- Email: [email protected]
New polarised neutron reflectometer with polarisation analysis PRISM
Physica B 283 (2000) 372}375
New polarised neutron re#ectometer with polarisation analysis PRISM C. Fermon , F. Ott *, G. Lego! , H. GlaK ttli , V. Wintenberger DRECAM/SPEC, CEA Saclay, 91191 Gif sur Yvette Cedex, France Laboratoire Le& on Brillouin, CEA/CNRS and DRECAM/SPEC bat-563, CEA Saclay, 91191 Gif sur Yvette Cedex, France
Abstract We present the new polarised neutron re#ectometer for the investigation of surface magnetism (PRISM), which has been built at the Laboratoire LeH on Brillouin. This polarised neutron re#ectometer with polarisation analysis has been optimised for the study of small magnetic samples (1 cm range). 2000 Elsevier Science B.V. All rights reserved. PACS: 61.12.Ha; 75.25.#z; 75.70.!i; 75.70.Ak Keywords: Neutron instruments; Re#ectometry; Polarised neutrons
1. Introduction The technique of neutron re#ectivity has emerged less than 15 years ago. It has appeared as a key technique in the study of polymers and magnetic thin "lms. Problems such as polymer interdiffusion and di-block copolymer ordering have been addressed [1}3]. In the last 10 years, neutron re#ectivity has been applied successfully to the study of magnetic multilayers and ultrathin "lms [4,5]. However, in order to push further the possibilities of neutron re#ectivity, it has appeared that improvements in the #ux and focalisation were necessary. With that aim, we have recently built a new polarised re#ectometer with polarisation analysis at the Laboratoire LeH on Brillouin dedicated to the study of magnetic thin "lms. * Corresponding author. Tel.: #33-1-69-08-61-21; fax: #331-69-08-82-61. E-mail address: [email protected] (F. Ott)
The di!erent ways we have used to increase the available neutron #ux are the following: E optimisation of the wavelength spectral width dj/j by using a multilayer monochromator, E focussing of a 100 mm high neutron beam onto a 15 mm high region (which is suitable for the study of small magnetic samples, E use of a position-sensitive detector (measurement of both spin states at the same time and access to o!-specular measurements).
2. Description of the optical set-up The OrpheH e neutron source is a 14 MW steadystate reactor generating a nominal #ux of 3;10 neutrons/cm/s. It is located at the Centre d'Etudes NucleH aires in Saclay, France. Neutrons are delivered to the spectrometers by neutron guides using 2h supermirrors [6]. The guides size
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 3 4 2 - 2
C. Fermon et al. / Physica B 283 (2000) 372}375
is 150;25 mm. The new polarised re#ectometer for the investigation of surface magnetism (PRISM) spectrometer is mounted on the guide G2 of the reactor. The end of the guide is used by a smallangle neutron scattering spectrometer. The #ux on this guide is shared between these two spectrometers. We use the #ux of the top 100 mm of the guide, the other 50 mm being used by the SANS spectrometer. The neutron beam is deviated and monochromatised by a 2 m mirror made of nickel}titanium multilayers. The direction of the monochromated beam makes an angle of 2.83 with
373
respect to the direction of the main guide (Fig. 1). This part of the guide has been built and mounted by the CILAS company. The angular deviation of 2.83 is however not su$cient to move the output beam away from the main guide at the sample position. Thus, we have mounted two 1.80 m long 2h super-mirror neutron guides at the output of the monochromator. The total beam deviation is thus 5.43 giving a beam separation of 1 m between the sample position and the main guide. A vertical focussing is obtained by a 8 m conical neutron guide made of h and 2h supermirrors (see
Fig. 1. Scheme of the experimental setup: (a) top view, (b) side view. The di!erent elements are multilayer monochromator (M), re#exion supermirrors (B), collimation slits, focussing guide (C), polariser (P), analyser (A).
