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Annealing temperature dependence on magnetoresistance of single and dual specular spin valve
Surface & Coatings Technology 193 (2005) 272 – 276 www.elsevier.com/locate/surfcoat
Annealing temperature dependence on magnetoresistance of single and dual specular spin valve S.Y. Yoon*, D.H. Lee, D.M. Jeon, D.H. Yoon, S.J. Suh Advanced Materials and Process Research Center for IT, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon, Gyunggi-do, 440-746, South Korea Available online 16 September 2004
Abstract We studied the changes of magnetoresistance ratio (MR ratio), exchange bias field (H ex), and microstructure with annealing in conventional, specular, and dual specular spin valves. MR ratio of conventional spin valve was 7.88% and that of specular spin valve was 11.8% at their optimal annealing conditions. Conventional spin valve had less thermal stability than specular spin valve due to the diffusion mainly of Mn at between 200 and 305 8C. Exchange bias field of both samples was improved rapidly with increasing temperature. Dual specular spin valve showed higher MR ratio than single specular spin valve due to two nano-oxide layers (NOLs) in the top- and bottompinned layers. D 2004 Elsevier B.V. All rights reserved. Keywords: Conventional; Specular; Dual specular spin valve; Diffusion
1. Introduction Giant magnetoresistance (GMR) thin multilayers have been used as the key element of read head in magnetic head areas. To improve a real density, further enhancement of MR ratio is desired. Conventional spin valve exhibits about 7–9% MR ratio, and this value rapidly decreases by reducing the free layer thickness below 5 nm. On the other hand, specular spin valve shows above 10% MR ratio due to the increase of the mean free path of up-spin electrons [1,2]. Specular spin valves, which are comprised of metallic antiferromagnete (Mn–Ir, Pt–Mn) and pinned layer containing nano-oxide layer (NOL), have been researched [3– 5] due to its merits, such as high MR ratio, high exchange bias field (H ex), and thin total thickness. But these reports focused on enhanced MR ratio and microstructure at a specific annealing temperature. However, the GMR conventional spin valves have a serious problem with thermal degradation, which is induced by * Corresponding author. Tel.: +82 31 290 7373; fax: +82 31 290 7377. E-mail address: [email protected] (S.Y. Yoon). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.07.096
the fabrication process of head. Particularly, this problem becomes more critical in the case of Mn-based spin valve, because the Mn can diffuse into other layers at temperatures above 280 8C [6]. In this study, we have investigated the variation of MR ratio, H ex, and microstructure as a function of annealing temperature of conventional and specular spin valve without NOL and with NOL, respectively. We have also studied the dual specular spin valve.
2. Experiments Spin valves of Si/Ta(5 nm)/Ni–Fe(3.5 nm)/Mn–Ir(6 nm)/Co–Fe(2 nm)/NOL/Co–Fe(2 nm)/Cu(2.5 nm)/Co– Fe(4 nm)/Ta(2.5 nm) with and without NOL were deposited on Si (100) substrate by autoprocess magnetron sputtering method. We also prepared the dual specular spin valve of Si/Ta(5 nm)/Ni–Fe(3.5 nm)/Mn–Ir(6 nm)/ Co–Fe(2 nm)/NOL/Co–Fe(2 nm)/Cu(2. 5 nm)/Co–Fe(4 nm)/Cu(2.5 nm)/Co–Fe(2 nm)/NOL/Co–Fe(2 nm)/Mn–Ir(6 nm)/Ta(2.5 nm). The Co–Fe layer in contact with the
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with NOL and without NOL layer with annealing temperature.
3. Results and discussion Fig. 1 shows MR curves of conventional spin valve with annealing temperature. And the inserted figure shows the change of resistance and H ex as a function of temperature. MR ratio increased from 6% to 7.88% until 200 8C and exhibited value a of 5.7% at 410 8C. And the minimum resistance decreased from 5.05 to 4.9 V until 200 8C and exhibited largely increased value of 5.33 V at 410 8C. Fig. 2 shows MR curves of specular spin valve with annealing temperature. MR ratio increased from 7.84% to 11.8% until 305 8C and exhibited a value of 10.8% at 410 8C. And, the minimum resistance decreased from 5.01 to 4.42 V until 305 8C and exhibited a value of 4.88 V at 410 8C. These results indicate that specular spin valve has more thermal stability than conventional spin valve. But the H ex of both samples increased with increasing annealing temperature.
Fig. 1. The MR curves of conventional spin valve with annealing temperature. n: conventional SV at as-deposited; o: conventional SV at 200 8C annealed; E: conventional SV at 305 8C annealed; and 5: conventional SV at 410 8C annealed. Inset shows the variation of H ex and resistance with annealing temperature.
