Mössbauer characterization of Fe-doped ZnO prepared by mechanical milling

Transcription

1 Hyperfine Interact (2007) 176:87 92 DOI /s Mössbauer characterization of Fe-doped ZnO prepared by mechanical milling L. Baum M. Meyer D. Richard L. C. Damonte L. Mendoza-Zélis Published online: 6 August 2008 Springer Science + Business Media B.V Abstract The preparation of massive oxides Zn 1 x Fe x O by means of mechanical milling in diverse gaseous atmospheres and starting with different powder mixtures was investigated. ZnO powder with 10 at.% admixture of Fe, FeO or Fe 2 O 3,were milled during 1, 4 or 16 h and characterized by X-ray diffraction and Mössbauer spectroscopy. Different phases were obtained according to the initial conditions (precursor used, atmosphere, etc.) and phase s formation enthalpies. Keywords Mechanical milling Mössbauer spectroscopy Fe-doped ZnO Diluted magnetic semiconductors 1 Introduction The wide band semiconductor ZnO, when doped with a small amount of a 3d transition metal (Mn, Co, Fe or Ni), may show room temperature ferromagnetism. In spite of abundant work on the subject, the mechanism responsible for such singular behaviour is not yet well understood and experimental evidences for the magnetic character are not conclusive [1]. Various methods like pulsed laser deposition, L. Baum M. Meyer D. Richard L. C. Damonte L. Mendoza-Zélis Departamento de Física, Universidad Nacional de La Plata, C.C.67 (1900), La Plata, Argentina L. C. Damonte (B) Departamento de Física Aplicada, Universidad Politécnica de Valencia, Camí de Vera s/n, 46071, Valencia, Spain damonte@fisica.unlp.edu.ar

2 88 L. Baum et al. electrodeposition and other chemical routes were used to prepare thin films of these materials [1, 2]. In a previous work [3] we have reported the preparation of Zn 1 x Co x O powders by mechanical alloying, a useful and versatile solid-state powder processing technique [4], and their magnetic characterization. Continuing that work and with the aim to contribute to the comprehension of the magnetic behaviour of these semiconductors, we start an investigation about the preparation of massive ternary oxides Zn 1 x Fe x O by means of mechanical milling in diverse gaseous atmospheres and starting with different powder mixtures. In the present case, 10 at.% α-fe or FeO or Fe 2 O 3 was added to ZnO and milled under air or Ar. The structural evolution with milling time was studied by X-ray diffraction, scanning electron microscopy (SEM) and Mössbauer spectroscopy in samples milled 1, 4 and 16 h. The aim of the present work is to look for the optimal conditions (atmosphere, starting materials, milling time) for the synthesis of Fe-doped ZnO semiconductor powders. These preliminary results will orient our future search of ferromagnetic behaviour in such mechanically alloyed semiconducting oxides. 2 Experimental The samples were prepared from mixtures of ZnO (Alfa Aesar, Johnson Matthey Co., 99.99) with 10 at.% of α-fe (Merk, 99.5) or FeO (Sigma Aldrich, 99.9, 10 mesh)orfe 2 O 3 (Johnson Matthey Co., 99.99, 15 mesh) powders. The samples with FeO and Fe were manipulated in a controlled atmosphere chamber (O 2 content less than a few parts per million). The starting powders were introduced in a cylindrical steel milling chamber together with one steel ball (φ =12mm),filledwithArat 0.2 MPa (air in the case of Fe 2 O 3 admixture) and sealed with an O ring. Each sample weighs 700 mg giving a ball to sample mass ratio 11.5:1. Milling was carried on with a horizontal oscillatory mill Retsch, at a fixed oscillation frequency of 32 Hz. X-ray diffraction patterns were obtained with CuK α radiationinthe20 2θ 80 range at 0.02 /s, using a Philips PW1710 diffractometer. Mössbauer spectra were measured using a conventional constant acceleration spectrometer at room temperature employing a 57 CoRh source, in transmission geometry. All quoted isomer shifts are given relative to α-fe. The morphology of the powders were also analyzed with a scanning electron microscopy (SEM), JSM-6300 (JEOL Scanning Microscopy) operating at 10 kv. 3 Results and discussion 3.1 XRD measurements Figure 1 shows the measured diffractograms. In all cases, the reflections coming from the added materials are still present after 1 h of milling; they are hardly discernible after 4 h and have completely disappeared after 16 h. In the sample with Fe 2 O 3,the spinel ZnFe 2 O 4 forms after prolonged milling. On the other hand, in the samples with FeO or Fe admixture, iron substitution in the wurtzite hexagonal structure (P6 3 mc) seems to occur after 16 h milling. In all cases, as milling proceeds, the diffraction

