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Room temperature ferromagnetic properties of Cu2O microcrystals
Journal of Alloys and Compounds 579 (2013) 572–575
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Room temperature ferromagnetic properties of Cu2O microcrystals G. Prabhakaran, Ramaswamy Murugan ⇑ Department of Physics, Pondicherry University, Puducherry 605 014, India
a r t i c l e
i n f o
Article history: Received 21 May 2013 Received in revised form 11 July 2013 Accepted 14 July 2013 Available online 23 July 2013 Keywords: Oxide materials Crystal growth Microstructure Scanning electron microscopy
a b s t r a c t Although there are many reports in the recent literatures regarding the observation of ferromagnetism in undoped and doped Cu2O; however, it is still a great challenge to produce room temperature ferromagnetic behaviour. We report room temperature ferromagnetic properties of octahedral, star/short hexapod, extended hexapod and microrod shape Cu2O microcrystals. Ferromagnetic ordering in the investigated Cu2O microcrystals might be induced via cation vacancy. The defects in these microcrystals have been analysed through Raman and photoluminescence studies. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Diluted magnetic semiconductors (DMSs) with room temperature ferromagnetic properties have been identified as an attractive material for potential application in spintronic devices [1]. Magnetic properties in bulk semiconductor lattices are primarily controlled by the incorporation of transition-metal impurities. In this context there are many reports in the literatures regarding the observation of room temperature ferromagnetism in doped and un-doped wide band gap oxides, e.g., HfO2 [2], TiO2 [3], ZnO [4,5], MgO [6], CaO [7] and SnO2 [8]. Magnetic properties of undoped and transition metal ion doped Cu2O nanocrystals with various morphology was reported earlier [9–13]. Ferromagnetic behaviour of Cu2O nanospheres at 5 K [9], room temperature ferromagnetism in Cu2O nanowires [10], room temperature ferromagnetism in metal ions doped Cu2O nanorods [11,13], and Cu2O flower like nanostructure [12] were reported. Phase pure defect free Cu2O is expected to exhibit diamagnetic properties since neither Cu1+ nor O2+ is magnetic and the d shell of Cu1+ is full. Although several reports revealed the ferromagnetism in doped and undoped Cu2O nanostructured materials, the exact origin of the observed ferromagnetic properties of Cu2O nanocrystals is still unclear. Systematic investigations indicated that the defects such as oxygen vacancy and/or cation vacancy, are crucial for the observed ferromagnetism in Cu2O-based materials [9,10]. The investigations on the magnetic properties of Cu2O microcrystals in the undoped and doped form with different morphology are scanty [9,12]. Herein we report room temperature ferromagnetic
⇑ Corresponding author. Tel.: +91 413 2654782. E-mail address: [email protected] (R. Murugan). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.07.094
properties of octahedral, star/short hexapod, extended hexapod and microrod shape Cu2O microcrystals. 2. Experimental Cu2O microcrystals with various shapes were prepared using Cu (CH3COO)2H2O and C6H12O6 by a simple chemical route as descried earlier [14]. To obtain Cu2O microcrystals with octahedral, star/short hexapod, extended hexapod and microrod shape the temperature and reaction time maintained was 60 °C for 5 h, 60 °C for 6 h, 70 °C for 4 h and 80 °C for 4 h, respectively. The crystal phase of the synthesized samples was characterized by an X-ray diffractometer (X’Pert PRO, PANalytical) with Cu Ka source, filtered by graphite. Thermal analysis (thermogravimetric/differential scanning calorimetry (TG/DSC)) of the synthesized Cu2O microcrystals was conducted using SDT Q600 (TA) in air with a heating rate of 10 °C/min. Confocal micro-Raman spectra have been recorded at room temperature in the range 50– 900 cm1 using a Raman microscope (Reneshaw inVia Reflex) with a 50 mW internal Ar ion laser source of wavelength 488 nm. The morphology of the prepared samples was investigated using scanning electron microscope (Hitachi S-3400 N). Magnetic measurements were done at room temperature by vibrating sample magnetometer (VSM) (Lake Shore, Model: 7404). The room temperature photoluminescence spectra were recorded by Fluorolog-FL3-11 (Jobin Yvon) spectroflurometer.
