V–Ni multilayered monochromators and supermirrors for cold neutrons

PERGAMON

Solid State Communications 111 (1999) 23–28

V–Ni multilayered monochromators and supermirrors for cold neutrons M. Maaza a,*, J.P. Chauvineau b, B. Pardo b, A. Raynal b, A. Menelle c, F. Bridou b, J. Corno b b

a Physics Department, University of the Witwatersrand, Wits 2050, Johannesburg, South Africa Institut d’Optique The´orique and Applique´e, Universite´ Paris-Sud, Bat. 503, 91403 Orsay Cedex, France c Laboratoire Leon Brillouin, Commissariat a l’Energie Atomique, 91191 Gif-sur-Yvette, France

Received 12 December 1998; accepted 15 March 1999 by R.T. Phillips

Abstract We present experimental neutron reflectivity results showing that it is possible to use V–Ni multilayered systems as cold neutron monochromators and supermirrors. Compared to the Ni–Ti optics devices, the V–Ni stacks present sharp interfaces characterised by a low interfacial roughness and very small interfacial diffusion. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Nanostructures; C. X-ray scattering; D. Optical properties; E. Neutron scattering

Neutrons are known as very effective tools for advanced condensed matter research, in magnetism, structural chemistry, and in biology. At present, thermal and cold neutrons intensity in research is limited due to the loss during their transport, monochromation and or their polarisation. Therefore, any possibility should be considered to increase the terminal neutron flux at the level of the spectrometers. Moreover, there are numerous programs for the construction of the next generation of pulsed and spallation sources brighter than the actual sources. From these one can quote: (i) ANS—the Advanced Neutron Source at Oak Ridge National Laboratory ANS [1]; (ii) KENS II-JHP—Japan Hadron Project, in Japan [2]; and (iii) ESS—the European Spallation Source in Europe [3]. Each of these projects aims for very high neutron * Corresponding author. Fax: 1 27-11-339-8262. E-mail address: [email protected] (M. Maaza)

fluxes; 5–10 times those at ILL for ANS while neutronic performances anticipated for ESS are about 30 times brighter than the ISIS-RAL, presently the world’s most powerful source of this type. The enhanced temporal brightness and resolution of these different neutron sources would allow to be made both a major impact on established fields and substantial contributions to new area of research. Thus, new challenges to make new neutron optics devices, presenting excellent optical performances coupled with an appreciable thermal stability and a long life-time, are encouraged. Recently, capillary optics are used to focus the neutron beams [4,5] and thin film Fabry–Perot resonators under total reflection are used to monochromate and to polarise cold ˚ [6–8]. Likewise, many efforts neutrons with l $ 8 A are made in the fabrication of thin films multilayered systems based on new couple of materials different from the traditional compounds. Thus, Ti–V periodic

0038-1098/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00133-7

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M. Maaza et al. / Solid State Communications 111 (1999) 23–28

multilayers are tested positively as beam-splitter in a neutron interferometry station [9] and TiX–FeCoV as polarising supermirrors [10]. In this way, V–Ni multilayered periodic and aperiodic systems were made. This brief letter deals with the experimental neutron reflectivity results obtained on the V–Ni multilayered monochromators and V–Ni supermirrors. The choice of these materials, that was pointed out indirectly in the past by Schaerpf [11], is justified by the following reasons: (i) in general, the interfaces of vanadium–metal multilayers present generally sharp profiles with a low interfacial diffusion and a low interfacial roughness as in the case of Mo–V, Eu–V and Fe–V systems [12–14]; (ii) and reasonably stable from thermal point of view [15]; (iii) the neutron refractive index contrast is high and almost close to that of the Ni–Ti couple; and (iv) as for Ti, V presents a high affinity with hydrogen which allows to improve the refractive index contrast [16] and thus the neutron reflectivity. One can note that the V–Ni multilayered periodic structures were studied ten years ago for the interaction of superconductivity and itinerant-electron magnetism by Homma et al. [17]. V–Ni multilayered monochromators and supermirrors of the various bi-layer number and periods were prepared by ion-beam sputtering technique at room temperature from V and Ni targets. The distance between the targets and the substrate was about 25 cm, assuring a thickness uniformity better than 1% on a 2 × 2 cm 2 area. The ion beam was extracted from a 3 cm diameter ion source. Prior to the deposition, the system was pumped with a cryogenic pump down to a pressure of about 10 28 mbar whereas the deposition is performed at a pressure of 2 × 10 24 mbar. The ion beam was neutralised by injecting electrons with a hot tungsten filament. The ion-beam sputtering was performed with argon ions with an energy of 1.2 keV and a current of 40 mA. The multilayers were deposited on optical quality float-glass substrates which were cleaned in a special solution and then rinsed several times in non-ionised water. Before sputter deposition, the substrate was etched for several minutes. The rate of deposition was 15

