Solid State Amorphisation in Magnetic Multilayers: the Interface Structure and the Electrical Transport Properties

\

Chaos\ Solitons + Fractals Vol[ 09\ No[ 01\ pp[ 1920Ð1933\ 0888 Þ 0888 Elsevier Science Ltd[ All rights reserved 9859Ð9668:88:, ! see front matter

Pergamon

PII] S9859!9668"87#99141!4

Solid State Amorphisation in Magnetic Multilayers] the Interface Structure and the Electrical Transport Properties T[ STOBIECKI\\a M[ KOPCEWICZb and F[ J[ CASTAN Oc a

Department of Electronics\ University of Mining + Metallurgy\ al[ Mickiewicza 29\ 29!948 Krakow\ Poland b Institute of Electronic Materials Technology\ Wolczynska 022\ 90!808 Warszawa\ Poland c Institute of Applied Magnetism "IMA#\ Laboratorio Salvador Velayos Apartado de Correos\ 044\ 17129 Las Rozas\ Madrid\ Spain

Abstract*The structure and electrical conductivity properties of the R[F[ sputtered Fe:Zr and Fe:Ti multilayers with ultrathin layer thicknesses\ in as!deposited states\ have been studied using X!ray di}raction\ low!angle X!ray and neutron re~ectometry\ conversion electron Mossbauer spectroscopy "CEMS#\ resis! tivity and magnetoresistivity measurements[ The thickness ratio "bdFe:dZr and dFe:dTi# of analysed multi!  to 599 A \ layers was 9[4 and 0\ the values of the bilayer thickness "ldFe¦dTi\Zr# was varied from 8 A maintaining constant the total thickness of the samples by controlling the number of bilayers[ The results  obtained from CEM!spectroscopy and X!ray di}raction show that Fe layers of the thickness below 19 A are alloyed forming an amorphous phase during deposition[ This amorphous phase is distributed in the plane between the crystalline sublayers as well as in the grain boundaries according with the proposed model of the interpretation of the electrical conductivity as a function of the bilayer thickness "l#[ Þ 0888 Elsevier Science Ltd[ All rights reserved[

0[ INTRODUCTION

In recent years\ thin _lm metallic multilayer structures "Ml|s# have become increasingly important in technological applications[ For instance\ permalloy thin _lm in the form of exchange coupled {spin valve structure| are used as magnetic recording heads ð0Ł and Co:Pt Ml|s seem to be promising as advanced magnetooptic materials[ The dimensions of technological devices are decreasing progressively[ As the individual layers in multilayer structures become thinner\ inter! faces comprise an increasingly large fraction of the volume of the material[ Therefore\ interface properties can dominate the magnetic\ electronic and structural properties of a thin multilayer[ For this reason\ fundamental studies of the interface structure and properties have signi_cant practical interest[ Solid state amorphisation reaction "SSAR# was discovered by Schwarz and Johnson in 0872 ð1Ł[ Since that time\ amorphisation reactions have been investigated in many binary alloy systems "see\ e[g[\ ref[ ð2Ł#\ including late transition metal "LTM] Fe\ Co\ Ni#:early transition metal "ETM] Hf\ Zr\ Ti# ð3Ð5Ł\ transition metal:rare earth metal "Co:Gd# ð6Ł and transition metal:metalloid pairs "FeB:Fe# ð7\ 8Ł[ Thin!_lm technology is ideal for studying SSAR because it allows us to prepare the initial state as a multilayer of alternating thin _lms of crystalline elemental com! ponents deposited on a suitable substrate[ In our recent papers on Fe:Zr and Fe:Ti Ml|s ð09\ 00Ł\

