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Synthesis, structure and ESR studies of Mg doped ZnAlO nanoparticles
Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Synthesis, structure and ESR studies of Mg doped ZnAlO nanoparticles Q1
O. Cakiroglu a, M. Acikgoz b, L. Arda b, D. Akcan b, N. Dogan c a b
Q3
c
Istanbul University, Hasan Ali Yucel Education Faculty, Beyazit, 34452 Istanbul, Turkey Bahcesehir University, Faculty of Arts and Sciences, Department of Mathematics & Computer Sciences, Besiktas, 34349 Istanbul, Turkey Gebze Institute of Technology, Department of Physics, 41400 Gebze/Kocaeli, Turkey
art ic l e i nf o
a b s t r a c t
Article history: Received 7 January 2014 Received in revised form 24 February 2014 Accepted 7 March 2014
Zn0.98 xMgxAl0.02O solutions with different (x ¼0.05, 0.10, 0.15, and 0.20) compositions were synthesized by the sol–gel technique using Zn, Mg and Al based alkoxide. The effects of highly Mg doping ratio on structure and magnetic properties were investigated systematically. The phase and the crystal structure of the Zn0.98 xMgxAl0.02O nanoparticles were characterized using X-ray diffraction. Scanning Electron Microscope and X-ray diffraction were utilized to understand the size and microstructure of samples. We observed the particle sizes of nanoparticles between 80 nm and 100 nm range. Furthermore, ESR spectra of Zn0.98 xMgxAl0.02O nanoparticles were collected at room temperature on a Bruker EMX model X-band spectrometer operating at a frequency of 9.50 GHz. It is observed that the critical concentration of Mg, x ¼ 0.1, has minimum g-factor and maximum line-width (W). & 2014 Published by Elsevier B.V.
Keywords: Magnetic materials Nanostructures Sol–gel growth Magnetic properties
1. Introduction Diluted magnetic semiconductors (DMSs) have attracted a great deal of interest for their potential technological applications to optoelectronics, spintronics, and microwave devices. Particularly, there has been an increasing interest to investigate the physical properties of binary ZnO doped with transition metal (TM) ions such as Mn, Co, V, Cr, Fe, and Ni. Many prominent researchers [1–5] have focused to explain the origin of magnetism in the DMSs materials and to correlate magnetic properties with the TM and deep level doping concentrations after the theoretical prediction of the possibility of above room temperature ferromagnetism [6,7]. Recently, co-doped ZnO materials forming a quadruple compound are receiving remarkable and increasing interest. It may be reasonably expected that the co-doping may result in some interesting properties. For example, strongly enhanced magnetic moments were obtained in ZnMnAlO by finding a clear relationship between the carrier density and magnetism [8]. Previously, Yang et al. investigated Zn1 x yMgxAlyO films prepared by pulsed laser deposition (PLD) and attained the band gap modulation and high conductivity as a result of Mg–Al co-doping [9]. Kim et al. studied the emission spectra of ZnMgAlO films through the cathodoluminescence (CL) measurement [10]. Moreover, Tanasoi et al. characterized and compared the catalytic activity of ZnMgAlO with the other mixed oxides containing transition metals [11]. Also, to analyze the effect of Al concentration on ZnMgAlO, we have recently studied the synthesis, characterization and ESR of powder Zn0.95 xMg0.05AlxO (x¼0, 0.01, 0.02, 0.05,
and 0.1) nanocrystals [12]. It was found that crystallite sizes decreased while microstrain increased with the increase in Al concentration and microstrain was less along the direction 〈0001〉 than along the basal directions 〈1010〉. Also, a new ESR line was seen as a result of the Al concentration in the ZnMgAlO system. After understanding the effect of Al concentration on ZnMgAlO nanoparticles in [12], in the present study we now aim to investigate the effect of Mg concentration on these quadruple nanoparticles by keeping the Al constant. For this, we have prepared the mixed oxides Zn0.98 xMgxAl0.02O as nano-polycrystalline powders by a simple sol–gel technique, which is the most promising for low cost, and low temperature processing technique to prepare complex oxide composition with high homogeneity. We have characterized the changes in the structure and microstructure using X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) and investigated magnetic properties using electron spin resonance (ESR) spectra of ZnMgAlO nanoparticle samples upon increasing Mg content.
