- Email: [email protected]
C self-assemblies
Materials Letters 122 (2014) 103–105
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Magnetic and electromagnetic properties of Fe3O4/C self-assemblies Feng Xiao, Chao Feng, Chuangui Jin n, Xianguo Liu, Le Pan, Ailin Xia Anhui Key Laboratory of Metal Materials and Processing, School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 October 2013 Accepted 8 January 2014 Available online 16 January 2014
Fe3O4/C self-assembled rod-like nanostructures with average diameter of 10 nm and length of 200 nm have been synthesized by the hydrothermal method. Fe3O4/C rod-like nanostructures were selfassembled by Fe3O4/C nanoparticles. The saturation magnetization of Fe3O4/C self-assemblies is 41.7 emu/g. Compared to Fe3O4 nanoparticles, the higher dielectric loss of Fe3O4/C self-assemblies is ascribed to the amorphous carbon layer. Due to the existence of carbon layer on Fe3O4, the real part of permeability of Fe3O4/C self-assemblies is higher than that of Fe3O4 nanoparticles at 2–16 GHz. Two overlapped peaks of imaginary part of permeability in Fe3O4/C self-assemblies are from the natural and exchange resonance. & 2014 Elsevier B.V. All rights reserved.
Keywords: Fe3O4 Carbon Nanocomposites Magnetic materials
1. Introduction During the last few years, with the rapid growth in utilization of electrical and electronic devices, the damage of ecological environment caused by electromagnetic (EM) radiation pollution and threat to the health of human have become more serious [1]. Much attention has been focused on EM-absorbing materials. Fe3O4 have been extensively investigated as EM-wave absorbers with low cost and strong absorption characteristics [2]. However, the Fe3O4 have main disadvantages such as high density and narrow absorption bandwidth. In order to improve the EM properties of Fe3O4, various Fe3O4-based nanocomposites including the conductive and nonmagnetic material have been widely investigated. Fe3O4/cabon [3], Fe3O4/TiO2 [4], Fe3O4/SiO2 [5], and Fe3O4/ SnO2 [6] have been reported and their EM properties have been studied. For magnetic materials, the key to enhancing microwave absorbing properties lies in the improvement of complex permeability. High permeability and high resonance frequency are both required. As is well known, the static permeability (μs ) and the resonance frequency (f r ) of the magnetic materials satisfy the equations: 2π f r ¼ γ H a and H a ¼ 4jK 1 j=3μs M s , where γ ¼2.8 GHz kOe 1 is the gyromagnetic ratio and Ha is the crystalline anisotropic field and coefficient, respectively [2,7,8]. Ha includes the volume anisotropic field, surface anisotropic field and shape anisotropic field contributions [2,7,8]. Therefore, the large shape anisotropy can effectively improve the initial permeability and resonance frequency. Fe3O4/C nanocomposites have been synthesis by different methods [9]. In this paper, we
n
Corresponding author. Tel.: þ 86 13665557919; fax: þ86 555 2311570. E-mail address: [email protected] (C. Jin).
0167-577X/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2014.01.016
report a new approach to synthesize Fe3O4/C self-assembled rod-like nanostructures by the simple hydrothermal method and investigate their magnetic and EM properties in detail. 2. Experimental Analytically pure reagents were used in this experiment. The Fe3O4 nanoparticles and Fe3O4/C self-assemblies were prepared without and with glucose at the same experimental conditions, respectively. The detailed processes were as follows: a mixture was prepared by the hydrated ferrous sulfate (FeSO4 7H2O, 2.78 g), glucose (C6H12O6, 2 g) and deionized water (50 ml). Then, ethyl alcohol (C2H5OH, 99%, 50 ml) and hydrazine hydrate (N2H4, 50%, 4 ml) were dissolved in the above mixture with continous stirring. The solution was transferred to an 150-ml Telfon-lined stainless autoclave. The autoclave was sealed and maintained at 180 1C for 6 h and then cooled to room temperature naturally. The products were collected by the centrifugal method and washed with deionized water and alcohol for three times, and finally dried in vacuum at 70 1C for 8 h. The obtained samples were characterized on a Brucker D8 X-ray powder diffractometer (XRD) with Cu Kα radiation. The morphology of the as synthesized samples was determined a JEOL2100F transmission electron microscope (TEM). Magnetic properties of the samples were carried out by vibrating sample magnetometer (VSM). For measurement of the EM properties, the samples were dispersed in paraffin homogeneously [1–3,6–12]. Then the composite was cut into toroidal shape with 7.00 mm outer diameter and 3.04 mm inner diameter. The EM parameters were measured for the composite containing 40 wt% sample, using an Agilent N5244A vector network analyzer (VNA). Coaxial method
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F. Xiao et al. / Materials Letters 122 (2014) 103–105
was used to determine the EM parameters of the toroidal samples in a frequency range of 2–18 GHz with a transverse EM mode. The VNA was calibrated for the full two-port measurement of reflection and transmission at each port. The complex permittivity and complex permeability were calculated from S-parameters tested by the vector network analyzer, using the simulation program of Reflection/Transmission Nicolson–Ross model [2].
