Defect-induced reversible ferromagnetism in Fe-doped ZnO semiconductor: An electronic structure and magnetization study

Materials Chemistry and Physics 123 (2010) 678–684

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Defect-induced reversible ferromagnetism in Fe-doped ZnO semiconductor: An electronic structure and magnetization study Arvind Samariya a , R.K. Singhal a,b,∗ , Sudhish Kumar c , Y.T. Xing b , Mariella Alzamora b , S.N. Dolia a , U.P. Deshpande d , T. Shripathi d , Elisa B. Saitovitch b a

Department of Physics, University of Rajasthan, Jaipur, Rajasthan 302004, India CBPF, Rua Dr.Xavier Sigaud 150, Urca, Rio de Janeiro, Brazil Department of Physics, ML Sukhadia University, Udaipur 313002, India d UGC-DAE CSR, University Campus, Indore 452001, India b c

a r t i c l e

i n f o

Article history: Received 14 January 2010 Received in revised form 14 April 2010 Accepted 7 May 2010 Keywords: Magnetic materials Sintering X-ray photoemission spectroscopy (XPS) Electronic structure

a b s t r a c t Effect of injection of hydrogen ions, followed by their evaporation, has been investigated in the Zn1−x Fex O (x = 0.02–0.07) pellets to throw some light on electronic structure and magnetic correlations. The XRD patterns show that x ≤ 0.05 samples are single phase and the Fe ions incorporate at the Zn2+ sites, while a secondary phase ZnFe2 O4 is detected for x ≥ 0.07. The 2% Fe-doped sample retains a paramagnetic ground state down to 50 K. Likewise, the 5% doped sample also shows paramagnetic state at 300 K but a weak ferromagnetic ordering stems from its cooling (Tc ∼ 160 K). Strikingly, the 5% doped sample, when annealed in hydrogen atmosphere, showed inducement of room temperature ferromagnetism. More significantly, the hydrogen-induced magnetism disappears upon evaporating the H ions by re-heating the sample. The magnetic ordering and the electronic properties exhibit a close parallelism/interplay. The X-ray photoemission spectroscopy results testify the Fe to be in mixed valent state (>2+) in paramagnetic state, however, the ferromagnetic transition stems only upon Fe3+ reducing to Fe2+ , accompanied by emergence of oxygen vacancies as a parallel electronic phenomenon. Origin of H-mediated ferromagnetism is discussed in the framework of cationic vs. anionic vacancies and it is suggested that oxygen vacancies play major role in mediating the coupling. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The dilute magnetic semiconductors (DMSs) have attracted enormous research interest because of their potential applications in spintronics and microelectronics [1–6]. However, for their realizable applications in microelectronic devices, these DMS systems must display ferromagnetism above room temperature. First-principle electronic structure calculations by Sato and Katayama-Yoshida [1] suggested that transition metal (TM)-doped ZnO would show ferromagnetism provided that the carriers produced by doping formed a partially filled spin–split impurity band. Dietl et al. [2] also predicted that wide band gap semiconductors like ZnO, GaN, etc. would show ferromagnetism above room temperature with nominal hole doping. Following these predictions, the TM-doped ZnO has been proposed to be a potential candidate for observing the room temperature ferromagnetism (RTFM). A number of researchers have successfully reported a Curie temper-

∗ Corresponding author at: Department of Physics, University of Rajasthan, JLN Marg, Jaipur, 302004, India. Tel.: +91 141 2545931; fax: +91 141 2701038. E-mail address: [email protected] (R.K. Singhal). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.05.036

ature (TC ) higher than room temperature [5–8] in many TM-doped ZnO samples with different morphologies. On the contrary, many groups have failed to observe the RTFM in these systems [9,10]. The conflicting results have led to the growing interest in both the theoretical [2] and the experimental [11] studies in TM-doped ZnO. A robust enhancement of ferromagnetic properties in Co-doped ZnO DMS systems, upon their hydrogenation, has further fuelled the research interest in these systems. However, the most of the reports are controversial; consequently the nature of magnetic coupling has not been established, as yet. Recently, a lot of research work has been reported on Fe-doped systems [12–18]. A wide range of contradictory experimental results, debating the success vs. failure of obtaining a TC above room temperature, have injected much excitement in the dopinginduced mechanism of the RTFM in this system. There are a tons of recent reports on hydrogenation studies in DMS systems, aimed at enhancing their magnetization. On contrary, there are hardly any attempts to establish whether the H-induced magnetization is a permanent feature or likely to degrade with heating/ageing. This aspect warrants a firm confirmation prior to planning their possible applications in spintronics and microelectronics. Motivated by this, we have systemati-