3 Mössbauer characterization of Fe-doped ZnO 89 ZnO FeO α -Fe ZnFe 2 O 4 Fe 2 O 3 Intensity ZnO+Fe 2 O 3 ZnO+FeO 1h 4h 16h 1h 4h 16h ZnO+α -Fe θ (degrees) 1h 4h 16h Fig. 1 XRD patterns for powders of ZnO milled during 1, 4 and 16 h with 10 at.% Fe, FeO and Fe 2 O 3 Table 1 Crystallite sizes and strains in samples milled 16 h, obtained from the XRD patterns with the Hall Williamson model Sampl Phase Size (nm) Strain ( 10 3 nm) ZnO + Fe 2 O 3 ZnFe 2 O 4 51 ± ± 0.02 ZnO 38 ± ± 0.02 ZnO + FeO ZnO 22 ± ± 0.01 ZnO + Fe ZnO 19 ± ± 0.01 peaks from ZnO broaden as a result of grain size reduction. From these results crystallite sizes and strains were evaluated using the Hall Williamson model for the 16 h ball-milled samples. The obtained parameters are shown in Table SEM images The microstructure of the different samples was analyzed by SEM. Figure 2 shows the SEM images at different milling times for mixture of hematite and alpha iron. Grain size reduction and homogenization is achieved for prolonged milling times. It is clearly observed (Fig. 2) that a different morphology is obtained when a spinel structure was formed.

4 90 L. Baum et al. a) b) c) a) b) c) Fig. 2 Images by Scanning electron microscopy for powders of ZnO doped with: top, Fe 2 O 3 after a 1, b 4andc 16 h of grinding; bottom, α-fe after a 1, b 4andc 16 h of grinding 3.3 Mössbauer spectroscopy A progressive reduction of the Fe 2 O 3, FeO and α-fe spectral contributions with milling time is observed (the corresponding spectra are not shown). In all cases, no absorption signal coming from Fe in the added phase, Fe, FeO or Fe 2 O 3 are observed in the MS spectra after 16 h milling (see Fig. 3). In the case of Fe 2 O 3,two interactions are observed, the main one corresponding to Fe in the spinel ZnFe 2 O 4 [5, 6]. The second one cannot be univocally assigned, although it may represent an oxygen deficient Fe environment or other possibilities discussed below. For samples with Fe and FeO the spectra are similar showing also two interactions, one to be assigned to Fe ions in the Zn sites within the zincite ZnO phase (site I). As shown in Fig. 3 two kinds of fits, each with two doublets, are possible and it is hard to decide among them with present statistics. The fitted parameters are shown in Table 2. For type A fits the quadrupole interaction assigned to Fe in ZnO (site I) would have δ = 0.7 mm/s and = 1.7 mm/s. While for type B, it would be δ = 1.1 mm/s and = 1.0 mm/s. The later values are consistent with Fe 2+ ions in tetrahedral coordination, as expected for substitutional sites in ZnO, and are similar to those observed by Ahn et al. [7] in samples prepared by solid state reaction at 1200 C. On the other hand, our spectra are similar to those reported recently by Lin et al. [8] for Fe-doped ZnO prepared by mechanical milling at low iron concentrations. The authors also fit their spectra with two quadrupole doublets assigning them to Fe +2 and Fe +3 cations at Zn sites in the zincite ZnO phase. The second one might be attributed to Fe in a cation site of wurtzite structure with oxygen vacancies (site II). This may offer an alternative interpretation to the

