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X-ray absorption near edge structure studies of Fe1−xNixOy thin films
Journal of Electron Spectroscopy and Related Phenomena 114–116 (2001) 545–548 www.elsevier.nl / locate / elspec
X-ray absorption near edge structure studies of Fe 12x Ni x O y thin films a, a a b c c C.L. Chang *, C.L. Chen , C.L. Dong , G. Chern , J.F. Lee , L.Y. Jang b
a Department of Physics, Tamkank University, Tamsui 251, Taiwan Department of Physics, Chung-Cheng University, Chiayi 621, Taiwan c Synchrotron Radiation Research Center, Hsin-Chu 300, Taiwan
Received 8 August 2000; received in revised form 22 September 2000; accepted 3 October 2000
Abstract We have performed Fe and Ni K-edge X-ray absorption near edge structure (XANES) measurement of a series of Fe 12x Ni x O y (0,x,1) spinel thin films grown by oxygen plasma assisted molecular beam epitaxy (MBE). The transition metal K-edge XANES provides information of the valence states of the absorbing atoms. We observe that the variation of Fe valence with the x value is consistent with the structural properties of this series of samples. The cation distribution in these films is dramatically different from the bulk samples. For x#0.5 Ni replaces Fe 31 and Fe 21 from the tetrahedral and the octahedral sites, respectively. For x.0.5, Ni replaces Fe 31 from tetrahedral and octahedral sites up to x50.5; for x.0.5, Ni ions only replace the Fe 31 ions of tetrahedral and octahedral symmetry up to x|0.7. Since the Ni valence is unaffected, a ligand hole must be created in order to balance the charges. 2001 Elsevier Science B.V. All rights reserved. Keywords: Ferrite; Electronic structure; XANES.
1. Introduction Various metal oxide systems including high temperature superconductor cuprates, colossal magnetoresistance (CMR) manganites, and other ferroelectric and ferromagnetic compounds have attracted enormous attentions in recent years. Especially interesting is the thin film of these materials due to their potential for device application and fundamental physical interests [1–4]. One important feature of these materials is that a mixed valence electronic state is formed during the *Corresponding author. Tel.: 1886-2-2620-9076; fax: 1886-22620-9917. E-mail address: [email protected] (C.L. Chang).
impurity doping process and this mixed valance state apparently plays an important role on the electromagnetic properties in these compounds. Relatively, some transition metal oxides such as Fe 3 O 4 , NiO and CoO have much simpler atomic arrangement but rich electrical and magnetic properties. For instance, iron oxide (Fe 3 O 4 ) itself has Fe 21 and Fe 31 ions and exhibits a metal-insulator transition at about 120 K (Verway transition) [5]. NiO and CoO are Mott insulators, which have antiferromagntic order and unexpectedly high resistivity [6]. By mixing these simple transition metal oxides, thin films of new compounds can be synthesized in a much more flexible and controllable way. A detailed study of the structure and valance states of transition metal oxides, such as Fe 12x Ni x O y (0,x,1), can be
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 00 )00298-X
546
C.L. Chang et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 545 – 548
done. This idea has been successfully applied on a series of oxide superlattices [7]. For instance, Fe 3 O 4 and NiO have been fabricated as multilayers with precise control on the modulated wavelength [8,9]. The observation of interesting magnetic and electrical properties in these Fe–Ni–O films were reported [10,11]. In a previous letter [12], we reported the study of cation distribution of a series of single-crystalline iron–nickel oxide alloy thin films (Fe 12x Ni x O y ) with various Fe–Ni concentration ratios. Our results are different from a bulk phase prediction for ferrite Fe 2 NiO 4 . It is usually believed, in that case, that all 21 21 Ni should replace Fe in the octahedral cites [13]. In order to elucidate the issue of charge distribution between Fe, Ni and O in this oxide system, which was not addressed in the previous studies [12], we have performed XANES measurements at Fe and Ni K-edges. It has been demonstrated that the transition metal K-edge XANES measurement provides information of the valence state by monitoring the energy of absorption edge relative to the standard oxides [14].
and seven Fe 12x Ni x O y (x50.15, 0.31, 0.35, 0.5, 0.56, 0.65, 0.71) films. The thickness of all oxide films is ˚ The base pressure of the deposition around 500 A. chamber was around 1310 28 Torr and the oxygen partial pressure was 5310 25 Torr during the growth. The substrate temperature was 2508C. A 20 keV RHEED (reflection high energy electron diffraction), with incident angle of |18, was used to monitor the growth. The chemical composition of Fe and Ni is then confirmed with an ex situ energy-dispersive-Xray analysis. X-ray adsorption spectroscopy measurements were performed at the wiggler beamline (17C) of Synchrotron Radiation Research Center (SRRC) in Taiwan. A double-crystal Si (111) monochromator with energy resolution of about 1 / 7000 was used. All the spectral measurements were taken by fluorescent yield mode at room temperature. Fe and Ni foil were used for energy calibrations.
