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Nuclear Instruments and Methods in Physics Research A 529 (2004) 90–93

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Neutron reflectivity and interface roughness in Ni/Ti andFeCoV/TiNx supermirrors

M. Senthil Kumara,*, P. B .onib, M. Horisbergerc

aDepartment of Physics, Indian Institute of Technology Bombay, Mumbai 400 076, IndiabPhysik Department E21, Technische Universit .at M .unchen, D-85747 Garching, Germany

cLaboratory for Neutron Scattering, ETH Zurich and Paul Scherrer Institute,CH-5232 Villigen PSI, Switzerland

Abstract

Supermirrors based on various multilayer systems such as Ni/Ti and FeCoV/TiNx are used in neutron optical

devices. In this paper, we report on investigations carried out in order to understand the influence of interface roughness

on the neutron reflectivity. In the case of Ni/Ti mirrors, the measured neutron reflectivity of mirrors having m ¼ 3:65(600 layers) that are prepared under various sputtering conditions shows that the interface roughness of the samples

sputtered only in Ar is large. To be specific, the thinner layers have low roughness when compared with thicker layers

and only about 150 layers on the top are responsible for the considerable reduction in the reflectivity. In the case of

FeCoV/TiNx mirrors, the use of TiGd/TiNx multilayer absorbers show considerable improvement in the performance

of the mirrors.

r 2004 Elsevier B.V. All rights reserved.

PACS: 03.75.B; 75.70; 68.55

Keywords: Supermirror; Multilayer; Interface roughness; Polarizer

1. Introduction

Supermirrors based on various multilayer sys-tems such as Ni/Ti and FeCoV/TiNx are used inneutron optical devices [1]. Our previous investiga-tions of the Ni/Ti multilayers with the objective toenhance their neutron reflection properties haveprovided us with well reflecting and mechanicallystable supermirrors [2]. Reactive sputtering of the

onding author. Tel.: +91-22-2576 7581; fax: +91-

0.

ddress: senthil@phy.iitb.ac.in (M. Senthil Kumar).

- see front matter r 2004 Elsevier B.V. All rights reserve

/j.nima.2004.04.184

Ni layers in an Ar/air mixture has been found toresult in smoother layers as compared to the layerssputtered only in Ar. Although the mechanicalproperties such as stress and embrittlement of thereactively sputtered samples have been understood,the reason for the improved neutron reflectionproperties was not clear. In order to understandthem it is necessary to obtain information on theinterfacial properties mainly the interface rough-ness which determines the reflectivity.

In the case of the FeCoV/TiNx polarizingmirrors, we have obtained high reflectivity andpolarization that are essential requirements for the

d.

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M. Senthil Kumar et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 90–93 91

successful use of the mirrors [1]. However, theperformance of these mirrors could be furtherenhanced by replacing the single TiGd absorberfilm with TiGd/TiNx multilayers [3]. In this article,we present the investigations carried out on the Ni/Ti and FeCoV/TiNx mirrors in order to under-stand the influence of interfaces on their neutronreflectivity.

Fig. 1. The neutron reflectivity data of a m ¼ 3:65 Ni/Ti

mirror (Sample A) sputtered in a partial pressure

pair ¼ 0:11� 10�3 mbar.

2. Experimental

The Ni/Ti and FeCoV/TiNx samples are pre-pared using a Leybold Z600 DC magnetronsputtering system. In the case of the Ni/Ti system,the sputtering of the Ti layers is carried out at anAr pressure pAr ¼ 0:44� 10�3 mbar. The Ni layersare sputtered at a constant partial pressurepAr ¼ 0:73� 10�3 mbar while the partial pressureof air, pair; is varied. The deposition is carried outby starting from the thinnest layers that contributeto the largest Q. Several m ¼ 3:65 mirrors with 600layers each were sputtered onto glass substrateswhere m ¼ 1 corresponds to the angle of totalreflection of Ni. The layer sequence for the mirrorswith gradually varying layer thickness was calcu-lated using the algorithm of Hayter and Mook [4].In the case of FeCoV/TiNx, the sputtering of theFeCoV layers is carried out in Ar at a pressurepAr ¼ 0:73� 10�3 mbar. The TiNx layers aresputtered at partial pressures of Ar and N2 thatare pAr ¼ 0:44� 10�3 mbar and pN2

¼ 0:02�10�3 mbar; respectively. We have prepared severalFeCoV/TiNx supermirrors having m ¼ 2:5: Forthe deposition of the mirrors, we deposited first theTiGd/TiNx bilayers as absorbing layers andthen the mirror layers. The absorber layers usedare [TiNx(3 nm)/TiGd(2.5 nm)]40, [TiNx(3 nm)/TiGd(5 nm)]20, [TiNx(3 nm)/ TiGd(20 nm)]5[TiNx(3 nm)/TiGd(50 nm)]2 and [TiNx(3 nm)/TiGd(100 nm)]1. The thickness of the individualTiGd layers and the number of bilayers areadjusted in such a way that the total TiGdthickness is maintained at 100 nm, in each sample.Both unpolarized and polarized neutron reflectiv-ity measurements on the samples are carried outusing the TOPSI reflectometer at the Swiss

spallation source SINQ in the Paul ScherrerInstitute.

