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Giant magnetoresistance in Fe and Co based spin valve structures
29 January 2001
Physics Letters A 279 (2001) 255–260 www.elsevier.nl/locate/pla
Giant magnetoresistance in Fe and Co based spin valve structures M. Guth a , G. Schmerber a , A. Dinia a,∗ , D. Muller b , H. Errahmani c a IPCMS-GEMM, ULP, UMR 46 CNRS, 23 rue du Loess, 67037 Strasbourg, France b Laboratoire PHASE, UPR 292 CNRS, 23 rue du Loess, 67037 Strasbourg, France c Laboratoire de Physique des Matériaux, Faculté des Sciences, BP 1014, Rabat, Morocco
Received 4 November 2000; accepted 27 November 2000 Communicated by V.M. Agranovich
Abstract Giant magnetoresistance (GMR) effects up to 7% have been observed in Fe/Cu/Co spin valve structures. The samples have been grown by ion beam sputtering at room temperature on a glass substrate covered by Cr buffer layer. The best results have been obtained for the sandwich in which a 0.5 nm Co thin layer has been deposited between Fe and Cu layer with the following structure: Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Cu2nm /Cr2nm. The role of the thin Co layer is to increase the spin-dependent scattering centers. Annealing treatments during 2 h have been performed on this sample at different temperature in order to study the thermal stability. A progressive increase of the GMR has been observed for the annealing at 200 and 250◦ C, while at 300◦ C, the GMR completely disappears. Rutherford back scattering (RBS) spectroscopy has been used to explain the physical origin of this drop, which is attributed to the total interdiffusion through grains boundaries between Fe, Co and Cu layers. 2001 Elsevier Science B.V. All rights reserved. PACS: 75.70.-i; 75.70.Cn; 75.70.Pa
1. Introduction Since its discovery [1], giant magnetoresistance (GMR) has remained at the center of an important part of current research in magnetism. This is partly due to its importance as a fundamental phenomenon as well as to its potential for applications. It has been early reported that the spin valve structures [2] are the most promising candidate for the next generation of read heads for ultrahigh density magnetic recording, because they present a very attractive magnetoresistance (MR) signal and sensitivity at low fields.
* Corresponding author. Tel.: (333)-88-10-70-67, fax: (333)-8810-72-49. E-mail address: [email protected] (A. Dinia).
Most of these structures require special precaution to guarantee that the mutual magnetization M-orientations in adjacent magnetic layers depend on the external field. The scheme favored the most is the socalled hard–soft system, in which layers with fixed magnetization are alternated with soft-magnetic detection layers. Ideally, the hard subsystems and the detection layer are magnetically decoupled. Therefore, the GMR in these structures is related to the relative angle between the magnetization of the hard and soft magnetic layers. There are various alternatives for realizing the hard– soft principle. The exchange-biased system [3] has been developed in which, in principle, the orientation of the magnetization in one magnetic layer is pinned by direct exchange coupling to an antiferromagnetic layer. These systems are currently used for data stor-
0375-9601/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 9 6 0 1 ( 0 0 ) 0 0 8 2 3 - 9
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M. Guth et al. / Physics Letters A 279 (2001) 255–260
age industry but present small disadvantages for their use for general sensor application with high demands of the operational temperature window, for the following reasons: (i) the anisotropy of these antiferromagnetic layers decreases monotonically towards their Néel temperatures being typically 150◦ C, i.e., much too small for sensor applications; (ii) the antiferromagnetic layer corrodes easily [4]. However, the NiMn has much higher Néel temperature of about 400◦ C and acceptable corrosion properties but it requires, however, a high temperature annealing in a magnetic field to establish the coupling [5]. Another candidate is NiO, which can be deposited in a reactive sputtering process, only with special control and attention [6]. Another interesting alternative for spin valve structures, which answers to the above limitations, corresponds to the scheme with the so-called artificial antiferromagnetic subsystem (AAF) by Van den Berg et al. [7], showing superior behavior, in particular for angle and position detector systems. A thin 1 nm thick Cu layer couples the AAF magnetic Co layers antiferromagnetically. This AAF subsystem is itself magnetically decoupled by an intermediate metallic layer, from the soft-magnetic detection layer. The aim of the present Letter is to show a very simple way to achieve a scheme of the so-called hard– soft system, with very interesting GMR values and a thermal stability until 250◦ C. In this case, the hard subsystem consists of one single hard magnetic layer decoupled by an intermediate metallic layer from one soft-magnetic detection layer.