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cally investigated the effect of injection of hydrogen ions on the magnetic properties vis à vis electronic structure of Zn1−x Fex O (x = 0.02–0.07) pellets using X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), resistance measurements and magnetization studies. Through hydrogen annealing, we observed robust inducement of RTFM in otherwise paramagnetic ZnO:Fe pellets. The hydrogen mediated magnetic transition is accompanied by electronic and transport properties with no structural deviations or creation of secondary phases, as evidenced by XRD and photoemission. Our findings reveal a route correlation of oxygen vacancies with the observed RTFM. In particular, we have also systematically investigated the re-heating consequences on the hydrogenated sample to check reversibility of the magnetization process. Interestingly, the H-induced RTFM and subsequent modifications viz. electronic structure and transport properties retrace back upon re-heating the hydrogenated sample in air. Our findings reveal a subtle interplay and close correlation of electronic properties with the magnetic ordering. 2. Experimental procedure The Fe-doped (2%, 5% and 7%) ZnO bulk samples were prepared using the conventional solid-state reaction method. The appropriate amounts of ZnO and Fe2 O3 (purity 99.999% each) were mixed and ground for 10 h, followed by their calcinations for 12 h at 900 ◦ C in a microprocessor controlled furnace (Linn, Germany). The specimens were then cooled slowly to the ambient temperatures. The resulting powders were ground and sintered for 12 h. The mixtures were then annealed in air at 600 ◦ C. The resulting lumps were further ground for 5 h and the pellets of 10 mm diameter were prepared using a hydraulic press. Hydrogenation of Zn0.95 Fe0.05 O was performed by heating the ground powders for 6 h at 600 ◦ C in alumina boat kept inside a quartz tube in continuously flowing hydrogen gas in a tubular reduction furnace. Then the powder was pressed into pellets form and further annealed in hydrogen atmosphere for nearly 4 h. For de-loading i.e. de-hydrogenation, the samples were re-heated for 6 h at 600 ◦ C inside quartz tube in air. The XRD patterns were collected on a Philips make X’Pert X-ray diffractometer equipped with CuK˛ radiation. The XRD patterns were recorded in the 2 range 20–90◦ with a step size of 0.02◦ . The Zn 2p, Fe 2p and O 1s core level XPS spectra were recorded on a VSW make X-ray photoelectron spectrometer using AlK˛ radiation. The resistances of the samples were measured on their rectangular bricks (4.0 mm × 2.0 mm × 0.5 mm) using a Keithley make Electrometer (Model 6517A). The magnetic field and the temperature dependent magnetization measurements were carried out on a PARC make vibrating sample magnetometer (VSM) and repeated on a Quantum Design (Versa Lab) VSM. To rule out the possibility of measurement errors, all the measurements were repeated.

3. Experimental results

Fig. 1. The XRD patterns for Zn1−x Fex O samples at 300 K. The indicated peak indexation corresponds to the wurtzite ZnO phase. Peaks marked with asterisk (*) are impurity ZnFe2 O4 peaks in the 7% Fe-doped sample.

dopants. The best fits were obtained when Fe atoms occupy the Zn site with a total preference, while these atoms occupying the O site gave poor fits. This confirms that Fe ions have well been incorporated into the ZnO matrix at Zn2+ sites and the Fe doping (up to 5%) in ZnO did not show any structural change. The XRD patterns of the 5% Fe-doped hydrogenated sample i.e. Zn0.95 Fe0.05 O:H and re-heated sample Zn0.95 Fe0.05 O:Ht were found to be replica of Zn0.95 Fe0.05 O. The fitted patterns of Zn0.95 Fe0.05 O:H are depicted in Fig. 2. During refinement of Zn0.95 Fe0.05 O:H, the oxygen occupancy was varied, which showed a remarkable (∼5.5%) oxygen deficiency as compared to the un-hydrogenated sample. 3.2. XPS data X-ray photoemission spectroscopy is known to yield reliable information on the oxidation states, electronic charge transfer and defects (cationic/anionic vacancies) in a multi-component system. Being an atomic specific technique, it is ideally suited for studying the dilute systems with high precession. The Zn 2p, Fe 2p and O 1s core level XPS spectra were recorded for Zn1−x Fex O (x = 0.0,

