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Polarized neutron reflectometry as a tool of investigation of surface and interface magnetism
Journal of Magnetism and Magnetic Materials 121 (1993) 112-115 North-Holland
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Polarized neutron reflectometry as a tool of investigation of surface and interface magnetism M. M~aza Laboratoire L~on Brillouin, Commissariat ?~l'Energie Atomique-Centre National de la Recherche Scientifique, Bat. 563, Centre d'Etudes Nucldaires de Saclay 91191, Gif-sur-Yvette, France This work is devoted to the use of grazing angle (unpolarized and polarized) neutron reflectometry for investigation of surface and interface phenomena. An example of a Ni-Ti multilayer is given to illustrate the possibilities of this technique and its sensitivity to the interracial diffusion and surface magnetism.
1. Introduction A range of phenomena analogous to those observed in classical optics are exhibited by cold neutrons such as refection, refraction and interference [1-6]. The optical interaction between slow neutrons and matter can be described by a refractive index which presents the same mathematical expression as that of X-rays. This refractive index is mainly related to the nuclear scattering length density and, hence, specular neutron reflection versus grazing angle or neutron waveCorrespondence to: Dr. M. M~aza, Laboratoire l_~on Brillouin, Commissariat ~ l'Energie Atomique-Centre National de la Recherche Scientifique, Bat 563, Centre d'Etudes Nucl6aires de Saclay 91191, Gif-sur-Yvette, France.
length can provide accurate information about the chemical composition of surface and interfaces. Due to the magnetic interaction of neutrons with the electron shell, magnetic materials have a neutron spin-dependent refractive index n + (spin up) and n - (spin down). This makes the polarized neutron refection sensitive to the surface magnetism with a spatial resolution that is on the nanometer scale [4]. In this paper, an example of a Ni-Ti multilayer studied by both unpolarized and polarized specular neutron reflectometry is given. This will allow us to illustrate the possibilities of this non-destructive technique in the study of surface and interface phenomena such as the interfacial diffusion and surface magnetism.
Spin Do wn
Ferromagnetic mirror
Spin ~p
Fig. 1. Neutron reflection on a magnetic bulk mirror device under a constant magnetic external field H. 0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
M. Maaza / Polarized neutron reflectometry
2. Theoretical background
the magnetization is constant are given by the Fresnel law [5]:
Technically, a cold neutron beam of de Broglie wavelength A is sent upon the sample surface at grazing angle 0 and is partially reflected. The reflection process is fully described in terms of the component of the neutron momentum perpendicular to the surface K z which is defined in vacuum as K z = [2re sin 0]/A. The reflectivity depends on the variation of n normal to the interface i n - - n ( z ) . For a magnetic and non-absorbing semi-infinite slab (magnetized in the plane of the reflecting surface), this refractive index is, thus, determined by both nuclear and magnetic densities; n is given by [7]: n += 1 - A2~4r[bn _+ C/Z]/2av,
(1)
where C is a constant (0.265 x 10-12/ZB/Cm) , ~4r is the number of scattering atoms per unit volume,/Z is the magnetic moment per atom, b n and C/x are the coherent nuclear and the magnetic scattering lengths respectively, b n varies randomly with different elements and isotopes and may be positive or negative [8]. This key property is exploited, particularly in the case of two neighbouring elements such as nickel and titanium for which b n is about 1.03 and - 0 . 3 4 x 10 -12 cm, respectively. The choice of sign, in eq. (1), is determined by whether the nuclear moment is aligned parallel [ + ] or antiparallel [ - ] to the applied external magnetic field. This equation shows that the refractive index is highly dispersive and in the case of a magnetised material (fig. 1), is neutron-spin dependent. This property is exploited in all optical neutron magnetic studies and applications [9,10]. Since b n may be of either sign, n+ may be larger or smaller than unity. In the latter case, total mirror reflection of neutrons may occur at momentum K z less than a critical angle K c+_ given closely by [1,4]: KC_+= 2f-(,n-M/'[ b n + C/z]).
113
(2)
During its propagation in the medium, the neutron wave behaves as an electromagnetic wave in S polarization state [7,11]. The spin reflectivities of a such magnetic semi-infinite slab in which
R + [ 0 / A ] = {sin 0 - f - [ n 2 - c o s 2 0 ] } /{sin 0 + ( - [ n 2 + - cos20]},
(3)
Generally, the studied samples are constituted by a stack of many layers, the reflectivity profile for such structure may be calculated using the optical matrix method for the reflection of light polarized perpendicular to the reflection plane [11]. In magnetic materials, the calculation of both spin up and spin down reflectivities R+ and R or the flipping ratio F[O/A] = [ R + / R -] as a function of K z = [2av sin 0]/A allows a determination of magnetic moment profile/Z of the considered sample.
