Mössbauer study on Zn 1 x Fe x O semiconductors prepared by high energy ball milling

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1 Hyperfine Interact (21) 195: DOI 1.17/s Mössbauer study on Zn 1 x Fe x O semiconductors prepared by high energy ball milling L. C. Damonte M. Meyer L. Baum L. A. Mendoza-Zélis Published online: 11 November 9 Springer Science + Business Media B.V. 9 Abstract We present the preparation of massive Zn 1 x Fe x O ternary oxides using the mechanical mill. The Fe atom is a particular dopant since it presents two different oxidation states which allow us to vary the starting materials: Fe 2 O 3, α-fe or FeO. Parameters such as initial concentrations, atmosphere and milling times were varied. X-ray diffraction and 57 Fe Mössbauer spectrometry (MS) were applied in order to analyze the structure evolution and iron incorporation in the wurtzite crystalline structure with milling time. At final stages, Fe atoms seem to be incorporated in the ZnO structure for those samples milled under Ar atmosphere. In all cases, two paramagnetic components, attributed to Fe atoms in both valence states, were observed by MS. Keywords Mechanical milling Mössbauer spectroscopy Fe-doped ZnO Diluted magnetic semiconductors 1 Introduction During the last years, the wide bandgap semiconductor ZnO doped with small quantities of magnetic atoms (Mn, Co, Fe and Ni) has attracted the attention since it may show room temperature ferromagnetism. However, the mechanism responsible of such singular ferromagnetic behaviour is not yet well understood and controversial experimental evidences of the magnetic character were reported [1 6]. L. C. Damonte M. Meyer L. Baum L. A. Mendoza-Zélis Departamento de Física, Universidad Nacional de La Plata, C.C.67 (19), La Plata, Argentina L. C. Damonte (B) IDF, Departamento de Física Aplicada, Universidad Politécnica de Valencia, Camí de Vera s/n, 4671, Valencia, Spain damonte@fisica.unlp.edu.ar

2 228 L.C. Damonte et al. Fig. 1 XRD patterns for ZnO powders with 5 at.% α-fe, FeO and Fe 2 O 3 after 16 h of milling under Ar atmosphere.column bar indicates the diffraction peaks for ZnO hexagonal structure (P6 3 mc), filled triangle corresponds to Fe 2 O 3 phase Fe 2 O 3 ZnO ZnO+5% Fe 2 O 3 Intensity (a.u.) ZnO+5% FeO ZnO +5% Fe θ We initiated our work in these diluted magnetic semiconductors by preparing Co-doped ZnO samples [7] and massive Zn 1 x Fe x O ternary oxides [8] usingthe mechanical mill. The Fe atom is a particular dopant since it presents two different oxidation states which allow us to vary the starting materials: Fe 2 O 3, α-fe or FeO. On the other hand, high energy ball milling has proved to be a useful and versatile solid-state powder processing technique [9] which has been previously used with similar aims [1, 11]. Continuing that work and contributing to the comprehension of the magnetic behaviour of these semiconductors, we investigated the preparation of massive ternary oxides Zn 1 x Fe x O using mechanical work in Ar atmosphere and different starting powder mixtures. Progressive iron incorporation in the wurtzite crystalline structure was studied by X-ray diffraction. 57 Fe Mössbauer spectrometry was applied in order to characterize the different ion phases formed and their magnetic properties. 2 Experimental Samples from mixtures of ZnO (Alfa Aesar, Johnson Matthey Co., 99.99) with 5 and 1 at.% of α-fe (Merk, 99.5) or FeO (Sigma Aldrich, 99.9, 1 mesh) or Fe 2 O 3 (Johnson Matthey Co., 99.99, 15 mesh) powders were prepared. All samples were