2. Experimental part: The mixed oxides Zn0.98 xMgxAl0.02O were prepared as polycrystalline nanoparticles with various compositions (0.05rxr0.20) using sol–gel technique. Zinc acetate dihydrate (C4H6O4Zn 2H2O), Mg 2, 4 pentanedionate ([CH3COCHC(O)CH3]2Mg∙2H2O, MgAcAc) and Al 2, 4 pentanedionate (Al(C5H7O2)3, AlAcAc) were used as precursor materials. Methanol (CH3OH) and acetyl acetone (C5H8O2) were used as solvents and chelating agent. The details of the
http://dx.doi.org/10.1016/j.jmmm.2014.03.032 0304-8853/& 2014 Published by Elsevier B.V.
Please cite this article as: O. Cakiroglu, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.03.032i
preparation of Zn0.98 xMgxAl0.02O nanoparticles were discussed in the previous studies [13–16]. The as prepared powders were ground and annealed individually in air at 600 1C using box furnace. XRD scans were recorded using a Rigaku diffractmeter with Cu Kα radiation. Microstructure properties of prepared samples were observed using scanning electron microscope (SEM) (JEOL, JSM5910LV). The thermal behavior of the xerogels of Zn0.98 x MgxAl0.02O solutions, which were dried at room temperature for 3 days, was studied by using differential thermal analysis (DTA), and thermogravimetric analysis (TG)/SII 7300 Exstar thermal analyzer system in air with a heating rate of 10 1C/min. ESR spectra of Zn0.98 xMgxAl0.02O nanoparticles were collected at room temperature on a Bruker EMX model X-band spectrometer operating at a frequency of 9.50 GHz. The static magnetic field was varied in the range 0–16000G. The field derivative of microwave power absorption (dW/dH1) was registered as a function of the applied magnetic field H1. All the samples were loaded into quartz ESR tubes.
evaporation of volatile organic component. The percentage of weight was 8%. The second slight endotermic peak appeared between 90 1C and 278 1C in DTA curve corresponding to the second weight decrease in the TG curve, in which the residual solvent and organics evaporated. The percentage of weight was 12%. As the temperature increases to 340 1C, a strong exotermic peak and at 375 1C a weak shoulder peak are observed in DTA curve, which correspond to a higher level of weight loss in the TG
MgO ZnO
3. Results and discussion 3.1. 0Thermal analysis Fig. 3. The XRD profile fitting resulting from Rietveld analysis of Zn0.98 xMgx Al0.02O, x Z 0.2 showing two phases (MgO and ZnO).
Exo
DTA and TG data were analyzed so as to understand the thermal reaction of ZnMgAlO nanoparticles. We analyzed the Zn0.93Mg0.05Al0.02O xerogels within the temperature range 50 and 1000 1C in air. Fig. 1 shows some exotermic and endotermic peaks until 510 1C. A strong endotermic peak is seen in the DTA curve between 50 1C and 90 1C corresponding to a first weight decrease in the TG curve, which is the removal of solvent and the
DTA (μV)
Weight (μg)
TG
Endo
DTA
Temperature (°C)
Fig. 1. TG–DTA curves of the Zn0.93Mg0.05Al0.02O xerogel.
1200 1000
Intensity (a.u )
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
O. Cakiroglu et al. / Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
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Zn0.73Mg0.25Al0.02 Zn0.78Mg0.20Al0.02
600
Zn0.83Mg0.15Al0.02
400
Zn0.88Mg0.1Al0.02
200
Zn0.93Mg0.05Al0.02
0 10
20
30
40
50
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2 Theta (°) Fig. 2. The X-ray diffraction patterns of the ZnMgAlO nanoparticles.
