Magnetic and structural study of Fe doped tin dioxide

ARTICLE IN PRESS Physica B 404 (2009) 2834–2837

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Magnetic and structural study of Fe doped tin dioxide V. Bilovol, A.M. Mudarra Navarro, C.E. Rodrı´guez Torres, F.H. Sa´nchez, A.F. Cabrera  Departamento de Fı´sica, Facultad de Ciencias Exactas, UNLP-IFLP-CCT, La Plata-CONICET, C.C 67, 1900 La Plata, Argentina

a r t i c l e in f o

Keywords: Semiconductor oxides Magnetic properties XAS

a b s t r a c t The structural and magnetic properties of Fe 10 at% doped SnO2 powders milled for different times have been investigated. XAS results demonstrate the dilution of Fe atoms in the rutile structure after 5 h of milling. Fe atoms have almost one oxygen vacancy near neighbour. At RT the sample presents the superposition of paramagnetic and ferromagnetic behaviours. When temperature decreases a progressive blocking process was observed. Below 100 K a field shift of hysteresis loops is evident indicating magnetic coupling between ferromagnetic/antiferromagnetic phases. Published by Elsevier B.V.

1. Introduction The origin of ordered magnetism in oxides of diluted magnetic semiconductors (DMSs) is a basic phenomenon not yet understood. It was found that there is a strong dependence of the magnetic properties of these diluted magnetic semiconductors on synthesis conditions and on their dimensionality. SnO2 doped with Fe is one of such DMSs. This system has been prepared by different methods such as chemical synthesis (powder samples) [1,2], pulsed-laser deposition (thin films) [3] or sol–gel synthesis (powders) [4]. In this work we explore the possibility of introducing the Fe atoms in the SnO2 rutile structure by mechano-synthesis. Different milling times were used to prepare the powders which were characterized by X-ray absorption spectroscopy (XAS) and magnetic measurements.

2. Experimental part Powders of rutile SnO2 and a-Fe2O3 (99.9% purity) with a nominal composition of 10 at% of Fe were mixed by mechanical alloying. The milling conditions are the same reported in Ref. [5]. Fresh powders were used for preparing 0.25, 0.5, 1, 1.5, 2 and 5 h milled samples. The magnetic characterizations were performed using standard SQUID equipment in the range from 5 to 300 K and applying a magnetic field of up to 5 T. The XAS measurements (EXAFS—extended X-ray absorption fine structure and XANES— X-ray absorption near edge structure) were carried out at room temperature (RT) in transmission mode at the Fe K-edge, using the Si (111) monochromator at the XAS1 beamline of LNLS  Corresponding author.

E-mail address: cabrera@fisica.unlp.edu.ar (A.F. Cabrera). 0921-4526/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.physb.2009.06.099

(Campinas, Brazil). The samples were previously characterized by X-ray diffraction (XRD) and Mo¨ssbauer Spectroscopy (MS) [5].

3. Results and discussion The XRD patterns of all samples showed reflections corresponding to the polycrystalline SnO2 rutile structure [5]. Reflection lines of hematite and bcc-Fe phases were also detected for short milling times (0.25, 0.5, 1 and 1.5 h). The Mo¨ssbauer spectra taken at RT confirmed these results, but, besides, revealed the existence of 4% of bcc-Fe and no traces of hematite in the sample milled during 2 h. No magnetically split subspectra were detected for 5 h milling time [5]. XANES spectra of samples milled 0.25, 0.5 and 1 h and of the aFe2O3 reference (not shown here) are qualitatively similar to the hematite one but with an increase of the pre-peak and a decrease of the white line. These facts reflect the presence of a fraction of metallic iron that was revealed by the MS results. No similarities between spectra of samples milled for more than 1 h and the reference oxides were found by XANES. Intermediate absorption energies between FeO and a-Fe2O3 references (see Fig. 1) were observed, indicating an oxidation state between 2+ and 3+. Fourier transformations (FT) of EXAFS spectra are shown in Fig. 2. Up to 0.5 h Fe atoms are mainly in the rhombohedral structure of hematite. We observed a reduction of the amplitude of the first peak (corresponding to Fe–O coordination) with milling time. The intensity of the peaks between 2 and 4 A˚ (corresponding to the second and the third coordination layers) also decreases with milling time. This fact could not be assigned only to the reduction of coordination number due to a grain size decrease but also to a substantial disorder around Fe atoms. After milling for 1 h peaks corresponding to the second and the third neighbours atoms in hematite practically disappear and peaks around 2.1 and 4.3 A˚ that can be assigned to Fe in bcc-Fe appear

ARTICLE IN PRESS V. Bilovol et al. / Physica B 404 (2009) 2834–2837

Absorption (a.u.)

