The local structure around Ge atoms in Ge-doped magnetite thin films

Abstract Distribution of Ge atoms between tetrahedral and octahedral sites in the spinel-type structure of Fe2.64Ge0.36O4 thin films fabricated by radio frequency sputtering with a composite target of magnetite and Ge has been investigated by extended X-ray absorption fine structure analysis. The local structural changes around the Ge atoms in the films induced by annealing at 573 and 873 K are discussed through comparison of the local structure for sintered crystalline Fe2.7Ge0.3O4 in which Ge atoms preferentially located at the tetrahedral site of the spinel-type structure. This work provides successful information on the structural change with magnetic property of the thin films as follows: the Ge atoms statistically distributed at the tetrahedral and octahedral sites of the as-synthesized films and preferentially occupied the tetrahedral site by annealing at 873 K corresponding to the increase in magnetization.


Introduction
Magnetite thin films have attracted much attention as devices exhibiting magnetic properties [1][2][3]. The properties have been investigated for the magnetite thin films synthesized by deposition methods such as molecularbeam epitaxy [4,5], dc magnetron reactive sputtering using an Fe target in Ar-O 2 gas mixture [6], and the pulsed-laser deposition using a magnetite target [7]. Among those interesting methods, the radio frequency (RF) sputtering method using composite targets is a promising method to easily obtain a film with stoichiometric magnetite film. As an example, magnetite thin films have been produced by the RF sputtering method using composite target of Fe 3 O 4 with 50% of Fe 2 O 3 [8] or Fe 3 O 4 with 5% of FeO [9]. In these cases, the target composition was intentionally deviated from the stoichiometry of magnetite toward oxygen excess or iron excess region to obtain the single-phase stoichiometric magnetite. On the contrary, thin films of magnetite prepared with a magnetite and Ge metal composite target exhibited superior magnetization properties [10], together with excellent resistance to oxidation, in particular ref. [11,12]. These interesting results prompted us to study the local structure around Ge atoms introduced in the magnetite thin films.
Magnetite (Fe 3 O 4 ) has the spinel-type structure expressed by the general structure formula of AB 2 O 4 with eight formula units in a cubic unit cell. A and B cations are located at the special positions 8a (A site) and 16d (B site) of the space group Fd3m. One-eighth of the tetrahedral and half of the octahedral interstices of the cubic-close packed oxide anions (32e) are the A and B sites, respectively. The cations at the A and B sites are coordinated by four and six oxide anions, respectively, with different A-O and B-O interatomic distances. In the case of magnetite, Fe 2+ cations usually prefer to occupy the octahedral B sites, and the structure is classified as an "inverse" spinel type. The structural differences are useful to determine the distribution of cations to the two sites. In this study, we investigate the local structure around the Ge atoms to clarify Ge atom distribution at the A and B sites in the magnetite thin film. substrate by the RF sputtering method using a composite target of 4-in.-diameter magnetite and 5 × 5 mm 2 Ge metal under 2.0 × 10 −3 Torr Ar gas atmosphere for 60 min in deposition time [10]. The obtained thin film sample was subsequently annealed at 573 or 873 K in vacuum for 1 h. X-ray diffraction measurements at a fixed incident angle of 3°were performed for the annealed thin film samples using X-ray diffractometer (Rigaku RINT-2200) with Cu Kα radiation. The chemical composition of the thin films was analyzed by energy dispersive spectroscopy (EDS; EDAX Phoenix).
A bulk sample of Fe 2.7 Ge 0.3 O 4 used for reference was prepared as follows. Appropriate amounts of chemical reagents FeO (99%), Fe 3 O 4 (99.9%), and GeO 2 (99.999%) (Furuuchi Chemical Co., Ltd) were mixed in an agate mortar with a pestle. The powder mixture was shaped in a cylindrical pellet and subsequently heated at 1,273 K in vacuum condition for 48 h. The obtained bulk sample was crushed, and a single crystal particle of 10 μm in diameter was taken for crystal structure analysis by single crystal X-ray diffraction.

Single crystal X-ray diffraction
Single crystal X-ray data were collected using a Rigaku XtaLAB Synergy-S diffractometer equipped with a HyPix-6000HE Hybrid Photon Counting detector and Mo microfocus sealed X-ray source. The data collection strategy was calculated within CrysAlis Pro (Rigaku OD, 2018) [13] to ensure desired data redundancy and completeness. Scattering factors for neutral atoms and anomalous dispersion coefficients were taken from International Tables for Crystallography Volume C (1992) [14]. The least-squares refinement was carried out using the software program SHELXL (Sheldrick, 2008) [15]. Crystal data and structural refinement details are summarized in Table 1.