80 Fig. 4. (a) SEM micrograph and (b) EDS of Zn0.93Mg0.05Al0.02O nanoparticles at 600 1C for 30 min.
Please cite this article as: O. Cakiroglu, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.03.032i
O. Cakiroglu et al. / Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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curve. It was supposed that the carbon-based materials were born out and the oxidation was occurred and ended at 510 1C. The percentage of third weight was 50%. 3.2. 0Structure analysis
3
nanoparticles increase. Referring to the SEM micrographs molten structure is observed at most for x ¼0.2 concentration (Fig. 7a). The particles are quasi spherical, agglomerating and dense. The nanoparticle sizes were observed approximately between 80 nm and 100 nm. The composition of Zn0.98 xMgxAl0.02 nanoparticles after annealing at 600 1C for 30 min is detected by EDS. EDS of Zn0.98 xMgxAl0.02O (x¼ 0.05, 0.1, 0.15, and 0.2) nanoparticles are shown in Figs. 4–7b. Zn, Mg and Al peaks are clearly seen and also Mg peaks increase with the increase in concentration. Zn, Mg and Al contents of the nanoparticles are the same as those in the preparation of samples.
The obtained powders were annealed at 600 1C from 30 min under air using box furnace. The X-ray diffraction of Zn0.98 xMgxAl0.02O (x¼0.05, 0.1, 0.15, and 0.2) nanoparticles annealed at 600 1C for 30 min in the air are shown in Fig. 2. The reflections correspond to a single phase ZnO with wurtite hexagonal structure (ICDD card No. 36-1451) of space group P63mc for xo0.2 concentration. Mg and Al ions incorporated substitutionally in the ZnO lattice replacing the Zn ions in the position 2b [17]. Applying the MAUD program [18], from Rietveld analysis, the occupation number of the Mg and Al ions in the position 2b is nearly equal to the intended value during the preparation for each value of xo0.2. Fig. 3 indicates two phases ZnO and MgO identified in the diffraction patterns for xZ0.2. The limited miscibility of Mg ions in the ZnO lattice are shown as in Fig. 3, which depicts the pattern fitting resulting from Rietveld quantitative phase analysis for x¼ 0.25. The microstructure of Zn0.98 xMgxAl0.02O nanoparticles with various compositions (0.05 rx r0.20) were examined by SEM as shown in Figs. 4–7a. As seen in the figures, when the Mg concentration is increased, the agglomeration and malting of
ESR measurements have been performed and analyzed through concentration dependence of the g-factor and the line-widths W of ESR spectra. Experimental and simulated X band ESR spectra of Zn0.98 xMgxAl0.02O powders with different doping concentration of Mg recorded at room temperature are shown in Fig. 8. It is seen that the ESR spectra are dominated by a strong peak at around 3450G (around g0 ¼1.97), which can be attributed to the conduction electrons located at the surface center of ZnO particles [19,20]. Upon the increasing of Mg concentration, in addition to this main resonance signal, a weak resonance signal is observed at around 3375G with a g-value close to 2.0013 (g0), which characterizes a
Fig. 5. (a) SEM micrograph and (b) EDS of Zn0.88Mg0.1Al0.02O nanoparticles at 600 1C for 30 min.
Fig. 6. (a) SEM micrograph and (b) EDS of Zn0.83Mg0.15Al0.02O nanoparticles at 600 1C for 30 min.
3.3. 0Magnetic analysis
Please cite this article as: O. Cakiroglu, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.03.032i
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O. Cakiroglu et al. / Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
new center with 4G. The same weak signal was also observed in Zn0.95 xMg0.05AlxO with the increasing of the Al concentration [12]. It shows that the presence of this signal does not depend on the variation in Mg or Al concentration and may be attributed to the Zn vacancy in the structure. A computer simulation of the ESR spectrum of each sample has been carried out to derive the related EPR parameters. The extracted spin Hamiltonian parameters (g ? and g jj ) with the corresponding line-width parameters (W ? and W jj ) for the powdered forms are given in Table 1. The line-widths of the ESR signals take different values between 3.6 and 10.4G when the Mg concentration is changed from 0.05 to 0.2. As can be seen in Table 1, the values of g ? and g jj are less than the free electron value ge (2.0023). For Zn0.83Mg0.15Al0.02O, g ? ¼g jj (isotropic) while the trend g jj 4 g ? (anisotropic) is obtained for the other samples. The isotropic parameter g 0 in Table 1 can be determined from the anisotropic one using the following equation: g 0 ¼ ð2g ? þ g jj Þ=3. For better visualization the variation of g-factors g ? , g jj , and g 0 with the concentration of Zn in Zn0.98 xMgxAl0.02O are shown in Fig. 9. Also, the dependence of the line-widths W ? and W jj of ESR spectra on Zn concentration is demonstrated in Fig. 10. It appears that the critical concentration of Mg for the observation of minimum g-factor and maximum W is x ¼0.1.