3. Results and discussion Fig. 1 shows the Fe K-edge XANES spectra for samples of different x values along with Fe 3 O 4 film. The main peaks between 7130 and 7135 eV is associated to the transition from Fe 1s to empty 4p states. The pre-edge feature near 7125 eV is associ-
2. Experimental Fe 3 O 4 , NiO and Fe 12x Ni x O y films are synthesized by a oxygen-plasma-assisted molecular beam epitaxy (MBE) which is similar to the previous study on the growth of metal oxide thin films and superlattices [15]. Relative to the pure oxide film, Fe and Ni are co-deposited in a plasma oxygen environment and assuming that all the metals are oxidized. Briefly, two high-temperature K-cells, containing high purity Fe and Ni, provide metallic vapor in an oxygen ˚/ plasma environment. The deposition rate, 20–40 A min, can be precisely controlled by the source temperature (|15508C) of the cells. A crystal thickness monitor originally characterizes the flux rates. Sufficient oxygen partial pressure is always provided but without intention on controlling oxygen during the growth. Mechanical shutters are used to block the flux so that a pure oxide film or an alloy oxide film with desired Fe–Ni ratio can be made. A total of nine films are made including a pure Fe 3 O 4 and NiO
Fig. 1. Fe K-edge XANES spectra for a series of Fe 12x Ni x O y samples along with Fe 3 O 4 film.
C.L. Chang et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 545 – 548
ated with the transition from Fe 1s to empty 3d states. The spectral shapes of all the curves are basically unaffected by x. However, the width of the main peak becomes sharper as x increases. The positions of the main peak, as indicated by the vertical arrows, shift systematically to the higher binding energy side as x increases from 0.15 to 0.50. No further shifts in peak position is observed for x higher than 0.5. There is no obvious variation observed in the pre-edge feature for different x value since the intensities are much weaker than that of the main peak. Fig. 2 plots the Ni K-edge spectra of these samples along with NiO film. No energy shift in the main absorption peaks is observed. The small difference of the peak position of NiO relative to the other spectra is due to the different (rock salt) structure. The spectral shape evolves slightly with Ni concentration due to the different symmetry of the sites which Ni occupies [12]. From the Fe K-edge spectra, it is clear that the average valence of Fe increases with the Ni concentration. At x50.5 the average Fe valence reaches a maximum, and there is no further increase in Fe valence for x.0.5. This result is consistent with the structural property of these films as reported in Ref. [12]. For x#0.5, Ni replaces Fe 21 and Fe 31 with a ratio of 2:1. Thus the ratio of Fe 21 to Fe 31 decreases and the average Fe valence increases. All the di-
547
valent Fe ions were replaced by Ni at x50.5. Therefore, for x.0.5, the average Fe valence is saturated to 31. At this point Ni can only replace Fe 31 from both the tetrahedral and octahedral sites. The change of the Fe valence from mixed Fe 21 / Fe 31 to single valent Fe 31 , with increasing x, causes the width of the main peak in Fig. 1 to become narrower. The unaffected Ni K-edge spectra for different Ni concentration indicates that the Ni valence remains constant and is closed to 21 as compared to the divalent Ni standard in NiO. Since the total valence of the cations increases with x, it is necessary for the O valence to increase in order to keep the system charge balanced. Oxygen holes must be created as Ni replaces Fe in these systems. This is consistent with the results of O K-edge measurement reported in Ref. [16].
4. Conclusion We have performed the Fe and Ni K-edge XANES measurements for a series of Ni ferrites thin films with different Ni concentrations. The average valence of Fe is increased with the Ni concentration, while the Ni valence in not affected by the composition variations. This change in Fe valence is balanced by the creation of oxygen holes.
Acknowledgements This work was supported by the National Science Council of the Republic of China through grant number NSC 89-2112-M-032-006.
References
Fig. 2. Ni K-edge XANES spectra of a series of Fe 12x Ni x O y samples along with NiO film.
