Layered magnetic structures: interlayer exchange coupling and giant magnetoresistance

Journal of Magnetism and Magnetic Materials 140-W

(1995) 1-8

Invited Paper

ELSEVIER

Layeredmagneticstructures:interlayerexchangecoupling andgiant magnetoresistance A. Fert a, P. Griinberg b-*, A. Barth&my

‘, F. Petroff a, W. Zinn b

a Universit6 Paris SI& 91405 Orsay Cedq France b Forschungszentrum Jiilich- IFF, 52425 Jiilich, Germany

Abstract Since the discovery of antiferromagnetic type interlayer coupling in 1986 and of ‘Giant Magnetoresistance’ in 1988, numerous systems heve been investigated. Here we give a critical review of the research on these phenomena and illustrate the development with some results from our groups in Orsay and Juelich.

1. I&oduction Although the possibility of an exchange interaction between ferromagnetic films across a metallic interlayer has been considered for a long time it was only around 1986 that it was clearly identified and characterized in Fe/Cr structures [1,2; and rare-earth based multitayers [3,4]. In 1990 the interest in the interlayer exchange was increased by the discovery of unexpected long range damped oscillations in the variation of the exchange constant with the thickness of the spacer layers [5] and a large number of systems have been investigated during the Iast years. The interest in exchange coupled mdtilayers was further enhanced by the discovery of the ‘giant magnetoresistance’ (GMR). Although there were already earlier reports of unusual magnetoresistive effects in layered structures I61 it was only around 1988 that GMR- effects were first clearly observed and characterized in antiferromagnetically W) coupled Fe/Cr systems [7,8]. For proper materials combinations it can also occur if antialignment is obtained by other means [9,10], for example by hysteresis effects. The theoretical interpretation of AF coupling and the GMR effect are somewhat related. Both effects are thought to be due to the propagation of spin-polarized electrons between the ferromagnetic films across the interlayer. The similarity is best expressed by the idea that !he coupling is established by a ‘spin current’ between the ferromagnetic films. This results in a torque which is due to the fact that electrons with spin up and down have different reflecticn coefficients at the interfaces [ll]. For the coupling there-

* Corresponding author. [email protected]

Fax:

+49-2461-512016;

email:

fore it is the reflection coefficients at the interfaces which are important whereas for GMR it is the diffuse scattering at the interfaces or in the bulk of the ferromagnetic films. While these mechanisms are widely accepted quantitative predictions for their size are often still by orders of magnitude different from the expetimental results which obscures also the relation between coupling and GMR. Anyway the promises of magnetoresistive effects for applications (magnetic sensors) have triggered a large number of experimental and theoretical research activities on both the coupling phenomena and the GMR. In this article we review our work on these topics, illustrate some aspects by results of our groups in Orsay and Juelich and discuss the current understanding of the field. 2. Iaterlnyer

coupling

A further milestone in the exploration of the hiterlayer coupling phenomena was the discovery that the damped oscillations can be multiperiodic, i.e. different a;cillation periods czn be superimposed. This was first found for the Fe/Cr- [12,13] and for the Fe/Au-system [14] and later on for many others. At the same time theories were developed which showed that the different oscillation p:?riods are connected with certain extremal distances :n the Fermi surface of the interlayer material. Adding to this distance f nG (G = nciprqcal lattice vector, n = in&$) is caUed atiasing. The smallest value obtained by afia&g is connected with the period observed in the experiment 115,161. However for complicated Fermi surfaces it is not always clear to which distance an obseF:eii oscill,tion period is related. For example in the Fe/Cr system two periods are observed. The short one could clearly be identified as due to the famous ‘nesting vector’ of Cr [12,13] whereas for the long period the related distanoe is still unclear. By

03t.W8853/95/$09.50 8 1995 Elsevier Science B.V. At1 rights reserved SSDI 0304-8853(94)00880-9

A. Fert et al. /Journal of Magnetism and Magnetic Materials 140-144 fl!WS) I-8

4.87ACI

5 UACI

61ACr

Fig. 1. Domain Patterns due to the different types of coupling in a Fe/G/Fe structure with wedged Cr-interlayer (in cooperation with J. McCord, A. Hubert, Univ. of Erlangen).

