- Email: [email protected]
Si multilayer
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 529 (2004) 94–97
Crystallization and magnetic properties of FeCo/Si multilayer S.J. Choa,*, H.Y. Kanga, C.H. Leea, Y.J. Kima, Thomas Kristb a
Korea Atomic Energy Institute, P.O.Box 105, Yuseong, Daejeon 305-600, Republic of Korea b Hahn-Meitner-Institut Berlin, Glienicker Str. 100, Berlin 14109, Germany
Abstract Triode sputtering is used to fabricate polarizing monochromators and supermirrors for neutrons with high quality. X-ray, polarized neutron scattering, in situ ellipsometry and SQUID are used to investigate the growth of sputtered FeCo–Si multilayer to determine the crystallization and its influence on magnetic properties. The surface energy during the layer deposition is found to have the most important influence on crystallinity. High energy on the substrate achieved with argon bombardment at a substrate bias potential of 70 V produces a pronounced crystallinity of Fe89Co11 layers and facilitates chemical bond in the interface. On the contrary low energy on substrate under bias potential +100 V and floating results in low crystallinity and no chemical bond. In a XRD study on monochromators deposited at a bias potential of 70 V, the (1 1 0) plane of Fe89Co11 is observed with nearly hundredfold intensity compared to the one with bias potential of +100 V or floating potential. In addition, the magnetic properties of the monochromators are influenced with the bias voltage as well. The minus bias potential leads to high coercivity but reduced remanence compared with floating. The monochromator with a positive bias potential of +100 V shows distinct magnetic anisotropy and in its soft direction a magnetic remanence of 94%. r 2004 Elsevier B.V. All rights reserved. PACS: 75.30.Gw; 75.70.-i; 75.70.Cn; 75.60.Ej Keywords: Neutron mirror; Remanent neutron mirror; Polarizing neutron
1. Introduction Following the invention of neutron supermirrors by F. Mezei [1] a large variety of these systems have been built, among them are polarizing supermirrors with Fe89Co11–Si multilayers [2]. Remanent supermirrors have recently been devel-
*Corresponding author. Tel.: +82-42-868-2914; fax: +8242-868-2080. E-mail address: [email protected] (S.J. Cho).
oped in several laboratories [3,4], because they require no field after being saturated. We found that a magnetically anisotropic layered structure is achieved also by choosing production parameters, especially a bias potential, which has an influence on the layer crystallinity. To study the processes during the growth of the Fe89Co11, the crystal formation depending upon the bias voltage was examined with XRD, and we attempted to manufacture an amorphous Fe89Co11 layer with a bias voltage. The magnetization of the s- and p-direction dependency of the
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.04.185
ARTICLE IN PRESS S.J. Cho et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 94–97
crystallization rate, which is varied with different bias voltages, is studied with a SQUID magnetometer and a polarized neutron reflectometer.
95
Table 1 Intensities for the (1 1 0)-reflection of Fe89Co11 depending on bias potentials Intensities (1 1 0)
2. Experimental The layers were produced in the triode sputtering machine at BENSC, Hahn-Meitner-Institut Berlin [5] where the substrates were fixed to a turnable substrate holder between two static targets. The model system used here was a monochromator with 10 bilayers of nominally ( Si and 120 A ( Fe89Co11 covered with a final 120 A ( layer of 120 A Si to prevent oxidation of the Fe89Co11 layer. It was grown on a float glass at a sputtering pressure of 1.3 10 3 mbar and a sputtering power of 0.25 kW. Measurements with polarized neutrons were done on the neutron reflectometer V14 at BENSC, ( using a neutron beam with a wavelength of 4.72 A, a polarization of 97%, and a divergence of 0.02 . The typical cross section was 5 0.4 mm2. A Mezei type flipper was used to reverse the spin state. Two He3 detectors were employed to simultaneously measure the beam reflected from and transmitted through a sample. For each sample, the hysteresis loops were measured at room temperature (300 K) with a SQUID magnetometer in two directions orthogonal to each other. The two directions are indicated with ‘‘p’’ and ‘‘s’’. The orientation ‘‘p’’ is parallel to the rotation axis of the sample holder and ‘‘s’’ perpendicular to it. For the measurement with SQUID, the samples are cut into pieces of 3.5 3.5 mm2 with a diamond saw.