 Corresponding author[ 1920

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it was shown that SSAR occurs during deposition when the thickness of the constituent layers is reduced[ The SSAR is driven by large negative heat of mixing between the components\ which results in the amorphous alloy having a lower free energy than the two!phase mixture of pure crystalline elemental phases and in di}usional asymmetry "i[e[\ one element di}uses anomalously fast into the other#[ The formation of intermetallic phases of lower free energy than the amorphous phase is inhibited by unfavourable kinetics[ The formation of amorphous phases in multilayered systems of ETMÐLTM is normally explained by planar growth ð3Ł between the crystalline sublayers[ Hollanders and Thijsse ð01Ł and Otto et al[ ð02Ł suggested that the amorphous phase\ after thermal treatment\ may also grow within the crystalline layers at the grain boundaries[ Recently\ the formation of amorphous iron was observed in Fe:Zr Ml|s by Dubowik et al[ ð03\ 04Ł and in Zr:Fe0−x Zrx :Zr trilayers by Geisler et al[ ð05Ł[ Moreover\ Zr\ Ti and Ta are used as a bu}er layer in the multilayer structures of {spin valve| sensors ð06Ł[ In this paper\ the interface structural properties and magnetic properties of Fe:Zr and Fe:Ti are presented with the aim of showing that the amorphousÐalloy phase is formed during depo! sition and is distributed in the plane between crystalline sublayers as well as in the grain boundaries[

1[ EXPERIMENT

Fe:Zr and Fe:Ti were prepared by the R[F[ sputtering technique in an Ar atmosphere "details are published elsewhere ð09Ł#[ The thickness ratios\ dFe:dZrb9[4\ 0 and dFe:dTib9[56\ 0\ correspond to the atomic compositions\ Fe49Zr49\ Fe55Zr23 and Fe49Ti49\ Fe59Ti39\ respectively\ of the paramagnetic "at room temperature# amorphous phase[ The range of modulation wavelength "ldFe¦dZr\Ti# was 9[8¾l¾79 nm for Fe:Zr and 4¾l¾79 nm for Fe:Ti[ Such samples exhibit a modulated atomic!crystalline:amorphous structure and ferro:paramagnetic ordering[

1[0[ Interface structure The structural properties of the sputtered Fe:Zr and Fe:Ti Ml|s in the as!deposited amorphous state have been studied in order to better understand the electrical conductivity and magnetic properties[ This chapter deals with a detailed structural characterisation using conversion electron Mossbauer spectroscopy "CEMS#\ X!ray di}raction\ as well as X!ray and neutron re~ectometry techniques "only for Fe:Zr Ml|s# of R[F[ sputter deposited Fe:Zr and Fe:Ti Ml|s[ 1[0[0[ CEMS results for Fe:Zr and Fe:Ti Ml|s[ The interface structure was systematically studied by CEMS in our previous papers ð09\ 00Ł[ It was found that in the CEM spectra of Fe:Zr "Fig[ 0#\ with dFe×19 nm\ only the crystalline phase "bcc!Fe# and\ with dFe¾0[2 nm\ only the amorphous paramagnetic phase were identi_ed ðFig[ 0"c#Ł[ However\ Fe:Ti Ml|s with dFe24 nm are crystalline and\ with dFe¾1 nm\ are amorphous "Fig[ 1#[ For the iron sublayer of thicknesses in the range 4 nm³dFe³14 nm\ the spectra of Fe:Zr and Fe:Ti Ml|s consist of a paramagnetic quadrupole doublet "QS# and three magnetically split components] bcc!Fe "HHf21[7 T# and two interfacial bcc!Fe"Zr\ Ti# alloy components with HHf18 T and HHf01 T ðFig[ 2"a\b#Ł[ In this case\ the relative spectral contributions of the QS doublet and of the magnetic sextets with HHf18 T and 01 T markedly increase at the expense of the bcc!Fe sextet[ The spin orientation in Fe layers\ determined from CEMS measurements\ shows strong alignment in the plane of the

Solid state amorphisation in magnetic multilayers

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Fig[ 0[ "a# CEM spectra of Fe:Zr Ml|s with b0[ "b# CEM spectra of Fe:Zr Ml|s with b9[4[ "c# The paramagnetic CEM spectra of Fe:Zr Ml|s with b9[4 "the results are presented in a reduced velocity range#[ The elemental layer thicknesses are given[

Fig[ 1[ "a# CEM spectra of Fe:Ti Ml|s with b0[ "b# CEM spectra of Fe:Ti Ml|s with b9[4[ The elemental layer thicknesses are given[

