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Si(1 1 1) films
Journal of Magnetism and Magnetic Materials 209 (2000) 208 } 210
Comparison of magnetic properties of ultrathin Co/Si(1 1 1) and Co/Ag/Si(1 1 1) "lms J.S. Tsay, Y.D. Yao*, Y. Liou, S.F. Lee, C.S. Yang Institute of Physics, Academia Sinica, Taipei 11529, Taiwan, ROC
Abstract The thermal stability of the magnetization of the Co/Ag/Si(1 1 1) "lm is lower than that of the Co/Si(1 1 1) "lm. From Auger electron spectroscopy, we demonstrate that Ag atoms in the Co/Ag/Si(1 1 1) "lm segregate to top layers below 350 K. The segregation of Ag atoms improves the di!usion of Co into Si(1 1 1) substrate. Annealing 10.5 ML Co/Si(1 1 1) "lm causes the easy axis of magnetization to transform from an in-plane to a cant out-of-plane. The in-plane magnetization of 10.5 ML in Co/Ag/Si(1 1 1) "lm persists after annealing. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Cobalt; Magneto-optic Kerr e!ect; Ultrathin "lm
1. Introduction In recent years the experimental and theoretical investigations of metal overlayers on silicon surface have witnessed a rapid growth since they are of fundamental and practical importance in the "eld of surface science, materials research and technology [1}6]. The magnetic properties of a magnetic "lm are usually modi"ed by the addition of another interface [7]. This phenomenon is of great scienti"c and technological interest. In this report, the magnetic properties of ultrathin Co "lms on Si(1 1 1) and Ag/Si(1 1 1) surfaces were comparatively investigated using in situ surface magneto-optic Kerr e!ect (SMOKE) and Auger electron spectroscopy (AES).
2. Experiment Experiments were conducted in an ultra-high vacuum (UHV) chamber with a background pressure better than 1]10~10 Torr. The Si(1 1 1) surface was cleaned by Ar ion bombardment and annealing cycles in the UHV
* Corresponding author. Tel.: #886-2-27899617; fax: #8862-27899636. E-mail address: [email protected] (Y.D. Yao)
chamber. The sputtering}annealing cycles were continued until a well-ordered 7]7 LEED pattern with bright, sharp spots and a low background was observed. A He}Ne laser with a wavelength of 632.8 nm was used as the light source in the SMOKE measurement. Experimental details have also been described elsewhere [5,8].
3. Results and discussion Fig. 1 shows temperature dependencies of the remnant Kerr intensities for 10.5 ML Co/Si(1 1 1) (circles) and 10.5 ML Co/8.4 ML Ag/Si(1 1 1) "lms (squares) on the longitudinal con"guration. As the temperature increases, the longitudinal Kerr intensity decreases monotonically for the 10.5 ML Co/Si(1 1 1) "lm. It vanishes at 600 K. For the 10.5 ML Co/8.4 ML Ag/Si(1 1 1) "lm, the behavior of the longitudinal Kerr intensity is similar to that of 10.5 ML Co/Si(1 1 1) "lm. It also decreases to zero. However, the temperature in which the Kerr intensity vanishes is reduced to 550 K. The thermal stability of the magnetization of the Co/Ag/Si(1 1 1) "lm is lower than that of the Co/Si(1 1 1) "lm. Fig. 2 shows temperature dependencies of the remnant Kerr intensities for 10.5 ML Co/Si(1 1 1) (circles) and 10.5 ML Co/8.4 ML Ag/Si(1 1 1) "lms (squares) on the polar con"guration. As the temperature increases, polar
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 6 8 9 - 7
J.S. Tsay et al. / Journal of Magnetism and Magnetic Materials 209 (2000) 208}210
Fig. 1. Temperature dependencies of the remnant Kerr intensities for 10.5 ML Co/Si(1 1 1) (circles) and 10.5 ML Co/Ag/Si(1 1 1) "lms (squares) on longitudinal con"guration.
Fig. 2. Temperature dependencies of the remnant Kerr intensities for 10.5 ML Co/Si(1 1 1) (circles) and 10.5 ML Co/Ag/Si(1 1 1) "lms (squares) on polar con"guration.