3 Zn 1 x Fe x O semiconductors prepared by high energy ball milling 229 Table 1 Lattice parameter (a, b) for doped-zno after 16 h of milling obtained from Rietveld refinement (determined from the fit with best agreement factors) Sample a(å) b(å) ZnO + Fe ZnO + FeO ZnO + Fe 2 O ZnO as received ZnO m.m. 1 h ZnO m.m. 5 h Results for non milled and milled pure ZnO (m.m. stands for mechanical milled) are included for comparison [8] Fig. 2 XRD patterns for ZnO powders with 1 at.% Fe 2 O 3 after 16 h of milling under air (top) orar(bottom) atmosphere. Column bar indicates the diffraction peaks for ZnO hexagonal structure (P6 3 mc), filled triangle corresponds to Fe 2 O 3 phase and filled circle corresponds to spinel ZnFe 2 O 4 phase Intensity (a.u.) in Air in Ar hexagonal ZnO Fe 2 O 3 ZnFe 2 O 4 ZnO+1% Fe 2 O θ manipulated in a Controlled Atmosphere Chamber (O 2 content less than a few ppm), introduced in a cylindrical steel milling chamber together with one steel ball (φ = 12 mm), filled with Ar at.2 MPa and sealed with an O ring. The ball to sample mass ratio was 11.5:1 and progressive milling was carried on using a horizontal oscillatory mill Retsch, at a fixed oscillation frequency of 32 Hz. X-ray diffraction patterns were obtained with CuK α radiation in the 2 2θ 8 range at.2 /s using a Philips PW171 diffractometer. 57 Fe 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.

4 23 L.C. Damonte et al. Fig. 3 Mössbauer spectra for: a ZnO+5 at.% FeO, b ZnO+5 at.% Fe 2 O 3, and c ZnO+1 at.% Fe 2 O 3 milled in Ar atmosphere a b c v (mm/s) data total fit ZnO(I) ZnO(II) hematite 3 Results and discussion 3.1 XRD measurements For all samples, as milling proceeds two main features can be observed: broadening of diffraction peaks due to grain size reduction and progressive disappearance of the initial added Fe-based materials. Figure 1 shows the resulting X-ray patterns for ZnO powders samples with 5 at.% α-fe, FeO and α-fe 2 O 3 after 16 h of milling. The characteristic diffraction peaks of ZnO hexagonal structure (P6 3 mc) are observed for all samples. For those samples milled with hematite, a slight contribution of this phase can still be appreciated. Lattice parameters obtained from Rietveld refinements show no significant changes with iron content with the exception of a slight increase in parameter a (Table 1) a similar behavior observed previously for milled pure ZnO [8]. Another sample of ZnO with 1 at.% Fe 2 O 3 was also prepared in Ar atmosphere, in order to compare with previous results done under air conditions [9]. While after prolonged mechanical milling in air the spinel structure ZnFe 2 O 4 forms, instead under Ar, the wurtzite structure is obtained (Fig. 2).

