- Email: [email protected]
The role of the magnetic structure and interface scattering in the GMR of magnetic multilayers
~ ELSEVIER
Journal of Magnetism and Magnetic Materials 173 (1997) 253-258
Journalof mngnetlsm ad magnetic ~i~ materials
The role of the magnetic structure and interface scattering in the GMR of magnetic multilayers M.J. Hall a, D.B. Jardine b, J.E. Evetts b'*, J.A. Leake b, R.E.
Somekh
Centre .,/br Electromagnetic and Time Metrologv, National Physical Laboratoo,, Teddington, Middlesex, TWI 1 olaf; UK b Department of Materials Science and Metallurgy, Pembroke Street, Cambridge, CB2 3QZ, UK
Received 6 February 1997
Abstract Room temperature giant magnetoresistance (GMR) has been investigated for Co/Cu(1 1 l ) multilayers grown by DC magnetron sputtering. The effect of annealing on the magnetic structure of multilayers with a nominal copper thickness of 0.9 nm has been studied, a strong linear correlation between the GMR and the volume fraction of ferromagnetic regions is observed. Such behaviour has been used to elucidate the scattering mechanism responsible for the GMR of this system, and suggests that s-d scattering into the split d-band of cobalt is the dominant mechanism responsible for the GMR of Co/Cu multilayers. This is in contrast to systems like Fe/Cr where experimental studies on the effect of interfacial mixing indicate that interfacial scattering resulting from spin-dependent interface potentials contributes strongly to the observed GMR. Kew,,ords: Sputtering; Co/Cu multilayers; Giant magnetoresistance; s-d scattering; Interface scattering
1. Introduction The fundamental origins of the giant magnetoresistance ( G M R ) effect exhibited by m a n y magnetic multilayers has presented challenges to both experimentalists and theoreticians. It is now well established that G M R is the result of an antiferromagnetic (AF) arrangement of the magnetic layers in zero field, with the resistivity d r o p result-
* Corresponding author. Tel.: + 44 1223 334364; fax: + 44 1223 334373; e-mail: [email protected].
ing from a reorientation to a ferromagnetic [FM) configuration upon the application of a sufficiently large external field. However, it remains unclear how the magnitude of the resulting G M R is dependent on the details of the multilayer magnetic and layer structures. The calculations of Levy and coworkers [1-3] attribute the G M R of Fe/Cr multilayers to spin-dependent interface potentials. Hall et al. [4] interpreted this as the effect of the strong spin dependence of electron scattering for chromium impurities in an iron host [5], which results in spin-dependent scattering at the interfaces. The origin of this is Friedel scattering [6], the resonant scattering of the virtual bound state of a chromium
0304-8853/97/$17.00 ~! 1997 Elsevier Science B.V. All rights reserved P l l S 0 3 0 4 - 8 8 53(97J001 67-4
254
M.J. Hall et al. / Journal of Magnetism and Magnetic Materials 173 (1997) 253-258
atom in the d-band of iron. Lenczowski et al. [7] have used the theories of Levy and co-workers to interpret the G M R effect in Co/Cu multilayers. In contrast, Edwards and co-workers [8-10] have shown that spin dependent s-d scattering into the split d-band of the magnetic layer can explain the observed effect. They used this approach to interpret the findings of Parkin et al. for both the Fe/Cr [11] and Co/Cu [12] systems. Clearly, there is a need for clarification. A further area of research has been into the details of the magnetic structure of the multilayers and how this relates to the measured GMR. Hall et al. [13] and Rupp et al. [14] have shown that the dependence of the G M R of Co/Cu multilayers on (1 - Mr~Ms)is linear. Here, the M r and Ms are the remanent and saturation magnetisations, respectively. In this paper we aim to show that such a plot can be used to determine if the spin-dependent scattering responsible for the G M R is due either to (1) s d scattering into the split d-band of the magnetic layer; or (2) the Friedel interface scattering introduced above and which has its own unique scattering asymmetry ~ (=p_/p+). Here p+ and p_ are the spin-up and spin-down resistivities. In doing this, we will illustrate the importance of monitoring the magnetic structure when interpreting the G M R effect. In Fig. 1 the different curves schematically show the expected behaviour for different scattering processes. In this figure the initial values of the G M R have been made equal for simplicity. In practice this will probably not be the case, however it is the trends that are important and not the absolute values. Straight line I is for the case of s~t scattering into the split magnetic d-band. In a highly perfect multilayer most of the sample is AF coupled in zero applied field. For this situation spin-up and spin-down electrons (the two spin coordinates) will be scattered in alternate magnetic layers. This is the result of the split magnetic d-band and the antiparallel arrangement of the layer magnetisation. Because of the split d-band an individual magnetic layer will only have spin-down electrons states available for scattering (strictly, this is only true at zero Kelvin and will be assumed here for simplicity). For two successive magnetic layers in an AF
configuration the local direction of magnetisation is reversed with respect to the spin coordinate of any electron. In one layer an electron which is spin-up with respect to the local magnetisation cannot be scattered. Then, on passing into the next layer the local magnetisation is reversed and the same electron will now have states available for scattering. Consequently, both spin coordinates can be scattered and both the zero-field resistivity and GMR, are high. In the presence of FM regions the direction of the local magnetisation does not reverse and the spin-up electrons do not have states available. The result of introducing such regions will be a decrease in the zero-field resistivity and consequently the GMR. Hence, for the case of s-d scattering into the split d-band of the magnetic layer the dependence of the G M R on (1 - Mr/M~) will be linear. For this type of scattering the associated scattering potential can result, for example, from phonons, stacking faults and point defects. The interface itself could also provide many scattering centres, with the spin dependence of the scattering arising solely from the split d-bands of the magnetic layers. Changes to the proportion of interface scattering alone will not effect the 80
II
I
0.0
I
I
I
I
1 - M~/Ms
[
I
I 1.0
Fig. 1. Schematic illustrating the dependence of the measured G M R on (1 - M r / M ~ ) for different scattering processes. For curves I and II the behaviour shown is in the presence of a changing proportion of interface scattering as an experimental sequence that progressively mixed the interface proceeded. The significance of the trends indicated by these curves is discussed fully in the text.