4. Conclusions
Fig. 7. (a) SEM micrograph and (b) EDS of Zn0.78Mg0.2Al0.02O nanoparticles at 600 1C for 30 min.
The mixed oxides Zn0.98 xMgxAl0.02O (0.05 rx r0.20) were prepared as polycrystalline nanoparticles using sol–gel technique. Rietveld analysis showed that the reflections of Mg doped ZnAlO nanoparticles correspond to ZnO hexagonal wurthzite structure without secondary phases for x o0.2 concentration, whereas for the higher concentration two phases of ZnO and MgO were obtained. The results of the ESR analysis indicate that the critical
Fig. 8. Experimental and simulated X band ESR spectra of (a) Zn0.78Mg0.2Al0.02O; (b) Zn0.83Mg0.15Al0.02O; (c) Zn0.88Mg0.1Al0.02O; and (d) Zn0.93Mg0.05Al0.02O samples at room temperature.
O. Cakiroglu et al. / Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Table 1 The values of the g-factor and line-width parameters derived from the simulations of the experimental ESR spectra.
Acknowledgments
Sample
g?
gjj
g0
W ? ðGÞ
W jj ðGÞ
The present work was supported by the Research Fund of Bahcesehir University.
Zn0.78Mg0.2Al0.02O Zn0.83Mg0.15Al0.02O Zn0.88Mg0.1Al0.02O Zn0.93Mg0.05Al0.02O
1.9704 1.9701 1.9673 1.9697
1.9741 1.9701 1.9692 1.9702
1.9716 1.9701 1.9679 1.9699
5.20 3.60 4.30 3.65
5.20 3.60 10.40 8.10
References
1,974
g⊥ g
1,973
g0 g-factor
1,972 1,971 1,970 1,969 1,968 1,967 0.78
0.80
0.82
0.84
0.86
0.88
0.90
0.92
0.94
Concentration of Zn Fig. 9. Variation of g-factors with the concentration of Zn in Zn0.98 xMgxAl0.02O nanoparticles.
11 10
[1] D.C. Look, Mater. Sci. Eng. B 80 (2001) 383 (references therein). [2] Ü. Özgür, Y.a.I. Alivov, C. Liu, A. Teke, M. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301 (references therein). [3] L. Arda, M. Acikgoz, N. Dogan, D. Akcan, O. Cakiroglu, J. Supercond Nov. Magn. 27 (2014) 799. [4] L. Arda, M. Açıkgöz, A. Güngör, J. Supercon. Nov. Mag. 25 (2012) 2701. [5] K.S. Syed Ali, R. Saravanan, S. Israel, M. Açıkgöz, L. Arda, Physica B 405 (2010) 1763. [6] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [7] Z.K. Heiba, L. Arda, M. Bakr Mohammed, Y. Nasser Mostafa, N. Dogan, J. Supercond Nov. Magn. 26 (2013) 3487. [8] X.H. Xu, H.J. Blythe, M. Ziese, A.J. Behan, J.R. Neal, A. Mokhtari, R.M. Ibrahim, A.M. Fox, G.A. Gehring, New J. Phys. 8 (2006) 135. [9] C. Yang, X.M. Li, X.D. Gao, X. Cao, R. Yang, Y.Z. Li, Solid State Commun. 151 (2011) 264. [10] I.S. Kim, B.T. Lee, Cryst. Growth Des. 10 (2010) 3273. [11] S. Tanasoi, G. Mitran, N. Tanchoux, T. Cacciaguerra, F. Fajula, I. Sandulescu, D. Tichit, I. Cezar Marcu, Appl. Catal. A – Gen. 395 (2011) 78. [12] L. Arda, M. Acikgoz, Z.K. Heiba, N. Dogan, D. Akcan, O. Cakiroglu, Solid State Commun. 170 (2013) 14. [13] Z.K. Heiba, L. Arda, Cryst. Res. Technol. 44 (8) (2009) 845. [14] L. Arda, O. Ozturk, E. Asikuzun, S. Ataoglu, Powder Technol. 235 (2013) 479. [15] M. Tosun, S. Ataoglu, L. Arda, O. Ozturk, E. Asikuzun, D. Akcan, O. Cakiroglu, Mater. Sci. Eng. A 590 (2014) 416. [16] Z.K. Heiba, L. Arda, J. Mol. Struct. 1022 (2012) 167. [17] Z.K. Heiba, L. Arda, Mohammed Bakr Mohammed, M.A. Al-Jalali, N. Dogan, J. Supercond. Nov. Magn. 26 (2013) 3299. [18] L. Lutterotti, Maud 2.072 〈http://www.mg.unitn.it/ Luttero/maud〉. [19] V.A. Niketenko, J. Appl. Spectrosc. 56 (1994) 783. [20] N.Y. Garces, N.C. Giles, L.E. Halliburton, G. Cantwell, D.B. Eason, D.C. Reynolds, D.D. Look, Appl. Phys. Lett. 80 (2002) 1334.
