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On the origin of the ferromagnetism in Zn0.8Mn0.2O having a higher Curie temperature than Zn0.8Co0.2O
Solid State Communications 152 (2012) 488–492
Contents lists available at SciVerse ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
On the origin of the ferromagnetism in Zn0.8 Mn0.2 O having a higher Curie temperature than Zn0.8 Co0.2 O Chia-Lung Tsai a , Yow-Jon Lin a,∗ , Jia-Hong Chen a , Hsing-Cheng Chang b , Ya-Hui Chen c , Lance Horng d , Yu-Tai Shih d a
Institute of Photonics, National Changhua University of Education, Changhua 500, Taiwan
b
Department of Automatic Control Engineering, Feng Chia University, Taichung 407, Taiwan
c
Ph. D. Program in Electrical and Communications Engineering, Feng Chia University, Taichung 407, Taiwan
d
Department of Physics, National Changhua University of Education, Changhua 500, Taiwan
article
info
Article history: Received 14 September 2011 Received in revised form 25 December 2011 Accepted 31 December 2011 by R. Merlin Available online 10 January 2012 Keywords: A. Thin films A. Semiconductors C. Point defects
abstract Zn0.8 Co0.2O and Zn0.8 Mn0.2 O films were deposited on substrates by a sol–gel technique. X-ray diffraction, field-emission scanning electron microscopy, photoluminescence, and ferromagnetism measurements were used to characterize these dilute magnetic semiconductors. It is shown that the ferromagnetic properties might be related to the formation of acceptor-like defects in the Zn0.8 Co0.2O and Zn0.8 Mn0.2 O films. It is found that ferromagnetic Zn0.8 Mn0.2 O has a higher Curie temperature than Zn0.8 Co0.2O . In addition, the higher ratio of grain-boundary area to grain volume of Zn0.8 Mn0.2 O than Zn0.8 Co0.2O indicates that grain boundaries and related acceptors are the intrinsic origin for ferromagnetism. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Recently, dilute magnetic semiconductors (DMSs) based on ZnO [1–14] have attracted a great deal of attention because of their application in magnetic semiconductor devices. In particular, ZnO-based DMSs have been predicted to show ferromagnetic (FM) behavior with Curie temperature (TC ) above room temperature, and they have a large magnetization [6–14], which makes them a promising candidate material for the next generation of spintronic devices utilizing electronically or optically controlled magnetism. Dietl et al. theoretically predicted that ZnO doped with magnetic atoms (Co, Mn, or Fe) would possess FM behavior with a high TC above room temperature [6]. Wang et al. found that the high-temperature (TC higher than 350 K) FM for Zn1−x Mnx O nanoparticles prepared by a sol–gel process was strongly related to defects in ZnO [8]. More recently, structural defects in transition metal-doped ZnO have been shown to play an important role in the occurrence and stability of ferromagnetism. Based on firstprinciples calculations, Yan et al. proposed that the Zn vacancy (VZn ) can induce room-temperature ferromagnetism in Mn-doped ZnO [15]. Ramachandran et al. found tuning of ferromagnetism
∗
Corresponding author. Tel.: +886 4 7232105x3379; fax: +886 4 7211153. E-mail address: [email protected] (Y.-J. Lin).
