- Email: [email protected]
Ge(1 1 1) films
Applied Surface Science 219 (2003) 88–92
Magnetic properties influenced by interfaces in ultrathin Co/Ge(1 0 0) and Co/Ge(1 1 1) films J.S. Tsaya,*, Y.D. Yaob, W.C. Chengc, T.K. Tsengc, K.C. Wangc, C.S. Yangb a
Department of Physics, Tunghai University, 181 Taichung-kang Road, Section 3, Taichung 407, Taiwan b Institute of Physics, Academia Sinica, Taipei 11529, Taiwan c Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 107, Taiwan Received 21 August 2002; accepted 20 December 2002
Abstract Magnetic properties influenced by interfaces in ultrathin Co/Ge(1 0 0) and Co/Ge(1 1 1) films with thickness below 28 monolayers (ML) have been studied using the surface magneto-optic Kerr effect (SMOKE) technique. In both systems, the nonferromagnetic layer, as an interface between Co and Ge, plays an important role during annealing. In general, ultrathin Co films with fixed total thickness but fabricated at different temperatures on the same substrate, their Kerr hysteresis loops disappear roughly at the same temperature. This suggests that the thickness of the interfacial layer could inversely prevent the diffusion between Co and Ge substrate. From the annealing studies for both systems with total film thickness of 28 monolayers, we have found that Kerr signal disappears at 375 K for Co/Ge(1 1 1) and 425 K for Co/Ge(1 0 0) films. This suggests that Co/ Ge(1 1 1) films possess a lower thermal stability than that of the Co/Ge(1 0 0) films. Our experimental data could be explained by different interfacial condition between Ge(1 0 0) and Ge(1 1 1), the different onset of interdiffusion, and the surface structure condition of Ge(1 0 0) and Ge(1 1 1). # 2003 Elsevier Science B.V. All rights reserved. PACS: 75.70.Ak; 78.20.Ls; 68.35.Fx Keywords: Surface magneto-optic Kerr effect; Metal–semiconductor magnetic heterostructures; Germanium; Cobalt; Ultrathin films
1. Introduction As device dimensions are promoted from submicrometer to nanometer scale in microelectronics applications, new fabrication processes and detailed understanding of materials become necessary to overcome the limitations of existed methods. Ge-contained alloys have received large interest in the passed years *
Corresponding author. Tel.: þ886-4-23590121x3461; fax: þ886-4-23594643. E-mail address: [email protected] (J.S. Tsay).
because they have emerged as challenging materials to improve the performance of Si-based devices [1–4]. The study of metal-semiconductor interaction is also of fundamental interest in material science [5–7]. Due to tremendous applications, such as spin transport and recording, nano-scale magnetism has received great attentions [7–10]. In our earlier work, we have investigated the magnetic behavior of the Co/Si films and compared the interfacial magnetism of Co/Si and Co/Ge species [10–13]. In this article, the influences of interfacial layers on the magnetic properties and compound formation for ultrathin Co films grown on
0169-4332/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00636-6
J.S. Tsay et al. / Applied Surface Science 219 (2003) 88–92
89
Ge(1 1 1) and Ge(1 0 0) surfaces have been systematically studied using the in situ surface magneto-optic Kerr effect (SMOKE) technique. It is shown that along with the rate deposition, which controls this interface compound layers between the different Ge substrate and the Co magnetic monolayer (ML), the temperature affects directly the behavior of the system, being these some of the important control parameters in the pursuit of tuning more complex magnetic systems [14].