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Fig[ 2[ "a# CEMS fractions of Fe:Zr vs[ Fe sublayer thickness[ "b# CEMS fractions of Fe:Ti vs[ Fe sublayer thickness[ The lines are visual guides only[

_lm ð09\ 00Ł[ However\ in the spectral components corresponding to the interfacial regions\ random spin orientation was observed[ 1[0[1[ X!ray wide an`le diffraction results for Fe:Zr Ml|s[ The FeÐZr system exists either as a crystalline intermetallic compound or as an amorphous alloy[ The amorphous FexZr0−x phase can be produced over a wide composition range "9[01³x³9[79# by sputtering deposition ð07\ 08Ł[ The Fe:Zr multilayer system is an interesting example for the exploration of structural proper! ties "and consequently the nature of the SSAR that occurs during deposition#\ since there is an  # and the hcp!Zr extreme mismatch between the lattice parameters of bcc!Fe "a1[76 A  \ c4[04 A  #[ This will allows us to distinguish between the two growth models of the "a2[12 A amorphous phase[ The _rst model assumes planar growth of the amorphous FeÐETM alloy phase ð3Ł and the second model suggests that an amorphous phase is formed during sputter deposition in the plane between layers as well as in the grain boundaries ð4\ 5\ 01\ 02Ł[ On the other hand\ the di}erence in the electronic densities and scattering length densities of bcc!Fe and hcp!Zr makes X!ray re~ectometry "XRR# and neutron re~ectometry ideal methods to examine the modulated structure of Fe:Zr Ml samples[ Structural\ electrical transport ð19Ł and magnetic properties ð10Ł are closely related and\ in this way\ a detailed study of SSAR in Fe:Zr Ml|s should be accompanied by a structural study that gives a better understanding of these results[ Fe:Zr systems are predominantly composed of closely packed "009# planes of bcc!Fe and "991# hcp and "099# hcp planes of Zr[ The plane spacings "d# calculated from the "009#Fe\ "099#Zr and  \ 1[460 A  and 1[790 A  \ respectively[ "991#Zr peak positions were about 1[917 A Figure 3 shows the XRD measurements of all the Fe:Zr samples produced using BraggÐ Brentano geometry and Co Ka radiation[ These measurements have been corrected for the di}raction intensity from the substrate and are presented over the range 29>¾1u¾59>[ The critical thickness\ dc\ of the Fe and Zr layers below which the multilayer appears to be  for both elements[ The fact that no satellite or superlattice peaks fully amorphous is around 19 A appear in the di}raction patterns is an indication of amorphous interfaces appearing in the plane between crystalline layers[ For large bilayer thickness "l×19 nm#\ the amount of amorphous phase can be considered as being negligible and the XRD intensity ratio of the Zr line to the Fe line for these samples is approximately 0 for samples with b0 and 1 for those with b9[4[ For

Solid state amorphisation in magnetic multilayers

1924

Fig[ 3[ XRD measurements of Fe:Zr multilayers with b9[4 and 0[ The elemental layer thicknesses are given[

Fig[ 4[ d!spacing and the size of the Fe and Zr crystallites vs[ modulation wavelength\ l\ for b0 "a# and 9[4 "b#[

small modulation wavelengths "l³19 nm#\ the di}raction lines of Fe "009# and Zr "991# broaden considerably Fig[ 3[ The Zr "099# peak disappears with decreasing l[ This e}ect is especially pronounced for b0[ The amount of amorphous phase increases with decreasing bilayer thickness\ as revealed by the appearance of a broad peak between the Fe and Zr lines[ In the XRD patterns for samples with b0\ the Zr peak intensity decreases more rapidly than that of Fe\ which suggests that the Zr layer is transformed faster into the amorphous phase than the Fe layer\ which is in good agreement with earlier di}usivity studies ð02Ł[ The di}usion results for Fe:Zr multilayers ð02Ł show that two mechanisms are responsible for the formation of an amorphous phase in as! deposited multilayer structures] planar growth ð3Ł and lateral growth of the amorphous phase due to fast di}usion into individual crystallites at the grain boundaries[ As was stated before\ it can be assumed that the layers in these sputtered samples have such a high density of grain boundaries that\ to a _rst approximation\ the grain size may be equivalent to the layer thickness[ From the Fe and Zr line broadening\ it is possible to estimate the size\ D\ of the Fe and Zr crystallites[ The results are shown in Fig[ 4"a\b#[ The size of the Fe and Zr