2. Experimental The samples were prepared at room temperature by ion beam sputtering (IBS) technique using a twogrid Kaufmann [8] ion source. The base pressure was about 5 × 10−9 mbar, and the working argon pressure was 2 × 10−4 mbar. The Ar+ ions are incident on the sputter target at 400 V at an angle of about 45◦ , and the beam current is around 5 mA. The growth of the deposited films was monitored by a vibrating quartz crystal oscillator, which is placed in close proximity to the substrates. The growth deposition rates were typically about 7 Å/min for Co, 4 Å/min for Cr, 4.2 Å/min for Fe and 8 Å/min for Cu.
The samples were deposited at room temperature on glass substrates covered by 3 nm Cr buffer layer. Then the Fe3nm /Cu3nm /Co5nm trilayer has been deposited and covered by Cu2nm /Cr2nm to protect the sample against oxidation. The magnetoresistance of the samples was measured with a low-frequency ac lock-in technique, and using a built-up compensation of zero magnetic-field resistance, with a conventional four in line goldplated contacts. At room temperature, the measurements were performed up to 20 kOe magnetic field applied in the plane, both parallel and perpendicular to the in-plane current direction, in order to detect any anisotropic magnetoresistance contribution. The magnetization measurements were carried out using an alternating gradient force magnetometer (AGFM) [9], with the magnetic field applied in the plane of the film and reaching the maximum of 13 kOe. Rutherford backscattering measurements have been performed with a 2 MeV 4 He+ beam produced by a van der Graaf particle accelerator. Special attention was paid to the analysis geometry, in order to obtain a high mass and depth resolution.
3. Results Fig. 1 shows magnetization and magnetoresistance loops of the Cr3nm /Fe3nm /Cu3nm /Co5nm /Cu2nm /Cr2nm sandwich. For the magnetization curve two hysteretic loops are distinguished: the first one with the rotation of the Fe magnetic layer for applied magnetic field above 50 Oe, and the second loop, for which the Co layer begins to rotate for magnetic field larger than 300 Oe. The magnetoresistance curve shows a similar behavior. Coming from the positive saturation down to zero field, the parallel configuration of the two ferromagnetic layers is conserved, and only a very small variation in the resistivity is observed. After reversing the applied magnetic field, the resistivity increases with increasing the magnetic field to reach a plateau for a 200 Oe applied magnetic field, with the GMR value of about 3%. In this plateau, where the antiparallel alignment of the two magnetic layers is fully achieved, the resistivity is constant until 350 Oe where a sudden resistivity drop is observed. This corresponds to the rotation of the Co magnetic layer inducing the
M. Guth et al. / Physics Letters A 279 (2001) 255–260
Fig. 1. Magnetization and magnetoresistance curves at room temperature for the Cr3nm /Fe3nm /Cu3nm /Co5nm /Cu2nm /Cr2nm sandwich. For the magnetoresistance curve, the magnetic field is applied in the film plane both parallel (solid line) and perpendicular (dashed line) to the current direction.
parallel configuration of both magnetic layers and the resistivity is constant. It is then interesting to notice that, this GMR value is comparable to the values obtained for the exchange biased system, and the magnetic window is large. The resistivity measurements have been done for both in-plane magnetic applied field parallel and perpendicular to the current direction. These variations reported in Fig. 1 give a clear evidence of a small anisotropic MR contribution of about 0.3%. To increase the GMR value for the sandwich presented above, we have tried to insert a thin Co magnetic layer between the soft Fe magnetic layer and the non-magnetic Cu layer. The role of this thin Co layer is to increase the scattering centers. A special attention has to be made to the thickness of the Co
257
Fig. 2. Magnetization and magnetoresistance curves at room temperature for the Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Cu2nm /Cr2nm sandwich. For the magnetoresistance curve, the magnetic field is applied in the film plane and perpendicular to the current direction.
