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FeNi bilayers
Applied Surface Science 252 (2006) 8611–8614 www.elsevier.com/locate/apsusc
The effects of Co-metal clusters on exchange bias for Co-doped NiO/FeNi bilayers Yaxin Wang a, Jie Xiong a, Yongjun Zhang b, Liang Sun a, Biao You a, Jun Du a, An Hu a, Mu Lu a,* a
National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, PR China b Institute of Solid State Physics of Jilin Normal University, Siping, Jilin 136000, PR China
Received 5 September 2005; received in revised form 29 November 2005; accepted 29 November 2005 Available online 6 January 2006
Abstract Co-doped NiO inhomogeneous films were synthesized by sputtering metallic Co chips and NiO together and the exchange bias of bilayers Codoped NiO/FeNi was investigated. When Co content was up to 25.2%, the exchange bias field HE at the room temperature increased to the maximum which was about three times compared to the undoped-bilayers. With further increase of Co content, the exchange bias field HE and blocking temperature TB decreased. Analysis suggests that the configuration of nanometer-sized Co-metal clusters enchased into NiO matrix played an important role in the change of magnetic behavior for the bilayers. # 2005 Elsevier B.V. All rights reserved. Keywords: Exchange bias; Co-metal clusters; Bilayers
1. Introduction Exchange bias arises from the exchange coupling between ferromagnetic (FM) and antiferromagnetic (AFM) layers. This coupling induces a shift of the hysteresis loop of FM layer along the magnetic field axis, often associated with an increase of coercivity. The exchange bias effect was extensively studied in FM/AF bilayers in recent years because of its applications in spintronics devices [1,2]. Simple model [3] suggested that AFM materials with larger anisotropies tended to induce larger loop shifts, while AFM with smaller anisotropy brought about important coercivity enhancements, while the microscopic origin of the loop shifts and coercivity enhancement are not clear at present. In this paper, Co-doped NiO inhomogenous films with different Co contents was prepared by sputtering metallic Co chips and NiO together in Ar ambience by magnetron sputtering system, which was different from the previous fabrication methods of reactive sputtering for CoNiO AFM materials [4,5]. Microstructure measure showed that
* Corresponding author. Tel.: +86 25 83596834; fax: +86 25 83595535. E-mail address: [email protected] (M. Lu). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.11.087
Co-metal clusters could be formed in AFM NiO matrix. The effects of the enchased configurations on magnetic behavior of the bilayers were discussed. 2. Experiment ˚ thick were grown on 50 A ˚ Co-doped NiO films with 240 A Ta on Si(1 0 0) using magnetron sputtering system in Ar ambience condition. The base pressure was 2.1 105 Pa and the argon pressure was stabilized at 0.4 Pa during deposition ˚ thick was deposited on the Co-doped process. FeNi film 100 A ˚ /s NiO film by dc magnetron sputtering at a growth rate of 1.1 A ˚ in the presence of a magnetic field of 20 kA/m, and 50 A Ta was used as a capping layer. NiO source was used and metallic Co chips were placed on NiO target. The Co-doped NiO films were deposited by rf sputtering method and the growth rate was ˚ /s. The Co contents were controlled by placing different 0.33 A amount of chips on NiO target and were given by atom ratio of Co/(Co + Ni) by ICP-IES. XPS was used to obtain atomistic information on Co dispersed in NiO. The hysteresis loops were taken with a vibrating sample magnetometer (VSM) and a superconducting quantum interference device (SQUID). The surface morphology of the film was achieved by field emission scanning electron microscopy (SEM).
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˚ /FeNi 100 A ˚ with different contents of Co at the room temperature: (a) 0%, (b) 9.26%, (c) 17.8%, (d) Fig. 1. The hysteresis loops of the bilayers Co-doped NiO 240 A 25.2%, (e) 38.8%, and (f) 46.1%. (g) The dependences of HE and HC on Co concentration at the room temperature.
