a study of amorphisation in fe-b multilayers by neutron reflectometry

Materials Science and Engineering A226-228 (1997) 90-94

MATERIALS SCIEMCE &

ENGlWEERlNO

A

A study of amorphisation in Fe-B multilayers by neutron reflectometry

R.J. Cooper a, J. Balogh b, N. Cowlam at*, T. Kern&y b ’ Department of Physics, University of Shef$eld, ShefJield, S3 7RH, UK

b Researcli Institute foi Solid Stale Physics, P.O. Box 49, H-1525 Budapest, H~cngmy

1. Introduction during preparation. Strong concentration gradients were also found in the interface regions. Typical results are shown in Fig. 1, which gives the Mijssbauer spectra Since its discovery in 1983 [I], the amorphisation

reaction in multilayer ‘diffusion couples’ has provided a very controlled and reproducible way of studying the so-called solid-state amorphisation reactions [2]. It is surprising that relatively little attention has been paid to transition metal-metalloid (TM-met) diffusion couples [3,4] (in comparison with TM-TM or TM-noble metal types), since TM*,met,, alloys are arguably the most important metallic glasses.

High-quality Fe-B multilayer samples have recently been produced by ultrahigh vacuum (UHV) vapour deposition in the laboratory of two of the authors (J.B. and T.K.) and have been examined by Mijssbauer spectroscopy, X-ray diffraction and electron mi- croscopy [5,6]. It has been shown that a layer of amorphous alloy is usually found at the Fe --+B (B + Fe) interfaces in the as-prepared samples and this is attributed to the unavoidable heating of the samples

1

550K 24h

L5 I . ‘ ’ 1 ’ 1

-7 -3 -1 I 3 5

Velocity (mm/s)

Fig. 1. Miissbauer spectra of a Fe,,B,, multilayer sample consisting of 22 repeats x (35 A iron + 10 A boron) (nominal), in the as-prepared and annealed states.

Abstract

The solid state ‘amorphisation’ reaction in Fe-B multilayer samples has been studied in a systematic programme. The Mtissbauer spectra of typical Fe-B multilayer samples contain the six-line spectrum of u-Fe, which disappears after a short period (z l-2 h) of annealing at temperatures of about 500-550 K. It is replaced by a paramagnetic component which can be confidently identified with the growth of the amorphous phase. Neutron reflectometry experiments have been made on the CRISP instrument at the ISIS neutron source to follow this amorphisation reaction directly. A Fe,,,B,,, multilayer sample has been given a real time, in-situ, annealing treatment at 537 K. The first-order 3ragg peak in the neutron reflectivity profiles R(Q) versus Q disappeared with increasing time of anneal, as the elemental layers interdiffused. The neutron data were successfully analysed with a structural model developed from the Miissbauer measurements. A second neutron experiment has been made on D17 reflectometer at the ILL, Grenoble, France, using pre-annealed samples of FeeOB,, multilayers. Two orders of Bragg peak have been observed in their neutron reflectivity profiles, the analysis of which confirms that the boron is the principle diffusing species in the reaction. 0 1997 Elsevier Science S.A.

KeytvoTds: Amorphization; Multilayers; Reflectometry

* Corresponding author.

0921-5093/97/$17.00 0 1997 Eisevier Science S.A. All rights reserved.

R.J. Cooper et al, /Materials Science and Engineering A.Z6-228 (1997) 90-94 91

for samples with 22 repeats of 35 A of iron and 28 w of boron. The spectrum for the as-prepared sample contains about 41% of the normal six-line spectrum of an a-Fe component, plus a second contribution from an amorphous phase which is shown by a dotted line in the figure. This phase (and the others which occur in these specimens) can be confidently identified, since the Miissbauer parameters of both crystalline and amorphous Fe3 alloys are well documented in the extensive literature on FeB metallic glasses. When this sample was annealed at 500 K, the u-Fe component disappeared from the Miissbauer spectrum after only 1 h and the characteristic spectrum of a paramagnetic amorphous phase with an estimated boron content of at least 50% emerged after 24 h at 550 K.