0038-1098/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.12.046
in Zn1−x Mnx O DMSs by controlling the concentrations of oxygen vacancies and substitutional Mn dopants [16]. In a previously reported result [3], we found that changes in cobalt concentration and the number of VZn are important issues for producing strong ferromagnetism in Zn1−x Cox O DMSs prepared by a sol–gel method. However, Liu et al. suggested that ferromagnetic coupling in the Zn–Co–O system was induced by electron charge transfer between Co 3d and donor defects [17]. It is found that FM coupling depends on the electron localization of acceptor or donor defects [3,15–17]. To date, the origin of the ferromagnetism in oxide DMSs remains a very controversial topic. Therefore, there remain a lot of open questions or controversial opinions. In this study, we report that FM behavior is observed for Zn0.8 Mn0.2 O or Zn0.8 Co0.2 O films prepared by a sol–gel method. We found that FM Zn0.8 Mn0.2 O has a higher TC than Zn0.8 Co0.2 O. The relationships between defects and the magnetic properties for Zn0.8 Mn0.2 O (Zn0.8 Co0.2 O) films were also researched in this study. 2. Experimental details Zn0.8 Co0.2 O (Zn0.8 Mn0.2 O) DMSs were prepared by a sol–gel method using Zn(CH3 COO)2 ·2H2 O and Co(CH3 COO)2 ·4H2 O (Mn (CH3 COO)2 ·4H2 O) as starting precursors and 2-methoxyethanol and monoethanolamine (MEA) as solvent and stabilizer. The molar ratio of MEA to zinc acetate was maintained at 1.0 and the
C.-L. Tsai et al. / Solid State Communications 152 (2012) 488–492
concentration of zinc acetate was 0.5 M. The molar ratio of dopant in the solution (Co/Zn) or (Mn/Zn) was 25%. The solution was stirred at 80 °C for 2 h to yield a clear and homogeneous solution, which served as the coating solution after cooling to room temperature. The solution was dropped onto Si or glass substrates, which were rotated at 2000 rpm for 20 s, and rotated again at 3000 rpm for another 20 s. These Si and glass substrates were ultrasonically cleaned for 10 min each in acetone, then in methanol, then in deionized water, and dried in nitrogen before spin coating. After deposition by spin coating, the films were dried at 300 °C for 10 min on a hotplate to evaporate the solvent and remove organic residuals. The procedures from coating to drying were repeated five times. The films were then inserted into a furnace and annealed in air at 500 °C for 30 min. The Zn0.8 Co0.2 O (Zn0.8 Mn0.2 O) film thickness, as estimated from field-emission scanning electron microscopy (FESEM), was about 100 nm. ZnO films were also prepared by a sol–gel method. The structures and morphologies of the ZnO, Zn0.8 Co0.2 O, and Zn0.8 Mn0.2 O films were studied using X-ray diffraction (XRD) and FESEM. The magnetization measurements were carried out using a superconducting quantum interference device magnetometer. To confirm the presence of acceptors in the Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films and verify the influence of the formation of acceptors on the FM properties, photoluminescence (PL) spectra were obtained from Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films prepared by a sol–gel spin-coating method on Si substrates. Using a He–Cd laser (the 325 nm line) as an excitation source, the PL band was noted for Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films at 10 and 300 K, respectively. From these results, the effect of defects on the magnetic property of Zn0.8 Co0.2 O (Zn0.8 Mn0.2 O) is discussed. 3. Results and discussions Fig. 1 shows the XRD patterns of ZnO, Zn0.8 Co0.2 O, and Zn0.8 Mn0.2 O films prepared by the sol–gel spin-coating method on glass substrates. The XRD pattern of the glass substrate is also shown in Fig. 1. The figure shows a comparison of the XRD pattern from pure ZnO and that for Zn0.8 Co0.2 O films, showing that the location of the (002) diffraction peak shifts towards lower angle for Zn0.8 Co0.2 O films. Comparing the Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films, we find that the (002) peak shifted toward low angle for Zn0.8 Mn0.2 O films. Based on the XRD data presented in Fig. 1, the c-axis lattice parameter (cf ) of the Zn0.8 Co0.2 O (Zn0.8 Mn0.2 O or ZnO) films has been calculated using Bragg’s law [18]. The cf value for ZnO, Zn0.8 Mn0.2 O, and Zn0.8 Co0.2 O is 5.2102, 5.270, 5.227 Å, respectively. The large cf value may be attributed to the incorporation of defects in the form of acceptor-like interstitial oxygen (Oi ) during deposition [11]. Remashan et al. found a relationship between the c-axis (the (002) plane), grain size, and point defects [19]. In addition, the relationship between cf and point defects has been investigated by Pant et al. [20] and Janotti and Van de Walle [21]. In the literature, peak shifts toward a lower 2θ value with respect to the bulk peak profiles have been attributed to compressive stress in the films [22,23]. This effect could result in increasing the interplanar spacing, thus leading to the observed decrease in the diffraction angle. It is seen that the cf value for Zn0.8 Mn0.2 O/glass or Zn0.8 Co0.2 O/glass samples is larger than that for ZnO/glass samples, implying that Co or Mn doping may lead to the presence of compressive stress along the c-axis in the films. Fig. 2 shows the FESEM images of ZnO, Zn0.8 Co0.2 O, and Zn0.8 Mn0.2 O films. We find a larger grain size of ZnO than of Zn0.8 Co0.2 O and a larger grain size of Zn0.8 Co0.2 O than of Zn0.8 Mn0.2 O. In addition, Fig. 2 shows the higher ratio of grainboundary area to grain volume of Zn0.8 Mn0.2 O than Zn0.8 Co0.2 O. As the main challenge for practical applications of DMS materials is the attainment of TC around or preferably above room
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Fig. 1. XRD patterns of glass, Zn0.2 Co0.8 O, Zn0.2 Mn0.8 O, and ZnO films. For the sake of clarity, the intensity for the ZnO film is multiplied by 0.4.
