- Email: [email protected]
Spin wave dynamics and domain structure in exchange coupled FeNi multilayers
a
Journal of Magnetism and Magnetic Materials 121 (1993) 303-305 North-Holland
Spin wave dynamics and domain structure in exchange coupled FeNi multilayers M. Schilberg, Th. K l e i n e f e l d Angewandte Physik, Universitiit Duisburg, D-4100 Duisburg, Germany
and
B. H i l l e b r a n d s 2. Physikalisches Institut, RWTH Aachen, D-5100 Aachen, Germany
Thin multilayer films of Fe and Ni consisting of up to five layers were prepared by e-beam evaporation with various thicknesses from 0.5 to 30 nm. Spin wave dynamicswere studied by means of Brillouin light spectroscopy. To analyze the data we performed model calculations for a stack of ferromagnetic layers including Rado-Weertman and Hoffmann boundary conditions for the surface anisotropy and the interlayer exchange, respectively. We were able to determine the interlayer exchange between Fe-Fe layers intermediate by a thin Ni layer. The coupling was always found to be ferromagnetic. Additionally, we applied Kerr microscopyto the multilayer films to investigate the magnetic domain structure and the magnetization process.
The exchange coupling of multilayered films has received great attention [1]. Basing on investigations of Invar materials, we focus our interest on pure ferromagnetic stacks, consisting of up to five layers of Fe or Ni. So we have two different ferromagnetic materials in close contact, evidently leading to a strong exchange coupling at the interfaces. One of our aims is to determine the thickness dependence of interlayer exchange A12 from Fe over a Ni spacer [7]. To study the magnetic characteristics of these films we used Brillouin spectroscopy, i.e. the inelastic scattering of light. For this purpose we illuminated the samples by the light of an Ar ÷ laser and analyzed the scattered light under crossed polarization to suppress scattering by phonons in a five-pass F a b r y - P e r o t interferometer. Because of very weak intensities photon counting is necessary. The experimental results Correspondence to: Dr. Th. Kleinefeld, Angewandte Physik, Universit~it Duisburg, Lotharstrasse 1, D-4100 Duisburg, Germany. Tel.: + 49-203-3792854; telefax: + 49-203-3793163. Email: [email protected].
are compared with model calculations [2]. They include anisotropy and exchange effects using the R a d o - W e e r t m a n and Hoffmann boundary conditions [3,4]. In such calculations it is possible to fit parameters such as magnetization, g-factor and exchange coupling strength. Our samples were e-beam evaporated under U H V conditions at room temperature [6]. Because of thermal stability we used polished sapphire substrates. From X-ray analysis all samples were found to be polycrystalline. At first we analyzed spin wave spectra for F e / N i / F e trilayers with thicknesses of 5 n m / x n m / 3 0 nm; x varies from 0.5 to 10. A good agreement within an experimental resolution of 1 GHz was found for the magnon frequencies with calculations by using bulk-values for magnetic properties, as can be seen in fig. 1. The thickness dependence of magnon frequency is plotted versus the Ni thickness. The squares indicate experimental values, the dots represent calculated ones. This agreement is the vindication of the study of a hypothetical FeFe double layer with varying interlayer exchange. The frequency dependence is corn-
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
M. Schilberg et al. / Spin wave dynamics and domain structure in FeNi multilayers
304
80 0 u
70
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.
.
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30
o
. . . . . . . .
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.
.
.
.
.
.
.
.
.
.
~. . . . . . . . . . . . . .
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8
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Fig. 1. Spin wave frequencies for several real FeNiFe systems are plotted vs. the Ni thickness at a constant field of 300 mT. Experimental values are indicated by squares, the simulated data-points are shown by dots. From bottom to top following modes can be identified: SSW 1, D a m o n - E s h b a c h mode, SSW 2 and SSW 3.