Mn–Ir layer will be referred to as the pinned layer, and the Co–Fe layer situated above Cu layer as the free layer. The seed layer Ta/NiFe was used to promote the (111) texture of antiferromagnetic Mn–Ir layer. The base pressure was less than 310 8 Torr. Mn81–Ir19 at.% was deposited from a Mn target with Ir chips attached to it. A Ni81–Fe19 wt.% and a Co90–Fe10 at.% alloy target were used for the corresponding layers. The samples were deposited under 5 mTorr of Ar at room temperature. In order to induce unidirectional anisotropy, we applied a magnetic field of 100 Oe to the samples during the process. A NOL was formed in the load lock chamber by exposing to pure oxygen gas. Samples were annealed for 30 min at a range from 200 to 410 8C under 3 kOe inplane magnetic field in a vacuum furnace (110 5 Torr), followed by cooling to room temperature for 2 h. The MR ratio is defined as MR ratio=[R(maximum) R(mini(minimum)]/R(minimum)100. And H ex was measured from the shift of the M–H loop away from the zero-field axis. Transmission Electron Microscopy (TEM), X-ray Diffractometer (XRD), and Auger Electron Spectroscopy (Auger Electron Spectroscopy) were used to analyze the microstructure and chemical structure of pinned layer
Fig. 2. The MR curves of specular spin valve with annealing temperature. n: specular SV at as-deposited; o: specular SV at 200 8C annealed; E: specular SV at 305 8C annealed; and 5: specular SV at 410 8C annealed. Inset shows the variation of H ex and resistance with annealing temperature.
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Fig. 3 shows the AES depth profile results of conventional spin valve at as-deposited and annealed at 410 8C. The AES revealed an amount of Mn and Co diffusion and intermixing between layers. There was a substantial amount of Mn diffused to both outward and inward, while Co diffused inward. And, Cu diffused slightly into the Co–Fe layer. The resistance of thin film is decreased by annealing process, because of defect healing and stress relief at initial stage of annealing. But resistance increased by diffusion process at a high annealing temperature. Our previous result showed that NOL acts as a diffusion barrier of Mn by forming Mn– Co–Fe–O in specular spin valve [7]. Fig. 1, Fig. 2, and Fig. 3 results indicate that diffusion starts at between 200 and 305 8C in conventional spin valve and above 305 8C in specular spin valve, respectively. It is considered that intermixing between Mn and Co leads to decreasing MR ratio by increasing spin-independent scattering. At present, it is unclear why the H ex of both samples increases with annealing temperature, despite the diffusion between Mn–Ir and Co–Fe may be harmful for producing high H ex. Fig. 4 shows the XRD diffraction patterns of specular and conventional spin valve as a function of annealing temperature. Both samples showed two strong (111) peaks. One of them is the (111) diffraction peak of the Fig. 4. XRD diffraction patterns of specular and conventional spin valve as a function of annealing temperature.
Fig. 3. AES depth profile of as-deposited and 410 8C annealed conventional spin valve.
Mn–Ir layer with FCC structure of around 2e`=41.68, and the other is the (111) diffraction peaks of the Co–Fe, Ni– Fe, and Cu layers with FCC structure of around 2h=43.58. The intensities of both samples slightly increased with annealing temperature. Although diffusion started with increasing temperature, growth of (111) texture in both samples may aid to produce high H ex. And, decrement of magnetic moment of pinned layer may be another reason of increasing H ex with annealing temperature. Fig. 5 shows the HR-TEM images of specular spin valves with annealing temperature. A narrow white band distinguishes itself from other layers. It corresponds to the thin-oxide-containing layer in Co–Fe. The thickness of the NOL in the pinned layer increased slightly from 0.7–1.4 nm (as-deposited) to 0.5–2 nm (200 8C annealed) and 1.87–2.1 nm (410 8C annealed). The microstructure of NOL did not vary with increasing temperature. But the NOL of the 410 8C annealed specular spin valve showed more firm and continuous layer than that of others. It may be due to the distribution of Mn in the NOL region by high annealing temperature [8]. Fig. 6 shows MR curves of dual specular spin valve with annealing temperature. The dual specular spin valve is consisted of two NOLs in both the top- and bottom-pinned
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Fig. 5. HR-TEM images of (a) as-deposited, (b) 200 8C annealed, and (c) 410 8C annealed.
layer. As the annealing temperature increased up to 305 8C, the MR ratio increased from 10.8% to 15.3%, and the resistance decreased from 3.142 to 2.9 V. And at 410 8C, MR showed 12.27% and resistance did 3.136 V. Until 305
8C, improved MR ratio is due to the increase of specular effect by the NOL and smooth interface formation at the Co–Fe/Mn–Ir interface. At 410 8C, decreased MR ratio and increased resistance are due to diffusion and intermixing
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ular, and dual specular spin valve with annealing temperature. Diffusion of Mn, Co, and Cu in conventional spin valve is the major reason of lower-optimum annealing temperature for highest MR ratio than that of specular spin valve. Dual specular spin valve showed higher MR ratio than single specular spin valve due to two NOLs in the top- and bottom-pinned layers.
