- Email: [email protected]
Ni multilayers
Journal of Magnetism and Magnetic Materials 104-107 (1992) 1822-1824 North-Holland ii
Ferromagnetic resonance studies of exchange-coupled Fe/Ni multilayers R. Krishnan '~, C. Sella b, M. Kaabouchi b, B. Ramamurthy Acharya ':, Shiva Prasad ': and N. Venkatramani ~ '~ Laboratoire de Magndtisme et Mat&iattr MagnE;tiques, CNRS, F-92195 Meudon, France t, Laboratoire de Physique des Mate;tiara', CNRS, F-q2195 Meudon, France " butian b~stitute of Technolo,~,% Bombay 400076, India
Multilayers of Fe/Ni have been deposited onto water-cooled glass substrates by adc triode under computer-controlled conditions. The layer thickness was in the range 20 to 200 ,~ and the total thickness about 600 ,~. FMR was observed at 9.8 GHz. The perpendicular spectra consisted of more than one mode. We have calculated the field positions of the modes and compared them with the experimental data and found that the latter were smaller by about 10%. In order to obtain a good agreement it is necessary, to suppose the presence of an alloy layer about 2(1 ,~ thick at each interface with an average magnetization on the order of 13 kG. Magnetic multilayers (ML) with very soft magnetic properties are interesting from the technological point of view and particularly for realizing magnetic heads for very high density recording. Typical ML systems that exhibit such properties arc F e / N i [ 1 ] / a n d F c / C o [2]. It is wcll known the magnetic propcrtics of the ML arc very sensitive to the chemical purity of the interfaces. Bcsidcs the usual characterizing methods such as X-ray and electron diffraction and so on, one could also utilizc the fcrromagnctic resonance (FMR) to get somc information about the quality of the layers and the intcrfaccs. Incidcntally FMR in cxchangcd-couplcd layers such as in F e / N i ML is particularly interesting. Wc describe in this papcr our FMR studics on F c / N i ML and show how by using a model developed by one of us [3] the spectra could be interpreted in terms of an alloy layer formed by the interfaciai mixing. F e / N i multilayers were prepared by sequential triode sputtering using a unit equipped with a computercontrolled thickness monitoring system based on the dependence of the deposition rate on the target current. The thickness calibration was done by measuring thc magnetization on a thick (500,4,) single layer of Fc and Ni. The sputter gas was argon and its pressure was kept at 0.7 m Torr. Water-cooled glass substrates wcre used. Thc thickness ratio R of Fc and Ni layers was cquai to one in most of the cases, though some sampics wcrc also made with R 4: 1. Thc individual iaycr thickncss was varicd from 20 to 200 ,~,. No buffcr layer was used and the top layer was Ni. FMR was observed, at 9.8 GHz with the external field applied both in the film plane and along thc film normal in addition to the usual magnetic studies. First let us rccall very briefly for the sake of completeness, the results of structural and magnetic studies [4]. For layer thicknesses t > 40 ,~, both Fe and Ni are
crystalline with bcc and fcc strucutres, respectively. For t = 20 A, the diffraction rings of the Fe bcc structure become larger and weaker than those of Ni. For t = 10 A, the bcc rings of Fe are no longer seen and a fcc single-phase Ni is observed indicating strong mixing. For ML samples with R = 1, the saturation oaagnctization 411M was inodcpendent of thc modulation length A tv~ + tNi >/80 A, and is closc to the c'dculatcd value. But for A < 80 ~, it starts decreasing, indicating some mixing at thc intcrfacc [4]. Let us discuss the FMR results for samples first with R ~-1 for which wc havc carricd out dctailcd analysis. Thc pcrpcndicu!,ar spectra consisted of more than one resonance mode. In some cases the line shape was not very symmetrical and the asymmetry was