X-ray absorption near edge structure studies of Fe1-xNixOy thin films

Article abstract of DOI:10.1016/S0368-2048(00)00298-X

We have performed Fe and Ni K-edge X-ray absorption near edge structure (XANES) measurement of a series of Fe_1-xNi_xO_y (03+ and Fe²⁺ from the tetrahedral and the octahedral sites, respectively. For x>0.5, Ni replaces Fe³⁺ from tetrahedral and octahedral sites up to x = 0.5; for x>0.5, Ni ions only replace the Fe³⁺ ions of tetrahedral and octahedral symmetry up to x approximately 0.7. Since the Ni valence is unaffected, a ligand hole must be created in order to balance the charges.

Full text of DOI:10.1016/S0368-2048(00)00298-X

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 Fe12xNixOy thin
films
a
a
b
c
c
C.L. Changa , , C.L. Chen , C.L. Dong , G. Chern , J.F. Lee , L.Y. Jang
*
aDepartment of Physics, Tamkank University, Tamsui 251, Taiwan
bDepartment of Physics, Chung-Cheng University, Chiayi 621, Taiwan
cSynchrotron 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
Fe12xNixOy (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 Fe31 and Fe21 from the tetrahedral and the
octahedral sites, respectively. For x.0.5, Ni replaces Fe31 from tetrahedral and octahedral sites up to x50.5; for x.0.5, Ni
ions only replace the Fe31 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
impurity doping process and this mixed valance state
apparently plays an important role on the electro-
magnetic properties in these compounds. Relatively,
some transition metal oxides such as Fe3O4, NiO and
CoO have much simpler atomic arrangement but rich
electrical and magnetic properties. For instance, iron
oxide (Fe3O4) itself has Fe21 and Fe31 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 Fe12xNixOy (0,x,1), can be
Various metal oxide systems including high tem-
perature superconductor cuprates, colossal magneto-
resistance (CMR) manganites, and other ferroelectric
and ferromagnetic compounds have attracted enorm-
ous 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: 1886-2-
2620-9917.
E-mail address: clchang@mail.tku.edu.tw (C.L. Chang).
0368-2048/01/$ – see front matter
PII: S0368-2048(00)00298-X
2001 Elsevier Science B.V. All rights reserved.

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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, Fe3O4
and NiO have been fabricated as multilayers with
precise control on the modulated wavelength [8,9].
The observation of interesting magnetic and electri-
cal 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 (Fe12xNixOy) with
various Fe–Ni concentration ratios. Our results are
different from a bulk phase prediction for ferrite
Fe2NiO4. It is usually believed, in that case, that all
Ni21 should replace Fe21 in the octahedral cites
[13].
In order to elucidate the issue of charge dis-
tribution 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 pro-
vides information of the valence state by monitoring
the energy of absorption edge relative to the standard
oxides [14].
and seven Fe12xNixOy (x50.15, 0.31, 0.35, 0.5, 0.56,
0.65, 0.71) films. The thickness of all oxide films is
˚
around 500 A. The base pressure of the deposition
chamber was around 131028 Torr and the oxygen
partial pressure was 531025 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-X-
ray analysis. X-ray adsorption spectroscopy measure-
ments 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 fluores-
cent 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 Fe3O4 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
Fe3O4, NiO and Fe12xNixOy 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 thick-
ness 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 Fe3O4 and NiO
Fig. 1. Fe K-edge XANES spectra for a series of Fe12xNixOy
samples along with Fe3O4 film.

C.L. Chang et al. / Journal of Electron Spectroscopy and Related Phenomena 114–116 (2001) 545–548
547
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].
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
Fe31 from both the tetrahedral and octahedral sites.
The change of the Fe valence from mixed Fe21 /Fe31
to single valent Fe31, 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].
From the Fe K-edge spectra, it is clear that the
average valence of Fe increases with the Ni con-
centration. 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 Fe21 and Fe31 with a
ratio of 2:1. Thus the ratio of Fe21 to Fe31 decreases
and the average Fe valence increases. All the di-
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 val-
ence of Fe is increased with the Ni concentration,
while the Ni valence in not affected by the com-
position 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
[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.
Fig. 2. Ni K-edge XANES spectra of a series of Fe12xNixOy
samples along with NiO film.

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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.
[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.
Lett. 68 (1992) 114.
[8] Y. Bando, S. Horii, T. Takada, Jpn. J. Appl. Phys. 17 (1978)
1037.
[13] S. Chikazumi, S.H. Charap, Physics of Magnetism, John
Wiley and Sons, New York, 1964, Chapter 5.
[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.
[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.
[11] J.B. Smathers, L.R. Testardi, Phys. Rev. Lett. 77 (1996)
1147.