Acknowledgement This work was supported by the Advanced Materials and Process Research Center for IT at SungkyunKwan University (Grant No. R12-2002-057-01001-0).
References
Fig. 6. The MR curves of dual specular spin valve with annealing temperature.
layer. These were major reasons of spin-independent scattering.
4. Conclusions We have investigated magnetic properties (MR ratio, H ex) and microstructure changes of conventional, spec-
[1] W.E. Egeloff Jr., P.J. Chen, C.J. Powell, J. Appl. Phys. 82 (1997) 6142. [2] H.J.M. Swagten, G.J. Strijkers, P.J.H. Bloemen, M.M.H. Willekens, W.J.M. de Jonge, Phys. Rev., B 53 (1996) 9108. [3] A. Veloso, P.P. Freitas, P. Wei, N.P. Barradas, J.C. Soares, B. Almeida, J.B. Sousa, Appl. Phys. Lett. 77 (2000) 1020. [4] M.F. Gillies, A.E.T. Kuiper, G.W.R. Leibbrandt, J. Appl. Phys. 89 (2001) 6922. [5] Y. Kamiguchi, H. Fukuzawa, K. Koui, Intermag Conference, DB-01 (1999). [6] Masafumi Takiguchi, Satoru Ishii, Eiji Makino, Akihiko Okabe, J. Appl. Phys. 87 (2000) 2169. [7] S.Y. Yoon, D.H. Lee, D.M. Jeon, J.H. Kim, D.H. Yoon, S.J. Suh, J. Magn. Magn. Mater. 272–276 (2004) 1879. [8] S.Y. Yoon, D.H. Lee, K.H. Jeong, D.H. Yoon, S.J. Suh, Mater. Sci. Forum 449–452 (2004) 1065.
Annealing temperature dependence on magnetoresistance of single and dual specular spin valve S.Y. Yoon*, D.H. Lee, D.M. Jeon, D.H. Yoon, S.J. Suh Advanced Materials and Process Research Center for IT, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon, Gyunggi-do, 440-746, South Korea Available online 16 September 2004
Abstract We studied the changes of magnetoresistance ratio (MR ratio), exchange bias field (H ex), and microstructure with annealing in conventional, specular, and dual specular spin valves. MR ratio of conventional spin valve was 7.88% and that of specular spin valve was 11.8% at their optimal annealing conditions. Conventional spin valve had less thermal stability than specular spin valve due to the diffusion mainly of Mn at between 200 and 305 8C. Exchange bias field of both samples was improved rapidly with increasing temperature. Dual specular spin valve showed higher MR ratio than single specular spin valve due to two nano-oxide layers (NOLs) in the top- and bottompinned layers. D 2004 Elsevier B.V. All rights reserved. Keywords: Conventional; Specular; Dual specular spin valve; Diffusion
1. Introduction Giant magnetoresistance (GMR) thin multilayers have been used as the key element of read head in magnetic head areas. To improve a real density, further enhancement of MR ratio is desired. Conventional spin valve exhibits about 7–9% MR ratio, and this value rapidly decreases by reducing the free layer thickness below 5 nm. On the other hand, specular spin valve shows above 10% MR ratio due to the increase of the mean free path of up-spin electrons [1,2]. Specular spin valves, which are comprised of metallic antiferromagnete (Mn–Ir, Pt–Mn) and pinned layer containing nano-oxide layer (NOL), have been researched [3– 5] due to its merits, such as high MR ratio, high exchange bias field (H ex), and thin total thickness. But these reports focused on enhanced MR ratio and microstructure at a specific annealing temperature. However, the GMR conventional spin valves have a serious problem with thermal degradation, which is induced by * Corresponding author. Tel.: +82 31 290 7373; fax: +82 31 290 7377. E-mail address: [email protected] (S.Y. Yoon). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.07.096
the fabrication process of head. Particularly, this problem becomes more critical in the case of Mn-based spin valve, because the Mn can diffuse into other layers at temperatures above 280 8C [6]. In this study, we have investigated the variation of MR ratio, H ex, and microstructure as a function of annealing temperature of conventional and specular spin valve without NOL and with NOL, respectively. We have also studied the dual specular spin valve.