Fig. 2. Shape of the output beam at the end of the guide and at 1 m. The Monte-Carlo simulation program can be found on WWW www-llb.cea.fr. It is compiled for Win95/NT platforms.
374
C. Fermon et al. / Physica B 283 (2000) 372}375
Figs. 1 and 2). The vertical focalisation optics is interrupted twice. These two interruptions make it possible to mount the polarising and #ipping systems. The vertical divergence of the beam at the output is about 1.23. 2.1. Polarisation and analysis system The beam is polarised by transmission polarisers made of Fe/Si multilayers deposited on 50 mm high silicon substrates. These mirrors are fabricated by Krist (HMI Berlin) [7]. The incidence angle on the mirrors is 0.33. The polarisation e$ciency is 0.97. The transmission of these polarisers is 70%. The polariser (P) is mounted 1.5 m away from the sample in a 50 mT "eld. The analysis system (A) is a polarising supermirror provided by PSI [8] mounted in a re#ection geometry.
Fig. 3. Re#ectivity of a monochromating mirror wavelength bandwidth is of order 5%, see inset).
(the
2.2. Flipping systems We use Mezei-type #ippers consisting of two orthogonal coils [9] built with aluminium wires in order to minimise absorption and di!usion. A `windowa is made in the winding of the coil creating the magnetic "eld perpendicular to the neutron beam in order to minimise the absorption. The absorption of each #ipping system is reduced to 3%. Both #ippers are located as far as possible from the polariser, the analyser and the sample position, in order to avoid any interference with the stray "elds created by the magnetic "elds applied on the sample. This is especially important when the magnetic "eld on the sample is varied (during hysteresis loop measurements for example [10]). The #ipping systems are the main cause of depolarisation of the beam and the normal #ipping ratio is about 35. This can be strongly enhanced by the use of cryo#ippers.
of the mounted monochromator (consisting of four mirrors) has a dj/j of 10%, perhaps due to a small misalignment of the mirrors. Such a large wavelength is only suitable for thin samples. In the case of thicker samples, it is possible to reduce the wavelength spread by collimating the entrance of the focussing system (using the slit S1, see Fig. 1). The minimum dj/j is about 3% but with only of the intensity. 2.4. Long wavelengths problem Due to the total re#ectivity plateau (Fig. 3), monochromatic multilayers always transmit longer wavelengths than the central wavelength of the device. The use of a transmission polariser allows us to suppress these long wavelengths by re#ection with enough e$ciency.
2.3. Monochromatisation system
2.5. Detection system
Fig. 3 shows a re#ectivity curve measured on one of the monochromators mirror. The inset shows the wavelength spectrum re#ected by the multilayer monochromator for an incidence angle of 1.553. The wavelength spectrum width is of about 5% (at FWHM). The wavelength spectrum measured out
The detection system currently consists of a BF gas detector. The maximum detection rate is 3;10 n/s. A micro-strip position-sensitive detector (PSD), 200;100 mm large, can be installed for o!-specular measurements without polarisation analysis.
C. Fermon et al. / Physica B 283 (2000) 372}375
Fig. 4. Re#ectivity curve on a silicon wafer of a surface of 8 cm. The measurement time was 5 min per point.
2.6. Angular resolution The horizontal resolution is usually set to 0.043 but can be varied between 0.013 and 0.103. The current #ux incident on the sample is 3;10 neutrons/cm/s after analysis on the detector with a resolution of dh"0.033. This #ux is available over a height of 15 mm. The #ux distribution is shown in Fig. 2. The background signal is 1 count/min above 2h"0.73.
3. Examples Fig. 4 shows the re#ectivity from a silicon substrate with a size of 8 cm. The counting time was 5 min per point at high angles. The accessible re#ectivity range is 10 with dh"0.033. Fig. 5 shows an example of measurement on a spin-valve system (FeMn/Co(5 nm)/Ru(2 nm)/Co(5 nm)/NiFe(30 nm) /Nb(100 nm)).