1.5

1.0

Fe2O3

0.5

FeO 1.5 h 2h 5h

0.0 7110

7120

7130

7140

E (eV) Fig. 1. XANES spectra for samples milled 1.5, 2 and 5 h compared with FeO and aFe2O3 references.

α−Fe2O3

FT (k2χ (k))

1.2

0.8

0.25 h 0.5 h 1h 1.5 h 2h 5h

bcc-Fe

bcc-Fe

0.4

We performed a fit of the 5 h milled sample spectrum, with four layers (see Table 2). We choose this sample because no evidence of spurious phases was detected by XAS, XRD and Mo¨ssbauer measurements. In Table 2 the obtained parameters from this fit together with the theoretical parameters for Sn in SnO2 are presented. The experimental and fitted curves are shown in Fig. 3. These results demonstrate that Fe atoms replace substitutionally Sn atoms in the rutile structure and indicate that the preparation conditions were optimal to promote the dilution of Fe in the rutile structure. Room temperature magnetic measurements on all samples present ferromagnetic (FM)-like behaviour (not shown here). The loops were fitted with one ferromagnetic and one paramagnetic (PM) component. The FM component was simulated using the function reported by Stearns [7] and the PM one by a straight line.

Table 1 Fitting results for the first oxygen shell around the Fe atom.

Fe2O3 0.25 h 0.5 h 1h 1.5 h 2h 5h

2

4 R (Å)

NN

R (A˚)

6 5.43 5.02 4.44 4.26 3.95 4.83

1.998 1.9992 2.0053 2.0042 1.9734 1.9783 1.9984

NN is the number of oxygen near neighbours located at a distance R from the central atom. The obtained value for s2 was 0.00834.

Table 2 Fitting parameters obtained from the fit of the 5 h milled sample spectrum. Atom type

Sample milled 5 h

O Sn O Sn

0.0

2835

Rutile-SnO2

R (A˚)

N

R (A˚)

N

1.9854 3.0942 3.6181 3.6856

4.83 2.47 5.82 7.93

2.056 3.186 3.591 3.709

6 2 4 8

N is the number of atoms located at a distance R from the central atom. Theoretical parameters for Sn in SnO2 are included for comparison purposes.

Fig. 2. Fourier transformation (FT) of k2w(k) obtained from 0.25, 0.5, 1, 1.5, 2 and 5 h milled samples and a-Fe2O3 reference in the interval 2–11 A˚ 1.

Data Fit 0.8

FT (k2χ (k))

(confirming the Mo¨ssbauer results [5]). After milling for 1.5 h no hematite peaks corresponding to the third and following coordinations are observed. The first and second hematite peaks coincide with those corresponding to Fe in SnO2 and in bcc-Fe, respectively, and could not be distinguished from them. This result indicates that after 1.5 h of milling a very low quantity of the starting hematite may remain in the samples. The IFEFFIT program [6] was used to fit the first shell data (Fe–O) with the parameters s2 (Debye Waller factor) and DE0 (shift in energy) correlated for all samples. In Table 1 the fitted parameters are shown. It can be observed that NN decreases with milling time up to 2 h. In the cases of samples milled 1, 1.5, and 2 h, the decrease could be reinforced due to the presence of a-Fe (no Fe–O exists). After 5 h of milling (no a-Fe presence) NN increases with regard to the 2 h milled sample but it remains lower than 6 (oxygen coordination of Sn in SnO2 structure) indicating the presence of oxygen vacancies as it is expected from charge balance.

0.4

0.0 0

2

4

6

8

R (Å) Fig. 3. The experimental and fitted curves of the sample milled for 5 h.

ARTICLE IN PRESS V. Bilovol et al. / Physica B 404 (2009) 2834–2837

M (emu/g)

5

0 300 K 100 K 10 K 5K

-5

0.024 0.022 0.020 M (emu/g)

