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Room temperature ferromagnetism in Zn–Mn–O
Solid State Communications 141 (2007) 641–644 www.elsevier.com/locate/ssc
Room temperature ferromagnetism in Zn–Mn–O D. Milivojevi´c ∗ , J. Blanuˇsa, V. Spasojevi´c, V. Kusigerski, B. Babi´c-Stoji´c Vinˇca Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia Received 13 December 2006; received in revised form 11 January 2007; accepted 19 January 2007 by H. Ohno Available online 25 January 2007
Abstract Zn–Mn–O semiconductor crystallites with nominal manganese concentration x = 0.01, 0.04 and 0.10 were synthesized by a solid state reaction method using oxalate precursors. A sintering procedure was carried out in air at 500 and 900 ◦ C. The samples were investigated by X-ray diffraction, magnetization measurements and electron paramagnetic resonance. X-ray diffraction spectra reveal that the dominant crystal phase in the Zn–Mn–O system corresponds to the wurtzite structure of ZnO. Room temperature ferromagnetism is observed in Zn–Mn–O samples with manganese concentrations x = 0.01 and 0.04 sintered at low temperature (500 ◦ C). Saturation magnetization in the x = 0.01 sample is found to be 0.03µ B /Mn at T = 300 K. The ferromagnetic phase seems to be developed by Zn diffusion into Mn-oxide grains. c 2007 Elsevier Ltd. All rights reserved.
PACS: 75.50.Pp Keywords: A. Magnetic semiconductors; A. Zn–Mn–O; D. Room temperature ferromagnetism
1. Introduction In recent years there has been a great interest in semiconducting materials that exhibit ferromagnetism above room temperature (RT) [1–3]. Dietl et al. [4] predicted ferromagnetism with a Curie temperature Tc above RT in ptype GaN and ZnO doped with Mn by a mean-field theory. The possible existence of a ferromagnetic state in p-type Mndoped ZnO and in n-type Fe-, Co- and Ni-doped ZnO was also predicted by a band calculation [5,6]. Since then intensive investigation of transition metal doped II–VI and III–V compounds has taken place. Mn-doped ZnO has attracted much attention due to disagreements about both the existence and the origin of RT ferromagnetism. Sharma et al. [7] observed hightemperature ferromagnetism in low-temperature processed bulk and thin film samples of Mn-doped ZnO and have discussed in favour of carrier-induced interactions between isolated Mn ions in ZnO. Contrary to this report, several authors have argued that high-temperature ferromagnetism in the low-temperature processed Mn-doped samples originated from an oxygenvacancy-stabilized metastable phase Mn2−x Znx O3−δ [8,9]. ∗ Corresponding author. Tel.: +381 64 276 3191; fax: +381 11 344 0100.
E-mail address: [email protected] (D. Milivojevi´c). c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.01.019
In addition, some recent studies show the absence of ferromagnetic ordering in bulk single phase Zn1−x Mnx O material down to 2 K [10–12]. Such inconsistent results have been also obtained for Mn-doped ZnO thin film samples which extend from paramagnetic [13] to spin-glass behaviour [14], and to low-temperature ferromagnetism [15]. Different results obtained for Zn–Mn–O by different methods and by different groups suggest a strong dependence of the magnetic properties of this system on the preparation conditions. In most of the previous investigations of bulk Zn–Mn–O samples the researchers followed the synthesis procedure described by Sharma et al. [7] starting from the same precursors, ZnO and MnO2 [8,9,16–18]. In this work we synthesized Zn–Mn–O samples using oxalate precursors in order to investigate the properties of such a prepared Zn–Mn–O system. 2. Experimental details Polycrystalline Zn–Mn–O samples were prepared by a solid state sintering route using zinc oxalate dihydrate (ZnC2 O4 ·2H2 O, 99.999%, Alfa Aesar) and manganese oxalate dihydrate (MnC2 O4 ·2H2 O, 99%, Alfa Aesar) as starting materials. Appropriate amounts of (ZnC2 O4 )1−x and (MnC2 O4 )x were mixed, pressed into pellets and calcinated at 400 ◦ C for 5 h in air. The calcinated samples were reground,