M.J. Hall et al. / Journal o[' Magnetism and Magnetic Materials 173 (1997) 253 258
magnitude of the G M R observed. This is because the spin-up and spin-down scattering rates will have increased by the same proportion. In curve lI of Fig. 1 the situation is for interface scattering that involves virtual bound states. Here the maximum in the G M R for a sequence of experiments that alter the degree of interface mixing does not necessarily occur at a value of (1 - M r / M J corresponding to 100% AF alignment. In this situation the introduction of FM regions will again lower the GMR. The distinction here is that the FM regions not only remove the possibility of spin-up electrons being scattered into the split dband of the magnetic layer, they also suppress the resonant scattering associated with virtual bound states. The origin of the initial increase in the G M R of curve II is the changes to the proportion of interface scattering. If the decrease in (1 - Mr/MJ is accompanied by an increase in the proportion of Friedel interface scattering, then the relative value of the interface-spin asymmetry is important. When this asymmetry is larger than that associated with the split d-band of the ferromagnetic layer, then the overall proportion of spin-dependent scattering can increase. The result of this is an increase in the GMR. Curve lI is one example of a family of curves, the exact forms of which depend on the detailed changes in the proportion of interface scattering upon annealing. What is clear is that the G M R would not initially decrease in the way illustrated by curve I. Curve III illustrates the situation where the G M R decreases. Here, one possibility is the progressive introduction of additional scattering which does not involve spin-dependent scattering parameters. An example of such a situation would be additional scattering within the copper layer since this would not depend on a split d-band. An additional effect which could result in the type of behaviour shown in III is the removal of virtual bound states upon annealing. Like before, this curve is just one example of a family of curves. Whatever the details of the structural changes upon annealing, it is clear that for curves II and Ill the G M R versus (1 - M~/MJ will not be linear. In what follows the variation of the electrical resistivity, GMR, magnetic structure and interface structure are reported for Co/Cu samples at various stages of annealing. From the observed
255
dependence of the G M R on (1 - Mr/MJ the results suggest that for this system electron scattering is spin dependent purely by virtue of the split d-bands of the cobalt layers. For the Fe/Cr system we will use these ideas along with published data to show that the scattering processes involved have a strong contribution from interface scattering with its own unique spin asymmetry.
2. Experimental details Two Co/Cu multilayers with a nominal structure of Fe(5.0)/[Co( 1.5)Cu(0.9)]30, where the thicknesses are in nm, have been deposited onto Si(1 0 01 substrates using UHV gettered DC magnetron sputtering. During deposition the substrates were at ambient temperature and rotated below two planar magnetrons which had been pre-sputtered to remove contaminants. For all runs the sputtering gas used was 6 N argon, further purified to 8 N, at a pressure of 0.5 Pa. Before deposition commenced the outgassing rate was typically 5 x 10- 11 Pa s Determination of the actual layer thicknesses was done using low-angle X-ray diffraction [XRDI to give the bilayer thickness. EDX data on the composition ratio was then used to determine the individual layer thicknesses. All magnetic and transport measurements on these samples were done at room temperature. Magnetisation loops were obtained using an Aerosonics 5001 vibrating sample magnetometer [VSMt. For resistance and G M R measurements a standard four-probe DC technique was used. Repeated measurements to determine the reproducibility of the resistance measurements indicated that a precision from one measurement to the next of better than 0.5% was possible. There was an absolute error of around 5"/,, arising from the film-thickness determination.
