multilayers measured by neutron reflectometry

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<ul><li><p>Iron self-diffusion in amorphous FeZr/ 57FeZr multilayers measured by neutron reflectometry</p><p>Mukul Gupta,1,* Ajay Gupta,2 J. Stahn,1 M. Horisberger,1 T. Gutberlet,1 and P. Allenspach11Laboratory for Neutron Scattering, ETHZ and PSI, Paul Scherrer Institute, Villigen CH-5232, Switzerland</p><p>2Inter-University Consortium for DAE Facilities, Khandwa Road, Indore IN-452017, India(Received 19 May 2004; revised manuscript received 7 September 2004; published 18 November 2004)</p><p>Self-diffusion of iron innaturalFe67Zr33/57Fe67Zr33 multilayers has been investigated by neutron reflectometry.</p><p>The as-deposited multilayer is amorphous in nature. It remains amorphous up to a temperature of 573 K andthereafter nanocrystallizes with an average grain size of 6 nm. The self-diffusion in the multilayers has beenmeasured after isothermal vacuum annealing below the nanocrystallization temperature by monitoring thedecay of the intensity of the first order Bragg peak, arising due to the isotopic periodicity. It has been found thatthe diffusivity at different temperatures follows an Arrhenius-type behavior with the preexponential factorD0=5310</p><p>181 m2 s1 and the activation energyE=0.380.05 eV, respectively. These values ofE and D0follow the well-knownED0 correlation and on the basis of this correlation it is suggested that diffusionmechanism in the present case is not highly collective but involves a rather small group of atoms.</p><p>DOI: 10.1103/PhysRevB.70.184206 PACS number(s): 66.30.Fq</p><p>I. INTRODUCTION</p><p>Amorphous metallic alloys also known as metallicglasses, are important from the point of their applications inindustry for computers, information technology, recordingmedia, etc.1 They differ from their crystalline counterparts bya nearly random arrangement of atoms and are an example ofthe paradigm of dense random packing which leads to a greatfundamental interest in these alloys.2 Generally, these alloysare metastable, and at elevated temperatures various atomicrearrangements take place above their crystallization tem-peratureTx, the atomic mobility increases drastically causinga rapid crystallization, and even belowTx the amorphousmatrix is internally unstable and transforms continuously toamorphous states of lower free energy. Transformation of astructure towards lower free energy is well-known as struc-tural relaxation of amorphous structure and leads to changesin most of the physical properties. Atomic transport proper-ties, such as diffusion are among the most affected duringstructural relaxation process and may change by several or-ders of magnitude.3 This leads to a strong need for an under-standing of the diffusion behavior in amorphous alloys. Be-cause of their limited thermal stability it was not possible tostudy long range atomic transport in these alloys.</p><p>Various attempts have been made to increase the thermalstability of these alloys using a multicomponent matrix inbulk metallic glasses and melt spun ribbons. Self-diffusionmeasurements have been performed in a series of alloys, e.g.,ZrCuNiTiBe,4 CoFeNbB,5 ZrCuNiAl6 (and Ref. 2 and refer-ences therein). Some studies have also been done in binaryamorphous alloys, e.g., NiZr,7 CoZr,8 TiZr,9 etc. at tempera-tures.470 K. Most of these studies have been done usingprofiling and sectioning techniques such as radiotracer tech-niques, secondary ion mass spectroscopy(SIMS), Rutherfordbackscattering(RBS), Auger electron spectroscopy(AES),etc. Since the depth resolution available with these tech-niques is of the order of a few nm, diffusion length less thanthat could not be probed. Generally, the crystallization tem-perature in amorphous alloys is around 700 K which gives</p><p>an upper limit for the diffusion annealing. Typical diffusionlengths at low temperaturess,400 Kd in a reasonable timewould be much shorter than the detection limit of cross-sectioning or depth-profiling techniques. In addition, forstudying diffusion in amorphous ultrathin filmssthickness, few nmd a technique with greater sensitivity is required asthe crystallization temperature in such layers is significantlylower as compared to bulk metallic glasses or melt-spun rib-bons.</p><p>Measurement of interdiffusion in compositionally modu-lated multilayer structures using x-ray scattering is one tech-nique to study diffusion lengths much shorter than the detec-tion limit of sectioning and profiling techniques.1013Severalattempts have been made to study interdiffusion in chemi-cally inhomogeneous multilayers. In a study by Mizoguchiet al.,14 interdiffusion and structural relaxation have beenstudied in 3d transition metal(TM)/Zr multilayers in thecomposition range of TM67Zr33 using x-ray diffraction(XRD) technique. Amorphization in these multilayers hasbeen achieved with a solid state reaction and diffusion mea-surements were performed at temperatures as low as 393 K.In another study Wanget al.15 have studied interdiffusion innanometer-scale multilayers using low-angle x-ray diffrac-tion in a series of polycrystalline binary alloys. While x-raydiffraction/reflection techniques can be successfully used forinterdiffusion studies in chemical-composition modulatedmultilayers, they cannot be used for studying self-diffusionin a chemically homogeneous structure because of electronicinteraction with x-rays.