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pelletized and sintered at 500 ◦ C and 900 ◦ C for 12 h in air to obtain Zn–Mn–O samples with a nominal manganese concentration x = 0.01, 0.04 and 0.10. Powder X-ray Diffraction (XRD) spectra were recorded on a Philips PW 1050 diffractometer using CuKα radiation. The magnetization measurements were carried out on a SQUID magnetometer (MPMS XL-5, Quantum Design) in the temperature range 5–300 K. Electron Paramagnetic Resonance (EPR) experiments were performed at room temperature on a Varian E-line spectrometer operating at a nominal frequency of 9.5 GHz. 3. Results and discussion XRD patterns of Zn–Mn–O samples with nominal manganese concentration x = 0.01, 0.04 and 0.10 sintered at 500 ◦ C in air and an x = 0.10 sample sintered at 900 ◦ C in air are presented in Fig. 1. The indices in spectrum (a) of Fig. 1 indicate the expected peak positions for the wurtzite crystal structure of ZnO, the vertical ticks and indices in spectrum (c) of Fig. 1 indicate the peak positions of the impurity phase with tetragonal crystal structure, and those in spectrum (d) indicate the peak positions of the impurity phase having cubic symmetry. It can be seen that dominant crystal phase in the sintered Zn–Mn–O samples is the wurtzite structure of ZnO (space group P63 mc). The XRD data were subject to Rietveld analysis. The lattice parameters for the samples with x = 0.01, 0.04 and 0.10 thermally treated at 400, 500 and 900 ◦ C in air are found to be very close to the crystal lattice parameters of ZnO, ˚ and c = 5.207 A ˚ [1]. No variation of the lattice paa = 3.250 A rameters with increasing manganese concentration is observed. Changes of the a and c parameters with increasing temperature of thermal treatment from 400 to 900 ◦ C are also not significant. These results confirm the earlier observations [9,11,19] that the solubility of Mn in the ZnO lattice is very low. The mean crystallite size determined from the width of the X-ray diffraction lines using Scherrer’s formula is found to increase with increasing sintering temperature: d ≈ 30 nm for the samples thermally treated at 400 ◦ C, d ≈ 45 nm for the samples sintered at 500 ◦ C and d ≈ 100 nm for the samples sintered at 900 ◦ C. In the Zn–Mn–O sample with x = 0.10 sintered at 500 ◦ C a secondary phase is clearly observed (Fig. 1(c)). We have indexed all the impurity XRD lines of this secondary phase to ZnMn2 O4 with tetragonal symmetry (space group I 41 /amd). In the Zn–Mn–O sample with x = 0.10 sintered at 900 ◦ C the observed impurity phase has been also identified as ZnMn2 O4 , but with cubic symmetry (space group Fd3 m) (Fig. 1(d)). A non-uniform distribution of Mn ions in the ZnO lattice has been observed so far in several studies [9, 20]. The appearance of the impurity phase with spinel structure (Zn1−x Mn(II)x )[Mn(III)]2 O4 having cubic symmetry was detected in the Mn-doped ZnO nanoparticles prepared by a co-precipitation method after annealing the 2% and 5% Mndoped samples at temperatures 1075 < T < 1275 K [20]. The tetragonal phase of ZnMn2 O4 was observed in the ZnMnO bulk sample with 1% Mn sintered in air at 900 ◦ C [9]. The appearance of the ZnMn2 O4 phase in our Zn–Mn–O samples
Fig. 1. X-ray diffraction patterns at T = 300 K for Zn–Mn–O: (a) x = 0.01 sintered at 500 ◦ C, (b) x = 0.04 sintered at 500 ◦ C, (c) x = 0.10 sintered at 500 ◦ C, (d) x = 0.10 sintered at 900 ◦ C. The indices in spectrum (a) indicate the peak positions for the wurtzite phase of ZnO, the vertical ticks and indices in spectrum (c) indicate the peak positions of the tetragonal ZnMn2 O4 , and those in spectrum (d) correspond to the cubic phase of ZnMn2 O4 .