layer in order to find a compromise between the increase of the GMR and the hardening of the soft layer. Several sandwiches have been prepared with different Co thickness, and the optimum value has been obtained for 0.5 nm of Co. The results of the optimized Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Cu2nm / Cr2nm sandwich are reported in Fig. 2. The GMR value is sensitively increased to reach 5.5%, while the magnetic window is decreased but still interesting. The increase of the GMR by inserting a Co scattering centers in close proximity to the soft magnetic layers has already been reported by Parkin et al. [10], and seems to be a general behavior. Attention has also been paid to the role of the protection layer. Several capping layers have been used to optimize the GMR values. For example, Fig. 3 shows the GMR and magnetization loops of
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Fig. 3. Magnetization and magnetoresistance curves at room temperature for the Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Ru1nm sandwich. For the magnetoresistance curve, the magnetic field is applied in the film plane and perpendicular to the current direction.
Fig. 4. Magnetization and magnetoresistance curves at room temperature for the Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Cu2nm /Cr2nm sandwich annealed at 250◦ C during 2 h. For the magnetoresistance curve, the magnetic field is applied in the film plane and perpendicular to the current direction.
the Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Ru1nm sandwich capped with 1 nm Ru. The GMR value of 4.5% of this sample is smaller than the value observed for the sandwich capped with Cu2nm /Cr2nm layers. The second difference between these two capping layers is the small decrease of the coercive field of the top Co layer. These two consequences can be easily understood since it is well known that the Co and Ru are miscible, and the interdiffusion is likely to occur at the Co/Ru interface. Therefore, the CoRu alloy at the outer interface decreases the spin-dependent scattering and the coercive field of the top layer. To optimize the magneto-transport properties, and to study the thermal stability, annealing at several temperatures have been performed on the as-deposited sandwich under nitrogen atmosphere during 2 h. After each annealing, the sample has been characterized by
magnetization and magnetoresistance measurements. For the annealed sample at 200◦ C, a sensitive increase of the GMR, which reaches 6%, is observed, while the hysteresis loop is roughly the same. Such increase in GMR is more pronounced after annealing at 250◦ C. As reported in Fig. 4, the GMR is close to 7%. Moreover, the coercive field of the top Co layer is enhanced, and as a consequence, the magnetic field window of the resistivity plateau is increased. For subsequent annealings at 300 and 350◦ C, the GMR drops to reach values around 0.5% (Fig. 5), while the magnetization curve shows only a unique loop with one coercive field, instead of two coercive fields, and no more distinction between the rotation of the soft Fe layer and the hard Co layer is observed. This behavior can be understood in terms of diffusion between Cu, Co and Fe. For the first annealing, the temperature
M. Guth et al. / Physics Letters A 279 (2001) 255–260
Fig. 5. Magnetization and magnetoresistance curves at room temperature for the Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Cu2nm /Cr2nm sandwich after annealing at 300◦ C (solid line) and 350◦ C (dashed line) during 2 h. For the magnetoresistance curve, the magnetic field is applied in the film plane and perpendicular to the current direction.
is not high enough to allow diffusion in the bulk of the layers, thus diffusion occurs only at the interfaces, which has as effect to smoothen the interfaces and to improve the GMR. As the annealing temperature is increased, the diffusion of Cu at the grain boundaries of Fe and Co, and of the Co at the Cu grain boundaries happens in the whole layers. It is well known that diffusion at grain boundaries has as consequence, the increase of the coercive field. This is exactly what happens in this case. The coercive fields of both Fe and Co layers are increased to reach the situation where the coercive fields for both layers are close each to other, and both Fe and Co moments rotate simultaneously, and the GMR is strongly decreased. In order to understand the origin of the strong variation in the magneto-transport properties after
259
Fig. 6. Rutherford backscattering spectra performed at room temperature for the Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Cu2nm /Cr2nm sandwich. (a) Spectrum of the as-deposited sandwich (solid line) and the simulated one (thick dashed line). In this simulated spectrum, the contribution of each single layer in the sandwiches is also presented (thin dashed line). (b) Spectra for both as-deposited (thin solid line) and annealed sandwich at 350◦ C during 2 h (bold solid line).