3. Results and discussion 3.1. Crystallization of the FeCo for various DC bias values For the normal sputtering process, the substrate holder is not grounded and it has the same potential as the plasma. Applying a negative DC bias to the substrate, the surface of the growing film is bombarded by argon ions, while with the positive DC bias by electrons. Both kinds of bias
+100 V Floating +35 V 0V 70 V
53.4 230.8 406.2 2409.4 3798.0
potential have an influence on the crystallization of the FeCo layers. The XRD study shows the high (1 1 0)-reflection intensities of the sample with the bias voltage 70 and 0 V relative to the sample with ‘floating’. A negative potential against the plasma leads to an increase of the energy which facilitates the growth of crystallites through the collisions of the argon ions and the layer atoms. On the contrary, for the sample manufactured with a bias of +100 V the (1 1 0)-reflection is hardly observable, because the electrons found near the substrate, colliding with the layer atoms, can disturb the layer crystal formation. The intensities of the (1 1 0)-reflection for the different bias potentials are calculated according to the formula of the modified Lorentzmodel and shown in Table 1. 3.2. Magnetic characteristics with different DC bias voltages The influence of DC bias on the magnetic characteristics of the layers is determined for 4 different monochromators formed by 10 bilayers ( Si and 120 A ( Fe89Co11 covered by a final of 120 A ( Si. Fig. 1, for the monochromator layer of 120 A fabricated with a positive potential of +100 V, shows a clear anisotropy of the magnetization. The electron bombardment at the substrate through the positive bias potential causes an increase of the remanence of about approximately 8% in comparison with the monochromator manufactured with floating potential. An amorphous phase was found in the samples manufactured with the bias potential of +100 V in the XRD study regarding the crystallization. Apparently, the crystallization
ARTICLE IN PRESS S.J. Cho et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 94–97
96 100
3500
100
Neutronreflexion in p direction 3000
spin up with 300G spin down with 300G spin up with 10G spin down with 10G
80
60
60
40
40
20
20
2500
Reflectivity
Remanence S Remanence P Coercivity S Coercivity P
Remanence[%]
Coercivity[Gauss]
80
2000 1500 1000 500
Bias -70V
Bias 0V
Floating
Bias +100V 0.2
Bias voltage
0.4
0.6
θ
0.8
Neutronreflexion in s direction 3000 spin up with 300G spin down with 300G spin up with 10G spin down with 10G
Reflectivity
2500 2000 1500 1000 500
0.2
3.3. Neutron reflectometer measurements of the multilayer with different magnetic fields The magnetic anisotropy, which was determined with the SQUID, was also studied with the polarized neutron reflectometer. For this the measurements were recorded for the different magnetic fields (300 and 10 G). It is expected that the magnetic scattering length of FeCo-alloy with a full magnetization amounts to 6.28 fm [6]. If the magnetization decreases, it reduces the magnetic scattering length. The monochromators manufactured with both bias voltages of –70 and +100 V were examined. The reflection of the sample manufactured with the bias of –70 V show only a slight difference between both the directions of s and p for the dependence of the magnetic field (Fig. 2). For a magnetic field of 10 G, the reflections of the spin up neutron-component
1.2
3500
Fig. 1. The coercivity and remanence in dependence of the bias potential: in each case both magnetic axes (s and p) in layer plane were plotted. The orientation ‘‘p’’ is parallel to the rotation axis of the sample holder and ‘‘s’’ perpendicular to it.
of this sample occurs very slowly, thereby an anisotropy of the magnetization is facilitated. For samples manufactured with negative bias between 0 and 70 V, the coercivity increases, but the remanence slightly reduces in comparison to the sample with floating potential. In Fig. 1 the magnetic characteristics are represented as a function of the bias potential.
1.0
0.4
0.6
θ
0.8
1.0
1.2
Fig. 2. Neutron reflection profile for the monochromators manufactured with bias 70 V.