ness for SSW1, SSW2 and the D a m o n - E s h b a c h mode. But there is still a strong asymmetric behaviour of the intensity of the scattering lines between the Stokes- and the anti-Stokes side in the spectra, i.e. the strongest peaks lie at different frequencies. There are no hints of a real asymmetry from the numerical point of view, so it could only be explained by hybridization of cou= pied spin wave modes. In the case of 15 nm layer thickness there is not a good correlation between experiment and calculation, although there are more low-frequency modes. This is surprising because from simulations it is known that this system reacts less sensitively to variations in general. In the other case the direct influence of exchange can only seen by magnons over 60 GHz. In addition to BriUouin spectroscopy we started measurements of the magnetic domain structure by using the longitudinal magneto-optic Kerr el-
pared with that for trilayers of varying Ni thickness, so that a new function could be extracted. In fig. 2, this is represented for SSW 2 (the standing spin wave with two nodes of the dynamic part of magnetization within the sample) by squares. A12 remains always positive indicating only ferromagnetic coupling. The best fit by least-squares fit analysis was found with a potential function
20
17.5 o 15 _o O D
12.5
Aa2/(mJ/m2) = 5 . 5 ( d N i / n m ) -0.92
D 0 0 []
E
The other spin wave modes yield the exponent being in these cases slightly less than - 1 . A refined analysis shows that an exponential function alone cannot approximate these data satisfactorily. Furthermore we investigated samples consisting of F e / N i / F e / N i / F e with 10 and 15 nm single layer thicknesses. The higher number of interfaces should enhance the exchange effects. But in this case the spectra are more difficult to interpret because of a greater manifold of spin wave modes. If one takes into account a value of AI~ = 20 m J / m 2 for the exchange coupling obtained at the Ni lattice constant - good agreement between calculation and measurement is obtained in the case of 10 nm single layer thick-
-0 0
(1)
7.5 2 <
l
o
oo 2.5
0
2.5
5 dm
7.5
10
[nm]
Fig. 2. T h e dependence of interlayer exchange A12 vs. spacer-thickness dNi for SSW 2 is indicated by squares. A least-squares fit yields the circles. For further information see text.
M. Schilberg et al. / Spin wave dynamics and domain structure in FeNi multilayers
fect. R e c e n t l y s o m e w o r k was d o n e to investigate t h e i n f l u e n c e o f e x c h a n g e c o u p l i n g on t h e m a g n e t i z a t i o n p r o c e s s a n d the r e l a t e d m a g n e t i c d o m a i n p a t t e r n s [8-10]. P o l a r i z a t i o n m i c r o s c o p y a n d ima g e p r o c e s s i n g e q u i p m e n t with e l e c t r o n i c contrast enhancement and nonmagnetic background s u p p r e s s i o n allows to analyze d o m a i n p a t t e r n s at t h e s a m p l e surfaces. D u e to an e x t r e m e l y w e a k l o n g i t u d i n a l K e r r r o t a t i o n in the r a n g e o f 0 = 10', i m a g e r e c o r d i n g results in p o o r c o n t r a s t p a t t e r n images, w h i c h still m a k e s the i n t e r p r e t a t i o n o f d a t a difficult. T h i s p r o b l e m m i g h t b e o v e r c o m e by c o a t i n g t h e s a m p l e surface with an a n t i r e f l e c tion layer [5]. This w o r k was s u p p o r t e d by t h e D F G , S F B 166. In this c o n t e x t we t h a n k H. M i i h l b a u e r for t h e p r e p a r a t i o n o f t h e samples.
305
References [1] P. Griinberg, R. Schreiber, Y. Pang, M.B. Brodsky and H. Sowers, Phys. Rev. Lett. 57 (1986) 2442. [2] B. Hillebrands, Phys. Rev. B 41 (1990) 530. [3] G.T. Rado and J.R. Weertman, J. Phys. Chem. Solids 11 (1959) 315. [4] F. Hoffmann, Phys. Stat. Sol. 41 (1970) 807. [5] H.H. Mende and Th. Kleinefeld, Appl. Phys. 20 (1979) 159. [6] R. Kordecki, R. Meckenstock, J. Pelzl, H. Miihlbauer, G. Dumpich and S. Nikitov, J. Appl. Phys. 70 (1991) 6418. [7] H. Litschke, M. Schilberg, Th. Kleinefeld and B. Hillebrands, J. Magn. Magn. Mater. 104-107 (1992) 1807. [8] M. Riihrig, R. Seh~ifer, A. Hubert, R. Mosler, J.A. Wolf, S. Demokritov and P. Griinberg, Phys. Stat. Sol. (a) 125 (1991) 635. [9] M. Goto, T. Sai, H. Tange and T. Kamimori, J. Magn. Magn. Mater. 104-107 (1992) 1789. [10] Q.M. Zhong, A.S. Arrott, B. Heinrich and Z. Celinski, J. Magn. Magn. Mater. 104-107 (1992) 1837.