These findings suggested that an independent confir- mation of the amorphisation reaction in these Fe-B samples by reflectometry methods [7] would be worth- while. In pulsed neutron reflectometry, the whole of the reflectivity profile R(Q) versus Q is recorded simulta- neously for a single setting of the instrument by using a ‘white’ incident beam and time-of-flight (ToF) analysis. Here, Q is the scattering vector Q = 47~ sin e//z in the usual notation. The reflectivity profile contains information about: (i) the composition of the sample from the position of the critical edge Q,; (ii) the total thickness T from the positions of the interference fringes Q,,, = (2z/T>m; and for a multilayer, (iii) the bilayer spacing d from the positions of the Bragg peaks Q, = (2njd)n. Information about the air-to-film roughness, the interface roughness and the film-to-substrate roughness can be obtained from a detailed analysis of R(Q) versus Q [7].

A Fe,&, multilayer sample consisting of 22 repeats of a nominal 35 A of iron and 10 A of boron, deposited on a silicon substrate was chosen by us [8] for neutron reflectometry measurements on the CRISP instrument [9] at ISIS source Rutherford Appleton Laboratory (RAL), Chilton, UK. Reflectivity profiles were collected sequentially for a period of 25 min each, in a real time in-situ experiment in which the sample was annealed at 537 K in a vacuum furnace. Tkey are shown in Fig. 2 measured out to a value Q z 2 A- I, which corresponds to a reflectivity value of R(Q) z 10 - 4. The vertical sequence of profiles corresponds to the increasing time of anneal t = 1.5, 3.0, 6.0, 16.5, 27.0 and 39.0 x lo3 s at 537 K. They contain: the critical edge $& at the left- hand extremity; interference fringes close to the critical edge and the first-order Bragg peak at Q, = 0.13 A --I. The reduction in intensity and the eventual disappear- ance of the first Bragg peak from the profile, as the constituent layers of the sample interdiffuse, is clearly shown in the figure.

The reflectivity profiles given in Fig. 2 were analysed [8] using the equation which describes the reduction in intensity I(t)/& of the n-th order satellite of a Bragg

l.e+12 -

l.e+il -

l.e+lO -

l.e+9 -

l.e+8 -

B l.e+7 -

'$ l.e+6 - a, 2 l.e+5 - 5: $j l.e+4 -

G l.e+3 - z ,2 l.e+2 - .z g l.e+l -

5 z l.e+O -

l.e-I -

l.e-2 -

l.e-3 -

l.e-4 -

l.e5 -

1.66 , I

0.05 0.10 0.15 0.20

Scattering vectotQ (Angstrom-l)

Fig. 2. The neutron reflectivity profiles R(Q) vs. Q measured on CRISP for the Fe,,B,, sample are shown. The top profile corresponds to room temperature and the vertical sequence to 1.5, 3.0, 6.0, 16.5, 27.0 and 39.0 x lo3 s of anneal at 537 K.

peak in the diffraction pattern of a sample with a periodic modulation of structure with wavelength n! PO12

Here, D is the average value of diffusivity D = xD, + (1 - s)D, for an A,B, -.~ alloy. The graph of ln(l(t)/lo) versus t was found to be linear and a value of the average diffusivity D = 1.2 x 1O-22 m2 s-’ (at 537 K) was obtained from its gradient by using the measured value of bilayer spacing d = 48.7 A IX]. The experimental reflectivity profiles were also compared with ones simulated for model multilayer structures, using com- puter programs based on optical matrix methods [ll]. The solid lines in Fig. 2 show the reflectivity profiles simulated from model structures based on the results of the Mijssbauer measurements. The input parameters for these simulations include the thickness of the individual layers, their scattering length density [7] and their interface roughness, as well as parameters describing the characteristics of the reflectometer. In every case, in- cluding the as-prepared sample, it was necessary to

92 R.J. Cooper et al. /Materials Science and Engineering A226-228 (1997) PO-94

include intermediate layers of amorphous Fe3 alloy at the Fe -+ B (B + Fe) interfaces. These simulations give a concise description of the evolution of the multilayer structure. In particular, since the width w of the amorphous layer, can be expressed as a function of time of anneal t [12],

d + Aw = Bt (2)

where the coefficient A is the diffusivity and the ratio 3/A gives the interface velocity. For short reaction times,

IV M (B/A)t (3)

so ‘a value of D can be obtained using the relation,

dw fD YG=X(l -X) (4)

Here, dwldt is the interface velocity; f is the fractional concentration range over which the amorphous phase exists and Fe,B, _ x is the average composition of the amorphous layer. The value of the average diffusivity in the Fe,,B,, sample obtained from the application of Eqs. (2)-(4) was D= 1.3 x 1O-22 m2 s-l, in excellent agreement with that obtained from the variation of the Bragg peak intensity. These two values are in good agreement with the value of D = 2 x 10 -‘* m2 s - l for the diffusion of boron in a Fe,,Ni,,B,, metallic glass at 537 K [13]. The atomic sizes and the free volume of a Fe,,Ni,,B,, metallic glass may be expected to be similar to those in the amorphous Fe,,B,, phase. This similar- ity implies that boron is the principal moving species in this reaction and the value of diffusivity of iron may be several orders of magnitude smaller.