25

˚ /min for V and Ni, respectively. In addition, and 20 A the targets were cleaned by pre-sputtering. The thickness of each deposited layer was measured during the sputtering by a calibrated quartz micro-balance, and a built-in soft X-ray reflectometer was used to control the periodicity of the stack [18,19]. The number of bilayers of V–Ni were fixed to 10, 12, 15 and 20 for the monochromators (periodic samples) and to 15 and 30 for the two supermirrors (aperiodic samples) [20]. ˚ The expected periods of monochromators are 60 A ˚ (NiV-2), 180 A ˚ (NiV-3) and 300 A ˚ (NiV-1), 90 A (NiV-4). First, grazing angle X-ray reflectometry measurements were performed on the samples to check their interfacial and periodicity qualities, using a sealed ˚ ). The simulated Cuka 1 tube source (l ˆ 1.5405 A profiles were calculated using the optical matrix method adapted for neutrons. The interfacial roughness and interdiffusion were treated as interfacial irregularities whose distribution is assumed to be gaussian with an r.m.s. roughness ks l [18,19]. As there is a high X-ray absorption in the two supermirrors and no substantial information can be obtained, Fig. 1 reports only the reflectivity profiles of the four monochromators with their corresponding simulations. As shown by the previous figure, the experimental profiles exhibit numerous well-defined Bragg peaks and contrasted Kiessig fringes pattern, confirming reasonably sharp composition at the interfaces and a high degree of stack regularity. The average inter˚ for different facial roughness is of the order of 4 A monochromators while the interfacial mixing layer was not required to simulate the reflectivity profiles. The difference between the theoretical and experimental refractive indices is not very high; it is of the order of 5 and 7% for V and Ni layers, respectively, for all samples. This could be due to the fact that Ni and V layers are polycrystalline (having some degree of preferred orientation) and amorphous, respectively. The neutron reflectivity measurements were conducted at ORPHEE-Laboratoire Leon Brillouin 14 MW reactor located at Saclay. The time of flight reflectometer EROS was used. The wavelength of the ˚ . The incident neutron beam varied from 3 to 25 A

Fig. 1. Experimental (WWWWW) and simulated (—) X-ray reflectivity profiles versus grazing angle incidence of the four V–Ni periodic ˚ ). multilayers (l ˆ 1.5405 A

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Fig. 2. Experimental (WWWWW) and simulated (—) neutron reflectivity profiles in logarithmic scale versus log Q of the four V–Ni periodic multilayers (u ˆ 28).

grazing angle u 0 was fixed to 28 with a corresponding angular resolution Du /u of 5 × 10 22. As in the case of X-rays, the simulation of the experimental neutron reflectivity profiles are calculated using the standard matrix method [11] by assuming rough and chemically uncontaminated interfaces and no random thickness errors in the periodicity (according to the former X-ray measurements). In Fig. 2, the experimental reflectivity profiles of the four monochromators in logarithmic scale versus log Q are shown together with their simulations (Table 1) where Q ˆ 2p sin(u )/l , is the normal component of the momentum. At very low Q, the regime of total reflection can be seen. In the vitreous region, the different Bragg

peaks, related to the artificial periodicity are well identified (Table 1). The small reflectivity of the even peaks is due to the g -ratio whose average value is of the order of 0.46 close to 0.5 [21,22] (g ˆ DV/(DV 1 DNi), DV and DNi are the thickness of V and Ni layers, respectively). The nuclear scattering length density of Ni layers remains constant ˚ 22), whereas that of V layers (NbNi < 9.0 × 10 26 A ˚ 22. As it is positive and varies from 0.0 to 0.7 × 10 26 A is well known, the theoretical value of pure vanadium is negative and is of the order of NbV < 20.5 × ˚ 22. As in the case of X-rays, there is, a priori 10 26 A no interfacial diffusion; the intermediate layer is not required to simulate the different reflectivity profiles