</p><p>Neutron reflectivity is a nondestructive technique, whichcan be used for studying self-diffusion in a chemically ho-mogeneous multilayer with a resolution as small as 0.1 nm,by taking advantage of isotopic labeling. Greeret al.10,16,17</p><p>have demonstrated the application of neutron reflectivity,measuring self-diffusion in amorphous NiZr multilayers. Inanother study by Bakeret al.18 self-diffusion of amorphous11B on 10B in isotopically enriched thin films of11B/10B onSi was investigated. It is rather surprising that since thenpractically no studies on self-diffusion measurements in me-</p><p>PHYSICAL REVIEW B 70, 184206(2004)</p><p>1098-0121/2004/70(18)/184206(7)/$22.50 2004 The American Physical Society70 184206-1</p></li><li><p>tallic multilayers using the neutron reflectivity techniquewere performed in spite of its unique potential. Another tech-nique through which scattering contrast between two differ-ent isotopes of an element can be obtained is nuclear reso-nance reflectivity (NRR) of synchrotron radiation.19,20</p><p>Possibility of using this technique for self-diffusion measure-ments of a Mssbauer active isotope was pointed out.20 In arecent study this technique has been used to study self-diffusion of 57Fe in some amorphous and nanocrystallinealloys.21</p><p>In the present study we have measured self-diffusion ofiron in an isotopic multilayer of FeZr/57FeZr in the amor-phous state. The self-diffusion measurements have been car-ried out measuring the neutron reflectivity of the multilayerafter isothermal vacuum annealing below the crystallizationtemperature. The height of the Bragg peak arising due toisotopic periodicity decays with annealing temperature andtime and depicts the self-diffusion and the activation energyfor a chemically homogeneous structure. In addition, a de-tailed fitting of the neutron reflectivity profile measured atroom temperature yields interdiffusion at ambient conditions.The results of the obtained diffusion behavior are presentedand discussed in this article.</p><p>II. EXPERIMENT</p><p>Amorphous FeZr isotopic multilayers have been preparedusing a magnetron sputtering system. Natural Fe and57Feenriched targets were sputtered alternatively to deposit themultilayer structure. The57Fe target was prepared by pastinga 57Fe foil (57Fe enrichment,95%) onto a natural iron tar-get. Small circular pieces of Zr(12 pieces) were pasted onthe natural Fe as well as57Fe in an area ratio of Fe to Zrabout 1:0.3. The multilayer with a nominal layer structureglass ssubstrated / fnaturalFeZrs9 nmd / 57FeZrs5 nmdg20 wasprepared. The composition of the film has been measuredusing x-ray photoelectron spectroscopy(XPS). The structuralcharacterizations of the film have been carried out usingx-ray reflectometry(XRR) and XRD techniques using a stan-dard x-ray diffractometer and CuKa radiation. Since theoverall thickness of the film is relatively small, the XRDpattern of the film has been measured in the asymmetricBraggBrentano geometry at grazing incidence so that thebackground from the glass substrate can be minimized(keeping the incident angle just above the critical edge).Prior to diffusion measurements the crystallization behaviorof the amorphous film has been studied using XRD afterannealing the film isochronally in a vacuum furnace with abase vacuum of the order of 106 mbar.</p><p>The self-diffusion measurements have been carried outusing neutron reflectivity after annealing the samples isother-mally at four different temperatures. The neutron reflectivitymeasurements have been carried out in the time-of-flightmode on the AMOR reflectometer and in theu-2u mode onthe MORPHEUS reflectometer both at SINQ/PSI.22 With anincoming wavelength band of 0.20.9 nm the reflectivitypattern was measured using two different angular settings inthe time of flight mode and on MORPHEUS using a mono-chromatic neutron beam with a wavelength of 0.474 nm.</p><p>III. RESULTS AND DISCUSSION</p><p>The composition of the cosputtered film was determinedusing XPS depth profiling. The average composition of thefilm was found to be Fe673Zr333 consistent with the arearatio of the targets used for sputtering. Figure 1 shows thex-ray diffraction pattern of the multilayer in the as-depositedstate and after isochronal annealing in the temperature rangeof 373673 K in a step of 100 K. The GIXRD pattern of theas-deposited film shows a broad hump centered around 2u=44 which is typical for the iron based amorphous alloys.1</p><p>The average interatomic distance can be estimated using therelationa=1.23l /2 sinu, whereu is taken to be the angle atthe center of the amorphous hump, and the factor 1.23 is ageometric factor which rationalizes the nearest neighbor dis-tance with the spacing between pseudo-close packedplanes.23 The calculation gives an