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Electronic structure of Cr doped Fe3O4 thin films by X-ray absorption near-edge structure spectroscopy
Solid State Communications 272 (2018) 48–52
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Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Communication
Electronic structure of Cr doped Fe3O4 thin films by X-ray absorption near-edge structure spectroscopy Chi-Liang Chen a, *, Chung-Li Dong b, Kandasami Asokan c, G. Chern d, C.L. Chang b a
National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan Department of Physics, Tamkang University, Tamsui, New Taipei City, Taiwan Inter University Accelerator Centre, New Delhi, 110067, India d Department of Physics, National Chung Cheng University, Chia-yi, Taiwan b c
A R T I C L E I N F O
A B S T R A C T
Communicated by Josep Fontcuberta
Present study reports the electronic structures of Cr doped Fe3O4 (Fe3-xCrxO4 (0 x 3) grown on MgO (100) substrates in the form of thin films fabricated by a plasma–oxygen assisted Molecular Beam Epitaxy (MBE). X-ray absorption near-edge structure (XANES) spectra at Cr & Fe L-, and O K-edges were used to understand the electronic structure: changes in the bonding nature, valence states, and site occupancies. Cr doping in Fe3O4 results in the change of charge transfer, crystal structure, and selective occupation of ions in octahedral and tetrahedral sites. Such change modifies the electrical and magnetic properties due to the covalency of Cr ions. The physical and chemical properties of ferrites are strongly dependent on the lattice site, ion size of dopant, and magnetic nature present at different structural symmetry of the spinel structure.
Keywords: A. Thin films B. MBE A. Magnetite E. X-ray absorption spectroscopy
1. Introduction Ferrites comprise of a group of iron oxides having a general formula MO-Fe2O3, where M is a divalent or trivalent metal ions. The parent compound, magnetite (Fe3O4), is a ferrimagnetic oxide with high spin polarization, half metallic and high Curie temperature. Magnetite has a great significance in large number of fields: magnetic recording media such as audio and videotape, and high-density digital recording disks, magnetic fluids, data storage, in the areas of medical care such as drug delivery systems (DDS), medical applications, including radiofrequency hyperthermia, photomagnetics, and magnetic resonance imaging (MRI), medical diagnostics and cancer therapy and microwave devices, magneto-optics devices, sensors, high frequency applications, catalysis and magnetic sensing. These ferrites have been investigated for their application in spintronic devices [1–3]. Very recently, novel applications have been explored in the development of ferrite catalysts for membrane (film) reactors, water-splitting and the generation of H2 as a clean fuel. The study in chromium doped magnetite could enhance the activity of the oxide for the H2O2 decomposition and increase the activity for the Fenton reaction, these higher activity is strongly correlated to the coupling of the redox pairs Fe3þ/Fe2þ with different valence state of Cr [4]. Since magnetic nanoparticles/films themselves have strong
magnetic properties and could easily aggregate together, for example; The magnetite nanoparticles with a high rate of metal substitution can be produced by biological processes by J. W Moon et al. [5] The Fe3O4 doped CdSe nanoparticles stabilized by various ligands to render them hydrophilicity [6], Co or Au doped Fe3O4 [7,8] and the magnetic hollow sphere [9] catalysts have been fabricated to improve catalyst efficiency by decreasing catalyst density and tailoring of energy band gap. Experimental investigations revealed that the spinel type structure of Fe3O4 has Fe ions in two inequivalent sites being in Fe3þ in tetrahedral (Td) sites and other a mixture of Fe2þ and Fe3þ in octahedral (Oh) sites [10]. In addition, different M mixed oxides may follow a simple atomic arrangement but possess rich electrical and magnetic properties [11,12]. Such substitution may bring about transformations of crystal structure and physical properties. The cation valences and site occupancies of substituting metal (Cr, Mn, Co and Ni)) in magnetite with different variations of phase transformation temperature have been investigated by hard x-ray absorption spectroscopy [13,14]. Substitution of Cr in magnetite (Fe3O4) results in Chromite (FeCr2O4). While magnetite is a typical magnetic spinel oxide with Curie temperature ~850 K, FeCr2O4 has low Curie temperature ~90 K but with conical spin structure and is a possible candidate for a multiferroic or a compound with high magnetoelectric effect [15–18]. The interest in the transformation from Fe3O4
* Corresponding author. National Synchrotron Radiation Research Center, 101 Hsin-Ann Rd., Hsinchu Science Parl, Hsinchu, 30076, Taiwan. E-mail address: [email protected] (C.-L. Chen). https://doi.org/10.1016/j.ssc.2018.01.011 Received 9 August 2017; Received in revised form 4 January 2018; Accepted 16 January 2018 Available online 31 January 2018 0038-1098/© 2018 Elsevier Ltd. All rights reserved.
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to FeCr2O4 in a series of solid solutions Fe3-xCrxO4 arises from the following: (1) FeCr2O4 is a nonlinear spin system with conical spiral states while Fe3O4 is a typical Neel antiferromagnetic; (2) FeCr2O4 is structurally a normal spinel while magnetite is an inverse spinel. The effect of cation distribution in the Oh site introduces extra charge hopping conductivity of magnetite and the metal-insulator Verwey transition remains very interesting. (3) Previous investigation of the bulk Fe2-xCrxO4 (0 < x < 2) have been mainly from the Mossbauer measurements [18, 19], which show that this system can be divided into three regions depending on the x value, namely, the inverse spinel (0 < x < 0.68), intermediate spinel (0.75 < x < 1.38) and the normal spinel (1.5 < x < 2.0). In previous spectroscopic studies on a variety of metal dopants like transition metals, Al, and Ag doped at Fe sites in spinel lattice suggested that some of dopants reduce the magnetic moments of Fe ions [20–23]. The properties of ferrites are different in thin films and bulk form and strongly depend on the preparation methods. The Cr doped Fe3O4 films grown by molecular beam epitaxy (MBE) exhibit these to be highly textured and of single phase [24]. Detailed structural and magnetic characteristics of these films have shown a strong dependence of the structure and magnetization on Cr content. These results indicate that the physical properties of these films start to deviate from the bulk at x > 0.7. In addition, beyond x > 1.5, a structural transformation is revealed which substantially affects the magnetic and electrical responses [24]. These discrepancies are attributed to the cation distribution which may originate from the low temperature growth process and the interaction between the film and substrate. Spectroscopic studies like X-ray absorption near-edge structure (XANES) at a specific absorption edge provides the local chemical environment of the absorbing atom within solids, liquids or gases. At the K (or L3,2-) edges of a selected atom, strong resonances appear below the ionisation potential which are assigned to transitions of the 1s (or 2p) electron to unoccupied molecular orbitals. An analysis of the intensity, the shape and the energy position of these resonances contains information about their electronic structure. From early report by D. S. Lee and G. Chern [24], XRD results are consistent with bulk materials at low Cr composition [18] and the resistivity of the films shows Arrhenius behaviors. The magnetic properties from the hysteresis loop show a transition of the anisotropy. Present study investigates the electronic structures of the Cr doped Fe3O4 thin films using XANES spectroscopy and correlates their electronic structure with the observed structural, magnetic and electrical properties considering the site occupation and valences of Fe and Cr ions and their bonding with O ions.