annealing, and to confirm the hypothesis given in the last paragraph, RBS measurements have been performed on the as-deposited and the 350◦ C annealed sandwiches. The spectra are reported in Fig. 6. As can be seen for the as-deposited sandwich in Fig. 6(a), which presents both the experimental and simulated spectra, the only separated peaks correspond to the 2 nm Cu capping layer at the energy of 1550 keV and the 3 nm Cr buffer layer at the energy of 1430 keV. Two other resolved peaks appear at the energy of 1475 and 1513 keV, respectively. The first one corresponds to the contributions of the 3 nm Fe layer and the 2 nm Cr capping layer. The second peak corresponds to the contributions of the 5 nm Co layer, which is the most
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important, and the 3 nm Cu decoupling layer. The effect of the annealing on the RBS spectra is evidenced in Fig. 6(b). For the sample annealed at 350◦ C, three important effects are observed: (i) a shift of the Cu/Co peak (at 1513 keV) and of the Cu capping layer peak (at 1550 keV) towards low energy; (ii) the decrease in the intensity of the Cu/Co peak at 1513 keV; (iii) the increase in the intensities of the all three other peaks at the energies 1430, 1475 and 1550 keV. The first effect, which corresponds to the shift towards small energies of the 1550 keV Cu capping layer and the 1513 keV Co/Cu active layers, can be easily understood since it is well known that diffusion of Cu through Co grain boundaries is likely to occur [11–13]. This means that there is a diffusion of the Cu atoms from the capping layer through Co grain boundaries, and of the Co atoms through Cu grain boundary for the Cu decoupling layer, and finally, of the Cu atoms from the decoupling layer and/or Co atoms through the Fe layer. Such explanations can also answer to the second and the third observed consequence of the annealing. The decrease in the intensity of the Cu/Co peak at 1513 keV can be explained by the diffusion of the Co through Cu grain boundaries, and of the Cu and Co through Fe grain boundaries. This is also confirmed by the strong increase in the intensity of the Fe peak at 1475 keV. These results are of good support to the hypothesis given to explain the magneto-transport properties change after annealing. We can, however, not give a quantitative analysis of the RBS spectra, due to the large number of layers and interfaces in our samples. The strong decrease of the GMR, and the observation of one coercive field, instead of two, is a good indication that the Fe/Cu/Co layers are completely intermixed and behave like one complex granular FeCuCo alloy layer.
4. Conclusion To conclude, we have shown that by using a very simple spin valve stack we can obtain an interesting GMR. The spin valve stack presents a good thermal
stability until 250◦C annealing temperature. These results are cheering but need to be improved for application purpose. RBS analysis performed on the as-deposited and the 350◦C annealed samples have shown that the observed change in the GMR and magnetic properties is mainly due to a strong diffusion between Cu, Co and Fe through grain boundaries. It is also important to add that RBS technique is a very sensitive tool to inform on diffusion mechanism.
Acknowledgements This work was supported by “la région d’Alsace” and SENSTRONIC company in Saverne, and by PICS (Projet international de coopération scientifique) contract between IPCMS-CNRS (French) and CNCPRST (Morocco).
References [1] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van-Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich, J. Chazelas, Phys. Rev. Lett. 61 (1988) 1417. [2] B. Dieny, V.S. Speriousu, S.S.P. Parkin, B.A. Gurney, D.R. Wilhoit, D. Mauri, Phys. Rev. B 43 (1991) 1297. [3] C. Tsang, R.E. Fontana, T. Lin, D.E. Heim, V.S. Spuriosu, B.A. Gurney, L. Williams, IEEE Trans. Magn. 30 (1994) 3801. [4] C. Tsang, K. Lee, Appl. Phys. 53 (1982) 2605. [5] S. Mao, S. Gangopadhyay, N. Amin, E. Murdock, Appl. Phys. Lett. 69 (1996) 3593. [6] W.F. Egelhoff Jr. et al., J. Appl. Phys. 79 (1996) 5277. [7] H.A.M. van den Berg, W. Clemens, G. Gieres, G. Rupp, M. Vieth, J. Wecker, S. Zoll, J. Magn. Magn. Mater. 165 (1996) 524. [8] H.R. Kaufmann, J.J. Cuomo, J.M.E. Harper, Vac. Sci. Technol. 21 (1982) 725. [9] P.J. Flanders, J. Appl. Phys. 63 (8) (1988) 3940. [10] S.S.P. Parkin, Phys. Rev. Lett. 71 (1993) 1641. [11] M. Hansen, in: Constitution of Binary Alloys, 2nd ed., McGraw-Hill, New York, 1958, p. 469. [12] J.G. Pellerin, S.G.H. Anderson, P.S. Ho, C. Wooten, K.R. Coffey, J.K. Howard, K. Brmak, J. Appl. Phys. 75 (10) (1994) 5052. [13] F. Bensmina, P. Humbert, A. Dinia, D. Muller, V.S. Sperious, B.A. Gurney, J. Magn. Magn. Mater. 198–199 (1999) 338.