average interatomic dis-tance in the present case equal to 0.2550.001 nm for theas-deposited sample. The inset in Fig. 1 shows the variationin the interatomic distance as a function of annealing tem-perature. As can be seen the interatomic distance shows adecrease with an increase of annealing temperature. Such adecrease indicates densification in the layer and is a directconsequence of structural relaxation which occurs due to an-nihilation of free volume during annealing. After annealingat 673 K, the amorphous hump converts into a relativelysharp peak indicating crystallization of the amorphous film.A detailed investigation of the peak shows that it is not pos-sible to fit the peak using a single function, instead the bestfit has been obtained using two Gaussian line shapesonecorresponding to the amorphous phase and the other to thecrystalline bcc-Fe phase. This indicates that the film has notbeen crystallized completely and represents a mixture ofamorphous and partially crystalline states. The width of thecrystalline peak has been used to calculate the average grain</p><p>FIG. 1. X-ray diffraction pattern of the glass(substrate)/fFeZr s9 nmd / 57FeZr s5 nmdg20 isotopic multilayer in the as-deposited state and after annealing at various temperatures as indi-cated in the figure. The measurements were carried out in the graz-ing incidence geometry using CuKa x-rays. The inset in the figureshows the change in interatomic distance as a function of annealingtemperature. The point corresponding to annealing at 673 K is afternanocrystallization.</p><p>GUPTA et al. PHYSICAL REVIEW B 70, 184206(2004)</p><p>184206-2</p></li><li><p>size using the Scherrer formula yielding a value of about6 nm. The area ratio of the two components can be used toestimate the amount of crystallization which indicates thatabout 50% of the film has converted into the nanocrystallinephase.</p><p>Neutron reflectivity, in combination with x-ray reflectivitycan be used to extract more information about the crystalli-zation process of the amorphous multilayer. Figure 2 showsthe neutron and the x-ray reflectivity pattern of the as-deposited multilayer. While a Bragg peak arising due to iso-topic periodicity can be seen clearly in the neutron reflectiv-ity pattern, no such structure can be seen in the x-rayreflectivity pattern. This shows that there is no chemical con-trast betweennaturalFeZr and57FeZr, as expected. In fact, thisis a prerequisite for studying self-diffusion in a chemicallyhomogeneous multilayer as any chemical contrast would af-fect the diffusion process significantly and the measured dif-fusivity would no longer be self-diffusion but mediated bychemical inhomogeneity. The fitting of the neutron reflectiv-ity data of the multilayer yields the following structure:Glass ssubstrated / fnaturalFeZrs9.1 nmd / 57FeZrs5.3 nmdg20,which is close to the designed nominal structure. The neutronreflectivity pattern was fitted using a computer program</p><p>based on Parratts formalism24 and it was found that the pat-tern could not be fitted assuming sharp interfaces; instead athin interlayer of thicknesss0.80.4d nm with the mean scat-tering length density of the two layers had to be introducedas interdiffused layer. This means that at room temperaturethere is some amount of interdiffusion in the multilayer. Thex-ray reflectivity pattern was fitted assuming a single layerwith a thin layer of scattering length density 50% of bulklayer, on the top of the film because of a possible oxidationof the film when exposed to the atmosphere.</p><p>Figure 3 shows the x-ray reflectivity pattern of themultilayer after annealing at 523, 573, and 673 K. It wasobserved that up to an annealing temperature of 523 K, thex-ray reflectivity pattern of the isotopic multilayer does notchange significantly; only the thickness corresponding to theoxide layer is found to increase with annealing temperature.Whereas, after annealing at 573 K, a small Bragg peak ap-pears indicating an evolution of chemical contrast betweenthe natural and57Fe layers. Further annealing at 673 Ksharpens this peak. As it is evident from the x-ray diffractionpattern of the multilayer that nanocrystalline Fe precipitatesout from the amorphous phase after annealing at 673 K andthe volume fraction of this nanocrystalline phase is about50%. This information was taken as an input parameter whilefitting the x-ray reflectivity pattern of the multilayer afterannealing at 673 K. Two thin layers of pure iron(density9095% of bulk iron) were introduced on both sides of natu-ral and57Fe layers with total thickness of this interlayer ap-proximately equal to half of the bilayer thickness and thissimple model, as shown in Fig. 4, gives a reasonably goodfitting of the x-ray reflectivity pattern. Figure 5 shows theneutron reflectivity pattern of the multilayer after annealingat 573 and 673 K and for comparison in the as-depositedstate (measured again inu-2u mode). Exactly the samemodel, as discussed above was applied for fitting the 673 Kannealed neutron reflectivit...</p></li></ul>

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