3. Results and discussion The X-ray diffraction patterns of all prepared samples exhibit the expected diffraction peaks of cubic Cu2O crystal structure. The XRD studies also indicated the absence of any diffraction peaks related to possible impure phase such as Cu and CuO. Fig. 1(a) illustrates the TG curves measured in air for synthesized Cu2O microcrystals with different shapes. The onset of oxidation temperature for octahedral, star/short hexapod and extended hexapod samples are, respectively, 304 °C, 312 °C and 400 °C. The offset of oxidation temperature for octahedral, star/short hexapod and extended hexapod samples are, respectively, 551 °C, 539 °C
G. Prabhakaran, R. Murugan / Journal of Alloys and Compounds 579 (2013) 572–575
573
expanded comparative view of room temperature magnetic hysteresis loop was shown in Fig. 3. Group theory analysis on perfect Cu2O crystal predicts 15 optical modes at the C point of the Brillouin zone with the following irreducible representation [17]:
CVib ¼ F 2u þ Eu þ 2F 1u þ A2u þ F 2g
Fig. 1. TG (a) and DSC (b) curves for a. Octahedral, b. Star/short hexapod, c. Extended hexapod and d. Microrods shape Cu2O microcrystals.
and 525 °C. A systematic increase in the onset and corresponding decrease in the offset temperature for oxidation of Cu2O microcrystals was observed during the evolution of morphology from octahedral to star/short hexapod and then to extended hexapod. Cu2O microrod shape crystals oxidize in the temperature range of 341–546 °C. The differential scanning calorimetric curves in Fig. 1(b) show maximum heat flow temperature at 467, 482, 470 and 523 °C for octahedral, star/short hexapod, extended hexapod and microrods, respectively. The observed difference in the oxidation temperature range of the Cu2O microcrystals might be due to difference in the surface area [15,16]. The SEM images and the corresponding magnetization vs. applied field curve (hysteresis loop) for the octahedral, star/short hexapod, extended hexapod and microrod shape Cu2O microcrystals are shown as Fig. 2(a,b), (c,d), (e,f) and (g,h), respectively. The observation of hysteresis loop (M–H) at room temperature shows an evidence of room temperature ferromagnetism in all the investigated Cu2O microcrystals. The observed saturation magnetization (Ms), coercivity (Hc) and remanent magnetization (Mr) for the Cu2O microcrystals measured at room temperature with different morphology are tabulated in Table 1. The saturation magnetization (Ms) for octahedral, star/ short hexapod, extended hexapod and microrod shape Cu2O microcrystals are, 4.7698, 2.0667, 1.9058 and 1.9614 103 emu/g, respectively. The saturation magnetization (4.7698 103 emu/g) is higher for the Cu2O microcrystal with octahedral morphology. From Table 1, it is also found that the coercivity (Hc) was least for the octahedral morphology compared to other morphology. The remanent magnetization (Mr) for the investigated Cu2O microcrystals is found to be in the range 305–363 106 emu/g. The
The two F1u modes are infrared-active, one F2g mode is Raman-active and all others are silent in both infrared and Raman spectra. Since the crystal has inversion symmetry, the mutual exclusion principle restricts the infrared-allowed modes are Raman forbidden and vice versa. As per group-theoretical analysis above, the Raman spectrum of a perfect Cu2O crystal should exhibit only Raman signal belonging to the three-fold degenerate F2g mode. The Raman spectra measured at room temperature of all the synthesized Cu2O microcrystals in the range of 50–900 cm1 shown as Fig. 4 vary significantly in the number of observed modes. The non-stoichiometry due to the formation of defects such as vacancies, interstitials or antisite defects may be the origin for this breakdown of the above selection rules. The reduction of local symmetry by point defects may diminish the distinction between Raman-allowed and Raman-forbidden lattice vibrations and also local Raman active modes may be introduced depending on the specific point defect and its compatibility with the lattice. The absence of characteristic strong peak at 290 cm1 weak band at 340 and 602 cm1 corresponding to the CuO phase further confirms the absence of CuO in all the investigated Cu2O crystals [18]. The weak band observed at around 515 cm1 as shown in Fig. 4 is the only allowed Raman mode (F2g) as per the selection rule. Based on the earlier investigations the bands observed in