2. Experiment The experiments were conducted in an ultrahigh vacuum (UHV) chamber equipped with low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoemission spectroscopy (XPS) and SMOKE. The background pressure of the UHV chamber was lower than 4 108 Pa. The various components are described in detailed elsewhere [10–13]. The Ge(1 1 1) and Ge(1 0 0) surfaces were cleaned by means of Arþ ion bombardment and subsequent annealing cycles. The chemical impurity on the substrate surfaces was monitored by AES. One ˚ (7.21014 atoms/ monolayer of cobalt equals to 0.8 A 2 14 ˚ (6.510 atoms/cm2) on Ge(1 1 1) cm ) and 0.7 A and Ge(1 0 0) surfaces, respectively. Annealing treatments have been done by directly passing electric currents through the Ge substrates. The sample temperature was measured by a K-type thermocouple. A He–Ne laser with a wavelength of 632.8 nm was used as the light source for the SMOKE measurement.
3. Results and discussion Thickness dependencies of the remanent Kerr intensities for ultrathin Co/Ge(1 0 0) and Co/Ge(1 1 1) films grown at 300 K are shown in Fig. 1. For a Co/Ge(1 0 0) system, the Kerr intensities are zero up to 12 ML at the initial stage of the deposition. As the coverage increases above roughly 15 ML, the linear behavior of the Kerr intensity indicates that the Curie temperature is well above the measurement temperature of 300 K. So pure cobalt could contribute to the magnetization with the same weight. Non-zero intercept from the straight-line extrapolation of the data for thickness above 16 ML shows the existence of
Fig. 1. Thickness dependencies of the remanent Kerr intensities for ultrathin Co/Ge(1 0 0) and Co/Ge(1 1 1) films grown and measured at 300 K. Non-zero intercept from the straight-line extrapolation of the data for thick films shows the existence of a nonferromagnetic Co/Ge compound layer.
a nonferromagnetic Co/Ge compound layer [11,15]. From Fig. 1, the compound layer at a Co/Ge(1 0 0) interface is about 6.2 ML. Spectroscopic evidences of the formation of a germanide were given for the room temperature deposition of cobalt on a Ge surface [15,16]. Thickness dependency of the remanent Kerr intensity for Co/Ge(1 1 1) films is quite similar to that of Co/Ge(1 0 0) while the Kerr intensity occurs abruptly at a lower coverage of roughly 10 ML. By comparing the thickness of nonferromagnetic layers at the interfaces of Co/Ge(1 0 0) and Co/Ge(1 1 1) obtained from the straight-line extrapolation of the Kerr intensities, we found that it is thicker in a Co/ Ge(1 0 0) system than that in a Co/Ge(1 1 1) system. This is explained by the difference of the densities of surface atoms on surfaces of Ge(1 0 0) (6.51014 atoms/cm2) and Ge(1 1 1) (7.21014 atoms/cm2). The looseness of the surface gives a more opportunity for a higher concentration of cobalt atoms into the substrate to form Co/Ge compounds. To obtain the structural information of the films, in situ LEED measurements have been performed. The LEED measurements were carried out at a sample temperature of 200 K. After cycles of argon ion sputtering and high temperature annealing, a wellordered 2 1 structure was observed for the Ge(1 0 0) substrates as shown in Fig. 2a. Deposition of cobalt overlayer causes the disappearance of the ordered surface structure. For cobalt films with coverage between 5 and 28 ML, only a diffused background
90
J.S. Tsay et al. / Applied Surface Science 219 (2003) 88–92
Fig. 2. LEED patterns for (a) a Ge(1 0 0) surface, (b) 7 ML Co/Ge(1 0 0) film, (c) a Ge(1 1 1) surface, and (d) 4.5 ML Co/Ge(1 1 1) film.