3. Results and discussion Fig.1 shows the XRD patterns of as-prepared samples, in which all the diffraction peaks can be indexed to the cubic phase of Fe3O4 (JCPDS 19-0629). The peaks from sample without glucose (Fig. 1 (a)) are stronger and narrower than those from sample with glucose (Fig. 1(b)). According to the Scherrer formula, the average size of the sample without glucose is estimated to be 22 nm, while the average size of the sample with glucose is 10 nm. The morphology of samples with/without glucose is given in Fig. 2. The sample without glucose whose average size is 25 nm exhibits some aggregation as shown in Fig. 2(a). Fig. 2(b) is the HRTEM image of nanoparticles in Fig. 2(a). The d-spacing calculated in Fig. 2(b) is 0.253 nm, corresponding to the lattice fringe {311} of
Fig. 1. XRD patterns of the as-prepared samples: (a) with and (b) without glucose.
Fig. 2. TEM images of (a) Fe3O4 nanoparticles and (c) Fe3O4/C self-assemblies, and HRTEM images of (b) Fe3O4 nanoparticles and (d) Fe3O4/C self-assemblies.
Fe3O4. In Fig. 2(c), sample with glucose exhibits rod-like arrays (about 200 nm in length) self-assembled by the Fe3O4 nanoparticles (about 8 nm in diameter). In Fig. 2(d), the HRTEM image of Fe3O4/C nanoparticles clearly shows the core/shell structure, with a crystalline core and an amorphous shell. In the core, the d-spacing of 0.253 nm corresponds to the lattice fringe {311} of Fe3O4. The shell can be determined to be carbon from the energydispersive spectrometry result. The size of Fe3O4/C nanoparticles is smaller than that of Fe3O4 nanoparticles, indicating that the carbon layer from the glucose may restrain the growth of Fe3O4 nanoparticles. The formation pocesses of Fe3O4 nanoparticles and Fe3O4/C self-assemblies were as follows: during the hydrothermal reaction process, Fe nanoparticles are obtained through redox at first, and then Fe nanoparticles are oxided to be Fe3O4 nanoparticles, the amorphous C from glucose carbonization is absorbed on the surface of Fe3O4 nanoparticles to form Fe3O4/C self-assemblies. The magnetic hysteresis loops of Fe3O4/C self-assemblies and Fe3O4 nanoparticles are shown in Fig. 3. The shape of loops and the existence of coercive field indicate ferromagnetic behavior of two samples. It is obvious that the saturation magnetization (MS) of Fe3O4/C self-assemblies (41.7 emu/g) is lower than that of Fe3O4 nanoparticles (48.7 emu/g), while the coercivity of the Fe3O4/C self-assemblies is higher than that of Fe3O4 nanoparticles as shown in the inset of Fig. 3. The phenomenon is ascribed to the fact that saturation magnetization drops monotonically with the size of nanoparticles. According to the Stoner–Wolhfarth theory, the coercivity (HC) of the nanosized materials with single-domain particles with uniaxial anisotropy depends on both the anisotropy constant (K) and the saturation magnetization (MS) Hc ¼ 2K=μ0 Ms, μ0 is the permeability constant of the vacuum [10]. In addition, the existence of nonmagnetic carbon layer also affects HC [11]. As shown in Fig. 4(a), the real part (ε0 )/the imaginary part (ε″) of permittivity of Fe3O4/C self-assemblies is lower/higher than that of Fe3O4 nanoparticles, respectively. According to the free electron theory ε″ 1=2πε0 ρf (ρ is the resistivity) [7,8], so the resistivity of Fe3O4/C self-assemblies is lower than that of Fe3O4 nanoparticles. Based on dielectric loss angle equationtg ε ¼ ε″=ε0 , the dielectric loss of Fe3O4/C self-assemblies is higher than that of Fe3O4 nanoparticles, which is helpful to the microwave absorption. In Fig. 4(b), the real part (μ0 ) of permeability of Fe3O4/C selfassemblies is higher than that of Fe3O4 nanoparticles at 2–16 GHz, because of the existence of carbon layer on Fe3O4. Generally, the imaginary part (μ″) of permeability represents magnetic energy dissipation, and a resonance peak of μ″ appearance means a high magnetic loss at a certain frequency. Compared to the Fe3O4 nanoparticles, the peaks of μ″ of Fe3O4/C self-assemblies shift to the high frequency, ascribed to the small size of Fe3O4/C
Fig. 3. Hysteresis loop for the Fe3O4 nanoparticles and Fe3O4/C self-assemblies at the room temperature. The inset shows the enlarged part at a low field range.