Physics Letters A 279 (2001) 255–260 www.elsevier.nl/locate/pla
Giant magnetoresistance in Fe and Co based spin valve structures M. Guth a , G. Schmerber a , A. Dinia a,∗ , D. Muller b , H. Errahmani c a IPCMS-GEMM, ULP, UMR 46 CNRS, 23 rue du Loess, 67037 Strasbourg, France b Laboratoire PHASE, UPR 292 CNRS, 23 rue du Loess, 67037 Strasbourg, France c Laboratoire de Physique des Matériaux, Faculté des Sciences, BP 1014, Rabat, Morocco
Received 4 November 2000; accepted 27 November 2000 Communicated by V.M. Agranovich
Abstract Giant magnetoresistance (GMR) effects up to 7% have been observed in Fe/Cu/Co spin valve structures. The samples have been grown by ion beam sputtering at room temperature on a glass substrate covered by Cr buffer layer. The best results have been obtained for the sandwich in which a 0.5 nm Co thin layer has been deposited between Fe and Cu layer with the following structure: Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Cu2nm /Cr2nm. The role of the thin Co layer is to increase the spin-dependent scattering centers. Annealing treatments during 2 h have been performed on this sample at different temperature in order to study the thermal stability. A progressive increase of the GMR has been observed for the annealing at 200 and 250◦ C, while at 300◦ C, the GMR completely disappears. Rutherford back scattering (RBS) spectroscopy has been used to explain the physical origin of this drop, which is attributed to the total interdiffusion through grains boundaries between Fe, Co and Cu layers. 2001 Elsevier Science B.V. All rights reserved. PACS: 75.70.-i; 75.70.Cn; 75.70.Pa
1. Introduction Since its discovery [1], giant magnetoresistance (GMR) has remained at the center of an important part of current research in magnetism. This is partly due to its importance as a fundamental phenomenon as well as to its potential for applications. It has been early reported that the spin valve structures [2] are the most promising candidate for the next generation of read heads for ultrahigh density magnetic recording, because they present a very attractive magnetoresistance (MR) signal and sensitivity at low fields.
* Corresponding author. Tel.: (333)-88-10-70-67, fax: (333)-8810-72-49. E-mail address: [email protected] (A. Dinia).