Kerr intensity for 10.5 ML Co/Si(1 1 1) increases roughly below 475 K. At higher temperature, it decreases to zero at 600 K. The Co "lm shows a canted out-of-plane easy axis after annealing. For the 10.5 ML Co/Ag/Si(1 1 1) "lm, we did not observe any polar Kerr signal below 625 K. The 10.5 ML Co/Ag/Si(1 1 1) "lm always shows an in-plane easy axis. A case similar to Co/Ag multilayer possessing in-plane anisotropy due to the interface anisotropy has also been reported [7]. The in-plane anisotropy is preferred for Co/Ag interface. This is the reason of the di!erent anisotropy. The ferromagnetic inactive layers at the Co/Si interface are formed due to intermixing of Co and Si; and can be reduced by an Ag bu!er layer [3]. The Ag layer serves as a bu!er layer which prevents the intermixing of Co and Si. However, comparing the temperature dependencies of the remnant Kerr intensities as depicted in Fig. 1, the temperature in which the Kerr intensity vanishes is reduced after adding an Ag bu!er layer. We can conclude that the thermal stability of the magnetization of the Co/Ag/Si(1 1 1) "lm is lower than that of the Co/Si(1 1 1) "lm. This means that the protective e!ect of the Ag bu!er
209
Fig. 3. Temperature dependence of the Auger signal for the 10.5 ML Co/Si(1 1 1) "lm. Auger signal of the Co decreases below 400 K. It keeps constant between 400 and 500 K, and decreases above 500 K.
layer functions only at low temperature. To understand the reason, we performed the AES study on the surface compositions of both "lms. Fig. 3 shows temperature dependence of the Auger signal of the 10.5 ML Co/Si(1 1 1) "lm. After annealing the "lm, the Auger signal of Co (LMM) decreases at temperature below 400 K. The Co Auger signal remains constant at temperature between 400 and 450 K. The #at part in the Auger signal versus temperature curve may be the result of the formation of a metastable Co}Si compound which prevents further silicide formation [5]. At higher temperature, it decreases monotonically. A rapid decrease occurs at temperature above 525 K. Comparing with Fig. 1, the corresponding Kerr intensity also decreases rapidly above 525 K. This indicates that the Co di!uses into Si substrate and the Co}Pt compound is formed. The Co}Si compound could reduce the magnetization of the Co "lm [8]. This causes the corresponding Kerr intensity to decrease rapidly to zero at temperatures above 600 K. Fig. 4 shows the temperature dependencies of the Auger signals for the 10.5 ML Co/8.4 ML Ag/Si(1 1 1) "lms. After annealing the "lm, the Auger signal of Ag (MNN) increases at temperature below 350 K. Simultaneously, Auger signal of the Co (LMM) slowly decreases. This means that Ag atoms segregate to the top layer even at 350 K. The decrease of the Co Auger signal (LMM) at temperature below 425 K is small. So the top layers contain almost pure Co. Comparing with Fig. 1, the corresponding Kerr intensity decreases slowly. The decrease in the Kerr intensity at higher temperature can be explained by the mean "eld theory. At temperature above 475 K, Co (LMM) signal decreases rapidly. This indicates that the Co di!uses into Si substrate and Co}Si compound forms. The Co}Si compound could reduce the magnetization of the Co "lm [8]. This causes the corresponding Kerr intensity to decrease rapidly to zero at temperature above 475 K. Comparing Figs. 3 and 4, the rapid decrease in Co Auger signal occurs at a lower
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J.S. Tsay et al. / Journal of Magnetism and Magnetic Materials 209 (2000) 208}210
a cant out-of-plane. The in-plane magnetization of 10.5 ML in Co/Ag/Si(1 1 1) "lm persists after annealing. From experimental results of temperature dependence of Auger signals, we demonstrate that Ag atoms in the Co/Ag/Si(1 1 1) "lm segregate to top layers below 350 K. The segregation of Ag atoms improves the di!usion of Co into Si(1 1 1) substrate. This implies that the thermal stability of the magnetization of the Co/Ag/Si(1 1 1) "lm is lower than that of the Co/Si(1 1 1) "lm.