careful investigations it could furlherrnore be established that this particular period is also independent of the orientation of the interlayer 1191. In less complicated cases like for interlayers of fee noble metals a connection with the Fermi-surface was possible and the periods were found to be orientation dependent [20] as expected. On the other hand it is still an unresolved puzzle that a period of about 10 w [21] is found for many interlayer materials, with rather different Fermi-surfaces. Hence generally it is found that the strength of the coupling shows damped oscillations with increasing interiayer riiickness. Often, but not neccessarily, also the type of coupling oscillates between ferro- and antiferromag netic. Superimposed to this - and dominating in the transition zones between ferro- and antiferromagnetic there can be a further type of coupling which wants to align the magnetizations of the films on both sides of the interlayer perpendicular to each other 122,231. Like the other types of coupling the %I’-type has an effect on magnetic domains as is demonstrated by Fig. 1, We see here domain patterns in a sample whose crosssection is displayed at the bottom of the figure. There are

two Fe-films separated by a Cr-interlayer whose thickness d, increases from the left to the right. Due to the changes in 4, we obtain various types of coupling which is refiected by the magnetic domains seen in t&e upper parts. On the top the original picture as observed by means of a Kerr microscope is displayed. The middle section shows rhe magnetization tiirections in the different domains as evaluated from the gray tones of the original. The relative orientation of the magnetizations of the two films reveals the associated types of coupling. it is F on the left hand side, of w-type in the middle and AF on the right hand side. Clearly there is a strong infiuence of the coupling on the shape of the domains walls which can be understood in the following way. In the present case with orientation of the sample plane parallel to a (100) type crystallographic plane the individual magnetizations of the Fe-films are always along one of the in-plane [MO]-type directions which are the easy axis. For the formation and shape of domains we have to consider the local nd magnetic moment. If it changes its direction from one domain 10 the next ihen the wall in between has a well defined orientation, in order to obtain flux closure. If the net moment vanishes, like for AF type coupling, or if it does not change its direction like between some domains in the 90”-type coupling range then the direction of the wall is arbitrary. The result is an irregularly shaped wall. Therefore in the F coupled range we find only straight, and in the AF coupled range only irregular walls. In the 90”-type coupling range both types of walls appear. From these criteria one can recognize the type of coupling qualitatively from the shape of the domain walls. For a quantitative description the coupling is treated phenomenologically by means of an expression for the interlayer exchange energy of the following kind:

Ml‘M2-J

E= I -J,m

= -J,cos(A,$) -Jz(cos(A~))*. (1) Here Ei is the interlayer coupling area1 energy density and A4 is the angle between the magnetizations of the films on both sides of the interlayer. If J1 dominates then from the minima of Eq. (1) the coupling is F (AF) for positive
A. Fert et al./Journal

dc,

of Magnetism and Magnetic Materials ML144

(monolaysr)

Fig. 2a-d. Interlayer coupling in Fe/Cr/Fe as P function of the Cr-thickness dcr at temperatures T as indicated. T, = substra:e temperature during preparation of the samples (261. If we neglect anisotropy and in IQ. (1) the term with .I,, we obtain for J, for a double layer with magnetic films of equal thickness d and magnetization M the foilowing explicit form: J, = ,woM&d/2

(2) where H, is the field where the M(H) curve saturates at high values of H. For a multilayer system with n magnetic films in the limit n 3 ~0 the factor of l/2 on the righthand side of E!q. (2) (which is correct for n = 2) has to be replaced by l/4. In the commonly used cgs units one has to set ,uu,= 1 in Eq. (2). The parameters of Eq. (1) can be determined from M(H)-curves and from the frequencies of coupled spinwave modes. As an example we show in Fig. 2 for Fe/Cr structures the dependence of J, + J, on the thickness d,, of the Cr-interlayer 1261. The differences in panels (a)-(d) are due to differences in the substrate temperatures T, during the preparation of the samples and to differences in the measurement temperature T at which the data were taken. For a sample made with T, = 293 R only an oscillation with a long period around 1.8 nm is observed. This is in agreement with the result of Fig. 1 which was obtained f’rom a sample with the same T, and agrees also with the observation made by Unguris et al. [12] for such samples. Panels (b)-(d) reveal that for samples made with T, = 523 K in addition to the long period a short period of w 2 MLCr is obssrved. Again this agrees with the observations of Ref. [12]. On the other hand we do not observe the ‘phase slips’ reported by Unguris et al. [12]. We were able to follow the 2ML oscillation up to interlayers with d,, t 50 ML but phase slips were never observed {ED]. Such phase slips are expected from theory and are due to the