Most of these structures require special precaution to guarantee that the mutual magnetization M-orientations in adjacent magnetic layers depend on the external field. The scheme favored the most is the socalled hard–soft system, in which layers with fixed magnetization are alternated with soft-magnetic detection layers. Ideally, the hard subsystems and the detection layer are magnetically decoupled. Therefore, the GMR in these structures is related to the relative angle between the magnetization of the hard and soft magnetic layers. There are various alternatives for realizing the hard– soft principle. The exchange-biased system [3] has been developed in which, in principle, the orientation of the magnetization in one magnetic layer is pinned by direct exchange coupling to an antiferromagnetic layer. These systems are currently used for data stor-
0375-9601/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 9 6 0 1 ( 0 0 ) 0 0 8 2 3 - 9
256
M. Guth et al. / Physics Letters A 279 (2001) 255–260
age industry but present small disadvantages for their use for general sensor application with high demands of the operational temperature window, for the following reasons: (i) the anisotropy of these antiferromagnetic layers decreases monotonically towards their Néel temperatures being typically 150◦ C, i.e., much too small for sensor applications; (ii) the antiferromagnetic layer corrodes easily [4]. However, the NiMn has much higher Néel temperature of about 400◦ C and acceptable corrosion properties but it requires, however, a high temperature annealing in a magnetic field to establish the coupling [5]. Another candidate is NiO, which can be deposited in a reactive sputtering process, only with special control and attention [6]. Another interesting alternative for spin valve structures, which answers to the above limitations, corresponds to the scheme with the so-called artificial antiferromagnetic subsystem (AAF) by Van den Berg et al. [7], showing superior behavior, in particular for angle and position detector systems. A thin 1 nm thick Cu layer couples the AAF magnetic Co layers antiferromagnetically. This AAF subsystem is itself magnetically decoupled by an intermediate metallic layer, from the soft-magnetic detection layer. The aim of the present Letter is to show a very simple way to achieve a scheme of the so-called hard– soft system, with very interesting GMR values and a thermal stability until 250◦ C. In this case, the hard subsystem consists of one single hard magnetic layer decoupled by an intermediate metallic layer from one soft-magnetic detection layer.
2. Experimental The samples were prepared at room temperature by ion beam sputtering (IBS) technique using a twogrid Kaufmann [8] ion source. The base pressure was about 5 × 10−9 mbar, and the working argon pressure was 2 × 10−4 mbar. The Ar+ ions are incident on the sputter target at 400 V at an angle of about 45◦ , and the beam current is around 5 mA. The growth of the deposited films was monitored by a vibrating quartz crystal oscillator, which is placed in close proximity to the substrates. The growth deposition rates were typically about 7 Å/min for Co, 4 Å/min for Cr, 4.2 Å/min for Fe and 8 Å/min for Cu.
The samples were deposited at room temperature on glass substrates covered by 3 nm Cr buffer layer. Then the Fe3nm /Cu3nm /Co5nm trilayer has been deposited and covered by Cu2nm /Cr2nm to protect the sample against oxidation. The magnetoresistance of the samples was measured with a low-frequency ac lock-in technique, and using a built-up compensation of zero magnetic-field resistance, with a conventional four in line goldplated contacts. At room temperature, the measurements were performed up to 20 kOe magnetic field applied in the plane, both parallel and perpendicular to the in-plane current direction, in order to detect any anisotropic magnetoresistance contribution. The magnetization measurements were carried out using an alternating gradient force magnetometer (AGFM) [9], with the magnetic field applied in the plane of the film and reaching the maximum of 13 kOe. Rutherford backscattering measurements have been performed with a 2 MeV 4 He+ beam produced by a van der Graaf particle accelerator. Special attention was paid to the analysis geometry, in order to obtain a high mass and depth resolution.
3. Results Fig. 1 shows magnetization and magnetoresistance loops of the Cr3nm /Fe3nm /Cu3nm /Co5nm /Cu2nm /Cr2nm sandwich. For the magnetization curve two hysteretic loops are distinguished: the first one with the rotation of the Fe magnetic layer for applied magnetic field above 50 Oe, and the second loop, for which the Co layer begins to rotate for magnetic field larger than 300 Oe. The magnetoresistance curve shows a similar behavior. Coming from the positive saturation down to zero field, the parallel configuration of the two ferromagnetic layers is conserved, and only a very small variation in the resistivity is observed. After reversing the applied magnetic field, the resistivity increases with increasing the magnetic field to reach a plateau for a 200 Oe applied magnetic field, with the GMR value of about 3%. In this plateau, where the antiparallel alignment of the two magnetic layers is fully achieved, the resistivity is constant until 350 Oe where a sudden resistivity drop is observed. This corresponds to the rotation of the Co magnetic layer inducing the
M. Guth et al. / Physics Letters A 279 (2001) 255–260
Fig. 1. Magnetization and magnetoresistance curves at room temperature for the Cr3nm /Fe3nm /Cu3nm /Co5nm /Cu2nm /Cr2nm sandwich. For the magnetoresistance curve, the magnetic field is applied in the film plane both parallel (solid line) and perpendicular (dashed line) to the current direction.