W⊥
9
W| |
8
W (G)
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5
7 6 5 4 3 0.78
0.80
0.82
0.84
0.86
0.88
0.90
0.92
0.94
Concentration of Zn Fig. 10. Dependence of the line-widths W of ESR spectra on Zn concentration for Zn0.98 xMgxAl0.02O nanoparticles.
concentration of Mg for these polycrystalline nanoparticles is x ¼0.1 to have minimum g-factor and maximum line-width W.
Please cite this article as: O. Cakiroglu, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.03.032i
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Synthesis, structure and ESR studies of Mg doped ZnAlO nanoparticles Q1
O. Cakiroglu a, M. Acikgoz b, L. Arda b, D. Akcan b, N. Dogan c a b
Q3
c
Istanbul University, Hasan Ali Yucel Education Faculty, Beyazit, 34452 Istanbul, Turkey Bahcesehir University, Faculty of Arts and Sciences, Department of Mathematics & Computer Sciences, Besiktas, 34349 Istanbul, Turkey Gebze Institute of Technology, Department of Physics, 41400 Gebze/Kocaeli, Turkey
art ic l e i nf o
a b s t r a c t
Article history: Received 7 January 2014 Received in revised form 24 February 2014 Accepted 7 March 2014
Zn0.98 xMgxAl0.02O solutions with different (x ¼0.05, 0.10, 0.15, and 0.20) compositions were synthesized by the sol–gel technique using Zn, Mg and Al based alkoxide. The effects of highly Mg doping ratio on structure and magnetic properties were investigated systematically. The phase and the crystal structure of the Zn0.98 xMgxAl0.02O nanoparticles were characterized using X-ray diffraction. Scanning Electron Microscope and X-ray diffraction were utilized to understand the size and microstructure of samples. We observed the particle sizes of nanoparticles between 80 nm and 100 nm range. Furthermore, ESR spectra of Zn0.98 xMgxAl0.02O nanoparticles were collected at room temperature on a Bruker EMX model X-band spectrometer operating at a frequency of 9.50 GHz. It is observed that the critical concentration of Mg, x ¼ 0.1, has minimum g-factor and maximum line-width (W). & 2014 Published by Elsevier B.V.