F. Xiao et al. / Materials Letters 122 (2014) 103–105
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Fig. 4. (a) Complex permittivity and (b) complex permeability of Fe3O4/C self-assemblies and Fe3O4 nanoparticles.
nanoparticles. In addition, two overlapped peaks in Fe3O4/C selfassemblies are also observed in Fig. 4(b), as reported elsewhere, one resonance peak of μ″ at 7 GHz is mainly attributed to the natural resonance originated from traditional static magnetic energy, and the other resonance peak at 12 GHz is attributed to energy exchange effect of magnetic nanoparticles from small size effect [12,13]. 4. Conclusions Fe3O4/C self-assemblies were synthesized by a simple hydrothermal method. The average diameter and length of Fe3O4/C selfassemblies is 10 and 200 nm, respectively. The MS of Fe3O4/C selfassemblies is lower than that of Fe3O4 nanoparticles. The dielectric loss of Fe3O4/C self-assemblies is higher than that of Fe3O4 nanoparticles. Due to the existence of carbon layer on Fe3O4, μ0 of Fe3O4/C self-assemblies is higher than that of Fe3O4 nanoparticles at 2–16 GHz. Two overlapped peaks of μ″ of Fe3O4/C selfassemblies are from natural and exchange resonance. Acknowledgments This study has been supported by the National Natural Science Foundation of China (Grant nos. 21071003 and 51201002).
References [1] Liu XG, Ou ZQ, Geng DY, Han Z, Jiang JJ, Liu W, et al. Carbon 2010;48(3):891–7. [2] Liu XG, Feng C, Or SW, Jin CG, Xiao F, Xia AL, et al. Mater Res Bull 2013;48 (10):3887–91. [3] Chen YJ, Xiao G, Wang TS, Ouyang QY, Qi LH, Ma Y, et al. J Phys Chem C 2011;115(28):13603–8. [4] Zhu CL, Zhang ML, Qiao YJ, Xiao G, Zhang F, Chen YJ, et al. J Phys Chem C 2010;114(39):16229–35. [5] Deng Y, Qi D, Deng C, Zheng X, Zhao DY. J Am Chem Soc 2008;130(1):28–9. [6] Chen YJ, Gao P, Wang RX, Zhu CL, Wang LJ, Cao MS, et al. J Phys Chem C 2009;113(23):10061–4. [7] Liu XG, Or SW, Ho SL, Cheung CC, Leung CM, Han Z, et al. J Alloys Compd 2011;509(6):2929–32. [8] Liu XG, Geng D, Shang P, Meng H, Yang F, Li B, et al. J Phys D: appl Phys 2008;41(17):175006 (175006-7). [9] Yan AG, Liu Y, Liu Y, Li X, Lei Z, Liu P. Mater Lett 2012;68:402–5. [10] Caruntu D, Caruntu G, O’Connor CJ. J Phys D: Appl Phys 2007;40(19):5801–9. [11] Liu XG, Geng DY, Meng H, Shang PJ, Zhang ZD. Appl Phys Lett 2008;92 (17):173117 (173117-3). [12] Ma J, Li J, Ni X, Zhang X, Huang J. Appl Phys Lett 2009;95(10):102505 (102505-3). [13] Pan H, Cheng X, Zhang C, Gong C, Yu L, Zhang J, et al. Appl Phys Lett 2013;102 (1):012410 (012410-4).
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Magnetic and electromagnetic properties of Fe3O4/C self-assemblies Feng Xiao, Chao Feng, Chuangui Jin n, Xianguo Liu, Le Pan, Ailin Xia Anhui Key Laboratory of Metal Materials and Processing, School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 October 2013 Accepted 8 January 2014 Available online 16 January 2014
Fe3O4/C self-assembled rod-like nanostructures with average diameter of 10 nm and length of 200 nm have been synthesized by the hydrothermal method. Fe3O4/C rod-like nanostructures were selfassembled by Fe3O4/C nanoparticles. The saturation magnetization of Fe3O4/C self-assemblies is 41.7 emu/g. Compared to Fe3O4 nanoparticles, the higher dielectric loss of Fe3O4/C self-assemblies is ascribed to the amorphous carbon layer. Due to the existence of carbon layer on Fe3O4, the real part of permeability of Fe3O4/C self-assemblies is higher than that of Fe3O4 nanoparticles at 2–16 GHz. Two overlapped peaks of imaginary part of permeability in Fe3O4/C self-assemblies are from the natural and exchange resonance. & 2014 Elsevier B.V. All rights reserved.
