- Email: [email protected]
CoNbZr multilayers
ELSEVIER
Journal of Magnetism and Magnetic Materials 133 (1994) 490-492
~H
journal of magnetism and magnetic materials
Influence of interdiffusion layer on magnetostriction of Fe/CoNbZr multilayers R. Zuberek a,,, H. Szymczak a, R. Krishnan b, M. Tessier b a Institute of Physics of the Polish Academy of Sciences, al. Lotnik6w 32 / 46, 02-668 Warszawa, Poland b Laboratoire de Magnetisme et Materiaux Magnetiques, CNRS, 1, Place A. Briand, 92 195 Meudon Cede:~, France
Abstract The results of measurements of the magnetostriction constant As of multilayer F e / C o N b Z r sputtered films are reported, performed at room temperature using the strain-modulated ferromagnetic resonance method. Experimental evidence for an interdiffusion layer in the multilayer with iron layer thickness of 12 nm is demonstrated. The magnetostriction of the total multilayer is determined by the balance in magnetostriction among Fe, CoNbZr and the interdiffusion layer.
1. Introduction Among materials suitable for magnetic recording heads the F e / C o N b Z r multilayers proposed by Dirne et al. [1], satisfy well all required conditions such as high magnetization, low coercivity, high permeability and a nearly zero magnetostriction constant. It is clear that the interfacial alloy formed during the preparation of these multilayers plays a key role in determining the final properties. The existence of the interdiffusion layer was shown by conversion electron M6ssbauer spectroscopy (CEMS). These studies show [2] that for an iron layer thickness tFe > 6 rim, the intermixing at the interface is either absent or negligible. But for tFe = 2.4 nm, intermixing occurs with a wider hyperfine field distribution. In most cases, the magnetostriction of the multilayers is found to change as the thickness t of the magnetic layers is changed. Since the above changes in magnetostriction depend on the thickness as t ~, it is attributed to the surface and called surface magnetostriction [3]. In some cases the linear dependence of magnetostriction on t 1 is observed to be due to the interdiffusion layers [4]. In principle, however, a
* Corresponding author. Tel: +48 (22) 43 06 34; fax: +48 (22) 43 09 26; e-mail:[email protected].
technique such as strain-modulated ferromagnetic resonance [5] provides the possibility to distinguish between intrinsic surface magnetostriction and interdiffusion effects. In this paper we report the influence of the interdiffusion layer in F e / a m CoNbZr multilayers on the magnetostriction constant.
2. Experimental details The Fe/amCo0.8Nb0.1Zr0.1 multilayers were prepared by sequential rf sputtering using a system that was first pumped to a pressure of 4 × 10 -7 Torr. The targets were pure Fe (99.99%) and an alloy of C o - N b Zr (CNZ). The deposition parameters were rf power 80 W and argon pressure 6 mTorr. Water-cooled glass substrates were used. The deposition rate was measured with a quartz crystal monitor, which was calibrated against the thickness obtained from Profilometer measurements on thicker layers. A double check was made for Fe by measuring also the magnetization on a single thick layer. The Fe layer thickness was varied in the range 2.4-12 nm and the CNZ layer thicknesses were kept at 12, 6, 4.8 and 2.4 rim. The composition of the CNZ layer was determined by electron microprobe analysis to be Co0.sNbo.~Zr0.~, which was also very close to that of the target. We chose this
0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00190-3
491
R. Zuberek et aL / Journal of Magnetism and Magnetic Materials 133 (1994) 490-492
composition because our earlier studies showed that the magnetostriction for this alloy was - 1.7 x 10 -7, which is indeed very small [6]. The magnetostriction constants were measured at room temperature by the strain-modulated ferromagnetic resonance (SMFMR) method at 9.10 GHz with external magnetic field and uniaxial stress parallel to the film plane and perpendicular to each other. In the SMFMR technique the strain, periodic in time, causes a modulation of the ferromagnetic resonance (FMR) line position. To obtain the value of the components of the magnetoelastic tensor one should compare the intensity of the SMFMR line with the intensity of the same line detected using the standard FMR technique. In addition to obtaining information on the magnetostriction constants, the SMFMR technique offers the opportunity to study the interface effects (including interdiffusion mixing); in particular, it can be used to separate the intrinsic surface magnetostriction from the interface induced effects. To perform such a separation, information is needed on the anisotropy of the magnetoelastic tensor [8]. Generally speaking SMFMR technique provides such a possibility (on the condition that one should be able to measure more than one component of the magnetoelastic tensor). In the case of F e / a m C o N b Z r multilayers we were unable to apply the procedure proposed in Ref. [8], but using SMFMR technique we estimated the magnetostriction of the interdiffusion layer. This parameter is of great importance in calculation of the magnetoelastic contribution to the effective anisotropy constant of the multilayers.