Keywords: Magnetic materials Nanostructures Sol–gel growth Magnetic properties
1. Introduction Diluted magnetic semiconductors (DMSs) have attracted a great deal of interest for their potential technological applications to optoelectronics, spintronics, and microwave devices. Particularly, there has been an increasing interest to investigate the physical properties of binary ZnO doped with transition metal (TM) ions such as Mn, Co, V, Cr, Fe, and Ni. Many prominent researchers [1–5] have focused to explain the origin of magnetism in the DMSs materials and to correlate magnetic properties with the TM and deep level doping concentrations after the theoretical prediction of the possibility of above room temperature ferromagnetism [6,7]. Recently, co-doped ZnO materials forming a quadruple compound are receiving remarkable and increasing interest. It may be reasonably expected that the co-doping may result in some interesting properties. For example, strongly enhanced magnetic moments were obtained in ZnMnAlO by finding a clear relationship between the carrier density and magnetism [8]. Previously, Yang et al. investigated Zn1 x yMgxAlyO films prepared by pulsed laser deposition (PLD) and attained the band gap modulation and high conductivity as a result of Mg–Al co-doping [9]. Kim et al. studied the emission spectra of ZnMgAlO films through the cathodoluminescence (CL) measurement [10]. Moreover, Tanasoi et al. characterized and compared the catalytic activity of ZnMgAlO with the other mixed oxides containing transition metals [11]. Also, to analyze the effect of Al concentration on ZnMgAlO, we have recently studied the synthesis, characterization and ESR of powder Zn0.95 xMg0.05AlxO (x¼0, 0.01, 0.02, 0.05,
and 0.1) nanocrystals [12]. It was found that crystallite sizes decreased while microstrain increased with the increase in Al concentration and microstrain was less along the direction 〈0001〉 than along the basal directions 〈1010〉. Also, a new ESR line was seen as a result of the Al concentration in the ZnMgAlO system. After understanding the effect of Al concentration on ZnMgAlO nanoparticles in [12], in the present study we now aim to investigate the effect of Mg concentration on these quadruple nanoparticles by keeping the Al constant. For this, we have prepared the mixed oxides Zn0.98 xMgxAl0.02O as nano-polycrystalline powders by a simple sol–gel technique, which is the most promising for low cost, and low temperature processing technique to prepare complex oxide composition with high homogeneity. We have characterized the changes in the structure and microstructure using X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) and investigated magnetic properties using electron spin resonance (ESR) spectra of ZnMgAlO nanoparticle samples upon increasing Mg content.
2. Experimental part: The mixed oxides Zn0.98 xMgxAl0.02O were prepared as polycrystalline nanoparticles with various compositions (0.05rxr0.20) using sol–gel technique. Zinc acetate dihydrate (C4H6O4Zn 2H2O), Mg 2, 4 pentanedionate ([CH3COCHC(O)CH3]2Mg∙2H2O, MgAcAc) and Al 2, 4 pentanedionate (Al(C5H7O2)3, AlAcAc) were used as precursor materials. Methanol (CH3OH) and acetyl acetone (C5H8O2) were used as solvents and chelating agent. The details of the
http://dx.doi.org/10.1016/j.jmmm.2014.03.032 0304-8853/& 2014 Published by Elsevier B.V.
Please cite this article as: O. Cakiroglu, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.03.032i
preparation of Zn0.98 xMgxAl0.02O nanoparticles were discussed in the previous studies [13–16]. The as prepared powders were ground and annealed individually in air at 600 1C using box furnace. XRD scans were recorded using a Rigaku diffractmeter with Cu Kα radiation. Microstructure properties of prepared samples were observed using scanning electron microscope (SEM) (JEOL, JSM5910LV). The thermal behavior of the xerogels of Zn0.98 x MgxAl0.02O solutions, which were dried at room temperature for 3 days, was studied by using differential thermal analysis (DTA), and thermogravimetric analysis (TG)/SII 7300 Exstar thermal analyzer system in air with a heating rate of 10 1C/min. ESR spectra of Zn0.98 xMgxAl0.02O nanoparticles were collected at room temperature on a Bruker EMX model X-band spectrometer operating at a frequency of 9.50 GHz. The static magnetic field was varied in the range 0–16000G. The field derivative of microwave power absorption (dW/dH1) was registered as a function of the applied magnetic field H1. All the samples were loaded into quartz ESR tubes.
evaporation of volatile organic component. The percentage of weight was 8%. The second slight endotermic peak appeared between 90 1C and 278 1C in DTA curve corresponding to the second weight decrease in the TG curve, in which the residual solvent and organics evaporated. The percentage of weight was 12%. As the temperature increases to 340 1C, a strong exotermic peak and at 375 1C a weak shoulder peak are observed in DTA curve, which correspond to a higher level of weight loss in the TG
MgO ZnO
3. Results and discussion 3.1. 0Thermal analysis Fig. 3. The XRD profile fitting resulting from Rietveld analysis of Zn0.98 xMgx Al0.02O, x Z 0.2 showing two phases (MgO and ZnO).