particularly marked on the lower field side of the highest field resonance mode. In F e / N i ML both the layers are ferromagnetic and the resonance spectra therefore results from excitation extending to the full thickness of the film in which the individual layers arc exchange coupled. In such case,;, in our opinion, it is not possible to treat them as a single homogeneous fiha and calculate the effective magnctization in the usual way and so on. In order to find the field positions and the intensities of the various modes, for such an exchange-coupied film, we have carried out a simple calcuiation similar to that done by Wilts and Prasad [3]. In this calculation the values of the wave vector k of the magnetic excitation were calculated in each layer for a given value of the applied field. The value of k would be real or imaginary, depending upon whether the value of the applied field is smaller or larger than the field for uniform precession for that particular layer. Using these k values the excitation throughout the film was mapped. The field for resonance was varied until a o
I1312-,',1~53/t}2/$(15.0i) ,~., 1902 - Elsevier Science Publishers B.V. All rights reserved
1823
R. Kristman et al. ,! FMR o]'exchange-coupled Fe / N: ,m!tilayers
value found for which the excitation satisfies the boundary conditions at all the interfaces and has the required number of crossings corresponding to that particular mode. The intensity of the modes were calculated once the total excitation was known using the standard expression [3]. A similar analysis for the FMR in amorphous C o - N b - Z r films has been made by us recently [5]. The calculation was carried out using the literature values of the magnetic parameters given in table 1. We also suppose that there is no perpendicular anisotropy as inferred from the in-plane loop studies [4]. Thc experimental values of the field corresponding to the highest field mode are compared with the theoretically calculated ones in table 1. It is seen that the experimental values are about 10% lower than the calculated ones when the layer thicknesses from the deposition conditions are used. However, if it is assumed that a part of Fe and Ni layers constitute an intermediate alloy layer with 411M and D values in between the values of Fe and Ni, the calculated value of the highest field mode is found to be reduced significantly. This field is tbund to bc strongly dependent on the thickness and the parameters of the intermediate alloy layer. Table 1 gives the intermediate layer thickness for which the best fit was obtained for all the resonancc mode fields and intensities for a particular multilayer. Fig. 1 shows a stick diagram of the experimental and theoretically calculated field positions and intensities for the sample 6(}/60 with five bilayers. It is seen from this figure that even with the present simple model, the agreement between the theoretical and experimental field positions and intensities is good. The above discussion leads us to believe that the interface between the Fc and Ni layers is not sharp. It is worth mentioning that one expects the Fe and Ni concentrations to vary continuously in the intermediate
Table 1 Calculated and experimental values of the highest field position in the perpendicular FMR spectra No.
1 2
t(Fe)/ t(Ni)i
Highest field position (Oe} Experi- Calculated
(,~}
mental
No With (A) diffusion diffusion
60/60 40/40
20226 19015
22437 21897
20769 191149
Layer thickness
20 19
Parameters: D(Ni)=364 meV ,~2; D(Fe)=281 meV 7X:; D(interlayer) = 300 meV ,~2 in sample 1 and 350 meV ,~,-" in sample 2. 411M(Ni)=6095 G, 4/IM(Fe) = 21450 G, 411M(interlayer)= 13000 G in sample I and 12000 G in sample 2. Note: The D and allM values for Fe and Ni were taken from rcf. [6].
O O-
,e-
EXPERIME NTAL O-
I
10 cca
I
!
I
18
14
22
Field (kOe)
4.,.a
g
~
THEORETICA L o o.