3. Annealing results and discussion An extensive annealing study on two samples with the same nominal structure and initial highfield room temperature resistivities of 18 Bf~cm have been performed. After each stage of annealing the magnetisation and G M R of the samples were
M.J. Hall et al. /Journal of Magnetism and Magnetic Materials 173 (1997) 253-258
256
measured. To investigate whether the details of the annealing history were important, sample A was annealed at increasing temperatures for various times and sample B at 220°C for increasing times. The full annealing history used can be found in Ref. [13]. After each stage of annealing the G M R of the samples always decreased. This was a result of the decrease in the zero-field resistivity, the high-field resistivity remained almost constant and changes in it did not contribute significantly to the G M R changes. The straight line I of Fig. 1 was for the case ofs d scattering into the cobalt split d-band. It is simple to show [15] that in this case the following relationship holds,
(1)
(~) ....... d ~ ( 1 --~)(~-)ideal"
The gradient of this line corresponds to the G M R of a perfect 100% AF multilayer. Fig. 2 shows a plot of the G M R versus (1 - Mr~Ms)for samples
65 60 55 50 45 /4~
40
Annealed for a total
35 30
~111 liP~lllllllllhllllllblJ~ 0.4
0.5
0.6
0.7
0.8
0.9
1-Mr/Ms Fig. 2. G M R versus (1 - M,/Ms) data for Co/Cu multilayers. The filled and open circles show the effect of annealing two samples A and B (see main text for structural details). The open square datum is for a sample C with the same nominal structure but deposited using different sputtering conditions. The significance of the agreement with the behaviour exhibited by samples A and B is discussed fully in the text.
A and B as the annealing proceeded. The agreement with Eq. (1) can be seen to be very good. The straight-line fit intercepts the G M R axis at a value of 3.8% corresponding to the cobalt anisotropic magnetoresistance. From the behaviour observed in such a plot it was argued earlier that the scattering processes involved can be determined. When discussing Fig. 1 it was shown that interface scattering through virtual bound states results in a dependence of the G M R on (1 - Mr/Ms) that is not linear. It is important to explain that this will only be the case if annealing changes the proportion of such interface states, otherwise the measured G M R will also depend linearly on (1 - Mr~Ms).It is therefore necessary to determine if annealing alters the interfaces between the layers so that the scattering process involved can be discussed. This can be done using low-angle X-ray studies and by monitoring the high-field resistivity. The latter corresponds to the multilayer resistivity in the fully FM state and its magnitude can therefore be used to determine if annealing altered the number of scattering centres. In general, if annealing introduced interdiffusion at the interface the high-field resistivity would increase. For the multilayers used in this study and other similar investigations the initial annealing stage was accompanied by a small decrease (~0.1%) in the saturation resistivity. One possible explanation of this is the healing of defects such as stacking faults upon annealing. Such a decrease in the resistivity could also arise from a sharpening of the interface composition profile upon annealing. While this could make a contribution to the observed change in the GMR, it is expected to be very small and restricted to the first anneal. Low-angle X-ray diffraction data was also taken at each stage of the annealing. The details can be found in Ref. [13] and only the conclusions will be stated here. From the relative intensities of the Bragg peaks associated with the multilayer structure, it was found that annealing had most probably not produced any changes at the interfaces along the growth direction. In order to establish whether a constant proportion of virtual bound states is contributing to the G M R for the data of samples A and B in Fig. 2, it is necessary to observe the effect of a more diffuse interface on the GMR. In Fig. 3 the low-angle
M.J. Hall et al. /Journal of Magnetism and Magnetic Materials' 173 (1997) 253- 258
107
lO~ 105 -~ 104 ~
10 3
lO2 101 10° 1
2
3
4
5
6
7
8
Angle in degrees20 Fig. 3. Low-angleX-ray data for samples A, curve (a), and C, curve (b). The drastically reduced intensity of the second-order peak in the profile of sample C, curve (b), is indicativeof more diffuse interfaces. In the figure, curve (a) has been shifted for clarity. X-ray data for a sample C is shown along with X-ray data for sample A. This sample had the same nominal layer thicknesses but sputtered under different conditions. Qualitatively, there is a clear difference between the interfaces of these two samples, with those of sample C being more diffuse [16]. A quantitative analysis using the SUPREX program of Schuller and co-workers [17] also concluded this to be the case. The open square data point in Fig. 2 is the G M R versus ( 1 - Mr~Ms) datum for sample C. It is clear that the relationship observed in Fig. 1 for samples A and B still holds. Highmore et al. [15] have published results on a series of Co/Cu multilayers made in the same system using a range of sputtering pressures. They observed a dependence of the G M R on (1 - M , . / Ms) in agreement with Eq. (1). For the G M R results they presented the samples exhibited a range of interface mixing and therefore different proportions of interface scattering. Along with the data for sample C, these results provide strong evidence that virtual bound states do not contribute to the G M R of Co/Cu multilayers. The situation for Fe/Cr is expected to be very different. In this case the interface will have its own
257
unique spin asymmetry because of the virtual bound states associated with chromium atoms in the iron layer at the interface. Due to the miscibility of this system, annealing has been found to cause interdiffusion [18] which will change the proportion of such interface scattering and consequently the magnitude of the GMR. If annealing also produced an increase in the fraction of F M regions as in the Co/Cu case, then as explained in the introduction the behaviour of G M R versus (1 - Mr/M~) would not be linear. Petroffet al. [18] and Obi et al. [19] have both shown that the G M R increased upon annealing. Without the magnetisation data it is not possible to establish if this is in agreement with the predicted behaviour, since an increase in the volume fraction of AF coupling would also produce such an increase. Korenivski et al. [20] have recently shown that the G M R of Fe/Cr multilayers increased upon annealing while the fraction of FM regions increased. Such results demonstrate that for Fe/Cr multilayers the interface is crucial to the overall GMR. It also clearly shows the benefit of plotting the G M R versus ( 1 - M../Ms), thus allowing the scattering processes involved to be determined.