Fig. 2. Temperature dependent magnetization in the ZFC state at 500 Oe for the Zn–Mn–O samples sintered at 500 ◦ C.
at sintering temperatures as low as 500 ◦ C is probably the result of fast decomposition of the starting materials used in the synthesis. The temperature dependence of magnetization under a magnetic field of 500 Oe for the Zn–Mn–O samples with x = 0.01, 0.04 and 0.10 sintered in air at 500 ◦ C for 12 h is presented in Fig. 2. At higher temperatures the magnetization of the x = 0.01 sample is flat and larger than that for the x = 0.04 and 0.10 samples. In addition, the sample with x = 0.01 shows a marked kink in its M(T ) dependence at about 65 K. Such properties of the Zn–Mn–O material stimulated us to investigate the magnetic field dependence of magnetization in the studied samples, which is shown in Figs. 3, 4 and 6. A hysteresis loop in the M(H ) dependence for the x = 0.01 sample sintered at 500 ◦ C is observed at T = 300 K (Fig. 3) with coercive field Hc = 800 Oe and remanent magnetization Mr = 0.005 emu/g (inset of Fig. 3). The M(H ) dependence carried out at T = 250 K for the x = 0.04 sample sintered at 500 ◦ C gives Hc = 50 Oe and Mr = 0.0001 emu/g. Contrary to this observation, there is no RT ferromagnetism in the sample
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Fig. 3. Field dependent magnetization curve at T = 300 K for the Zn–Mn–O sample with x = 0.01 sintered at 500 ◦ C. The inset shows a detail of the loop in the low field region.
Fig. 5. EPR spectra at T = 300 K for the Zn–Mn–O samples: (a) x = 0.01 sintered at 500 ◦ C, (b) x = 0.04 sintered at 500 ◦ C, (c) x = 0.10 sintered at 500 ◦ C, (d) x = 0.01 sintered at 900 ◦ C.
Fig. 4. Field dependent magnetization at T = 300 K for the Zn–Mn–O samples with x = 0.10 sintered at 500 ◦ C and with x = 0.01 sintered at 900 ◦ C.
x = 0.10 sintered at 500 ◦ C (Fig. 4). The M(H ) dependence for this sample is a linear function in the magnetic field range up to 50 kOe, indicating the paramagnetic origin of the magnetization at T = 300 K. RT ferromagnetism does not exist either in the x = 0.01 sample sintered at 900 ◦ C. Its magnetization can be ascribed entirely to the paramagnetic component (Fig. 4). The EPR spectra of the Zn–Mn–O samples at RT are presented in Fig. 5. A broad resonance appears in the samples with x = 0.01 and 0.04 sintered at 500 ◦ C at the lower field side which disappears in the x = 0.10 sample sintered at 500 ◦ C and in the x = 0.01 sample sintered at 900 ◦ C. The broad signal is attributed to a ferromagnetic phase in this material. At the same time the EPR spectrum arising from the paramagnetic moments of isolated Mn ions is detected at higher field side in the form of fine and hyperfine lines in all the Zn–Mn–O samples (Fig. 5). The isolated Mn ions do not contribute to ferromagnetic ordering. Subtracting the paramagnetic component from the total magnetization for the x = 0.01 and x = 0.04 samples sintered at 500 ◦ C, we evaluated ferromagnetic component of the magnetization with saturation value Ms = 0.0195 emu/g for x = 0.01 at T = 300 K and Ms = 0.0013 emu/g
Fig. 6. Ferromagnetic component of the magnetization obtained after subtracting the paramagnetic contribution for the Zn–Mn–O samples with x = 0.01 at T = 300 K and with x = 0.04 at T = 250 K, both sintered at 500 ◦ C.