3. Result and discussion The hysteresis loops at the room temperature for the bilayers ˚ )/FeNi (100 A ˚ ) with different contents of Co-doped NiO (240 A of Co are shown in Fig. 1a–f and Fig. 1g shows the dependences of exchange bias field HE and coercivity HC on Co contents. As the content of Co increases, exchange bias field HE increases gradually to a maximum of 11.6 kA/m at the content of Co 25.2%, and HE is about three times large compared to the undoped film. HE decreases with further increase of Co content, and coercivity HC almost linearly increases. The hysteresis loops of the bilayers with various Co contents were measured at different temperatures. The variations of exchange bias field HE and blocking temperature TB are given in Fig. 2. With the increase of the temperature, the exchange bias field HE decreases (Fig. 2a). The blocking temperature TB decreases with the increase of Co content (Fig. 2b). The blocking temperature TB of the undoped NiO/FeNi bilayers is lower than TN of the bulk NiO. The result is consistent with the previous studies and different explanations have been given [6,7].
As mentioned above, Co-doped films were prepared by placing some Co chips on NiO target; thus, we believe that the phenomenon might be relative to the configuration of Co-doped ˚ Co-doped NiO with NiO. Single layer films of thickness 300 A Co content 17.8% and 25.2%, respectively, were prepared at the same condition for the observation of microstructure. SEM image of single layer film with 25.2% Co is shown in Fig. 3. Some small light spots are dispersed inhomogenously and the magnified image shows size is about 10 nm (inset in Fig. 3). We speculate that the light spots are possibly Co-metal clusters. Fig. 4 is the spectra of Co 2p XPS of single film with 25.2% Co-doped NiO. At the surface of the sample, the binding energy at positions 780.2 eV and 796 eV corresponds to Co 2p3/2 and 2p1/2 peaks, respectively, which is characteristic of CoO [8]. After low energy Ar ions etch the surface of the film, Co 2p peaks become narrow and move to the low positions (Fig. 4b). The peak of Co 2p3/2 occurring at binding energy 778.2 eV corresponds to metallic Co [8]. Thus, the metallic content of Co increases and that of CoO decreases. The results of XPS show that metallic Co was oxided slightly at the surface. Generally,
˚ )/FeNi (100 A ˚ ) with various Co contents. (b) The Fig. 2. (a) The dependence of exchange bias field HE on the temperature for the bilayers Co-doped NiO (240 A change of blocking temperature TB with Co content.
Y. Wang et al. / Applied Surface Science 252 (2006) 8611–8614
Fig. 3. Field-emission scanning electron microscopic image for the 25.2% Codoped NiO film, showing nanometer-sized clusters dispersed in a NiO matrix. The inset is the corresponding magnified image.
the metallic Co is not oxided during the process of sputtering because the fabrication of the sample was performed in Ar ambience [9]. When the volume percentage of CoO was very low in CoNiO, the influence of CoO on HE was not obvious [5]. Therefore, even though metallic Co was slightly oxided during sputtering, it could be not the main factor for the enhancement of HE of Co-doped NiO/FeNi bilayers. In our experiment the sample was taken out of sputtering cavity for measurement, which was the possible cause for surfacial oxidation. Two kinds of Co-doped NiO single layer film (no FeNi layer): sample A—17.8% Co-doped NiO film and sample B—25.2% Co-doped NiO film measured the magnetic property by SQUID at room temperature. The magnetic field was applied in the sample plane. The diamagnetic signals of Si substrate were deducted. The results are shown in Fig. 5. Based on the data measured by ICP-IES, taking the Co density 8.9 g/cm3 and the NiO density 6.67 g/cm3, we could deduce the volume percentage of Co in samples A and B to be 11.5% and 16.8%, respectively. Then the Co volume in sample A is 5.11 105 mm3, and 6.61 105 mm3 for sample B. We took the magnetization of Co 1.4 106 A/m at