2. Structural models for the amorphisation reaction

The neutron and MSssbauer measurements which are described above, together provide very satisfactory evidence that a genuine ‘amorphisation’ reaction takes place in these Fe-B multilayer samples. However, a real time, in-situ reflectometry experiment is feasible on the CRISP instrument at present only if a single con- stant setting of the angle of glancing incidence is used throughout. This meant that only the first-order Bragg peak was observed in the neutron reflectivity profiles shown in Fig, 2, on account of the need to record the critical edge Q, for the purpose of normalisation. Be- cause the reflectivity profiles contains just this single Bragg peak, they can only be shown to be consistent with the model of the solid-state reaction developed from the Miissbauer measurements and cannot be used to distinguish between different models of the reaction, in which the parental layers are consumed in different ways.

This point is illustrated schematically in Fig. 3, in which the simulated reflectivity profiles are given for two different models for the consumption of the iron and boron layers in a solid-state reaction in an Fe-B multilayer sample, as a function of a notional time of anneal. The as-prep>red specimen was Shosen to have 20 repeats of 100 A of iron and 60 A of boron. Its simulated reflectivity profile is given at the top of each column of the figure and it contains three orders of Bragg peak over the range of scattering vector Q specified in the simulations. The left-hand column of the figure shows the evolution of the reflectivity profile in the case when the iron layers are consumed more rapidly in the reaction than the boron layers. The reflectivity profile at the bottom of the column, which lacks any Bragg peaks, represents the totally interdiffused sample which consist of a homogeneous amorphous phase. The right-hand column of the figure shows the. converse case, in which the boron layers are consumed more rapidly in the reaction than the iron layers. The reflectivity profiles for the initial and final stages of the reaction are of course the same, but the change in intensity of the second- and third-order Bragg peaks with increasing time of anneal is quite different from that observed in the first simulation. These simulations formed the basis of a second neutron reflectometry experiment on pre-annealed Fe-B multilayer samples which will be discussed in the section below.

l.e-1

lx-2 /

1 .e-3

l.a-4 ;

I.%13 . I

0.00 0.55 0.10 0.00 0.05 0.10

Scattering vector Q (Angstrom”)

Fig. 3. Simulated reflectivity profiles are given for the solid-state reaction in an Fe-B multilayer sample consisting of 20 repeats x (100 f\ iron + 60 A boron). The left-hand column represents the evolution when the iron layers are consumed more rapidly than the boron layers and the right-hand column represents the converse case.

R.J. Cooper ei al. /Materials Science and Engineering A226-225 (1997) 90-94 93

1 .e+O

1 .e-I

1 .e-2

1 .e-3

1 .e-6

1 .e-7

1 .e-8

1 .e-9 0.00 0.05 0.10 0.15 0.20 0.25 0.30

Scattering vector Q (Angstrom-l) Fig. 4. The neutron reflectivity profiles R(Q) vs. Q for the Fe,,B,, samples measured on DI7 are shown: (a) as-prepared sample; (b) sample annealed for 3.3 x lo3 s at 537 K.

3. Sample preparation and experimental method

The Fe6,B4,, multilayer samples measured on the D17 neutron reflectometer at the ILL, Grenoble, France, were from the same series used in the preliminary experiment on CRISP which is described above. They were deposited on to silicon substrates (cooled to liquid nitrogen temperature) from the vapour phase in a UHV chamber (10 -7 Pa) using an evaporation rate of 1 A s-i. The layer thickness was controlled by a quartz oscillator and the samples consisted of 20 repeats of 35 A of iron and 15 A of boron (nominal). The maximum variation of thickness within the plane of the layers was less than 1 A. Reflectometry data will be presented below on two examples, an as-prepared Fe,,B,, multilayer and a second Fe,,B,, sample from the same series which had been pre-annealed for 3.3 x lo3 s at 537 K in a vacuum of 10m5 Pa.