3. Illustrative example Conceptually, there are two ways of performing the neutron reflectivity experiment [12]. The first is called 0 - 2 0 geometry in which the reflectivity of a monochromatic neutron beam is measured as function of the incident angle 0. The second is called the time-of-flight technique. In this case, the reflectivity of a fixed incidence angle beam is measured as a function of the neutron wavelength. The neutron wavelength of incident cold neutrons is analyzed using a multichannel time analyzer. This simulates an analogous experiment at a pulsed source. This type of technique is perfectly suited to measurements on liquid interfaces which cannot be inclined towards the incident beam. Moreover, the fixed sample geometry ensures a constant sample illumination. The measurement of the neutron reflectivity profile of a surface at K greater than the critical K c provides a sensitive chemical and magnetic probe through the nuclear and magnetic scattering length densities ~/Zbn and ~/zC/Z. To illustrate the difference between the nuclear and magnetic neutron reflections, a N i - T i multilayer constituted of 10 bilayers, deposited onto a borosilicate glass by magnetron dc was studied. The thickness of nickel and titanium layers mea-
114
M. M~aza / Polarized neutron reflectometry
C
~J~#,
I~;
"-,
°
Bragg peaks-
xoJ .......
•
Theor.
O.t,
o
SimuL
a
I 1.12
I 1.8/,
I 2.56
t 3.28
K=[2~r sinOo/,~] (lO-2A-1) K~
0.28
0.64
1
K=[2rc sinOo/).] (10-2~-1) Fig. 2. (a) Observed grazing angle unpolarized neutron reflectometry profiles (log R) of the 10 [400 ,~ Ni-400 A Ti]/borosilicate glass multilayer (o) versus the momentum K = 2~r sin 0/A and the corresponding theoretical (e) and simulated (,,, ) unl~larized neutron reflectivity profiles; (b) Observed grazing angle polarized neutron reflectometry profiles (log R) of the 10 [400 ~d~i-400 .~ Ti]/borosilicate glass multilayer versus the momentum K = 2"~ sin 0/A: spin up (o) and spin down (rn).
/the = r/the sured during the sputtering are r~Ni '~Ti = 400 ,~. To investigate the overall characteristics of this periodic stack, the superlattice is first scanned on the unpolarized neutron reflectometer D E S I R localized at the L6on Brillouin Laboratory at 0 0 = 2 . 4 x 10 -2 rad [13,14]. This reflectometer uses the time-of-flight technique and logarithmic reflectivity Log R is recorded versus K. Figure 2 shows a comparison of the theoretical, experimental and simulated neutron reflectivity profiles. Two main regions are observed: the total reflection plateau and the vitreous region.
Log R(K) displays many Bragg peaks in this last region. The spectral positions of these interference fringes provide a mean to check the multilayer period. In the simulation using a computer program based on Abel,s matrix method [5], the N i - T i and Ti-glass substrate interfaces are assumed to be perfectly sharp and without a mixing layer. A serie of simulations is therefore carried out, varying the layer thickness and the nuclear scattering length density of each layer until a reasonable fit is obtained. According to this hypothesis, the nickel and titanium layers can be considered to be nominally about dNisim_-- dTisim = 330 ,~. Thus, the simulated value of the multilayer period is then A s~ = 660 against Athe = 800 .~ which is the expected value. Taking into account the great discrepancy ([A t h e - Asim]/Athe= 17.5%), this suggests that some diffusion has taken place during the sputtering. The simulated nuclear scattering length densities are X b sim .= +9.4 X 10 6 ~ - 2 and ,///'bTi sim = - - 0 . 5 X 106 N/~ A -2 respectively. One can note that A/'b~m corresponds to pure Ni whereas JVb~ m is less negative than that of pure titanium. This confirms the diffusion process of nickel into titanium [14] underlined above. Concerning the region near the total reflection plateau, there is a significant difference between the experimental data and the simulated curve in the region 1.2 X 10 -2 .~-1 < K < 1.6 x 10 -2 .~-1. The simulation of this part of the experimental curve is consistent with the existence of a superficial layer on the top of the glass. The superficial layer thickness is of the order of o 650 A. If this layer exists really, it could be caused during the mechanical polishing of the borosilicate glass substrate [15]. For simulation of polarized neutron reflectivity profiles below, we will take into account this superficial layer. Moreover, the reflectivity of the experimental Bragg peak is smaller than that of simulated ones. This indicates that the interfaces are not perfect; the mean value of the interfacial roughness is of the order of 17 ,~ [16]. Secondly, polarized neutron measurements are carried out on the D17 reflectometer at Institut Laue-Langevin [17]. The spin up and down reflectivity profiles are performed by a 0 - 2 0 scan with a fixed neutron wavelength )t = 7.2 A [17].