for the x = 0.04 sample at T = 250 K (Fig. 6). Taking into account the manganese concentration in the x = 0.01 sample determined by an atomic absorption method, which is a little less than that used in the synthesis, ∼0.8 at.%, and estimated saturation magnetization at RT, the average magnetic moment per Mn ion is found to be 0.03µ B /Mn at 300 K. The small value of the magnetic moment per Mn ion was reported for the bulk Mn-doped ZnO [7,21]
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and also for Mn-doped ZnO nanoparticles [20]. This result suggests that there is competition between antiferromagnetic and ferromagnetic interactions of the Mn ions. Due to antiferromagnetic competition ferromagnetic interactions are suppressed, resulting in magnetization of the Mn ions which is much smaller than the theoretical value. Experimental results of this work show that the RT ferromagnetism in the Zn–Mn–O samples does not originate from the ZnMn2 O4 phase. The ZnMn2 O4 phase with tetragonal symmetry was observed in the x = 0.10 sample sintered at 500 ◦ C, but the RT ferromagnetism in this sample was not detected. On the other hand, the ZnMn2 O4 phase was not observed in the x = 0.01 sample where, for a sintering temperature of 500 ◦ C, the ferromagnetic component is the dominant part of the RT magnetization. The EPR spectra show the existence of isolated Mn ions in the ZnO lattice. The absence of dependence of the ZnO lattice parameter on the manganese concentration and sintering temperature indicates that only some of the Mn ions have diffused into the ZnO lattice. It appears that only p-type defects in Mn2+ :ZnO can produce high-TC ferromagnetism. This property was predicted theoretically [4] and shown experimentally by doping of Mn2+ :ZnO with nitrogen [22]. On the contrary, n-type defects in Mn2+ :ZnO introduced by Zn vapour diffusion [22] or by hydrogen annealing [23] do not stabilize long range Mn–Mn ferromagnetic coupling. Such properties of Mn2+ :ZnO can be explained within a model proposed by Kittilstved et al. [22]. The Zn–Mn–O samples studied in this work were not intentionally doped with any kind of impurity, so one could not expect the appearance of ferromagnetic ordering of the Mn ion magnetic moments in the ZnO lattice. We have subjected manganese oxalate to the same thermal treatment as that used in the preparation of the Zn–Mn–O samples. After sintering of MnC2 O4 at 500 ◦ C for 12 h, the Mn5 O8 phase was observed by X-ray diffraction. This phase is also expected to exist in a very small amount in the Zn–Mn–O samples sintered at 500 ◦ C. The RT ferromagnetism in the samples prepared at low sintering temperature (500 ◦ C) seems to arise by diffusion of Zn into Mn-oxide grains. The appearance of Zn diffusion into Mn-oxides has been observed so far in several works [8,16,17]. Diffusion of Zn into Mnoxides such as Mn5 O8 or MnO2 favours reduction into other oxides with lower Mn oxidation state like Mn2 O3 , making the coexistence of phases with mixed valence states quite possible. 4. Conclusion Room temperature ferromagnetism has been observed in Zn–Mn–O samples with lower manganese concentration, x = 0.01 and x = 0.04, sintered at a low temperature (500 ◦ C). The structural and magnetic properties of the Zn–Mn–O
samples show that it is a non-homogeneous material. X-ray diffraction, magnetization and EPR studies suggest that the room temperature ferromagnetism arises from diffusion of Zn into the Mn-oxide grains. Acknowledgments Financial support for this study was granted by the Ministry of Science and Environmental Protection of the Republic of Serbia, project nos 141013 and 141027. References [1] S.J. Pearton, W.H. Heo, M. Ivill, D.P. Norton, T. Steiner, Semicond. Sci. Technol. 19 (2004) R59. [2] S.J. Pearton, C.R. Abernathy, M.E. Overberg, G.T. Thaler, D.P. Norton, J. Appl. Phys. 93 (2003) 1. [3] T. Dietl, Semicond. Sci. Technol. 17 (2002) 377. [4] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [5] K. Sato, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 39 (2000) L555. [6] K. Sato, H. Katayama-Yoshida, Semicond. Sci. Technol. 17 (2002) 367. [7] P. Sharma, A. Gupta, K.V. Rao, F.J. Owens, R. Sharma, R. Ahuja, J.M. Osorio Guillen, B. Johansson, G.A. Gehring, Nat. Mater. 