show a slight decrease of approximately 3% for both the directions of s and p, and those of a spin down component show a slight increase. Additionally, the critical angle of the spin down component rises by about 0.05 . This result corresponds to the magnetic characteristics, which were determined through the SQUID-measurement for both the directions of s and p. In the case of the sample manufactured with a bias of +100 V, an apparent difference between the reflections of the s- and p-direction is observed (Fig. 3). In the measurement with 10 G, the critical angle for the total reflection and the reflectivity of the Bragg peak shows an obvious reduction in the
ARTICLE IN PRESS S.J. Cho et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 94–97
4. Conclusion
3500
Neutronreflexion in p direction 3000
spin up with 300G spin down with 300G spin up with10G spin down with 10G
Reflektivity
2500 2000 1500 1000 500
0.2
0.4
0.6
θ
0.8
1.0
1.2
3500
Neutronreflexion in s direction 3000
spin up with 300G spin down with 300G spin up with 10G spin down with 10G
Reflectivity
2500 2000 1500 1000 500
0.2
97
By employing X-ray and polarized neutron scattering and SQUID-magnetometer to investigate the growth of sputtered Fe89Co11–Si multilayer, it was found that the crystallization of the FeCo alloy is influenced by the substrate surface energy. The sample manufactured with a bias of +100 V shows a very weak crystallization, which causes the magnetic anisotropy and an increase of the remanence by 8% with the SQUID magnetometer, while no difference is found for the samples with a 0 and 70 V. In order to examine this magnetic characteristic with the polarizing neutron reflectometer measured under a magnetic field of 300 and 10 G. The sample with a bias of +100 V shows clearly the anisotropy in two directions (p and s) in the neutron measurement as well. It appears that with this sputtering condition of the bias potential of +100 V the remanence polarizing neutron monochromator can be fabricated. This fact provides us with the possibility of making a remanent polarizing neutron supermirror.
References 0.4
0.6
θ
0.8
1.0
1.2
Fig. 3. Neutron reflection profile for the monochromators manufactured with bias +100 V.
p direction in comparison to the measurement for a magnetic field of 300 G, while no difference is observed in the s direction for both magnetic fields.
[1] F. Mezei, Commun. Phys. 1 (1976) 81; F. Mezei, P. Dagleish, Commun. Phys. 2 (1977) 41. [2] Th. Krist, C. Pappas, Th. Keller, F. Mezei, Phys. B 213 & 214 (1995) 939. [3] N.K. Pleshanov, Phys. B 297 (2001) 131. [4] N.K. Pleshanov, V. Bodnarchuk, R. Gaehler, D.A. Korneev, A. Menelle, S.V. Metelev, V.M. Pusenkov, A.F. Schebetov, V.A. Ul’yanov, Phys. B 297 (2001) 126. [5] D. Clemens, Th. Krist, P. Schubert-Bischoff, J. Hoffmann, F. Mezei, Phys. Scr. 50 (1994) 195. [6] D.J. Mueller, Diploma Thesis, TU Berlin, 1993, unpublished.
Nuclear Instruments and Methods in Physics Research A 529 (2004) 94–97
Crystallization and magnetic properties of FeCo/Si multilayer S.J. Choa,*, H.Y. Kanga, C.H. Leea, Y.J. Kima, Thomas Kristb a
Korea Atomic Energy Institute, P.O.Box 105, Yuseong, Daejeon 305-600, Republic of Korea b Hahn-Meitner-Institut Berlin, Glienicker Str. 100, Berlin 14109, Germany
Abstract Triode sputtering is used to fabricate polarizing monochromators and supermirrors for neutrons with high quality. X-ray, polarized neutron scattering, in situ ellipsometry and SQUID are used to investigate the growth of sputtered FeCo–Si multilayer to determine the crystallization and its influence on magnetic properties. The surface energy during the layer deposition is found to have the most important influence on crystallinity. High energy on the substrate achieved with argon bombardment at a substrate bias potential of 70 V produces a pronounced crystallinity of Fe89Co11 layers and facilitates chemical bond in the interface. On the contrary low energy on substrate under bias potential +100 V and floating results in low crystallinity and no chemical bond. In a XRD study on monochromators deposited at a bias potential of 70 V, the (1 1 0) plane of Fe89Co11 is observed with nearly hundredfold intensity compared to the one with bias potential of +100 V or floating potential. In addition, the magnetic properties of the monochromators are influenced with the bias voltage as well. The minus bias potential leads to high coercivity but reduced remanence compared with floating. The monochromator with a positive bias potential of +100 V shows distinct magnetic anisotropy and in its soft direction a magnetic remanence of 94%. r 2004 Elsevier B.V. All rights reserved. PACS: 75.30.Gw; 75.70.-i; 75.70.Cn; 75.60.Ej Keywords: Neutron mirror; Remanent neutron mirror; Polarizing neutron
1. Introduction Following the invention of neutron supermirrors by F. Mezei [1] a large variety of these systems have been built, among them are polarizing supermirrors with Fe89Co11–Si multilayers [2]. Remanent supermirrors have recently been devel-
*Corresponding author. Tel.: +82-42-868-2914; fax: +8242-868-2080. E-mail address: [email protected] (S.J. Cho).
oped in several laboratories [3,4], because they require no field after being saturated. We found that a magnetically anisotropic layered structure is achieved also by choosing production parameters, especially a bias potential, which has an influence on the layer crystallinity. To study the processes during the growth of the Fe89Co11, the crystal formation depending upon the bias voltage was examined with XRD, and we attempted to manufacture an amorphous Fe89Co11 layer with a bias voltage. The magnetization of the s- and p-direction dependency of the
0168-9002/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2004.04.185
ARTICLE IN PRESS S.J. Cho et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 94–97
crystallization rate, which is varied with different bias voltages, is studied with a SQUID magnetometer and a polarized neutron reflectometer.