M. M~aza / Polarized neutron reflectometry
Figure 2b shows the spin dependent neutron reflectivities measured under the saturation nickel magnetic field. As in the unpolarized configuration, many Bragg peaks are observed. According to the dispersion relation (eq. (1)), the spin up peaks are shifted relative to the down ones. Moreover, the spin up reflectivity becomes less than the spin down at the third-order Bragg position. This behaviour observed at KB(k= 3)--o 1 0.51 × 10 -2 A - , is in fact characteristic of an interfacial diffusion. This is most easily explained by assuming that a diffusion process takes place between the nickel and titanium layers [18]. This is in agreement with the previous unpolarized results. As indicated above, the critical parameters K~ and K~- are different; their average values are 0.31 and 0.27 x 10 -2 .~-a respectively. This indicates that nickel stays magnetic in this range of thickness and behaves as a bulk. This typical example Can be generalized for thin films, to check the smallest thickness for which the film stays magnetic. 4. Conclusion
First, the essential background of the grazing angle (polarized and unpolarized) neutron reflectometry technique was outlined. From an experimental point of view, this method provides a non-destructive and effective way to study surface and interface phenomena such as diffusion, chemical contamination process and magnetic changes. As the neutron refractive index is linked directly to magnetic moment per atom, this method allows the determination of absolute magnetic moment within magnetic films and its orientation relative to the external perturbation.
115
I am indebted to Dr. O. Sch~ierft from Institut Laue-Langevin for the polarized neutron reflectivity measurements.
References [1] V.F. Sears, in: Neutron Optics (Clarendon, Oxford, 1988) p. 36. [2] E. Fermi and W. Zinn, Phys. Rev. 70 (1946) 103. [3] H. Rauch and D. Petraschek, in: Topics in Current Physics, vol. 6 (Springer, Berlin, 1978) chap. 9. [4] G.P. Felcher, Phys. Rev. B 24 (1981) 1595. [5] J. Penfold and R.K. Thomas, J. Phys.: Condens. Mater. 2 (1980) 1369. [6] J. Penfold, Physica B 173 (1991) 1. [7] M.L. Goldberg and F. Seitz, Phys. Rev. 71 (1947) 295. [8] G.E. Bacon, Neutron Diffraction, 3rd ed. (Clarendon, Oxford, 1975) 31. [9] J. Hayter and H.A. Mook, J. Appl. Cryst. 22 (1989) 35. [10] G.P. Felcher, R. Kleb, D.G. Hinks and W.H. Potter, Phys. Rev. B 29 (1984) 4843. [11] M. Born and E. Wolf, in: Principles in Optics (Pergamon, Oxford, 1970) p. 243. [12] B. Farnoux, IAEA Proc. IAEA-CN-46/20, Jiilich (1985) 14. [13] B. Farnoux, Proc. Mat. Res. Soc. Symp., vol. 166 (1990) 95. [14] M. M~aza, to appear in Proc. Int. Fermi School on Industrial Applications of Neutrons, Lerici, Italy (22-30 June, 1991). [15] M. M~aza, C. Sella, B. Farnoux, F. Samuel and P. Trocellier, Proceedings in Physics 61, eds. H. Zabel, I.K. Robinson (Springer, Berlin, 1992) 195. [16] C. Sella, M. M~taza, M. Kfiabouchi, B. Pardo and F. Bridou, to appear in SPIE Proc. Conf. 1738, eds. C.F. Majkrzack and J.L. Wood (1992). [17] H.J. Lauter, J.A.C. Bland, R.D. Bateson and A.D. Johnson, Proceedings in Physics 61, eds. H. Zabel and I.K. Robinson (Springer, 1992) p. 219. [18] C.F. Majkrzack and G. Shirane, J. de Phys. 43 (1982) C7-215.