Acknowledgements Fig. 4. Temperature dependence of the Auger signal for the (10.5 ML Co/8.4 ML Ag)/Si(1 1 1) "lm. Auger signal of the Co (triangles) keeps constant below 475 K and decreases above 475 K. Auger signal of the Ag (crosses) increases below 350 K and slightly decreases above 450 K.
temperature for 10.5 ML Co/Ag/Si(1 1 1) "lm than 10.5 ML Co/Si(1 1 1) "lm. So the added Ag layer improves the di!usion of Co into Si(1 1 1) substrate. This may be due to the segregation of Ag atoms which damage the structure of the Co layer. This indicates that the thermal stability of the magnetization of the Co/Ag/Si(1 1 1) "lm is lower than that of the Co/Si(1 1 1) "lm. As the temperature increases above 450 K, Ag (MNN) signal slightly decreases. This is due to the di!usion of Ag into Si substrate.
4. Conclusion Annealing 10.5 ML Co/Si(1 1 1) "lm causes the easy axis of magnetization to transform from an in-plane to
This work was supported by the National Science Council of ROC under Grant No. NSC 88-2112-M-001038.
References [1] D. Leong, M. Harry, K.J. Reeson, K.P. Homewood, Nature 387 (1997) 686. [2] M.O. Aboelfotoh, A.D. Marwick, J.L. Freeouf, Phys. Rev. B 49 (1994) 10753. [3] T. Meyer, H. Von Kanel, Phys. Rev. Lett. 78 (1997) 3133. [4] K. Prabhakaran, K. Sumitomo, T. Ogino, Appl. Phys. Lett. 70 (1997) 607. [5] J.S. Tsay, Y.D. Yao, Appl. Phys. Lett. 74 (1999) 1311. [6] A.E. Dolbak, B.Z. Olshanetsky, S.A. Teys, Surf. Sci. 373 (1997) 43. [7] F.J.A. den Broeder, W. Hoving, P.J.H. Bloemen, J. Magn. Magn. Mater. 93 (1991) 562. [8] J.S. Tsay, C.S. Yang, Y.D. Yao, Y. Liou, S.F. Lee, Jpn. J. Appl. Phys. 37 (1998) 1065.
Comparison of magnetic properties of ultrathin Co/Si(1 1 1) and Co/Ag/Si(1 1 1) "lms J.S. Tsay, Y.D. Yao*, Y. Liou, S.F. Lee, C.S. Yang Institute of Physics, Academia Sinica, Taipei 11529, Taiwan, ROC
Abstract The thermal stability of the magnetization of the Co/Ag/Si(1 1 1) "lm is lower than that of the Co/Si(1 1 1) "lm. From Auger electron spectroscopy, we demonstrate that Ag atoms in the Co/Ag/Si(1 1 1) "lm segregate to top layers below 350 K. The segregation of Ag atoms improves the di!usion of Co into Si(1 1 1) substrate. Annealing 10.5 ML Co/Si(1 1 1) "lm causes the easy axis of magnetization to transform from an in-plane to a cant out-of-plane. The in-plane magnetization of 10.5 ML in Co/Ag/Si(1 1 1) "lm persists after annealing. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Cobalt; Magneto-optic Kerr e!ect; Ultrathin "lm
1. Introduction In recent years the experimental and theoretical investigations of metal overlayers on silicon surface have witnessed a rapid growth since they are of fundamental and practical importance in the "eld of surface science, materials research and technology [1}6]. The magnetic properties of a magnetic "lm are usually modi"ed by the addition of another interface [7]. This phenomenon is of great scienti"c and technological interest. In this report, the magnetic properties of ultrathin Co "lms on Si(1 1 1) and Ag/Si(1 1 1) surfaces were comparatively investigated using in situ surface magneto-optic Kerr e!ect (SMOKE) and Auger electron spectroscopy (AES).