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the room temperature [10], then the calculated saturation magnetization is 7.15 105 emu (1 emu = 103 A m2) for sample A, and 9.25 105 emu for sample B. The values are in good agreement with the data measured in Fig. 5a and b. The calculation result shows metal Co are contained in NiO. Combining with the results of SEM and XPS, we believe that nanometer size Co-metal clusters are dispersed in NiO matrix. When Co content is less than 25.2%, these Co clusters are dispersed in NiO matrix like ‘‘magnetic islands’’, and they do not coalesce, being magnetically isolated from each other and show ferromagnetic feature at the room temperature. Many small domains could form in antiferromagnetic NiO layer due to Co-metal cluster dispersed in NiO matrix when FM layer of bilayers NiO/FeNi reverses. According to the analysis of Malozemoff [11], the exchange bias is inversely proportional to the size of domain in the antiferromagnet. Therefore, The pinning of small-size domain wall enhances the exchange bias. Moreover, the enchasing microstructure increases pinning interfaces between the FM and AFM, which contributes also to the enhancement of exchange bias. The superparamagnetic behavior of some small-size Co clusters would lead to the reduction of blocking temperature TB. For our experimental setting the volume of islands occurring superparamagnetic behavior can be expressed as [12] V¼
kB lnð109 tÞ T K
We take K = 4.5 105 J/m3 [13] and t 102 s for the time scale of the measurement of TB and TB = 378 K (105 8C) (25.2% Co-doped in Fig. 2b). Thus, V is theoretically estimated 293 nm3. Therefore, we think that the reduction of TB may be the result of superparamagnetic behavior of some Co clusters with the volume smaller than 293 nm3. While Co content is larger than 25.2%, a magnetic connection among the Co clusters forms a large area Co region in NiO. And the pinning sites decrease and a few large domains form in AFM. The pinning of domain wall in AFM decreases, while the coupling between FM layer FeNi and metallic Co enhances. Therefore, exchange bias field HE and blocking temperature TB decrease steeply and coercivity HC increases.
Fig. 4. The spectra of Co 2p XPS of single film with 25.2% Co-doped NiO for (a) the surface and (b) after low energy Ar ions etching.
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Fig. 5. M–H hysteresis loops for: (a) 17.8% and (b) 25.2% Co-doped NiO film measured by SQUID at the room temperature, respectively. The insets are the corresponding magnified loops.
4. Conclusion
References
We prepared Co-doped NiO/FeNi bilayers by sputtering metallic Co chips and NiO together. The enhancement of the exchange bias HE was attributed to the information of Co-metal clusters enchased in NiO matrix. We believed that the microstructure of enchasing Co-metal clusters increased pinning interfaces between FM and AFM, and led to the formation of many small-size domains when FM layer reversed in AFM, which contributed to the enhancement of the exchange bias. The superparamagnetic behavior may be an important factor of TB reduction.
[1] T. Gredig, I.N. Krivorotov, P. Eames, E.D. Dahlberg, Appl. Phys. Lett. 81 (2002) 1270. [2] L. Thomas, B. Negulescu, J. Appl. Phys. 93 (2003) 8606. [3] W.H. Meiklejohn, C.P. Bean, Phys. Rev. 105 (1957) 904. [4] C.H. Lai, W.E. Bailey, R.L. White, T.C. Anthony, J. Appl. Phys. 81 (1997) 4990. [5] M.J. Carey, A.E. Berkowitz, Appl. Phys. Lett. 60 (1992) 3060. [6] S. Soeya, S. Nakamura, T. Iamgawa, S. Narishige, J. Appl. Phys. 77 (1995) 5838. [7] S. Soeya, H. Hoshiya, M. Fuyama, J. Appl. Phys. 80 (1996) 1006. [8] J. Haber, L. Ungier, J. Electron Spectrosc. Relat. Phenom. 12 (1977) 305. [9] H.D. Chopra, B.J. Hockey, P.J. Chen, R.D. McMichael, W.F. Egelhoff, J. Appl. Phys. 81 (1997) 4017. [10] E.P. Wohlfarth, Ferromagnetic Materials (Part 1), North-Holland, New York, 1980 (Chapter 1). [11] A.P. Malozemoff, J. Appl. Phys. 63 (1988) 3874. [12] P.J. van der Zaag, Y. Ijiri, J.A. Borchers, L.F. Feiner, R.M. Wolf, J.M. Gaines, R.W. Erwin, M.A. Verheijen, Phys. Rev. Lett. 84 (2000) 6102. [13] D. Jiles, Introduction to Magnetism, Chapman and Hall, London, UK, 1990, p. 134.
Acknowledgments This work is supported partly by the State Key Project of Fundamental Research under Grant No. 001CB 610602, the National Natural Science Foundation of China Grant No. 10174032, the Natural Science Foundation of Jiangsu Province No. BK2001203, and the Chinese Ministry of Education.