95
Table 1 Intensities for the (1 1 0)-reflection of Fe89Co11 depending on bias potentials Intensities (1 1 0)
2. Experimental The layers were produced in the triode sputtering machine at BENSC, Hahn-Meitner-Institut Berlin [5] where the substrates were fixed to a turnable substrate holder between two static targets. The model system used here was a monochromator with 10 bilayers of nominally ( Si and 120 A ( Fe89Co11 covered with a final 120 A ( layer of 120 A Si to prevent oxidation of the Fe89Co11 layer. It was grown on a float glass at a sputtering pressure of 1.3 10 3 mbar and a sputtering power of 0.25 kW. Measurements with polarized neutrons were done on the neutron reflectometer V14 at BENSC, ( using a neutron beam with a wavelength of 4.72 A, a polarization of 97%, and a divergence of 0.02 . The typical cross section was 5 0.4 mm2. A Mezei type flipper was used to reverse the spin state. Two He3 detectors were employed to simultaneously measure the beam reflected from and transmitted through a sample. For each sample, the hysteresis loops were measured at room temperature (300 K) with a SQUID magnetometer in two directions orthogonal to each other. The two directions are indicated with ‘‘p’’ and ‘‘s’’. The orientation ‘‘p’’ is parallel to the rotation axis of the sample holder and ‘‘s’’ perpendicular to it. For the measurement with SQUID, the samples are cut into pieces of 3.5 3.5 mm2 with a diamond saw.
3. Results and discussion 3.1. Crystallization of the FeCo for various DC bias values For the normal sputtering process, the substrate holder is not grounded and it has the same potential as the plasma. Applying a negative DC bias to the substrate, the surface of the growing film is bombarded by argon ions, while with the positive DC bias by electrons. Both kinds of bias
+100 V Floating +35 V 0V 70 V
53.4 230.8 406.2 2409.4 3798.0
potential have an influence on the crystallization of the FeCo layers. The XRD study shows the high (1 1 0)-reflection intensities of the sample with the bias voltage 70 and 0 V relative to the sample with ‘floating’. A negative potential against the plasma leads to an increase of the energy which facilitates the growth of crystallites through the collisions of the argon ions and the layer atoms. On the contrary, for the sample manufactured with a bias of +100 V the (1 1 0)-reflection is hardly observable, because the electrons found near the substrate, colliding with the layer atoms, can disturb the layer crystal formation. The intensities of the (1 1 0)-reflection for the different bias potentials are calculated according to the formula of the modified Lorentzmodel and shown in Table 1. 3.2. Magnetic characteristics with different DC bias voltages The influence of DC bias on the magnetic characteristics of the layers is determined for 4 different monochromators formed by 10 bilayers ( Si and 120 A ( Fe89Co11 covered by a final of 120 A ( Si. Fig. 1, for the monochromator layer of 120 A fabricated with a positive potential of +100 V, shows a clear anisotropy of the magnetization. The electron bombardment at the substrate through the positive bias potential causes an increase of the remanence of about approximately 8% in comparison with the monochromator manufactured with floating potential. An amorphous phase was found in the samples manufactured with the bias potential of +100 V in the XRD study regarding the crystallization. Apparently, the crystallization
ARTICLE IN PRESS S.J. Cho et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 94–97
96 100
3500
100
Neutronreflexion in p direction 3000
spin up with 300G spin down with 300G spin up with 10G spin down with 10G
80
60
60
40
40
20
20
2500
Reflectivity
Remanence S Remanence P Coercivity S Coercivity P
Remanence[%]
Coercivity[Gauss]
80
2000 1500 1000 500
Bias -70V
Bias 0V
Floating
Bias +100V 0.2
Bias voltage
0.4
0.6
θ
0.8
Neutronreflexion in s direction 3000 spin up with 300G spin down with 300G spin up with 10G spin down with 10G
Reflectivity
2500 2000 1500 1000 500
0.2
3.3. Neutron reflectometer measurements of the multilayer with different magnetic fields The magnetic anisotropy, which was determined with the SQUID, was also studied with the polarized neutron reflectometer. For this the measurements were recorded for the different magnetic fields (300 and 10 G). It is expected that the magnetic scattering length of FeCo-alloy with a full magnetization amounts to 6.28 fm [6]. If the magnetization decreases, it reduces the magnetic scattering length. The monochromators manufactured with both bias voltages of –70 and +100 V were examined. The reflection of the sample manufactured with the bias of –70 V show only a slight difference between both the directions of s and p for the dependence of the magnetic field (Fig. 2). For a magnetic field of 10 G, the reflections of the spin up neutron-component
1.2
3500
Fig. 1. The coercivity and remanence in dependence of the bias potential: in each case both magnetic axes (s and p) in layer plane were plotted. The orientation ‘‘p’’ is parallel to the rotation axis of the sample holder and ‘‘s’’ perpendicular to it.