temperature, we conducted a detailed study of the magnetization of Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films. Fig. 3 shows magnetization curves at 10 and 300 K for Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films prepared by the sol–gel spin-coating method on Si substrates. The magnetization curves were obtained with the applied field parallel to the plane of the samples. In Fig. 3, the hysteresis loops show clear FM behavior of the Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films. It is worth noting that the FM signal for Zn0.8 Mn0.2 O films at 10 (300) K was stronger than that for Zn0.8 Co0.2 O films at 10 (300) K. Fig. 4 shows the temperature dependence of the residual magnetization (Mr ) of Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films. A freehand extrapolation of the data to higher temperature for Mr → 0 yields TC ≈ 440 K (536 K) for Zn0.8 Co0.2 O (Zn0.8 Mn0.2 O) [24]. Lee et al. [25], Wang et al. [8], and Kittilstved and Gamelin [7] pointed out that TC > 350 K for ZnMnO and ZnCoO samples. Sharma et al. achieved FM with a TC of 425 K for ZnMnO samples [26]. We find that FM Zn0.8 Mn0.2 O has a higher TC than Zn0.8 Co0.2 O. The enhanced ferromagnetism in the grain boundaries is likely caused by increased defects [9]. Straumal et al. suggested that the grain boundaries and related defects are the intrinsic origin for room-temperature ferromagnetism [12]. Gu et al. found that the FM correlations are strongly influenced by the crystal structure [13]. Manivannan et al. pointed out that p-type doping in ZnMnO may be necessary to stabilize the FM state [10]. Mounkachi et al. found that higher values of TC are attained for high concentration of vacancy defect sites in ZnMnO [27]. Kim et al. stated that the lattice distortion is a crucial factor for VZn -induced ferromagnetism [28]. Wang et al. suggested that the adsorbed oxygen in the grain boundary caused the p-type conductivity, and that the high Hall mobility of p-type ZnO film was due to the quasi-two-dimensional hole gas, which was induced by the negatively charged interface states in the boundaries [29]. Based on the previously reported results [9,10,12,13,27–29], we deduce that acceptor-like defects (that is, Oi , VZn or oxygen-substituting zinc atoms (OZn )) may cause the enhancement of the FM characteristics, thus increasing TC . In addition, the higher ratio of grain-boundary area to grain volume of Zn0.8 Mn0.2 O than Zn0.8 Co0.2 O in Fig. 2 indicates that grain boundaries and related acceptors are the intrinsic origin for the ferromagnetism. Based on the experimental results, we suggest that defective grain boundaries have a key role with respect to the FM properties. PL spectra of Zn0.8 Co0.2 O films are shown in Fig. 5. For our samples, strong PL spectra were observed at 10 K whereas
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Fig. 2. FESEM images of ZnO, Zn0.2 Co0.8 O, and Zn0.2 Mn0.8 O films. The scale bar is 200 nm.
Fig. 3. Magnetization curves for Zn0.2 Co0.8 O and Zn0.2 Mn0.8 O films at (a) 10 and (b) 300 K.