Fig. 1. The stick diagram comparing the theoretical and experimental values of the field positions of the modes for the sample 60/60. The arrows indicate the positions of the low intensity modes. layer leading to a gradual variation of the properties which has been approximated here by a constant value for the simplicity of the calculation. So the magnetic parameters wc have specified for this alloy should be treated as some sort of average value. Of course it is difficult to refine the calculation any more. For instance, if one calculates the magnetization of the ML. by varying the thickness of the alloy layer from 10 to 15 A and the average magnetization from 10 to 13 kG then one ring that the calculated values agree to within 10% o, ,no experimental value indicating the inability of this method to justify the choice of the parameters. Wc arc carrying out conversion electron M6ssbauer spectroscopic studies, which could hopefully throw more light on this problem. Wc havc also studied the samples whcrc t(Fc) was fixed at 4(} A and t(Ni) was varied. It was found that the field position of the highest mode decreased with increasing t(Ni), in agreement with our calculation. As the perpendicular mode in some samples arc asymmetric as wc have mentioned before, wc have plotted the variation of the line width of the parallel mode as a function of the modulation in fig. 2. It is seen that it increases sharply for .1 = 2(} A, indicating inhomogcnity due to a strong modulation of the composition with composition gradient. In conclusion we have carried out FMR studies in F e / N i multilaycrs. By a simple model, treating them as exchanged-coupled layers, and assuming the presence of an alloy layer, wc have calculated the field positions of the absorption modes as well as their intensities. The agreement is quite satisfactory, The alloy layer thickness is in the range of 15 to 20 A at each interface. o
! 824
R. Kris/man et al. / FMR o.I"exchange-coupled Fe / Ni multilayers
0
pour la Promotion de la Rcchcrchc Avanc6c), New Dcihi, and is gratefully acknowledged.
200.
e-
-r" ~"
References
100
U.i
z .d
io MODULATION
'f~i"
20
A nm
Fig. 2. The modulation dependence of the parallel resonance line width. Tiffs work was performed under the contract no. 181)-01 from The lndo Frcnch Ccntcr for thc Promotion of Advanccd Rcscarch (Centre Franco lndicn
[1] Y. Nagi and M. Senda, Japan. J. Appl. Phys. 26 (1987) L1514. [2] M. Senda and Y. Nagi, Appl. Phys. Lett. 52 (1988) 672. [3] C.H. Wilts and S. Prasad, IEEE Trans. Magn. MAG-17 (1981 ) 2405. [4] R. Krishnan, H.O. Gupta, H. Lassri, C. Sella and M. Kaabouchi, Joint M M M and Interrnag. Conf. Pittsburgh, 1991 J. Appl. Phys. to be published. [5] R. Krishnan, M. Nailli,Tessier, B. Ramamurthy Acharya, Shiva Prasad and N. Venkatramani, J. Magn. Magn. Mater. 93 (1991) 257. [6] C. Kittel, Solid State Physics (Wiley, New York} 5th edn.
Ferromagnetic resonance studies of exchange-coupled Fe/Ni multilayers R. Krishnan '~, C. Sella b, M. Kaabouchi b, B. Ramamurthy Acharya ':, Shiva Prasad ': and N. Venkatramani ~ '~ Laboratoire de Magndtisme et Mat&iattr MagnE;tiques, CNRS, F-92195 Meudon, France t, Laboratoire de Physique des Mate;tiara', CNRS, F-q2195 Meudon, France " butian b~stitute of Technolo,~,% Bombay 400076, India
Multilayers of Fe/Ni have been deposited onto water-cooled glass substrates by adc triode under computer-controlled conditions. The layer thickness was in the range 20 to 200 ,~ and the total thickness about 600 ,~. FMR was observed at 9.8 GHz. The perpendicular spectra consisted of more than one mode. We have calculated the field positions of the modes and compared them with the experimental data and found that the latter were smaller by about 10%. In order to obtain a good agreement it is necessary, to suppose the presence of an alloy layer about 2(1 ,~ thick at each interface with an average magnetization on the order of 13 kG. Magnetic multilayers (ML) with very soft magnetic properties are interesting from the technological point of view and particularly for realizing magnetic heads for very high density recording. Typical ML systems that exhibit such properties arc F e / N i [ 1 ] / a n d F c / C o [2]. It is wcll known the magnetic propcrtics of the ML arc very sensitive to the chemical purity of the interfaces. Bcsidcs the usual characterizing methods such as X-ray and electron diffraction and so on, one could also utilizc the fcrromagnctic resonance (FMR) to get somc information about the quality of the layers and the intcrfaccs. Incidcntally FMR in cxchangcd-couplcd layers such as in F e / N i ML is particularly interesting. Wc describe in this papcr our FMR studics on F c / N i ML and show how by using a model developed by one of us [3] the spectra could be interpreted in terms of an alloy layer formed by the interfaciai mixing. F e / N i multilayers were prepared by sequential triode sputtering using a unit equipped with a computercontrolled