2. Experiments Spin valves of Si/Ta(5 nm)/Ni–Fe(3.5 nm)/Mn–Ir(6 nm)/Co–Fe(2 nm)/NOL/Co–Fe(2 nm)/Cu(2.5 nm)/Co– Fe(4 nm)/Ta(2.5 nm) with and without NOL were deposited on Si (100) substrate by autoprocess magnetron sputtering method. We also prepared the dual specular spin valve of Si/Ta(5 nm)/Ni–Fe(3.5 nm)/Mn–Ir(6 nm)/ Co–Fe(2 nm)/NOL/Co–Fe(2 nm)/Cu(2. 5 nm)/Co–Fe(4 nm)/Cu(2.5 nm)/Co–Fe(2 nm)/NOL/Co–Fe(2 nm)/Mn–Ir(6 nm)/Ta(2.5 nm). The Co–Fe layer in contact with the
S.Y. Yoon et al. / Surface & Coatings Technology 193 (2005) 272–276
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with NOL and without NOL layer with annealing temperature.
3. Results and discussion Fig. 1 shows MR curves of conventional spin valve with annealing temperature. And the inserted figure shows the change of resistance and H ex as a function of temperature. MR ratio increased from 6% to 7.88% until 200 8C and exhibited value a of 5.7% at 410 8C. And the minimum resistance decreased from 5.05 to 4.9 V until 200 8C and exhibited largely increased value of 5.33 V at 410 8C. Fig. 2 shows MR curves of specular spin valve with annealing temperature. MR ratio increased from 7.84% to 11.8% until 305 8C and exhibited a value of 10.8% at 410 8C. And, the minimum resistance decreased from 5.01 to 4.42 V until 305 8C and exhibited a value of 4.88 V at 410 8C. These results indicate that specular spin valve has more thermal stability than conventional spin valve. But the H ex of both samples increased with increasing annealing temperature.
Fig. 1. The MR curves of conventional spin valve with annealing temperature. n: conventional SV at as-deposited; o: conventional SV at 200 8C annealed; E: conventional SV at 305 8C annealed; and 5: conventional SV at 410 8C annealed. Inset shows the variation of H ex and resistance with annealing temperature.
Mn–Ir layer will be referred to as the pinned layer, and the Co–Fe layer situated above Cu layer as the free layer. The seed layer Ta/NiFe was used to promote the (111) texture of antiferromagnetic Mn–Ir layer. The base pressure was less than 310 8 Torr. Mn81–Ir19 at.% was deposited from a Mn target with Ir chips attached to it. A Ni81–Fe19 wt.% and a Co90–Fe10 at.% alloy target were used for the corresponding layers. The samples were deposited under 5 mTorr of Ar at room temperature. In order to induce unidirectional anisotropy, we applied a magnetic field of 100 Oe to the samples during the process. A NOL was formed in the load lock chamber by exposing to pure oxygen gas. Samples were annealed for 30 min at a range from 200 to 410 8C under 3 kOe inplane magnetic field in a vacuum furnace (110 5 Torr), followed by cooling to room temperature for 2 h. The MR ratio is defined as MR ratio=[R(maximum) R(mini(minimum)]/R(minimum)100. And H ex was measured from the shift of the M–H loop away from the zero-field axis. Transmission Electron Microscopy (TEM), X-ray Diffractometer (XRD), and Auger Electron Spectroscopy (Auger Electron Spectroscopy) were used to analyze the microstructure and chemical structure of pinned layer
Fig. 2. The MR curves of specular spin valve with annealing temperature. n: specular SV at as-deposited; o: specular SV at 200 8C annealed; E: specular SV at 305 8C annealed; and 5: specular SV at 410 8C annealed. Inset shows the variation of H ex and resistance with annealing temperature.
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Fig. 3 shows the AES depth profile results of conventional spin valve at as-deposited and annealed at 410 8C. The AES revealed an amount of Mn and Co diffusion and intermixing between layers. There was a substantial amount of Mn diffused to both outward and inward, while Co diffused inward. And, Cu diffused slightly into the Co–Fe layer. The resistance of thin film is decreased by annealing process, because of defect healing and stress relief at initial stage of annealing. But resistance increased by diffusion process at a high annealing temperature. Our previous result showed that NOL acts as a diffusion barrier of Mn by forming Mn– Co–Fe–O in specular spin valve [7]. Fig. 1, Fig. 2, and Fig. 3 results indicate that diffusion starts at between 200 and 305 8C in conventional spin valve and above 305 8C in specular spin valve, respectively. It is considered that intermixing between Mn and Co leads to decreasing MR ratio by increasing spin-independent scattering. At present, it is unclear why the H ex of both samples increases with annealing temperature, despite the diffusion between Mn–Ir and Co–Fe may be harmful for producing high H ex. Fig. 4 shows the XRD diffraction patterns of specular and conventional spin valve as a function of annealing temperature. Both samples showed two strong (111) peaks. One of them is the (111) diffraction peak of the Fig. 4. XRD diffraction patterns of specular and conventional spin valve as a function of annealing temperature.
Fig. 3. AES depth profile of as-deposited and 410 8C annealed conventional spin valve.
Mn–Ir layer with FCC structure of around 2e`=41.68, and the other is the (111) diffraction peaks of the Co–Fe, Ni– Fe, and Cu layers with FCC structure of around 2h=43.58. The intensities of both samples slightly increased with annealing temperature. Although diffusion started with increasing temperature, growth of (111) texture in both samples may aid to produce high H ex. And, decrement of magnetic moment of pinned layer may be another reason of increasing H ex with annealing temperature. Fig. 5 shows the HR-TEM images of specular spin valves with annealing temperature. A narrow white band distinguishes itself from other layers. It corresponds to the thin-oxide-containing layer in Co–Fe. The thickness of the NOL in the pinned layer increased slightly from 0.7–1.4 nm (as-deposited) to 0.5–2 nm (200 8C annealed) and 1.87–2.1 nm (410 8C annealed). The microstructure of NOL did not vary with increasing temperature. But the NOL of the 410 8C annealed specular spin valve showed more firm and continuous layer than that of others. It may be due to the distribution of Mn in the NOL region by high annealing temperature [8]. Fig. 6 shows MR curves of dual specular spin valve with annealing temperature. The dual specular spin valve is consisted of two NOLs in both the top- and bottom-pinned
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Fig. 5. HR-TEM images of (a) as-deposited, (b) 200 8C annealed, and (c) 410 8C annealed.
layer. As the annealing temperature increased up to 305 8C, the MR ratio increased from 10.8% to 15.3%, and the resistance decreased from 3.142 to 2.9 V. And at 410 8C, MR showed 12.27% and resistance did 3.136 V. Until 305
8C, improved MR ratio is due to the increase of specular effect by the NOL and smooth interface formation at the Co–Fe/Mn–Ir interface. At 410 8C, decreased MR ratio and increased resistance are due to diffusion and intermixing
276
S.Y. Yoon et al. / Surface & Coatings Technology 193 (2005) 272–276
ular, and dual specular spin valve with annealing temperature. Diffusion of Mn, Co, and Cu in conventional spin valve is the major reason of lower-optimum annealing temperature for highest MR ratio than that of specular spin valve. Dual specular spin valve showed higher MR ratio than single specular spin valve due to two NOLs in the top- and bottom-pinned layers.
Acknowledgement This work was supported by the Advanced Materials and Process Research Center for IT at SungkyunKwan University (Grant No. R12-2002-057-01001-0).
References
Fig. 6. The MR curves of dual specular spin valve with annealing temperature.
layer. These were major reasons of spin-independent scattering.
4. Conclusions We have investigated magnetic properties (MR ratio, H ex) and microstructure changes of conventional, spec-
[1] W.E. Egeloff Jr., P.J. Chen, C.J. Powell, J. Appl. Phys. 82 (1997) 6142. [2] H.J.M. Swagten, G.J. Strijkers, P.J.H. Bloemen, M.M.H. Willekens, W.J.M. de Jonge, Phys. Rev., B 53 (1996) 9108. [3] A. Veloso, P.P. Freitas, P. Wei, N.P. Barradas, J.C. Soares, B. Almeida, J.B. Sousa, Appl. Phys. Lett. 77 (2000) 1020. [4] M.F. Gillies, A.E.T. Kuiper, G.W.R. Leibbrandt, J. Appl. Phys. 89 (2001) 6922. [5] Y. Kamiguchi, H. Fukuzawa, K. Koui, Intermag Conference, DB-01 (1999). [6] Masafumi Takiguchi, Satoru Ishii, Eiji Makino, Akihiko Okabe, J. Appl. Phys. 87 (2000) 2169. [7] S.Y. Yoon, D.H. Lee, D.M. Jeon, J.H. Kim, D.H. Yoon, S.J. Suh, J. Magn. Magn. Mater. 272–276 (2004) 1879. [8] S.Y. Yoon, D.H. Lee, K.H. Jeong, D.H. Yoon, S.J. Suh, Mater. Sci. Forum 449–452 (2004) 1065.