without LEED spot patterns was observed. As an example, Fig. 2b shows the LEED pattern for 7 ML Co/Ge(1 0 0) film. The structure of the interfacial layers is disordered. This could be explained by the large misfit between Co and Ge(1 0 0) lattice, and tendency of interdiffusion resulting in the formation of Co/Ge compound. Clean Ge(1 1 1) surface exhibits a well-ordered c(2 8) structure as shown in Fig. 2c. After Co deposition, the surface structure changes to a 1 1 ordered structure. For Co coverage below 25 ML, the 1 1 structure retains. However, the LEED spots become dimmer as the Co coverage increases. Considering the topmost layer, 1 1 Ge(1 1 1) surface shows a triangular structure with the nearest-neighbor distance about 0.245 nm [17]. These structure data are closed to those of Co(0 0 0 1) structure where the nearest-neighbor distance is about 0.250 nm. Co adatoms could grow epitaxially under this circumstance. The interface of Co/Ge(1 1 1) is then in a more ordered structure. From the LEED measurements, we can conclude that the interfacial layer for Co/Ge(1 0 0)
is rougher than that for Co/Ge(1 1 1). This result is attributed to the difference of the surface atomic densities on Ge(1 0 0) and Ge(1 1 1), and confirms the different thickness for the nonferromagnetic layers. Thermal evolution of the magnetic properties for Co/Ge(1 0 0) films prepared at different substrate temperatures was investigated by SMOKE. For each sample, thermal annealing was started from the temperature of deposition with a step of 25 K up to 450 K by passing electric currents through the sample. At each temperature step, 3 min were taken for the Kerr loop measurements. In general, for Co films with fixed thickness but fabricated at different substrate temperatures, the hysteresis loops disappear roughly at the same temperature. This suggests that the thickness of the nonferromagnetic interfacial layer could inversely prevent the diffusion between Co and the underlying Ge substrate. As an example, Fig. 3 depicts the thermal evolution of the hysteresis loops for 28 ML Co/ Ge(1 0 0) deposited at 300 K. As the substrate temperature increases, the height of the hysteresis loop
J.S. Tsay et al. / Applied Surface Science 219 (2003) 88–92
91
Fig. 3. Thermal evolution of the magnetic hysteresis for 28 ML Co/Ge(1 0 0) on the longitudinal configuration. The hysteresis disappears at temperatures above 425 K.
Fig. 4. Temperature dependence of the remanent Kerr intensities for 28 ML Co/Ge(1 0 0) films deposited at indicated temperatures. The Kerr intensities vanish at the same temperature.
decreases monotonically. The hysteresis loop disappears at temperatures above 425 K. No hysteresis could be observed after the sample is then cooled down to low temperatures. The evolution of the hysteresis upon annealing is irreversible. Fig. 4 depicts the temperature dependence of the remanent Kerr intensities on the longitudinal configuration for 28 ML Co/ Ge(1 0 0) films deposited at 120, 200, 300, and 400 K. As the substrate temperature increases, all the Kerr intensities decrease monotonically at temperatures roughly below 375 K. At higher temperatures, Kerr intensities decrease rapidly until zero. Corresponding intensities of Co Auger signals show a drop around 400 K from our AES experiments. This indicates a strong diffusion of Co into the Ge substrate. For all the samples, the Kerr intensity vanishes at the same temperature of 425 K. XPS measurements of this system show a drop of the Co signal followed by a slow decrease around the same temperature [3]. Therefore, we believe that the temperature is high enough for Co adatoms to overcome the energy barrier of the interdiffusion into the Ge(1 0 0) substrate at 425 K. No pure cobalt island exists on the top of the surface and results in the disappearance of the ferromagnetism. As a comparison of the annealing effects on the magnetic properties of Co/Ge(1 0 0) and Co/Ge(1 1 1) films, Fig. 5 depicts the temperature dependencies of the longitudinal Kerr intensities for 28 ML-thick Co films deposited at room temperature. When the annealing temperature increases, both the Kerr intensities
decrease and drop to zero at high temperatures. For the Co/Ge(1 1 1) film, the temperature where ferromagnetism disappears occurs at 375 K, i.e. 50 K lower than in the Co/Ge(1 0 0) system. LEED measurements in Fig. 2 reveal that Co/Ge(1 0 0) exhibits a disordered structure for the interfacial layers. SMOKE measurements during the Co deposition in Fig. 1 show that the interdiffusion is stronger in Co/Ge(1 0 0) than in Co/Ge(1 1 1). Therefore we can conclude that Co/Ge(1 0 0) forms a rougher interface and the possibility of accumulation of atoms to form micro-size clusters is higher. The clustering process increases the possible cluster sizes. The formation of some larger clusters causes a higher thermal stability of this system.