2 (2003) 673. [8] D.C. Kundaliya, S.B. Ogale, S.E. Lofland, S. Dhar, C.J. Metting, S.R. Shinde, Z. Ma, B. Varughese, K.V. Ramanujachary, L. SalamancaRiba, T. Vankatesan, Nat. Mater. 3 (2004) 709. [9] J. Zhang, R. Skomski, D.J. Sellmyer, J. Appl. Phys. 97 (2005) 10D303. [10] S. Kolesnik, B. Dabrowski, J. Mais, J. Appl. Phys. 95 (2004) 2582. [11] G. Lawes, A.S. Risbud, A.P. Ramirez, R. Seshadri, Phys. Rev. B 71 (2005) 045201. [12] H.W. Zhang, E.W. Shi, Z.Z. Chen, X.C. Liu, B. Xiao, Solid State Commun. 137 (2006) 272. [13] A. Tiwari, C. Jin, A. Kvit, D. Kumar, J.F. Muth, J. Narayan, Solid State Commun. 121 (2002) 371. [14] T. Fukumura, J. Zhengwu, M. Kawasaki, Appl. Phys. Lett. 78 (2001) 958. [15] S.W. Jung, S.J. An, G.C. Yi, C.U. Jung, S.I. Lee, S. Cho, Appl. Phys. Lett. 80 (2002) 4561. [16] J.L. Costa-Kr¨amer, F. Briones, J.F. Fern´andez, A.C. Caballero, M. Villegas, M. Diaz, M.A. Garcia, A. Hernando, Nanotechnology 16 (2005) 214. [17] M.A. Garcia, M.L. Ruiz-Gonz´ales, A. Quesada, J.L. Costa-Kr¨amer, J.F. Fern´andez, S.J. Khatib, A. Wennberg, A.C. Cabarello, M.S. Mart´ınGonz´ales, M. Villegas, F. Briones, J.M. Gonz´ales-Calbet, A. Hernando, Phys. Rev. Lett. 94 (2005) 217206. [18] W. Chen, L.F. Zhao, Y.Q. Wang, J.H. Miao, S. Liu, Z.C. Xia, S.L. Yuan, Solid State Commun. 134 (2005) 827. [19] M. Kakazey, M. Vlasova, M. Dominguez-Pati˜no, J. Kliava, T. Tomila, J. Amer. Ceram. Soc. 89 (2006) 1458. [20] O.D. Jayakumar, H.G. Salunke, R.M. Kadam, M. Mohapatra, G. Yaswant, S.K. Kulshreshtha, Nanotechnology 17 (2006) 1278. [21] W. Chen, L.F. Zhao, Y.Q. Wang, J.H. Miao, S. Liu, Z.C. Xia, S.L. Yuan, Appl. Phys. Lett. 87 (2005) 042507. [22] K.R. Kittilstved, N.S. Norberg, D.R. Gamelin, Phys. Rev. Lett. 94 (2005) 147209. [23] A. Manivannan, P. Dutta, G. Glaspell, M.S. Seehra, J. Appl. Phys. 99 (2006) 08M110.
Room temperature ferromagnetism in Zn–Mn–O D. Milivojevi´c ∗ , J. Blanuˇsa, V. Spasojevi´c, V. Kusigerski, B. Babi´c-Stoji´c Vinˇca Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia Received 13 December 2006; received in revised form 11 January 2007; accepted 19 January 2007 by H. Ohno Available online 25 January 2007
Abstract Zn–Mn–O semiconductor crystallites with nominal manganese concentration x = 0.01, 0.04 and 0.10 were synthesized by a solid state reaction method using oxalate precursors. A sintering procedure was carried out in air at 500 and 900 ◦ C. The samples were investigated by X-ray diffraction, magnetization measurements and electron paramagnetic resonance. X-ray diffraction spectra reveal that the dominant crystal phase in the Zn–Mn–O system corresponds to the wurtzite structure of ZnO. Room temperature ferromagnetism is observed in Zn–Mn–O samples with manganese concentrations x = 0.01 and 0.04 sintered at low temperature (500 ◦ C). Saturation magnetization in the x = 0.01 sample is found to be 0.03µ B /Mn at T = 300 K. The ferromagnetic phase seems to be developed by Zn diffusion into Mn-oxide grains. c 2007 Elsevier Ltd. All rights reserved.
PACS: 75.50.Pp Keywords: A. Magnetic semiconductors; A. Zn–Mn–O; D. Room temperature ferromagnetism
1. Introduction In recent years there has been a great interest in semiconducting materials that exhibit ferromagnetism above room temperature (RT) [1–3]. Dietl et al. [4] predicted ferromagnetism with a Curie temperature Tc above RT in ptype GaN and ZnO doped with Mn by a mean-field theory. The possible existence of a ferromagnetic state in p-type Mndoped ZnO and in n-type Fe-, Co- and Ni-doped ZnO was also predicted by a band calculation [5,6]. Since then intensive investigation of transition metal doped II–VI and III–V compounds has taken place. Mn-doped ZnO has attracted much attention due to disagreements about both the existence and the origin of RT ferromagnetism. Sharma et al. [7] observed hightemperature ferromagnetism in low-temperature processed bulk and thin film samples of Mn-doped ZnO and have discussed in favour of carrier-induced interactions between isolated Mn ions in ZnO. Contrary to this report, several authors have argued that high-temperature ferromagnetism in the low-temperature processed Mn-doped samples originated from an oxygenvacancy-stabilized metastable phase Mn2−x Znx O3−δ [8,9]. ∗ Corresponding author. Tel.: +381 64 276 3191; fax: +381 11 344 0100.