Fig. 1. Cr L3-edge XANES spectra of a series of Fe3-xCrxO4, x ¼ 0.0–2.4, thin films and Cr2O3/MgO standard films. These spectra are normalized to the peak of highest intensity (A3).
2. Experimental The thin films of Cr doped Fe3O4 in the stochiometric formula, Fe3(0 x 3) thin films were fabricated on MgO (001) substrates by a plasma–oxygen assisted Molecular Beam Epitaxy (MBE) [24]. MBE has been commonly used in the fabrication of oxide thin films and superlattices. The thicknesses of these films were at ~120 nm. X-ray diffraction (XRD) was used to confirm the characterization and ascertain the crystalline phase and quality of the films. XANES measurements were performed at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The XANES at Cr and Fe L-, and O K-edges were measured at the high-energy spherical grating monochromator (HSGM) BL20A1 (resolving power E/ΔE ¼ 8000) in ultrahigh vacuum (<5 109 Torr) in the total electron yield mode. Fe3O4 and Cr2O3 were used as reference compounds apart from various oxides of Fe and Cr for calibration, reliability of measurements and determination of the different electronic states of these compounds. xCrxO4
3. Results and discussion The structural results based on X-ray diffraction, electrical and magnetic properties have been published elsewhere [24]. Fig. 1 shows the normalized XANES spectra at Cr L3-edges of Cr doped Fe3O4 compounds.
Fig. 2. Fe L3,2-edge (2p3/2, 49
1/2→3d
transition).
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Solid State Communications 272 (2018) 48–52
spectra (Fig. 1) thus show that the unoccupied 3d state on Cr3þ has an in-plane orbital polarization. As a result of the Cr occupied in Oh site at x > 1.5, the two degenerates 3d eg unoccupied orbitals split into the d23z2-r (A3 peak) and d2x 2-y (A4 peak) states. In other words, the evolution of A4 unravels the in-plane character of the unoccupied d2x 2-y orbital of Cr3þ in the a-b plane CrO4 of Oh symmetry. The feature A3, on the other hand, is dominated by the z character from the d23z2-r orbital of Cr3þ that agrees with the intuitive expectation of the CrO2 bonding in Oh symmetry directed along the c axis. Therefore, the increase splitting A3 and A4 change would indicate that the lattice is shrinking. With 0.3 < x<0.9, the spectral shapes are similar to Cr Oh symmetry about 10Dq~0.5eV as per the calculations by de Groot et al. [26] and van der Laan et al. [27]. Above results confirms that trivalent Cr ions replaces the Fe (Oh) site in the Fe3O4 at x 1.5 as the 10Dq energy spitting increases with x. Further increase in x, towards Cr2O3, results in the line shapes of spectra gradually transform to broad towards the Oh symmetry. The normalized XANES spectra at Fe L3-edge of these mixed oxides are shown in Fig. 2. These spectra arise due to the transition from the 2p3/ 2, 1/2 levels to the 3d unoccupied state with spin-orbit coupling. The L3edge of Fe3O4 contains three prominent features marked B1, B2,and B3 corresponding to Fe2þ in Oh site, Fe3þ in Td site and Fe3þ in Oh site, respectively [20,28,29]. Owing to the orbital orientation, structural symmetry, and the p-d hybridization in the unoccupied states. The weak
These spectra were normalized to the peak of highest intensity along with Cr2O3/MgO film. The spectral features of Cr L3–edge consist of a main peak and shoulders at the lower energy side. These are consistent with the calculated crystal-field 10 Dq results for Cr (3d3 and 3d4L, where the L is the ligand form of oxygen 2p state) states in the Oh symmetry [25–27]. Two main peaks are found to dominate the spectral evolution features that are marked A1,2 (2p→t2g) and A3,4 (2p→eg) at lower and higher photon energies. The eg splitting into two peaks A3 and A4 are contributions associated with Cr 3d23z2-r and d2x 2-y from the Jahn-Teller effect (ΔEJ-T~0.6 eV at x ¼ 1.5) for the electronic configuration 3d4 with high spin state in Oh symmetry, i.e. no Jahn-teller effect is expected in the d3 state. The crystal structure and their physical properties depend on the Fe site symmetry. Cr3þ ion has an electronic configuration of t32g e0g. It usually occupies the octahedral sites in spinel structure which theoretically has no orbital angular momentum. Similarly, Fe ion has a zero orbital momentum at the B site with electronic configuration of t32ge2g in the spinel structure. One would expect that there is weak magnetic properties due to Cr substitution and these opposite trivalent spins are found to destroy the ferrimagnetic structure. However, Cr substitutions can induce variations of the exchange coupling into this mixed spinel ferrite system. When the Cr3þ ions are doped in the octahedral sites, the crystal field splitting of 10Dq distortion effects are seen in the spectra of the films for x > 1.5 which resembles to the standard Cr2O3 spectrum. The additional peaks arise when the ground state of Cr 3d3 (t32ge0g) is slightly distorted from the Oh symmetry resulting in 10Dq energy level splitting [25–27]. The x-ray absorption spectroscopy is more sensitive the structural symmetry than in house XRD [24]. Therefore, These XANES Cr L3-edge
feature B4 is a contribution from the 3 dL, where L denotes a hole in the O 2p valence band. The spectra exhibit main peak at 708.5 eV and a shoulder at 707 eV attributed to the crystal-field splitting. The Cr3þ prefers to replace the same valence state in Oh symmetry in the crystalline as bulk like systems [18,19]. Additionally, the evolution of B1 feature is shown in Fig. 3(a). When Cr3þ replaces Fe3þ of Oh site, the B3 feature decreases in the case of inverse-spinel, while B1 remains the same. In order to estimate the variation of B1 feature, the normalized the spectra at main feature peak B3 were fitted with Gaussian functions and ratio of B1/B3 are shown in Fig. 3(b). It is observed that the ratio of B1/B3 is
Fig. 3. (a) An expanded view of the peak B1 region form Fe L3-edge of Fig. 2 to show that the peak diminishes beyond x ¼ 0.9. (b) The ratios of the intensity of the B2/B3 and B1/B3 peaks as a function of Cr concentration (x) and comparisons with c-axis lattice parameters of Fe3-xCrxO4/MgO thin films at x≦1.5.