{a.u.}
J
b
=,
I
[
300
600
I _
900 lq[0e)
Fig. 1. (a) FMR and (b) SMFMR spectra of Fe/CoNbZr multilayer with thicknesses tFe = 2.4 nm and tcNz = 2.4 nm.
fully analyzed the FMR and SMFMR, decomposing them into two lines in order to obtain correct information about the intensity of the individual lines. Since the magnetostriction constant of the CoNbZr sublayer is very small (of the order of 10-7), we have assumed
(a.u]
J
3. Results and discussion
Typical spectra of FMR and SMFMR for the samples with tFe -----2.4 and 12 nm are shown on Figs. 1 and 2. For thin sublayers (tFe = 2.4 nm) the measured lines are symmetric and similar to those observed in our previous studies [5-8]. A completely different picture was observed for thick sublayers. It can be seen from Fig. 2 that in this case the measured lineshape obtained using the SMFMR technique is quite different from that obtained by the FMR method. The unusual shape of the SMFMR line indicates that it comes from superposition of two lines being very close in the parallel field configuration and having opposite polarization. The opposite polarization of these lines results from the different signs of the magnetostriction constants of magnetic layers creating resonance lines. One should expect that for the extreme case of two lines with the same resonance field but with opposite polarizations, the intensity of the SMFMR line will be strongly reduced, yielding wrong information about the magnetostriction constants. We have therefore care-
[l
I
Cl
[
I
I
400 600 800 1000 H (0el Fig. 2. (a) FMR and (b) SMFMR spectra of Fe/CoNbZr multilayer with thicknesses t F e = 12 nm and tcrqz ~ 12 nm.
R. Zuberek et al. /Journal of Magnetism and Magnetic Materials 133 (1994) 490-492
492
Table 1 Experimental and calculated data for the magnetostriction constant of Fe/CoNbZr multilayers for different sublayer thicknesses tFe (,~)
ten z (~)
4~rM~ (kG)
A~ (×106 )
A~(calc) (×106 )
120 120 120 72 60 36 36 24 24
120 60 36 60 24 60 36 48 24
14 16 17 14 16 13 14 12 14
- 0.5 1.2 0.5 0.8 2.7 6.3 5.9 6.5 15.3
- 0.30 -0.34 0.37 2.08 4.35 5.49 7.37 8.51 12.85
that observed S M F M R line is a superposition of two lines, o n e from F e sublayers a n d o n e from interdiffusion layers. This a t t r i b u t i o n is crucial for the i n t e r p r e tation of our e x p e r i m e n t a l data. In o r d e r to o b t a i n f u r t h e r p r o o f of o u r i n t e r p r e t a t i o n we investigated several samples (see T a b l e 1) with various thicknesses of C o N b Z r sublayers (but probably with t h e same thickness of interdiffusion layer). Careful analysis of the intensity of d e c o m p o s e d lines confirms the proposed i n t e r p r e t a t i o n . A s s u m i n g t h a t t h e m a g n e t o s t r i c t i o n of the total multilayer is d e t e r m i n e d by the b a l a n c e in m a g n e tostriction a m o n g t h e Fe, C N Z and interdiffusion layers, a~ can b e expressed as [4]: As = ( 1 / ( t F e + tCNZ)){AFetFe + ACNZtCN Z +tx(2Ax-Ave-
ACNC)},
(11
w h e r e tve a n d t e n z are the p r o g r a m m e d Fe a n d C N Z layer thicknesses, t x is the thickness of interdiffusion layer, A~ is the m e a s u r e d m a g n e t o s t r i c t i o n of the total film, a n d Ax is the e s t i m a t e d value for the interdiffusion layer. F o r Ave value - 7 × 10 . 6 [7] a n d for AcN z
value - 1.7 × 10 . 7 are used. It was also assumed, in a c c o r d a n c e with e x p e r i m e n t a l data [2], that the thickness of the F e C o interdiffusion layer is t x ~ 1 nm. With this a s s u m p t i o n the e s t i m a t e d value of magnetostriction for interdiffusion F e C o layer is very high and equal 36 × 10 6. T h e results of m e a s u r e m e n t s and calculations are s u m m a r i z e d in T a b l e 1. T h e results suggest that the interface layer of F e C o gives the d o m i n a n t c o n t r i b u t i o n to As. It can be seen that the effective m a g n e t o s t r i c t i o n c o n s t a n t s of the F c / C o N b Z r multilayers are nearly zero over a wide r a n g e of Fe layer and of a m o r p h o u s C o N b Z r layer thicknesses.
