Physica B 248 (1998) 349-354 ADAM, the new reflectometer at the ILL A. Schreyer *, R. Siebrecht , U. Englisch , U. Pietsch , H. Zabel Experimentalphysik (Festko(rperphysik), Ruhr-Universita(t Bochum, Postfach 102148, D-44780 Bochum, Germany Institut fu(r Festko(rperphysik, Universita(t Potsdam, D-14415 Potsdam, Germany Abstract The new reflectometer ADAM at the ILL is described and some of the results obtained in the first year of operation are presented. These include a reflectivity of a Si wafer over eight orders of magnitude, a measurement of a thick [ Fe/ Fe] isotope superlattice, and a polarised reflectivity of a Co/Cu multilayer. The instrument is now available to outside users. 1998 Elsevier Science B.V. All rights reserved. Keywords: Neutron reflectometry; Polarisation analysis; Instrumentation Throughout the last decade layered materials of February 1996. In May 1996 ADAM saw its first unprecedented structural quality have become neutrons. Since then, the machine has been system- available. Many fascinating new properties have atically tested, first with an unpolarised beam, and been found in thin-film systems including polymers, since September 1996 with a polarised beam and biological membranes, and magnetic multilayers polarisation analysis. and superlattices. Neutron reflectometry (NR) was ADAM is situated on ILL's neutron guide H 53 developed to make use of the well-known advant- which is fed by a liquid deuterium cold source. In ages of the neutron as a probe for the study of such Fig. 1 a schematic sketch of the instrument is thin-film systems [1]. Currently NR is one of the shown. Making use of a focusing HOPG mono- fastest growing neutron techniques. chromator a fixed wavelength of The new reflectometer ADAM (Advanced Dif- "4.4 A> is se- lected. A Be filter removes any higher harmonics fractometer for the Analysis of Materials) at the ILL in Grenoble/France is one of the latest addi- /n, n"2, 3, . . . which are also reflected by the monochromator. Optionally, the Be filter can be tions to the fleet. After a design and construction removed, allowing the use of phase of about one year at the Ruhr-Universitaet /2 [2] to extend the scattering vector range. Two motorised collimation Bochum, the instrument was moved to the ILL in slits, positioned 2 m apart, provide the high col- limation required for reflectometry experiments. Being a fixed wavelength instrument, specular re- flectivity measurements are carried out by standard * Corresponding author. Tel.: #49 234 7003 625; fax: #49 /2 scans, whereas off-specular measurements can 234 7094 173; e-mail: andreas.schreyer@ruhr-uni-bochum.de. be performed, e.g. by -rocks. The scattering plane 0921-4526/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 2 6 3 - 4 350 A. Schreyer et al. / Physica B 248 (1998) 349-354 Fig. 1. Schematic side view of the ADAM reflectometer at the ILL in Grenoble/France. is horizontal, i.e. the samples are mounted vertically reduce the measuring times by a factor of two. on a motorised heavy load goniometer head which A two-dimensional position-sensitive detector will provides all motions for the alignment of the become available in 1998 significantly reducing sample. counting times for off-specular measurements. As detailed e.g. in Refs. [3,4] polarised neutron As sample environment a customised elec- reflectometry (PNR) is ideally suited for the study tromagnet and cryofurnace combination is avail- of magnetic thin-film systems. For this purpose, able. For measurements on large samples a sample optional supermirror polarisers and flippers in holder with an integrated heater up to 150°C can be front of and behind the sample are available. The provided. Instrument and sample environment are use of transmission polarisers [5] obtained from controlled by the commercial SPEC diffractometer the HMI in Berlin/Germany avoids the need to control software running under LINUX on a realign the diffractometer in the polarised mode. Pentium PC. Furthermore, it will be possible to detect two Although primarily designed for small-angle re- polarisation cross-sections at the same time by si- flectometry, ADAM can access 2 angles up to multaneously measuring the transmitted and the about 140°. Thus, scattering vectors Q up to reflected beam of the analysing supermirror behind 5.4 A>\ can be reached when using the sample. For this purpose two He pencil de- /2 (i.e. with- out the Be filter). This allows the investigation of tectors are mounted in the well-shielded detector ordering phenomena on atomic scales (via Bragg housing. One of these detectors is motorised to scattering) at high Q and on the nanometer scale allow