Fig. 4. O K-edge XANES spectra of a series of Fe3xCrxO4 thin films and the reference Cr2O3/MgO and Fe3O4/MgO films. These spectra were normalized to the main peak at 540eV. 50
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nearly independent in the range of 0.3 x 0.9. The decrease in the intensity of spectral feature B1 implies reduction in the unoccupied electronic states of Oh site at x > 0.9. Apart from this, the shoulder of the B2 feature also reduces in intensity with x 0.9. The intensity reduction at spectral features of B1 and B2 is likely due to charge transfer (CT) effect from Oh site to Td site by the oxygen bridge of Fe(Oh)-O-Fe(Td). Using a 3þ 3þ 2þ 3þ 3þ simple formula ½ðFe2þ y Fe1y ÞB2 Td fðFe1y Fey ÞB1 ðFe1x Crx ÞB3 gOh O4 ,
experience similar strain from the mismatch with respect to MgO substrate that becomes even larger as x increases. The previous studies on the bulk have concluded that the change of the lattice parameters is associated with a transformation from inverse spinel to normal spinel as x increases in Fe3-xCrxO4 compounds and that this transformation may be divided into three regions (as shown in the figure). It should be noted that the ionic size is [Cr3þ]<[Fe3þ]<[Feþ2]. Reduction in the lattice parameters in the range of x < 0.7 result from the replacement of larger Fe3þ by smaller Cr3þ in the Oh site. However, the increase of the lattice parameters with x > 0.7 indicates that extra electron populations move from Fe in Oh to Fe in Td site (see below). In other words, Fe2þ is forced into the Td site and inverse spinel gradually changes to intermediate spinel. Because the interstitial occupation of Cr at Td site is more favorable than the Oh site is also a reason for the lattice parameter of Fe3-xCrxO4 starts increasing with x > 0.7. The lower lattice parameters of the present films is because of the lattice matching of MgO substrate as compared to the bulk results from the literature [24]. However, continue to decrease at 0.9 < x < 1.5 is different from the bulk results as present by M.Robbins et al. [18], although with a weak tendency to increase at x ¼ 0.9, and reach to a much lower value ~8.22 Å at x ¼ 1.5, Fig. 3(b). This inconsistency can be understood as the cation distribution resulting by XANES spectra which induces change transfer as Cr3þ continuously substitutes the at Oh in 0.9 < x<1.5. The formation changes of spinel in these films. The preference of the cation distribution for the inverse phase may be associated with the crystalline stability but is not very clear at this moment. The structural variation results probably from the low temperature growth and substrate effect. Increasing Cr continuously in the present films may cause instability of the spinel structure and a crossover from spinel to corundum (Cr2O3) phase occurring at x~1.5. Moreover, pure Fe3O4 film (x ¼ 0) shows small strain but basically retain reasonable physical characteristic which is similar to the previous reports [20]. In the small Cr concentration region (x < 0.9), the films retain the spinel structure. However, the lattice parameter starts to show deviation from the bulk with x > 0.9. The present spectroscopic results clearly confirm that these films have spinel structure (Fe3O4-like) at low Cr concentrations (x < 0.9) and corundum structure (Cr2O3-like) at large Cr content (x > 2.1) and an intermediate region (1.2 < x < 1.8) connecting these two regions. These discrepancies are attributed to cation distribution which may be due to the low temperature growth and substrate effect. A recent study of Mn3-xCrxO4 films on SrTiO3 (001), which has larger lattice mismatch between films and substrate, shows more stable spinel phase but the films are polycrystalline [35].
where y represents the charge transfer (C-T) reactions, one understands that Fe valence is reduced in the Td (i.e. from trivalent to divalent Fe) site with a complete reaction at x ¼ 0.9 becomes a normal spinel-like (intermediate spinel) structure. With further substitution x > 0.9, a steep reduction of the B2 peak is observed and with further increase of x shows a minimum at x ¼ 1.5 and the ratio of B1/B3 also reveal a maximum at the same concentration of x around 0.9. Thus the Fe3þ in Oh (B3) is fully occupied and Cr3þ continuously substitutes the Fe2þ at Oh (B1). Therefore, the reduction of the B1 and B2 intensities was observed as the Oh sites are replaced by Cr and the Fe valence changes between the Td and Oh sites, simultaneously for x 1.5. In addition, the variations of the B2/B3 ratio are very much similar to the c-axis lattice parameters when the x 1.5 as Fig. 3(b) shown, and are different from the bulk behavior as discuss by D. S. Lee et al. [24]. The line shapes are nearly identical from x ¼ 1.5 to 2.4 and closely resemble that of hematite Fe2O3 as in the case of eskolaite Cr2O3 crystal system at x ¼ 2.4 and 2.7. These results are consistent with the XRD data and XANES results at Cr L-edges in which no major change was seen in the spinel structure up to x ¼ 1.5. Fig. 4 shows the XANES spectra at O K-edge of Cr doped Fe3O4 thin films with different x values. These absorption spectra correspond to the transition from O 1s to the 2p states with various overlapping electronic orbital states. Thus, one needs to consider, the Fe 3dO 2p and Cr 3d–O 2p hybridizations as Cr replaces Fe. The spectral features exhibit a complex shapes consisting of three major components labeled by C1, C2 and C3 in the figures. The relatively strong pre-peak feature in the photon energy region 528–533 eV is due to the transition of O 1s to the unoccupied O 2p states hybridized with Fe (Cr) 3d states. The main peak in the region 533–543 eV is due to the transition of O 1s to higher-energy unoccupied states of O p character hybridized with the metal 4sp band. Owing to the Oh crystal field splitting, the t2g and eg bands are observed. The C1 and C2 peaks are assigned to the splitting of Fe eg band t2g band, the C3 and C4 peaks are attributed to the Cr 3d3 and 3d4L unoccupied features [30–34]. A strong dependence of the intensities of C3 and C4 on the Cr concentration is evident. These correspond to the octahedral crystal field (10 Dq) and weak Jahn-Teller (EJ-T) effect on the eg band splitting in the electronic configuration of Fe 3d5 (low spin) or 3d6 (high spin) in the ground state as well as due to the trivalent Cr in the thin films as discussed above in case of the Cr L-edge. As Cr is doped into the Fe site at x~1.2, the intensity changes and the peaks broaden due to ΔEJ-T. Both of these indicate that the two degenerates 3d eg orbitals split in the energy band of Fe due to the d23z2-r and d2x 2-y states [34], that respectively hybridize with O 2p orbitals. This is consistent with the results of XANES spectra at Fe and Cr L-edges. Owing to different orientations of the 3d orbitals in the Td and Oh crystal symmetries of Fe-O and Cr-O, the energy bands experience a complex crystal field splitting. A large difference is observed between the C3 peaks at x ¼ 1.2 and 2.1 in the O K-edge spectra. The observed changes in the O K-edge spectra arise from the Cr 3d–O 2p hybridization. This also supported by the observations of the Cr L-edge data where the Fe cation has been substituted by Cr3þ. Up to 50% Cr substitution results change in the feature C3 which corresponds to t2g band of Cr while the small changes in the feature C4 corresponding to eg band attributed to the Cr2O3. The consistent observations in the magnetic properties are attributed to the presence of Cr [24]. Results from magnetic measurements also indicate that Cr3þ replaces 3þ Fe from B site and the spinel retains the reverse structure in the range 0 < x < 0.6. At x ¼ 0.9, results show that Cr3þ starts to distribute at the A site and becomes an intermediate spinel state [24]. The deviation of the lattice parameter along the c-axis is expected to decrease if the films
4. Conclusion The electronic structures of Cr doped Fe3O4 (Fe3-xCrxO4 (0 x 3) grown on MgO (100) substrates in the form of thin films fabricated by a plasma–oxygen assisted MBE were investigated. The XANES spectra at Cr, Fe L-, and O K-edges show changes in the valence states, crystal structure and site occupancies of Cr, and Fe ions. The electrical and magnetic properties of Cr doped in Fe3O4 were understood based on above results. Present study shows that Fe3O4-FeCr2O4 thin films exhibit unique characteristics which are different from other substituted ferrites: Cr is a typical trivalent cation and preferentially occupies at site with Oh symmetry while it replaces Fe in Fe3-xCrxO4. The divalent Fe occupied in the Td site and the compound becomes normal spinel at high Cr concentration. The Jahn-Teller ions in this case with a tetragonal distortion are also appeared as Cr replace the Oh site with 3d4L state and more complicated spin states such as spiral configurations. The unusual magnetic respond in these thin films maybe due to the cation distribution and C-T effect are different from the bulk and the low temperature fabrication by MBE process. It is important to note that an intermediate state (0.3 < x < 1.5), connecting Fe3O4 and FeCr2O4 from inverse to normal spinel, are also identified in this study.