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crystallites increases with l and is smaller than\ or comparable to\ the thickness of the Fe and Zr sublayer in as!deposited _lms Fig[ 4[ The average grain size of Fe is larger than that of Zr for b0 ðFig[ 2"a#Ł[ This fact can be explained by grain boundary di}usion ð02Ł\ in which Zr crystallites convert to the amorphous state faster than the Fe ones[ Further structural analysis in the present and following sections was undertaken only for Fe:Zr Ml|s with b9[4[ The relative fractions of the bcc!Fe\ hcp!Zr and amorphous FeÐZr phases can be estimated from the integrated intensity of the XRD lines[ A pseudo!Voigt shape for the di}raction peaks was used in the calculations[ Reasonable values of the volume fractions can be obtained since we know that the average atomic composition of the analysed samples is Fe49 Zr49[ It was only possible to estimate the relative increment of the amorphous phase in these samples in the absence of an absolute intensity measurement[  \ 02:15 A  Examples of these _ttings\ as well as the experimental XRD measurements of the 2:5 A  samples\ corrected for the intensity di}racted from the substrate\ are presented in and 14:49 A  −0\ where Q3p sin u:lCo "u is the scattering angle and lCo Fig[ 5 over the range 0[64¾Q¾2[4 A is the X!ray wavelength of Co Ka#[  sample has a broad halo while\ for the 02:15 A  sample\ a single The pattern for the 2:5 A  −0 is observed\ which can be identi_ed as a Zr Bragg peak from the XRD maximum at Q¼1[2 A  sample[ The Fe "009# Bragg peak is observed at Q¼2[0 A  −0 only for spectrum for the 14:49 A the latter sample Fig[ 5[ This picture clearly demonstrates the polycrystalline structure with randomly oriented Fe and Zr grains and the quality of the samples produced by the R[F[ sputtering technique[ Figure 6 shows a cross!section of a high resolution transmission electron microscopy picture of the  sample[ We can see an apparent contrast between the Fe and the Zr layers as well as 049:299 A the good layering of the sample[ 1[0[2[ X!ray wide an`le diffraction results for Fe:Ti Ml|s[ Under equilibrium conditions\ the bulk FeÐTi system exists either as a crystalline intermetallic compound or as an amorphous alloy[ The only known crystalline intermetallic compounds are bcc!FeTi and hcp!Fe1Ti ð11\ 12Ł[ However\ because of the low solubility of Ti in Fe "a few percent#\ only the iron!rich crystalline FeTi alloy with the bcc structure can be formed ð13\ 14Ł[ The solubility range can be extended to

Fig[ 5[ XRD measurements as well as _tting for Fe:Zr samples with Fe layer thickness around the critical thickness of  [ Open circles correspond to the experimental data\ while solid and dashed lines to the _tting[ 19 A

Solid state amorphisation in magnetic multilayers

1926

 sample "courtesy of Dr[ J[ Zweck\ University of Fig[ 6[ Cross!section transmission electron micrograph of a 049:299 A Regensburg\ Germany#[

Fig[ 7[ XRD patterns of FeTi multilayers for b0 "a# and 9[56 "b#[

about 19) by fast quenching ð14Ł[ The amorphous FexTi0−x phase can be produced over a wide composition range "9[14³x³9[79# by vapour quenching or sputtering deposition ð15Ł[ The XRD patterns of Fe:Ti were recorded at the same experimental conditions as for the Fe:Zr Ml|s[ Figure 7"a\b# present di}ractograms of as!deposited Fe:Ti Ml|s with b0 and b9[56\ respectively[

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Fig[ 8[ d!spacing and the size\ D\ of Fe and Ti crystallites for b0 "a# and 9[56 "b#[