no PL was observed at 300 K. This implies that the Co dopant quenches the room-temperature PL emission. The lack of PL may be due to dopant complexes acting as nonradiative centers [30]. The peak at 3.387 eV is attributed to free exciton (FX) recombination [31]. The peak at 3.370 eV is assigned to bound exciton (BX) recombination [31]. The peak at 3.315 eV is attributed to FX emission accompanied by first-order longitudinal optical (1LO) phonons [31]. The peak at 3.243 eV is attributed to FX emission accompanied by second-order longitudinal optical
(2LO) phonons. The peak at 3.171 eV is attributed to FX emission accompanied by third-order longitudinal optical (3LO) phonons. 1− The peak at ∼3.0 eV is attributed to VZn -related emission [32]. 2− The peak at ∼2.7 eV is attributed to VZn -related emission [32]. The peak at ∼2.4 eV is attributed to OZn -related emission [33]. The peak at ∼2.1 eV is attributed to Oi -related emission [5,34,35]. According to the PL results, we find the presence of acceptors (i.e., VZn , OZn , and Oi ) in the Zn0.8 Co0.2 O film. The peaks are determined by Gaussian fitting. Fig. 6 shows PL spectra of
C.-L. Tsai et al. / Solid State Communications 152 (2012) 488–492
Fig. 4. Temperature dependence of the residual magnetization (Mr ) of Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films.
Fig. 5. PL spectra of Zn0.8 Co0.2 O films at 10 and 300 K. Inset: The acceptor-related emissions measured at 10 K.
Zn0.8 Mn0.2 O films. Five emission peaks (peak positions at ∼3.56, ∼3.0, ∼2.7, ∼2.4, and ∼2.1 eV) at 10 K were noted. The peaks are determined by Gaussian fitting. The peak at ∼3.56 eV measured at 10 K is the band-edge luminescence (BEL). However, Fig. 6 shows negligible BEL at 300 K, implying that the Mn dopant quenches the room-temperature BEL. In addition, we find the presence of acceptors (i.e., VZn , Oi , and OZn ) in the Zn0.8 Mn0.2 O film. However, the intensity of the OZn -related (or Oi -related) peak of the Zn0.8 Mn0.2 O film is higher than that of the Zn0.8 Co0.2 O film. This can be explained since cf of Zn0.8 Mn0.2 O is larger than that of Zn0.8 Co0.2 O [11]. In addition, the larger band-gap energy of Zn0.8 Mn0.2 O at 10 K than of ZnO may be attributed to the charge-transfer transition between donor and acceptor ionization levels of Mn ions and the band continuum [36,37]. According to the results presented in Figs. 3–6, we deduce that increases in the number of Oi and OZn in Zn0.8 Mn0.2 O may lead to the enhancement of ferromagnetism, thus increasing TC .
491
Fig. 6. PL spectra of Zn0.8 Mn0.2 O films at 10 and 300 K.
Without charged dopants being introduced into the system, the acceptor-like defects may play a crucial role in the enhancement of FM properties. Consequently, the presence of the stronger ferromagnetic signal for Zn0.8 Mn0.2 O films than Zn0.8 Co0.2 O films is attributed to the formation of more acceptors. FM coupling depends on the electron localization of acceptor defects. We suggest that the number of acceptors is an important issue for producing strong ferromagnetism in Zn0.8 Co0.2 O or Zn0.8 Mn0.2 O DMSs prepared by a sol–gel method. On the other hand, for ZnMnO (ZnCoO) samples, the value of J1 /kB was evaluated as −15 (−27) K, based on the temperature dependence of the magnetization, as shown in [38,39]. Han et al. pointed out that the antiferromagnetic coupling constant extracted was J1 /kB = −40 K for ZnCoO samples [40]. kB is the Boltzman constant, and the negative value of the exchange integral J1 indicates that the interactions between Mn (Co) ions in ZnMnO (ZnCoO) are antiferromagnetic. We note that the magnitude of the exchange integral J1 for ZnCoO is about two times larger than that for ZnMnO. Lewicki et al. found that the values of the exchange integrals for Co-based DMS alloys are about three times larger than those for their Mn-based counterparts [41]. It is shown that a weak antiferromagnetic exchange coupling has a significant contribution to FM ZnMnO with a high TC . 4. Conclusions Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films were deposited on substrates by a sol–gel technique. It is shown that the magnetic properties might be related to the formation of acceptor-like defects in the Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films. It is worth noting that the FM signal for Zn0.8 Mn0.2 O films was stronger than that for Zn0.8 Co0.2 O films, owing to the presence of more acceptors (that is, Oi and OZn ) in the Zn0.8 Mn0.2 O film. In addition, the higher ratio of grainboundary area to grain volume of Zn0.8 Mn0.2 O than of Zn0.8 Co0.2 O suggests that grain boundaries and related acceptors have key roles with respect to FM properties. Acknowledgment The authors acknowledge support in the form of grants from the National Science Council of Taiwan (Contract No. 100-2112-M018-003-MY3).