the Raman spectra may be assigned as lattice modes at about 92 cm1 (T2u), 110 cm1 (Eu), 150 cm1 (T1u LO), and 630 and 647 cm1 (T1u TO, LO) [18,19]. The strong Raman peaks at 218 cm1 (2Eu) and 415 cm1 are assigned to multiphonon Raman scattering. The second-order overtone Raman mode observed at 218 cm1 is usually appeared as most intense Raman mode in the reported Raman spectrum of Cu2O crystals [19]. However in the present study as shown in Fig. 4 the peak at 150 cm1 appeared as most intense and sharp peak irrespective of the morphology of all the investigated Cu2O micro crystals. The weak Raman peak at 110 cm1 and very strong peak at 150 cm1 actually corresponds to infrared allowed mode in perfect Cu2O crystal [19]. The appearance of infrared active band in the Raman spectrum as most intense peak at 150 cm1 strongly suggest the breakdown of selection rule due to the possible defect in the investigated Cu2O microcrystals. The defect in the microcrystals was also confirmed by the room temperature photoluminescence spectroscopy by exciting the sample at 514 nm. The photoluminescence spectra shown in Fig. 5 clearly revealed a broad peak at around 612 nm for all the investigated Cu2O microcrystals. The earlier photoluminescence studies on Cu2O indicated that the broad peak observed around 612 nm has been attributed to the cationic vacancy present in the material [20,21]. Usually, the magnetic property depends on extrinsic effects, such as the presence of magnetic impurities or intrinsic crystal defects such as vacancies. The ab initio DFT calculations indicated that the ferromagnetic property possibly arise from the presence of cation vacancies in the ideal lattice of diamagnetic Cu2O [9]. Reports on Cu2O nanostructures also indicated that the defects in the form of cation vacancies in these materials are the origin for the observed ferromagnetism in these materials [10]. The slight variation observed in the magnetic properties of the prepared samples might
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0.006
M (emu/g)
0.004
(b)
0.002 0.000 -0.002 -0.004 -0.006
-4000
-2000
0
2000
4000
2000
4000
2000
4000
2000
4000
H (G) 0.002
(d)
M (emu/g)
0.001 0.000 -0.001 -0.002
-4000
-2000
0
F (G)
M (emu/g)
0.002
(f)
0.001 0.000 -0.001 -0.002
-4000
-2000
0
F (G) 0.002
(h)
M (emu/g)
0.001 0.000 -0.001 -0.002
-4000
-2000
0
F (G) Fig. 2. Typical SEM micrographs and corresponding room temperature magnetic hysteresis loop (M–H) of octahedral (a,b), star/short hexapod (c,d), extended hexapod (e,f) and microrod (g,h) shape Cu2O microcrystals.
G. Prabhakaran, R. Murugan / Journal of Alloys and Compounds 579 (2013) 572–575
be due to the variations of the vacancy concentration in the Cu2O microcrystals with different morphology.
Table 1 Magnetic properties of the Cu2O microcrystals. Hc (Oe) Octahedron Star/short hexapod Extended hexapod Microrod
74.301 105.93 104.93 104.14
575
Ms (emu/g)
Mr (emu/g) 3
4.7698 10 2.0667 103 1.9058 103 1.9614 103
362.94 106 332.84 106 305.75 106 309.51 106
4. Conclusion In summary, herein we report the observation of room temperature ferromagnetism in octahedral, star/short hexapod, extended hexapod and microrod shape Cu2O microcrystals. The experimental results revealed that the FM order in octahedral Cu2O microcrystal appears to be slightly different from other morphology. The magnetic properties of the star/short hexapod, extended hexapod and microrod shape Cu2O microcrystals appears to be almost morphology independent. The preliminary TG/DSC, photoluminescence, Raman spectroscopic and vibrating sample magnetometer result indicated the defect induced ferromagnetism present in the synthesized Cu2O microcrystals. However a detailed X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and positron annihilation analysis (PAS) are essential to gain further knowledge on the origin of this ferromagnetism in these microcrystals. Acknowledgements
Fig. 3. The expanded view of room temperature magnetic hysteresis loop (M–H) of octahedral (a), star/short hexapod (b), extended hexapod (c) and microrod (d) shape Cu2O microcrystals.
R.M. acknowledges the financial support of UGC, New Delhi, India [F.No. 41-918/2012 (SR) dated 23.07.2012]. The authors acknowledge CIF, Pondicherry University, India for extending the instrumentation facilities. References
Fig. 4. Raman spectra of octahedral (a), star/short hexapod (b), extended hexapod (c), microrod (d) shape Cu2O microcrystals.