These results show that the Fe:Ti system is predominantly composed of closely packed "009# planes of bcc!Fe and "099# hcp!Ti[ The plane spacings "d# calculated from the "009# Fe and "099# Ti peak positions were about 9[1917 nm and 9[1447 nm for Fe and Ti\ respectively[ The d!spacing in the Ti layers is smaller than the bulk value of the "099# hcp!Ti peak\ with the maximum deviation of about 5) for l59 nm ðFig[ 8"a\b#Ł[ For the Fe layers\ the d!spacing is almost independent of l for l×19 nm but\ for l³19 nm\ d decreases by about 0[6) as compared with the bulk value ðFig[ 8"a\b#Ł[ For both _lm compositions\ the size of the Fe crystallites is comparable to the thickness of the Fe sublayer\ while the size of the Ti crystallites is systematically smaller than the corresponding value of the thickness of Ti sublayer over the whole range of l ðFig[ 8"a\b#Ł[ For all l\ the average size of Fe crystallites is larger than that of the Ti ones because of the grain boundary di}usion during which the Ti grains convert more quickly to the amorphous state than the Fe ones[ This e}ect can easily be seen in the XRD patterns] the intensity of the "099# hcp!Ti peak decreases much faster than that of the "009# bcc!Fe peak "Figure 7 and Fig[ 8# when the elemental layer thickness becomes smaller[ 1[0[3[ Modulated structure] X!ray and neutron re~ectometry[ The modulated structure of the series of Fe:Zr multilayer samples with thickness ratio 9[4 and approximate average atomic composition Fe49 Zr49 was analysed using X!ray and neutron re~ectometry techniques[ Multilayer samples with Fe layer thicknesses below the critical thickness\ dc\ were analysed using X!ray re~ectometry\ while the modulation structure of samples with Fe layer thicknesses above dc was examined by neutron re~ectometry[ Figure 7 shows the X!ray re~ectometry "XRR# measurements  and 02:15 A  samples[ of the 2:5 A  sample displays a Fresnel fall!o} which decays as Q−3^ The re~ectivity pro_le of the 2:5 A however\ the interference fringes whose positions correspond to the total thickness of the sample are not visible ð16Ł\ perhaps because the upper surface of the sample is rough[ A weak Bragg  −0 is observed\ from which the layer period can be deduced[ The value of the peak at Q9[603 A bilayer thickness obtained in such a manner is in good agreement with the nominal bilayer thickness[ On the other hand\ up to three orders of Bragg peaks can be resolved in the re~ectivity  sample\ which provides strong evidence for the bilayer spacing not being pro_le of the 02:15 A close to the nominal one[

Solid state amorphisation in magnetic multilayers

1928

Table 0[ Electronic and scattering length densities for bcc!Fe and hcp!Zr[

bcc!Fe hcp!Zr

 −1# Electronic density "×09−5 A

 −1# Scattering length density "×09−5 A

51[02 37[2

7[90 2[96

 sample can be Moreover\ according to the above XRD and XRR measurements\ the 2:5 A regarded as a modulated structure composed of Fe!based and Zr!based amorphous phases[ In  sample can be described as a modulated structure composed of both the same way\ the 02:15 A an amorphous Fe!based and a crystalline Zr!based phase[ In order to discuss these results\ the electronic density and scattering length density of bcc!Fe and hcp!Zr have been calculated "Table 0#[ Furthermore\ to study the nature of the Fe!based phases in the analysed samples\ _tting and simulation of this data "shown in Fig[ 09# were carried out by using the Born approximation[ This model neglects high order re~ections in the multilayer structure ð16Ł[ The _tting parameters were] the layer thicknesses\ the roughness of the interfaces and the critical wave vector "Qc# which is related to the electronic density[ This approximation yields good _ts for experimental data with Q×Qc[ The results obtained from these _tting are shown in Table 1[ The values of the interface roughness agree with those reported earlier for D[C[!sputtered and electron beam evaporated amorphous Fe:Zr multilayer samples ð17Ł[ Conclusions can be drawn from the results presented in Table 1[ "a# The electronic density obtained for the Fe!based layers is lower than that of bcc!Fe shown in Table 0[ It can be inferred from this that the Fe!based layers become FeZr amorphous alloys[

 and 2:15 A  Fe:Zr multilayer samples[ The inset shows the Fig[ 09[ X!ray re~ectivity measurements and _ts for 2:5 A  sample[ weak Bragg peak obtained for the 2:5 A