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Contents lists available at SciVerse ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
On the origin of the ferromagnetism in Zn0.8 Mn0.2 O having a higher Curie temperature than Zn0.8 Co0.2 O Chia-Lung Tsai a , Yow-Jon Lin a,∗ , Jia-Hong Chen a , Hsing-Cheng Chang b , Ya-Hui Chen c , Lance Horng d , Yu-Tai Shih d a
Institute of Photonics, National Changhua University of Education, Changhua 500, Taiwan
b
Department of Automatic Control Engineering, Feng Chia University, Taichung 407, Taiwan
c
Ph. D. Program in Electrical and Communications Engineering, Feng Chia University, Taichung 407, Taiwan
d
Department of Physics, National Changhua University of Education, Changhua 500, Taiwan
article
info
Article history: Received 14 September 2011 Received in revised form 25 December 2011 Accepted 31 December 2011 by R. Merlin Available online 10 January 2012 Keywords: A. Thin films A. Semiconductors C. Point defects
abstract Zn0.8 Co0.2O and Zn0.8 Mn0.2 O films were deposited on substrates by a sol–gel technique. X-ray diffraction, field-emission scanning electron microscopy, photoluminescence, and ferromagnetism measurements were used to characterize these dilute magnetic semiconductors. It is shown that the ferromagnetic properties might be related to the formation of acceptor-like defects in the Zn0.8 Co0.2O and Zn0.8 Mn0.2 O films. It is found that ferromagnetic Zn0.8 Mn0.2 O has a higher Curie temperature than Zn0.8 Co0.2O . In addition, the higher ratio of grain-boundary area to grain volume of Zn0.8 Mn0.2 O than Zn0.8 Co0.2O indicates that grain boundaries and related acceptors are the intrinsic origin for ferromagnetism. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Recently, dilute magnetic semiconductors (DMSs) based on ZnO [1–14] have attracted a great deal of attention because of their application in magnetic semiconductor devices. In particular, ZnO-based DMSs have been predicted to show ferromagnetic (FM) behavior with Curie temperature (TC ) above room temperature, and they have a large magnetization [6–14], which makes them a promising candidate material for the next generation of spintronic devices utilizing electronically or optically controlled magnetism. Dietl et al. theoretically predicted that ZnO doped with magnetic atoms (Co, Mn, or Fe) would possess FM behavior with a high TC above room temperature [6]. Wang et al. found that the high-temperature (TC higher than 350 K) FM for Zn1−x Mnx O nanoparticles prepared by a sol–gel process was strongly related to defects in ZnO [8]. More recently, structural defects in transition metal-doped ZnO have been shown to play an important role in the occurrence and stability of ferromagnetism. Based on firstprinciples calculations, Yan et al. proposed that the Zn vacancy (VZn ) can induce room-temperature ferromagnetism in Mn-doped ZnO [15]. Ramachandran et al. found tuning of ferromagnetism
∗
Corresponding author. Tel.: +886 4 7232105x3379; fax: +886 4 7211153. E-mail address: [email protected] (Y.-J. Lin).