Fig. 5. Photoluminescence spectra of octahedral (a), star/short hexapod (b), extended hexapod (c), microrod (d) shape Cu2O microcrystals.
[1] S.A. Wolf, D.D. Awschalom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Science 294 (2001) 1488–1495. [2] M. Venkatesan, C.B. Fitzgerald, J.M.D. Coey, Nature 430 (2004) 630. [3] N.H. Hong, J. Sakai, N. Poirot, V. Brize, Phys. Rev. B 73 (2006) 132404. [4] Q. Xu, H. Schmidt, S. Zhou, K. Potzger, M. Helm, H. Hochmuth, M. Lorenz, A. Setzer, P. Esquinazi, C. Meinecke, M. Grundmann, Appl. Phys. Lett. 92 (2008) 082508. [5] J.B. Yi, C.C. Lim, G.Z. Xing, H.M. Fan, L.H. Van, S.L. Huang, K.S. Yang, X.L. Huang, X.B. Qin, B.Y. Wang, T. Wu, L. Wang, H.T. Zhang, X.Y. Gao, T. Liu, A.T.S. Wee, Y.P. Feng, J. Ding, Phys. Rev. Lett. 104 (2010) 137201. [6] C. Martinez-Boubeta, J.I. Beltran, LI. Balcells, Z. Konstantinovic, S. Valencia, D. Schmitz, J. Arbiol, S. Estrade, J. Cornil, B. Martinez, Phys. Rev. B 82 (2010) 024405. [7] J. Osorio-Guillen, S. Lany, S.V. Barabash, A. Zunger, Phys. Rev. Lett. 96 (2006) 107203. [8] N.H. Hong, N. Poirot, J. Sakai, Phys. Rev. B 77 (2008) 033205. [9] C. Chen, L. He, L. Lai, H. Zhang, J. Lu, L. Guo, Y. Li, J. Phys.: Condens. Matter 21 (2009) 145601. [10] L. Liao, B. Yan, Y.F. Hao, G.Z. Xing, J.P. Liu, B.C. Zao, Z.X. Shen, T. Wu, L. Wang, J.T.L. Thong, C.M. Li, W. Huang, T. Yu, Appl. Phys. Lett. 94 (2009) 113106. [11] A. Ahmed, N.S. Gajbhiye, S. Kurian, J. Solid State Chem. 183 (2010) 2248–2251. [12] A. Ahmed, N.S. Gajbhiye, J. Solid State Chem. 184 (2011) 30–35. [13] A. Ahmed, N.S. Gajbhiye, J. Solid State Chem. 183 (2010) 3100–3104. [14] G. Prabhakaran, R. Murugan, Cryst. Eng. Commun. 14 (2012) 8338–8341. [15] Z. Zhang, H. Che, J. Gao, Y. Wang, X. She, J. Sun, P. Gunawan, Z. Zhong, F. Su, Catal. Sci. Technol. 2 (2012) 1207–1212. [16] Z. Zhang, H. Che, Y. Wang, J. Gao, L. Zhao, X. She, J. Sun, P. Gunawan, Z. Zhong, F. Su, Ind. Eng. Chem. Res. 51 (2012) 1264–1274. [17] A. Sanson, Solid State Commun. 151 (2011) 1452–1454. [18] B.K. Meyer, A. Polity, D. Reppin, M. Becker, P. Hering, P.J. Klar, Th. Sander, C. Reindl, J. Benz, M. Eickhoff, C. Heilinger, M. Heinemann, J. Blasing, A. Krost, S. Shokovets, C. Muller, C. Ronning, Phys. Status Solidi B 249 (2012) 1487–1509. [19] H. Solache-Carranco, G. Juarez-Diaz, A. Esparza-Garcia, M. Briseno-Garcia, M. Galvan- Arellano, J. Martinez-Juarez, G. Romero-Paredes, R. Pena-Sierra, J. Lumin. 129 (2009) 1483–1487. [20] T. Ito, T. Masumi, J. Phys. Soc. Jpn. 66 (1997) 2185–2193. [21] H. Shi, K. Yu, F. Sun, Z. Zhu, Cryst. Eng. Commun. 14 (2012) 278–285.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Room temperature ferromagnetic properties of Cu2O microcrystals G. Prabhakaran, Ramaswamy Murugan ⇑ Department of Physics, Pondicherry University, Puducherry 605 014, India
a r t i c l e
i n f o
Article history: Received 21 May 2013 Received in revised form 11 July 2013 Accepted 14 July 2013 Available online 23 July 2013 Keywords: Oxide materials Crystal growth Microstructure Scanning electron microscopy