fN!?5) I-8

3

noncommensurate nesting vector at the Fermi surface of Cr. Their absence in Fig. 2(b)-(d) therefore is an mdication that the associated sample is not as perfect as those on the Fe-whiskers [12]. This is not surprising because the whiskers provide the best possible conditions for the growth of a Fe/Cr structure. Further attention has been paid to the phase of the oscillations for small interlayer thickness. Heir&h et al. [24] observe the maxima of AF coupling between d, = 4MLand d cr = 13 ML at an odd number of Cr monolayers. This is in agreement with the result found by Unguris et al. 1121. From symmetry then there must be an additional phase slip below d, = 5 ML. In Fig. 2 these q axima are halfway in between an odd and an even number of Cr ML. This is very Piely the result of an experimental uncertaiuty in d, due to the fact that for Fig. 2 wedge type interlayers have been used. As mentioned the oscillation with a period of I*I 2ML of Cr is observed up to rather large values of &. Thii can be attributed to the fact that ir is connected with the nesting vector [Zs]. For complete nesting to which we are close here one expects a decrease of the oscillation amplitudes only with l/d,, (d,, = interlayer thickness). Without nesting the decrease is stronger as it goes with l/&‘. Apparently the oscillations of panel (b)-(d) sit on a negative background which is strongly temperatur? dependent. This is in dkzgrmzent with Ref. [12] where the oscillations (for d, 2 0.5 nm) seem to include a sign change. On the other hand a negative offset was also found by Heimich et aI. [24] and is also expected from theory [18]. However a more detailed evaluation revealed E26] that what appears as an antiferromagnetic bias in Fig. 2 is in most parts !JO*-coupling. This example shows that many aspects are still controversial here, even if one compares only the results from different experimental groups. Concerning the 90”-type or biquadratic coupling there seems to be so far agreement that it is strongfy temperature dependent [14,29]. Various good suggestions have been made on its microscopic origin, but since the question essentially is still open we do not want to go here further into this topic. A so far unresolved problem in the comparison of theory and experiment is also the fact that the calculated coupling strengths generally are much larger (about a factor of 50 to 100) than what is found experimentally. Whereas most theoreticians agree on this @stilt thetr cxplb nations differ and reach from partial confinement [%I to interface roughness 1311 and lack of precision in the computations [32]. Comparing the result displayed in panels (a) and (b) we see that the oscillation amplihrde did not much increase in panel (b), despite the fact that the sample of(b) is of much better quality than that of (a). Otherwise a short period oscillation would not have been observed in (b). Therefore we think that the measured coupling strength here is an intrinsic property and not greatly modified by &I imperfect sample.

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of Magnetism

and Magnetic Materials 140-144 (1995) 1-8

ing [35]. Along the grain boundaries in the Cu interlayer bridges of Co can form and hence ‘bridge’ or ‘pinhole’ coupling could obscure ail true coupling phenomena[36]. Finally ‘Unambiguous Evidence of Oscillatory Magnetic Coupling in . . . Co/Au(ll ~/CO Trilayers’ [37] estab-

lished that coupling is possiblealso along the [ill] direction. Although there is disagreement between theory and experiment concerning the size of the coupling strength its increase with the filling of d- or s-states of the interlayer material found experimentally [21] is in good agreement with the theoretical expectation and thought to be due to

4 -40

*#)

-10

-x)

0

10

300 rwd’(kO~

Magi%

differencesin band matching[38]. Recent interpretationsof interlayer coupling empha-

l

Fig. 3. Xesistivity versus applied field for Fe30 &Tr@Ol) superlattices; H, is the field necessary to align the magnetic moments of the Fe layers [7].

Another puzzle which occurred in these investigations from the beginning was the fact that long period osciiiations appeared to be rather easy to observe on sputtered, poiycrystailine samples whereas groups we11 known for careful MBE work had problems in finding them. This was in particular true for Co/01 structures with the [Ill] growth direction 133,341. From the Fermi surface of a fee type noble metal one expects in the [ill] direction only one oscillation period but there is a peculiarity because the electrons mediating the coupling propagate in this case

under an angle of about 60” with respectto the sample normal [28] and not para’!el to it like e.g. for the IlOO]growth direction. Therefo:.e one would expect that in this case the electron wave could be Gsturbed by lateral inhomogeneities and the coupling would in the same way be attenuated [28]. Meanwhile it has beet, established that in the Co/Cu (111) case during growth there is a natural trend towards twinning which is connectet! with fee and hcp type stack-