Ah"
Polarized neutron reflectometry as a tool of investigation of surface and interface magnetism M. M~aza Laboratoire L~on Brillouin, Commissariat ?~l'Energie Atomique-Centre National de la Recherche Scientifique, Bat. 563, Centre d'Etudes Nucldaires de Saclay 91191, Gif-sur-Yvette, France This work is devoted to the use of grazing angle (unpolarized and polarized) neutron reflectometry for investigation of surface and interface phenomena. An example of a Ni-Ti multilayer is given to illustrate the possibilities of this technique and its sensitivity to the interracial diffusion and surface magnetism.
1. Introduction A range of phenomena analogous to those observed in classical optics are exhibited by cold neutrons such as refection, refraction and interference [1-6]. The optical interaction between slow neutrons and matter can be described by a refractive index which presents the same mathematical expression as that of X-rays. This refractive index is mainly related to the nuclear scattering length density and, hence, specular neutron reflection versus grazing angle or neutron waveCorrespondence to: Dr. M. M~aza, Laboratoire l_~on Brillouin, Commissariat ~ l'Energie Atomique-Centre National de la Recherche Scientifique, Bat 563, Centre d'Etudes Nucl6aires de Saclay 91191, Gif-sur-Yvette, France.
length can provide accurate information about the chemical composition of surface and interfaces. Due to the magnetic interaction of neutrons with the electron shell, magnetic materials have a neutron spin-dependent refractive index n + (spin up) and n - (spin down). This makes the polarized neutron refection sensitive to the surface magnetism with a spatial resolution that is on the nanometer scale [4]. In this paper, an example of a Ni-Ti multilayer studied by both unpolarized and polarized specular neutron reflectometry is given. This will allow us to illustrate the possibilities of this non-destructive technique in the study of surface and interface phenomena such as the interfacial diffusion and surface magnetism.
Spin Do wn
Ferromagnetic mirror
Spin ~p
Fig. 1. Neutron reflection on a magnetic bulk mirror device under a constant magnetic external field H. 0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
M. Maaza / Polarized neutron reflectometry
2. Theoretical background
the magnetization is constant are given by the Fresnel law [5]:
Technically, a cold neutron beam of de Broglie wavelength A is sent upon the sample surface at grazing angle 0 and is partially reflected. The reflection process is fully described in terms of the component of the neutron momentum perpendicular to the surface K z which is defined in vacuum as K z = [2re sin 0]/A. The reflectivity depends on the variation of n normal to the interface i n - - n ( z ) . For a magnetic and non-absorbing semi-infinite slab (magnetized in the plane of the reflecting surface), this refractive index is, thus, determined by both nuclear and magnetic densities; n is given by [7]: n += 1 - A2~4r[bn _+ C/Z]/2av,
(1)
where C is a constant (0.265 x 10-12/ZB/Cm) , ~4r is the number of scattering atoms per unit volume,/Z is the magnetic moment per atom, b n and C/x are the coherent nuclear and the magnetic scattering lengths respectively, b n varies randomly with different elements and isotopes and may be positive or negative [8]. This key property is exploited, particularly in the case of two neighbouring elements such as nickel and titanium for which b n is about 1.03 and - 0 . 3 4 x 10 -12 cm, respectively. The choice of sign, in eq. (1), is determined by whether the nuclear moment is aligned parallel [ + ] or antiparallel [ - ] to the applied external magnetic field. This equation shows that the refractive index is highly dispersive and in the case of a magnetised material (fig. 1), is neutron-spin dependent. This property is exploited in all optical neutron magnetic studies and applications [9,10]. Since b n may be of either sign, n+ may be larger or smaller than unity. In the latter case, total mirror reflection of neutrons may occur at momentum K z less than a critical angle K c+_ given closely by [1,4]: KC_+= 2f-(,n-M/'[ b n + C/z]).
113
(2)
During its propagation in the medium, the neutron wave behaves as an electromagnetic wave in S polarization state [7,11]. The spin reflectivities of a such magnetic semi-infinite slab in which
R + [ 0 / A ] = {sin 0 - f - [ n 2 - c o s 2 0 ] } /{sin 0 + ( - [ n 2 + - cos20]},
(3)
Generally, the studied samples are constituted by a stack of many layers, the reflectivity profile for such structure may be calculated using the optical matrix method for the reflection of light polarized perpendicular to the reflection plane [11]. In magnetic materials, the calculation of both spin up and spin down reflectivities R+ and R or the flipping ratio F[O/A] = [ R + / R -] as a function of K z = [2av sin 0]/A allows a determination of magnetic moment profile/Z of the considered sample.