a b s t r a c t Although there are many reports in the recent literatures regarding the observation of ferromagnetism in undoped and doped Cu2O; however, it is still a great challenge to produce room temperature ferromagnetic behaviour. We report room temperature ferromagnetic properties of octahedral, star/short hexapod, extended hexapod and microrod shape Cu2O microcrystals. Ferromagnetic ordering in the investigated Cu2O microcrystals might be induced via cation vacancy. The defects in these microcrystals have been analysed through Raman and photoluminescence studies. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Diluted magnetic semiconductors (DMSs) with room temperature ferromagnetic properties have been identified as an attractive material for potential application in spintronic devices [1]. Magnetic properties in bulk semiconductor lattices are primarily controlled by the incorporation of transition-metal impurities. In this context there are many reports in the literatures regarding the observation of room temperature ferromagnetism in doped and un-doped wide band gap oxides, e.g., HfO2 [2], TiO2 [3], ZnO [4,5], MgO [6], CaO [7] and SnO2 [8]. Magnetic properties of undoped and transition metal ion doped Cu2O nanocrystals with various morphology was reported earlier [9–13]. Ferromagnetic behaviour of Cu2O nanospheres at 5 K [9], room temperature ferromagnetism in Cu2O nanowires [10], room temperature ferromagnetism in metal ions doped Cu2O nanorods [11,13], and Cu2O flower like nanostructure [12] were reported. Phase pure defect free Cu2O is expected to exhibit diamagnetic properties since neither Cu1+ nor O2+ is magnetic and the d shell of Cu1+ is full. Although several reports revealed the ferromagnetism in doped and undoped Cu2O nanostructured materials, the exact origin of the observed ferromagnetic properties of Cu2O nanocrystals is still unclear. Systematic investigations indicated that the defects such as oxygen vacancy and/or cation vacancy, are crucial for the observed ferromagnetism in Cu2O-based materials [9,10]. The investigations on the magnetic properties of Cu2O microcrystals in the undoped and doped form with different morphology are scanty [9,12]. Herein we report room temperature ferromagnetic
⇑ Corresponding author. Tel.: +91 413 2654782. E-mail address: [email protected] (R. Murugan). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.07.094
properties of octahedral, star/short hexapod, extended hexapod and microrod shape Cu2O microcrystals. 2. Experimental Cu2O microcrystals with various shapes were prepared using Cu (CH3COO)2H2O and C6H12O6 by a simple chemical route as descried earlier [14]. To obtain Cu2O microcrystals with octahedral, star/short hexapod, extended hexapod and microrod shape the temperature and reaction time maintained was 60 °C for 5 h, 60 °C for 6 h, 70 °C for 4 h and 80 °C for 4 h, respectively. The crystal phase of the synthesized samples was characterized by an X-ray diffractometer (X’Pert PRO, PANalytical) with Cu Ka source, filtered by graphite. Thermal analysis (thermogravimetric/differential scanning calorimetry (TG/DSC)) of the synthesized Cu2O microcrystals was conducted using SDT Q600 (TA) in air with a heating rate of 10 °C/min. Confocal micro-Raman spectra have been recorded at room temperature in the range 50– 900 cm1 using a Raman microscope (Reneshaw inVia Reflex) with a 50 mW internal Ar ion laser source of wavelength 488 nm. The morphology of the prepared samples was investigated using scanning electron microscope (Hitachi S-3400 N). Magnetic measurements were done at room temperature by vibrating sample magnetometer (VSM) (Lake Shore, Model: 7404). The room temperature photoluminescence spectra were recorded by Fluorolog-FL3-11 (Jobin Yvon) spectroflurometer.