The effects of Co-metal clusters on exchange bias for Co-doped NiO/FeNi bilayers Yaxin Wang a, Jie Xiong a, Yongjun Zhang b, Liang Sun a, Biao You a, Jun Du a, An Hu a, Mu Lu a,* a
National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, PR China b Institute of Solid State Physics of Jilin Normal University, Siping, Jilin 136000, PR China
Received 5 September 2005; received in revised form 29 November 2005; accepted 29 November 2005 Available online 6 January 2006
Abstract Co-doped NiO inhomogeneous films were synthesized by sputtering metallic Co chips and NiO together and the exchange bias of bilayers Codoped NiO/FeNi was investigated. When Co content was up to 25.2%, the exchange bias field HE at the room temperature increased to the maximum which was about three times compared to the undoped-bilayers. With further increase of Co content, the exchange bias field HE and blocking temperature TB decreased. Analysis suggests that the configuration of nanometer-sized Co-metal clusters enchased into NiO matrix played an important role in the change of magnetic behavior for the bilayers. # 2005 Elsevier B.V. All rights reserved. Keywords: Exchange bias; Co-metal clusters; Bilayers
1. Introduction Exchange bias arises from the exchange coupling between ferromagnetic (FM) and antiferromagnetic (AFM) layers. This coupling induces a shift of the hysteresis loop of FM layer along the magnetic field axis, often associated with an increase of coercivity. The exchange bias effect was extensively studied in FM/AF bilayers in recent years because of its applications in spintronics devices [1,2]. Simple model [3] suggested that AFM materials with larger anisotropies tended to induce larger loop shifts, while AFM with smaller anisotropy brought about important coercivity enhancements, while the microscopic origin of the loop shifts and coercivity enhancement are not clear at present. In this paper, Co-doped NiO inhomogenous films with different Co contents was prepared by sputtering metallic Co chips and NiO together in Ar ambience by magnetron sputtering system, which was different from the previous fabrication methods of reactive sputtering for CoNiO AFM materials [4,5]. Microstructure measure showed that
* Corresponding author. Tel.: +86 25 83596834; fax: +86 25 83595535. E-mail address: [email protected] (M. Lu). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.11.087
Co-metal clusters could be formed in AFM NiO matrix. The effects of the enchased configurations on magnetic behavior of the bilayers were discussed. 2. Experiment ˚ thick were grown on 50 A ˚ Co-doped NiO films with 240 A Ta on Si(1 0 0) using magnetron sputtering system in Ar ambience condition. The base pressure was 2.1 105 Pa and the argon pressure was stabilized at 0.4 Pa during deposition ˚ thick was deposited on the Co-doped process. FeNi film 100 A ˚ /s NiO film by dc magnetron sputtering at a growth rate of 1.1 A ˚ in the presence of a magnetic field of 20 kA/m, and 50 A Ta was used as a capping layer. NiO source was used and metallic Co chips were placed on NiO target. The Co-doped NiO films were deposited by rf sputtering method and the growth rate was ˚ /s. The Co contents were controlled by placing different 0.33 A amount of chips on NiO target and were given by atom ratio of Co/(Co + Ni) by ICP-IES. XPS was used to obtain atomistic information on Co dispersed in NiO. The hysteresis loops were taken with a vibrating sample magnetometer (VSM) and a superconducting quantum interference device (SQUID). The surface morphology of the film was achieved by field emission scanning electron microscopy (SEM).
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˚ /FeNi 100 A ˚ with different contents of Co at the room temperature: (a) 0%, (b) 9.26%, (c) 17.8%, (d) Fig. 1. The hysteresis loops of the bilayers Co-doped NiO 240 A 25.2%, (e) 38.8%, and (f) 46.1%. (g) The dependences of HE and HC on Co concentration at the room temperature.