Journal of Magnetism and Magnetic Materials 121 (1993) 303-305 North-Holland
Spin wave dynamics and domain structure in exchange coupled FeNi multilayers M. Schilberg, Th. K l e i n e f e l d Angewandte Physik, Universitiit Duisburg, D-4100 Duisburg, Germany
and
B. H i l l e b r a n d s 2. Physikalisches Institut, RWTH Aachen, D-5100 Aachen, Germany
Thin multilayer films of Fe and Ni consisting of up to five layers were prepared by e-beam evaporation with various thicknesses from 0.5 to 30 nm. Spin wave dynamicswere studied by means of Brillouin light spectroscopy. To analyze the data we performed model calculations for a stack of ferromagnetic layers including Rado-Weertman and Hoffmann boundary conditions for the surface anisotropy and the interlayer exchange, respectively. We were able to determine the interlayer exchange between Fe-Fe layers intermediate by a thin Ni layer. The coupling was always found to be ferromagnetic. Additionally, we applied Kerr microscopyto the multilayer films to investigate the magnetic domain structure and the magnetization process.
The exchange coupling of multilayered films has received great attention [1]. Basing on investigations of Invar materials, we focus our interest on pure ferromagnetic stacks, consisting of up to five layers of Fe or Ni. So we have two different ferromagnetic materials in close contact, evidently leading to a strong exchange coupling at the interfaces. One of our aims is to determine the thickness dependence of interlayer exchange A12 from Fe over a Ni spacer [7]. To study the magnetic characteristics of these films we used Brillouin spectroscopy, i.e. the inelastic scattering of light. For this purpose we illuminated the samples by the light of an Ar ÷ laser and analyzed the scattered light under crossed polarization to suppress scattering by phonons in a five-pass F a b r y - P e r o t interferometer. Because of very weak intensities photon counting is necessary. The experimental results Correspondence to: Dr. Th. Kleinefeld, Angewandte Physik, Universit~it Duisburg, Lotharstrasse 1, D-4100 Duisburg, Germany. Tel.: + 49-203-3792854; telefax: + 49-203-3793163. Email: [email protected].
are compared with model calculations [2]. They include anisotropy and exchange effects using the R a d o - W e e r t m a n and Hoffmann boundary conditions [3,4]. In such calculations it is possible to fit parameters such as magnetization, g-factor and exchange coupling strength. Our samples were e-beam evaporated under U H V conditions at room temperature [6]. Because of thermal stability we used polished sapphire substrates. From X-ray analysis all samples were found to be polycrystalline. At first we analyzed spin wave spectra for F e / N i / F e trilayers with thicknesses of 5 n m / x n m / 3 0 nm; x varies from 0.5 to 10. A good agreement within an experimental resolution of 1 GHz was found for the magnon frequencies with calculations by using bulk-values for magnetic properties, as can be seen in fig. 1. The thickness dependence of magnon frequency is plotted versus the Ni thickness. The squares indicate experimental values, the dots represent calculated ones. This agreement is the vindication of the study of a hypothetical FeFe double layer with varying interlayer exchange. The frequency dependence is corn-
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
M. Schilberg et al. / Spin wave dynamics and domain structure in FeNi multilayers
304
80 0 u
70
•
.
.
n
[q
40 "o. . . . . .
30
o
. . . . . . . .
~ ,~ o . . . . . .
t~
.
.
.
.
.
.
.
.
.
.
~. . . . . . . . . . . . . .
°1 ,2
20 2
4 dm
6
8
0
[nm]
Fig. 1. Spin wave frequencies for several real FeNiFe systems are plotted vs. the Ni thickness at a constant field of 300 mT. Experimental values are indicated by squares, the simulated data-points are shown by dots. From bottom to top following modes can be identified: SSW 1, D a m o n - E s h b a c h mode, SSW 2 and SSW 3.