5 Mössbauer characterization of Fe-doped ZnO 91 ZnO+0.1Fe3O2 16h fit A ZnO+0.1Fe mm 16h Counts (a.u.) fit B ZnO+0.1Fe mm 16h fit A ZnO+0.1FeO mm 16h fit B ZnO+0.1FeO mm 16h v (mm/s) Fig. 3 Mössbauer spectra for: a spinel ZnFe 2 O 4, b ZnO + Fe 2 O 3, c ZnO+FeOandd ZnO + α-fe. The results from least-squared fitting procedure (explain in text) are also shown observed second interaction in all the studied cases. It is worth to mention that no magnetic interactions were observed in any of our samples. On the contrary, ferromagnetic sextets have been observed by Mössbauer spectroscopy in Fe-implanted ZnO samples, as reported very recently [9, 10].

6 92 L. Baum et al. Table 2 Fitted hyperfine parameters for the Mössbauer spectra of 16 h ball-milled samples Sample Phase Isomer shift Quadrupole splitting Line width Fraction (%) δ (mm/s) Δ (mm/s) Ɣ (mm/s) ZnO + Fe 2 O 3 ZnFe 2 O ± ± ± ± 8 Site II 0.46 ± ± ± 17 ZnO + FeO Site I 0.72 ± ± ± ± 6 Fit A Site II 0.48 ± ± ± ± 7 ZnO + FeO Site I 1.07 ± ± ± ± 5 Fit B Site II 0.01 ± ± ± ± 4 ZnO + α-fe Site I 0.73 ± ± ± ± 10 Fit A Site II 0.45 ± ± ± ± 17 ZnO + α-fe Site I 1.05 ± ± ± ± 8 Fit B Site II 0.07 ± ± ± ± 10 4 Conclusions Ternary nanocrystalline Zn 1 x Fe x O oxides were successfully synthesized by mechanical work starting from ZnO with α-fe or FeO admixtures. The observed MS parameters are within the expected values for such oxides. No definitive conclusions could be arrived at for the secondary observed phase. On the other hand, when the starting material is Fe 2 O 3 the formation of the spinel structure could not be avoided and Fe does not enter in the wurtzite structure. In all cases, no magnetic structures were found in the studied composition range and further work in samples with lesser proportion of Fe, eventually using enriched 57 Fe, is needed. References 1. Coey, J.M.D., Venkatesan, M., Fitzgerald, C.B.: Nat. Mater. 4, 172 (2005) 2. Norton, D.P., Pearton, S.J., Hebard, A.F., Theeodoroponlow, N., Boatner, L.A., Wilson, R.G.: Appl. Phys. Lett. 82, 239 (2003) 3. Damonte, L.C., Hernández-Fenollosa, M.A., Meyer, M., Mendoza-Zélis, L., Marí, B.: Physica B: Condens. Matter 398, (2007) 4. Suryanarayana, C.: Prog. Mater. Sci. 46, (2001) 5. Verdier, T., Nachbaur, V., Jean, M.: J. Solid State Chem. 178, (2005) 6. Chinnasamy, C.N., Narayanasamy, A., Ponpandian, N., Chattopadhyay, K., Guérault, H., Greneche, J.-M.: J. Phys.: Condens. Matter 12, (2000) 7. Ahn, G.Y., Park, S.I., Kim, S.J., Kim, C.S.: J. Magn. Magn. Mater. 304, e498 e500 (2006) 8. Lin, Y., Jiang, D., Lin, F., Shi, W., Ma, X.: J. Alloys Compd. 436, (2007) 9. Potzger, K., Zhou, S., Reuther, H., Mücklich, A., Eichhom, F., Schell, N., Skorupa, W., Helm, M., Fassbender, J., Hermannsdörfer, T., Papageorgiou, T.P.: Appl. Phys. Lett. 88, (2006) 10. Weyer, G., Gunnlaugsson, H.P., Mantovan, R., Fanciulli, M., Naidoo, D., Bharuth-Ramand, K., Agne, T.: J. Appl. Phys. 102, (2007)