Fig. 5. Temperature dependencies of the remanent Kerr intensities for 28 ML-thick Co/Ge(1 0 0) and Co/Ge(1 1 1) films.
92
J.S. Tsay et al. / Applied Surface Science 219 (2003) 88–92
In contract, Co/Ge(1 1 1) shows a smooth interface which serves a more possibility of a contact between Co and the underlying Ge(1 1 1) substrate. This should be the reason of the observation of a lower thermal stability in this system. In summary, by comparing the thickness of nonferromagnetic layers at the interfaces of Co/Ge(1 0 0) and Co/Ge(1 1 1), we found that it is thicker in Co/Ge(1 0 0) system than that in Co/Ge(1 1 1) system. This is explained by the difference of the densities of surface atoms on Ge(1 0 0) and Ge(1 1 1) surfaces. In general, ultrathin Co films with fixed total thickness (including Co and nonferromagnetic layers) but fabricated at different temperatures on the same substrate, their Kerr hysteresis loops disappear roughly at the same temperature upon annealing up to 450 K. For instance, the disappearance temperature was observed at 425 K for all the Co/Ge(1 0 0) films with a thickness of 28 ML. This suggests that the thickness of the interfacial layer could inversely prevent the diffusion between Co and the Ge substrate. From the annealing studies between room temperature and 450 K for samples with total film thickness of 28 ML, we have found that the Co/Ge(1 1 1) films possesses a lower thermal stability than that of the Co/Ge(1 0 0) films. This result could be explained by different interfacial conditions between Ge(1 0 0) and Ge(1 1 1).
References [1] H. van Kanal, Mater. Sci. Rep. 8 (1992) 193. [2] P.T. Goeller, B.I. Boyanov, D.E. Sayers, R.J. Nemanich, A.F. Myers, E.B. Steel, J. Mater. Res. 14 (1999) 4372. [3] K. Prabhakaran, T. Ogino, Appl. Surf. Sci. 100–101 (1996) 518. [4] K. Prabhakaran, K. Sumitomo, T. Ogino, Appl. Phys. Lett. 70 (1997) 607. [5] G. Rossi, Surf. Sci. Rep. 7 (1987) 1. [6] D. Leong, M. Harry, K.J. Reeson, K.P. Homewood, Nature 387 (1997) 686. [7] Y. Shibayama, H. Sato, T. Enoki, M. Endo, Phys. Rev. Lett. 84 (2000) 1744. [8] K.J. Kirk, Contemp. Phys. 41 (2000) 61. [9] W. Wulfhekel, F. Zavaliche, F. Porrati, H.P. Oepen, J. Kirschner, Europhys. Lett. 49 (2000) 651. [10] J.S. Tsay, Y.D. Yao, Appl. Phys. Lett. 74 (1999) 1311. [11] J.S. Tsay, C.S. Yang, Y.D. Yao, Y. Liou, S.F. Lee, Jpn. J. Appl. Phys. Part 1 37 (1998) 5976. [12] J.S. Tsay, Y.D. Yao, K.C. Wang, W.C. Cheng, C.S. Yang, Surf. Sci. 507–510 (2002) 498. [13] J.S. Tsay, Y.D. Yao, K.C. Wang, W.C. Cheng, C.S. Yang, J. Appl. Phys. 91 (2002) 8766. [14] J.P. Bucher, L.A. Bloomfield, Int. Mod. Phys. B 7 (1993) 1079. [15] G.A. Smith, L. Luo, S. Hashimoto, W.M. Gibson, N. Lewis, J. Vac. Sci. Technol. A 7 (1989) 1475. [16] G. Rangelov, P. Augustin, J. Stober, T. Fauster, Phys. Rev. B 49 (1994) 7535. [17] C. Kittel, Introduction to Solid State Physics, sixth ed., Wiley, Singapore, 1986.