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Acknowledgments
[15] S. Bordacs, D. Varjas, I. Kezsmarki, G. Mihaly, L. Baldassarre, A. Abouelsayed, C.A. Kuntscher, K. Ohgushi, Y. Tokura, Phys. Rev. Lett. 103 (2009) 077205. [16] I. Kim, Y.S. Oh, Y. Liu, S.H. Chun, J.S. Lee, K.T. Ko, J.H. Park, J.H. Chung, K.H. Kim, Appl. Phys. Lett. 94 (2009) 042505. [17] K. Singh, A. Maignan, C. Simon, C. Martin, Appl. Phys. Lett. 99 (2011) 172903. [18] M. Robbins, G.K. Wertheim, R.C. Sherwood, D.N.E. Buchanan, J. Phys. Chem. Solid. 32 (1971) 717. [19] H.J. Yearuan, J.M. Kortright, R.H. Langenheim, J. Chem. Phys. 22 (1954) 1196. [20] C.L. Chen, C.L. Dong, S.M. Rao, G. Chern, M.C. Chen, M. K.Wu, C.L. Chang, J. Phys. Condens. Matter 20 (2008) 255236. [21] Y.S. Liu, J.L. Chen, C.L. Chen, C.L. Dong, D.S. Lee, G. Chern, C.L. Chang, J. Alloy. Comp. 442 (2007) 259–261. [22] S.H. Liu, H.M. Tsai, C.W. Pao, J.W. Chiou, D.C. Ling, W.F. Pong, M.-H. Tsai, H.J. Lin, L.Y. Jang, J.F. Lee, J.H. Hsu, W.J. Wang, C.J. Hsu, Appl. Phys. Lett. 89 (2006) 092112. [23] C.K. Yang, J.W. Chiou, H.M. Tsai, C.W. Pao, J.C. Jan, S.C. Ray, C.L. Yeh, K.C. Huang, H.C. Hsueh, W.F. Pong, M.-H. Tsai, H.H. Hsieh, H.J. Lin, T.Y. Hou, J.H. Hsu, Appl. Phys. Lett. 86 (2005) 062504. [24] D.S. Lee, G. Chern, Phys. B Condens. Matter 407 (2012) 297–301. [25] C. Theil, J. van Elp, F. Folkmann, Phys. Rev. B 59 (1999) 7931. [26] F.M.F. de Groot, J.C. Fuggle, B.T. Thole, G.A. Sawatzky, Phys. Rev. B 42 (1990) 5459. [27] G. Van der Lann, I.W. Kirkman, J. Phys. Condens. Matter 4 (1992) 4189. [28] A. Mirone, M. Sacchi, S. Gota, Phys. Rev. B 61 (2000) 13540. [29] 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 Commun. 109 (1999) 599. [30] T. Kendelewicz, P. Liu, C.S. Doyle, G.E. Brown Jr., E.J. Nelson, S.A. Chambers, Surf. Sci. 424 (1999) 219. [31] G. Chern, C.L. Chang, C.L. Chen, C.L. Dong, J. Vac. Sci. Technol. 17 (4) (1999) 1630. [32] C. Colliex, T. Manoubi, C. Ortiz, Phys. Rev. B 44 (1991) 11402. [33] F.M.F. de Groot, M. Grioni, J.C. Fuggle, J. Ghijsen, G.A. Sawatzky, H. Petersen, Phys. Rev. B 40 (1989) 5715. [34] A.J. Millis, Nature 392 (1998) 147. [35] Y.-C. Jhuang, K.M. Kuo, G. Chern, J. Appl. Phys. 109 (2011) 07D714.
This work was supported by the National Science Council of Taiwan under Contract NSC 102-2112-M-001-004-MY3 and 101-2112-M-213004-MY3. The authors acknowledge the NSRRC staffs, Drs. J. F. Lee and J. M. Chen, for useful discussions and experimental support and the excellent cooperation and the members of Prof. C. L. Chang's laboratory during measurements. References c, J. Fabian, J.D. Sharma, Rev. Mod. Phys. 76 (2004) 323. [1] I. Zuti [2] M. Ziese, Rep. Prog. Phys. 65 (2002) 143. [3] J.M.D. Coey, A.E. Berkowitz, Ll Balcells, F.F. Putris, F.T. Parker, Appl. Phys. Lett. 72 (1998) 734. [4] F. Magalh~ aes, M.C. Pereira, S.E.C. Botrel, J.D. Fabris, W.A. Macedo, R. Mendonca, R.M. Lago, L.C.A. Oliveira, Appl. Catal. Gen. 332 (2007) 115–123. [5] J.-W. Moon, Y. Roh, R.J. Lauf, H. Vali, L.W. Yeary, T.J. Phelps, J. Microbiol. Meth. 70 (2007) 150–158. [6] G. Jie, J. Yuan, Anal. Chem. 84 (6) (2012) 2811. [7] A. Lv, C. Hu, Y. Nie, J. Qu, Appl. Catal. B Environ. 117–118 (2012) 246–252. [8] S. G omez-Quero, F. C ardenas-Lizana, M.A. Keane, Nanotechnology 23 (2012) 294002. [9] S.-W. Cao, Y.-J. Zhu, M.-Y. Ma, L. Li, L. Zhang, J. Phys. Chem. C 112 (6) (2008) 1851. [10] T. Terada, Nature 91 (1913) 213. [11] Y. Yamasaki, S. Miyasaka, Y. Kaneko, J.-P. He, T. Arima, Y. Tokura, Phys. Rev. Lett. 96 (2006), 207204. [12] K. Ohgushi, Y. Okimoto, T. Ogasaeara, S. Miyasaka, Y. Tokura, J. Phys. Soci. Japan 77 (2008), 034713. [13] X. Liang, Y. Zhong, S. Zhu, H. He, P. Yuan, J. Zhu, Z. Jiang, Solid State Sci. 15 (2013) 115–122. [14] X. Liang, Y. Zhong, S. Zhu, H. He, P. Yuan, J. Zhu, Z. Jiang, Chem. Eng. J. 191 (2012) 177–184.