easy alignment with the beam which is reflec- (via reflectometry measurements) at small Q in one ted by the analysing supermirror. The ability to experiment and within the same sample environ- measure two cross-sections at the same time can ment. For example, such information has been A. Schreyer et al. / Physica B 248 (1998) 349-354 351 of crucial importance for the understanding of the In Fig. 3 data taken from an epitaxial Fe/ Fe correlation between the Cr antiferromagnetic order superlattice of nominal composition Pd 50 A>/ Fe and the coupling between the Fe layers in Fe/Cr 90 A>[ Fe 10 A>/ Fe 90 A>] superlattices [6]. /MgO (substrate size 4 cm ) grown by sputtering are shown. The sample To demonstrate the instrument's dynamic range had been obtained from the University of Up- we show a reflectivity measurement of a Si wafer of psala/Sweden. Ranging over nearly six orders of 5 diameter in Fig. 2. To minimise background, it magnitude, a Q range up to 0.4 A>\ and six super- was essential to put the sample into a vacuum lattice peaks were obtained. Although the expected chamber eliminating air scattering. The remaining nominal overall thickness D off-specular intensity was measured independently of the sample is quite large, Kiessig fringes rapidly oscillating at and subtracted. The resulting specular reflectivity Q essentially corresponds to a Fresnel reflectivity "2 /D are easily resolved up to 0.11 A>\ (see insets). The overall thickness D with a broad bump superimposed which can be "1653 A> determined from the Kiessig fringes agrees well ascribed to the native oxide of the Si. Clearly, with the nominal value. Good agreement is also reflectivities up to more than 0.5 A>\ and down found between the superlattice period deter- into the 10\ range can already be achieved with mined from the separation of the superlattice reflec- ADAM. tions and the nominal superlattice period. In the first year of operation at the ILL measure- However, upon close inspection of the higher-order ments from magnetic, isotope, and polymer multi- reflections a second component is observed, indic- layers have already been performed. Whereas the ating a partial deviation from the nominal superla- latter data are discussed elsewhere in these pro- ttice period. A more detailed and quantitative ceedings [9] we will present examples of the other account on this work will be given elsewhere. For experiments to highlight the capabilities of the in- the present purpose, we would like to point out that strument. such a structural analysis can only be carried out Fig. 2. Reflectivity of a Si wafer measured over nearly eight orders of magnitude with ADAM. For details see text. 352 A. Schreyer et al. / Physica B 248 (1998) 349-354 Fig. 3. Reflectivity of an epitaxially sputtered superlattice of nominal composition Pd 50 A>/ Fe 90 A>[ Fe 10 A>/ Fe 90 A>] /MgO (substrate size 4 cm ). In the insets blowups of the measured reflectivity are shown making the very short period Kiessig fringes visible. The line is a guide to the eye. with neutrons, since no scattering contrast between figure. Thus, by reorienting the sample in a small isotopes for the same element exists for X-rays. guide field the sensitivity of PNR to the orientation Furthermore, this example demonstrates the high of the in-plane magnetisation can be studied. This Q-resolution and intensity of ADAM, easily allow- orientational sensitivity is depicted schematically in ing to resolve Kiessig fringes from thick samples. the right part of Fig. 4. The axis marked NSF Clearly, much thicker samples could be investi- (non-spin flip) is parallel to the polarisation direc- gated which might be interesting, e.g., for industrial tion P of the incident neutrons. It indicates that the coatings. spin state of the incident neutrons is not flipped by In our final example we want to demonstrate a magnetisation oriented parallel to this axis. The ADAMs capability for polarised measurements. In axis marked SF (spin flip), on the other hand, indi- Fig. 4 we show two reflectivities measured with cates spin flip of the neutrons for any magnetisation polarisation analysis on a sputtered [Cu 15 A>/Co components along this axis. Thus, by separating 34 A>] /Al O multilayer in the two different sche- NSF and SF scattering via polarisation analysis, matically depicted orientations. The sample has PNR is sensitive to the orientation of any in-plane ferromagnetically aligned in-plane magnetisations magnetisation. A more detailed account of the un- in the Co layers. Furthermore, it exhibits the useful derlying theory can be found e.g. in Refs. [3,4]. The feature of a strong uniaxial anisotropy