- Email: [email protected]
A new technique for polarized neutron diffraction
Physica B 267}268 (1999) 341}343
A new technique for polarized neutron di!raction S.-H. Lee *, C.F. Majkrzak NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA University of Maryland, College Park, MD 20742, USA
Abstract We have developed a new technique for polarized neutron di!raction utilizing a two-dimensional, area-sensitive detector. An area-sensitive detector, used in conjunction with a transmission polarizer in the scattered beam, allows us to simultaneously measure non-spin-#ip (NSF) and spin-#ip (SF) scattering processes and therefore increases the data collection rate by a factor of two. 1999 Elsevier Science B.V. All rights reserved. Keywords: Polarized neutron di!raction; Area-sensitive detector
Polarized neutron di!raction is a well-established probe for the investigation of the spin con"guration of a system [1}3]. It can distinguish magnetic and nuclear structural contributions because of di!erent selection rules for non-spin-#ip (NSF) and spin-#ip (SF) scattering processes. Fig. 1 shows a conventional geometry for polarized neutron di!raction. A monochromatic beam is polarized in one-spin eigenstate (!) by the forward transmission polarizer and can be subsequently rotated adiabatically to the other spin state (#) by a #ipper. The spin state of the beam scattered from the sample is then analyzed with the rear #ipper and polarizer combination. Each of four possible channels, (o!, o!), (on, on), (o!, on) and (on, o!), is measured by turning the front and rear #ippers on or o!. If the instrumental e$ciencies of the forward and rear polarizers and #ippers were perfect, then * Corresponding address: NIST Center for Neutron Research, NIST, Gaithersburg, MD 20899, USA. Tel.: #1-301-975-4257; fax: #1-301-921-9847; e-mail: [email protected].
the measured intensities would correspond to the spin-dependent cross sections, p, one-to-one: I(o!, o! )Pp(!!); I(on, on)Pp(##); I(o+, on)Pp(!#); and I(on, o! )Pp(#!) [4}6]. In this technique, each channel is measured sequentially, which signi"cantly reduces the rate at which data can be collected at a given sample scattering angle. In this paper, we report a new technique for polarized neutron di!raction utilizing a two-dimensional area-sensitive detector, which increases the data rate by a factor of two. The idea is simple: since the transmission polarizer passes neutrons with one-spin state (!), straight through and undeviated, but re#ects neutrons with the other spin state (#) by a few degrees, then if we place a twodimensional area sensitive detector (ASD) in a two-axis mode as shown in Fig. 2, we can measure neutrons scattered by the sample into both spin states at the same time. The separation of the re#ected beam from the transmitted beam on the ASD, D, is determined by the distance from the polarizer
0921-4526/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 0 4 0 - X
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S.-H. Lee, C.F. Majkrzak / Physica B 267}268 (1999) 341}343
Fig. 2. Experimental setup for polarized neutron di!raction measurements with an area-sensitive detector (ASD). Note that in this setup we need only one #ipper to measure all four channels (!!), (!#), (#!) and (##). Fig. 1. Experimental setup for conventional polarized neutron di!raction measurements with transmission polarizers.
to the ASD, D, and the re#ection angle, h which depends on the wavelength of the neutron, j: D&h ) D. D should be su$ciently smaller than the width of the beam. In our case, the polarizer was optimized for a j of 4 As and longer. To measure all four spin-dependent processes, this technique requires only a single #ipper located in front of or upstream of the sample. For instance, if the #ipper is placed in the incident beam as shown in Fig. 2, when the front #ipper is o!, the transmitted beam measures the (o!, o!) channel, and the re#ected beam the (o!, on) channel. In this case, the ASD measures both (o!, o!) and (o!, on) channels simultaneously, thereby eliminating the need for the rear #ipper used in the conventional set-up. When the front #ipper is on, the transmitted beam
corresponds to (on, o! ), and the re#ected beam to (on, on). To illustrate how well this technique works, we show in Fig. 3 the data for a nuclear Bragg re#ection in La Sr NiO [7,8], which were obtained on the spin polarized inelastic neutron scattering (SPINS) spectrometer at the NIST Center for Neutron Research. Details of experimental con"gurations are described in Ref. [9]. Fig. 3 shows the horizontal dependence of the ASD intensities which are summed vertically. The circles are the data obtained with the front #ipper in the `o! a state and the squares are obtained with the #ipper in the `ona state. When the #ipper is o!, the stronger peak should correspond to the (!!) NSF channel, which is the transmitted beam. For this purely nuclear peak, we should expect no scattering in the SF channel. The weaker peak at the left is contamination due to imperfect polarizing
S.-H. Lee, C.F. Majkrzak / Physica B 267}268 (1999) 341}343
343
However, for other systems, such as a ferromagnetic one, in which the interference between nuclear and magnetic scattering is present in the NSF processes, we should correct the data for the re#ectivity. However, if we put another #ipper in the scattered beam and measure all four channels, then the four spin-dependent cross sections can be evaluated without explicitly correcting for the polarizer re#ectivity [4]. In summary, this new technique provides us with a better opportunity to investigate the magnetic nature of a system with weak signals. Fig. 3. The horizontal dependence of intensities of Fig. 2 integrated along the vertical pixels of the ASD. The circles are the data obtained with the front #ipper o! and the squares are the data with the front #ipper on. Solid lines are Gaussian "ts.