of this sample occurs very slowly, thereby an anisotropy of the magnetization is facilitated. For samples manufactured with negative bias between 0 and 70 V, the coercivity increases, but the remanence slightly reduces in comparison to the sample with floating potential. In Fig. 1 the magnetic characteristics are represented as a function of the bias potential.
1.0
0.4
0.6
θ
0.8
1.0
1.2
Fig. 2. Neutron reflection profile for the monochromators manufactured with bias 70 V.
show a slight decrease of approximately 3% for both the directions of s and p, and those of a spin down component show a slight increase. Additionally, the critical angle of the spin down component rises by about 0.05 . This result corresponds to the magnetic characteristics, which were determined through the SQUID-measurement for both the directions of s and p. In the case of the sample manufactured with a bias of +100 V, an apparent difference between the reflections of the s- and p-direction is observed (Fig. 3). In the measurement with 10 G, the critical angle for the total reflection and the reflectivity of the Bragg peak shows an obvious reduction in the
ARTICLE IN PRESS S.J. Cho et al. / Nuclear Instruments and Methods in Physics Research A 529 (2004) 94–97
4. Conclusion
3500
Neutronreflexion in p direction 3000
spin up with 300G spin down with 300G spin up with10G spin down with 10G
Reflektivity
2500 2000 1500 1000 500
0.2
0.4
0.6
θ
0.8
1.0
1.2
3500
Neutronreflexion in s direction 3000
spin up with 300G spin down with 300G spin up with 10G spin down with 10G
Reflectivity
2500 2000 1500 1000 500
0.2
97
By employing X-ray and polarized neutron scattering and SQUID-magnetometer to investigate the growth of sputtered Fe89Co11–Si multilayer, it was found that the crystallization of the FeCo alloy is influenced by the substrate surface energy. The sample manufactured with a bias of +100 V shows a very weak crystallization, which causes the magnetic anisotropy and an increase of the remanence by 8% with the SQUID magnetometer, while no difference is found for the samples with a 0 and 70 V. In order to examine this magnetic characteristic with the polarizing neutron reflectometer measured under a magnetic field of 300 and 10 G. The sample with a bias of +100 V shows clearly the anisotropy in two directions (p and s) in the neutron measurement as well. It appears that with this sputtering condition of the bias potential of +100 V the remanence polarizing neutron monochromator can be fabricated. This fact provides us with the possibility of making a remanent polarizing neutron supermirror.
References 0.4
0.6
θ
0.8
1.0
1.2
Fig. 3. Neutron reflection profile for the monochromators manufactured with bias +100 V.
p direction in comparison to the measurement for a magnetic field of 300 G, while no difference is observed in the s direction for both magnetic fields.
[1] F. Mezei, Commun. Phys. 1 (1976) 81; F. Mezei, P. Dagleish, Commun. Phys. 2 (1977) 41. [2] Th. Krist, C. Pappas, Th. Keller, F. Mezei, Phys. B 213 & 214 (1995) 939. [3] N.K. Pleshanov, Phys. B 297 (2001) 131. [4] N.K. Pleshanov, V. Bodnarchuk, R. Gaehler, D.A. Korneev, A. Menelle, S.V. Metelev, V.M. Pusenkov, A.F. Schebetov, V.A. Ul’yanov, Phys. B 297 (2001) 126. [5] D. Clemens, Th. Krist, P. Schubert-Bischoff, J. Hoffmann, F. Mezei, Phys. Scr. 50 (1994) 195. [6] D.J. Mueller, Diploma Thesis, TU Berlin, 1993, unpublished.