- Email: [email protected]
The 3He polarizing filter on the neutron reflectometer D17
ARTICLE IN PRESS
Physica B 385–386 (2006) 1134–1137 www.elsevier.com/locate/physb
The 3He polarizing filter on the neutron reflectometer D17 K.H. Andersena, R. Cubitta, H. Humblota, D. Julliena, A. Petoukhova, F. Tasseta, C. Schanzerb, V.R. Shahb, A.R. Wildesa, a
Institut Laue-Langevin, 6 r. Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France Physics Department, TU Muenchen, James-Fransk-Str. 1, 85748 Garching, Germany
b
Abstract The D17 neutron reflectometer at the Institut Laue-Langevin, Grenoble, can now carry out neutron polarization analysis experiments with a polarized 3He gas filter. The cell containing the gas is housed in the evacuated detector tank. The gas polarization decays with time and must be replenished periodically. A novel method to accomplish this in situ, known as local filling, has been developed to minimize down-time on the instrument. The instrument is now optimized for the rapid collection of off-specular scattering with polarization analysis, complementary to a polarizing supermirror analyser which is used for measurements of specular reflectivity. Cell performance, test and experimental data are presented, along with methods for analysing the data collected using the filter. r 2006 Elsevier B.V. All rights reserved. PACS: 61.12.Ha; 07.60.Hv; 28.20.Cz Keywords: Polarized neutrons; Reflectometry; Polarized 3He
Neutron scattering techniques with polarization analysis are powerful tools for the study of magnetic structures, and have been used with great success when the experimental data can be analysed within the first Born approximation. More recently, a great deal of interest has been expressed in the magnetic properties of thin films, multilayers and laterally patterned materials. These materials are investigated for fundamental studies, due to the effects of confining the dimensions on the physical properties, and for future technology, as samples may be engineered to have certain characteristics. Neutron reflectometry and grazing incidence techniques are well suited to measure and characterize these materials, particularly when combined with neutron polarization analysis. Knowing this, the Institut Laue-Langevin (ILL), Grenoble, has made a major rebuild of the D17 instrument to become a dedicated reflectometer [1]. One of the standard modes of operation for this versatile instrument is with a polarizing monochromator, delivering neutrons of waveCorresponding author. Tel.: +33 4 76 20 70 37; fax: +33 4 76 48 39 06.
E-mail address: [email protected] (A.R. Wildes). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.05.391
˚ with 97% polarization. The instrument length 5:5 0:2 A has a large multi-detector array, housed in a vacuum tank, enabling the rapid measurement of off-specular scattering due to magnetic roughness, domain walls, non-trivial magnetic structures and in-plane correlations between lateral structures. The polarization of the scattered neutrons is analysed using a specially designed polarized 3 He spin filter, developed by the neutron optics group at the ILL [2], with the aid of a radio-frequency neutron flipper between the sample and the filter. The spin filter is circular in cross-section with the front and back windows being made from single crystal silicon to minimize any small angle scattering of the neutrons. It is wide enough to cover the angle subtended by the detector from the sample, and is 10 cm thick (see Fig. 1). It sits in a magnetic field, created by a mu-metal ‘‘magic’’ box, with a homogeneity of dB=B5 104 [3]. The ensemble is placed in the vacuum tank, which is then pumped to o105 bar to reduce air scattering. The nominal pressure of gas in the cell is 0:6 bar which, for 65% 3He gas polarization, gives 92% neutron polarization.
ARTICLE IN PRESS K.H. Andersen et al. / Physica B 385–386 (2006) 1134–1137
1135
Fig. 2. Schematic showing the process of refreshing the 3He gas on D17 via the local filling technique.
Fig. 1. The D17 3He polarizing cell. Attached to the inlet is the nonmagnetic, pneumatic valve used in the local filling process.