e$ciency, the possibility of which was mentioned above. The e!ective e$ciency was determined to be 0.790(1) using the top "ve points each of the transmitted and re#ected beams. When the front #ipper is on, as shown in squares, the positions of the stronger and the weaker peaks are switched. The stronger peak is now the (on, on) channel. The (on, on) peak is weaker than the (o!, o! ) peak, which indicates that the re#ectivity of the polarizer is less than one or 0.798(1). For collinear antiferromagnetic systems such as La Sr NiO in which the NSF scattering cross sections for a magnetic Bragg re#ection are identical to each other, as are the SF cross sections, the polarizer re#ectivity cancels out and the (o!, o! ) and (on, on) signals can be combined as can the (o!, on) and (on, o! ) if the e$ciency of the #ipper can be taken as unity, which is a good approximation in the present example.
The e$ciency of our #ipper was measured to be 1.000(5).
Acknowledgements Work at SPINS is based upon activities supported by the National Science Foundation under Agreement No. DMR-9423101.
References [1] G.E. Bacon, Neutron Di!raction, Clarendon Press, Oxford, 1975. [2] S.W. Lovesey, Theory of Neutron Scattering from Condensed Matter, Clarendon Press, Oxford, 1984. [3] R.M. Moon, T. Riste, W.C. Koehler, Phys. Rev. 181 (1969) 920. [4] C.F. Majkrzak, Physica B 221 (1996) 342. [5] C.F. Majkrzak et al., in: C.F. Majkrzak, J. Wood (Eds.), SPIE Proceedings, vol. 90, SPIE, Bellingham, WA, 1992. [6] C.F. Majkrzak, Physica B 213 & 214 (1995) 904. [7] C.H. Chen, S-W. Cheong, Phys. Rev. Lett. 67 (1991) 1791. [8] S.-H. Lee, S-W. Cheong, Phys. Rev. Lett. 79 (1997) 2514. [9] S.-H. Lee, C.F. Majkrzak, J. Neutron Res. (1998), submitted for publication.
A new technique for polarized neutron di!raction S.-H. Lee *, C.F. Majkrzak NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA University of Maryland, College Park, MD 20742, USA
Abstract We have developed a new technique for polarized neutron di!raction utilizing a two-dimensional, area-sensitive detector. An area-sensitive detector, used in conjunction with a transmission polarizer in the scattered beam, allows us to simultaneously measure non-spin-#ip (NSF) and spin-#ip (SF) scattering processes and therefore increases the data collection rate by a factor of two. 1999 Elsevier Science B.V. All rights reserved. Keywords: Polarized neutron di!raction; Area-sensitive detector
Polarized neutron di!raction is a well-established probe for the investigation of the spin con"guration of a system [1}3]. It can distinguish magnetic and nuclear structural contributions because of di!erent selection rules for non-spin-#ip (NSF) and spin-#ip (SF) scattering processes. Fig. 1 shows a conventional geometry for polarized neutron di!raction. A monochromatic beam is polarized in one-spin eigenstate (!) by the forward transmission polarizer and can be subsequently rotated adiabatically to the other spin state (#) by a #ipper. The spin state of the beam scattered from the sample is then analyzed with the rear #ipper and polarizer combination. Each of four possible channels, (o!, o!), (on, on), (o!, on) and (on, o!), is measured by turning the front and rear #ippers on or o!. If the instrumental e$ciencies of the forward and rear polarizers and #ippers were perfect, then * Corresponding address: NIST Center for Neutron Research, NIST, Gaithersburg, MD 20899, USA. Tel.: #1-301-975-4257; fax: #1-301-921-9847; e-mail: [email protected].
the measured intensities would correspond to the spin-dependent cross sections, p, one-to-one: I(o!, o! )Pp(!!); I(on, on)Pp(##); I(o+, on)Pp(!#); and I(on, o! )Pp(#!) [4}6]. In this technique, each channel is measured sequentially, which signi"cantly reduces the rate at which data can be collected at a given sample scattering angle. In this paper, we report a new technique for polarized neutron di!raction utilizing a two-dimensional area-sensitive detector, which increases the data rate by a factor of two. The idea is simple: since the transmission polarizer passes neutrons with one-spin state (!), straight through and undeviated, but re#ects neutrons with the other spin state (#) by a few degrees, then if we place a twodimensional area sensitive detector (ASD) in a two-axis mode as shown in Fig. 2, we can measure neutrons scattered by the sample into both spin states at the same time. The separation of the re#ected beam from the transmitted beam on the ASD, D, is determined by the distance from the polarizer
0921-4526/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 0 4 0 - X
342
S.-H. Lee, C.F. Majkrzak / Physica B 267}268 (1999) 341}343
Fig. 2. Experimental setup for polarized neutron di!raction measurements with an area-sensitive detector (ASD). Note that in this setup we need only one #ipper to measure all four channels (!!), (!#), (#!) and (##). Fig. 1. Experimental setup for conventional polarized neutron di!raction measurements with transmission polarizers.