4. C o n c l u s i o n s
In an attempt to understand more clearly the details of the scattering processes responsible for the G M R of Co/Cu multilayers, we have shown the usefulness of plotting the G M R versus (1 - Mr~Ms).By considering the detailed behaviour of such plots in the presence of different proportions of interface scattering it is possible to determine if the interface has its own unique spin asymmetry or if the spin-dependent scattering due to the split d-band of the ferromagnet are responsible for the GMR. From the observed linear dependence, it is possible to conclude that for the Co/Cu multilayer system the scattering involved is solely s-d scattering into the split d-band of the cobalt. In the Fe/Cr case, data from other groups show that the interface has its own spin asymmetry which is crucial in determining the magnitude of the GMR. It is important to emphasise that in the Co/Cu case interface scattering still occurs but it
258
M.J. Hall et al. /Journal of Magnetism and Magnetic Materials 173 (1997) 253 258
does not have its own spin asymmetry as for the Fe/Cr system.
Acknowledgements The work presented in this paper was funded by the EPSRC. DBJ would further like to thank GEC for his EPSRC CASE studentship award.
References [1] P.M. Levy, S. Zhang, A. Fert, Phys. Rev. Lett. 65 (1990) 1643. [2] S. Zhang, P.M. Levy, A. Fert, Phys. Rev. B 45 (1992) 8689. [3] P.M. Levy, S. Zhang, J. Magn. Magn. Mater. 93 (1991) 67. [4] M.J. Hall, B.J. Hickey, M.A. Howson, M.J. Walker, J. Xu, D. Greig, N. Wiser, Phys. Rev. B 47 (1993) 12785. [5] A. Fert, 1.A. Campbell, J. Phys. F 6 (1976) 849. [6] J. Friedel, in: W. Marshall (Ed.), Proc. Int. School in Physics 'Enrico Fermi', Course XXXVII, Theory of Magnetism in Transition Metals, Academic Press, New York, 1967, p. 283. [7] S.K.J. Lenczowski, M.A.M. Gijs, J.B. Giesbers, R.J.M. van de Veerdonk, W.J.M. de Jonge, Phys. Rev. B 50 (1994) 9982.
[8] D.M. Edwards, J. Mathon, R.B. Muniz, IEEE Trans. Magn. 27 (1991) 3548. [9] J. Mathon, Contemp. Phys. 32 (1991) 143. [10] D.M. Edwards, J. Mathon, J. Magn. Magn. Mater. 93 (1991) 85. [11] S.S.P. Parkin, N. More, K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [12] S,S.P. Parkin, R. Bhadra, K.P. Roche, Phys. Rev. Lett. 66 (1991) 2152. [13] M.J. Hall, E.D. Whitton, D.B. Jardine, R.E. Somekh, J.E. Evetts, J.A. Leake, Thin Solid Films 275 (1996) 195. [14] G. Rupp, H.A.M. van den Berg, IEEE Trans. Magn. 29 (1993) 3102. [15] R.J. Highmore, W.C. Shih, R.E. Somekh, J.E. Evetts, J. Magn. Magn. Mater. 116 (1992) 249. [16] E. Fullerton, D,M. Kelly, J. Guimpel, I.K. Schuller, Y. Brunynseraede, Phys. Rev. Lett. 68 (1992) 859. [17] E. Fullerton, I.K. Schuller, H. Vanderstraeten, Y. Bruynseraede, Phys. Rev. B 45 (1992) 9292. [18] F. Petroff, A. Barthelemy, A. Hamzic, A. FerL P. Etienne, S. Lequien, G. Creuzet, J. Magn. Magn. Mater. 93 (1991~ 95. [19] Y. Obi, K. Takanashi, Y. Mitani, N. Tsuda, H. Fujimori, J. Magn. Magn. Mater. 104-107 (1992) 1747. [20] V. Korenivski, K.V. Rao, D.M. Kelly, I.K. Schuller, K.K Larsen, J. Bottiger, J. Magn. Magn. Mater. 140-144 (1995) 549.