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Communication
Electronic structure of Cr doped Fe3O4 thin films by X-ray absorption near-edge structure spectroscopy Chi-Liang Chen a, *, Chung-Li Dong b, Kandasami Asokan c, G. Chern d, C.L. Chang b a
National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan Department of Physics, Tamkang University, Tamsui, New Taipei City, Taiwan Inter University Accelerator Centre, New Delhi, 110067, India d Department of Physics, National Chung Cheng University, Chia-yi, Taiwan b c
A R T I C L E I N F O
A B S T R A C T
Communicated by Josep Fontcuberta
Present study reports the electronic structures of Cr doped Fe3O4 (Fe3-xCrxO4 (0 x 3) grown on MgO (100) substrates in the form of thin films fabricated by a plasma–oxygen assisted Molecular Beam Epitaxy (MBE). X-ray absorption near-edge structure (XANES) spectra at Cr & Fe L-, and O K-edges were used to understand the electronic structure: changes in the bonding nature, valence states, and site occupancies. Cr doping in Fe3O4 results in the change of charge transfer, crystal structure, and selective occupation of ions in octahedral and tetrahedral sites. Such change modifies the electrical and magnetic properties due to the covalency of Cr ions. The physical and chemical properties of ferrites are strongly dependent on the lattice site, ion size of dopant, and magnetic nature present at different structural symmetry of the spinel structure.
Keywords: A. Thin films B. MBE A. Magnetite E. X-ray absorption spectroscopy
1. Introduction Ferrites comprise of a group of iron oxides having a general formula MO-Fe2O3, where M is a divalent or trivalent metal ions. The parent compound, magnetite (Fe3O4), is a ferrimagnetic oxide with high spin polarization, half metallic and high Curie temperature. Magnetite has a great significance in large number of fields: magnetic recording media such as audio and videotape, and high-density digital recording disks, magnetic fluids, data storage, in the areas of medical care such as drug delivery systems (DDS), medical applications, including radiofrequency hyperthermia, photomagnetics, and magnetic resonance imaging (MRI), medical diagnostics and cancer therapy and microwave devices, magneto-optics devices, sensors, high frequency applications, catalysis and magnetic sensing. These ferrites have been investigated for their application in spintronic devices [1–3]. Very recently, novel applications have been explored in the development of ferrite catalysts for membrane (film) reactors, water-splitting and the generation of H2 as a clean fuel. The study in chromium doped magnetite could enhance the activity of the oxide for the H2O2 decomposition and increase the activity for the Fenton reaction, these higher activity is strongly correlated to the coupling of the redox pairs Fe3þ/Fe2þ with different valence state of Cr [4]. Since magnetic nanoparticles/films themselves have strong
magnetic properties and could easily aggregate together, for example; The magnetite nanoparticles with a high rate of metal substitution can be produced by biological processes by J. W Moon et al. [5] The Fe3O4 doped CdSe nanoparticles stabilized by various ligands to render them hydrophilicity [6], Co or Au doped Fe3O4 [7,8] and the magnetic hollow sphere [9] catalysts have been fabricated to improve catalyst efficiency by decreasing catalyst density and tailoring of energy band gap. Experimental investigations revealed that the spinel type structure of Fe3O4 has Fe ions in two inequivalent sites being in Fe3þ in tetrahedral (Td) sites and other a mixture of Fe2þ and Fe3þ in octahedral (Oh) sites [10]. In addition, different M mixed oxides may follow a simple atomic arrangement but possess rich electrical and magnetic properties [11,12]. Such substitution may bring about transformations of crystal structure and physical properties. The cation valences and site occupancies of substituting metal (Cr, Mn, Co and Ni)) in magnetite with different variations of phase transformation temperature have been investigated by hard x-ray absorption spectroscopy [13,14]. Substitution of Cr in magnetite (Fe3O4) results in Chromite (FeCr2O4). While magnetite is a typical magnetic spinel oxide with Curie temperature ~850 K, FeCr2O4 has low Curie temperature ~90 K but with conical spin structure and is a possible candidate for a multiferroic or a compound with high magnetoelectric effect [15–18]. The interest in the transformation from Fe3O4
* Corresponding author. National Synchrotron Radiation Research Center, 101 Hsin-Ann Rd., Hsinchu Science Parl, Hsinchu, 30076, Taiwan. E-mail address: [email protected] (C.-L. Chen). https://doi.org/10.1016/j.ssc.2018.01.011 Received 9 August 2017; Received in revised form 4 January 2018; Accepted 16 January 2018 Available online 31 January 2018 0038-1098/© 2018 Elsevier Ltd. All rights reserved.
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to FeCr2O4 in a series of solid solutions Fe3-xCrxO4 arises from the following: (1) FeCr2O4 is a nonlinear spin system with conical spiral states while Fe3O4 is a typical Neel antiferromagnetic; (2) FeCr2O4 is structurally a normal spinel while magnetite is an inverse spinel. The effect of cation distribution in the Oh site introduces extra charge hopping conductivity of magnetite and the metal-insulator Verwey transition remains very interesting. (3) Previous investigation of the bulk Fe2-xCrxO4 (0 < x < 2) have been mainly from the Mossbauer measurements [18, 19], which show that this system can be divided into three regions depending on the x value, namely, the inverse spinel (0 < x < 0.68), intermediate spinel (0.75 < x < 1.38) and the normal spinel (1.5 < x < 2.0). In previous spectroscopic studies on a variety of metal dopants like transition metals, Al, and Ag doped at Fe sites in spinel lattice suggested that some of dopants reduce the magnetic moments of Fe ions [20–23]. The properties of ferrites are different in thin films and bulk form and strongly depend on the preparation methods. The Cr doped Fe3O4 films grown by molecular beam epitaxy (MBE) exhibit these to be highly textured and of single phase [24]. Detailed structural and magnetic characteristics of these films have shown a strong dependence of the structure and magnetization on Cr content. These results indicate that the physical properties of these films start to deviate from the bulk at x > 0.7. In addition, beyond x > 1.5, a structural transformation is revealed which substantially affects the magnetic and electrical responses [24]. These discrepancies are attributed to the cation distribution which may originate from the low temperature growth process and the interaction between the film and substrate. Spectroscopic studies like X-ray absorption near-edge structure (XANES) at a specific absorption edge provides the local chemical environment of the absorbing atom within solids, liquids or gases. At the K (or L3,2-) edges of a selected atom, strong resonances appear below the ionisation potential which are assigned to transitions of the 1s (or 2p) electron to unoccupied molecular orbitals. An analysis of the intensity, the shape and the energy position of these resonances contains information about their electronic structure. From early report by D. S. Lee and G. Chern [24], XRD results are consistent with bulk materials at low Cr composition [18] and the resistivity of the films shows Arrhenius behaviors. The magnetic properties from the hysteresis loop show a transition of the anisotropy. Present study investigates the electronic structures of the Cr doped Fe3O4 thin films using XANES spectroscopy and correlates their electronic structure with the observed structural, magnetic and electrical properties considering the site occupation and valences of Fe and Cr ions and their bonding with O ions.