E-mail address: [email protected] (D. Milivojevi´c). c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.01.019
In addition, some recent studies show the absence of ferromagnetic ordering in bulk single phase Zn1−x Mnx O material down to 2 K [10–12]. Such inconsistent results have been also obtained for Mn-doped ZnO thin film samples which extend from paramagnetic [13] to spin-glass behaviour [14], and to low-temperature ferromagnetism [15]. Different results obtained for Zn–Mn–O by different methods and by different groups suggest a strong dependence of the magnetic properties of this system on the preparation conditions. In most of the previous investigations of bulk Zn–Mn–O samples the researchers followed the synthesis procedure described by Sharma et al. [7] starting from the same precursors, ZnO and MnO2 [8,9,16–18]. In this work we synthesized Zn–Mn–O samples using oxalate precursors in order to investigate the properties of such a prepared Zn–Mn–O system. 2. Experimental details Polycrystalline Zn–Mn–O samples were prepared by a solid state sintering route using zinc oxalate dihydrate (ZnC2 O4 ·2H2 O, 99.999%, Alfa Aesar) and manganese oxalate dihydrate (MnC2 O4 ·2H2 O, 99%, Alfa Aesar) as starting materials. Appropriate amounts of (ZnC2 O4 )1−x and (MnC2 O4 )x were mixed, pressed into pellets and calcinated at 400 ◦ C for 5 h in air. The calcinated samples were reground,
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pelletized and sintered at 500 ◦ C and 900 ◦ C for 12 h in air to obtain Zn–Mn–O samples with a nominal manganese concentration x = 0.01, 0.04 and 0.10. Powder X-ray Diffraction (XRD) spectra were recorded on a Philips PW 1050 diffractometer using CuKα radiation. The magnetization measurements were carried out on a SQUID magnetometer (MPMS XL-5, Quantum Design) in the temperature range 5–300 K. Electron Paramagnetic Resonance (EPR) experiments were performed at room temperature on a Varian E-line spectrometer operating at a nominal frequency of 9.5 GHz. 3. Results and discussion XRD patterns of Zn–Mn–O samples with nominal manganese concentration x = 0.01, 0.04 and 0.10 sintered at 500 ◦ C in air and an x = 0.10 sample sintered at 900 ◦ C in air are presented in Fig. 1. The indices in spectrum (a) of Fig. 1 indicate the expected peak positions for the wurtzite crystal structure of ZnO, the vertical ticks and indices in spectrum (c) of Fig. 1 indicate the peak positions of the impurity phase with tetragonal crystal structure, and those in spectrum (d) indicate the peak positions of the impurity phase having cubic symmetry. It can be seen that dominant crystal phase in the sintered Zn–Mn–O samples is the wurtzite structure of ZnO (space group P63 mc). The XRD data were subject to Rietveld analysis. The lattice parameters for the samples with x = 0.01, 0.04 and 0.10 thermally treated at 400, 500 and 900 ◦ C in air are found to be very close to the crystal lattice parameters of ZnO, ˚ and c = 5.207 A ˚ [1]. No variation of the lattice paa = 3.250 A rameters with increasing manganese concentration is observed. Changes of the a and c parameters with increasing temperature of thermal treatment from 400 to 900 ◦ C are also not significant. These results confirm the earlier observations [9,11,19] that the solubility of Mn in the ZnO lattice is very low. The mean crystallite size determined from the width of the X-ray diffraction lines using Scherrer’s formula is found to increase