[1] J. Kwo, M. Hong, D.J. Trevvor, R.M. Fleming, A.E. White, R.C. Farrow, A.R. Kortan, K.T. Short, Appl. Phys. Lett. 53 (1988) 2683. [2] B.H. Moeckly, S.E. Rusek, D.K. Lathrop, R.A. Buhrman, J. Li, J.W. Mayer, Appl. Phys. Lett. 57 (1990) 1687. [3] M. Kanai, T. Kawai, S. Kawai, Appl. Phys. Lett. 57 (1990) 198. [4] R. von Helmholt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [5] E.J.W. Verwey, P.W. Haayman, Physica 8 (1941) 979.
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C.L. Chang et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 545 – 548
[6] D. Adler, Solid State Phys. 21 (1968) 1. [7] G. Chern, S.D. Berry, H. Mathias, L.R. Testardi, Phys. Rev. Lett. 68 (1992) 114. [8] Y. Bando, S. Horii, T. Takada, Jpn. J. Appl. Phys. 17 (1978) 1037. [9] D.M. Lind, S.D. Berry, G. Chern, H. Mathias, L.R. Testardi, Phys. Rev. B 45 (1992) 1838. [10] J.A. Borchers, R.W. Erwin, S.D. Berry, D.M. Lind, J.F. Ankner, E. Lochner, K.A. Show, D. Hilton, Phys. Rev. B 51 (1995) 8276. [11] J.B. Smathers, L.R. Testardi, Phys. Rev. Lett. 77 (1996) 1147.
[12] C.L. Chang, G. Chern, C.L. Chen, H.H. Hsieh, C.L. Dong, W.F. Pong, C.H. Chao, H.C. Chien, S.L. Chang, Solid State Cummun. 109 (1999) 599. [13] S. Chikazumi, S.H. Charap, Physics of Magnetism, John Wiley and Sons, New York, 1964, Chapter 5. [14] M. Croft, D. Sills, M. Greenblatt, C. Lee, S.-W. Cheong, K.V. Ramanujachary, D. Tran, Phys. Rev. B 55 (1997) 8726. [15] G. Chern, Y.R. Chean, Jpn. J. Appl. Phys. 36 (1997) 2813. [16] G. Chern, C.L. Chang, C.L. Chen, C.L. Dong, J. Vac. Sci. Technol. A 17 (1999) 1630.
X-ray absorption near edge structure studies of Fe 12x Ni x O y thin films a, a a b c c C.L. Chang *, C.L. Chen , C.L. Dong , G. Chern , J.F. Lee , L.Y. Jang b
a Department of Physics, Tamkank University, Tamsui 251, Taiwan Department of Physics, Chung-Cheng University, Chiayi 621, Taiwan c Synchrotron Radiation Research Center, Hsin-Chu 300, Taiwan
Received 8 August 2000; received in revised form 22 September 2000; accepted 3 October 2000
Abstract We have performed Fe and Ni K-edge X-ray absorption near edge structure (XANES) measurement of a series of Fe 12x Ni x O y (0,x,1) spinel thin films grown by oxygen plasma assisted molecular beam epitaxy (MBE). The transition metal K-edge XANES provides information of the valence states of the absorbing atoms. We observe that the variation of Fe valence with the x value is consistent with the structural properties of this series of samples. The cation distribution in these films is dramatically different from the bulk samples. For x#0.5 Ni replaces Fe 31 and Fe 21 from the tetrahedral and the octahedral sites, respectively. For x.0.5, Ni replaces Fe 31 from tetrahedral and octahedral sites up to x50.5; for x.0.5, Ni ions only replace the Fe 31 ions of tetrahedral and octahedral symmetry up to x|0.7. Since the Ni valence is unaffected, a ligand hole must be created in order to balance the charges. 2001 Elsevier Science B.V. All rights reserved. Keywords: Ferrite; Electronic structure; XANES.