Acknowledgement. This work was s u p p o r t e d in part by the State C o m m i t t e e for Scientific R e s e a r c h u n d e r G r a n t No. 2 0495 91 01.
References [1] F.W.A. Dirne, F.J.A. den Broeder, J.A.M. Tolboom, tt.J. de Wit and C.H.M. Witmer, Appl. Phys. Lett. 53 (19881 2386. [2] R. Krishnan and J.P. Eymery, J. Magn. Magn. Mater. 104-107 (1992) 1893. [3] H. Szymczak, R. Zuberek, R. Krishnan, M. Tcssier, K.B. Youn, and C. Sella, Dig. 12th ICMFS, Le Creusot 1-5 Aug. 1988, France, p. 266. [4] F.W.A. Dirne and C.J.M. Denissen, J. Magn. Magn. Mater. 78 (1989) 122. [5] R. Zuberek, H. Szymczak, R. Krishnan, K.B. Youn, and C. Sella, IEEE Trans. Magn. 23 (1987) 3699. [6] R. Zuberek, H. Szymczak, D. Zymierska, G. Suran and M. Naili, J. Magn. Magn. Mater. 104-107 (19921 117. [7] R. Zuberek, H. Szymczak, R. Krishnan, C. Sella and M. Kaabouchi, J. Magn. Magn. Mater. 121 (1993) 5111. [8] H. Szymczak, R. Zuberek, R. Krishnan, C. Sella and M. Kaabouchi, Intermag'93, IEEE Trans. Magn. (1993) to be published.
Journal of Magnetism and Magnetic Materials 133 (1994) 490-492
~H
journal of magnetism and magnetic materials
Influence of interdiffusion layer on magnetostriction of Fe/CoNbZr multilayers R. Zuberek a,,, H. Szymczak a, R. Krishnan b, M. Tessier b a Institute of Physics of the Polish Academy of Sciences, al. Lotnik6w 32 / 46, 02-668 Warszawa, Poland b Laboratoire de Magnetisme et Materiaux Magnetiques, CNRS, 1, Place A. Briand, 92 195 Meudon Cede:~, France
Abstract The results of measurements of the magnetostriction constant As of multilayer F e / C o N b Z r sputtered films are reported, performed at room temperature using the strain-modulated ferromagnetic resonance method. Experimental evidence for an interdiffusion layer in the multilayer with iron layer thickness of 12 nm is demonstrated. The magnetostriction of the total multilayer is determined by the balance in magnetostriction among Fe, CoNbZr and the interdiffusion layer.
1. Introduction Among materials suitable for magnetic recording heads the F e / C o N b Z r multilayers proposed by Dirne et al. [1], satisfy well all required conditions such as high magnetization, low coercivity, high permeability and a nearly zero magnetostriction constant. It is clear that the interfacial alloy formed during the preparation of these multilayers plays a key role in determining the final properties. The existence of the interdiffusion layer was shown by conversion electron M6ssbauer spectroscopy (CEMS). These studies show [2] that for an iron layer thickness tFe > 6 rim, the intermixing at the interface is either absent or negligible. But for tFe = 2.4 nm, intermixing occurs with a wider hyperfine field distribution. In most cases, the magnetostriction of the multilayers is found to change as the thickness t of the magnetic layers is changed. Since the above changes in magnetostriction depend on the thickness as t ~, it is attributed to the surface and called surface magnetostriction [3]. In some cases the linear dependence of magnetostriction on t 1 is observed to be due to the interdiffusion layers [4]. In principle, however, a
* Corresponding author. Tel: +48 (22) 43 06 34; fax: +48 (22) 43 09 26; e-mail:[email protected].
technique such as strain-modulated ferromagnetic resonance [5] provides the possibility to distinguish between intrinsic surface magnetostriction and interdiffusion effects. In this paper we report the influence of the interdiffusion layer in F e / a m CoNbZr multilayers on the magnetostriction constant.