thickness monitoring system based on the dependence of the deposition rate on the target current. The thickness calibration was done by measuring thc magnetization on a thick (500,4,) single layer of Fc and Ni. The sputter gas was argon and its pressure was kept at 0.7 m Torr. Water-cooled glass substrates wcre used. Thc thickness ratio R of Fc and Ni layers was cquai to one in most of the cases, though some sampics wcrc also made with R 4: 1. Thc individual iaycr thickncss was varicd from 20 to 200 ,~,. No buffcr layer was used and the top layer was Ni. FMR was observed, at 9.8 GHz with the external field applied both in the film plane and along thc film normal in addition to the usual magnetic studies. First let us rccall very briefly for the sake of completeness, the results of structural and magnetic studies [4]. For layer thicknesses t > 40 ,~, both Fe and Ni are
crystalline with bcc and fcc strucutres, respectively. For t = 20 A, the diffraction rings of the Fe bcc structure become larger and weaker than those of Ni. For t = 10 A, the bcc rings of Fe are no longer seen and a fcc single-phase Ni is observed indicating strong mixing. For ML samples with R = 1, the saturation oaagnctization 411M was inodcpendent of thc modulation length A tv~ + tNi >/80 A, and is closc to the c'dculatcd value. But for A < 80 ~, it starts decreasing, indicating some mixing at thc intcrfacc [4]. Let us discuss the FMR results for samples first with R ~-1 for which wc havc carricd out dctailcd analysis. Thc pcrpcndicu!,ar spectra consisted of more than one resonance mode. In some cases the line shape was not very symmetrical and the asymmetry was particularly marked on the lower field side of the highest field resonance mode. In F e / N i ML both the layers are ferromagnetic and the resonance spectra therefore results from excitation extending to the full thickness of the film in which the individual layers arc exchange coupled. In such case,;, in our opinion, it is not possible to treat them as a single homogeneous fiha and calculate the effective magnctization in the usual way and so on. In order to find the field positions and the intensities of the various modes, for such an exchange-coupied film, we have carried out a simple calcuiation similar to that done by Wilts and Prasad [3]. In this calculation the values of the wave vector k of the magnetic excitation were calculated in each layer for a given value of the applied field. The value of k would be real or imaginary, depending upon whether the value of the applied field is smaller or larger than the field for uniform precession for that particular layer. Using these k values the excitation throughout the film was mapped. The field for resonance was varied until a o
I1312-,',1~53/t}2/$(15.0i) ,~., 1902 - Elsevier Science Publishers B.V. All rights reserved
1823
R. Kristman et al. ,! FMR o]'exchange-coupled Fe / N: ,m!tilayers
value found for which the excitation satisfies the boundary conditions at all the interfaces and has the required number of crossings corresponding to that particular mode. The intensity of the modes were calculated once the total excitation was known using the standard expression [3]. A similar analysis for the FMR in amorphous C o - N b - Z r films has been made by us recently [5]. The calculation was carried out using the literature values of the magnetic parameters given in table 1. We also suppose that there is no perpendicular anisotropy as inferred from the in-plane loop studies [4]. Thc experimental values of the field corresponding to the highest field mode are compared with the theoretically calculated ones in table 1. It is seen that the experimental values are about 10% lower than the calculated ones when the layer thicknesses from the deposition conditions are used. However, if it is assumed that a part of Fe and Ni layers constitute an intermediate alloy layer with 411M and D values in between the values of Fe and Ni, the calculated value of the highest field mode is found to be reduced significantly. This field is tbund to bc strongly dependent on the thickness and the parameters of the intermediate alloy layer. Table 1 gives the intermediate layer thickness for which the best fit was obtained for all the resonancc mode fields and intensities for a particular multilayer. Fig. 1 shows a stick diagram of the experimental and theoretically calculated field positions and intensities for the sample 6(}/60 with five bilayers. It is seen from this figure that even with the present simple model, the agreement between the theoretical and experimental field positions and intensities is good. The above discussion leads us to believe that the interface between the Fc and Ni layers is not sharp. It is worth mentioning that one expects the Fe and Ni concentrations to vary continuously in the intermediate
Table 1 Calculated and experimental values of the highest field position in the perpendicular FMR spectra No.