Journal of Magnetism and Magnetic Materials 173 (1997) 253-258
Journalof mngnetlsm ad magnetic ~i~ materials
The role of the magnetic structure and interface scattering in the GMR of magnetic multilayers M.J. Hall a, D.B. Jardine b, J.E. Evetts b'*, J.A. Leake b, R.E.
Somekh
Centre .,/br Electromagnetic and Time Metrologv, National Physical Laboratoo,, Teddington, Middlesex, TWI 1 olaf; UK b Department of Materials Science and Metallurgy, Pembroke Street, Cambridge, CB2 3QZ, UK
Received 6 February 1997
Abstract Room temperature giant magnetoresistance (GMR) has been investigated for Co/Cu(1 1 l ) multilayers grown by DC magnetron sputtering. The effect of annealing on the magnetic structure of multilayers with a nominal copper thickness of 0.9 nm has been studied, a strong linear correlation between the GMR and the volume fraction of ferromagnetic regions is observed. Such behaviour has been used to elucidate the scattering mechanism responsible for the GMR of this system, and suggests that s-d scattering into the split d-band of cobalt is the dominant mechanism responsible for the GMR of Co/Cu multilayers. This is in contrast to systems like Fe/Cr where experimental studies on the effect of interfacial mixing indicate that interfacial scattering resulting from spin-dependent interface potentials contributes strongly to the observed GMR. Kew,,ords: Sputtering; Co/Cu multilayers; Giant magnetoresistance; s-d scattering; Interface scattering
1. Introduction The fundamental origins of the giant magnetoresistance ( G M R ) effect exhibited by m a n y magnetic multilayers has presented challenges to both experimentalists and theoreticians. It is now well established that G M R is the result of an antiferromagnetic (AF) arrangement of the magnetic layers in zero field, with the resistivity d r o p result-
* Corresponding author. Tel.: + 44 1223 334364; fax: + 44 1223 334373; e-mail: [email protected].
ing from a reorientation to a ferromagnetic [FM) configuration upon the application of a sufficiently large external field. However, it remains unclear how the magnitude of the resulting G M R is dependent on the details of the multilayer magnetic and layer structures. The calculations of Levy and coworkers [1-3] attribute the G M R of Fe/Cr multilayers to spin-dependent interface potentials. Hall et al. [4] interpreted this as the effect of the strong spin dependence of electron scattering for chromium impurities in an iron host [5], which results in spin-dependent scattering at the interfaces. The origin of this is Friedel scattering [6], the resonant scattering of the virtual bound state of a chromium
0304-8853/97/$17.00 ~! 1997 Elsevier Science B.V. All rights reserved P l l S 0 3 0 4 - 8 8 53(97J001 67-4
254
M.J. Hall et al. / Journal of Magnetism and Magnetic Materials 173 (1997) 253-258
atom in the d-band of iron. Lenczowski et al. [7] have used the theories of Levy and co-workers to interpret the G M R effect in Co/Cu multilayers. In contrast, Edwards and co-workers [8-10] have shown that spin dependent s-d scattering into the split d-band of the magnetic layer can explain the observed effect. They used this approach to interpret the findings of Parkin et al. for both the Fe/Cr [11] and Co/Cu [12] systems. Clearly, there is a need for clarification. A further area of research has been into the details of the magnetic structure of the multilayers and how this relates to the measured GMR. Hall et al. [13] and Rupp et al. [14] have shown that the dependence of the G M R of Co/Cu multilayers on (1 - Mr~Ms)is linear. Here, the M r and Ms are the remanent and saturation magnetisations, respectively. In this paper we aim to show that such a plot can be used to determine if the spin-dependent scattering responsible for the G M R is due either to (1) s d scattering into the split d-band of the magnetic layer; or (2) the Friedel interface scattering introduced above and which has its own unique scattering asymmetry ~ (=p_/p+). Here p+ and p_ are the spin-up and spin-down resistivities. In doing this, we will illustrate the importance of monitoring the magnetic structure when interpreting the G M R effect. In Fig. 1 the different curves schematically show the expected behaviour for different scattering processes. In this figure the initial values of the G M R have been made equal for simplicity. In practice this will probably not be the case, however it is the trends that are important and not the absolute values. Straight line I is for the case of s~t scattering into the split magnetic d-band. In a highly perfect multilayer most of the sample is AF coupled in zero applied field. For this situation spin-up and spin-down electrons (the two spin coordinates) will be scattered in alternate magnetic layers. This is the result of the split magnetic d-band and the antiparallel arrangement of the layer magnetisation. Because of the split d-band an individual magnetic layer will only have spin-down electrons states available for scattering (strictly, this is only true at zero Kelvin and will be assumed here for simplicity). For two successive magnetic layers in an AF
configuration the local direction of magnetisation is reversed with respect to the spin coordinate of any electron. In one layer an electron which is spin-up with respect to the local magnetisation cannot be scattered. Then, on passing into the next layer the local magnetisation is reversed and the same electron will now have states available for scattering. Consequently, both spin coordinates can be scattered and both the zero-field resistivity and GMR, are high. In the presence of FM regions the direction of the local magnetisation does not reverse and the spin-up electrons do not have states available. The result of introducing such regions will be a decrease in the zero-field resistivity and consequently the GMR. Hence, for the case of s-d scattering into the split d-band of the magnetic layer the dependence of the G M R on (1 - Mr/M~) will be linear. For this type of scattering the associated scattering potential can result, for example, from phonons, stacking faults and point defects. The interface itself could also provide many scattering centres, with the spin dependence of the scattering arising solely from the split d-bands of the magnetic layers. Changes to the proportion of interface scattering alone will not effect the 80
II
I
0.0
I
I
I
I
1 - M~/Ms
[
I
I 1.0
Fig. 1. Schematic illustrating the dependence of the measured G M R on (1 - M r / M ~ ) for different scattering processes. For curves I and II the behaviour shown is in the presence of a changing proportion of interface scattering as an experimental sequence that progressively mixed the interface proceeded. The significance of the trends indicated by these curves is discussed fully in the text.