-400

-309

-203

-lal

0

So [lti

100

ml

3al

sizes the similarity of a magnetic double layer (the simplest system to have coupling) with an optical resonator, like a Fabry-Perot interferometer. For the magnetic case the light wave is replaced by an electron wave and in addition the reflectivities at the interfaces are considered to be spin-dependent[11,15,25,39]. With this in mind one would obviously also expect osciiIations not only upon changing the thickness of the nonmagnetic but also of the magnetic films. This has indeed recently been confirmed experimentally [40]. The work described so far has all been on metallic materials - for the magnetic films as well as for the interlayers. Recently F as well as AF coupling has been

found across insulating and semiconductinginterlayers also [41,42]. This coupling is much weaker than the one across metallic interlayers as intuitively expected. Furthermore its strength increases at higher temperatures and/or irradiation with light which is consistent with the idea that the free electrons in the interlayer play an important role. There is still a debate[43,44] whether the light induced

effect is connectedwith optical transitions or finds a somewhat simpler explanation in the heating of the sample by the light.

Loo

-503

Fig. 4. Resistivity versus tieId for Fe/Cr/Fe(llO)

-000

-5fxl

0

330

moo

1500

B, [lo’ Tl

Tl

trilayers. The magnetic field

displays the anisotropic MK effect of a 250 8, thick Fe film [PI.

is

applied along (100) in (a) and (110) in (b). Also (b)

A. Fert et al./.lournal 3. Giant magnetoresistance

ofMagnetism and Magnetic Materials

IGMR)

In Figs. 3 and 4, we show the first observations of GMR in multilayers [7,8]. The results are on antiferromagnetically coupled Fe/Cr superlattices and one sees that the resistivity drops dramatically when an applied field aligns the magnetic moments of successive layers. The GMR is generally ascribed to the interplay between spin dependent scattering in successive magnetic layers. As will be discussed below, the conditions for the interplay is that the distance between the layers is relatively small in comparison with the electron mean free path (MFP). In addition there must exist some way to change the relative orientation of the magnetizations in adjacent layers by applying a magnetic field. In the most classical case, an antiferromagnetic WI arrangement is changed into a ferromagnetic (F) one by the applied field. This AF arrangement can be provided by AF interlayer exchange, as in Figs. 3 and 4. In this case, oscillations of the interlayer exchange as a function of the thickness of the non-magnetic layers give rise to oscillations of the MR ratio, see Fig. 5 for example. But an AF arrangement can aho be obtained in other ways, by giving different coercivities to successive magnetic layers, or by pinning the magnetization of some layers in the so-called spin valve systems [9,10], A non-saturated MR can also be observed if there is only a random arrangement of the magnetic moments in successive layers at low field. The GMR is currently attributed to the spin dependent conduction properties of the ferromagnetic metals [7,46,47]. In a ferromagnetic metal the electricat current is carried by the spin t (majority) and spin 1 (minority) eleclrons in hvo approximately independent channels [45]. The conductivities of the two channels can be very different, especially when the ferromagnetic metal contains impurities with strongly spin dependent scattering cross sections. In magnetic multilayers electron scattering by imperfect interfaces or by impurities or defects within the magtletic layers

Fig. 5. killatory variation of the MR ratio as a function of the thic’kneaaof Co in Co15 &uClll~ multilayers. The black circles and the squares are for 4.2 and 300 K respectively. From Mosca et al. [331.

140-144 fI@S) l-8

M NM M

AF

5 #

NM M

F

Fig. 6. Schematic picture of the GMR mechanism. Tbe electron trajectories between two scatter&s arc represented by straight lines and the scatter& by abrupt changes in direction (for simplicity the figure is drawn with only interface scattering). ‘IAe signs .I- and - are for electrons spins s, = + and -l/2 respectively. The arrows represent the majority spin direction in the magne:ic layers. In the ferromagnetic (FJ configuration at the eidlt, the spin + electrons are weakly scattered everywhere, which gives a shorl circuit effect and leads to a small resistivity. In the AF configuration at the left, each spin direction is scattered in every second magnetic layer, there is no short circuit effect and the resistivity is higher. The schematic is for MFP much larger than tha layer thickness.