2. Experimental details The Fe/amCo0.8Nb0.1Zr0.1 multilayers were prepared by sequential rf sputtering using a system that was first pumped to a pressure of 4 × 10 -7 Torr. The targets were pure Fe (99.99%) and an alloy of C o - N b Zr (CNZ). The deposition parameters were rf power 80 W and argon pressure 6 mTorr. Water-cooled glass substrates were used. The deposition rate was measured with a quartz crystal monitor, which was calibrated against the thickness obtained from Profilometer measurements on thicker layers. A double check was made for Fe by measuring also the magnetization on a single thick layer. The Fe layer thickness was varied in the range 2.4-12 nm and the CNZ layer thicknesses were kept at 12, 6, 4.8 and 2.4 rim. The composition of the CNZ layer was determined by electron microprobe analysis to be Co0.sNbo.~Zr0.~, which was also very close to that of the target. We chose this
0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00190-3
491
R. Zuberek et aL / Journal of Magnetism and Magnetic Materials 133 (1994) 490-492
composition because our earlier studies showed that the magnetostriction for this alloy was - 1.7 x 10 -7, which is indeed very small [6]. The magnetostriction constants were measured at room temperature by the strain-modulated ferromagnetic resonance (SMFMR) method at 9.10 GHz with external magnetic field and uniaxial stress parallel to the film plane and perpendicular to each other. In the SMFMR technique the strain, periodic in time, causes a modulation of the ferromagnetic resonance (FMR) line position. To obtain the value of the components of the magnetoelastic tensor one should compare the intensity of the SMFMR line with the intensity of the same line detected using the standard FMR technique. In addition to obtaining information on the magnetostriction constants, the SMFMR technique offers the opportunity to study the interface effects (including interdiffusion mixing); in particular, it can be used to separate the intrinsic surface magnetostriction from the interface induced effects. To perform such a separation, information is needed on the anisotropy of the magnetoelastic tensor [8]. Generally speaking SMFMR technique provides such a possibility (on the condition that one should be able to measure more than one component of the magnetoelastic tensor). In the case of F e / a m C o N b Z r multilayers we were unable to apply the procedure proposed in Ref. [8], but using SMFMR technique we estimated the magnetostriction of the interdiffusion layer. This parameter is of great importance in calculation of the magnetoelastic contribution to the effective anisotropy constant of the multilayers.
{a.u.}
J
b
=,
I
[
300
600
I _
900 lq[0e)
Fig. 1. (a) FMR and (b) SMFMR spectra of Fe/CoNbZr multilayer with thicknesses tFe = 2.4 nm and tcNz = 2.4 nm.
fully analyzed the FMR and SMFMR, decomposing them into two lines in order to obtain correct information about the intensity of the individual lines. Since the magnetostriction constant of the CoNbZr sublayer is very small (of the order of 10-7), we have assumed
(a.u]
J
3. Results and discussion
Typical spectra of FMR and SMFMR for the samples with tFe -----2.4 and 12 nm are shown on Figs. 1 and 2. For thin sublayers (tFe = 2.4 nm) the measured lines are symmetric and similar to those observed in our previous studies [5-8]. A completely different picture was observed for thick sublayers. It can be seen from Fig. 2 that in this case the measured lineshape obtained using the SMFMR technique is quite different from that obtained by the FMR method. The unusual shape of the SMFMR line indicates that it comes from superposition of two lines being very close in the parallel field configuration and having opposite polarization. The opposite polarization of these lines results from the different signs of the magnetostriction constants of magnetic layers creating resonance lines. One should expect that for the extreme case of two lines with the same resonance field but with opposite polarizations, the intensity of the SMFMR line will be strongly reduced, yielding wrong information about the magnetostriction constants. We have therefore care-
[l
I
Cl
[
I
I
400 600 800 1000 H (0el Fig. 2. (a) FMR and (b) SMFMR spectra of Fe/CoNbZr multilayer with thicknesses t F e = 12 nm and tcrqz ~ 12 nm.