3. Illustrative example Conceptually, there are two ways of performing the neutron reflectivity experiment [12]. The first is called 0 - 2 0 geometry in which the reflectivity of a monochromatic neutron beam is measured as function of the incident angle 0. The second is called the time-of-flight technique. In this case, the reflectivity of a fixed incidence angle beam is measured as a function of the neutron wavelength. The neutron wavelength of incident cold neutrons is analyzed using a multichannel time analyzer. This simulates an analogous experiment at a pulsed source. This type of technique is perfectly suited to measurements on liquid interfaces which cannot be inclined towards the incident beam. Moreover, the fixed sample geometry ensures a constant sample illumination. The measurement of the neutron reflectivity profile of a surface at K greater than the critical K c provides a sensitive chemical and magnetic probe through the nuclear and magnetic scattering length densities ~/Zbn and ~/zC/Z. To illustrate the difference between the nuclear and magnetic neutron reflections, a N i - T i multilayer constituted of 10 bilayers, deposited onto a borosilicate glass by magnetron dc was studied. The thickness of nickel and titanium layers mea-
114
M. M~aza / Polarized neutron reflectometry
C
~J~#,
I~;
"-,
°
Bragg peaks-
xoJ .......
•
Theor.
O.t,
o
SimuL
a
I 1.12
I 1.8/,
I 2.56
t 3.28
K=[2~r sinOo/,~] (lO-2A-1) K~
0.28
0.64
1
K=[2rc sinOo/).] (10-2~-1) Fig. 2. (a) Observed grazing angle unpolarized neutron reflectometry profiles (log R) of the 10 [400 ,~ Ni-400 A Ti]/borosilicate glass multilayer (o) versus the momentum K = 2~r sin 0/A and the corresponding theoretical (e) and simulated (,,, ) unl~larized neutron reflectivity profiles; (b) Observed grazing angle polarized neutron reflectometry profiles (log R) of the 10 [400 ~d~i-400 .~ Ti]/borosilicate glass multilayer versus the momentum K = 2"~ sin 0/A: spin up (o) and spin down (rn).
/the = r/the sured during the sputtering are r~Ni '~Ti = 400 ,~. To investigate the overall characteristics of this periodic stack, the superlattice is first scanned on the unpolarized neutron reflectometer D E S I R localized at the L6on Brillouin Laboratory at 0 0 = 2 . 4 x 10 -2 rad [13,14]. This reflectometer uses the time-of-flight technique and logarithmic reflectivity Log R is recorded versus K. Figure 2 shows a comparison of the theoretical, experimental and simulated neutron reflectivity profiles. Two main regions are observed: the total reflection plateau and the vitreous region.
Log R(K) displays many Bragg peaks in this last region. The spectral positions of these interference fringes provide a mean to check the multilayer period. In the simulation using a computer program based on Abel,s matrix method [5], the N i - T i and Ti-glass substrate interfaces are assumed to be perfectly sharp and without a mixing layer. A serie of simulations is therefore carried out, varying the layer thickness and the nuclear scattering length density of each layer until a reasonable fit is obtained. According to this hypothesis, the nickel and titanium layers can be considered to be nominally about dNisim_-- dTisim = 330 ,~. Thus, the simulated value of the multilayer period is then A s~ = 660 against Athe = 800 .~ which is the expected value. Taking into account the great discrepancy ([A t h e - Asim]/Athe= 17.5%), this suggests that some diffusion has taken place during the sputtering. The simulated nuclear scattering length densities are X b sim .= +9.4 X 10 6 ~ - 2 and ,///'bTi sim = - - 0 . 5 X 106 N/~ A -2 respectively. One can note that A/'b~m corresponds to pure Ni whereas JVb~ m is less negative than that of pure titanium. This confirms the diffusion process of nickel into titanium [14] underlined above. Concerning the region near the total reflection plateau, there is a significant difference between the experimental data and the simulated curve in the region 1.2 X 10 -2 .~-1 < K < 1.6 x 10 -2 .~-1. The simulation of this part of the experimental curve is consistent with the existence of a superficial layer on the top of the glass. The superficial layer thickness is of the order of o 650 A. If this layer exists really, it could be caused during the mechanical polishing of the borosilicate glass substrate [15]. For simulation of polarized neutron reflectivity profiles below, we will take into account this superficial layer. Moreover, the reflectivity of the experimental Bragg peak is smaller than that of simulated ones. This indicates that the interfaces are not perfect; the mean value of the interfacial roughness is of the order of 17 ,~ [16]. Secondly, polarized neutron measurements are carried out on the D17 reflectometer at Institut Laue-Langevin [17]. The spin up and down reflectivity profiles are performed by a 0 - 2 0 scan with a fixed neutron wavelength )t = 7.2 A [17].