2. Experiment Experiments were conducted in an ultra-high vacuum (UHV) chamber with a background pressure better than 1]10~10 Torr. The Si(1 1 1) surface was cleaned by Ar ion bombardment and annealing cycles in the UHV
* Corresponding author. Tel.: #886-2-27899617; fax: #8862-27899636. E-mail address: [email protected] (Y.D. Yao)
chamber. The sputtering}annealing cycles were continued until a well-ordered 7]7 LEED pattern with bright, sharp spots and a low background was observed. A He}Ne laser with a wavelength of 632.8 nm was used as the light source in the SMOKE measurement. Experimental details have also been described elsewhere [5,8].
3. Results and discussion Fig. 1 shows temperature dependencies of the remnant Kerr intensities for 10.5 ML Co/Si(1 1 1) (circles) and 10.5 ML Co/8.4 ML Ag/Si(1 1 1) "lms (squares) on the longitudinal con"guration. As the temperature increases, the longitudinal Kerr intensity decreases monotonically for the 10.5 ML Co/Si(1 1 1) "lm. It vanishes at 600 K. For the 10.5 ML Co/8.4 ML Ag/Si(1 1 1) "lm, the behavior of the longitudinal Kerr intensity is similar to that of 10.5 ML Co/Si(1 1 1) "lm. It also decreases to zero. However, the temperature in which the Kerr intensity vanishes is reduced to 550 K. The thermal stability of the magnetization of the Co/Ag/Si(1 1 1) "lm is lower than that of the Co/Si(1 1 1) "lm. Fig. 2 shows temperature dependencies of the remnant Kerr intensities for 10.5 ML Co/Si(1 1 1) (circles) and 10.5 ML Co/8.4 ML Ag/Si(1 1 1) "lms (squares) on the polar con"guration. As the temperature increases, polar
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 6 8 9 - 7
J.S. Tsay et al. / Journal of Magnetism and Magnetic Materials 209 (2000) 208}210
Fig. 1. Temperature dependencies of the remnant Kerr intensities for 10.5 ML Co/Si(1 1 1) (circles) and 10.5 ML Co/Ag/Si(1 1 1) "lms (squares) on longitudinal con"guration.
Fig. 2. Temperature dependencies of the remnant Kerr intensities for 10.5 ML Co/Si(1 1 1) (circles) and 10.5 ML Co/Ag/Si(1 1 1) "lms (squares) on polar con"guration.
Kerr intensity for 10.5 ML Co/Si(1 1 1) increases roughly below 475 K. At higher temperature, it decreases to zero at 600 K. The Co "lm shows a canted out-of-plane easy axis after annealing. For the 10.5 ML Co/Ag/Si(1 1 1) "lm, we did not observe any polar Kerr signal below 625 K. The 10.5 ML Co/Ag/Si(1 1 1) "lm always shows an in-plane easy axis. A case similar to Co/Ag multilayer possessing in-plane anisotropy due to the interface anisotropy has also been reported [7]. The in-plane anisotropy is preferred for Co/Ag interface. This is the reason of the di!erent anisotropy. The ferromagnetic inactive layers at the Co/Si interface are formed due to intermixing of Co and Si; and can be reduced by an Ag bu!er layer [3]. The Ag layer serves as a bu!er layer which prevents the intermixing of Co and Si. However, comparing the temperature dependencies of the remnant Kerr intensities as depicted in Fig. 1, the temperature in which the Kerr intensity vanishes is reduced after adding an Ag bu!er layer. We can conclude that the thermal stability of the magnetization of the Co/Ag/Si(1 1 1) "lm is lower than that of the Co/Si(1 1 1) "lm. This means that the protective e!ect of the Ag bu!er
209
Fig. 3. Temperature dependence of the Auger signal for the 10.5 ML Co/Si(1 1 1) "lm. Auger signal of the Co decreases below 400 K. It keeps constant between 400 and 500 K, and decreases above 500 K.