1 2
t(Fe)/ t(Ni)i
Highest field position (Oe} Experi- Calculated
(,~}
mental
No With (A) diffusion diffusion
60/60 40/40
20226 19015
22437 21897
20769 191149
Layer thickness
20 19
Parameters: D(Ni)=364 meV ,~2; D(Fe)=281 meV 7X:; D(interlayer) = 300 meV ,~2 in sample 1 and 350 meV ,~,-" in sample 2. 411M(Ni)=6095 G, 4/IM(Fe) = 21450 G, 411M(interlayer)= 13000 G in sample I and 12000 G in sample 2. Note: The D and allM values for Fe and Ni were taken from rcf. [6].
O O-
,e-
EXPERIME NTAL O-
I
10 cca
I
!
I
18
14
22
Field (kOe)
4.,.a
g
~
THEORETICA L o o.
Fig. 1. The stick diagram comparing the theoretical and experimental values of the field positions of the modes for the sample 60/60. The arrows indicate the positions of the low intensity modes. layer leading to a gradual variation of the properties which has been approximated here by a constant value for the simplicity of the calculation. So the magnetic parameters wc have specified for this alloy should be treated as some sort of average value. Of course it is difficult to refine the calculation any more. For instance, if one calculates the magnetization of the ML. by varying the thickness of the alloy layer from 10 to 15 A and the average magnetization from 10 to 13 kG then one ring that the calculated values agree to within 10% o, ,no experimental value indicating the inability of this method to justify the choice of the parameters. Wc arc carrying out conversion electron M6ssbauer spectroscopic studies, which could hopefully throw more light on this problem. Wc havc also studied the samples whcrc t(Fc) was fixed at 4(} A and t(Ni) was varied. It was found that the field position of the highest mode decreased with increasing t(Ni), in agreement with our calculation. As the perpendicular mode in some samples arc asymmetric as wc have mentioned before, wc have plotted the variation of the line width of the parallel mode as a function of the modulation in fig. 2. It is seen that it increases sharply for .1 = 2(} A, indicating inhomogcnity due to a strong modulation of the composition with composition gradient. In conclusion we have carried out FMR studies in F e / N i multilaycrs. By a simple model, treating them as exchanged-coupled layers, and assuming the presence of an alloy layer, wc have calculated the field positions of the absorption modes as well as their intensities. The agreement is quite satisfactory, The alloy layer thickness is in the range of 15 to 20 A at each interface. o
! 824
R. Kris/man et al. / FMR o.I"exchange-coupled Fe / Ni multilayers
0
pour la Promotion de la Rcchcrchc Avanc6c), New Dcihi, and is gratefully acknowledged.
200.
e-
-r" ~"
References
100
U.i
z .d
io MODULATION
'f~i"
20
A nm
Fig. 2. The modulation dependence of the parallel resonance line width. Tiffs work was performed under the contract no. 181)-01 from The lndo Frcnch Ccntcr for thc Promotion of Advanccd Rcscarch (Centre Franco lndicn
[1] Y. Nagi and M. Senda, Japan. J. Appl. Phys. 26 (1987) L1514. [2] M. Senda and Y. Nagi, Appl. Phys. Lett. 52 (1988) 672. [3] C.H. Wilts and S. Prasad, IEEE Trans. Magn. MAG-17 (1981 ) 2405. [4] R. Krishnan, H.O. Gupta, H. Lassri, C. Sella and M. Kaabouchi, Joint M M M and Interrnag. Conf. Pittsburgh, 1991 J. Appl. Phys. to be published. [5] R. Krishnan, M. Nailli,Tessier, B. Ramamurthy Acharya, Shiva Prasad and N. Venkatramani, J. Magn. Magn. Mater. 93 (1991) 257. [6] C. Kittel, Solid State Physics (Wiley, New York} 5th edn.