The polarization of the gas, PHe falls exponentially with time according to the equation [4] PHe ðtÞ ¼ P0He expðt=t1 Þ,
(1)
where P0He is the initial polarization of the gas and t1 is a characteristic relaxation time. While the t1 for the spin filter, when in place on the instrument, is 100 h, it is highly desirable to refresh the 3He gas every day during an experiment. On other instruments at the ILL this is done by having many filters and swapping one on the instrument with another, freshly filled. This would be extremely inconvenient to do on D17 as it would involve breaking the vacuum and opening the detector tank, changing the filter, then re-evacuating the tank—a process that takes some hours to perform. Consequently, a novel method for changing the gas without opening the vacuum tank has been developed. The method, known as local filling, leaves the filter in place and evacuates and refills it via a capillary that traverses the wall of the vacuum tank. The method is shown schematically in Fig. 2. A buffer cell is filled with freshly polarized gas at Tyrex, the ILL polarized 3He filling station [5]. The buffer cell is then immediately brought to the instrument and attached to the capillary and to a vacuum pump using a three-way valve. The old gas in the spin filter is evacuated using the pump. The spin filter is then filled via a free expansion of the gas from the buffer cell. A pneumatic, non-magnetic valve seals the filter, which sits in the most homogenous part of the magnetic environment, ensuring that the t1 is as large as possible. The entire process takes no more than 30 min and has been done with magnetic fields up to 0.5 T on the sample. The depolarization of the 3He gas between the moment the
Fig. 3. The typical transmission and polarization of the D17 3He cell. The data are fitted with functions derived from Eq. (2). The fits give P0He ¼ 67% and t1 ¼ 120 h.
buffer cell leaves Tyrex to the end of the transfer process is less than 2%. The spin-dependent neutron transmission of the spin filter for unpolarized neutrons, T, is given by [4] T ðl; tÞ ¼ 12 expð½3 Helsa ðlÞð1 PHe ðtÞÞÞ, 3
3
(2)
where [ He] is the number density of the He atoms, l is the filter length and sa ðlÞ is the absorption cross-section for unpolarized neutrons of wavelength l. The superscript, , refers to the transmission of neutrons with spins that are parallel ðþÞ or antiparallel ðÞ to the 3He polarization. The transmission, and thus the neutron polarization, are timedependent and are monitored during an experiment by regular measurements of the flipping ratio of the main beam. Fig. 3 shows the typical performance of the filter over a 24-h period on D17. The total neutron polarization and transmission of the instrument were simultaneously
ARTICLE IN PRESS K.H. Andersen et al. / Physica B 385–386 (2006) 1134–1137
1136
fitted with equations derived from Eq. (2). The fitted parameters are consistent with expected values, with a 3He pressure of 0.6 bar, a monochromator polarization of 97%, P0He ¼ 67% and t1 ¼ 120 h. The time dependence impacts on the manner of conducting an experiment. Reflectivity falls quickly with increasing angle, therefore scans are always started at the largest angles. The weakest scattering is thus measured first, when the transmission is better. Scans are kept relatively short and are repeated for adequate statistics. This creates time to periodically measure the transmission for calibration, and gives confidence in subsequent analysis as data for a given measurement should all coincide after being corrected for the time dependence of the filter. To properly correct for the time dependence, the measurements of the various flipper states are done in pairs, mirrored about a central time. For example, if measuring with one flipper at one position in angle, four measurements would be made in the order flipper off– on–on–off. Adding the data for equivalent flipper states gives the average counts at the central time. The transmission of the filter is then calculated at that time to correct the data.
Correcting the data is done via a matrix method [6,7]. The measured data, I, can be expressed as a linear combination of the neutron polarization-dependent crosssections, S, where the coefficients are given by the efficiencies of the polarizer, the analyser, and the flippers. The equations can be expressed in matrix form (3)
½S ¼ ½A½F A ½P½F P ½I,
where ½P; ½A; ½F P ; ½F A are matrices containing the efficiencies of the polarizer, analyser, flipper before the sample and flipper after the sample, respectively. ½S is thus determined by multiplying the measured data by the appropriate matrices. When using the spin filter as an analyser, the appropriate matrix is 0 þ 1 T T 0 0 B T Tþ 0 0 C 1 B C ½A ¼ þ 2 (4) B C. þ 2@ 0 0 T T A ðT Þ ðT Þ 0
0
T
Tþ
Applying Eqs. (3) and (4) with the appropriate T ðtÞ simultaneously corrects the data for both the polarization and the transmission of the cell at time t. 2.5
2.5
2
2
1.5
1.5
θin
θin
FeCoV/NiO/FeCoV thin film sandwich
1
1 Σ--
Σ++ 0.5
0.5 0.5
1
1.5
2
0.5
1
2.5
2.5
2
2
1.5
1.5
θin
θin
1.5
2
θout
θout
1
1 Σ-+
Σ+0.5
0.5 0.5
1.5
1 θout
2
0.5
1
1.5
2
θout
Fig. 4. Data from a recent experiment using the 3He cell on D17. The off-specular scattering is similar in magnitude between spin flip and non-spin flip, meaning that it is magnetic.