Fig. 1. Cr L3-edge XANES spectra of a series of Fe3-xCrxO4, x ¼ 0.0–2.4, thin films and Cr2O3/MgO standard films. These spectra are normalized to the peak of highest intensity (A3).
2. Experimental The thin films of Cr doped Fe3O4 in the stochiometric formula, Fe3(0 x 3) thin films were fabricated on MgO (001) substrates by a plasma–oxygen assisted Molecular Beam Epitaxy (MBE) [24]. MBE has been commonly used in the fabrication of oxide thin films and superlattices. The thicknesses of these films were at ~120 nm. X-ray diffraction (XRD) was used to confirm the characterization and ascertain the crystalline phase and quality of the films. XANES measurements were performed at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The XANES at Cr and Fe L-, and O K-edges were measured at the high-energy spherical grating monochromator (HSGM) BL20A1 (resolving power E/ΔE ¼ 8000) in ultrahigh vacuum (<5 109 Torr) in the total electron yield mode. Fe3O4 and Cr2O3 were used as reference compounds apart from various oxides of Fe and Cr for calibration, reliability of measurements and determination of the different electronic states of these compounds. xCrxO4
3. Results and discussion The structural results based on X-ray diffraction, electrical and magnetic properties have been published elsewhere [24]. Fig. 1 shows the normalized XANES spectra at Cr L3-edges of Cr doped Fe3O4 compounds.
Fig. 2. Fe L3,2-edge (2p3/2, 49
1/2→3d
transition).
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spectra (Fig. 1) thus show that the unoccupied 3d state on Cr3þ has an in-plane orbital polarization. As a result of the Cr occupied in Oh site at x > 1.5, the two degenerates 3d eg unoccupied orbitals split into the d23z2-r (A3 peak) and d2x 2-y (A4 peak) states. In other words, the evolution of A4 unravels the in-plane character of the unoccupied d2x 2-y orbital of Cr3þ in the a-b plane CrO4 of Oh symmetry. The feature A3, on the other hand, is dominated by the z character from the d23z2-r orbital of Cr3þ that agrees with the intuitive expectation of the CrO2 bonding in Oh symmetry directed along the c axis. Therefore, the increase splitting A3 and A4 change would indicate that the lattice is shrinking. With 0.3 < x<0.9, the spectral shapes are similar to Cr Oh symmetry about 10Dq~0.5eV as per the calculations by de Groot et al. [26] and van der Laan et al. [27]. Above results confirms that trivalent Cr ions replaces the Fe (Oh) site in the Fe3O4 at x 1.5 as the 10Dq energy spitting increases with x. Further increase in x, towards Cr2O3, results in the line shapes of spectra gradually transform to broad towards the Oh symmetry. The normalized XANES spectra at Fe L3-edge of these mixed oxides are shown in Fig. 2. These spectra arise due to the transition from the 2p3/ 2, 1/2 levels to the 3d unoccupied state with spin-orbit coupling. The L3edge of Fe3O4 contains three prominent features marked B1, B2,and B3 corresponding to Fe2þ in Oh site, Fe3þ in Td site and Fe3þ in Oh site, respectively [20,28,29]. Owing to the orbital orientation, structural symmetry, and the p-d hybridization in the unoccupied states. The weak
These spectra were normalized to the peak of highest intensity along with Cr2O3/MgO film. The spectral features of Cr L3–edge consist of a main peak and shoulders at the lower energy side. These are consistent with the calculated crystal-field 10 Dq results for Cr (3d3 and 3d4L, where the L is the ligand form of oxygen 2p state) states in the Oh symmetry [25–27]. Two main peaks are found to dominate the spectral evolution features that are marked A1,2 (2p→t2g) and A3,4 (2p→eg) at lower and higher photon energies. The eg splitting into two peaks A3 and A4 are contributions associated with Cr 3d23z2-r and d2x 2-y from the Jahn-Teller effect (ΔEJ-T~0.6 eV at x ¼ 1.5) for the electronic configuration 3d4 with high spin state in Oh symmetry, i.e. no Jahn-teller effect is expected in the d3 state. The crystal structure and their physical properties depend on the Fe site symmetry. Cr3þ ion has an electronic configuration of t32g e0g. It usually occupies the octahedral sites in spinel structure which theoretically has no orbital angular momentum. Similarly, Fe ion has a zero orbital momentum at the B site with electronic configuration of t32ge2g in the spinel structure. One would expect that there is weak magnetic properties due to Cr substitution and these opposite trivalent spins are found to destroy the ferrimagnetic structure. However, Cr substitutions can induce variations of the exchange coupling into this mixed spinel ferrite system. When the Cr3þ ions are doped in the octahedral sites, the crystal field splitting of 10Dq distortion effects are seen in the spectra of the films for x > 1.5 which resembles to the standard Cr2O3 spectrum. The additional peaks arise when the ground state of Cr 3d3 (t32ge0g) is slightly distorted from the Oh symmetry resulting in 10Dq energy level splitting [25–27]. The x-ray absorption spectroscopy is more sensitive the structural symmetry than in house XRD [24]. Therefore, These XANES Cr L3-edge
feature B4 is a contribution from the 3 dL, where L denotes a hole in the O 2p valence band. The spectra exhibit main peak at 708.5 eV and a shoulder at 707 eV attributed to the crystal-field splitting. The Cr3þ prefers to replace the same valence state in Oh symmetry in the crystalline as bulk like systems [18,19]. Additionally, the evolution of B1 feature is shown in Fig. 3(a). When Cr3þ replaces Fe3þ of Oh site, the B3 feature decreases in the case of inverse-spinel, while B1 remains the same. In order to estimate the variation of B1 feature, the normalized the spectra at main feature peak B3 were fitted with Gaussian functions and ratio of B1/B3 are shown in Fig. 3(b). It is observed that the ratio of B1/B3 is
Fig. 3. (a) An expanded view of the peak B1 region form Fe L3-edge of Fig. 2 to show that the peak diminishes beyond x ¼ 0.9. (b) The ratios of the intensity of the B2/B3 and B1/B3 peaks as a function of Cr concentration (x) and comparisons with c-axis lattice parameters of Fe3-xCrxO4/MgO thin films at x≦1.5.