3.1. Characterization data The indexed XRD patterns for pure ZnO and Fe-doped samples Zn1−x Fex O (x = 0.02, 0.05 and 0.07) are presented in Fig. 1. All these samples crystallize in the wurtzite type hexagonal structure belonging to the space group P63mc (No. 186, Z = 2). No peaks of other impurity phases were observed in the patterns of ZnO, Zn0.98 Fe0.02 O and Zn0.95 Fe0.05 O. However, small peaks corresponding to cubic ZnFe2 O4 spinel phase were observed in the patterns of Zn0.93 Fe0.07 O indicated by asterisk (*) in Fig. 1, therefore, we did not include this sample for further studies. For the samples with x ≥ 0.07, the intensities of the peaks corresponding to ZnFe2 O4 phase were found to increase. The Rietveld profile refinements were carried out using the FULLPROF Program [19] in the wurtzite hexagonal structure with each atom residing on the 2b Wyckoff position: Zn atoms at (1/3 2/3 0) and the O atoms at (1/3 2/3 z) co-ordinates. The refined values of the structural parameters along with the agreement factors are listed in Table 1. The obtained values of the unit cell parameters for pure ZnO are in very good agreement with the reported standards (JCPDS card No. 36-1451, a = 3.249 Å and c = 5.206 Å). During the refinement process, the Fe occupancies were varied for the Zn and O sites to locate the exact site of the

Fig. 2. The refined XRD pattern of Zn0.95 Fe0.05 O:H. The observed (calculated) profiles are shown by open circle (solid line) curves. The short vertical marks represent allowed Bragg reflections in the wurtzite ZnO phase. The lower curve is the difference plot.

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Table 1 The refined structural parameters for Zn1−x Fex O. Space group: P63mc (No. 186, Z = 2). The relative oxygen bulk content estimated by the O 1s XPS spectra is also shown. Sample

a (Å)

c (Å)

V (Å3 )

Average distance Zn–O (Å)

ZnO Zn0.98 Fe0.02 O Zn0.95 Fe0.05 O Zn0.95 Fe0.05 O:H

3.2485 (5) 3.2450 (3) 3.2391 (3) 3.2389 (4)

5.2054 (5) 5.1988 (5) 5.1892 (5) 5.1894 (4)

47.662 (7) 47.411 (8) 47.148 (7) 47.149 (6)

1.9785 1.9724 1.9692 1.9692

0.02 and 0.05) samples. The pellets were scraped uniformly until the feature coming from carbon contamination of the surface (C 1s peak) got minimized. The vacuum in the chamber was kept at ∼4.4 × 10−10 Torr, hence, the cleanliness of the samples was conserved during the experiment. The spectra were recorded only after ensuring a clean sample surface. No shift was observed due to charging of the samples. All the XPS peaks were calibrated by C 1s (284.6 eV). 3.2.1. Zn 2p XPS data The Zn 2p XPS spectra for ZnO, Zn0.98 Fe0.02 O, Zn0.95 Fe0.05 O and Zn0.95 Fe0.05 O:H are displayed in Fig. 3. The main peaks observed at 1022.6 and 1045.9 eV, correspond to the binding energy of Zn 2p3/2 and 2p1/2, respectively. All the spectra show symmetric single peaks and no change in peak positions is noticed upon increasing the Fe content or hydrogenation. The symmetric single peaks rule out the possibility of multiple components of Zn in these samples. The peak positions of Zn 2p3/2 and 2p1/2 in all the samples match closely with the standard values of ZnO [20], indicating that Zn atoms retain the 2+ oxidation state. The intensity under the 2p3/2 and 2p1/2 peaks lower upon increasing the Fe doping re-confirming that the Fe ions incorporate at the Zn site. 3.2.2. Fe 2p XPS data Fig. 4 shows the Fe 2p XPS spectra for the samples Zn0.98 Fe0.02 O, Zn0.95 Fe0.05 O and Zn0.95 Fe0.05 O:H. The main peaks observed at 710.2 and 723.1 eV correspond to the binding energy of Fe 2p3/2 and 2p1/2, respectively. No additional peaks corresponding to metal clusters or particles are noticed in any of the spectra. The peak positions corresponding to Fe2+ state (in FeO) occur at 709.8 and 722.7 eV, respectively, while for the Fe3+ state (in Fe2 O3 ) these occur at 710.7 and 724 eV, respectively, as per the standard line positions chart of our VSW make XPS set-up. We must state that