ARTICLE IN PRESS K.H. Andersen et al. / Physica B 385–386 (2006) 1134–1137
The spin filter and the local filling technique have already been successfully used for a number of scheduled experiments on D17. Fig. 4 shows the results of a recent experiment on a FeCoV/NiO/FeCoV thin film sandwich, being two ferromagnetic layers separated by an antiferromagnetic spacer. The figure shows the polarizationdependent scattering, corrected for instrument efficiency and filter transmission, measured at a magnetic state during magnetization reversal. As expected, the specular scattering (yin ¼ yout ) is almost entirely non-spin flip, giving confidence that the data has been properly corrected. Significant off-specular scattering is observed in all four polarization states in roughly equivalent quantities. This is expected for purely magnetic scattering when the magnetism is isotropic in the plane of the sample, and the data are currently being analysed in the context of magnetic roughness due to the formation of lateral domains. The data represent the combination of four 2 h scans and the quality of the data is very good. The same sample has been recently re-measured with a more optimized configuration and equivalently high quality data was collected in 2 h. In conclusion, the D17 reflectometer at the ILL is now well optimized for measuring off-specular neutron scatter-
1137
ing with polarization analysis. The D17 spin filter is of high quality, consistently having high neutron polarization and transmission. The local filling technique is an efficient and relatively non-invasive method for guaranteeing the best conditions for measurement, and the techniques for collecting and treating data have been developed and shown to be correct. Future developments by the polarized 3 He group at the ILL will improve the option further, with potentially higher 3He polarization and improved local filling apparatus. The local filling technique also represents an important step towards steady-state 3He filters, where the filter transmission can be kept constant as a function of time [8]. References [1] [2] [3] [4] [5] [6] [7] [8]
R. Cubitt, G. Fragneto, Appl. Phys. A 74 (2002) S329. A. Petoukhov, et al., these proceedings. A. Petoukhov, et al., Nucl. Instr. and Meth. A 560 (2006) 480. R. Surkau, et al., Nucl. Instr. and Meth. A 384 (1997) 444. K.H. Andersen, et al., Physica B 356 (2005) 103. A.R. Wildes, Rev. Sci. Instrum. 70 (1999) 4241. A.R. Wildes, Neutron News, in press. D. Jullien, et al., in preparation.
Physica B 385–386 (2006) 1134–1137 www.elsevier.com/locate/physb
The 3He polarizing filter on the neutron reflectometer D17 K.H. Andersena, R. Cubitta, H. Humblota, D. Julliena, A. Petoukhova, F. Tasseta, C. Schanzerb, V.R. Shahb, A.R. Wildesa, a
Institut Laue-Langevin, 6 r. Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France Physics Department, TU Muenchen, James-Fransk-Str. 1, 85748 Garching, Germany
b
Abstract The D17 neutron reflectometer at the Institut Laue-Langevin, Grenoble, can now carry out neutron polarization analysis experiments with a polarized 3He gas filter. The cell containing the gas is housed in the evacuated detector tank. The gas polarization decays with time and must be replenished periodically. A novel method to accomplish this in situ, known as local filling, has been developed to minimize down-time on the instrument. The instrument is now optimized for the rapid collection of off-specular scattering with polarization analysis, complementary to a polarizing supermirror analyser which is used for measurements of specular reflectivity. Cell performance, test and experimental data are presented, along with methods for analysing the data collected using the filter. r 2006 Elsevier B.V. All rights reserved. PACS: 61.12.Ha; 07.60.Hv; 28.20.Cz Keywords: Polarized neutrons; Reflectometry; Polarized 3He
Neutron scattering techniques with polarization analysis are powerful tools for the study of magnetic structures, and have been used with great success when the experimental data can be analysed within the first Born approximation. More recently, a great deal of interest has been expressed in the magnetic properties of thin films, multilayers and laterally patterned materials. These materials are investigated for fundamental studies, due to the effects of confining the dimensions on the physical properties, and for future technology, as samples may be engineered to have certain characteristics. Neutron reflectometry and grazing incidence techniques are well suited to measure and characterize these materials, particularly when combined with neutron polarization analysis. Knowing this, the Institut Laue-Langevin (ILL), Grenoble, has made a major rebuild of the D17 instrument to become a dedicated reflectometer [1]. One of the standard modes of operation for this versatile instrument is with a polarizing monochromator, delivering neutrons of waveCorresponding author. Tel.: +33 4 76 20 70 37; fax: +33 4 76 48 39 06.