4. Conclusion We have presented the new polarised neutron re#ectometer PRISM at the Laboratoire LeH on Brillouin. This spectrometer o!ers enhanced performances for the study of small magnetic samples (with surfaces of the order of 1 cm). The next step for increasing the #ux will be the rebuilding of the
375
Fig. 5. Re#ectivity of a spin-valve system (MnFe/Co/Ru/Co/NiFe) in a 0.3 T "eld. The measurement time was 3 min per point on a 1 cm sample.
part after the sample: a vacuum tank and a higher analyser. A factor 4 in counting time will then be obtained. The use of an analyser working in re#ection and in transmission will also slightly reduce the counting time.
Acknowledgements This work has been supported by the European Commission in the frame of the XENNI network and of the TMR network Dynaspin (FMRXCT97-0124).
References [1] J. Penfold, R.K. Thomas, J. Phys.: Condens. Matter 2 (1990) 1369. [2] T.P. Russel, Mater. Sci. Rep. 5 (1990) 171. [3] T.P. Russel, Physica B 221 (1996) 267. [4] C.F. Majkrzak, J.W. Cable, J. Kwo, M. Hong, D.B. McWhan, Y. Yafet, J. Waszcak, Phys. Rev. Lett. 56 (1986) 2700. [5] G.P. Felcher, R.O. Hilleke, R.K. Crawford, J. Haumann, R. Kleb, G. Ostrowski, Rev. Sci. Instr. 58 (1987) 609. [6] P. BoK ni, Physica B 234}236 (1997) 522. [7] Th. Krist, F. Klose, G.P. Felcher, Physica B 248 (1998) 372. [8] P. BoK ni et al., Physica B 267&268 (1999) 320. [9] F. Mezei, Z. Phys. 255 (1972) 146. [10] C. Fermon, S. Gray, G. Lego!, V. Mathet, S. Mathieu, F. Ott, M. Viret, P. Warin, Physica B 241}243 (1998) 1055.
New polarised neutron re#ectometer with polarisation analysis PRISM C. Fermon , F. Ott *, G. Lego! , H. GlaK ttli , V. Wintenberger DRECAM/SPEC, CEA Saclay, 91191 Gif sur Yvette Cedex, France Laboratoire Le& on Brillouin, CEA/CNRS and DRECAM/SPEC bat-563, CEA Saclay, 91191 Gif sur Yvette Cedex, France
Abstract We present the new polarised neutron re#ectometer for the investigation of surface magnetism (PRISM), which has been built at the Laboratoire LeH on Brillouin. This polarised neutron re#ectometer with polarisation analysis has been optimised for the study of small magnetic samples (1 cm range). 2000 Elsevier Science B.V. All rights reserved. PACS: 61.12.Ha; 75.25.#z; 75.70.!i; 75.70.Ak Keywords: Neutron instruments; Re#ectometry; Polarised neutrons
1. Introduction The technique of neutron re#ectivity has emerged less than 15 years ago. It has appeared as a key technique in the study of polymers and magnetic thin "lms. Problems such as polymer interdiffusion and di-block copolymer ordering have been addressed [1}3]. In the last 10 years, neutron re#ectivity has been applied successfully to the study of magnetic multilayers and ultrathin "lms [4,5]. However, in order to push further the possibilities of neutron re#ectivity, it has appeared that improvements in the #ux and focalisation were necessary. With that aim, we have recently built a new polarised re#ectometer with polarisation analysis at the Laboratoire LeH on Brillouin dedicated to the study of magnetic thin "lms. * Corresponding author. Tel.: #33-1-69-08-61-21; fax: #331-69-08-82-61. E-mail address: [email protected] (F. Ott)
The di!erent ways we have used to increase the available neutron #ux are the following: E optimisation of the wavelength spectral width dj/j by using a multilayer monochromator, E focussing of a 100 mm high neutron beam onto a 15 mm high region (which is suitable for the study of small magnetic samples, E use of a position-sensitive detector (measurement of both spin states at the same time and access to o!-specular measurements).