Fig. 3. The Zn 2p3/2 XPS spectra for ZnO, Zn0.98 Fe0.02 O, Zn0.95 Fe0.05 O and Zn0.95 Fe0.05 O:H.

Positional parameter, occupancy of oxygen z

N

0.3850 (5) 0.3727 (8) 0.3812 (9) 0.3813 (8)

1.00 1.00 1.00 0.945

RBragg

2

Relative O content (±2%)

3.85 3.15 3.42 3.36

1.66 1.87 1.85 1.79

– 1.00 0.98 0.93

these energy values may vary slightly from machine to machine. Therefore, for exact referencing, one must use the values measured with same set-up under similar calibration conditions. In all our samples the peak positions lie in between the two values (Fe2+ and Fe3+ ), implying that the Fe ions are in the mixed valence state. For Zn0.95 Fe0.05 O, the peaks do not show any observable positional change with respect to Zn0.98 Fe0.02 O, indicating that the Fe valence states are identical for the two compositions. Of course, the intensities of the peaks show an overall growth with Fe content, which was natural. On contrary, the hydrogenated sample Zn0.95 Fe0.05 O:H shows a remarkable shift to the lower energy side (Fig. 4). This testifies that hydrogenation causes the Fe oxidation state to transform from 3+ to 2+ state, which is a noticeable electronic change. To estimate the relative weights of the two oxidation states, the Gaussian were fit to 2p3/2 as well as 2p1/2 peaks. The intensities of Gaussians corresponding to Fe2+ and Fe3+ states show that Fe2+ is dominant in Zn0.95 Fe0.05 O (Fig. 5). Notably, in Zn0.95 Fe0.05 O:H, the spectral weight shifts from Fe3+ to Fe2+ , re-confirming that hydrogenation causes a transformation of Fe valence from 3+ to 2+. The percent quantity of Fe2+ estimated from the relative areas of the Gaussian peaks under the Fe2+ and Fe3+ has been displayed in Table 2. A similar transformation of Fe valence, followed by a magnetization enhancement, has been reported by Mössbauer studies in Fe-doped ZnO [21]. 3.2.3. O 1s XPS data The Zn 2p XPS spectra confirmed Zn to retain the divalent state upon Fe doping while the Fe 2p XPS spectra confirmed the Fe valence to be more than 2+. These two observations weigh against the charge neutrality in ZnO host lattice! Therefore, the O 1s spectra were also recorded to look at the oxygen electronic states. The O 1s spectrum for Zn0.95 Fe0.05 O shown in Fig. 6a is asymmetric indicating the presence of some multi-component oxygen species in the near-surface region of the sample. Three Gaus-

Fig. 4. The Fe 2p3/2 and 2p1/2 XPS spectra for Zn0.98 Fe0.02 O, Zn0.95 Fe0.05 O and Zn0.95 Fe0.05 O:H.