3. Results and discussion

The intensity of the reflected neutrons of twom ¼ 3:65 mirrors composed of 600 layers is shownin Figs. 1 and 2. The Ni layers of sample A aresputtered in an Ar/air mixture whereas sample B issputtered in Ar alone. The figures clearly demon-strate how effective the reactive sputtering is inimproving the reflected intensity that yields anaverage interface roughness of srms=0.9 nm forsample A. From Fig. 2, we note that the reflectedintensity is rather low. It is also important to notefrom the figures that both A and B show the sameintensity near the cut-off Q value indicating thatthe layers immediately on the substrate have thesame small roughness. It appears, for sample B, atfirst sight that only the layers contributing to the Q

range 0.07–0.08 nm�1 have nearly the same rough-ness as that of sample A. But the simulation resultsare quite surprising. In the simulation, the rough-ness of the interfaces is assumed to vary. Fig. 2(b)shows the roughness model assumed for thesimulations and the solid line in Fig. 2(a) showsthe corresponding simulated intensity. The goodagreement between experiment and simulationindicates that the roughness does not increase

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Fig. 2. (a) The neutron reflectivity data of a m ¼ 3:65 Ni/Ti

mirror (Sample B) sputtered only in Ar. (b) Interface roughness

in this mirror as a function of layer number.

M. Senthil Kumar et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 90–9392

with increasing number of layers but it increasesonly after a certain number of layers is reached.The sharp increase in roughness may be due tocumulative roughness (because of increased grainsize) that develops when the individual layerthickness reaches a certain threshold value. Thissharp rise in the roughness is the important featurethat has been deduced from the simulations. Theflat region of roughness shown for the layers470–600 that contribute to the low Q values closeto the total reflection is representative since wecould not fit this part very well. As the roughnessbecomes cumulative only above a certain layerthickness, it may be possible to enhance theneutron reflectivity of the mirrors by reducingthe roughness of the thicker layers. We havealso tried to simulate data by assuming differentroughness models such as linear and expo-nential increase in roughness with increasing layer

number and we have found a strong disagreementbetween experiment and simulation. We have alsoobserved a disagreement when we slightly shiftthe sharp rise shown in Fig. 2(b) to other regionsof the layer number. The disagreement was alsonoticed when the slope of the sharp rise is changedslightly.

We have also investigated the FeCoV/TiNx

polarizing mirrors with a view to improving theirreflectivity. In the case of the FeCoV/TiNx

mirrors, we have obtained high reflectivity andpolarization [1]. In these mirrors, we have usedsingle TiGd films of about 50 nm as absorbinglayers. When we used thicker TiGd films thereflectivity was found to decrease. This is because avery thick absorber film generally results in anincreased roughness of the film that in turnincreases the interface roughness of the mirrorlayers.

In our investigations on the Ni/Ti mirrorsreported above, we have observed that the inter-face roughness of thinner layers is rather smallwhen compared with thicker layers. From ourearlier investigation of depositing thick single filmunderlayers in Ni/Ti mirrors also resulted in adecrease in the reflectivity with increasing thick-ness of the underlayers [5]. Thus, in the presentinvestigation, we have used TiGd/TiNx multilayerabsorbers in which the total TiGd thickness canbe increased much more than that is possible witha single TiGd film [3] while the thickness ofthe individual TiGd layers of the multilayer canbe kept as low as 2.5 nm. We have preparedseveral FeCoV/TiNx supermirrors having TiGd/TiNx bilayers as absorbing layers. The neutronreflectivity data of a m ¼ 2:5 polarizing mirrorwith [TiNx(3 nm)/TiGd(2.5 nm)]40 absorber layersis shown in Fig. 3(a). The reflectivity data ofa sample with [TiNx(3 nm)/TiGd(100 nm)]1 isshown in Fig. 3(b) for comparison. It can be seenfrom the figure that the up spin reflectivity of themirror shown in Fig. 3(a) is larger than that isshown in Fig. 3(b). The reflectivity data of themirrors with various other multilayer absorbersreported in this article are almost similar and theirreflectivities at large Q are comparable with thesamples having TiGd(50 nm) single films reportedby us earlier [1].

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Fig. 3. (a) Neutron reflectivity data of a m ¼ 2:5 FeCoV/TiNx

mirror deposited on [TiNx(3 nm)/TiGd(2.5 nm)]40 multilayer

absorber. The solid and open circles represent the up and down

spin reflectivities. The open triangles represent the polarization.

(b) Neutron reflectivity data of a m ¼ 2:5 FeCoV/TiNx mirror

deposited on [TiNx(3 nm)/TiGd(100 nm)]1 absorber.

M. Senthil Kumar et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 90–93 93

4. Conclusion

We have shown that special care has to be takento reduce the thickness of the thicker layerscomprising the supermirror structure because theyare prone to become very rough due to the largegrain size. In Ni/Ti supermirrors the grain size canbe reduced by reactive sputtering, while in theFeCoV/TiNx multilayers the roughness that isinduced by the absorbing layer can be reduced byan absorbing multilayer consisting of TiGd/TiNx.In addition, the polarization at low Q is enhanced.

Acknowledgements

This work was partially performed at the SINQ,Paul Scherrer Institute, Villigen, Switzerland.

References

[1] P. B .oni, D. Clemens, M. Senthil Kumar, C. Pappas, Physica

B 267–268 (1999) 320.

[2] M. Senthil Kumar, P. B .oni, D. Clemens, J. Appl. Phys. 84

(1998) 6940.

[3] M. Senthil Kumar, P. B .oni, 1999 (private communication).

[4] J.B. Hayter, H.A. Mook, J. Appl. Crystallogr. 22 (1989) 35.

[5] M. Senthil Kumar, P. B .oni, D. Clemens, M. Horisberger,

Physica B 276–278 (2000) 142.