can be spin dependent. The schematic of Fig, 6 illustrates the mechanism of the GMR in the simple &nit where electrons have MFP much larger than the layer thicknesses and can feel the relative orientation of the magnetization in successive layers. When the thickness of the non magnetic layers becomes larger than the MFP (- 1ti2 A), the interplay betweer: successive magnetic layers disappears and the GMR vanishes. An additional reason for the decrease of the GMR at increasing thickness comes from the importance of interface spin dependent scattering: as the thickness increases, the interface density decreases, which also contributes to the decrease of the GMR. Several theoretical models based on the above mechanism have been worked out. The first, by Camley and BarnaS, was semi-classical, that is expressing the spin dependent scattering at interfaces by a spin dependent transmission coefficient and using a Boltunarm equation formalism [46]. Then quantum models, calculating the scattering of electron waves by spin dependent potentials have also been developed [47]. The models of the first generation - semi&ssical or quantum - were assuming free electrons propagating in some distribution of scattering potentials. More recent models take into account the spin dependent potential steps of the superlattice potential [48,49]. Although the specular scattering by these periodic steps does not give rise to resistivity or magnetoresistance by itself, they can influence significantly the magnetoresistance induced by the diffuse spin dependent scattering 148,491. Most models have been worked out for the low temperature limit where the momentum transfer between the two channels by spin-flip scattering can be neglected. At finite temperature, it is importmint to introduce the spin-nip electron-magnon scattering to account for the temperature dependence of the GMR. Examples of fit of

6

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of Magnetism and Magnetic Materials 140-144 hY5) l-8

experimental results at both zero and finite temperature can be found in Ref. [50]. A general conclusion from various fits of theoretical model with the experimental thickness dependence of the GMR is that, at least for canonical systems such as Fe/Cr or Co/Cu, it is necessary to assume strongly spin-dependent interface scattering [50-533. This predominance of interface scattering for the GMR has been confirmed by some more direct experiments with very thin layers of another metal inserted at interfaces [53-561. In Co/Cu, for example, it turns out that inserting 3 or 4 A of Fe between Co and Cu suppresses most of magnetoresistance, which can be ascribed to the removal of the spin filtering properties of the Co/Cu interface [S3]. In a similar way, inserting ultrathin Co layers at the interfaces of NiFe/Cu multilayers introduces Co/Cu interfaces and enhances significantly the GMR [54-X]. Another direct consequence of the importance of interface scattering is the influence of the interface roughness 157-591. Whereas the role of spin dependent scattering by interfaces is well recognized, there has been some controversy about the existence of additional contributions from spin dependent scattering within the magnetic layers [54,55,60]. We think that some moderate contribution from ‘bulk scattering’ should generally exist. Recent results on the enhancement of the GMR in Fe/Cr by doping the Fe layers with Cr impurities seem to bring a direct proof of the existence of such contributions [till. Another result that we want to mention is the GMR change of sign obtained by inserting ultrathin layers of Cr every second layer of Fe in Fe/u multilayers. The spin asymmetries of the electron scattering in the doped and undoped Fe layers are opposite (i.e. a < 1 and (Y > 1 respectively), so that there is short circuit by one of the spin directions and low resistivity when the magnetizations of successive layers are antiferromagnetically arranged

b21. The microscopic mechanism of the spin dependent scattering by interfaces is still not very clear. Why is the GMR larger in Fe/Cr than in Co/r, in Co/u or Co/Ag than in Co/Au? Simple models have been developed but are not realistic enough to account for all the experimental results [63]. We think that a realistic treatment of such a band matching problem requires a realistic numerical calculation of the band structure of the multilayer and of the electron scattering by rough interfaces. The ab initio calculations developed by Butler et al, [64], although still imperfect, represent certainly the right way. So far wq have discussed GMR with the Current In the Plane of the layers (the so-called CIP geometry). GMR effects have been now observed also with !he Current Perpendicular to the Plane of the layers, the so-called CPP geometry [65,66]. A very interesting point is that the CPP-GMR raises fundamental problems quite different from those of the CIP-GMR [67-701. The model of Valet and Fert [67] describes the spin accumulation effects that