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Table 1 Simulation parameters of the different V–Ni neutron reflectivity profiles: Dx, Nbx, s x are the layer thickness, nuclear scattering length density and interfacial roughness of material x, respectively; Qk, Rk are the spectral position and the reflectivity of the Bragg peak of order k, respectively (only intense first peaks are considered) NiV-1

NiV-2

(a) Theoretical values ˚) DNi (A 30 45 ˚) DV (A 30 45 g -ratio 0.5 0.5 ˚ 22) NbNi (10 26 A 9.4 9.4 ˚ 22) 2 0.5 2 0.5 NbV (10 26 A (b) Experimental and simulation values ˚) DNi (A 36 54 ˚) DV (A 26 34 g -ratio 0.24 0.39 ˚ 22) NbNi (10 26 A 9.0 9.0 ˚ 22) 0.0 0.5 NbV (10 26 A ˚) s Ni (A 0 0 ˚) s V (A 0 0 Bi-layer number 20 15 ˚ 21) Qkˆ1 (10 22 A 5.03 3.66 Rkˆ1 (%) p1 11.6 ˚ 21) Qkˆ2 (10 22 A 1.01 7.18 Rkˆ2 (%) p1 p1 ˚ 21) 1.52 10.8 Qkˆ3 (10 22 A Rkˆ3 (%) p1 p1

NiV-3

NiV-4

90 90 0.5 9.4 2 0.5

150 150 0.5 9.4 2 0.5

90 90 0.5 9.0 0.5 0 0 12 1.97 72 3.90 p1 5.31 2.5

150 148 0.5 9.0 0.7 5 5 10 1.30 99.5 2.24 4.4 3.26 16.2

which was not the case for Ni–Ti and Co–Ti multilayered systems (the thickness of the intermediate was ˚ ) [11,15,16]. The tendency of the slight 15–20 A variation of NbV suggests that the V layers are contaminated by residual gases possessing a positive nuclear scattering length such as oxygen [15]. This deduction can be explained, as in the case of Ti, by the affinity of vanadium with oxygen and hydrogen [23]. Concerning the reflected Bragg peaks, their broadening is insignificant due to the consequential reproducibility of the period. Their reflectivity varies from 2.8 to 99.5% as indicated in Table 1 (if we take into account only the intense peaks i.e. the low orders) suggesting thus, the possibility to use these multilayered systems to monochromate white neutron beams. In order to further investigate the efficiency of these V–Ni to transport neutron beams, as underlined previously, two supermirrors were realized following the Hayter–Mook algorithm for the layer thickness gradient [24]. Fig. 3 reports their corresponding linear reflectivity versus Q. One can note

Fig. 3. Experimental neutron reflectivity profiles in linear scale of the two V–Ni supermirrors SM1 and SM2 versus Q (u ˆ 28).

that, in spite of the incoherence of the vanadium, there is a real extension of the total reflection region (supermirror effect). The critical Q values are of the ˚ 21 for SM1 and SM2, order of 1.58 and 1.75 × 10 22 A c respectively i.e. 1.46Q Ni and 1.61Q cNi (Q cNi is the critical value of the usual Ni neutron guides). Such values are comparable to that obtained with Ni–Ti supermirrors [11]. One can note the existence of a deep region between the natural critical region and the artificial one for both supermirrors. This abrupt and wide decay in the reflectivity profiles is due to the absence of the well-known Ni buffer layer [11,20,24]. This layer was omitted to find out what is the real part of the V–Ni supermirror structure on the extension of the total reflection plateau. Likewise, one can remark that the decay of the reflectivity in this intermediate region is slightly deeper for the SM2 than for SM1, suggesting some absorption in V layers, especially the thickest ones. In summary, the performance of both V–Ni monochromators and supermirrors can be compared to the usual Ni–Ti ones. One can expect that the efficiency of these V–Ni neutron optics devices could be improved by hydrogenation of V layers.

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Acknowledgements Fruitful discussions with Professors O. Schaerpf, C. Sella and P. Croce are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

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