ness for SSW1, SSW2 and the D a m o n - E s h b a c h mode. But there is still a strong asymmetric behaviour of the intensity of the scattering lines between the Stokes- and the anti-Stokes side in the spectra, i.e. the strongest peaks lie at different frequencies. There are no hints of a real asymmetry from the numerical point of view, so it could only be explained by hybridization of cou= pied spin wave modes. In the case of 15 nm layer thickness there is not a good correlation between experiment and calculation, although there are more low-frequency modes. This is surprising because from simulations it is known that this system reacts less sensitively to variations in general. In the other case the direct influence of exchange can only seen by magnons over 60 GHz. In addition to BriUouin spectroscopy we started measurements of the magnetic domain structure by using the longitudinal magneto-optic Kerr el-
pared with that for trilayers of varying Ni thickness, so that a new function could be extracted. In fig. 2, this is represented for SSW 2 (the standing spin wave with two nodes of the dynamic part of magnetization within the sample) by squares. A12 remains always positive indicating only ferromagnetic coupling. The best fit by least-squares fit analysis was found with a potential function
20
17.5 o 15 _o O D
12.5
Aa2/(mJ/m2) = 5 . 5 ( d N i / n m ) -0.92
D 0 0 []
E
The other spin wave modes yield the exponent being in these cases slightly less than - 1 . A refined analysis shows that an exponential function alone cannot approximate these data satisfactorily. Furthermore we investigated samples consisting of F e / N i / F e / N i / F e with 10 and 15 nm single layer thicknesses. The higher number of interfaces should enhance the exchange effects. But in this case the spectra are more difficult to interpret because of a greater manifold of spin wave modes. If one takes into account a value of AI~ = 20 m J / m 2 for the exchange coupling obtained at the Ni lattice constant - good agreement between calculation and measurement is obtained in the case of 10 nm single layer thick-
-0 0
(1)
7.5 2 <
l
o
oo 2.5
0
2.5
5 dm
7.5
10
[nm]
Fig. 2. T h e dependence of interlayer exchange A12 vs. spacer-thickness dNi for SSW 2 is indicated by squares. A least-squares fit yields the circles. For further information see text.
M. Schilberg et al. / Spin wave dynamics and domain structure in FeNi multilayers
fect. R e c e n t l y s o m e w o r k was d o n e to investigate t h e i n f l u e n c e o f e x c h a n g e c o u p l i n g on t h e m a g n e t i z a t i o n p r o c e s s a n d the r e l a t e d m a g n e t i c d o m a i n p a t t e r n s [8-10]. P o l a r i z a t i o n m i c r o s c o p y a n d ima g e p r o c e s s i n g e q u i p m e n t with e l e c t r o n i c contrast enhancement and nonmagnetic background s u p p r e s s i o n allows to analyze d o m a i n p a t t e r n s at t h e s a m p l e surfaces. D u e to an e x t r e m e l y w e a k l o n g i t u d i n a l K e r r r o t a t i o n in the r a n g e o f 0 = 10', i m a g e r e c o r d i n g results in p o o r c o n t r a s t p a t t e r n images, w h i c h still m a k e s the i n t e r p r e t a t i o n o f d a t a difficult. T h i s p r o b l e m m i g h t b e o v e r c o m e by c o a t i n g t h e s a m p l e surface with an a n t i r e f l e c tion layer [5]. This w o r k was s u p p o r t e d by t h e D F G , S F B 166. In this c o n t e x t we t h a n k H. M i i h l b a u e r for t h e p r e p a r a t i o n o f t h e samples.
305
References [1] P. Griinberg, R. Schreiber, Y. Pang, M.B. Brodsky and H. Sowers, Phys. Rev. Lett. 57 (1986) 2442. [2] B. Hillebrands, Phys. Rev. B 41 (1990) 530. [3] G.T. Rado and J.R. Weertman, J. Phys. Chem. Solids 11 (1959) 315. [4] F. Hoffmann, Phys. Stat. Sol. 41 (1970) 807. [5] H.H. Mende and Th. Kleinefeld, Appl. Phys. 20 (1979) 159. [6] R. Kordecki, R. Meckenstock, J. Pelzl, H. Miihlbauer, G. Dumpich and S. Nikitov, J. Appl. Phys. 70 (1991) 6418. [7] H. Litschke, M. Schilberg, Th. Kleinefeld and B. Hillebrands, J. Magn. Magn. Mater. 104-107 (1992) 1807. [8] M. Riihrig, R. Seh~ifer, A. Hubert, R. Mosler, J.A. Wolf, S. Demokritov and P. Griinberg, Phys. Stat. Sol. (a) 125 (1991) 635. [9] M. Goto, T. Sai, H. Tange and T. Kamimori, J. Magn. Magn. Mater. 104-107 (1992) 1789. [10] Q.M. Zhong, A.S. Arrott, B. Heinrich and Z. Celinski, J. Magn. Magn. Mater. 104-107 (1992) 1837.