parallel configuration of both magnetic layers and the resistivity is constant. It is then interesting to notice that, this GMR value is comparable to the values obtained for the exchange biased system, and the magnetic window is large. The resistivity measurements have been done for both in-plane magnetic applied field parallel and perpendicular to the current direction. These variations reported in Fig. 1 give a clear evidence of a small anisotropic MR contribution of about 0.3%. To increase the GMR value for the sandwich presented above, we have tried to insert a thin Co magnetic layer between the soft Fe magnetic layer and the non-magnetic Cu layer. The role of this thin Co layer is to increase the scattering centers. A special attention has to be made to the thickness of the Co
257
Fig. 2. Magnetization and magnetoresistance curves at room temperature for the Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Cu2nm /Cr2nm sandwich. For the magnetoresistance curve, the magnetic field is applied in the film plane and perpendicular to the current direction.
layer in order to find a compromise between the increase of the GMR and the hardening of the soft layer. Several sandwiches have been prepared with different Co thickness, and the optimum value has been obtained for 0.5 nm of Co. The results of the optimized Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Cu2nm / Cr2nm sandwich are reported in Fig. 2. The GMR value is sensitively increased to reach 5.5%, while the magnetic window is decreased but still interesting. The increase of the GMR by inserting a Co scattering centers in close proximity to the soft magnetic layers has already been reported by Parkin et al. [10], and seems to be a general behavior. Attention has also been paid to the role of the protection layer. Several capping layers have been used to optimize the GMR values. For example, Fig. 3 shows the GMR and magnetization loops of
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M. Guth et al. / Physics Letters A 279 (2001) 255–260
Fig. 3. Magnetization and magnetoresistance curves at room temperature for the Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Ru1nm sandwich. For the magnetoresistance curve, the magnetic field is applied in the film plane and perpendicular to the current direction.
Fig. 4. Magnetization and magnetoresistance curves at room temperature for the Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Cu2nm /Cr2nm sandwich annealed at 250◦ C during 2 h. For the magnetoresistance curve, the magnetic field is applied in the film plane and perpendicular to the current direction.
the Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Ru1nm sandwich capped with 1 nm Ru. The GMR value of 4.5% of this sample is smaller than the value observed for the sandwich capped with Cu2nm /Cr2nm layers. The second difference between these two capping layers is the small decrease of the coercive field of the top Co layer. These two consequences can be easily understood since it is well known that the Co and Ru are miscible, and the interdiffusion is likely to occur at the Co/Ru interface. Therefore, the CoRu alloy at the outer interface decreases the spin-dependent scattering and the coercive field of the top layer. To optimize the magneto-transport properties, and to study the thermal stability, annealing at several temperatures have been performed on the as-deposited sandwich under nitrogen atmosphere during 2 h. After each annealing, the sample has been characterized by
magnetization and magnetoresistance measurements. For the annealed sample at 200◦ C, a sensitive increase of the GMR, which reaches 6%, is observed, while the hysteresis loop is roughly the same. Such increase in GMR is more pronounced after annealing at 250◦ C. As reported in Fig. 4, the GMR is close to 7%. Moreover, the coercive field of the top Co layer is enhanced, and as a consequence, the magnetic field window of the resistivity plateau is increased. For subsequent annealings at 300 and 350◦ C, the GMR drops to reach values around 0.5% (Fig. 5), while the magnetization curve shows only a unique loop with one coercive field, instead of two coercive fields, and no more distinction between the rotation of the soft Fe layer and the hard Co layer is observed. This behavior can be understood in terms of diffusion between Cu, Co and Fe. For the first annealing, the temperature
M. Guth et al. / Physics Letters A 279 (2001) 255–260
Fig. 5. Magnetization and magnetoresistance curves at room temperature for the Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Cu2nm /Cr2nm sandwich after annealing at 300◦ C (solid line) and 350◦ C (dashed line) during 2 h. For the magnetoresistance curve, the magnetic field is applied in the film plane and perpendicular to the current direction.