R. Zuberek et al. /Journal of Magnetism and Magnetic Materials 133 (1994) 490-492
492
Table 1 Experimental and calculated data for the magnetostriction constant of Fe/CoNbZr multilayers for different sublayer thicknesses tFe (,~)
ten z (~)
4~rM~ (kG)
A~ (×106 )
A~(calc) (×106 )
120 120 120 72 60 36 36 24 24
120 60 36 60 24 60 36 48 24
14 16 17 14 16 13 14 12 14
- 0.5 1.2 0.5 0.8 2.7 6.3 5.9 6.5 15.3
- 0.30 -0.34 0.37 2.08 4.35 5.49 7.37 8.51 12.85
that observed S M F M R line is a superposition of two lines, o n e from F e sublayers a n d o n e from interdiffusion layers. This a t t r i b u t i o n is crucial for the i n t e r p r e tation of our e x p e r i m e n t a l data. In o r d e r to o b t a i n f u r t h e r p r o o f of o u r i n t e r p r e t a t i o n we investigated several samples (see T a b l e 1) with various thicknesses of C o N b Z r sublayers (but probably with t h e same thickness of interdiffusion layer). Careful analysis of the intensity of d e c o m p o s e d lines confirms the proposed i n t e r p r e t a t i o n . A s s u m i n g t h a t t h e m a g n e t o s t r i c t i o n of the total multilayer is d e t e r m i n e d by the b a l a n c e in m a g n e tostriction a m o n g t h e Fe, C N Z and interdiffusion layers, a~ can b e expressed as [4]: As = ( 1 / ( t F e + tCNZ)){AFetFe + ACNZtCN Z +tx(2Ax-Ave-
ACNC)},
(11
w h e r e tve a n d t e n z are the p r o g r a m m e d Fe a n d C N Z layer thicknesses, t x is the thickness of interdiffusion layer, A~ is the m e a s u r e d m a g n e t o s t r i c t i o n of the total film, a n d Ax is the e s t i m a t e d value for the interdiffusion layer. F o r Ave value - 7 × 10 . 6 [7] a n d for AcN z
value - 1.7 × 10 . 7 are used. It was also assumed, in a c c o r d a n c e with e x p e r i m e n t a l data [2], that the thickness of the F e C o interdiffusion layer is t x ~ 1 nm. With this a s s u m p t i o n the e s t i m a t e d value of magnetostriction for interdiffusion F e C o layer is very high and equal 36 × 10 6. T h e results of m e a s u r e m e n t s and calculations are s u m m a r i z e d in T a b l e 1. T h e results suggest that the interface layer of F e C o gives the d o m i n a n t c o n t r i b u t i o n to As. It can be seen that the effective m a g n e t o s t r i c t i o n c o n s t a n t s of the F c / C o N b Z r multilayers are nearly zero over a wide r a n g e of Fe layer and of a m o r p h o u s C o N b Z r layer thicknesses.
Acknowledgement. This work was s u p p o r t e d in part by the State C o m m i t t e e for Scientific R e s e a r c h u n d e r G r a n t No. 2 0495 91 01.
References [1] F.W.A. Dirne, F.J.A. den Broeder, J.A.M. Tolboom, tt.J. de Wit and C.H.M. Witmer, Appl. Phys. Lett. 53 (19881 2386. [2] R. Krishnan and J.P. Eymery, J. Magn. Magn. Mater. 104-107 (1992) 1893. [3] H. Szymczak, R. Zuberek, R. Krishnan, M. Tcssier, K.B. Youn, and C. Sella, Dig. 12th ICMFS, Le Creusot 1-5 Aug. 1988, France, p. 266. [4] F.W.A. Dirne and C.J.M. Denissen, J. Magn. Magn. Mater. 78 (1989) 122. [5] R. Zuberek, H. Szymczak, R. Krishnan, K.B. Youn, and C. Sella, IEEE Trans. Magn. 23 (1987) 3699. [6] R. Zuberek, H. Szymczak, D. Zymierska, G. Suran and M. Naili, J. Magn. Magn. Mater. 104-107 (19921 117. [7] R. Zuberek, H. Szymczak, R. Krishnan, C. Sella and M. Kaabouchi, J. Magn. Magn. Mater. 121 (1993) 5111. [8] H. Szymczak, R. Zuberek, R. Krishnan, C. Sella and M. Kaabouchi, Intermag'93, IEEE Trans. Magn. (1993) to be published.