Keywords: Fe3O4 Carbon Nanocomposites Magnetic materials
1. Introduction During the last few years, with the rapid growth in utilization of electrical and electronic devices, the damage of ecological environment caused by electromagnetic (EM) radiation pollution and threat to the health of human have become more serious [1]. Much attention has been focused on EM-absorbing materials. Fe3O4 have been extensively investigated as EM-wave absorbers with low cost and strong absorption characteristics [2]. However, the Fe3O4 have main disadvantages such as high density and narrow absorption bandwidth. In order to improve the EM properties of Fe3O4, various Fe3O4-based nanocomposites including the conductive and nonmagnetic material have been widely investigated. Fe3O4/cabon [3], Fe3O4/TiO2 [4], Fe3O4/SiO2 [5], and Fe3O4/ SnO2 [6] have been reported and their EM properties have been studied. For magnetic materials, the key to enhancing microwave absorbing properties lies in the improvement of complex permeability. High permeability and high resonance frequency are both required. As is well known, the static permeability (μs ) and the resonance frequency (f r ) of the magnetic materials satisfy the equations: 2π f r ¼ γ H a and H a ¼ 4jK 1 j=3μs M s , where γ ¼2.8 GHz kOe 1 is the gyromagnetic ratio and Ha is the crystalline anisotropic field and coefficient, respectively [2,7,8]. Ha includes the volume anisotropic field, surface anisotropic field and shape anisotropic field contributions [2,7,8]. Therefore, the large shape anisotropy can effectively improve the initial permeability and resonance frequency. Fe3O4/C nanocomposites have been synthesis by different methods [9]. In this paper, we
n
Corresponding author. Tel.: þ 86 13665557919; fax: þ86 555 2311570. E-mail address: [email protected] (C. Jin).
0167-577X/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2014.01.016
report a new approach to synthesize Fe3O4/C self-assembled rod-like nanostructures by the simple hydrothermal method and investigate their magnetic and EM properties in detail. 2. Experimental Analytically pure reagents were used in this experiment. The Fe3O4 nanoparticles and Fe3O4/C self-assemblies were prepared without and with glucose at the same experimental conditions, respectively. The detailed processes were as follows: a mixture was prepared by the hydrated ferrous sulfate (FeSO4 7H2O, 2.78 g), glucose (C6H12O6, 2 g) and deionized water (50 ml). Then, ethyl alcohol (C2H5OH, 99%, 50 ml) and hydrazine hydrate (N2H4, 50%, 4 ml) were dissolved in the above mixture with continous stirring. The solution was transferred to an 150-ml Telfon-lined stainless autoclave. The autoclave was sealed and maintained at 180 1C for 6 h and then cooled to room temperature naturally. The products were collected by the centrifugal method and washed with deionized water and alcohol for three times, and finally dried in vacuum at 70 1C for 8 h. The obtained samples were characterized on a Brucker D8 X-ray powder diffractometer (XRD) with Cu Kα radiation. The morphology of the as synthesized samples was determined a JEOL2100F transmission electron microscope (TEM). Magnetic properties of the samples were carried out by vibrating sample magnetometer (VSM). For measurement of the EM properties, the samples were dispersed in paraffin homogeneously [1–3,6–12]. Then the composite was cut into toroidal shape with 7.00 mm outer diameter and 3.04 mm inner diameter. The EM parameters were measured for the composite containing 40 wt% sample, using an Agilent N5244A vector network analyzer (VNA). Coaxial method
104
F. Xiao et al. / Materials Letters 122 (2014) 103–105
was used to determine the EM parameters of the toroidal samples in a frequency range of 2–18 GHz with a transverse EM mode. The VNA was calibrated for the full two-port measurement of reflection and transmission at each port. The complex permittivity and complex permeability were calculated from S-parameters tested by the vector network analyzer, using the simulation program of Reflection/Transmission Nicolson–Ross model [2].
3. Results and discussion Fig.1 shows the XRD patterns of as-prepared samples, in which all the diffraction peaks can be indexed to the cubic phase of Fe3O4 (JCPDS 19-0629). The peaks from sample without glucose (Fig. 1 (a)) are stronger and narrower than those from sample with glucose (Fig. 1(b)). According to the Scherrer formula, the average size of the sample without glucose is estimated to be 22 nm, while the average size of the sample with glucose is 10 nm. The morphology of samples with/without glucose is given in Fig. 2. The sample without glucose whose average size is 25 nm exhibits some aggregation as shown in Fig. 2(a). Fig. 2(b) is the HRTEM image of nanoparticles in Fig. 2(a). The d-spacing calculated in Fig. 2(b) is 0.253 nm, corresponding to the lattice fringe {311} of
Fig. 1. XRD patterns of the as-prepared samples: (a) with and (b) without glucose.
Fig. 2. TEM images of (a) Fe3O4 nanoparticles and (c) Fe3O4/C self-assemblies, and HRTEM images of (b) