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Enhancement of MR ratios using thin oxide layers in PtMn and α-Fe2O3-based spin valves
LETTER TO THE EDITOR
Journal of Magnetism and Magnetic Materials 210 (2000) L20}L24
Letter to the Editor
Enhancement of MR ratios using thin oxide layers in PtMn and a-Fe O -based spin valves 2 3 H. Sakakima*, M. Satomi, Y. Sugita, Y. Kawawake Central Research Laboratory, Matsushita Electric Industrial Co., Ltd., 3-4, Hikaridai, Seika-cho, Souraku-gun, Kyoto 619-02, Japan Received 29 June 1999; received in revised form 21 September 1999
Abstract Spin valves having thin oxide layers in the pinned and/or free layers were prepared by sputtering. MR ratios of the spin valves were increased from 8.1 to 11.9% by inserting the oxide layer into the pinned layer in Ta/PtMn/CoFe/Cu/CoFe/ Ta spin valves. MR ratio of 13.9% and considerably large sheet *R of 2.55 ) were obtained in the PtMn-based spin valves having the oxide layer in the pinned and free layer. Larger MR ratio of 17.3% and the sheet *R of 1.3 ) were obtained in the PtMn-based dual-type spin valves having the oxide layer in both pinned layers. a-Fe O based spin 2 3 valves having thin oxide layers were also prepared. MR ratios of the spin valves were increased from 11.9 to 14.3% by inserting the oxide layer into the free layer in a-Fe O /CoFe/Cu/CoFe/Ta spin valves. The enhancement of the MR 2 3 ratios may be attributed to the specular scattering e!ect of the conduction electrons by the thin oxide layers. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 75.70.p Keywords: GMR; Spin valves; Specular scattering
1. Introduction Spin valves having large MR ratios and high thermal stability are required for GMR heads. Spin valves using NiO [1,2] or a-Fe O [3,4] as the 2 3 pinning layers showed large MR ratios owing to the specular scattering e!ect [1,5]. However, the pinning "elds of these spin valves were not large enough [6] for practical applications and synthetic ferrimagnetic layers [7] were used for these spin
* Corresponding author. Tel.: #81-774-98-2538; fax: #81774-98-2585. E-mail address: [email protected] (H. Sakakima)
valves to increase the pinning "elds. But the synthetic ferrimagnetic layers reduced the specular scattering e!ect and caused the decreases of MR ratios [6]. Recently, a new type of spin valves having pinned layers with a nano oxide layer (NOL) was proposed to enhance the specular scattering e!ect and large MR ratios, such as 16%, were reported for the IrMn-based spin valves [8]. The authors studied PtMn-based spin valves [9] having thin oxide layers inserted into the pinned and/or free layers, as the PtMn-based spin valves show better thermal stability than IrMn-based spin valves. a-Fe O -based spin valves having thin ox2 3 ide layers were also studied for comparison.
0304-8853/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 7 6 8 - 4
LETTER TO THE EDITOR
H. Sakakima et al. / Journal of Magnetism and Magnetic Materials 210 (2000) L20}L24
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2. Experimental procedure The spin valves were prepared onto Si substrates by sputtering, using alloy targets of Ta, Cu, Co Fe and PtMn, and sintered targets of a0.9 0.1 Fe O . The thin oxide layers were prepared using 2 3 oxide targets. Base pressure and sputtering Ar gas pressure were (1}5)]10~9 Torr and 0.8 mTorr, respectively. Deposition rates were 0.3}0.4 nm/min for Cu, CoFe and PtMn, and 0.2 nm/min for Ta and a-Fe O . MR ratios of the spin valves were 2 3 measured by the DC four-point probe method by applying a magnetic "eld of 0.5}5 kOe in the "lm plane at RT. The PtMn-based spin valves were annealed at 2803C for 0.5}2.5 h with applying a magnetic "eld of 500 Oe, in a vacuum of 10~5 Torr.
3. Results and discussion The basic structure of the spin valves is Ta/AF ("PtMn or a-Fe O )/PL/Cu/FL/Ta, where PL 2 3 and FL denote pinned and free layer, respectively, and several types of the spin valves as shown in Fig. 1 were prepared to study the specular scattering e!ect of the inserted oxide layer. The thin oxide layer (OL) was inserted into the pinned layer for type-A, and inserted into the free and pinned layer for type-B, and inserted into both pinned layers for type-C of dual spin-valve structure. FL and PL are composed of CoFe for the basic structure. Firstly, spin valves of type-A having PL" CoFe/OL(t)/CoFe and FL"CoFe were studied. Fig. 2 shows the dependence of MR ratio, sheet resistance, R, sheet *R, and switching "eld of the pinned layer, H , on the oxide layer (OL) thickness, 1 t (nm), for the spin valves after annealing at 2803C for 1.5 h by applying a magnetic "eld. Fig. 3 shows MR curves of the spin valves with t"0 and 0.6. The MR ratio and sheet *R increased drastically from 8.1% and 1.17 ) to 11.2% and 1.62 ) with the insertion of 0.6 nm thick OL, and the values slightly increased with t ('0.6 nm). H increased with 1 t(0.6 nm, then decreased with t'1 nm and R increased slightly with the increase of t. The drastic increase of MR ratios and sheet *R with the insertion of OL is considered to originate in the specular
Fig. 1. Structures of the spin valves used for this study.
scattering of the conduction electrons by OL. MR ratios of these spin valves are thermally stable. For example, MR ratio of the spin valves with 1 nm thick OL was 12% even after annealing at 2803C for 2.5 h. Then spin valves of type-B having PL"CoFe/ OL(1.4)/CoFe and FL"CoFe/OL(1)/CoFe and type-C having PL"CoFe/OL(1)/CoFe and FL"CoFe were studied. Fig. 4 shows the MR curves for these spin valves. The MR ratio and sheet *R were 13.9% and 2.55 ) for type-B and 17.3% and 1.3 ) for type-C, respectively. A drastic increase of sheet *R by 0.9 ), as well as an increase of MR ratios by 2%, was achieved by changing the
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Fig. 3. MR curves for (a) Ta/PtMn/CoFe/Cu/CoFe/Ta (basic structure) and (b) Ta/PtMn/CoFe/OL/CoFe/Cu/CoFe/Ta (type-A) spin valves.
Fig. 2. (a) Dependence of MR ratio, and sheet resistance, R, on the OL thickness, t. (b) Dependence of sheet *R and H on the 1 OL thickness, t.
spin-valve structure from types-A to B. The enhancement of sheet *R and MR ratios by the insertion of OL-F in type-B may be explained by assuming that both OL-F and OL-P contribute the specular scattering of the conduction electrons in type-B, while only OL-P contributes the specular scattering in type-A. Larger MR ratios were obtained for type-C spin valves. In this case, dual structure and the specular scattering e!ect caused by both OL-P layers are considered to contribute the enhancement of the MR ratios. Decrease in
sheet *R is attributed to the increase of "lm thickness due to the dual spin-valve structure. Synthetic type spin valves having 1}1.4 nm thick oxide layers were also studied for types-A and B. CoFe/Ru/CoFe/OL/CoFe was used as the pinned layer for both types of spin valves. As for the free layer, CoFe was used for type-A and CoFe/ OL/CoFe was used for type-B. Fig. 5 shows the MR curves for the spin valves after annealing at 2803C. The MR ratios and sheet *R were 10% and 1.47 ) for type-A and 11.4% and 1.87 ) for type-B, respectively. These values are larger than those of conventional PtMn based spin valves, but are smaller than those of spin valves without synthetic layers as shown in Figs. 3b, 4a and b. Shunting e!ect of CoFe/Ru/CoFe synthetic layers may contribute to the decrease of MR ratios and sheet *R. Magnetic "eld strength of 500 Oe used for the
LETTER TO THE EDITOR
H. Sakakima et al. / Journal of Magnetism and Magnetic Materials 210 (2000) L20}L24
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Fig. 5. MR curves for (a) Ta/PtMn/CoFe/Ru/CoFe/OL/ CoFe/Cu/CoFe/Ta (type-A) and (b) Ta/PtMn/CoFe/ Ru/CoFe/OL/CoFe/Cu/CoFe/OL/CoFe/Ta (type-B). Fig. 4. MR curves for (a) Ta/PtMn/CoFe/OL/CoFe/Cu/CoFe/ OL/CoFe (type-B) and (b) Ta/PtMn/CoFe/OL/CoFe/Cu/CoFe/ Cu/CoFe/OL/CoFe/PtMn/Ta (type-C).
annealing might be insu$cient to align the pinned layers. Lastly, spin valves composed of a-Fe O / 2 3 PL/Cu/FL/Ta were studied, where PL is composed of CoFe and FL is composed of CoFe or CoFe/ OL(1)/CoFe. The MR ratios and sheet *R were increased from 11.9% and 2.35 ) to 14.3% and 3.22 ) by inserting OL in the free layer. In these spin valves, total metallic layer thickness is small, and the specular scattering occurs at the interfaces of a-Fe O /CoFe and CoFe/OL. Therefore, 2 3 large *R is obtained in these spin valves. However, H of these spin valves was about 100 Oe, which is 1 much smaller than those of PtMn-based spin valves.
4. Summary PtMn and a-Fe O -based spin valves having 2 3 specular re#ective oxide layers, OL, in the pinned or free layers were studied. These spin valves showed larger MR ratios, 12}17% and larger sheet *R, 1.7}3.2 ), than spin valves without OL. PtMn-based spin valves are thermally stable than IrMn-based spin valves, but the MR ratios of conventional PtMn-based spin valves were not su$ciently high as IrMn-based spin valves. Therefore, PtMn-based spin valves having OL in this study, having large MR ratios and high thermal stability, are promising GMR "lms for spin-valve heads. Acknowledgements Part of this work was performed under the management of ASET in MITI's R&D program supported by NEDO.
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References [1] [2] [3] [4] [5]
H.J. Swagten et al., Phys. Rev. B 53 (1996) 9108. W.F. Egelhof et al., J. Appl. Phys. 79 (1996) 5277. N. Hasegawa et al., IEEE Trans. Magn. 32 (1996) 4918. Y. Sugita et al., Jpn. J. Appl. Phys. 37 (1998) 5984. H. Sakakima et al., J. Magn. Magn. Mater. 198}199 (1999) 9.
[6] Y. Sugita et al., Digest of INTERMAG'99 FC-07 (1999). [7] V.S. Speriosu et al., Digest of INTERMAG'96 AA-04 (1996). [8] Y. Kamiguchi et al., Digest of INTERMAG'99 DB-01 (1999). [9] M. Saito et al., Digest of INTERMAG'97 HA-06 (1997).
Journal of Magnetism and Magnetic Materials 210 (2000) L20}L24
Letter to the Editor
Enhancement of MR ratios using thin oxide layers in PtMn and a-Fe O -based spin valves 2 3 H. Sakakima*, M. Satomi, Y. Sugita, Y. Kawawake Central Research Laboratory, Matsushita Electric Industrial Co., Ltd., 3-4, Hikaridai, Seika-cho, Souraku-gun, Kyoto 619-02, Japan Received 29 June 1999; received in revised form 21 September 1999
Abstract Spin valves having thin oxide layers in the pinned and/or free layers were prepared by sputtering. MR ratios of the spin valves were increased from 8.1 to 11.9% by inserting the oxide layer into the pinned layer in Ta/PtMn/CoFe/Cu/CoFe/ Ta spin valves. MR ratio of 13.9% and considerably large sheet *R of 2.55 ) were obtained in the PtMn-based spin valves having the oxide layer in the pinned and free layer. Larger MR ratio of 17.3% and the sheet *R of 1.3 ) were obtained in the PtMn-based dual-type spin valves having the oxide layer in both pinned layers. a-Fe O based spin 2 3 valves having thin oxide layers were also prepared. MR ratios of the spin valves were increased from 11.9 to 14.3% by inserting the oxide layer into the free layer in a-Fe O /CoFe/Cu/CoFe/Ta spin valves. The enhancement of the MR 2 3 ratios may be attributed to the specular scattering e!ect of the conduction electrons by the thin oxide layers. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 75.70.p Keywords: GMR; Spin valves; Specular scattering
1. Introduction Spin valves having large MR ratios and high thermal stability are required for GMR heads. Spin valves using NiO [1,2] or a-Fe O [3,4] as the 2 3 pinning layers showed large MR ratios owing to the specular scattering e!ect [1,5]. However, the pinning "elds of these spin valves were not large enough [6] for practical applications and synthetic ferrimagnetic layers [7] were used for these spin
* Corresponding author. Tel.: #81-774-98-2538; fax: #81774-98-2585. E-mail address: [email protected] (H. Sakakima)
valves to increase the pinning "elds. But the synthetic ferrimagnetic layers reduced the specular scattering e!ect and caused the decreases of MR ratios [6]. Recently, a new type of spin valves having pinned layers with a nano oxide layer (NOL) was proposed to enhance the specular scattering e!ect and large MR ratios, such as 16%, were reported for the IrMn-based spin valves [8]. The authors studied PtMn-based spin valves [9] having thin oxide layers inserted into the pinned and/or free layers, as the PtMn-based spin valves show better thermal stability than IrMn-based spin valves. a-Fe O -based spin valves having thin ox2 3 ide layers were also studied for comparison.
0304-8853/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 7 6 8 - 4
LETTER TO THE EDITOR
H. Sakakima et al. / Journal of Magnetism and Magnetic Materials 210 (2000) L20}L24
L21
2. Experimental procedure The spin valves were prepared onto Si substrates by sputtering, using alloy targets of Ta, Cu, Co Fe and PtMn, and sintered targets of a0.9 0.1 Fe O . The thin oxide layers were prepared using 2 3 oxide targets. Base pressure and sputtering Ar gas pressure were (1}5)]10~9 Torr and 0.8 mTorr, respectively. Deposition rates were 0.3}0.4 nm/min for Cu, CoFe and PtMn, and 0.2 nm/min for Ta and a-Fe O . MR ratios of the spin valves were 2 3 measured by the DC four-point probe method by applying a magnetic "eld of 0.5}5 kOe in the "lm plane at RT. The PtMn-based spin valves were annealed at 2803C for 0.5}2.5 h with applying a magnetic "eld of 500 Oe, in a vacuum of 10~5 Torr.
3. Results and discussion The basic structure of the spin valves is Ta/AF ("PtMn or a-Fe O )/PL/Cu/FL/Ta, where PL 2 3 and FL denote pinned and free layer, respectively, and several types of the spin valves as shown in Fig. 1 were prepared to study the specular scattering e!ect of the inserted oxide layer. The thin oxide layer (OL) was inserted into the pinned layer for type-A, and inserted into the free and pinned layer for type-B, and inserted into both pinned layers for type-C of dual spin-valve structure. FL and PL are composed of CoFe for the basic structure. Firstly, spin valves of type-A having PL" CoFe/OL(t)/CoFe and FL"CoFe were studied. Fig. 2 shows the dependence of MR ratio, sheet resistance, R, sheet *R, and switching "eld of the pinned layer, H , on the oxide layer (OL) thickness, 1 t (nm), for the spin valves after annealing at 2803C for 1.5 h by applying a magnetic "eld. Fig. 3 shows MR curves of the spin valves with t"0 and 0.6. The MR ratio and sheet *R increased drastically from 8.1% and 1.17 ) to 11.2% and 1.62 ) with the insertion of 0.6 nm thick OL, and the values slightly increased with t ('0.6 nm). H increased with 1 t(0.6 nm, then decreased with t'1 nm and R increased slightly with the increase of t. The drastic increase of MR ratios and sheet *R with the insertion of OL is considered to originate in the specular
Fig. 1. Structures of the spin valves used for this study.
scattering of the conduction electrons by OL. MR ratios of these spin valves are thermally stable. For example, MR ratio of the spin valves with 1 nm thick OL was 12% even after annealing at 2803C for 2.5 h. Then spin valves of type-B having PL"CoFe/ OL(1.4)/CoFe and FL"CoFe/OL(1)/CoFe and type-C having PL"CoFe/OL(1)/CoFe and FL"CoFe were studied. Fig. 4 shows the MR curves for these spin valves. The MR ratio and sheet *R were 13.9% and 2.55 ) for type-B and 17.3% and 1.3 ) for type-C, respectively. A drastic increase of sheet *R by 0.9 ), as well as an increase of MR ratios by 2%, was achieved by changing the
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H. Sakakima et al. / Journal of Magnetism and Magnetic Materials 210 (2000) L20}L24
Fig. 3. MR curves for (a) Ta/PtMn/CoFe/Cu/CoFe/Ta (basic structure) and (b) Ta/PtMn/CoFe/OL/CoFe/Cu/CoFe/Ta (type-A) spin valves.
Fig. 2. (a) Dependence of MR ratio, and sheet resistance, R, on the OL thickness, t. (b) Dependence of sheet *R and H on the 1 OL thickness, t.
spin-valve structure from types-A to B. The enhancement of sheet *R and MR ratios by the insertion of OL-F in type-B may be explained by assuming that both OL-F and OL-P contribute the specular scattering of the conduction electrons in type-B, while only OL-P contributes the specular scattering in type-A. Larger MR ratios were obtained for type-C spin valves. In this case, dual structure and the specular scattering e!ect caused by both OL-P layers are considered to contribute the enhancement of the MR ratios. Decrease in
sheet *R is attributed to the increase of "lm thickness due to the dual spin-valve structure. Synthetic type spin valves having 1}1.4 nm thick oxide layers were also studied for types-A and B. CoFe/Ru/CoFe/OL/CoFe was used as the pinned layer for both types of spin valves. As for the free layer, CoFe was used for type-A and CoFe/ OL/CoFe was used for type-B. Fig. 5 shows the MR curves for the spin valves after annealing at 2803C. The MR ratios and sheet *R were 10% and 1.47 ) for type-A and 11.4% and 1.87 ) for type-B, respectively. These values are larger than those of conventional PtMn based spin valves, but are smaller than those of spin valves without synthetic layers as shown in Figs. 3b, 4a and b. Shunting e!ect of CoFe/Ru/CoFe synthetic layers may contribute to the decrease of MR ratios and sheet *R. Magnetic "eld strength of 500 Oe used for the
LETTER TO THE EDITOR
H. Sakakima et al. / Journal of Magnetism and Magnetic Materials 210 (2000) L20}L24
L23
Fig. 5. MR curves for (a) Ta/PtMn/CoFe/Ru/CoFe/OL/ CoFe/Cu/CoFe/Ta (type-A) and (b) Ta/PtMn/CoFe/ Ru/CoFe/OL/CoFe/Cu/CoFe/OL/CoFe/Ta (type-B). Fig. 4. MR curves for (a) Ta/PtMn/CoFe/OL/CoFe/Cu/CoFe/ OL/CoFe (type-B) and (b) Ta/PtMn/CoFe/OL/CoFe/Cu/CoFe/ Cu/CoFe/OL/CoFe/PtMn/Ta (type-C).
annealing might be insu$cient to align the pinned layers. Lastly, spin valves composed of a-Fe O / 2 3 PL/Cu/FL/Ta were studied, where PL is composed of CoFe and FL is composed of CoFe or CoFe/ OL(1)/CoFe. The MR ratios and sheet *R were increased from 11.9% and 2.35 ) to 14.3% and 3.22 ) by inserting OL in the free layer. In these spin valves, total metallic layer thickness is small, and the specular scattering occurs at the interfaces of a-Fe O /CoFe and CoFe/OL. Therefore, 2 3 large *R is obtained in these spin valves. However, H of these spin valves was about 100 Oe, which is 1 much smaller than those of PtMn-based spin valves.
4. Summary PtMn and a-Fe O -based spin valves having 2 3 specular re#ective oxide layers, OL, in the pinned or free layers were studied. These spin valves showed larger MR ratios, 12}17% and larger sheet *R, 1.7}3.2 ), than spin valves without OL. PtMn-based spin valves are thermally stable than IrMn-based spin valves, but the MR ratios of conventional PtMn-based spin valves were not su$ciently high as IrMn-based spin valves. Therefore, PtMn-based spin valves having OL in this study, having large MR ratios and high thermal stability, are promising GMR "lms for spin-valve heads. Acknowledgements Part of this work was performed under the management of ASET in MITI's R&D program supported by NEDO.
LETTER TO THE EDITOR
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H. Sakakima et al. / Journal of Magnetism and Magnetic Materials 210 (2000) L20}L24
References [1] [2] [3] [4] [5]
H.J. Swagten et al., Phys. Rev. B 53 (1996) 9108. W.F. Egelhof et al., J. Appl. Phys. 79 (1996) 5277. N. Hasegawa et al., IEEE Trans. Magn. 32 (1996) 4918. Y. Sugita et al., Jpn. J. Appl. Phys. 37 (1998) 5984. H. Sakakima et al., J. Magn. Magn. Mater. 198}199 (1999) 9.
[6] Y. Sugita et al., Digest of INTERMAG'99 FC-07 (1999). [7] V.S. Speriosu et al., Digest of INTERMAG'96 AA-04 (1996). [8] Y. Kamiguchi et al., Digest of INTERMAG'99 DB-01 (1999). [9] M. Saito et al., Digest of INTERMAG'97 HA-06 (1997).