. In our work the multilayers are grown by molecular beam epitaxy (MBE) techniques on a Cu( 11 1) single-crystal substrates. None of the resulting multilayers showed any evidence of the oscillatory AF coupling being sought. We have concluded that (1 1 l)-oriented crystallites in the sputtered multilayers make little if any contribution to the observed oscillatory AF coupling. However, oscillatory AF coupling does occur in MBE-grown multilayers on Cu(100). and its &pendace on Cu thickness is remarkably similar to that of the sputterdeposited multilayers. This suggests that (100)-oriented grains in the sputter-deposited multilayers may be partly, or perhaps even largely, responsible for the AF coupling. Supporting this suggestion are preliminary x-ray diffraction pole-figure measurements we have made on three of the sputter-deposited multilayers which indicate that the tendency to (1 11) texture is not extremely strong, and that other crystalline grains are present.

I. INTRODUCTION

Antiferromagnetic (AF) coupling played a key role in the recent discovery of the "Giant Magnetoresistance (Gm) Effect" in magnetic multilayers [ll. In certain multilayers consisting of alternating ferromagnetic-nonferromagnetic films, adjacent ferromagnetic films couple to each other antiferromagnetically through the nonferromagnetic film separating them [1]-[8]. When an extemal magnetic field of sufficient strength to overcome this coupling is applied and the ferromagnetic films align, a change in the electrical resistance of unprecedented magnitude is observed, the so-called GMR effect [1]-[7]. Values for this magnetoresistance (MR) change in excess of 60% have been reported in Co/Cu/Co multilayers at m m temperature [3]. Such "giant" values could be of great technological importance in device applications such as thin-film heads or as a nonvolatile form of dynamic random access memory [8]. However, to date "giant" MR values are always accompanied by AF coupling strengths so large (21 kOe) as to be impractical for devices. An intense search is presently underway in labs around the world to find ways to reduce the AF coupling strength while maintaining "giant" MR values [1]-[9].

Manuscript received February 17, 1992. This work was supported, in part, by the Office of Naval Research, contract number NOW14-9 1 -F-0044.

II. EXPERIMENTAL

The basic elements of the magneto-opticd Kerr effect instrumentation, the MBE system, and the sample growth procedures have been published previously [lo]. In the p s n t work we have found that a buffer layer of -9-14 A (minimum) Cu is important for achieving surfaxis of sufficient qualily to observe the highest levels of AF coupling on Cu(100).

III. RESULTS AND DISCUSSION

Our purpose in the present work is to study the crystallographic dependence of oscillatory AF coupling in Co/Cu/Co multilayers and in Fe/cu/Fe multilayers. For this work we used a Cu( 11 1) single-crystal substrate since the samples which exhibit the GMR effect were reported to exhibit predominant (1 1 1) crystallographic texture. Our work is in part an extension of earlier studies of Fe/CwFe multilayers on Cu(100) in which oscillatory AF coupling was found [lo]. A remarkable aspect of the oscillatory AF coupling found on the Cu(100) substrates was its resemblance to the results later reported for the nominally (1 1 1)-textured samples exhibiting the GMR effect [4].

Figure 1 presents a comparison of the two sets of data. Although Fig. 1 compares different properties (this was all that was available), there is general agreement that in these systems the oscillations in AF coupling are commensurate with oscillations in the magnetoresistance ratio[ 21441.

In a series of results which stand in stark contrast to those of Fig. 1, we did not find any evidence of oscillatory AF coupling in the present series of 14 Fe/Cu/Fe multilayers

9 0.1

2 / / I O 0-

Cu Ih icknk A

16

Fig. 1 A comparison of the dependence of AF coupling strength in MBE-grown FelCulFe multilayers on Cu(100) hom Ref. [lo] and the MR ratio for sputterdeposited multilayas of Ref.. [4].

0018-9464/92$03.00 0 1992 EEE

2743

Supporting our conclusions are a series of 18 Co/Cu/Co multilayers we have grown on Cu(lll), again using the instrumentation and methods we devised to achieve the best possible epitaxy. In our Kerr-effect studies we found that none of these multilayers exhibited any oscillatory AF coupling.

Concerning the possibility of non-oscillatory AF coupling, it should be noted that we did observe strongly skewed polar hysteresis loops for Cu spacer-layer thickness of 2-2~A. While this data might, on its own, be taken as evidence of non-oscillatory AF coupling, we think it is not because, concurrently, we observed square in-plane hysteresis loops. It seems more likely that this behavior is due to the magnetic moments being canted almost in-plane.

Also supporting our interpretation is the fact that other groups have found oscillatory AF coupling on Cu(100) but not on Cu(lll)[llI-[141.

There are preliminary reports of what appears, tenratively. to be strong but non-oscillatory AF coupling on Cu(l11).[151,[16] aneffectpredicted bytheory [17]. Wefound no evidence of this coupling in our work.

Further supporting evidence for our conclusion that minority -(loo)-oriented grains are responsible for the oscillatory AF coupling comes in the form of x-ray diffraction pole figure measurements we have made on three of the original sputter-deposited Co/Cu/Co multilayers of Ref. [3]. The pole figures indicate that the tendency to (1 1 1)-texture is not extremely strong and that other crystalline grain orientations are present.