1. Introduction Various metal oxide systems including high temperature superconductor cuprates, colossal magnetoresistance (CMR) manganites, and other ferroelectric and ferromagnetic compounds have attracted enormous attentions in recent years. Especially interesting is the thin film of these materials due to their potential for device application and fundamental physical interests [1–4]. One important feature of these materials is that a mixed valence electronic state is formed during the *Corresponding author. Tel.: 1886-2-2620-9076; fax: 1886-22620-9917. E-mail address: [email protected] (C.L. Chang).
impurity doping process and this mixed valance state apparently plays an important role on the electromagnetic properties in these compounds. Relatively, some transition metal oxides such as Fe 3 O 4 , NiO and CoO have much simpler atomic arrangement but rich electrical and magnetic properties. For instance, iron oxide (Fe 3 O 4 ) itself has Fe 21 and Fe 31 ions and exhibits a metal-insulator transition at about 120 K (Verway transition) [5]. NiO and CoO are Mott insulators, which have antiferromagntic order and unexpectedly high resistivity [6]. By mixing these simple transition metal oxides, thin films of new compounds can be synthesized in a much more flexible and controllable way. A detailed study of the structure and valance states of transition metal oxides, such as Fe 12x Ni x O y (0,x,1), can be
0368-2048 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0368-2048( 00 )00298-X
546
C.L. Chang et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 545 – 548
done. This idea has been successfully applied on a series of oxide superlattices [7]. For instance, Fe 3 O 4 and NiO have been fabricated as multilayers with precise control on the modulated wavelength [8,9]. The observation of interesting magnetic and electrical properties in these Fe–Ni–O films were reported [10,11]. In a previous letter [12], we reported the study of cation distribution of a series of single-crystalline iron–nickel oxide alloy thin films (Fe 12x Ni x O y ) with various Fe–Ni concentration ratios. Our results are different from a bulk phase prediction for ferrite Fe 2 NiO 4 . It is usually believed, in that case, that all 21 21 Ni should replace Fe in the octahedral cites [13]. In order to elucidate the issue of charge distribution between Fe, Ni and O in this oxide system, which was not addressed in the previous studies [12], we have performed XANES measurements at Fe and Ni K-edges. It has been demonstrated that the transition metal K-edge XANES measurement provides information of the valence state by monitoring the energy of absorption edge relative to the standard oxides [14].
and seven Fe 12x Ni x O y (x50.15, 0.31, 0.35, 0.5, 0.56, 0.65, 0.71) films. The thickness of all oxide films is ˚ The base pressure of the deposition around 500 A. chamber was around 1310 28 Torr and the oxygen partial pressure was 5310 25 Torr during the growth. The substrate temperature was 2508C. A 20 keV RHEED (reflection high energy electron diffraction), with incident angle of |18, was used to monitor the growth. The chemical composition of Fe and Ni is then confirmed with an ex situ energy-dispersive-Xray analysis. X-ray adsorption spectroscopy measurements were performed at the wiggler beamline (17C) of Synchrotron Radiation Research Center (SRRC) in Taiwan. A double-crystal Si (111) monochromator with energy resolution of about 1 / 7000 was used. All the spectral measurements were taken by fluorescent yield mode at room temperature. Fe and Ni foil were used for energy calibrations.
3. Results and discussion Fig. 1 shows the Fe K-edge XANES spectra for samples of different x values along with Fe 3 O 4 film. The main peaks between 7130 and 7135 eV is associated to the transition from Fe 1s to empty 4p states. The pre-edge feature near 7125 eV is associ-
2. Experimental Fe 3 O 4 , NiO and Fe 12x Ni x O y films are synthesized by a oxygen-plasma-assisted molecular beam epitaxy (MBE) which is similar to the previous study on the growth of metal oxide thin films and superlattices [15]. Relative to the pure oxide film, Fe and Ni are co-deposited in a plasma oxygen environment and assuming that all the metals are oxidized. Briefly, two high-temperature K-cells, containing high purity Fe and Ni, provide metallic vapor in an oxygen ˚/ plasma environment. The deposition rate, 20–40 A min, can be precisely controlled by the source temperature (|15508C) of the cells. A crystal thickness monitor originally characterizes the flux rates. Sufficient oxygen partial pressure is always provided but without intention on controlling oxygen during the growth. Mechanical shutters are used to block the flux so that a pure oxide film or an alloy oxide film with desired Fe–Ni ratio can be made. A total of nine films are made including a pure Fe 3 O 4 and NiO
Fig. 1. Fe K-edge XANES spectra for a series of Fe 12x Ni x O y samples along with Fe 3 O 4 film.