M. M~aza / Polarized neutron reflectometry
Figure 2b shows the spin dependent neutron reflectivities measured under the saturation nickel magnetic field. As in the unpolarized configuration, many Bragg peaks are observed. According to the dispersion relation (eq. (1)), the spin up peaks are shifted relative to the down ones. Moreover, the spin up reflectivity becomes less than the spin down at the third-order Bragg position. This behaviour observed at KB(k= 3)--o 1 0.51 × 10 -2 A - , is in fact characteristic of an interfacial diffusion. This is most easily explained by assuming that a diffusion process takes place between the nickel and titanium layers [18]. This is in agreement with the previous unpolarized results. As indicated above, the critical parameters K~ and K~- are different; their average values are 0.31 and 0.27 x 10 -2 .~-a respectively. This indicates that nickel stays magnetic in this range of thickness and behaves as a bulk. This typical example Can be generalized for thin films, to check the smallest thickness for which the film stays magnetic. 4. Conclusion
First, the essential background of the grazing angle (polarized and unpolarized) neutron reflectometry technique was outlined. From an experimental point of view, this method provides a non-destructive and effective way to study surface and interface phenomena such as diffusion, chemical contamination process and magnetic changes. As the neutron refractive index is linked directly to magnetic moment per atom, this method allows the determination of absolute magnetic moment within magnetic films and its orientation relative to the external perturbation.
115
I am indebted to Dr. O. Sch~ierft from Institut Laue-Langevin for the polarized neutron reflectivity measurements.
References [1] V.F. Sears, in: Neutron Optics (Clarendon, Oxford, 1988) p. 36. [2] E. Fermi and W. Zinn, Phys. Rev. 70 (1946) 103. [3] H. Rauch and D. Petraschek, in: Topics in Current Physics, vol. 6 (Springer, Berlin, 1978) chap. 9. [4] G.P. Felcher, Phys. Rev. B 24 (1981) 1595. [5] J. Penfold and R.K. Thomas, J. Phys.: Condens. Mater. 2 (1980) 1369. [6] J. Penfold, Physica B 173 (1991) 1. [7] M.L. Goldberg and F. Seitz, Phys. Rev. 71 (1947) 295. [8] G.E. Bacon, Neutron Diffraction, 3rd ed. (Clarendon, Oxford, 1975) 31. [9] J. Hayter and H.A. Mook, J. Appl. Cryst. 22 (1989) 35. [10] G.P. Felcher, R. Kleb, D.G. Hinks and W.H. Potter, Phys. Rev. B 29 (1984) 4843. [11] M. Born and E. Wolf, in: Principles in Optics (Pergamon, Oxford, 1970) p. 243. [12] B. Farnoux, IAEA Proc. IAEA-CN-46/20, Jiilich (1985) 14. [13] B. Farnoux, Proc. Mat. Res. Soc. Symp., vol. 166 (1990) 95. [14] M. M~aza, to appear in Proc. Int. Fermi School on Industrial Applications of Neutrons, Lerici, Italy (22-30 June, 1991). [15] M. M~aza, C. Sella, B. Farnoux, F. Samuel and P. Trocellier, Proceedings in Physics 61, eds. H. Zabel, I.K. Robinson (Springer, Berlin, 1992) 195. [16] C. Sella, M. M~taza, M. Kfiabouchi, B. Pardo and F. Bridou, to appear in SPIE Proc. Conf. 1738, eds. C.F. Majkrzack and J.L. Wood (1992). [17] H.J. Lauter, J.A.C. Bland, R.D. Bateson and A.D. Johnson, Proceedings in Physics 61, eds. H. Zabel and I.K. Robinson (Springer, 1992) p. 219. [18] C.F. Majkrzack and G. Shirane, J. de Phys. 43 (1982) C7-215.