3. Result and discussion The hysteresis loops at the room temperature for the bilayers ˚ )/FeNi (100 A ˚ ) with different contents of Co-doped NiO (240 A of Co are shown in Fig. 1a–f and Fig. 1g shows the dependences of exchange bias field HE and coercivity HC on Co contents. As the content of Co increases, exchange bias field HE increases gradually to a maximum of 11.6 kA/m at the content of Co 25.2%, and HE is about three times large compared to the undoped film. HE decreases with further increase of Co content, and coercivity HC almost linearly increases. The hysteresis loops of the bilayers with various Co contents were measured at different temperatures. The variations of exchange bias field HE and blocking temperature TB are given in Fig. 2. With the increase of the temperature, the exchange bias field HE decreases (Fig. 2a). The blocking temperature TB decreases with the increase of Co content (Fig. 2b). The blocking temperature TB of the undoped NiO/FeNi bilayers is lower than TN of the bulk NiO. The result is consistent with the previous studies and different explanations have been given [6,7].
As mentioned above, Co-doped films were prepared by placing some Co chips on NiO target; thus, we believe that the phenomenon might be relative to the configuration of Co-doped ˚ Co-doped NiO with NiO. Single layer films of thickness 300 A Co content 17.8% and 25.2%, respectively, were prepared at the same condition for the observation of microstructure. SEM image of single layer film with 25.2% Co is shown in Fig. 3. Some small light spots are dispersed inhomogenously and the magnified image shows size is about 10 nm (inset in Fig. 3). We speculate that the light spots are possibly Co-metal clusters. Fig. 4 is the spectra of Co 2p XPS of single film with 25.2% Co-doped NiO. At the surface of the sample, the binding energy at positions 780.2 eV and 796 eV corresponds to Co 2p3/2 and 2p1/2 peaks, respectively, which is characteristic of CoO [8]. After low energy Ar ions etch the surface of the film, Co 2p peaks become narrow and move to the low positions (Fig. 4b). The peak of Co 2p3/2 occurring at binding energy 778.2 eV corresponds to metallic Co [8]. Thus, the metallic content of Co increases and that of CoO decreases. The results of XPS show that metallic Co was oxided slightly at the surface. Generally,
˚ )/FeNi (100 A ˚ ) with various Co contents. (b) The Fig. 2. (a) The dependence of exchange bias field HE on the temperature for the bilayers Co-doped NiO (240 A change of blocking temperature TB with Co content.
Y. Wang et al. / Applied Surface Science 252 (2006) 8611–8614
Fig. 3. Field-emission scanning electron microscopic image for the 25.2% Codoped NiO film, showing nanometer-sized clusters dispersed in a NiO matrix. The inset is the corresponding magnified image.
the metallic Co is not oxided during the process of sputtering because the fabrication of the sample was performed in Ar ambience [9]. When the volume percentage of CoO was very low in CoNiO, the influence of CoO on HE was not obvious [5]. Therefore, even though metallic Co was slightly oxided during sputtering, it could be not the main factor for the enhancement of HE of Co-doped NiO/FeNi bilayers. In our experiment the sample was taken out of sputtering cavity for measurement, which was the possible cause for surfacial oxidation. Two kinds of Co-doped NiO single layer film (no FeNi layer): sample A—17.8% Co-doped NiO film and sample B—25.2% Co-doped NiO film measured the magnetic property by SQUID at room temperature. The magnetic field was applied in the sample plane. The diamagnetic signals of Si substrate were deducted. The results are shown in Fig. 5. Based on the data measured by ICP-IES, taking the Co density 8.9 g/cm3 and the NiO density 6.67 g/cm3, we could deduce the volume percentage of Co in samples A and B to be 11.5% and 16.8%, respectively. Then the Co volume in sample A is 5.11 105 mm3, and 6.61 105 mm3 for sample B. We took the magnetization of Co 1.4 106 A/m at