The existence of a RT ferromagnetic component in samples milled for 0.25, 0.5, 1, 1.5 and 2 h was expected due to the presence of metallic iron. Although the sample with the highest quantity of bcc-Fe (about 18% from MS data) is the 1 h milled one, the sample with the highest saturation magnetization was the sample milled during 1.5 h. The 1.5 h sample extra contribution may come from a small amount of MS undetected FM ordered Fe3+ or Fe2+. From the analysis of the hysteresis loops two stages of the evolution of the samples with the milling time can be distinguished. A first step with a contribution to the magnetization from bcc-Fe phase and a second one in which the magnetization value decreases due to the disappearance of the phases mentioned above. For 5 h milling time the magnetization curve does not saturate and the PM contribution becomes more significant. According to the structural and hyperfine results the 5 h milled sample is the unique in which the Fe atoms are totally incorporated into the rutile structure, and then it is the best sample to study the magnetic behaviour. Consequently M vs H measurements were performed at 300, 200, 100, 50, 10 and 5 K (some of them are shown in Fig. 4). The hysteresis loops were reproduced with two contributions: a FM component plus a PM one, the latter using a straight line for temperatures between 50 and 300 K and the Brillouin function [8] for 5 and 10 K. The temperature dependence of the magnetization M(T), under zero-field-cooling (ZFC) and field-cooling (FC) in an applied field of 20 Oe is shown in Fig. 5 where an irreversible behaviour is observed. The MZFC(T) curve presents a very broad maximum (around 175 K) which is no longer observed in the measurements under a 1000 Oe applied magnetic field (not shown here) as it is expected for a transition between superparamagnetic and blocked regimes. On the other hand, MZFC(T) increases below 15 K, a feature that can be detected even when the 1000 Oe field is applied. Then, although the 5 h milled M vs H curves show mainly ordered and paramagnetic contributions, the MZFC curve reveals a superparamagnetic-like behaviour indicating the presence of unblocked moments at RT. When the temperature decreases the thermal fluctuations slow down and the moments block progressively according to their anisotropy energy barrier distribution (depending on the particle size distributions), giving rise to the irreversibility behaviour and to the broad maximum observed in the MZFC(T) curves. Furthermore, at low temperature (To15 K) an increment in the magnetization is observed indicating the appearance of a new contribution to the magnetization. This last

0.018 FC

0.016 0.014 0.012 ZFC 0.010 0.008 0

50

100

150

200

250

300

T (K) Fig. 5. ZFC–FC curves taken under a 20 Oe applied external field.

800

Hc (Oe)

2836

600

400 Left branch (module) Right branch 200

0

50

100

150

200

250

300

T (K) Fig. 6. Coercive field temperature dependence of both branches of the M vs H curve for the 5 h sample.

contribution has not been taken into account in the fit of the M(H) curves for the sake of simplicity. It is worth to point out that below 100 K the hysteresis loops are asymmetric (they are shifted to negative H values). This behaviour results in different HC (coercive magnetic field) fitted values for each branch of the M vs H curve (Fig. 6). This shift indicates a magnetic coupling between ferro and antiferromagnetic phases (exchange bias effect). This phenomenon occurs at temperatures below the point where the MZFC curve falls down. From the M vs H curves taken at 10 and 5 K the J values (total angular momentum quantum number) of the paramagnetic component were 1.9 and 0.8, respectively. These estimated values are lower than those expected for Fe ions in 3+ and 2+ oxidation states, it may be because we have not taken into account the magnetic contribution that was observed below 15 K in the MZFC curves.

4. Conclusions -40000

0 H (Oe)

40000

Fig. 4. M vs H cycles of the 5 h milled sample taken at 300, 100, 10 and 5 K.

The milling time needed to produce, from high purity rutile SnO2 and a-Fe2O3 powders, a Sn0.9Fe0.1O2 rutile structure solid solution was explored. It was found that under our

ARTICLE IN PRESS V. Bilovol et al. / Physica B 404 (2009) 2834–2837

mechanosynthesis conditions the optimal Fe incorporation was obtained after 5 h of milling. For this sample XAS measurements indicate that Fe substitutes for Sn in the SnO2 rutile structure with an oxidation state between 2+ and 3+, having almost one oxygen near neighbour vacancy. Magnetic measurements show a complex behaviour with temperature. At RT the M(H) curves show ordered and paramagnetic contributions, but when the temperature decreases a progressive blocking process is observed from the M(T) curves. At temperatures below 100 K the coercive field increases up to several hundreds of Oe and a shift of the hysteresis cycles is observed, which indicates the appearance of ferro–antiferromagnetic interactions giving rise to the exchange bias effect.

Acknowledgements We appreciate financial support of CONICET, Argentina (PIP 6005) and Laboratorio Nacional de Luz Sincrotro´n (Campinas,

2837

Brazil). The DC magnetic measurements were performed using the Argentinean RN3M facilities. References [1] [2] [3] [4] [5]

J. Sauma, et al., Thin Solid Films 515 (2007) 8653. A. Punnoose, et al., Physical Review B 72 (2005) 054402. J.M. Coey, et al., Applied Physics Letters 84 (8) (2004). K. Nomura, et al., Physical Review B 75 (2007) 184411. V. Bilovol, A.M. Mudarra Navarro, C.E. Rodrı´guez Torres, F.H. Sa´nchez, A.F. Cabrera, Hyperfine Interactions 179 (1–3) (2007) 381. [6] J.M. Newville, Journal of Synchrotron Radiation 8 (2001) 322. [7] M.B. Stearns, Y. Cheng, Journal of Applied Physics 75 (10) (1994). [8] B.D. Cullity, Introduction to Magnetic Materials 3 (1972) 105.