M.J. Hall et al. / Journal o[' Magnetism and Magnetic Materials 173 (1997) 253 258
magnitude of the G M R observed. This is because the spin-up and spin-down scattering rates will have increased by the same proportion. In curve lI of Fig. 1 the situation is for interface scattering that involves virtual bound states. Here the maximum in the G M R for a sequence of experiments that alter the degree of interface mixing does not necessarily occur at a value of (1 - M r / M J corresponding to 100% AF alignment. In this situation the introduction of FM regions will again lower the GMR. The distinction here is that the FM regions not only remove the possibility of spin-up electrons being scattered into the split dband of the magnetic layer, they also suppress the resonant scattering associated with virtual bound states. The origin of the initial increase in the G M R of curve II is the changes to the proportion of interface scattering. If the decrease in (1 - Mr/MJ is accompanied by an increase in the proportion of Friedel interface scattering, then the relative value of the interface-spin asymmetry is important. When this asymmetry is larger than that associated with the split d-band of the ferromagnetic layer, then the overall proportion of spin-dependent scattering can increase. The result of this is an increase in the GMR. Curve lI is one example of a family of curves, the exact forms of which depend on the detailed changes in the proportion of interface scattering upon annealing. What is clear is that the G M R would not initially decrease in the way illustrated by curve I. Curve III illustrates the situation where the G M R decreases. Here, one possibility is the progressive introduction of additional scattering which does not involve spin-dependent scattering parameters. An example of such a situation would be additional scattering within the copper layer since this would not depend on a split d-band. An additional effect which could result in the type of behaviour shown in III is the removal of virtual bound states upon annealing. Like before, this curve is just one example of a family of curves. Whatever the details of the structural changes upon annealing, it is clear that for curves II and Ill the G M R versus (1 - M~/MJ will not be linear. In what follows the variation of the electrical resistivity, GMR, magnetic structure and interface structure are reported for Co/Cu samples at various stages of annealing. From the observed
255
dependence of the G M R on (1 - Mr/MJ the results suggest that for this system electron scattering is spin dependent purely by virtue of the split d-bands of the cobalt layers. For the Fe/Cr system we will use these ideas along with published data to show that the scattering processes involved have a strong contribution from interface scattering with its own unique spin asymmetry.
2. Experimental details Two Co/Cu multilayers with a nominal structure of Fe(5.0)/[Co( 1.5)Cu(0.9)]30, where the thicknesses are in nm, have been deposited onto Si(1 0 01 substrates using UHV gettered DC magnetron sputtering. During deposition the substrates were at ambient temperature and rotated below two planar magnetrons which had been pre-sputtered to remove contaminants. For all runs the sputtering gas used was 6 N argon, further purified to 8 N, at a pressure of 0.5 Pa. Before deposition commenced the outgassing rate was typically 5 x 10- 11 Pa s Determination of the actual layer thicknesses was done using low-angle X-ray diffraction [XRDI to give the bilayer thickness. EDX data on the composition ratio was then used to determine the individual layer thicknesses. All magnetic and transport measurements on these samples were done at room temperature. Magnetisation loops were obtained using an Aerosonics 5001 vibrating sample magnetometer [VSMt. For resistance and G M R measurements a standard four-probe DC technique was used. Repeated measurements to determine the reproducibility of the resistance measurements indicated that a precision from one measurement to the next of better than 0.5% was possible. There was an absolute error of around 5"/,, arising from the film-thickness determination.