0038-1098/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.12.046
in Zn1−x Mnx O DMSs by controlling the concentrations of oxygen vacancies and substitutional Mn dopants [16]. In a previously reported result [3], we found that changes in cobalt concentration and the number of VZn are important issues for producing strong ferromagnetism in Zn1−x Cox O DMSs prepared by a sol–gel method. However, Liu et al. suggested that ferromagnetic coupling in the Zn–Co–O system was induced by electron charge transfer between Co 3d and donor defects [17]. It is found that FM coupling depends on the electron localization of acceptor or donor defects [3,15–17]. To date, the origin of the ferromagnetism in oxide DMSs remains a very controversial topic. Therefore, there remain a lot of open questions or controversial opinions. In this study, we report that FM behavior is observed for Zn0.8 Mn0.2 O or Zn0.8 Co0.2 O films prepared by a sol–gel method. We found that FM Zn0.8 Mn0.2 O has a higher TC than Zn0.8 Co0.2 O. The relationships between defects and the magnetic properties for Zn0.8 Mn0.2 O (Zn0.8 Co0.2 O) films were also researched in this study. 2. Experimental details Zn0.8 Co0.2 O (Zn0.8 Mn0.2 O) DMSs were prepared by a sol–gel method using Zn(CH3 COO)2 ·2H2 O and Co(CH3 COO)2 ·4H2 O (Mn (CH3 COO)2 ·4H2 O) as starting precursors and 2-methoxyethanol and monoethanolamine (MEA) as solvent and stabilizer. The molar ratio of MEA to zinc acetate was maintained at 1.0 and the
C.-L. Tsai et al. / Solid State Communications 152 (2012) 488–492
concentration of zinc acetate was 0.5 M. The molar ratio of dopant in the solution (Co/Zn) or (Mn/Zn) was 25%. The solution was stirred at 80 °C for 2 h to yield a clear and homogeneous solution, which served as the coating solution after cooling to room temperature. The solution was dropped onto Si or glass substrates, which were rotated at 2000 rpm for 20 s, and rotated again at 3000 rpm for another 20 s. These Si and glass substrates were ultrasonically cleaned for 10 min each in acetone, then in methanol, then in deionized water, and dried in nitrogen before spin coating. After deposition by spin coating, the films were dried at 300 °C for 10 min on a hotplate to evaporate the solvent and remove organic residuals. The procedures from coating to drying were repeated five times. The films were then inserted into a furnace and annealed in air at 500 °C for 30 min. The Zn0.8 Co0.2 O (Zn0.8 Mn0.2 O) film thickness, as estimated from field-emission scanning electron microscopy (FESEM), was about 100 nm. ZnO films were also prepared by a sol–gel method. The structures and morphologies of the ZnO, Zn0.8 Co0.2 O, and Zn0.8 Mn0.2 O films were studied using X-ray diffraction (XRD) and FESEM. The magnetization measurements were carried out using a superconducting quantum interference device magnetometer. To confirm the presence of acceptors in the Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films and verify the influence of the formation of acceptors on the FM properties, photoluminescence (PL) spectra were obtained from Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films prepared by a sol–gel spin-coating method on Si substrates. Using a He–Cd laser (the 325 nm line) as an excitation source, the PL band was noted for Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films at 10 and 300 K, respectively. From these results, the effect of defects on the magnetic property of Zn0.8 Co0.2 O (Zn0.8 Mn0.2 O) is discussed. 3. Results and discussions Fig. 1 shows the XRD patterns of ZnO, Zn0.8 Co0.2 O, and Zn0.8 Mn0.2 O films prepared by the sol–gel spin-coating method on glass substrates. The XRD pattern of the glass substrate is also shown in Fig. 1. The figure shows a comparison of the XRD pattern from pure ZnO and that for Zn0.8 Co0.2 O films, showing that the location of the (002) diffraction peak shifts towards lower angle for Zn0.8 Co0.2 O films. Comparing the Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films, we find that the (002) peak shifted toward low angle for Zn0.8 Mn0.2 O films. Based on the XRD data presented in Fig. 1, the c-axis lattice parameter (cf ) of the Zn0.8 Co0.2 O (Zn0.8 Mn0.2 O or ZnO) films has been calculated using Bragg’s law [18]. The cf value for ZnO, Zn0.8 Mn0.2 O, and Zn0.8 Co0.2 O is 5.2102, 5.270, 5.227 Å, respectively. The large cf value may be attributed to the incorporation of defects in the form of acceptor-like interstitial oxygen (Oi ) during deposition [11]. Remashan et al. found a relationship between the c-axis (the (002) plane), grain size, and point defects [19]. In addition, the relationship between cf and point defects has been investigated by Pant et al. [20] and Janotti and Van de Walle [21]. In the literature, peak shifts toward a lower 2θ value with respect to the bulk peak profiles have been attributed to compressive stress in the films [22,23]. This effect could result in increasing the interplanar spacing, thus leading to the observed decrease in the diffraction angle. It is seen that the cf value for Zn0.8 Mn0.2 O/glass or Zn0.8 Co0.2 O/glass samples is larger than that for ZnO/glass samples, implying that Co or Mn doping may lead to the presence of compressive stress along the c-axis in the films. Fig. 2 shows the FESEM images of ZnO, Zn0.8 Co0.2 O, and Zn0.8 Mn0.2 O films. We find a larger grain size of ZnO than of Zn0.8 Co0.2 O and a larger grain size of Zn0.8 Co0.2 O than of Zn0.8 Mn0.2 O. In addition, Fig. 2 shows the higher ratio of grainboundary area to grain volume of Zn0.8 Mn0.2 O than Zn0.8 Co0.2 O. As the main challenge for practical applications of DMS materials is the attainment of TC around or preferably above room
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Fig. 1. XRD patterns of glass, Zn0.2 Co0.8 O, Zn0.2 Mn0.8 O, and ZnO films. For the sake of clarity, the intensity for the ZnO film is multiplied by 0.4.