Exo
DTA and TG data were analyzed so as to understand the thermal reaction of ZnMgAlO nanoparticles. We analyzed the Zn0.93Mg0.05Al0.02O xerogels within the temperature range 50 and 1000 1C in air. Fig. 1 shows some exotermic and endotermic peaks until 510 1C. A strong endotermic peak is seen in the DTA curve between 50 1C and 90 1C corresponding to a first weight decrease in the TG curve, which is the removal of solvent and the
DTA (μV)
Weight (μg)
TG
Endo
DTA
Temperature (°C)
Fig. 1. TG–DTA curves of the Zn0.93Mg0.05Al0.02O xerogel.
1200 1000
Intensity (a.u )
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
O. Cakiroglu et al. / Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
800
Zn0.73Mg0.25Al0.02 Zn0.78Mg0.20Al0.02
600
Zn0.83Mg0.15Al0.02
400
Zn0.88Mg0.1Al0.02
200
Zn0.93Mg0.05Al0.02
0 10
20
30
40
50
60
70
2 Theta (°) Fig. 2. The X-ray diffraction patterns of the ZnMgAlO nanoparticles.
80 Fig. 4. (a) SEM micrograph and (b) EDS of Zn0.93Mg0.05Al0.02O nanoparticles at 600 1C for 30 min.
Please cite this article as: O. Cakiroglu, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.03.032i
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curve. It was supposed that the carbon-based materials were born out and the oxidation was occurred and ended at 510 1C. The percentage of third weight was 50%. 3.2. 0Structure analysis
3
nanoparticles increase. Referring to the SEM micrographs molten structure is observed at most for x ¼0.2 concentration (Fig. 7a). The particles are quasi spherical, agglomerating and dense. The nanoparticle sizes were observed approximately between 80 nm and 100 nm. The composition of Zn0.98 xMgxAl0.02 nanoparticles after annealing at 600 1C for 30 min is detected by EDS. EDS of Zn0.98 xMgxAl0.02O (x¼ 0.05, 0.1, 0.15, and 0.2) nanoparticles are shown in Figs. 4–7b. Zn, Mg and Al peaks are clearly seen and also Mg peaks increase with the increase in concentration. Zn, Mg and Al contents of the nanoparticles are the same as those in the preparation of samples.
The obtained powders were annealed at 600 1C from 30 min under air using box furnace. The X-ray diffraction of Zn0.98 xMgxAl0.02O (x¼0.05, 0.1, 0.15, and 0.2) nanoparticles annealed at 600 1C for 30 min in the air are shown in Fig. 2. The reflections correspond to a single phase ZnO with wurtite hexagonal structure (ICDD card No. 36-1451) of space group P63mc for xo0.2 concentration. Mg and Al ions incorporated substitutionally in the ZnO lattice replacing the Zn ions in the position 2b [17]. Applying the MAUD program [18], from Rietveld analysis, the occupation number of the Mg and Al ions in the position 2b is nearly equal to the intended value during the preparation for each value of xo0.2. Fig. 3 indicates two phases ZnO and MgO identified in the diffraction patterns for xZ0.2. The limited miscibility of Mg ions in the ZnO lattice are shown as in Fig. 3, which depicts the pattern fitting resulting from Rietveld quantitative phase analysis for x¼ 0.25. The microstructure of Zn0.98 xMgxAl0.02O nanoparticles with various compositions (0.05 rx r0.20) were examined by SEM as shown in Figs. 4–7a. As seen in the figures, when the Mg concentration is increased, the agglomeration and malting of
ESR measurements have been performed and analyzed through concentration dependence of the g-factor and the line-widths W of ESR spectra. Experimental and simulated X band ESR spectra of Zn0.98 xMgxAl0.02O powders with different doping concentration of Mg recorded at room temperature are shown in Fig. 8. It is seen that the ESR spectra are dominated by a strong peak at around 3450G (around g0 ¼1.97), which can be attributed to the conduction electrons located at the surface center of ZnO particles [19,20]. Upon the increasing of Mg concentration, in addition to this main resonance signal, a weak resonance signal is observed at around 3375G with a g-value close to 2.0013 (g0), which characterizes a
Fig. 5. (a) SEM micrograph and (b) EDS of Zn0.88Mg0.1Al0.02O nanoparticles at 600 1C for 30 min.