Fig. 4. O K-edge XANES spectra of a series of Fe3xCrxO4 thin films and the reference Cr2O3/MgO and Fe3O4/MgO films. These spectra were normalized to the main peak at 540eV. 50
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nearly independent in the range of 0.3 x 0.9. The decrease in the intensity of spectral feature B1 implies reduction in the unoccupied electronic states of Oh site at x > 0.9. Apart from this, the shoulder of the B2 feature also reduces in intensity with x 0.9. The intensity reduction at spectral features of B1 and B2 is likely due to charge transfer (CT) effect from Oh site to Td site by the oxygen bridge of Fe(Oh)-O-Fe(Td). Using a 3þ 3þ 2þ 3þ 3þ simple formula ½ðFe2þ y Fe1y ÞB2 Td fðFe1y Fey ÞB1 ðFe1x Crx ÞB3 gOh O4 ,
experience similar strain from the mismatch with respect to MgO substrate that becomes even larger as x increases. The previous studies on the bulk have concluded that the change of the lattice parameters is associated with a transformation from inverse spinel to normal spinel as x increases in Fe3-xCrxO4 compounds and that this transformation may be divided into three regions (as shown in the figure). It should be noted that the ionic size is [Cr3þ]<[Fe3þ]<[Feþ2]. Reduction in the lattice parameters in the range of x < 0.7 result from the replacement of larger Fe3þ by smaller Cr3þ in the Oh site. However, the increase of the lattice parameters with x > 0.7 indicates that extra electron populations move from Fe in Oh to Fe in Td site (see below). In other words, Fe2þ is forced into the Td site and inverse spinel gradually changes to intermediate spinel. Because the interstitial occupation of Cr at Td site is more favorable than the Oh site is also a reason for the lattice parameter of Fe3-xCrxO4 starts increasing with x > 0.7. The lower lattice parameters of the present films is because of the lattice matching of MgO substrate as compared to the bulk results from the literature [24]. However, continue to decrease at 0.9 < x < 1.5 is different from the bulk results as present by M.Robbins et al. [18], although with a weak tendency to increase at x ¼ 0.9, and reach to a much lower value ~8.22 Å at x ¼ 1.5, Fig. 3(b). This inconsistency can be understood as the cation distribution resulting by XANES spectra which induces change transfer as Cr3þ continuously substitutes the at Oh in 0.9 < x<1.5. The formation changes of spinel in these films. The preference of the cation distribution for the inverse phase may be associated with the crystalline stability but is not very clear at this moment. The structural variation results probably from the low temperature growth and substrate effect. Increasing Cr continuously in the present films may cause instability of the spinel structure and a crossover from spinel to corundum (Cr2O3) phase occurring at x~1.5. Moreover, pure Fe3O4 film (x ¼ 0) shows small strain but basically retain reasonable physical characteristic which is similar to the previous reports [20]. In the small Cr concentration region (x < 0.9), the films retain the spinel structure. However, the lattice parameter starts to show deviation from the bulk with x > 0.9. The present spectroscopic results clearly confirm that these films have spinel structure (Fe3O4-like) at low Cr concentrations (x < 0.9) and corundum structure (Cr2O3-like) at large Cr content (x > 2.1) and an intermediate region (1.2 < x < 1.8) connecting these two regions. These discrepancies are attributed to cation distribution which may be due to the low temperature growth and substrate effect. A recent study of Mn3-xCrxO4 films on SrTiO3 (001), which has larger lattice mismatch between films and substrate, shows more stable spinel phase but the films are polycrystalline [35].
where y represents the charge transfer (C-T) reactions, one understands that Fe valence is reduced in the Td (i.e. from trivalent to divalent Fe) site with a complete reaction at x ¼ 0.9 becomes a normal spinel-like (intermediate spinel) structure. With further substitution x > 0.9, a steep reduction of the B2 peak is observed and with further increase of x shows a minimum at x ¼ 1.5 and the ratio of B1/B3 also reveal a maximum at the same concentration of x around 0.9. Thus the Fe3þ in Oh (B3) is fully occupied and Cr3þ continuously substitutes the Fe2þ at Oh (B1). Therefore, the reduction of the B1 and B2 intensities was observed as the Oh sites are replaced by Cr and the Fe valence changes between the Td and Oh sites, simultaneously for x 1.5. In addition, the variations of the B2/B3 ratio are very much similar to the c-axis lattice parameters when the x 1.5 as Fig. 3(b) shown, and are different from the bulk behavior as discuss by D. S. Lee et al. [24]. The line shapes are nearly identical from x ¼ 1.5 to 2.4 and closely resemble that of hematite Fe2O3 as in the case of eskolaite Cr2O3 crystal system at x ¼ 2.4 and 2.7. These results are consistent with the XRD data and XANES results at Cr L-edges in which no major change was seen in the spinel structure up to x ¼ 1.5. Fig. 4 shows the XANES spectra at O K-edge of Cr doped Fe3O4 thin films with different x values. These absorption spectra correspond to the transition from O 1s to the 2p states with various overlapping electronic orbital states. Thus, one needs to consider, the Fe 3dO 2p and Cr 3d–O 2p hybridizations as Cr replaces Fe. The spectral features exhibit a complex shapes consisting of three major components labeled by C1, C2 and C3 in the figures. The relatively strong pre-peak feature in the photon energy region 528–533 eV is due to the transition of O 1s to the unoccupied O 2p states hybridized with Fe (Cr) 3d states. The main peak in the region 533–543 eV is due to the transition of O 1s to higher-energy unoccupied states of O p character hybridized with the metal 4sp band. Owing to the Oh crystal field splitting, the t2g and eg bands are observed. The C1 and C2 peaks are assigned to the splitting of Fe eg band t2g band, the C3 and C4 peaks are attributed to the Cr 3d3 and 3d4L unoccupied features [30–34]. A strong dependence of the intensities of C3 and C4 on the Cr concentration is evident. These correspond to the octahedral crystal field (10 Dq) and weak Jahn-Teller (EJ-T) effect on the eg band splitting in the electronic configuration of Fe 3d5 (low spin) or 3d6 (high spin) in the ground state as well as due to the trivalent Cr in the thin films as discussed above in case of the Cr L-edge. As Cr is doped into the Fe site at x~1.2, the intensity changes and the peaks broaden due to ΔEJ-T. Both of these indicate that the two degenerates 3d eg orbitals split in the energy band of Fe due to the d23z2-r and d2x 2-y states [34], that respectively hybridize with O 2p orbitals. This is consistent with the results of XANES spectra at Fe and Cr L-edges. Owing to different orientations of the 3d orbitals in the Td and Oh crystal symmetries of Fe-O and Cr-O, the energy bands experience a complex crystal field splitting. A large difference is observed between the C3 peaks at x ¼ 1.2 and 2.1 in the O K-edge spectra. The observed changes in the O K-edge spectra arise from the Cr 3d–O 2p hybridization. This also supported by the observations of the Cr L-edge data where the Fe cation has been substituted by Cr3þ. Up to 50% Cr substitution results change in the feature C3 which corresponds to t2g band of Cr while the small changes in the feature C4 corresponding to eg band attributed to the Cr2O3. The consistent observations in the magnetic properties are attributed to the presence of Cr [24]. Results from magnetic measurements also indicate that Cr3þ replaces 3þ Fe from B site and the spinel retains the reverse structure in the range 0 < x < 0.6. At x ¼ 0.9, results show that Cr3þ starts to distribute at the A site and becomes an intermediate spinel state [24]. The deviation of the lattice parameter along the c-axis is expected to decrease if the films
4. Conclusion The electronic structures of Cr doped Fe3O4 (Fe3-xCrxO4 (0 x 3) grown on MgO (100) substrates in the form of thin films fabricated by a plasma–oxygen assisted MBE were investigated. The XANES spectra at Cr, Fe L-, and O K-edges show changes in the valence states, crystal structure and site occupancies of Cr, and Fe ions. The electrical and magnetic properties of Cr doped in Fe3O4 were understood based on above results. Present study shows that Fe3O4-FeCr2O4 thin films exhibit unique characteristics which are different from other substituted ferrites: Cr is a typical trivalent cation and preferentially occupies at site with Oh symmetry while it replaces Fe in Fe3-xCrxO4. The divalent Fe occupied in the Td site and the compound becomes normal spinel at high Cr concentration. The Jahn-Teller ions in this case with a tetragonal distortion are also appeared as Cr replace the Oh site with 3d4L state and more complicated spin states such as spiral configurations. The unusual magnetic respond in these thin films maybe due to the cation distribution and C-T effect are different from the bulk and the low temperature fabrication by MBE process. It is important to note that an intermediate state (0.3 < x < 1.5), connecting Fe3O4 and FeCr2O4 from inverse to normal spinel, are also identified in this study.