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the room temperature [10], then the calculated saturation magnetization is 7.15 105 emu (1 emu = 103 A m2) for sample A, and 9.25 105 emu for sample B. The values are in good agreement with the data measured in Fig. 5a and b. The calculation result shows metal Co are contained in NiO. Combining with the results of SEM and XPS, we believe that nanometer size Co-metal clusters are dispersed in NiO matrix. When Co content is less than 25.2%, these Co clusters are dispersed in NiO matrix like ‘‘magnetic islands’’, and they do not coalesce, being magnetically isolated from each other and show ferromagnetic feature at the room temperature. Many small domains could form in antiferromagnetic NiO layer due to Co-metal cluster dispersed in NiO matrix when FM layer of bilayers NiO/FeNi reverses. According to the analysis of Malozemoff [11], the exchange bias is inversely proportional to the size of domain in the antiferromagnet. Therefore, The pinning of small-size domain wall enhances the exchange bias. Moreover, the enchasing microstructure increases pinning interfaces between the FM and AFM, which contributes also to the enhancement of exchange bias. The superparamagnetic behavior of some small-size Co clusters would lead to the reduction of blocking temperature TB. For our experimental setting the volume of islands occurring superparamagnetic behavior can be expressed as [12] V¼
kB lnð109 tÞ T K
We take K = 4.5 105 J/m3 [13] and t 102 s for the time scale of the measurement of TB and TB = 378 K (105 8C) (25.2% Co-doped in Fig. 2b). Thus, V is theoretically estimated 293 nm3. Therefore, we think that the reduction of TB may be the result of superparamagnetic behavior of some Co clusters with the volume smaller than 293 nm3. While Co content is larger than 25.2%, a magnetic connection among the Co clusters forms a large area Co region in NiO. And the pinning sites decrease and a few large domains form in AFM. The pinning of domain wall in AFM decreases, while the coupling between FM layer FeNi and metallic Co enhances. Therefore, exchange bias field HE and blocking temperature TB decrease steeply and coercivity HC increases.
Fig. 4. The spectra of Co 2p XPS of single film with 25.2% Co-doped NiO for (a) the surface and (b) after low energy Ar ions etching.
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Fig. 5. M–H hysteresis loops for: (a) 17.8% and (b) 25.2% Co-doped NiO film measured by SQUID at the room temperature, respectively. The insets are the corresponding magnified loops.
4. Conclusion
References
We prepared Co-doped NiO/FeNi bilayers by sputtering metallic Co chips and NiO together. The enhancement of the exchange bias HE was attributed to the information of Co-metal clusters enchased in NiO matrix. We believed that the microstructure of enchasing Co-metal clusters increased pinning interfaces between FM and AFM, and led to the formation of many small-size domains when FM layer reversed in AFM, which contributed to the enhancement of the exchange bias. The superparamagnetic behavior may be an important factor of TB reduction.
[1] T. Gredig, I.N. Krivorotov, P. Eames, E.D. Dahlberg, Appl. Phys. Lett. 81 (2002) 1270. [2] L. Thomas, B. Negulescu, J. Appl. Phys. 93 (2003) 8606. [3] W.H. Meiklejohn, C.P. Bean, Phys. Rev. 105 (1957) 904. [4] C.H. Lai, W.E. Bailey, R.L. White, T.C. Anthony, J. Appl. Phys. 81 (1997) 4990. [5] M.J. Carey, A.E. Berkowitz, Appl. Phys. Lett. 60 (1992) 3060. [6] S. Soeya, S. Nakamura, T. Iamgawa, S. Narishige, J. Appl. Phys. 77 (1995) 5838. [7] S. Soeya, H. Hoshiya, M. Fuyama, J. Appl. Phys. 80 (1996) 1006. [8] J. Haber, L. Ungier, J. Electron Spectrosc. Relat. Phenom. 12 (1977) 305. [9] H.D. Chopra, B.J. Hockey, P.J. Chen, R.D. McMichael, W.F. Egelhoff, J. Appl. Phys. 81 (1997) 4017. [10] E.P. Wohlfarth, Ferromagnetic Materials (Part 1), North-Holland, New York, 1980 (Chapter 1). [11] A.P. Malozemoff, J. Appl. Phys. 63 (1988) 3874. [12] P.J. van der Zaag, Y. Ijiri, J.A. Borchers, L.F. Feiner, R.M. Wolf, J.M. Gaines, R.W. Erwin, M.A. Verheijen, Phys. Rev. Lett. 84 (2000) 6102. [13] D. Jiles, Introduction to Magnetism, Chapman and Hall, London, UK, 1990, p. 134.
Acknowledgments This work is supported partly by the State Key Project of Fundamental Research under Grant No. 001CB 610602, the National Natural Science Foundation of China Grant No. 10174032, the Natural Science Foundation of Jiangsu Province No. BK2001203, and the Chinese Ministry of Education.