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Fig. 5. The Gaussian fittings in Fe 2p3/2 and 2p1/2 XPS spectra showing relative weights of Fe2+ and Fe3+ states.

sians were fitted to the O 1s spectra of all the samples. The most intense first peak at ∼531.2 eV is due to the O 1s bulk contribution from the oxide samples which is our main focus. The next two high-energy peaks are due to the surface contaminations [8]. Fig. 6b compares the intensities of the O 1s Gaussian for samples Zn0.98 Fe0.02 O, Zn0.95 Fe0.05 O, Zn0.95 Fe0.05 O:H and Zn0.95 Fe0.05 O:Ht (re-heated in air). The fractional oxygen bulk contents in the samples, with respect to Zn0.98 Fe0.02 O, are displayed in Table 1. The oxygen content is nearly same for Zn0.98 Fe0.02 O and Zn0.95 Fe0.05 O. Interestingly, the hydrogenated sample Zn0.95 Fe0.05 O:H shows a significant (∼7%) oxygen depletion as compared to the as-prepared Zn0.95 Fe0.05 O, however, upon re-heating the hydrogenated sample in air, the oxygen content is recovered. The observed oxygen depletion in hydrogenated sample is re-confirmation of the XRD results which indicated a substantial oxygen deficiency. The oxygen depletion being a direct measure of oxygen vacancies actually holds the key to understand the charge neutrality factor (discussed later). 3.3. Resistance data For resistance measurements, the samples were transformed into rectangular bricks of exactly uniform dimensions (4.0 mm × 2.0 mm × 0.5 mm) and the silver spot electrodes were made on the bricks across their lengths. The obtained values of resistance for the samples are shown in Table 2. The value of resistance for hydrogenated sample Zn0.95 Fe0.05 O:H shows a significant decrease (∼3 orders of magnitude) as compared to the as-prepared

one. Interestingly, the resistance again increases after re-heating the sample in air. This indicates that extra charge carriers induced upon hydrogenations, disappear upon re-heating the sample in air. 3.4. Magnetization data Fig. 7 shows the room temperature magnetization vs. applied magnetic field (M–H) curves for the pure ZnO and Zn0.98 Fe0.02 O. The pure ZnO shows a typical diamagnetic behaviour, on the other hand, Zn0.98 Fe0.02 O shows a paramagnetic state. Both the samples retained the same magnetic states down to 50 K. Linear shape of the M–H curve for Zn0.95 Fe0.05 O (Fig. 8) is also indicative of paramagnetic state at 300 K. However, upon cooling down to 50 K it shows a hysterisis loop, indicative of a weak ferromagnetic ordering. Of course, the shape of the curve does not rule out the presence of some paramagnetic contribution which must be separated out for making any quantitative estimation. Fig. 9 shows the deconvoluted paramagnetic and ferromagnetic contributions in the sample obtained by fitting the curves, using the method described elsewhere [8]. Inset of Fig. 7 shows the magnetization as a function of temperature for Zn0.95 Fe0.05 O under a field of 0.1 T revealing that the sample is ferromagnetic below a Curie point of ∼160 K. While the cooling of paramagnetic sample Zn0.95 Fe0.05 O inculcates only marginal ferromagnetic ordering, in contrast, the hydrogenation annealing of the sample caused a noticeable inducement of room temperature ferromagnetism (Fig. 10). The ordering strengthens further upon cooling down the sample. More impor-

Table 2 The saturation magnetic moment (Ms ), coercivity, remanence, the resistance values and the percent quantity of Fe (2+) estimated from Fe XPS spectra for Zn1−x Fex O samples. Sample

ZnO Zn0.98 Fe0.02 O Zn0.95 Fe0.05 O Zn0.95 Fe0.05 O:H Zn0.95 Fe0.05 O:Ht

Ms (emu g−1 )

Remanence (emu g−1 )

Coercivity (T)

300 K

50 K

300 K

50 K

300 K

50 K

Dia Para Para 1.4 Para

Dia Para 0.04 1.6 0.41

– – – 0.031 –

– – 0.019 0.075 0.017

– – – 0.23 –

– – 0.005 0.41 0.044

Resistance ()

Fe (2+) (% weight)

100 K 650 G 550 G 0.8 G 525 G

– 60 62 85 63

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Fig. 8. The M–H curves for Zn0.95 Fe0.05 O at 300 and 50 K. The inset shows low-field data for better clarity to observe the coercovity and remanene.