The neutron measurements were made on the D17 reflectometer at the ILL, Grenoble [14]. In contrast to the CRISP (ToF) instrument, D17 has a monochro- matic incident beam and the sample table and multide- tector move in a 8:20 mode in the horizontal plane. The neutron counts are naturally high at Q values close to the critical edge Q, but at higher Q values it is

necessary to increase the data acquisition times to maintain good counting statistics. In a typical reflectometry experiment, a range of scattering vectors 0.004 < Q < 0.28 k 1 was covered in five scans for a total time of approximately 7 h, of which 92% was spent on the region 0.098 < Q < 0.28 A-‘. (This may be compared with 25 min to record the individual reflectivity profiles shown in Fig. 2, over a smaller range of Q.) The first of the five scans was normalised to unit reflectivity from the region before the critical edge Q,, and the subsequent scans combined together to make a continuous reflectivity profile using normalisation constants which take into account the different areas of sample illuminated at different angles. Fig. 4 shows the reflectivity profiles obtained on D17 for the two Fe,,B,, multilayer samples specified above, which are well defined even at small values of reflectivity R(Q) = 10 -‘- 10 - 5, because of the long counting times devoted to the large Q regions. Comparison of the two pronles shows that the annealing has produced a reduction in the intensity of the first-order Bragg peak and has caused the second-order Bragg peak to disappear, as the elemental layers have interdiffused. A comparison between Figs. 3 and 4 suggests that the model involving the more rapid consumption of the boron

94 R.J. Cooper et al. /Materials Scietzce and Enginewiug A226-228 (1997) 90-94

Table 1 4. Discussion and conclusions Parameters used in calculating the reflectivity profiles (-) in Fig. 4

Layer b/Y (low6 A-‘) Thickness (A) Roughness (A) The solid-state ‘amorphisation’ reaction in Fe-B multilayer samples has been studied by Miissbauer spectroscopy and neutron reflectometry. The MBss- bauer measurements provide clear evidence for the presence of an amorphous FeB phase at the interfaces in the as-prepared samples which grows on annealing to give a wholly amorphous sample. The data from an in-situ neutron reflectometry experiment have been analysed satisfactorily with a structural model based on the MGssbauer data. This has yielded values of the diffusivity D which show that the boron is the principal moving species in the reaction. A second neutron reflectometry experiment has confirmed that the boron layers are consumed more rapidly than the iron layers in the solid-state reaction in Fe-B multilayers.

As-prepared Fe6,B,, multilayer sample Fe 8.6 25 2 x-FeB 7.4 8 2 B 6.9 13 2 cc-FeB 1.4 8 2

Annealed Fe6,B,, multilayer sample Fe 8.6 23 a-FeB 7.4 14 B 6.9 3 cc-FeB 7.4 14

12 11 11 11

layers is probably closest to the actual amorphisation reaction in these Fe-B multilayers. To test this, the two profiles in Fig. 4 were again compared with ones simulated using the optical matrix programs [ll]. The solid lines in Fig. 4 show the best fits obtained and the parameters describing the structural models used are given in Table 1. In both cases, it was again necessary to include intermediate layers of amorphous FeB alloy at the Fe+! (B--f Fe) interfaces. Approximately 30% (16 A t 54 A) of the as-prepared Fe,,B,, sample had already transformed to the amorphous phase, in comparison with 20% of the as-prepared Fe,,B,, sample measured on CRISP [8]. In this latter case, the greater number of reflectivity profiles measured have allowed an estimate to be made of the ideal ‘zero time’ elemental bilayer thickness (30’ A of iron and 19 A of boron) which may exist before any mixing takes place at the interfaces. The heating of the samples during preparation which promotes this mixing can also be estimated to be equivalent to approximately 3.3 min of annealing at 537 K. These estimates cannot be made so easily for the Fe,,B,, sample, as there are only two reflectivity profiles to work with. Table 1 shows that about 52% of the annealed Fe,,B,, sample has transformed to the amorphous phase and the thickness of the boron layer (3 A) is less than in the Fe,,B,, sample at a similar stage of its reaction. This low value may be correlated with the artificially high values of interface roughness for the annealed sample. We have tentatively associated this with a final, inhomogeneous consumption of the boron layers within the plane, which the simulation is incapable of imitating satisfactorily [8].

Acknowledgements

The neutron measurements were performed with the help of Dr D. Bucknall at ISIS and Drs Hans Lamer and Bob Cubitt at ILL; R.J.C. acknowledges the re- ceipt an EPSRC studentship, and T.K. and J.B. finan- cial support of OTKA grants T4464 and T4469.

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a study of amorphisation in fe-b multilayers by neutron reflectometry

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