Fig. 6. (a) SEM micrograph and (b) EDS of Zn0.83Mg0.15Al0.02O nanoparticles at 600 1C for 30 min.
3.3. 0Magnetic analysis
Please cite this article as: O. Cakiroglu, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.03.032i
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O. Cakiroglu et al. / Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
new center with 4G. The same weak signal was also observed in Zn0.95 xMg0.05AlxO with the increasing of the Al concentration [12]. It shows that the presence of this signal does not depend on the variation in Mg or Al concentration and may be attributed to the Zn vacancy in the structure. A computer simulation of the ESR spectrum of each sample has been carried out to derive the related EPR parameters. The extracted spin Hamiltonian parameters (g ? and g jj ) with the corresponding line-width parameters (W ? and W jj ) for the powdered forms are given in Table 1. The line-widths of the ESR signals take different values between 3.6 and 10.4G when the Mg concentration is changed from 0.05 to 0.2. As can be seen in Table 1, the values of g ? and g jj are less than the free electron value ge (2.0023). For Zn0.83Mg0.15Al0.02O, g ? ¼g jj (isotropic) while the trend g jj 4 g ? (anisotropic) is obtained for the other samples. The isotropic parameter g 0 in Table 1 can be determined from the anisotropic one using the following equation: g 0 ¼ ð2g ? þ g jj Þ=3. For better visualization the variation of g-factors g ? , g jj , and g 0 with the concentration of Zn in Zn0.98 xMgxAl0.02O are shown in Fig. 9. Also, the dependence of the line-widths W ? and W jj of ESR spectra on Zn concentration is demonstrated in Fig. 10. It appears that the critical concentration of Mg for the observation of minimum g-factor and maximum W is x ¼0.1.
4. Conclusions
Fig. 7. (a) SEM micrograph and (b) EDS of Zn0.78Mg0.2Al0.02O nanoparticles at 600 1C for 30 min.
The mixed oxides Zn0.98 xMgxAl0.02O (0.05 rx r0.20) were prepared as polycrystalline nanoparticles using sol–gel technique. Rietveld analysis showed that the reflections of Mg doped ZnAlO nanoparticles correspond to ZnO hexagonal wurthzite structure without secondary phases for x o0.2 concentration, whereas for the higher concentration two phases of ZnO and MgO were obtained. The results of the ESR analysis indicate that the critical
Fig. 8. Experimental and simulated X band ESR spectra of (a) Zn0.78Mg0.2Al0.02O; (b) Zn0.83Mg0.15Al0.02O; (c) Zn0.88Mg0.1Al0.02O; and (d) Zn0.93Mg0.05Al0.02O samples at room temperature.
O. Cakiroglu et al. / Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎ Table 1 The values of the g-factor and line-width parameters derived from the simulations of the experimental ESR spectra.
Acknowledgments
Sample
g?
gjj
g0
W ? ðGÞ
W jj ðGÞ
The present work was supported by the Research Fund of Bahcesehir University.
Zn0.78Mg0.2Al0.02O Zn0.83Mg0.15Al0.02O Zn0.88Mg0.1Al0.02O Zn0.93Mg0.05Al0.02O
1.9704 1.9701 1.9673 1.9697
1.9741 1.9701 1.9692 1.9702
1.9716 1.9701 1.9679 1.9699
5.20 3.60 4.30 3.65
5.20 3.60 10.40 8.10
References
1,974
g⊥ g
1,973
g0 g-factor
1,972 1,971 1,970 1,969 1,968 1,967 0.78
0.80
0.82
0.84
0.86
0.88
0.90
0.92
0.94
Concentration of Zn Fig. 9. Variation of g-factors with the concentration of Zn in Zn0.98 xMgxAl0.02O nanoparticles.
11 10
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W⊥
9
W| |
8
W (G)
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7 6 5 4 3 0.78
0.80
0.82
0.84
0.86
0.88
0.90
0.92
0.94
Concentration of Zn Fig. 10. Dependence of the line-widths W of ESR spectra on Zn concentration for Zn0.98 xMgxAl0.02O nanoparticles.
concentration of Mg for these polycrystalline nanoparticles is x ¼0.1 to have minimum g-factor and maximum line-width W.
Please cite this article as: O. Cakiroglu, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.03.032i