Magnetic properties influenced by interfaces in ultrathin Co/Ge(1 0 0) and Co/Ge(1 1 1) films J.S. Tsaya,*, Y.D. Yaob, W.C. Chengc, T.K. Tsengc, K.C. Wangc, C.S. Yangb a
Department of Physics, Tunghai University, 181 Taichung-kang Road, Section 3, Taichung 407, Taiwan b Institute of Physics, Academia Sinica, Taipei 11529, Taiwan c Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei 107, Taiwan Received 21 August 2002; accepted 20 December 2002
Abstract Magnetic properties influenced by interfaces in ultrathin Co/Ge(1 0 0) and Co/Ge(1 1 1) films with thickness below 28 monolayers (ML) have been studied using the surface magneto-optic Kerr effect (SMOKE) technique. In both systems, the nonferromagnetic layer, as an interface between Co and Ge, plays an important role during annealing. In general, ultrathin Co films with fixed total thickness but fabricated at different temperatures on the same substrate, their Kerr hysteresis loops disappear roughly at the same temperature. This suggests that the thickness of the interfacial layer could inversely prevent the diffusion between Co and Ge substrate. From the annealing studies for both systems with total film thickness of 28 monolayers, we have found that Kerr signal disappears at 375 K for Co/Ge(1 1 1) and 425 K for Co/Ge(1 0 0) films. This suggests that Co/ Ge(1 1 1) films possess a lower thermal stability than that of the Co/Ge(1 0 0) films. Our experimental data could be explained by different interfacial condition between Ge(1 0 0) and Ge(1 1 1), the different onset of interdiffusion, and the surface structure condition of Ge(1 0 0) and Ge(1 1 1). # 2003 Elsevier Science B.V. All rights reserved. PACS: 75.70.Ak; 78.20.Ls; 68.35.Fx Keywords: Surface magneto-optic Kerr effect; Metal–semiconductor magnetic heterostructures; Germanium; Cobalt; Ultrathin films
1. Introduction As device dimensions are promoted from submicrometer to nanometer scale in microelectronics applications, new fabrication processes and detailed understanding of materials become necessary to overcome the limitations of existed methods. Ge-contained alloys have received large interest in the passed years *
Corresponding author. Tel.: þ886-4-23590121x3461; fax: þ886-4-23594643. E-mail address: [email protected] (J.S. Tsay).
because they have emerged as challenging materials to improve the performance of Si-based devices [1–4]. The study of metal-semiconductor interaction is also of fundamental interest in material science [5–7]. Due to tremendous applications, such as spin transport and recording, nano-scale magnetism has received great attentions [7–10]. In our earlier work, we have investigated the magnetic behavior of the Co/Si films and compared the interfacial magnetism of Co/Si and Co/Ge species [10–13]. In this article, the influences of interfacial layers on the magnetic properties and compound formation for ultrathin Co films grown on
0169-4332/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00636-6
J.S. Tsay et al. / Applied Surface Science 219 (2003) 88–92
89
Ge(1 1 1) and Ge(1 0 0) surfaces have been systematically studied using the in situ surface magneto-optic Kerr effect (SMOKE) technique. It is shown that along with the rate deposition, which controls this interface compound layers between the different Ge substrate and the Co magnetic monolayer (ML), the temperature affects directly the behavior of the system, being these some of the important control parameters in the pursuit of tuning more complex magnetic systems [14].
2. Experiment The experiments were conducted in an ultrahigh vacuum (UHV) chamber equipped with low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoemission spectroscopy (XPS) and SMOKE. The background pressure of the UHV chamber was lower than 4 108 Pa. The various components are described in detailed elsewhere [10–13]. The Ge(1 1 1) and Ge(1 0 0) surfaces were cleaned by means of Arþ ion bombardment and subsequent annealing cycles. The chemical impurity on the substrate surfaces was monitored by AES. One ˚ (7.21014 atoms/ monolayer of cobalt equals to 0.8 A 2 14 ˚ (6.510 atoms/cm2) on Ge(1 1 1) cm ) and 0.7 A and Ge(1 0 0) surfaces, respectively. Annealing treatments have been done by directly passing electric currents through the Ge substrates. The sample temperature was measured by a K-type thermocouple. A He–Ne laser with a wavelength of 632.8 nm was used as the light source for the SMOKE measurement.