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T[ STOBIECKI et al[ Table 1[ Fitting results of the XRR measurements shown in Fig[ 7[ Bilayer # thickness "A

 2:5 A

7[8

 02:15 A

30

Layer label

# Layer thickness "A

Electronic density  −1# "×09−5 A

Interface # roughness "A

Fe!based Zr!based Fe!based Zr!based

2 4[8 07[3 11[5

38[63 42[68 36[6 31[0

1[4 2 3[4 3[4

 sample\ as "b# On the other hand\ the increase of the Fe!based layer thickness in the 02:15 A compared with the nominal Fe layer thickness\ can also be interpreted as a result of alloying during sputter deposition[ These conclusions con_rm that the planar growth of the amorphous Fe layer is impossible[ The modulated structure of Fe:Zr multilayer samples with nominal bilayer thicknesses above  were analysed using the neutron re~ectometry technique[ The neutron re~ectometry 49 A  \ 49:099 A  and 099:199 A  samples are shown in Fig[ 00[ measurements for the 14:49 A As in the case of the previous XRR measurements\ no interference fringes are observed\ suggesting rough upper surfaces of all the samples[ Fitting of these data "shown in Fig[ 00# were also made by using the Born approximation[ In the case of neutron re~ectometry data\ the _tting parameters were] the thickness of each sublayer\ the interface roughness and the scattering length density[ The results of the calculations and _tting procedures are shown in Table 2[ The agreement between the bilayer thickness calculated from the neutron re~ectometry measurements and the nominal bilayer thickness is not as good as for smaller bilayer thicknesses\ perhaps because the error is larger for greater deposition periods[ On the other hand\ the scattering length densities obtained from the _tting to the neutron re~ectivity measurements are in good agreement with the values of this parameter for bcc!Fe and hcp!Zr shown in Table 0[ Nevertheless\ the volume fraction of the amorphous phase formed during deposition in the  \ 49:099 A  and 099:199 A  samples can be estimated from the knowledge of roughness 14:49 A calculated from the above neutron re~ectivity _tting results[ Considering the roughness region as an amorphous alloy and using the experimental thicknesses of these samples\ calculated from

 \ 49:099 A  \ 099:199 A  and 049:299 A  Fe:Zr Fig[ 00[ Neutron re~ectometry "dotted line# and _ts "solid line# for 14:49 A multilayers[

Solid state amorphisation in magnetic multilayers

1930

Table 2[ Fitting results of the neutron re~ectivity measurements shown in Fig[ 00[

Sample

Bilayer thickness # "A

 14:49 A

40

 49:099 A

85

 099:199 A

199

Layer label

# Layer thickness "A

Scattering length density  −1# "×09−5 A

Fe Zr Fe Zr Fe Zr

06 23 17 57 69 029

7[25 1[75 7[25 1[75 7[03 2[91

Interface roughness # "A 09 09 6 6 09 04

Fig[ 01[ Volume fraction of amorphous phase in a series of Fe:Zr samples with b9[4[ Open circles represent the data obtained from XRD measurements and full squares correspond to the results obtained from re~ectometry measurements[ The line is shown as a visual guide[

step pro_ling\ the volume fraction of the amorphous phase can be estimated[ Finally\ to compare the calculated results obtained from the XRD and neutron re~ectivity data\ Fig[ 01 shows the volume fraction of the amorphous phase for di}erent thicknesses of the Fe layer calculated from the XRD measurements and the neutron re~ectometry measurements[ Despite the small number of data points\ there is a good agreement between the data obtained by the analysis of both the di}raction and neutron re~ectometry results[ We can conclude that the amount of amorphous phase increases with decreasing bilayer thickness due to the SSAR that occurs during deposition[