One important point should be made about using x-ray diffraction to determine sample texture. The common assumption that all Crystalline grains are of equal size may be incorrect and may lead to extreme errors of interpreration. Recall that. if the grain diameters are on the order of or less than the coherence width of the x-ray beam, as is usually the case for sputtered multilayers, then the x-ray diffraction intensity from a grain is proportional to she square of the number of atoms in the grain. This proportionality makes grain size very important. For example, it is likely that the dominant grains in a film are dominant because they grow faster, and the minority grains are thus on average smaller in diameter. To illusaate the dramatic consequences this can have imagine a surface that is 50% (1 11) and 50% (100) by area. However, let the (100) grains be 10 times smaller in diameter. To be equal in total area to the (1 1 1) grains, the (100) grains would be 100 times more numerous [as if the (1 11) grains grew faster and merged into large units while the (100) M n s remained small inclusions surrounded by large (1 11) areas] Then the relative x-ray diffraction intensities would be 12@111 = 100~(0.01~)(0.51)/1-(1~) = 0.005. where051 is the 200/111 x-ray structure factor. Note further, the 0.005 relative intensity of the 200 reflection will be bmder by about a factor of 10, both in the scattering plane and perpendicular to it [18]. This 100-fold increase in solid angle will reduce the 200 peak intensity by 100. Thus the relative peak intensity will be 0.00005, well below the detection threshold of conventional diffractometers. Yet, this is a surface that is 50% (loo)! Clearly, x-ray diffraction cannot establish texture without prior knowledge of the distribution of grain sizes.

grown on Cu(ll1) using the Same inshumentation as in Ref. [lo]. These multilayers were grown by a variety of techniques including those of Ref. [lO],which produce the highest quality of epitaxy possible, as well as ones intended to mimic the conditions of magnetron sputtering. This included the introduction of background gases, warming the substrate during growth, and flooding the substrate with an electron beam. A Variety of post-growth annealing trearments was also aaempted in the search for oscillatory AF coupling.

The failure of these experiments to reproduce the oscillatory AF coupling of Ref. [4]. together with the excellent agreement in Fig. 1 between Refs. 141 and [lo] suggests that in the sputterdeposited multilayers a minority component of (100)-oriented crystalline grains is responsible for the observed oscillatory AF coupling. It is noteworthy in this regard that the peak AF coupling strength in the sputter-deposited multilayers of Ref. [4] is a mere 0.095 ergs/cm2 at 4 K, while in the MBE-grown multilayers on Cu(100) of Ref. [lo] it is 0.32 erggcm2 at 300 K, and in our latest work 0.40 ergs/cm2 using a 30 A buffer layer. Moreover, our values are for Fe bilayers and should approximately double in a superlattice due to the two-sided effect. Clearly, a minority component of -(100)-oriented crystalline grains in the sputterdeposited multilayers could be responsible for the obser~ed oscillatory AF coupling.

Within a given Fe layer in the sputter-deposited multilayers, magnetic alignment probably occurs horizontally through the film, crossing grain boundaries. Thus, Fe in (1 1 1)-oriented grains probably aligns with Fe in adjoining (100)-glains.

Figure 2 presents a comparison analogous to that in Fig. 1 for the Co/Cu/Co system. Although the fmt peak differs by -3 A, the second and third peaks are in excellent agreement. This degree of agreement is not bad considering that very different grbwth techniques were used (which may produce different growth morphologies) and that thickness calibrations could differ slightly. (It is not uncommon for a group to change quoted thicknesses by 20% upon performing a recalibration, although few like to discuss it publicly.)

' t I \ I t 0.2 s --L I I fl o_

zot lot- I n.

colculco lo., 1 ' 0

0 I O 20 30 40 50 60

Cu thickness, A

Fig. 2 A comparison of the dependence of AF coupliig strength in MBE-grown Co/Cu/Co multilayers on Cu(100) from Ref. [I21 and the MR ratio for sputter-deposited multilayem of Ref. [3].

2744 If our conclusions are correct. the non-(lW)-oriented

grains are still likely to be primarily responsible for the giant magnetoresistance effect. Therefore. reducing the concentration of -(lOO)-oriented grains in the multilayer might achieve the much sought after goal of lowering the AF coupling strength while maintaining the giant magnetoresistance efftxt.

IV. CONCLUSIONS

The major points of this work can be summarized as follows.

1) No evidence of oscillatory antiferromagnetic coupling is observed for MBE-grown Fe/Cu/Fe and Co/Cu/Co multilayers on Cu( 1 11) substrates.

2) Point 1) seems to conflict with the oscillatory antiferromagnetic coupling observed for Fe/Cu/Fe and Co/Cu/co multilayers reportedly exhibiting (1 1 1)-texture and sputter-deposited on Si-wafers [31,[41.

3) Preliminary x-ray diffraction pole-figure data on magnetron grown multilayers suggest the tendency to (1 11) texture is not extremely strong and other crystalline grains are present.

4) Oscillatory antiferromagnetic coupling very similar to that of Refs. [3].[4] has been observed for MBE-grown Fe/Cu/Fe [lo] and Co/Cu/Co [121,[131 multilayers on Cu( 100) substrates.

5 ) Points 1)- 4) suggest that a minority constituent of -( 100)-oriented crystalline grains present in the sputter-deposited multilayers could be partly, or perhaps even largely, responsible for the oscillatory AF coupling.

6) Reducing the concentration of -( 100)-oriented crystalline grains may be one way to lower the antiferromagnetic coupling strength while maintaining the giant magnetoresistance effect.

ACKNOWLEDGMENTS

The authors gratefully acknowledge very helpful conversations with Drs. R. Coehoom. P. M. Levy, R. F. C. Farrow, B. Heinrich, S. S. P. Parkin, and F. Herman. We wish to thank Drs. R. F. C. Farrow and S. S. P. Parkin for loaning us several of their Co/Cu/Co multilayer samples for x-ray diffraction pole-figure analysis.

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