3. Results and discussion Thickness dependencies of the remanent Kerr intensities for ultrathin Co/Ge(1 0 0) and Co/Ge(1 1 1) films grown at 300 K are shown in Fig. 1. For a Co/Ge(1 0 0) system, the Kerr intensities are zero up to 12 ML at the initial stage of the deposition. As the coverage increases above roughly 15 ML, the linear behavior of the Kerr intensity indicates that the Curie temperature is well above the measurement temperature of 300 K. So pure cobalt could contribute to the magnetization with the same weight. Non-zero intercept from the straight-line extrapolation of the data for thickness above 16 ML shows the existence of
Fig. 1. Thickness dependencies of the remanent Kerr intensities for ultrathin Co/Ge(1 0 0) and Co/Ge(1 1 1) films grown and measured at 300 K. Non-zero intercept from the straight-line extrapolation of the data for thick films shows the existence of a nonferromagnetic Co/Ge compound layer.
a nonferromagnetic Co/Ge compound layer [11,15]. From Fig. 1, the compound layer at a Co/Ge(1 0 0) interface is about 6.2 ML. Spectroscopic evidences of the formation of a germanide were given for the room temperature deposition of cobalt on a Ge surface [15,16]. Thickness dependency of the remanent Kerr intensity for Co/Ge(1 1 1) films is quite similar to that of Co/Ge(1 0 0) while the Kerr intensity occurs abruptly at a lower coverage of roughly 10 ML. By comparing the thickness of nonferromagnetic layers at the interfaces of Co/Ge(1 0 0) and Co/Ge(1 1 1) obtained from the straight-line extrapolation of the Kerr intensities, we found that it is thicker in a Co/ Ge(1 0 0) system than that in a Co/Ge(1 1 1) system. This is explained by the difference of the densities of surface atoms on surfaces of Ge(1 0 0) (6.51014 atoms/cm2) and Ge(1 1 1) (7.21014 atoms/cm2). The looseness of the surface gives a more opportunity for a higher concentration of cobalt atoms into the substrate to form Co/Ge compounds. To obtain the structural information of the films, in situ LEED measurements have been performed. The LEED measurements were carried out at a sample temperature of 200 K. After cycles of argon ion sputtering and high temperature annealing, a wellordered 2 1 structure was observed for the Ge(1 0 0) substrates as shown in Fig. 2a. Deposition of cobalt overlayer causes the disappearance of the ordered surface structure. For cobalt films with coverage between 5 and 28 ML, only a diffused background
90
J.S. Tsay et al. / Applied Surface Science 219 (2003) 88–92
Fig. 2. LEED patterns for (a) a Ge(1 0 0) surface, (b) 7 ML Co/Ge(1 0 0) film, (c) a Ge(1 1 1) surface, and (d) 4.5 ML Co/Ge(1 1 1) film.