1[1[ Electrical conductivity The electrical conductivity\ s\ of as!deposited Fe:Zr and Fe:Ti _lms\ as a function of 0:l\ is shown in Fig[ 02"a\b#[ Our experimental data\ which are similar to those reported by Rodmacq et al[ for Fe:Ti ð18Ł and by Otto et al[ for Fe:Zr ð02Ł\ were interpreted in terms of the model which assumes that the amorphous phase is formed during sputter deposition not only at the plane between the crystalline layers of Fe and Ti but that is also distributed in the grain boundaries due to stronger di}usion in the grain boundaries than in the volume ð02Ł[ The resulting conductivity can be written as

1931

T[ STOBIECKI et al[

Fig[ 02[ "a# Conductivity at RT of Fe:Zr of Fe:Zr vs[ reciprocal of the modulation wavelength[ "b# Conductivity at RT of Fe:Ti of Fe:Zr vs[ reciprocal of the modulation wavelength[ The curves are according to eq[ "0#[

sam "5Z9 ¦FFe "dFe −1Z9 #¦FZr\Ti "dZr\Ti −3Z9 ##\ s l

"0#

where Z90 nm is the initial thickness of the crystalline Fe sublayer which was reduced by the thickness of the amorphous alloy interface sublayer[ Ri ¦0¦fi "0−Ri # Fi  Ri ¦0−fi "0−Ri #

"1#

is the crystallites distribution function\ where grains are in the form of a very thin cylinder "1! dimension model# in the i!th sublayer "iFe\Zr\Ti#\ Riri:ram "ram199 mV=cm\ rTi029 mV=cm\ rZr090 mV=cm\ rFe29 mV=cm# and fi "l#Vi"C#:Vi"tot# "determined from XRD and CEMS measurements# is the relative amount of crystalline phase in crystalline sublayer\ i[ If fi9\ then ssam^ if 0:l:9 and fi0\ then eq[ "0# transforms into a parallel connection of resistances and bsFe ¦sTi [ 0¦b

s

"2#

1[2[ Ma`netoresistivity The anisotropic magnetoresistivity "MR# ratio was measured in the con_guration cor! responding to the magnetic _eld being perpendicular to the current in the plane of the sample[ For Fe:Zr and Fe:Ti\ MR is independent of b and increases with decreasing Fe sublayer thickness "Figure 03# due to di}erent magnetic states of iron[ From CEMS\ three crystalline ferromagnetic phases were found] bcc!Fe "HHf22 T# and two interfacial states of Fe "HHf18 T and HHf01 T#[ The contributions of the interfacial states of Fe increase with decreasing Fe sublayer thickness and reach a maximum at dFe4 nm "see Fig[ 2#[ The MR of Fe:Ti is twice as large as that of Fe:Zr and\ in both systems\ suddenly vanishes "dFe³1 nm# when the ferro:paramagnetic structure transforms to amorphous paramagnetic alloy[

Solid state amorphisation in magnetic multilayers

1932

Fig[ 03[ Magnetoresistivity of Fe:Zr and Fe:Ti vs[ Fe sublayer thickness[

2[ CONCLUSIONS

From the investigations of the interface structure using CEMS\ the relative contributions of crystalline\ amorphous and interfacial phases of Fe were determined[ We have shown\ from wide! angle X!ray di}raction\ X!ray and neutron re~ectometry and from the measurements of the electrical conductivity and the magnetoresistivity of Fe:Zr and Fe:Ti Ml|s\ that the amorphous state is formed during deposition and is distributed in the plane between crystalline sublayers as well as in the grain boundaries[ Fe:Zr with l¾0[2 nm and Fe:Ti with dFe¾1[4 nm may be treated as entirely amorphous[ Acknowled`ements*This work was supported by the State Committee for Scienti_c Research from project T00B 94700 and the statutory research of the Institute of Electronic Materials Technology of Warsaw[ One of us "F[C[# would like to thank Prof[ M[R[J[ Gibbs\ Dr[ H[J[ Blythe and Dr[ N[ Cowlam for fruitful discussions[

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