3. Results and discussion The X-ray diffraction patterns of all prepared samples exhibit the expected diffraction peaks of cubic Cu2O crystal structure. The XRD studies also indicated the absence of any diffraction peaks related to possible impure phase such as Cu and CuO. Fig. 1(a) illustrates the TG curves measured in air for synthesized Cu2O microcrystals with different shapes. The onset of oxidation temperature for octahedral, star/short hexapod and extended hexapod samples are, respectively, 304 °C, 312 °C and 400 °C. The offset of oxidation temperature for octahedral, star/short hexapod and extended hexapod samples are, respectively, 551 °C, 539 °C
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573
expanded comparative view of room temperature magnetic hysteresis loop was shown in Fig. 3. Group theory analysis on perfect Cu2O crystal predicts 15 optical modes at the C point of the Brillouin zone with the following irreducible representation [17]:
CVib ¼ F 2u þ Eu þ 2F 1u þ A2u þ F 2g
Fig. 1. TG (a) and DSC (b) curves for a. Octahedral, b. Star/short hexapod, c. Extended hexapod and d. Microrods shape Cu2O microcrystals.
and 525 °C. A systematic increase in the onset and corresponding decrease in the offset temperature for oxidation of Cu2O microcrystals was observed during the evolution of morphology from octahedral to star/short hexapod and then to extended hexapod. Cu2O microrod shape crystals oxidize in the temperature range of 341–546 °C. The differential scanning calorimetric curves in Fig. 1(b) show maximum heat flow temperature at 467, 482, 470 and 523 °C for octahedral, star/short hexapod, extended hexapod and microrods, respectively. The observed difference in the oxidation temperature range of the Cu2O microcrystals might be due to difference in the surface area [15,16]. The SEM images and the corresponding magnetization vs. applied field curve (hysteresis loop) for the octahedral, star/short hexapod, extended hexapod and microrod shape Cu2O microcrystals are shown as Fig. 2(a,b), (c,d), (e,f) and (g,h), respectively. The observation of hysteresis loop (M–H) at room temperature shows an evidence of room temperature ferromagnetism in all the investigated Cu2O microcrystals. The observed saturation magnetization (Ms), coercivity (Hc) and remanent magnetization (Mr) for the Cu2O microcrystals measured at room temperature with different morphology are tabulated in Table 1. The saturation magnetization (Ms) for octahedral, star/ short hexapod, extended hexapod and microrod shape Cu2O microcrystals are, 4.7698, 2.0667, 1.9058 and 1.9614 103 emu/g, respectively. The saturation magnetization (4.7698 103 emu/g) is higher for the Cu2O microcrystal with octahedral morphology. From Table 1, it is also found that the coercivity (Hc) was least for the octahedral morphology compared to other morphology. The remanent magnetization (Mr) for the investigated Cu2O microcrystals is found to be in the range 305–363 106 emu/g. The
The two F1u modes are infrared-active, one F2g mode is Raman-active and all others are silent in both infrared and Raman spectra. Since the crystal has inversion symmetry, the mutual exclusion principle restricts the infrared-allowed modes are Raman forbidden and vice versa. As per group-theoretical analysis above, the Raman spectrum of a perfect Cu2O crystal should exhibit only Raman signal belonging to the three-fold degenerate F2g mode. The Raman spectra measured at room temperature of all the synthesized Cu2O microcrystals in the range of 50–900 cm1 shown as Fig. 4 vary significantly in the number of observed modes. The non-stoichiometry due to the formation of defects such as vacancies, interstitials or antisite defects may be the origin for this breakdown of the above selection rules. The reduction of local symmetry by point defects may diminish the distinction between Raman-allowed and Raman-forbidden lattice vibrations and also local Raman active modes may be introduced depending on the specific point defect and its compatibility with the lattice. The absence of characteristic strong peak at 290 cm1 weak band at 340 and 602 cm1 corresponding to the CuO phase further confirms the absence of CuO in all the investigated Cu2O crystals [18]. The weak band observed at around 515 cm1 as shown in Fig. 4 is the only allowed Raman mode (F2g) as per the selection rule. Based on the earlier investigations the bands observed in the Raman spectra may be assigned as lattice modes at about 92 cm1 (T2u), 110 cm1 (Eu), 150 cm1 (T1u LO), and 630 and 647 cm1 (T1u TO, LO) [18,19]. The strong Raman peaks at 218 cm1 (2Eu) and 415 cm1 are assigned to multiphonon