Fig. 6. The O 1s XPS spectrum for (a) Zn0.95 Fe0.05 O along with the Gaussian fits and (b) the O 1s Gaussian peaks for Zn0.98 Fe0.02 O, Zn0.95 Fe0.05 O, Zn0.95 Fe0.05 O:H and the re-heated sample Zn0.95 Fe0.05 O:Ht.

tantly, upon re-heating the sample in air for ∼6 h the M–H curves turned back into a linear shape (upper inset, Fig. 10), indicating its complete reversal to paramagnetic ground state at 300 K. Since the Fe atoms are well dissolved into the ZnO lattice without forming any iron oxide phase, the ferromagnetism observed in Zn0.95 Fe0.05 O upon cooling and an RTFM observed upon its hydrogenation can be claimed to be an intrinsic bulk property.

Fig. 7. The magnetic hysteresis (M–H) loops of ZnO and Zn0.98 Fe0.02 O at 300 K.

Fig. 9. The M–H curve for Zn0.95 Fe0.05 O at 50 K along with paramagnetic and ferromagnetic contributions obtained by fittings. The inset shows the M–T curve for Zn0.95 Fe0.05 O showing its Tc ∼ 160 K.

Fig. 10. A comparison of high field M–H curves for Zn0.95 Fe0.05 O:H at 300 and 50 K. The upper inset shows M–H curve of re-heated sample Zn0.95 Fe0.05 O:Ht, indicating a paramagnetic state.

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4. Discussion of results and possible causes of the RTFM 4.1. Fe mixed valence From the electronic charge neutrality and stoichiometric viewpoints when the Fe ions incorporate at the Zn2+ site in ZnO lattice, it must exist as divalent (Fe2+ ). But our XPS results confirmed a mixed valence state of Fe (valence > 2), which is in agreement with the other reports [21,22]. The existence of Fe3+ in these samples warrants an explanation. The presence of Fe3+ can be predicted to arise as a consequence of hole doping process created by cation (Zn) vacancies, since the cation vacancies near Fe atoms are likely to promote Fe2+ to Fe3+ . However, as far as the inducement of RTFM is concerned, our magnetization results reveal that the RTFM is realized only upon hydrogenation of Zn0.95 Fe0.05 O, which is accompanied by Fe3+ transforming to Fe2+ . Hence, the reduction of Fe valence state from 3+ to 2+ valence state seems to have strong correlation with the ferromagnetic ordering. On the contrary, our results indicate that cationic vacancies as evidenced by co-existence of Fe3+ state do not favour the inducement of RTFM in the Fe-doped ZnO system. However, we would revert to this point ahead, in light of the oxygen vacancy correlations observed. 4.2. Role of oxygen vacancies It was significant that the RTFM observed in Zn0.95 Fe0.05 O:H was followed by oxygen depletion in parallel with Fe reducing from 3+ to 2+ state. The oxygen deletion in the doped ZnO indicates creation of oxygen vacancies. These oxygen vacancies are known to create shallow donor states and thus would add electrons to the system, doping the n-type carriers. On contrary, the cation vacancies create acceptor states and thus dope the p-type carriers. Our resistance measurements indicated a drastic fall in resistance in the hydrogenated Zn0.95 Fe0.05 O:H indicating evolution of extra charge carriers. Unfortunately, on the basis of resistance results alone, we cannot fix the nature of carriers, nevertheless, the observed correlation of ferromagnetism with the oxygen vacancies, strongly reveals that the anionic vacancies are very important for the RTFM in the Fe-doped ZnO. This prediction is substantiated by the similar conclusions drawn in regard to other doped ZnO and TiO2 systems [23–26]. The theoretical calculations in regard of Co-doped ZnO system [27] show that while Co2+ can interact only at short ranges, the long-range magnetic interaction can be achieved as a result of the interplay between Co2+ and the Co2+ –oxygen vacancy (CoV) pairs via conduction electrons through moderate doping. This mechanism seems to hold for the ZnFeO system too, due to the oxygen vacancies and the magnetization correlations observed in Fe-doped ZnO matrix.