to the ASD, D, and the re#ection angle, h which depends on the wavelength of the neutron, j: D&h ) D. D should be su$ciently smaller than the width of the beam. In our case, the polarizer was optimized for a j of 4 As and longer. To measure all four spin-dependent processes, this technique requires only a single #ipper located in front of or upstream of the sample. For instance, if the #ipper is placed in the incident beam as shown in Fig. 2, when the front #ipper is o!, the transmitted beam measures the (o!, o!) channel, and the re#ected beam the (o!, on) channel. In this case, the ASD measures both (o!, o!) and (o!, on) channels simultaneously, thereby eliminating the need for the rear #ipper used in the conventional set-up. When the front #ipper is on, the transmitted beam
corresponds to (on, o! ), and the re#ected beam to (on, on). To illustrate how well this technique works, we show in Fig. 3 the data for a nuclear Bragg re#ection in La Sr NiO [7,8], which were obtained on the spin polarized inelastic neutron scattering (SPINS) spectrometer at the NIST Center for Neutron Research. Details of experimental con"gurations are described in Ref. [9]. Fig. 3 shows the horizontal dependence of the ASD intensities which are summed vertically. The circles are the data obtained with the front #ipper in the `o! a state and the squares are obtained with the #ipper in the `ona state. When the #ipper is o!, the stronger peak should correspond to the (!!) NSF channel, which is the transmitted beam. For this purely nuclear peak, we should expect no scattering in the SF channel. The weaker peak at the left is contamination due to imperfect polarizing
S.-H. Lee, C.F. Majkrzak / Physica B 267}268 (1999) 341}343
343
However, for other systems, such as a ferromagnetic one, in which the interference between nuclear and magnetic scattering is present in the NSF processes, we should correct the data for the re#ectivity. However, if we put another #ipper in the scattered beam and measure all four channels, then the four spin-dependent cross sections can be evaluated without explicitly correcting for the polarizer re#ectivity [4]. In summary, this new technique provides us with a better opportunity to investigate the magnetic nature of a system with weak signals. Fig. 3. The horizontal dependence of intensities of Fig. 2 integrated along the vertical pixels of the ASD. The circles are the data obtained with the front #ipper o! and the squares are the data with the front #ipper on. Solid lines are Gaussian "ts.
e$ciency, the possibility of which was mentioned above. The e!ective e$ciency was determined to be 0.790(1) using the top "ve points each of the transmitted and re#ected beams. When the front #ipper is on, as shown in squares, the positions of the stronger and the weaker peaks are switched. The stronger peak is now the (on, on) channel. The (on, on) peak is weaker than the (o!, o! ) peak, which indicates that the re#ectivity of the polarizer is less than one or 0.798(1). For collinear antiferromagnetic systems such as La Sr NiO in which the NSF scattering cross sections for a magnetic Bragg re#ection are identical to each other, as are the SF cross sections, the polarizer re#ectivity cancels out and the (o!, o! ) and (on, on) signals can be combined as can the (o!, on) and (on, o! ) if the e$ciency of the #ipper can be taken as unity, which is a good approximation in the present example.
The e$ciency of our #ipper was measured to be 1.000(5).
Acknowledgements Work at SPINS is based upon activities supported by the National Science Foundation under Agreement No. DMR-9423101.
References [1] G.E. Bacon, Neutron Di!raction, Clarendon Press, Oxford, 1975. [2] S.W. Lovesey, Theory of Neutron Scattering from Condensed Matter, Clarendon Press, Oxford, 1984. [3] R.M. Moon, T. Riste, W.C. Koehler, Phys. Rev. 181 (1969) 920. [4] C.F. Majkrzak, Physica B 221 (1996) 342. [5] C.F. Majkrzak et al., in: C.F. Majkrzak, J. Wood (Eds.), SPIE Proceedings, vol. 90, SPIE, Bellingham, WA, 1992. [6] C.F. Majkrzak, Physica B 213 & 214 (1995) 904. [7] C.H. Chen, S-W. Cheong, Phys. Rev. Lett. 67 (1991) 1791. [8] S.-H. Lee, S-W. Cheong, Phys. Rev. Lett. 79 (1997) 2514. [9] S.-H. Lee, C.F. Majkrzak, J. Neutron Res. (1998), submitted for publication.