is not high enough to allow diffusion in the bulk of the layers, thus diffusion occurs only at the interfaces, which has as effect to smoothen the interfaces and to improve the GMR. As the annealing temperature is increased, the diffusion of Cu at the grain boundaries of Fe and Co, and of the Co at the Cu grain boundaries happens in the whole layers. It is well known that diffusion at grain boundaries has as consequence, the increase of the coercive field. This is exactly what happens in this case. The coercive fields of both Fe and Co layers are increased to reach the situation where the coercive fields for both layers are close each to other, and both Fe and Co moments rotate simultaneously, and the GMR is strongly decreased. In order to understand the origin of the strong variation in the magneto-transport properties after
259
Fig. 6. Rutherford backscattering spectra performed at room temperature for the Cr3nm /Fe3nm /Co0.5nm /Cu3nm /Co5nm /Cu2nm /Cr2nm sandwich. (a) Spectrum of the as-deposited sandwich (solid line) and the simulated one (thick dashed line). In this simulated spectrum, the contribution of each single layer in the sandwiches is also presented (thin dashed line). (b) Spectra for both as-deposited (thin solid line) and annealed sandwich at 350◦ C during 2 h (bold solid line).
annealing, and to confirm the hypothesis given in the last paragraph, RBS measurements have been performed on the as-deposited and the 350◦ C annealed sandwiches. The spectra are reported in Fig. 6. As can be seen for the as-deposited sandwich in Fig. 6(a), which presents both the experimental and simulated spectra, the only separated peaks correspond to the 2 nm Cu capping layer at the energy of 1550 keV and the 3 nm Cr buffer layer at the energy of 1430 keV. Two other resolved peaks appear at the energy of 1475 and 1513 keV, respectively. The first one corresponds to the contributions of the 3 nm Fe layer and the 2 nm Cr capping layer. The second peak corresponds to the contributions of the 5 nm Co layer, which is the most
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important, and the 3 nm Cu decoupling layer. The effect of the annealing on the RBS spectra is evidenced in Fig. 6(b). For the sample annealed at 350◦ C, three important effects are observed: (i) a shift of the Cu/Co peak (at 1513 keV) and of the Cu capping layer peak (at 1550 keV) towards low energy; (ii) the decrease in the intensity of the Cu/Co peak at 1513 keV; (iii) the increase in the intensities of the all three other peaks at the energies 1430, 1475 and 1550 keV. The first effect, which corresponds to the shift towards small energies of the 1550 keV Cu capping layer and the 1513 keV Co/Cu active layers, can be easily understood since it is well known that diffusion of Cu through Co grain boundaries is likely to occur [11–13]. This means that there is a diffusion of the Cu atoms from the capping layer through Co grain boundaries, and of the Co atoms through Cu grain boundary for the Cu decoupling layer, and finally, of the Cu atoms from the decoupling layer and/or Co atoms through the Fe layer. Such explanations can also answer to the second and the third observed consequence of the annealing. The decrease in the intensity of the Cu/Co peak at 1513 keV can be explained by the diffusion of the Co through Cu grain boundaries, and of the Cu and Co through Fe grain boundaries. This is also confirmed by the strong increase in the intensity of the Fe peak at 1475 keV. These results are of good support to the hypothesis given to explain the magneto-transport properties change after annealing. We can, however, not give a quantitative analysis of the RBS spectra, due to the large number of layers and interfaces in our samples. The strong decrease of the GMR, and the observation of one coercive field, instead of two, is a good indication that the Fe/Cu/Co layers are completely intermixed and behave like one complex granular FeCuCo alloy layer.
4. Conclusion To conclude, we have shown that by using a very simple spin valve stack we can obtain an interesting GMR. The spin valve stack presents a good thermal
stability until 250◦C annealing temperature. These results are cheering but need to be improved for application purpose. RBS analysis performed on the as-deposited and the 350◦C annealed samples have shown that the observed change in the GMR and magnetic properties is mainly due to a strong diffusion between Cu, Co and Fe through grain boundaries. It is also important to add that RBS technique is a very sensitive tool to inform on diffusion mechanism.
Acknowledgements This work was supported by “la région d’Alsace” and SENSTRONIC company in Saverne, and by PICS (Projet international de coopération scientifique) contract between IPCMS-CNRS (French) and CNCPRST (Morocco).
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