temperature, we conducted a detailed study of the magnetization of Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films. Fig. 3 shows magnetization curves at 10 and 300 K for Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films prepared by the sol–gel spin-coating method on Si substrates. The magnetization curves were obtained with the applied field parallel to the plane of the samples. In Fig. 3, the hysteresis loops show clear FM behavior of the Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films. It is worth noting that the FM signal for Zn0.8 Mn0.2 O films at 10 (300) K was stronger than that for Zn0.8 Co0.2 O films at 10 (300) K. Fig. 4 shows the temperature dependence of the residual magnetization (Mr ) of Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films. A freehand extrapolation of the data to higher temperature for Mr → 0 yields TC ≈ 440 K (536 K) for Zn0.8 Co0.2 O (Zn0.8 Mn0.2 O) [24]. Lee et al. [25], Wang et al. [8], and Kittilstved and Gamelin [7] pointed out that TC > 350 K for ZnMnO and ZnCoO samples. Sharma et al. achieved FM with a TC of 425 K for ZnMnO samples [26]. We find that FM Zn0.8 Mn0.2 O has a higher TC than Zn0.8 Co0.2 O. The enhanced ferromagnetism in the grain boundaries is likely caused by increased defects [9]. Straumal et al. suggested that the grain boundaries and related defects are the intrinsic origin for room-temperature ferromagnetism [12]. Gu et al. found that the FM correlations are strongly influenced by the crystal structure [13]. Manivannan et al. pointed out that p-type doping in ZnMnO may be necessary to stabilize the FM state [10]. Mounkachi et al. found that higher values of TC are attained for high concentration of vacancy defect sites in ZnMnO [27]. Kim et al. stated that the lattice distortion is a crucial factor for VZn -induced ferromagnetism [28]. Wang et al. suggested that the adsorbed oxygen in the grain boundary caused the p-type conductivity, and that the high Hall mobility of p-type ZnO film was due to the quasi-two-dimensional hole gas, which was induced by the negatively charged interface states in the boundaries [29]. Based on the previously reported results [9,10,12,13,27–29], we deduce that acceptor-like defects (that is, Oi , VZn or oxygen-substituting zinc atoms (OZn )) may cause the enhancement of the FM characteristics, thus increasing TC . In addition, the higher ratio of grain-boundary area to grain volume of Zn0.8 Mn0.2 O than Zn0.8 Co0.2 O in Fig. 2 indicates that grain boundaries and related acceptors are the intrinsic origin for the ferromagnetism. Based on the experimental results, we suggest that defective grain boundaries have a key role with respect to the FM properties. PL spectra of Zn0.8 Co0.2 O films are shown in Fig. 5. For our samples, strong PL spectra were observed at 10 K whereas
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Fig. 2. FESEM images of ZnO, Zn0.2 Co0.8 O, and Zn0.2 Mn0.8 O films. The scale bar is 200 nm.
Fig. 3. Magnetization curves for Zn0.2 Co0.8 O and Zn0.2 Mn0.8 O films at (a) 10 and (b) 300 K.
no PL was observed at 300 K. This implies that the Co dopant quenches the room-temperature PL emission. The lack of PL may be due to dopant complexes acting as nonradiative centers [30]. The peak at 3.387 eV is attributed to free exciton (FX) recombination [31]. The peak at 3.370 eV is assigned to bound exciton (BX) recombination [31]. The peak at 3.315 eV is attributed to FX emission accompanied by first-order longitudinal optical (1LO) phonons [31]. The peak at 3.243 eV is attributed to FX emission accompanied by second-order longitudinal optical
(2LO) phonons. The peak at 3.171 eV is attributed to FX emission accompanied by third-order longitudinal optical (3LO) phonons. 1− The peak at ∼3.0 eV is attributed to VZn -related emission [32]. 2− The peak at ∼2.7 eV is attributed to VZn -related emission [32]. The peak at ∼2.4 eV is attributed to OZn -related emission [33]. The peak at ∼2.1 eV is attributed to Oi -related emission [5,34,35]. According to the PL results, we find the presence of acceptors (i.e., VZn , OZn , and Oi ) in the Zn0.8 Co0.2 O film. The peaks are determined by Gaussian fitting. Fig. 6 shows PL spectra of
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Fig. 4. Temperature dependence of the residual magnetization (Mr ) of Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films.