with increasing sintering temperature: d ≈ 30 nm for the samples thermally treated at 400 ◦ C, d ≈ 45 nm for the samples sintered at 500 ◦ C and d ≈ 100 nm for the samples sintered at 900 ◦ C. In the Zn–Mn–O sample with x = 0.10 sintered at 500 ◦ C a secondary phase is clearly observed (Fig. 1(c)). We have indexed all the impurity XRD lines of this secondary phase to ZnMn2 O4 with tetragonal symmetry (space group I 41 /amd). In the Zn–Mn–O sample with x = 0.10 sintered at 900 ◦ C the observed impurity phase has been also identified as ZnMn2 O4 , but with cubic symmetry (space group Fd3 m) (Fig. 1(d)). A non-uniform distribution of Mn ions in the ZnO lattice has been observed so far in several studies [9, 20]. The appearance of the impurity phase with spinel structure (Zn1−x Mn(II)x )[Mn(III)]2 O4 having cubic symmetry was detected in the Mn-doped ZnO nanoparticles prepared by a co-precipitation method after annealing the 2% and 5% Mndoped samples at temperatures 1075 < T < 1275 K [20]. The tetragonal phase of ZnMn2 O4 was observed in the ZnMnO bulk sample with 1% Mn sintered in air at 900 ◦ C [9]. The appearance of the ZnMn2 O4 phase in our Zn–Mn–O samples
Fig. 1. X-ray diffraction patterns at T = 300 K for Zn–Mn–O: (a) x = 0.01 sintered at 500 ◦ C, (b) x = 0.04 sintered at 500 ◦ C, (c) x = 0.10 sintered at 500 ◦ C, (d) x = 0.10 sintered at 900 ◦ C. The indices in spectrum (a) indicate the peak positions for the wurtzite phase of ZnO, the vertical ticks and indices in spectrum (c) indicate the peak positions of the tetragonal ZnMn2 O4 , and those in spectrum (d) correspond to the cubic phase of ZnMn2 O4 .
Fig. 2. Temperature dependent magnetization in the ZFC state at 500 Oe for the Zn–Mn–O samples sintered at 500 ◦ C.
at sintering temperatures as low as 500 ◦ C is probably the result of fast decomposition of the starting materials used in the synthesis. The temperature dependence of magnetization under a magnetic field of 500 Oe for the Zn–Mn–O samples with x = 0.01, 0.04 and 0.10 sintered in air at 500 ◦ C for 12 h is presented in Fig. 2. At higher temperatures the magnetization of the x = 0.01 sample is flat and larger than that for the x = 0.04 and 0.10 samples. In addition, the sample with x = 0.01 shows a marked kink in its M(T ) dependence at about 65 K. Such properties of the Zn–Mn–O material stimulated us to investigate the magnetic field dependence of magnetization in the studied samples, which is shown in Figs. 3, 4 and 6. A hysteresis loop in the M(H ) dependence for the x = 0.01 sample sintered at 500 ◦ C is observed at T = 300 K (Fig. 3) with coercive field Hc = 800 Oe and remanent magnetization Mr = 0.005 emu/g (inset of Fig. 3). The M(H ) dependence carried out at T = 250 K for the x = 0.04 sample sintered at 500 ◦ C gives Hc = 50 Oe and Mr = 0.0001 emu/g. Contrary to this observation, there is no RT ferromagnetism in the sample
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Fig. 3. Field dependent magnetization curve at T = 300 K for the Zn–Mn–O sample with x = 0.01 sintered at 500 ◦ C. The inset shows a detail of the loop in the low field region.
Fig. 5. EPR spectra at T = 300 K for the Zn–Mn–O samples: (a) x = 0.01 sintered at 500 ◦ C, (b) x = 0.04 sintered at 500 ◦ C, (c) x = 0.10 sintered at 500 ◦ C, (d) x = 0.01 sintered at 900 ◦ C.
Fig. 4. Field dependent magnetization at T = 300 K for the Zn–Mn–O samples with x = 0.10 sintered at 500 ◦ C and with x = 0.01 sintered at 900 ◦ C.