along an orientational sensitivity of PNR has also lead to the easy axis marked `EA' in the right-hand side of the term "vector magnetometry" for the technique. A. Schreyer et al. / Physica B 248 (1998) 349-354 353 Fig. 4. Reflectivity of a [Cu 15 A>/Co 34 A>] /Al O multilayer measured on ADAM using the polarisation analysis option. On the right-hand side the scattering geometry is depicted with the scattering vector Q perpendicular to the film plane and the NSF (non-spin flip) and SF (spin flip) axes, as described in the text. (a) and (b) show the measured reflectivities for the two sample orientations shown on the right. A recent study of collinear and non-collinear coup- other hand, the sample is oriented to cause max- ling in exchange-coupled superlattices with this imum SF scattering. Consistent with theory, strong technique can be found in Refs. [7,8]. degenerate R>\ and R\> reflectivities are found. The data in Fig. 4a were taken with the sample Furthermore, the splitting of the NSF reflectivities oriented with the magnetisation parallel to the has completely vanished since the magnetisation NSF axis. Consequently, no SF scattering is ob- component along the NSF axis is zero. The ob- served. The splitting of the two NSF reflectivities served NSF reflectivity now results from purely R>> and R\\ is a measure for the magnitude of nuclear scattering. The data exhibit a superlattice the magnetisation of the sample. Here, the super- peak due to the periodic structure of the sample scripts indicate the polarisation state in front of and and five Kiessig fringes due to the sample's finite behind the sample, respectively. In Fig. 4b, on the thickness. Only in R\\ in Fig. 4a hardly any 354 A. Schreyer et al. / Physica B 248 (1998) 349-354 structure is observed in the data. This is due to thank our technical staff F. Adams and J. Pod- a very similar scattering length density of the Co, schwadek as well as J. Meermann and his work- the Cu and the substrate for this case. A more shop crew in Bochum, whose expertise and thorough discussion, including quantitative fits to efficiency was essential for ADAM's short design, equivalent data from the same sample taken on construction, and set-up phase. Last, but not the another reflectometer can be found in Ref. [3]. least we thank B. Hoervarson and P. Isberg of the For the present purpose, we want to demonstrate University of Uppsala for providing the Fe/ Fe ADAMs capability to perform such measurements superlattice. This project is funded by the German with high quality. Notably, at least five orders of BMBF (03-ZA4BC2-3). magnitude in reflectivity are accessible although the sample has a surface of only 2 cm . The mea- sured splitting of nearly three orders of magnitude References between the two NSF reflectivities at the superla- [1] For an overview, see e.g.: Proc. 4th Int. Conf. on Surface ttice peak around 0.13 A>\ indicates a good effi- X-ray and Neutron Scattering, Lake Geneva, 1995, Physica ciency of the polarisation elements. Flipping ratios B 221 (1996) as well as older proceedings of that conference up to 54 are achieved, i.e. the polarisation efficiency series: Physica B 198 (1994); Surface X-Ray and Neutron is larger than 98%. Scattering, H. Zabel, I.K. Robinson (Eds.), Springer, Berlin, In conclusion, we present a new high flux reflec- 1992. [2] Wavelengths corresponding to harmonics higher than n"2 tometer to an international user community. Two are not transmitted to the monochromator by the guide. modes of access are in effect. First, 30% of the [3] A. Schreyer, J. Phys. Soc. Japan 65 (Suppl.) A (1996) 113, ADAM beam time is made available via the stan- also see http://www.ep4.ruhr-uni-bochum.de/people/as. dard ILL proposal scheme. Second, German users [4] H. Zabel, Physica B 198 (1994) 156. can obtain beam time via the `Verbundforschung'. [5] T. Krist, C. Papas, Th. Keller, F. Mezei, Physica B 213&214 (1995) 939. For details see http://www.ep4.ruhr-uni-bochum. [6] A. Schreyer, C.F. Majkrzak, Th. Zeidler, T. Schmitte, de/adam.html. P. Bo¨deker, K. Theis-Bro¨hl, A. Abromeit, J. A. Dura, T. Watanabe, Phys. Rev. Lett. 79 (1997) 4914. [7] A. Schreyer, J.F. Ankner, Th. Zeidler, H. Zabel, C.F. Majkrzak, M. Scha¨fer, P. Gru¨nberg, Europhys. Lett. 32 Acknowledgement (1995) 595. [8] A. Schreyer, J.F. Ankner, Th. Zeidler, H. Zabel, M. Scha¨fer, We gratefully acknowledge the help of A. Ma- J.A. Wolf, P. Gru¨nberg, C.F. Majkrzak, Phys. Rev. B 52 (1995) 16066. gerl, who provided many expert tips in mono- [9] U. Englisch, F. Penacorada, I. Samolenko, U. Pietsch, chromator design and alignment. Furthermore, we These Proceedings, Physica B 248 (1998).