Fig. 5. PL spectra of Zn0.8 Co0.2 O films at 10 and 300 K. Inset: The acceptor-related emissions measured at 10 K.
Zn0.8 Mn0.2 O films. Five emission peaks (peak positions at ∼3.56, ∼3.0, ∼2.7, ∼2.4, and ∼2.1 eV) at 10 K were noted. The peaks are determined by Gaussian fitting. The peak at ∼3.56 eV measured at 10 K is the band-edge luminescence (BEL). However, Fig. 6 shows negligible BEL at 300 K, implying that the Mn dopant quenches the room-temperature BEL. In addition, we find the presence of acceptors (i.e., VZn , Oi , and OZn ) in the Zn0.8 Mn0.2 O film. However, the intensity of the OZn -related (or Oi -related) peak of the Zn0.8 Mn0.2 O film is higher than that of the Zn0.8 Co0.2 O film. This can be explained since cf of Zn0.8 Mn0.2 O is larger than that of Zn0.8 Co0.2 O [11]. In addition, the larger band-gap energy of Zn0.8 Mn0.2 O at 10 K than of ZnO may be attributed to the charge-transfer transition between donor and acceptor ionization levels of Mn ions and the band continuum [36,37]. According to the results presented in Figs. 3–6, we deduce that increases in the number of Oi and OZn in Zn0.8 Mn0.2 O may lead to the enhancement of ferromagnetism, thus increasing TC .
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Fig. 6. PL spectra of Zn0.8 Mn0.2 O films at 10 and 300 K.
Without charged dopants being introduced into the system, the acceptor-like defects may play a crucial role in the enhancement of FM properties. Consequently, the presence of the stronger ferromagnetic signal for Zn0.8 Mn0.2 O films than Zn0.8 Co0.2 O films is attributed to the formation of more acceptors. FM coupling depends on the electron localization of acceptor defects. We suggest that the number of acceptors is an important issue for producing strong ferromagnetism in Zn0.8 Co0.2 O or Zn0.8 Mn0.2 O DMSs prepared by a sol–gel method. On the other hand, for ZnMnO (ZnCoO) samples, the value of J1 /kB was evaluated as −15 (−27) K, based on the temperature dependence of the magnetization, as shown in [38,39]. Han et al. pointed out that the antiferromagnetic coupling constant extracted was J1 /kB = −40 K for ZnCoO samples [40]. kB is the Boltzman constant, and the negative value of the exchange integral J1 indicates that the interactions between Mn (Co) ions in ZnMnO (ZnCoO) are antiferromagnetic. We note that the magnitude of the exchange integral J1 for ZnCoO is about two times larger than that for ZnMnO. Lewicki et al. found that the values of the exchange integrals for Co-based DMS alloys are about three times larger than those for their Mn-based counterparts [41]. It is shown that a weak antiferromagnetic exchange coupling has a significant contribution to FM ZnMnO with a high TC . 4. Conclusions Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films were deposited on substrates by a sol–gel technique. It is shown that the magnetic properties might be related to the formation of acceptor-like defects in the Zn0.8 Co0.2 O and Zn0.8 Mn0.2 O films. It is worth noting that the FM signal for Zn0.8 Mn0.2 O films was stronger than that for Zn0.8 Co0.2 O films, owing to the presence of more acceptors (that is, Oi and OZn ) in the Zn0.8 Mn0.2 O film. In addition, the higher ratio of grainboundary area to grain volume of Zn0.8 Mn0.2 O than of Zn0.8 Co0.2 O suggests that grain boundaries and related acceptors have key roles with respect to FM properties. Acknowledgment The authors acknowledge support in the form of grants from the National Science Council of Taiwan (Contract No. 100-2112-M018-003-MY3).
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