3. Annealing results and discussion An extensive annealing study on two samples with the same nominal structure and initial highfield room temperature resistivities of 18 Bf~cm have been performed. After each stage of annealing the magnetisation and G M R of the samples were
M.J. Hall et al. /Journal of Magnetism and Magnetic Materials 173 (1997) 253-258
256
measured. To investigate whether the details of the annealing history were important, sample A was annealed at increasing temperatures for various times and sample B at 220°C for increasing times. The full annealing history used can be found in Ref. [13]. After each stage of annealing the G M R of the samples always decreased. This was a result of the decrease in the zero-field resistivity, the high-field resistivity remained almost constant and changes in it did not contribute significantly to the G M R changes. The straight line I of Fig. 1 was for the case ofs d scattering into the cobalt split d-band. It is simple to show [15] that in this case the following relationship holds,
(1)
(~) ....... d ~ ( 1 --~)(~-)ideal"
The gradient of this line corresponds to the G M R of a perfect 100% AF multilayer. Fig. 2 shows a plot of the G M R versus (1 - Mr~Ms)for samples
65 60 55 50 45 /4~
40
Annealed for a total
35 30
~111 liP~lllllllllhllllllblJ~ 0.4
0.5
0.6
0.7
0.8
0.9
1-Mr/Ms Fig. 2. G M R versus (1 - M,/Ms) data for Co/Cu multilayers. The filled and open circles show the effect of annealing two samples A and B (see main text for structural details). The open square datum is for a sample C with the same nominal structure but deposited using different sputtering conditions. The significance of the agreement with the behaviour exhibited by samples A and B is discussed fully in the text.
A and B as the annealing proceeded. The agreement with Eq. (1) can be seen to be very good. The straight-line fit intercepts the G M R axis at a value of 3.8% corresponding to the cobalt anisotropic magnetoresistance. From the behaviour observed in such a plot it was argued earlier that the scattering processes involved can be determined. When discussing Fig. 1 it was shown that interface scattering through virtual bound states results in a dependence of the G M R on (1 - Mr/Ms) that is not linear. It is important to explain that this will only be the case if annealing changes the proportion of such interface states, otherwise the measured G M R will also depend linearly on (1 - Mr~Ms).It is therefore necessary to determine if annealing alters the interfaces between the layers so that the scattering process involved can be discussed. This can be done using low-angle X-ray studies and by monitoring the high-field resistivity. The latter corresponds to the multilayer resistivity in the fully FM state and its magnitude can therefore be used to determine if annealing altered the number of scattering centres. In general, if annealing introduced interdiffusion at the interface the high-field resistivity would increase. For the multilayers used in this study and other similar investigations the initial annealing stage was accompanied by a small decrease (~0.1%) in the saturation resistivity. One possible explanation of this is the healing of defects such as stacking faults upon annealing. Such a decrease in the resistivity could also arise from a sharpening of the interface composition profile upon annealing. While this could make a contribution to the observed change in the GMR, it is expected to be very small and restricted to the first anneal. Low-angle X-ray diffraction data was also taken at each stage of the annealing. The details can be found in Ref. [13] and only the conclusions will be stated here. From the relative intensities of the Bragg peaks associated with the multilayer structure, it was found that annealing had most probably not produced any changes at the interfaces along the growth direction. In order to establish whether a constant proportion of virtual bound states is contributing to the G M R for the data of samples A and B in Fig. 2, it is necessary to observe the effect of a more diffuse interface on the GMR. In Fig. 3 the low-angle
M.J. Hall et al. /Journal of Magnetism and Magnetic Materials' 173 (1997) 253- 258
107
lO~ 105 -~ 104 ~
10 3
lO2 101 10° 1
2
3
4
5
6
7
8
Angle in degrees20 Fig. 3. Low-angleX-ray data for samples A, curve (a), and C, curve (b). The drastically reduced intensity of the second-order peak in the profile of sample C, curve (b), is indicativeof more diffuse interfaces. In the figure, curve (a) has been shifted for clarity. X-ray data for a sample C is shown along with X-ray data for sample A. This sample had the same nominal layer thicknesses but sputtered under different conditions. Qualitatively, there is a clear difference between the interfaces of these two samples, with those of sample C being more diffuse [16]. A quantitative analysis using the SUPREX program of Schuller and co-workers [17] also concluded this to be the case. The open square data point in Fig. 2 is the G M R versus ( 1 - Mr~Ms) datum for sample C. It is clear that the relationship observed in Fig. 1 for samples A and B still holds. Highmore et al. [15] have published results on a series of Co/Cu multilayers made in the same system using a range of sputtering pressures. They observed a dependence of the G M R on (1 - M , . / Ms) in agreement with Eq. (1). For the G M R results they presented the samples exhibited a range of interface mixing and therefore different proportions of interface scattering. Along with the data for sample C, these results provide strong evidence that virtual bound states do not contribute to the G M R of Co/Cu multilayers. The situation for Fe/Cr is expected to be very different. In this case the interface will have its own
257
unique spin asymmetry because of the virtual bound states associated with chromium atoms in the iron layer at the interface. Due to the miscibility of this system, annealing has been found to cause interdiffusion [18] which will change the proportion of such interface scattering and consequently the magnitude of the GMR. If annealing also produced an increase in the fraction of F M regions as in the Co/Cu case, then as explained in the introduction the behaviour of G M R versus (1 - Mr/M~) would not be linear. Petroffet al. [18] and Obi et al. [19] have both shown that the G M R increased upon annealing. Without the magnetisation data it is not possible to establish if this is in agreement with the predicted behaviour, since an increase in the volume fraction of AF coupling would also produce such an increase. Korenivski et al. [20] have recently shown that the G M R of Fe/Cr multilayers increased upon annealing while the fraction of FM regions increased. Such results demonstrate that for Fe/Cr multilayers the interface is crucial to the overall GMR. It also clearly shows the benefit of plotting the G M R versus ( 1 - M../Ms), thus allowing the scattering processes involved to be determined.