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netism in it. Interestingly, the hydrogen-induced magnetization disappears upon re-heating the sample in air. A parallel behaviour is observed in the resistance of samples and the oxygen content in them, as discussed. The resistance results indicate that the charge carriers induced upon hydrogenation disappears upon reheating the sample in air. More interestingly, the hydrogenation causes a significant oxygen depletion that is direct indicator of oxygen vacancies in the sample, however, upon re-heating in air the oxygen content is recovered. The recovery of original (paramagnetic) ground state along with the regain of oxygen content and resistance values upon re-heating clearly indicate that H ions escape/evaporate from the samples upon their re-heating. A similar behaviour has been reported in Co-doped ZnO and TiO2 systems [28,29] too. To further establish the reproducibility of H-induced RTFM, the re-heated sample was again hydrogenated for 6 h at 600 ◦ C, which showed a recovery of the same robust ferromagnetism. Thus our results clearly point out that ferromagnetism in Fe-doped ZnO lattice can be induced upon hydrogenation, removed upon re-heating and can be re-inducted by its re-hydrogenation. In metals and their oxides, the injected hydrogen prefers to reside at the interstitial sites i.e. the grain boundaries [30]. In view of these facts, the H-induced oxygen depletion is most likely to take place at the grain boundaries of these polycrystalline samples. This in turn would cause the migration of oxygen from lattice to the grain boundary side therefore creating the oxygen vacancy assisted grain boundary in the sample and in turn reduce the average grain size, which is a favourable situation for the ferromagnetism in these DMS systems [31]. However, this requires detailed studies on this system. Finally, a thorough survey of the literature reveals that depending on sample preparation parameters, the same material system may display the ferromagnetic, the paramagnetic or the spin glass behaviour on same morphology. In different morphologies these findings are even more horribly controversial. We do not claim our results to be any exception in this regard; therefore, we do not claim these results to show a perfect quantitative agreement with the bulk samples prepared by other researchers by different heating/annealing methods, with regard to magnetization parameters like saturation magnetization, coercivity, Tc , etc. However, our findings enable us to make an important comment that the oxygen vacancies are the basic building units that play an effective role, if not the sole role, in inducing the RTFM in the DMS systems. Further investigation with regard to Fe 3d–O 2p hybridization using high resolution X-ray absorption and emission techniques can throw further light on electronic and magnetic correlations.

5. Conclusion 4.3. Charge neutrality factor The observed oxygen depletion upon hydrogenation and its recovery upon re-heating the sample in air not only explain the RTFM correlations but also can take into account the electronic charge neutrality factor. In this system, the reduction of Fe valence state from 3+ to 2+ state, accompanied by creation of oxygen vacancies, are the two electronic phenomena which can encounter each other to conserve the lattice charge neutrality. Further, since the Zn (2+) state remains unaffected upon Fe doping, we suggest that oxygen depletion owns the sole responsibility of charge neutrality factor in the lattice. 4.4. Reversibility of ferromagnetism Our results clearly indicate that hydrogenation of paramagnetic Zn0.95 Fe0.05 O causes inducement of room temperature ferromag-

Effect of insertion of hydrogen ions followed by their evaporation has been investigated in Fe-doped ZnO pellets to understand the electronic structure and magnetic property correlations. The XRD patterns exhibit no structural deviations up to 5% Fe doping and the Fe ions substitute the Zn2+ sites. However, for higher level (7%) Fe doping a secondary phase of ZnFe2 O4 starts to form. The 2% Fe-doped sample shows a paramagnetic ground state. Similarly, the 5% Fe-doped sample is paramagnetic at 300 K but a weak ferromagnetism induces upon its cooling (Tc ∼ 160 K). Upon hydrogenation the Zn0.95 Fe0.05 O shows inducement of room temperature ferromagnetism but the hydrogen-induced magnetization disappears upon re-heating the sample in air. The XPS results indicate the Fe to be in mixed valent state (>2), however, the ferromagnetism is induced only when Fe transforms from 3+ to 2+, accompanied by creation of oxygen vacancies. The observed ferromagnetism is compared in terms of cationic vs. anionic vacancies but the oxygen

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vacancies seem to play crucial role in mediating the long-range coupling. Acknowledgments RKS is grateful to TWAS, Italy and CNPq Brazil for associate membership. Part of the work was performed at UGC-DAE-CSR, Indore, India. We also thank DST FIST, India for the experimental facilities. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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