x = 0.10 sintered at 500 ◦ C (Fig. 4). The M(H ) dependence for this sample is a linear function in the magnetic field range up to 50 kOe, indicating the paramagnetic origin of the magnetization at T = 300 K. RT ferromagnetism does not exist either in the x = 0.01 sample sintered at 900 ◦ C. Its magnetization can be ascribed entirely to the paramagnetic component (Fig. 4). The EPR spectra of the Zn–Mn–O samples at RT are presented in Fig. 5. A broad resonance appears in the samples with x = 0.01 and 0.04 sintered at 500 ◦ C at the lower field side which disappears in the x = 0.10 sample sintered at 500 ◦ C and in the x = 0.01 sample sintered at 900 ◦ C. The broad signal is attributed to a ferromagnetic phase in this material. At the same time the EPR spectrum arising from the paramagnetic moments of isolated Mn ions is detected at higher field side in the form of fine and hyperfine lines in all the Zn–Mn–O samples (Fig. 5). The isolated Mn ions do not contribute to ferromagnetic ordering. Subtracting the paramagnetic component from the total magnetization for the x = 0.01 and x = 0.04 samples sintered at 500 ◦ C, we evaluated ferromagnetic component of the magnetization with saturation value Ms = 0.0195 emu/g for x = 0.01 at T = 300 K and Ms = 0.0013 emu/g
Fig. 6. Ferromagnetic component of the magnetization obtained after subtracting the paramagnetic contribution for the Zn–Mn–O samples with x = 0.01 at T = 300 K and with x = 0.04 at T = 250 K, both sintered at 500 ◦ C.
for the x = 0.04 sample at T = 250 K (Fig. 6). Taking into account the manganese concentration in the x = 0.01 sample determined by an atomic absorption method, which is a little less than that used in the synthesis, ∼0.8 at.%, and estimated saturation magnetization at RT, the average magnetic moment per Mn ion is found to be 0.03µ B /Mn at 300 K. The small value of the magnetic moment per Mn ion was reported for the bulk Mn-doped ZnO [7,21]
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and also for Mn-doped ZnO nanoparticles [20]. This result suggests that there is competition between antiferromagnetic and ferromagnetic interactions of the Mn ions. Due to antiferromagnetic competition ferromagnetic interactions are suppressed, resulting in magnetization of the Mn ions which is much smaller than the theoretical value. Experimental results of this work show that the RT ferromagnetism in the Zn–Mn–O samples does not originate from the ZnMn2 O4 phase. The ZnMn2 O4 phase with tetragonal symmetry was observed in the x = 0.10 sample sintered at 500 ◦ C, but the RT ferromagnetism in this sample was not detected. On the other hand, the ZnMn2 O4 phase was not observed in the x = 0.01 sample where, for a sintering temperature of 500 ◦ C, the ferromagnetic component is the dominant part of the RT magnetization. The EPR spectra show the existence of isolated Mn ions in the ZnO lattice. The absence of dependence of the ZnO lattice parameter on the manganese concentration and sintering temperature indicates that only some of the Mn ions have diffused into the ZnO lattice. It appears that only p-type defects in Mn2+ :ZnO can produce high-TC ferromagnetism. This property was predicted theoretically [4] and shown experimentally by doping of Mn2+ :ZnO with nitrogen [22]. On the contrary, n-type defects in Mn2+ :ZnO introduced by Zn vapour diffusion [22] or by hydrogen annealing [23] do not stabilize long range Mn–Mn ferromagnetic coupling. Such properties of Mn2+ :ZnO can be explained within a model proposed by Kittilstved et al. [22]. The Zn–Mn–O samples studied in this work were not intentionally doped with any kind of impurity, so one could not expect the appearance of ferromagnetic ordering of the Mn ion magnetic moments in the ZnO lattice. We have subjected manganese oxalate to the same thermal treatment as that used in the preparation of the Zn–Mn–O samples. After sintering of MnC2 O4 at 500 ◦ C for 12 h, the Mn5 O8 phase was observed by X-ray diffraction. This phase is also expected to exist in a very small amount in the Zn–Mn–O samples sintered at 500 ◦ C. The RT ferromagnetism in the samples prepared at low sintering temperature (500 ◦ C) seems to arise by diffusion of Zn into Mn-oxide grains. The appearance of Zn diffusion into Mn-oxides has been observed so far in several works [8,16,17]. Diffusion of Zn into Mnoxides such as Mn5 O8 or MnO2 favours reduction into other oxides with lower Mn oxidation state like Mn2 O3 , making the coexistence of phases with mixed valence states quite possible. 4. Conclusion Room temperature ferromagnetism has been observed in Zn–Mn–O samples with lower manganese concentration, x = 0.01 and x = 0.04, sintered at a low temperature (500 ◦ C). The structural and magnetic properties of the Zn–Mn–O
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