Raman scattering. The second-order overtone Raman mode observed at 218 cm1 is usually appeared as most intense Raman mode in the reported Raman spectrum of Cu2O crystals [19]. However in the present study as shown in Fig. 4 the peak at 150 cm1 appeared as most intense and sharp peak irrespective of the morphology of all the investigated Cu2O micro crystals. The weak Raman peak at 110 cm1 and very strong peak at 150 cm1 actually corresponds to infrared allowed mode in perfect Cu2O crystal [19]. The appearance of infrared active band in the Raman spectrum as most intense peak at 150 cm1 strongly suggest the breakdown of selection rule due to the possible defect in the investigated Cu2O microcrystals. The defect in the microcrystals was also confirmed by the room temperature photoluminescence spectroscopy by exciting the sample at 514 nm. The photoluminescence spectra shown in Fig. 5 clearly revealed a broad peak at around 612 nm for all the investigated Cu2O microcrystals. The earlier photoluminescence studies on Cu2O indicated that the broad peak observed around 612 nm has been attributed to the cationic vacancy present in the material [20,21]. Usually, the magnetic property depends on extrinsic effects, such as the presence of magnetic impurities or intrinsic crystal defects such as vacancies. The ab initio DFT calculations indicated that the ferromagnetic property possibly arise from the presence of cation vacancies in the ideal lattice of diamagnetic Cu2O [9]. Reports on Cu2O nanostructures also indicated that the defects in the form of cation vacancies in these materials are the origin for the observed ferromagnetism in these materials [10]. The slight variation observed in the magnetic properties of the prepared samples might
574
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0.006
M (emu/g)
0.004
(b)
0.002 0.000 -0.002 -0.004 -0.006
-4000
-2000
0
2000
4000
2000
4000
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4000
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4000
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(d)
M (emu/g)
0.001 0.000 -0.001 -0.002
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0
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M (emu/g)
0.002
(f)
0.001 0.000 -0.001 -0.002
-4000
-2000
0
F (G) 0.002
(h)
M (emu/g)
0.001 0.000 -0.001 -0.002
-4000
-2000
0
F (G) Fig. 2. Typical SEM micrographs and corresponding room temperature magnetic hysteresis loop (M–H) of octahedral (a,b), star/short hexapod (c,d), extended hexapod (e,f) and microrod (g,h) shape Cu2O microcrystals.
G. Prabhakaran, R. Murugan / Journal of Alloys and Compounds 579 (2013) 572–575
be due to the variations of the vacancy concentration in the Cu2O microcrystals with different morphology.
Table 1 Magnetic properties of the Cu2O microcrystals. Hc (Oe) Octahedron Star/short hexapod Extended hexapod Microrod
74.301 105.93 104.93 104.14
575
Ms (emu/g)
Mr (emu/g) 3
4.7698 10 2.0667 103 1.9058 103 1.9614 103
362.94 106 332.84 106 305.75 106 309.51 106
4. Conclusion In summary, herein we report the observation of room temperature ferromagnetism in octahedral, star/short hexapod, extended hexapod and microrod shape Cu2O microcrystals. The experimental results revealed that the FM order in octahedral Cu2O microcrystal appears to be slightly different from other morphology. The magnetic properties of the star/short hexapod, extended hexapod and microrod shape Cu2O microcrystals appears to be almost morphology independent. The preliminary TG/DSC, photoluminescence, Raman spectroscopic and vibrating sample magnetometer result indicated the defect induced ferromagnetism present in the synthesized Cu2O microcrystals. However a detailed X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS) and positron annihilation analysis (PAS) are essential to gain further knowledge on the origin of this ferromagnetism in these microcrystals. Acknowledgements
Fig. 3. The expanded view of room temperature magnetic hysteresis loop (M–H) of octahedral (a), star/short hexapod (b), extended hexapod (c) and microrod (d) shape Cu2O microcrystals.
R.M. acknowledges the financial support of UGC, New Delhi, India [F.No. 41-918/2012 (SR) dated 23.07.2012]. The authors acknowledge CIF, Pondicherry University, India for extending the instrumentation facilities. References
Fig. 4. Raman spectra of octahedral (a), star/short hexapod (b), extended hexapod (c), microrod (d) shape Cu2O microcrystals.
Fig. 5. Photoluminescence spectra of octahedral (a), star/short hexapod (b), extended hexapod (c), microrod (d) shape Cu2O microcrystals.
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