2. Description of the optical set-up The OrpheH e neutron source is a 14 MW steadystate reactor generating a nominal #ux of 3;10 neutrons/cm/s. It is located at the Centre d'Etudes NucleH aires in Saclay, France. Neutrons are delivered to the spectrometers by neutron guides using 2h supermirrors [6]. The guides size
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 0 ) 0 0 3 4 2 - 2
C. Fermon et al. / Physica B 283 (2000) 372}375
is 150;25 mm. The new polarised re#ectometer for the investigation of surface magnetism (PRISM) spectrometer is mounted on the guide G2 of the reactor. The end of the guide is used by a smallangle neutron scattering spectrometer. The #ux on this guide is shared between these two spectrometers. We use the #ux of the top 100 mm of the guide, the other 50 mm being used by the SANS spectrometer. The neutron beam is deviated and monochromatised by a 2 m mirror made of nickel}titanium multilayers. The direction of the monochromated beam makes an angle of 2.83 with
373
respect to the direction of the main guide (Fig. 1). This part of the guide has been built and mounted by the CILAS company. The angular deviation of 2.83 is however not su$cient to move the output beam away from the main guide at the sample position. Thus, we have mounted two 1.80 m long 2h super-mirror neutron guides at the output of the monochromator. The total beam deviation is thus 5.43 giving a beam separation of 1 m between the sample position and the main guide. A vertical focussing is obtained by a 8 m conical neutron guide made of h and 2h supermirrors (see
Fig. 1. Scheme of the experimental setup: (a) top view, (b) side view. The di!erent elements are multilayer monochromator (M), re#exion supermirrors (B), collimation slits, focussing guide (C), polariser (P), analyser (A).
Fig. 2. Shape of the output beam at the end of the guide and at 1 m. The Monte-Carlo simulation program can be found on WWW www-llb.cea.fr. It is compiled for Win95/NT platforms.
374
C. Fermon et al. / Physica B 283 (2000) 372}375
Figs. 1 and 2). The vertical focalisation optics is interrupted twice. These two interruptions make it possible to mount the polarising and #ipping systems. The vertical divergence of the beam at the output is about 1.23. 2.1. Polarisation and analysis system The beam is polarised by transmission polarisers made of Fe/Si multilayers deposited on 50 mm high silicon substrates. These mirrors are fabricated by Krist (HMI Berlin) [7]. The incidence angle on the mirrors is 0.33. The polarisation e$ciency is 0.97. The transmission of these polarisers is 70%. The polariser (P) is mounted 1.5 m away from the sample in a 50 mT "eld. The analysis system (A) is a polarising supermirror provided by PSI [8] mounted in a re#ection geometry.
Fig. 3. Re#ectivity of a monochromating mirror wavelength bandwidth is of order 5%, see inset).
(the
2.2. Flipping systems We use Mezei-type #ippers consisting of two orthogonal coils [9] built with aluminium wires in order to minimise absorption and di!usion. A `windowa is made in the winding of the coil creating the magnetic "eld perpendicular to the neutron beam in order to minimise the absorption. The absorption of each #ipping system is reduced to 3%. Both #ippers are located as far as possible from the polariser, the analyser and the sample position, in order to avoid any interference with the stray "elds created by the magnetic "elds applied on the sample. This is especially important when the magnetic "eld on the sample is varied (during hysteresis loop measurements for example [10]). The #ipping systems are the main cause of depolarisation of the beam and the normal #ipping ratio is about 35. This can be strongly enhanced by the use of cryo#ippers.
of the mounted monochromator (consisting of four mirrors) has a dj/j of 10%, perhaps due to a small misalignment of the mirrors. Such a large wavelength is only suitable for thin samples. In the case of thicker samples, it is possible to reduce the wavelength spread by collimating the entrance of the focussing system (using the slit S1, see Fig. 1). The minimum dj/j is about 3% but with only of the intensity. 2.4. Long wavelengths problem Due to the total re#ectivity plateau (Fig. 3), monochromatic multilayers always transmit longer wavelengths than the central wavelength of the device. The use of a transmission polariser allows us to suppress these long wavelengths by re#ection with enough e$ciency.