5 Zn 1 x Fe x O semiconductors prepared by high energy ball milling 231 Table 2 Fitted hyperfine parameters for the Mössbauer spectra of 16 h ball-milled samples Sample Phase Isomer Quadrupole Line width Hyperfine Absorption or site shift δ splitting Ɣ (mm/s) magnetic area (mm/s) or shift 2ε field (T) proportion (mm/s) (%) ZnO+5%FeO ZnO(I).25 ±.3.82 ±.5.6 ±.6 46 ± 17 ZnO(II).87 ± ±.4.6 ±.6 54 ± 17 ZnO+5%Fe 2 O 3 ZnO(I).26 ±.1.92 ±.2.61 ±.2 46 ± 6 ZnO(II).97 ± ±.3.61 ±.2 25 ± 7 Fe 2 O 3.38 ±.2.21 ± ±.2 29 ± 6 ZnO+1%Fe 2 O 3 ZnO(I).26 ±.1.86 ±.1.57 ±.1 82 ± 4 ZnO(II).89 ± ±.9.57 ±.1 7 ± 5 Fe 2 O 3.4 ±.4.23 ± ±.2 11 ± 4 ZnO+1%Fe 2 O 3 ZnO(I).32 ±.2.57 ±.3.58 ±.6 45 ± 1 milled in air [9] ZnFe 2 O 4.47 ± ±.3.51 ±.2 56 ± Mössbauer spectrometry For all samples, a progressive evolution with milling time from the characteristic hyperfine features for FeO or Fe 2 O 3 to two similar paramagnetic signals is observed. In Fig. 3, the Mössbauer spectra corresponding to the most prolonged milling time for ZnO mixtures with 5 at.% FeO and Fe 2 O 3, are displayed. Both cases can be successfully described with similar quadrupole interactions. For the case of hematite less than a 3% of Fe atoms experienced the characteristic sextet for this oxide, assuming the same values of recoilless f factor. In consequence, there is a good agreement with X-ray diffraction results. The resulting hyperfine parameters obtained from least squared fittings are displayed in Table 2. The observed two quadrupole interactions can be identified with two ion sites in the wurtzite ZnO structure in Fe 2+ (ZnO(I)) and Fe +3 (ZnO(II)) state. These values are consistent with the proposed fit B in a previous work on mixtures of ZnO with 1% of additional Fe-based phase [9]. The (ZnO(I)) called values are consistent with Fe 2+ ions in tetrahedral coordination, as expected for substitutional sites in ZnO. Similar results were observed by Ahn et al. [12] in samples prepared by solid state reaction at 1, C and Lin et al. [9] for Fe-doped ZnO prepared by mechanical milling at low iron concentrations. As it was discussed by Verdier et al. [5] ontheformationonfe 2+ during milling hematite with ZnO to obtain the spinel ZnFe 2 O 4, we must analyzed the presence of both valence state of Fe in our results. Initially Fe is on the valence state Fe +2 in FeO and Fe +3 in Fe 2 O 3 andinstateif coming from the milling tools. At the final step of milling both valence states are found for both experiments, and when the milling was done under air as well. It was argued [5] that ZnO presence and tools contamination may be responsible of such redox reduction reaction, but since these facts occur independently of atmosphere and initial precursors, we state that the mechanical work itself, by the introduction of different kinds of defects, does originates the variety of valence states in Fe ions. However some additional experiments and ab initio calculations will be done in order to elucidate this point. Under air milling conditions favour the formation of spinel structure ZnFe 2 O 4,in agreement with results of previous section. Figure 4 exhibits the Mössbauerspectrum

6 232 L.C. Damonte et al. Fig. 4 Mössbauer spectra for ZnO+1 at.%fe 2 O 3 milled in air [8] exp spinel undet. fit v (mm/s) of ZnO+1 at.%fe 2 O 3 after 16 h of air-milling: the obtained parameters agree with those reported in literature for this phase [13]. 4 Conclusions Fe-doped ZnO oxides were successfully synthesized by high energy ball milling under Ar atmosphere irrespective of concentration and kind of starting materials (FeO or α-fe 2 O 3 ). Among the milling conditions, the atmosphere is the more important parameter for the final product phases. Fe atoms with different ionization states, substitute Zn atoms in tetragonal sites leading to two quadrupole splitting sites: Zn(I) Fe +2 and ZnO(II) Fe +3. In all cases, no final magnetic hyperfine structures were observed in the studied composition range. References 1. Coey, J.M.D., Venkatesan, M., Fitzgerald, C.B.: Nat. Matters 4, 172 (5) 2. Norton, D.P., Pearton, S.J., Hebard, A.F., Theeodoroponlow, N., Boatner, L.A., Wilson, R.G.: Appl. Phys. Lett. 82, 239 (3) 3. Lin, Y., Jiang, D., Lin, F., Shi, W., Ma, X.: J. Alloys Compd. 436, 3 33 (7) 4. 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, 5258 (6) 5. Weyer, G., Gunnlaugsson, H.P., Mantovan, R., Fanciulli, M., Naidoo, D., Bharuth-Ramand, K., Agne, T.: J. Appl. Phys. 12, (7) 6. Ahn, G.Y., Park, S.-I., Kim, C.S.: J. Magn. Magn. Mater. 33, e329 e331 (6) 7. Damonte, L.C., Hernández-Fenollosa, M.A., Meyer, M., Mendoza-Zélis, L., Marí, B.: Phys. B Condens. Matter 398, (7) 8. Damonte, L.C., Mendoza-Zélis, L., Marí, B., Hernández-Fenollosa, M.A.: Powder Technol. 148, (4)

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