E-mail address: [email protected] (A.R. Wildes). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.05.391
˚ with 97% polarization. The instrument length 5:5 0:2 A has a large multi-detector array, housed in a vacuum tank, enabling the rapid measurement of off-specular scattering due to magnetic roughness, domain walls, non-trivial magnetic structures and in-plane correlations between lateral structures. The polarization of the scattered neutrons is analysed using a specially designed polarized 3 He spin filter, developed by the neutron optics group at the ILL [2], with the aid of a radio-frequency neutron flipper between the sample and the filter. The spin filter is circular in cross-section with the front and back windows being made from single crystal silicon to minimize any small angle scattering of the neutrons. It is wide enough to cover the angle subtended by the detector from the sample, and is 10 cm thick (see Fig. 1). It sits in a magnetic field, created by a mu-metal ‘‘magic’’ box, with a homogeneity of dB=B5 104 [3]. The ensemble is placed in the vacuum tank, which is then pumped to o105 bar to reduce air scattering. The nominal pressure of gas in the cell is 0:6 bar which, for 65% 3He gas polarization, gives 92% neutron polarization.
ARTICLE IN PRESS K.H. Andersen et al. / Physica B 385–386 (2006) 1134–1137
1135
Fig. 2. Schematic showing the process of refreshing the 3He gas on D17 via the local filling technique.
Fig. 1. The D17 3He polarizing cell. Attached to the inlet is the nonmagnetic, pneumatic valve used in the local filling process.
The polarization of the gas, PHe falls exponentially with time according to the equation [4] PHe ðtÞ ¼ P0He expðt=t1 Þ,
(1)
where P0He is the initial polarization of the gas and t1 is a characteristic relaxation time. While the t1 for the spin filter, when in place on the instrument, is 100 h, it is highly desirable to refresh the 3He gas every day during an experiment. On other instruments at the ILL this is done by having many filters and swapping one on the instrument with another, freshly filled. This would be extremely inconvenient to do on D17 as it would involve breaking the vacuum and opening the detector tank, changing the filter, then re-evacuating the tank—a process that takes some hours to perform. Consequently, a novel method for changing the gas without opening the vacuum tank has been developed. The method, known as local filling, leaves the filter in place and evacuates and refills it via a capillary that traverses the wall of the vacuum tank. The method is shown schematically in Fig. 2. A buffer cell is filled with freshly polarized gas at Tyrex, the ILL polarized 3He filling station [5]. The buffer cell is then immediately brought to the instrument and attached to the capillary and to a vacuum pump using a three-way valve. The old gas in the spin filter is evacuated using the pump. The spin filter is then filled via a free expansion of the gas from the buffer cell. A pneumatic, non-magnetic valve seals the filter, which sits in the most homogenous part of the magnetic environment, ensuring that the t1 is as large as possible. The entire process takes no more than 30 min and has been done with magnetic fields up to 0.5 T on the sample. The depolarization of the 3He gas between the moment the
Fig. 3. The typical transmission and polarization of the D17 3He cell. The data are fitted with functions derived from Eq. (2). The fits give P0He ¼ 67% and t1 ¼ 120 h.
buffer cell leaves Tyrex to the end of the transfer process is less than 2%. The spin-dependent neutron transmission of the spin filter for unpolarized neutrons, T, is given by [4] T ðl; tÞ ¼ 12 expð½3 Helsa ðlÞð1 PHe ðtÞÞÞ, 3
3
(2)
where [ He] is the number density of the He atoms, l is the filter length and sa ðlÞ is the absorption cross-section for unpolarized neutrons of wavelength l. The superscript, , refers to the transmission of neutrons with spins that are parallel ðþÞ or antiparallel ðÞ to the 3He polarization. The transmission, and thus the neutron polarization, are timedependent and are monitored during an experiment by regular measurements of the flipping ratio of the main beam. Fig. 3 shows the typical performance of the filter over a 24-h period on D17. The total neutron polarization and transmission of the instrument were simultaneously
ARTICLE IN PRESS K.H. Andersen et al. / Physica B 385–386 (2006) 1134–1137
1136
fitted with equations derived from Eq. (2). The fitted parameters are consistent with expected values, with a 3He pressure of 0.6 bar, a monochromator polarization of 97%, P0He ¼ 67% and t1 ¼ 120 h. The time dependence impacts on the manner of conducting an experiment. Reflectivity falls quickly with increasing angle, therefore scans are always started at the largest angles. The weakest scattering is thus measured first, when the transmission is better. Scans are kept relatively short and are repeated for adequate statistics. This creates time to periodically measure the transmission for calibration, and gives confidence in subsequent analysis as data for a given measurement should all coincide after being corrected for the time dependence of the filter. To properly correct for the time dependence, the measurements of the various flipper states are done in pairs, mirrored about a central time. For example, if measuring with one flipper at one position in angle, four measurements would be made in the order flipper off– on–on–off. Adding the data for equivalent flipper states gives the average counts at the central time. The transmission of the filter is then calculated at that time to correct the data.