2.3. Monochromatisation system
2.5. Detection system
Fig. 3 shows a re#ectivity curve measured on one of the monochromators mirror. The inset shows the wavelength spectrum re#ected by the multilayer monochromator for an incidence angle of 1.553. The wavelength spectrum width is of about 5% (at FWHM). The wavelength spectrum measured out
The detection system currently consists of a BF gas detector. The maximum detection rate is 3;10 n/s. A micro-strip position-sensitive detector (PSD), 200;100 mm large, can be installed for o!-specular measurements without polarisation analysis.
C. Fermon et al. / Physica B 283 (2000) 372}375
Fig. 4. Re#ectivity curve on a silicon wafer of a surface of 8 cm. The measurement time was 5 min per point.
2.6. Angular resolution The horizontal resolution is usually set to 0.043 but can be varied between 0.013 and 0.103. The current #ux incident on the sample is 3;10 neutrons/cm/s after analysis on the detector with a resolution of dh"0.033. This #ux is available over a height of 15 mm. The #ux distribution is shown in Fig. 2. The background signal is 1 count/min above 2h"0.73.
3. Examples Fig. 4 shows the re#ectivity from a silicon substrate with a size of 8 cm. The counting time was 5 min per point at high angles. The accessible re#ectivity range is 10 with dh"0.033. Fig. 5 shows an example of measurement on a spin-valve system (FeMn/Co(5 nm)/Ru(2 nm)/Co(5 nm)/NiFe(30 nm) /Nb(100 nm)).
4. Conclusion We have presented the new polarised neutron re#ectometer PRISM at the Laboratoire LeH on Brillouin. This spectrometer o!ers enhanced performances for the study of small magnetic samples (with surfaces of the order of 1 cm). The next step for increasing the #ux will be the rebuilding of the
375
Fig. 5. Re#ectivity of a spin-valve system (MnFe/Co/Ru/Co/NiFe) in a 0.3 T "eld. The measurement time was 3 min per point on a 1 cm sample.
part after the sample: a vacuum tank and a higher analyser. A factor 4 in counting time will then be obtained. The use of an analyser working in re#ection and in transmission will also slightly reduce the counting time.
Acknowledgements This work has been supported by the European Commission in the frame of the XENNI network and of the TMR network Dynaspin (FMRXCT97-0124).
References [1] J. Penfold, R.K. Thomas, J. Phys.: Condens. Matter 2 (1990) 1369. [2] T.P. Russel, Mater. Sci. Rep. 5 (1990) 171. [3] T.P. Russel, Physica B 221 (1996) 267. [4] C.F. Majkrzak, J.W. Cable, J. Kwo, M. Hong, D.B. McWhan, Y. Yafet, J. Waszcak, Phys. Rev. Lett. 56 (1986) 2700. [5] G.P. Felcher, R.O. Hilleke, R.K. Crawford, J. Haumann, R. Kleb, G. Ostrowski, Rev. Sci. Instr. 58 (1987) 609. [6] P. BoK ni, Physica B 234}236 (1997) 522. [7] Th. Krist, F. Klose, G.P. Felcher, Physica B 248 (1998) 372. [8] P. BoK ni et al., Physica B 267&268 (1999) 320. [9] F. Mezei, Z. Phys. 255 (1972) 146. [10] C. Fermon, S. Gray, G. Lego!, V. Mathet, S. Mathieu, F. Ott, M. Viret, P. Warin, Physica B 241}243 (1998) 1055.