without LEED spot patterns was observed. As an example, Fig. 2b shows the LEED pattern for 7 ML Co/Ge(1 0 0) film. The structure of the interfacial layers is disordered. This could be explained by the large misfit between Co and Ge(1 0 0) lattice, and tendency of interdiffusion resulting in the formation of Co/Ge compound. Clean Ge(1 1 1) surface exhibits a well-ordered c(2 8) structure as shown in Fig. 2c. After Co deposition, the surface structure changes to a 1 1 ordered structure. For Co coverage below 25 ML, the 1 1 structure retains. However, the LEED spots become dimmer as the Co coverage increases. Considering the topmost layer, 1 1 Ge(1 1 1) surface shows a triangular structure with the nearest-neighbor distance about 0.245 nm [17]. These structure data are closed to those of Co(0 0 0 1) structure where the nearest-neighbor distance is about 0.250 nm. Co adatoms could grow epitaxially under this circumstance. The interface of Co/Ge(1 1 1) is then in a more ordered structure. From the LEED measurements, we can conclude that the interfacial layer for Co/Ge(1 0 0)
is rougher than that for Co/Ge(1 1 1). This result is attributed to the difference of the surface atomic densities on Ge(1 0 0) and Ge(1 1 1), and confirms the different thickness for the nonferromagnetic layers. Thermal evolution of the magnetic properties for Co/Ge(1 0 0) films prepared at different substrate temperatures was investigated by SMOKE. For each sample, thermal annealing was started from the temperature of deposition with a step of 25 K up to 450 K by passing electric currents through the sample. At each temperature step, 3 min were taken for the Kerr loop measurements. In general, for Co films with fixed thickness but fabricated at different substrate temperatures, the hysteresis loops disappear roughly at the same temperature. This suggests that the thickness of the nonferromagnetic interfacial layer could inversely prevent the diffusion between Co and the underlying Ge substrate. As an example, Fig. 3 depicts the thermal evolution of the hysteresis loops for 28 ML Co/ Ge(1 0 0) deposited at 300 K. As the substrate temperature increases, the height of the hysteresis loop
J.S. Tsay et al. / Applied Surface Science 219 (2003) 88–92
91
Fig. 3. Thermal evolution of the magnetic hysteresis for 28 ML Co/Ge(1 0 0) on the longitudinal configuration. The hysteresis disappears at temperatures above 425 K.
Fig. 4. Temperature dependence of the remanent Kerr intensities for 28 ML Co/Ge(1 0 0) films deposited at indicated temperatures. The Kerr intensities vanish at the same temperature.
decreases monotonically. The hysteresis loop disappears at temperatures above 425 K. No hysteresis could be observed after the sample is then cooled down to low temperatures. The evolution of the hysteresis upon annealing is irreversible. Fig. 4 depicts the temperature dependence of the remanent Kerr intensities on the longitudinal configuration for 28 ML Co/ Ge(1 0 0) films deposited at 120, 200, 300, and 400 K. As the substrate temperature increases, all the Kerr intensities decrease monotonically at temperatures roughly below 375 K. At higher temperatures, Kerr intensities decrease rapidly until zero. Corresponding intensities of Co Auger signals show a drop around 400 K from our AES experiments. This indicates a strong diffusion of Co into the Ge substrate. For all the samples, the Kerr intensity vanishes at the same temperature of 425 K. XPS measurements of this system show a drop of the Co signal followed by a slow decrease around the same temperature [3]. Therefore, we believe that the temperature is high enough for Co adatoms to overcome the energy barrier of the interdiffusion into the Ge(1 0 0) substrate at 425 K. No pure cobalt island exists on the top of the surface and results in the disappearance of the ferromagnetism. As a comparison of the annealing effects on the magnetic properties of Co/Ge(1 0 0) and Co/Ge(1 1 1) films, Fig. 5 depicts the temperature dependencies of the longitudinal Kerr intensities for 28 ML-thick Co films deposited at room temperature. When the annealing temperature increases, both the Kerr intensities
decrease and drop to zero at high temperatures. For the Co/Ge(1 1 1) film, the temperature where ferromagnetism disappears occurs at 375 K, i.e. 50 K lower than in the Co/Ge(1 0 0) system. LEED measurements in Fig. 2 reveal that Co/Ge(1 0 0) exhibits a disordered structure for the interfacial layers. SMOKE measurements during the Co deposition in Fig. 1 show that the interdiffusion is stronger in Co/Ge(1 0 0) than in Co/Ge(1 1 1). Therefore we can conclude that Co/Ge(1 0 0) forms a rougher interface and the possibility of accumulation of atoms to form micro-size clusters is higher. The clustering process increases the possible cluster sizes. The formation of some larger clusters causes a higher thermal stability of this system.