C.L. Chang et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 545 – 548
ated with the transition from Fe 1s to empty 3d states. The spectral shapes of all the curves are basically unaffected by x. However, the width of the main peak becomes sharper as x increases. The positions of the main peak, as indicated by the vertical arrows, shift systematically to the higher binding energy side as x increases from 0.15 to 0.50. No further shifts in peak position is observed for x higher than 0.5. There is no obvious variation observed in the pre-edge feature for different x value since the intensities are much weaker than that of the main peak. Fig. 2 plots the Ni K-edge spectra of these samples along with NiO film. No energy shift in the main absorption peaks is observed. The small difference of the peak position of NiO relative to the other spectra is due to the different (rock salt) structure. The spectral shape evolves slightly with Ni concentration due to the different symmetry of the sites which Ni occupies [12]. From the Fe K-edge spectra, it is clear that the average valence of Fe increases with the Ni concentration. At x50.5 the average Fe valence reaches a maximum, and there is no further increase in Fe valence for x.0.5. This result is consistent with the structural property of these films as reported in Ref. [12]. For x#0.5, Ni replaces Fe 21 and Fe 31 with a ratio of 2:1. Thus the ratio of Fe 21 to Fe 31 decreases and the average Fe valence increases. All the di-
547
valent Fe ions were replaced by Ni at x50.5. Therefore, for x.0.5, the average Fe valence is saturated to 31. At this point Ni can only replace Fe 31 from both the tetrahedral and octahedral sites. The change of the Fe valence from mixed Fe 21 / Fe 31 to single valent Fe 31 , with increasing x, causes the width of the main peak in Fig. 1 to become narrower. The unaffected Ni K-edge spectra for different Ni concentration indicates that the Ni valence remains constant and is closed to 21 as compared to the divalent Ni standard in NiO. Since the total valence of the cations increases with x, it is necessary for the O valence to increase in order to keep the system charge balanced. Oxygen holes must be created as Ni replaces Fe in these systems. This is consistent with the results of O K-edge measurement reported in Ref. [16].
4. Conclusion We have performed the Fe and Ni K-edge XANES measurements for a series of Ni ferrites thin films with different Ni concentrations. The average valence of Fe is increased with the Ni concentration, while the Ni valence in not affected by the composition variations. This change in Fe valence is balanced by the creation of oxygen holes.
Acknowledgements This work was supported by the National Science Council of the Republic of China through grant number NSC 89-2112-M-032-006.
References
Fig. 2. Ni K-edge XANES spectra of a series of Fe 12x Ni x O y samples along with NiO film.
[1] J. Kwo, M. Hong, D.J. Trevvor, R.M. Fleming, A.E. White, R.C. Farrow, A.R. Kortan, K.T. Short, Appl. Phys. Lett. 53 (1988) 2683. [2] B.H. Moeckly, S.E. Rusek, D.K. Lathrop, R.A. Buhrman, J. Li, J.W. Mayer, Appl. Phys. Lett. 57 (1990) 1687. [3] M. Kanai, T. Kawai, S. Kawai, Appl. Phys. Lett. 57 (1990) 198. [4] R. von Helmholt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71 (1993) 2331. [5] E.J.W. Verwey, P.W. Haayman, Physica 8 (1941) 979.
548
C.L. Chang et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 545 – 548
[6] D. Adler, Solid State Phys. 21 (1968) 1. [7] G. Chern, S.D. Berry, H. Mathias, L.R. Testardi, Phys. Rev. Lett. 68 (1992) 114. [8] Y. Bando, S. Horii, T. Takada, Jpn. J. Appl. Phys. 17 (1978) 1037. [9] D.M. Lind, S.D. Berry, G. Chern, H. Mathias, L.R. Testardi, Phys. Rev. B 45 (1992) 1838. [10] J.A. Borchers, R.W. Erwin, S.D. Berry, D.M. Lind, J.F. Ankner, E. Lochner, K.A. Show, D. Hilton, Phys. Rev. B 51 (1995) 8276. [11] J.B. Smathers, L.R. Testardi, Phys. Rev. Lett. 77 (1996) 1147.
[12] C.L. Chang, G. Chern, C.L. Chen, H.H. Hsieh, C.L. Dong, W.F. Pong, C.H. Chao, H.C. Chien, S.L. Chang, Solid State Cummun. 109 (1999) 599. [13] S. Chikazumi, S.H. Charap, Physics of Magnetism, John Wiley and Sons, New York, 1964, Chapter 5. [14] M. Croft, D. Sills, M. Greenblatt, C. Lee, S.-W. Cheong, K.V. Ramanujachary, D. Tran, Phys. Rev. B 55 (1997) 8726. [15] G. Chern, Y.R. Chean, Jpn. J. Appl. Phys. 36 (1997) 2813. [16] G. Chern, C.L. Chang, C.L. Chen, C.L. Dong, J. Vac. Sci. Technol. A 17 (1999) 1630.