4. C o n c l u s i o n s
In an attempt to understand more clearly the details of the scattering processes responsible for the G M R of Co/Cu multilayers, we have shown the usefulness of plotting the G M R versus (1 - Mr~Ms).By considering the detailed behaviour of such plots in the presence of different proportions of interface scattering it is possible to determine if the interface has its own unique spin asymmetry or if the spin-dependent scattering due to the split d-band of the ferromagnet are responsible for the GMR. From the observed linear dependence, it is possible to conclude that for the Co/Cu multilayer system the scattering involved is solely s-d scattering into the split d-band of the cobalt. In the Fe/Cr case, data from other groups show that the interface has its own spin asymmetry which is crucial in determining the magnitude of the GMR. It is important to emphasise that in the Co/Cu case interface scattering still occurs but it
258
M.J. Hall et al. /Journal of Magnetism and Magnetic Materials 173 (1997) 253 258
does not have its own spin asymmetry as for the Fe/Cr system.
Acknowledgements The work presented in this paper was funded by the EPSRC. DBJ would further like to thank GEC for his EPSRC CASE studentship award.
References [1] P.M. Levy, S. Zhang, A. Fert, Phys. Rev. Lett. 65 (1990) 1643. [2] S. Zhang, P.M. Levy, A. Fert, Phys. Rev. B 45 (1992) 8689. [3] P.M. Levy, S. Zhang, J. Magn. Magn. Mater. 93 (1991) 67. [4] M.J. Hall, B.J. Hickey, M.A. Howson, M.J. Walker, J. Xu, D. Greig, N. Wiser, Phys. Rev. B 47 (1993) 12785. [5] A. Fert, 1.A. Campbell, J. Phys. F 6 (1976) 849. [6] J. Friedel, in: W. Marshall (Ed.), Proc. Int. School in Physics 'Enrico Fermi', Course XXXVII, Theory of Magnetism in Transition Metals, Academic Press, New York, 1967, p. 283. [7] S.K.J. Lenczowski, M.A.M. Gijs, J.B. Giesbers, R.J.M. van de Veerdonk, W.J.M. de Jonge, Phys. Rev. B 50 (1994) 9982.
[8] D.M. Edwards, J. Mathon, R.B. Muniz, IEEE Trans. Magn. 27 (1991) 3548. [9] J. Mathon, Contemp. Phys. 32 (1991) 143. [10] D.M. Edwards, J. Mathon, J. Magn. Magn. Mater. 93 (1991) 85. [11] S.S.P. Parkin, N. More, K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [12] S,S.P. Parkin, R. Bhadra, K.P. Roche, Phys. Rev. Lett. 66 (1991) 2152. [13] M.J. Hall, E.D. Whitton, D.B. Jardine, R.E. Somekh, J.E. Evetts, J.A. Leake, Thin Solid Films 275 (1996) 195. [14] G. Rupp, H.A.M. van den Berg, IEEE Trans. Magn. 29 (1993) 3102. [15] R.J. Highmore, W.C. Shih, R.E. Somekh, J.E. Evetts, J. Magn. Magn. Mater. 116 (1992) 249. [16] E. Fullerton, D,M. Kelly, J. Guimpel, I.K. Schuller, Y. Brunynseraede, Phys. Rev. Lett. 68 (1992) 859. [17] E. Fullerton, I.K. Schuller, H. Vanderstraeten, Y. Bruynseraede, Phys. Rev. B 45 (1992) 9292. [18] F. Petroff, A. Barthelemy, A. Hamzic, A. FerL P. Etienne, S. Lequien, G. Creuzet, J. Magn. Magn. Mater. 93 (1991~ 95. [19] Y. Obi, K. Takanashi, Y. Mitani, N. Tsuda, H. Fujimori, J. Magn. Magn. Mater. 104-107 (1992) 1747. [20] V. Korenivski, K.V. Rao, D.M. Kelly, I.K. Schuller, K.K Larsen, J. Bottiger, J. Magn. Magn. Mater. 140-144 (1995) 549.