F--1 d-

ta=6nm

Bilayer

No. N

Fig. 7. Square root of the CPP-GMR (see Eq. (3)) versus bilayernumber N for Co/& Co/CuMn7%, CojCuPt6% multilajfes for 6 nm thick Co layers and a fixed total thickness. The solid curves are fits to erp&sions of the VF model [4;] with I,, = 2.8 nm and 8 nm tar CuMn7% and Cupt6Y0 respective!y. From Yang et al. [72]. are important for the CPP-geometry and not for the CIP. Whereas the scaling length of the CIP-geometry is the MFP A, the unique scaling length of the CPP-geometry is the spin diffusion length, 1,, w (Ah,&‘/’ where A, is the spin MFP (in usual materials, I,, = 103-104 8, a A * lo2 A). In the usual limit in which the layer thicknesses are much smaller than Iat, the expressions if the CPP-GMR in the VF model [67i-be.come- very simple and similar to those derived in a phenomenological model by Pratt ar:d coworkers [71]. More precisely, in this limit, the squaz root of the MR becomes a linear function of the density of in!erfaces. For example, in the case where the thickness of the magnetic layers is fixed: 6

W)

- R(F)]RW)

= m

(3)

where RfAF) (R(F)) is the resistance of the AF (F) state for a unit surface and a fLxed total thickness L, N is the number of bilayers within L and A is a parameter depending on the thickness of the magnetic layers. This linear variation has been observed for extensive series of samples 1711. On the other hand, if the spin diffusion length is shortened and is not much longer than the layer thickness, a reduction of the MR and a strong deviation from a linear variation in N are predicted by the VF model [67]. This behavior has been clearly observed (see Fig. 7) by Yang et al. 1721 who have doped the non-magnetic layers with Mn or Pt impurities to shorten I,,. It turns out that CPP-MR experiments can be used to determine spin diffusion lengths and spin relaxation times. In 1992 GMR effects have also been observed for granular materials in which magnetic clusters are embedded in a metal [74,75]. The resistivity drops when the magnetic moments of the clusters are aligned by an ap-

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plied field. The GMR of granular systems such as CoAg or CoCu can be very large but a relatively large field is generally necessary to overcome the anisotropy energy of the ciusters and saturate the GMR. This explains why, for applications, several attempts have been made to prepare intermediate structures having some advantages of the granular materials (weak sensitivity to defects and parasitic couplings) with also the low field of the multilayers. Interesting results have been obtained with structures including permalloy islands obtained by annealing NiFe/Ag multilayers [76]. Other interesting systems for GMR effects at very low field are hybrid structures including both continuous permalloy layers and Co clusters (discontinuous layers). With such structures, MR slopes of about 6% per Oersted at 4.2 K have been obtained [77]. Most magnetic nanostructures (based on multilayers or clusters) investigated up to now include only metallic components. What begins to be developed now are magnetic nanostructures including not only metals but also semiconductor or insulators. We have already mentioned the recent observation of light induced interlayer coupling in ferromagnetic metal/semiconductor structures 141-441. Concerning the transport properties, an interesting problem is the spin dependent tunneling between ferromagnetic metals through an insulator or between a ferromagnetic metal and a semiconductor excited by circularly polarized light [78]. Other interesting problems are related to the injection of spin polarized electror,s into a semiconductor m. We conclude with a few words on the applications of the GMR. Applications to various magnetic sensors, in particular read heads, begin to appear. For most of these applications, the GMR effects have to be obtained at low field. This has led to specially designed structures switching from AF to F configurations in small fields. The performances of the magnetoresistive devices based on GMR turn out to be definitely better than those of conventional materials [80]. For the future, more sophisticated applications can be predicted [73]. Various applications can also be expected from structures inc!uding magnetic materials and semiconductors and based on spin polarized transport effects [791. Ackmvledgements: We want to thank all our colleagues or students who have contributed to our work, in particular J.R. ChBdress, J.L. Duvail, J.M. George, D.K. Lottis, R. Morel, K. Ounadjela, L. Steren at Orsay, R. Schreiber, G. Binasch, M. Vohl, F. Saurenbach, U. Walz, S. Demokritov, J.A. Wolf, A. Fuss, R. SchZfer, M. Schiifer, Q. Leng at Jiilich, V. 00s at Orsay and Jiilich, P. Etienne, A. Friederich, F. Nguyen Van Dau, A. Schuhl, T. Valet at Thomson-CSF, M. Baibich, D.H. Mosca, L.G. Pereira at Porto Alegre, J. Bass, P. Holcdy, R. Loloee, W.P. Pratt, P.A. Schroeder at Michigan State University, P.M. Levy at New York University, J. BarnaS at Pozna6, M.B. Brodsky, H. Sowers, SD. Bader, M. Grimsditch at Argonne, Y. Pang at Bejing, A. Hubert, M. Riihrig, J. McCord at

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