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Acknowledgments
[15] S. Bordacs, D. Varjas, I. Kezsmarki, G. Mihaly, L. Baldassarre, A. Abouelsayed, C.A. Kuntscher, K. Ohgushi, Y. Tokura, Phys. Rev. Lett. 103 (2009) 077205. [16] I. Kim, Y.S. Oh, Y. Liu, S.H. Chun, J.S. Lee, K.T. Ko, J.H. Park, J.H. Chung, K.H. Kim, Appl. Phys. Lett. 94 (2009) 042505. [17] K. Singh, A. Maignan, C. Simon, C. Martin, Appl. Phys. Lett. 99 (2011) 172903. [18] M. Robbins, G.K. Wertheim, R.C. Sherwood, D.N.E. Buchanan, J. Phys. Chem. Solid. 32 (1971) 717. [19] H.J. Yearuan, J.M. Kortright, R.H. Langenheim, J. Chem. Phys. 22 (1954) 1196. [20] C.L. Chen, C.L. Dong, S.M. Rao, G. Chern, M.C. Chen, M. K.Wu, C.L. Chang, J. Phys. Condens. Matter 20 (2008) 255236. [21] Y.S. Liu, J.L. Chen, C.L. Chen, C.L. Dong, D.S. Lee, G. Chern, C.L. Chang, J. Alloy. Comp. 442 (2007) 259–261. [22] S.H. Liu, H.M. Tsai, C.W. Pao, J.W. Chiou, D.C. Ling, W.F. Pong, M.-H. Tsai, H.J. Lin, L.Y. Jang, J.F. Lee, J.H. Hsu, W.J. Wang, C.J. Hsu, Appl. Phys. Lett. 89 (2006) 092112. [23] C.K. Yang, J.W. Chiou, H.M. Tsai, C.W. Pao, J.C. Jan, S.C. Ray, C.L. Yeh, K.C. Huang, H.C. Hsueh, W.F. Pong, M.-H. Tsai, H.H. Hsieh, H.J. Lin, T.Y. Hou, J.H. Hsu, Appl. Phys. Lett. 86 (2005) 062504. [24] D.S. Lee, G. Chern, Phys. B Condens. Matter 407 (2012) 297–301. [25] C. Theil, J. van Elp, F. Folkmann, Phys. Rev. B 59 (1999) 7931. [26] F.M.F. de Groot, J.C. Fuggle, B.T. Thole, G.A. Sawatzky, Phys. Rev. B 42 (1990) 5459. [27] G. Van der Lann, I.W. Kirkman, J. Phys. Condens. Matter 4 (1992) 4189. [28] A. Mirone, M. Sacchi, S. Gota, Phys. Rev. B 61 (2000) 13540. [29] 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 Commun. 109 (1999) 599. [30] T. Kendelewicz, P. Liu, C.S. Doyle, G.E. Brown Jr., E.J. Nelson, S.A. Chambers, Surf. Sci. 424 (1999) 219. [31] G. Chern, C.L. Chang, C.L. Chen, C.L. Dong, J. Vac. Sci. Technol. 17 (4) (1999) 1630. [32] C. Colliex, T. Manoubi, C. Ortiz, Phys. Rev. B 44 (1991) 11402. [33] F.M.F. de Groot, M. Grioni, J.C. Fuggle, J. Ghijsen, G.A. Sawatzky, H. Petersen, Phys. Rev. B 40 (1989) 5715. [34] A.J. Millis, Nature 392 (1998) 147. [35] Y.-C. Jhuang, K.M. Kuo, G. Chern, J. Appl. Phys. 109 (2011) 07D714.
This work was supported by the National Science Council of Taiwan under Contract NSC 102-2112-M-001-004-MY3 and 101-2112-M-213004-MY3. The authors acknowledge the NSRRC staffs, Drs. J. F. Lee and J. M. Chen, for useful discussions and experimental support and the excellent cooperation and the members of Prof. C. L. Chang's laboratory during measurements. References c, J. Fabian, J.D. Sharma, Rev. Mod. Phys. 76 (2004) 323. [1] I. Zuti [2] M. Ziese, Rep. Prog. Phys. 65 (2002) 143. [3] J.M.D. Coey, A.E. Berkowitz, Ll Balcells, F.F. Putris, F.T. Parker, Appl. Phys. Lett. 72 (1998) 734. [4] F. Magalh~ aes, M.C. Pereira, S.E.C. Botrel, J.D. Fabris, W.A. Macedo, R. Mendonca, R.M. Lago, L.C.A. Oliveira, Appl. Catal. Gen. 332 (2007) 115–123. [5] J.-W. Moon, Y. Roh, R.J. Lauf, H. Vali, L.W. Yeary, T.J. Phelps, J. Microbiol. Meth. 70 (2007) 150–158. [6] G. Jie, J. Yuan, Anal. Chem. 84 (6) (2012) 2811. [7] A. Lv, C. Hu, Y. Nie, J. Qu, Appl. Catal. B Environ. 117–118 (2012) 246–252. [8] S. G omez-Quero, F. C ardenas-Lizana, M.A. Keane, Nanotechnology 23 (2012) 294002. [9] S.-W. Cao, Y.-J. Zhu, M.-Y. Ma, L. Li, L. Zhang, J. Phys. Chem. C 112 (6) (2008) 1851. [10] T. Terada, Nature 91 (1913) 213. [11] Y. Yamasaki, S. Miyasaka, Y. Kaneko, J.-P. He, T. Arima, Y. Tokura, Phys. Rev. Lett. 96 (2006), 207204. [12] K. Ohgushi, Y. Okimoto, T. Ogasaeara, S. Miyasaka, Y. Tokura, J. Phys. Soci. Japan 77 (2008), 034713. [13] X. Liang, Y. Zhong, S. Zhu, H. He, P. Yuan, J. Zhu, Z. Jiang, Solid State Sci. 15 (2013) 115–122. [14] X. Liang, Y. Zhong, S. Zhu, H. He, P. Yuan, J. Zhu, Z. Jiang, Chem. Eng. J. 191 (2012) 177–184.
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