X-ray absorption spectroscopy
The local structures around the Ge atoms for the film samples and the reference Fe 2.7 Ge 0.3 O 4 bulk sample were characterized by X-ray absorption spectroscopy (XAS). Measurements of X-ray absorption spectra in X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) regions around Ge K absorption edge (11,103 eV = 1.7789 × 10 −15 J) in a fluorescence-yield mode were carried out using laboratory X-ray absorption spectrometer (R-XAS Looper; Rigaku Co., Ltd) [16]. White X-ray emitted from an unsealed X-ray tube with a Mo target and LaB 6 filament was monochromized using a Johansson-type bent monochromator with Si 620 for XANES and Si 400 for EXAFS measurements. The monochromized X-ray was irradiated on the sample pass through an Ar gas-sealed proportional counter as an intensity monitor for incident X-ray. The Fe and Ge Kα fluorescence and elastic scattering X-rays emitted from the sample were detected with a silicon drift detector (XSDD50-01Be; TechnoAP Co. Ltd). The Ge Kα fluorescence was electrically filtered by a digital spectrometer (APU101X; TechnoAP Co. Ltd). The collected XAFS spectra were characterized using the Demeter package [17] including software Athena for data processing and Artemis for determination of structure parameters with FEFF 6 code [18].

Results and discussion
3.1 Thin film formation Figure 1 shows the X-ray diffraction patterns of the assynthesized and annealed thin film samples. These pattern profiles indicated that all thin film samples consist only of crystalline phase of the spinel structure. The At first, we started to refine the structural model with the random distribution of the Ge atoms both at the tetrahedral A and octahedral B sites with the chemical constraints. Converged occupation parameters for Ge at the A and B sites were 0.263(4) and 0.019, respectively. These results readily indicated the preferred distribution of Ge at the A site. The final positional and displacement parameters are presented in Table 2. The values of structural parameters of magnetite [19] are also summarized in Tables 1 and 2. The calculated interatomic distances are listed in Table 3 together with the reported values of Fe 2 GeO 4 [20]. It should be added that the preferred residence of Ge at the A site was also observed in the case of brunogeierite Fe 2+ 2 GeO 4 , from Tsumeb, Namibia [20]. The results of single crystal X-ray diffraction of Fe 2.7 Ge 0.3 O 4 suggest the mixed occupation of Ge and Fe atoms at the tetrahedral A site. Because the discrimination of Fe 2+ and Fe 3+ is difficult in the case of the ordinary single crystal X-ray diffraction, the present paper discusses their distribution based on the interatomic distances realized in the crystalline structure. In magnetite structure, the tetrahedral A sites are occupied by Fe 3+ and the octahedral B sites are by Fe 2+ and Fe 3+ [19]. This information encourages us to simulate the tetrahedral A-O distance of the present sample using the distances of Ge-O(IV) and Fe 3+ -O(IV) of brunogeierite and magnetite together with the occupation parameter of 0.263. The calculated tetrahedral A-O distance of 0.1858 nm agreed well with the observed 0.18574 (8)  On the contrary, the XANES spectrum profiles of the as-synthesized and 573 K annealed films were similar to each other, but different from the profile of the 873 K annealed film. These results readily suggest that local atomic environment around Ge in the as-synthesized film is not similar to that coordinated tetrahedrally by four oxygens, and that the local structure is not changed by annealing at relatively low temperature. The k 3 -weighted EXAFS spectra obtained from the X-ray absorption measurements at Ge K absorption edge for the Fe 2.64 Ge 0.36 O 4 thin film samples and reference Fe 2.7 Ge 0.3 O 4 are shown in Figure 3. The oscillation feature of the thin film sample annealed at 873 K was again quite similar to that of Fe 2.7 Ge 0.3 O 4 . On the contrary, the k 3 -weighted EXAFS spectra of the film samples as-synthesized and annealed at 573 K showed different behavior together with rather broad oscillation nature at the wave vector region over 100 nm −1 , in particular. These results suggest the appearance of relatively complex structure with several additional interatomic correlations for the two film samples, which differ from that found in Fe 2.7 Ge 0.3 O 4 with the strong preference of tetrahedrally coordinated Ge (see Appendix).
The radial structure functions (RSFs) calculated from the k 3 -weighted EXAFS spectra in Figure 3 are shown in Figure 4. In the region of nearest neighboring Ge-O correlation of RSFs for the thin film samples as-synthesized and annealed at 573 K, a small shoulder b appeared at the farther side of Ge-O(IV) peak a corresponding to 0.1767 nm in distance determined by EXAFS analysis (see Table A1 in Appendix). In the region of cation-cation correlations, a shoulder e appeared at the closer side of A-B correlation peak c corresponding to 0.34845 nm in distance calculated from the lattice constant of    Table A1 and the calculated k 3 -weighted EXAFS spectrum including all correlations and the spectra corresponding to contribution of each correlation are shown in the left side of Figure A1 with the RSFs obtained by Fourier transformation of the EXAFS spectra in the wave number region from 28 to 128 nm −1 on the right side. For the region of cation-cation correlations in the RSF, calculated profile using the structure parameter values for A-B correlation c and A-A correlation d obtained from the lattice constant was in good agreement with the observed one. The nearest neighboring Ge-O (correlation a) distance (0.1767 nm) obtained by the least-squares fitting showed good agreement with the Ge-O(IV) distance realized in brunogeierite Fe 2+ 2 GeO 4 .