layer functions only at low temperature. To understand the reason, we performed the AES study on the surface compositions of both "lms. Fig. 3 shows temperature dependence of the Auger signal of the 10.5 ML Co/Si(1 1 1) "lm. After annealing the "lm, the Auger signal of Co (LMM) decreases at temperature below 400 K. The Co Auger signal remains constant at temperature between 400 and 450 K. The #at part in the Auger signal versus temperature curve may be the result of the formation of a metastable Co}Si compound which prevents further silicide formation [5]. At higher temperature, it decreases monotonically. A rapid decrease occurs at temperature above 525 K. Comparing with Fig. 1, the corresponding Kerr intensity also decreases rapidly above 525 K. This indicates that the Co di!uses into Si substrate and the Co}Pt compound is formed. The Co}Si compound could reduce the magnetization of the Co "lm [8]. This causes the corresponding Kerr intensity to decrease rapidly to zero at temperatures above 600 K. Fig. 4 shows the temperature dependencies of the Auger signals for the 10.5 ML Co/8.4 ML Ag/Si(1 1 1) "lms. After annealing the "lm, the Auger signal of Ag (MNN) increases at temperature below 350 K. Simultaneously, Auger signal of the Co (LMM) slowly decreases. This means that Ag atoms segregate to the top layer even at 350 K. The decrease of the Co Auger signal (LMM) at temperature below 425 K is small. So the top layers contain almost pure Co. Comparing with Fig. 1, the corresponding Kerr intensity decreases slowly. The decrease in the Kerr intensity at higher temperature can be explained by the mean "eld theory. At temperature above 475 K, Co (LMM) signal decreases rapidly. This indicates that the Co di!uses into Si substrate and Co}Si compound forms. The Co}Si compound could reduce the magnetization of the Co "lm [8]. This causes the corresponding Kerr intensity to decrease rapidly to zero at temperature above 475 K. Comparing Figs. 3 and 4, the rapid decrease in Co Auger signal occurs at a lower
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a cant out-of-plane. The in-plane magnetization of 10.5 ML in Co/Ag/Si(1 1 1) "lm persists after annealing. From experimental results of temperature dependence of Auger signals, we demonstrate that Ag atoms in the Co/Ag/Si(1 1 1) "lm segregate to top layers below 350 K. The segregation of Ag atoms improves the di!usion of Co into Si(1 1 1) substrate. This implies that the thermal stability of the magnetization of the Co/Ag/Si(1 1 1) "lm is lower than that of the Co/Si(1 1 1) "lm.
Acknowledgements Fig. 4. Temperature dependence of the Auger signal for the (10.5 ML Co/8.4 ML Ag)/Si(1 1 1) "lm. Auger signal of the Co (triangles) keeps constant below 475 K and decreases above 475 K. Auger signal of the Ag (crosses) increases below 350 K and slightly decreases above 450 K.
temperature for 10.5 ML Co/Ag/Si(1 1 1) "lm than 10.5 ML Co/Si(1 1 1) "lm. So the added Ag layer improves the di!usion of Co into Si(1 1 1) substrate. This may be due to the segregation of Ag atoms which damage the structure of the Co layer. This indicates that the thermal stability of the magnetization of the Co/Ag/Si(1 1 1) "lm is lower than that of the Co/Si(1 1 1) "lm. As the temperature increases above 450 K, Ag (MNN) signal slightly decreases. This is due to the di!usion of Ag into Si substrate.
4. Conclusion Annealing 10.5 ML Co/Si(1 1 1) "lm causes the easy axis of magnetization to transform from an in-plane to
This work was supported by the National Science Council of ROC under Grant No. NSC 88-2112-M-001038.
References [1] D. Leong, M. Harry, K.J. Reeson, K.P. Homewood, Nature 387 (1997) 686. [2] M.O. Aboelfotoh, A.D. Marwick, J.L. Freeouf, Phys. Rev. B 49 (1994) 10753. [3] T. Meyer, H. Von Kanel, Phys. Rev. Lett. 78 (1997) 3133. [4] K. Prabhakaran, K. Sumitomo, T. Ogino, Appl. Phys. Lett. 70 (1997) 607. [5] J.S. Tsay, Y.D. Yao, Appl. Phys. Lett. 74 (1999) 1311. [6] A.E. Dolbak, B.Z. Olshanetsky, S.A. Teys, Surf. Sci. 373 (1997) 43. [7] F.J.A. den Broeder, W. Hoving, P.J.H. Bloemen, J. Magn. Magn. Mater. 93 (1991) 562. [8] J.S. Tsay, C.S. Yang, Y.D. Yao, Y. Liou, S.F. Lee, Jpn. J. Appl. Phys. 37 (1998) 1065.