Correcting the data is done via a matrix method [6,7]. The measured data, I, can be expressed as a linear combination of the neutron polarization-dependent crosssections, S, where the coefficients are given by the efficiencies of the polarizer, the analyser, and the flippers. The equations can be expressed in matrix form (3)
½S ¼ ½A½F A ½P½F P ½I,
where ½P; ½A; ½F P ; ½F A are matrices containing the efficiencies of the polarizer, analyser, flipper before the sample and flipper after the sample, respectively. ½S is thus determined by multiplying the measured data by the appropriate matrices. When using the spin filter as an analyser, the appropriate matrix is 0 þ 1 T T 0 0 B T Tþ 0 0 C 1 B C ½A ¼ þ 2 (4) B C. þ 2@ 0 0 T T A ðT Þ ðT Þ 0
0
T
Tþ
Applying Eqs. (3) and (4) with the appropriate T ðtÞ simultaneously corrects the data for both the polarization and the transmission of the cell at time t. 2.5
2.5
2
2
1.5
1.5
θin
θin
FeCoV/NiO/FeCoV thin film sandwich
1
1 Σ--
Σ++ 0.5
0.5 0.5
1
1.5
2
0.5
1
2.5
2.5
2
2
1.5
1.5
θin
θin
1.5
2
θout
θout
1
1 Σ-+
Σ+0.5
0.5 0.5
1.5
1 θout
2
0.5
1
1.5
2
θout
Fig. 4. Data from a recent experiment using the 3He cell on D17. The off-specular scattering is similar in magnitude between spin flip and non-spin flip, meaning that it is magnetic.
ARTICLE IN PRESS K.H. Andersen et al. / Physica B 385–386 (2006) 1134–1137
The spin filter and the local filling technique have already been successfully used for a number of scheduled experiments on D17. Fig. 4 shows the results of a recent experiment on a FeCoV/NiO/FeCoV thin film sandwich, being two ferromagnetic layers separated by an antiferromagnetic spacer. The figure shows the polarizationdependent scattering, corrected for instrument efficiency and filter transmission, measured at a magnetic state during magnetization reversal. As expected, the specular scattering (yin ¼ yout ) is almost entirely non-spin flip, giving confidence that the data has been properly corrected. Significant off-specular scattering is observed in all four polarization states in roughly equivalent quantities. This is expected for purely magnetic scattering when the magnetism is isotropic in the plane of the sample, and the data are currently being analysed in the context of magnetic roughness due to the formation of lateral domains. The data represent the combination of four 2 h scans and the quality of the data is very good. The same sample has been recently re-measured with a more optimized configuration and equivalently high quality data was collected in 2 h. In conclusion, the D17 reflectometer at the ILL is now well optimized for measuring off-specular neutron scatter-
1137
ing with polarization analysis. The D17 spin filter is of high quality, consistently having high neutron polarization and transmission. The local filling technique is an efficient and relatively non-invasive method for guaranteeing the best conditions for measurement, and the techniques for collecting and treating data have been developed and shown to be correct. Future developments by the polarized 3 He group at the ILL will improve the option further, with potentially higher 3He polarization and improved local filling apparatus. The local filling technique also represents an important step towards steady-state 3He filters, where the filter transmission can be kept constant as a function of time [8]. References [1] [2] [3] [4] [5] [6] [7] [8]
R. Cubitt, G. Fragneto, Appl. Phys. A 74 (2002) S329. A. Petoukhov, et al., these proceedings. A. Petoukhov, et al., Nucl. Instr. and Meth. A 560 (2006) 480. R. Surkau, et al., Nucl. Instr. and Meth. A 384 (1997) 444. K.H. Andersen, et al., Physica B 356 (2005) 103. A.R. Wildes, Rev. Sci. Instrum. 70 (1999) 4241. A.R. Wildes, Neutron News, in press. D. Jullien, et al., in preparation.