Fig. 5. Temperature dependencies of the remanent Kerr intensities for 28 ML-thick Co/Ge(1 0 0) and Co/Ge(1 1 1) films.
92
J.S. Tsay et al. / Applied Surface Science 219 (2003) 88–92
In contract, Co/Ge(1 1 1) shows a smooth interface which serves a more possibility of a contact between Co and the underlying Ge(1 1 1) substrate. This should be the reason of the observation of a lower thermal stability in this system. In summary, by comparing the thickness of nonferromagnetic layers at the interfaces of Co/Ge(1 0 0) and Co/Ge(1 1 1), we found that it is thicker in Co/Ge(1 0 0) system than that in Co/Ge(1 1 1) system. This is explained by the difference of the densities of surface atoms on Ge(1 0 0) and Ge(1 1 1) surfaces. In general, ultrathin Co films with fixed total thickness (including Co and nonferromagnetic layers) but fabricated at different temperatures on the same substrate, their Kerr hysteresis loops disappear roughly at the same temperature upon annealing up to 450 K. For instance, the disappearance temperature was observed at 425 K for all the Co/Ge(1 0 0) films with a thickness of 28 ML. This suggests that the thickness of the interfacial layer could inversely prevent the diffusion between Co and the Ge substrate. From the annealing studies between room temperature and 450 K for samples with total film thickness of 28 ML, we have found that the Co/Ge(1 1 1) films possesses a lower thermal stability than that of the Co/Ge(1 0 0) films. This result could be explained by different interfacial conditions between Ge(1 0 0) and Ge(1 1 1).
References [1] H. van Kanal, Mater. Sci. Rep. 8 (1992) 193. [2] P.T. Goeller, B.I. Boyanov, D.E. Sayers, R.J. Nemanich, A.F. Myers, E.B. Steel, J. Mater. Res. 14 (1999) 4372. [3] K. Prabhakaran, T. Ogino, Appl. Surf. Sci. 100–101 (1996) 518. [4] K. Prabhakaran, K. Sumitomo, T. Ogino, Appl. Phys. Lett. 70 (1997) 607. [5] G. Rossi, Surf. Sci. Rep. 7 (1987) 1. [6] D. Leong, M. Harry, K.J. Reeson, K.P. Homewood, Nature 387 (1997) 686. [7] Y. Shibayama, H. Sato, T. Enoki, M. Endo, Phys. Rev. Lett. 84 (2000) 1744. [8] K.J. Kirk, Contemp. Phys. 41 (2000) 61. [9] W. Wulfhekel, F. Zavaliche, F. Porrati, H.P. Oepen, J. Kirschner, Europhys. Lett. 49 (2000) 651. [10] J.S. Tsay, Y.D. Yao, Appl. Phys. Lett. 74 (1999) 1311. [11] J.S. Tsay, C.S. Yang, Y.D. Yao, Y. Liou, S.F. Lee, Jpn. J. Appl. Phys. Part 1 37 (1998) 5976. [12] J.S. Tsay, Y.D. Yao, K.C. Wang, W.C. Cheng, C.S. Yang, Surf. Sci. 507–510 (2002) 498. [13] J.S. Tsay, Y.D. Yao, K.C. Wang, W.C. Cheng, C.S. Yang, J. Appl. Phys. 91 (2002) 8766. [14] J.P. Bucher, L.A. Bloomfield, Int. Mod. Phys. B 7 (1993) 1079. [15] G.A. Smith, L. Luo, S. Hashimoto, W.M. Gibson, N. Lewis, J. Vac. Sci. Technol. A 7 (1989) 1475. [16] G. Rangelov, P. Augustin, J. Stober, T. Fauster, Phys. Rev. B 49 (1994) 7535. [17] C. Kittel, Introduction to Solid State Physics, sixth ed., Wiley, Singapore, 1986.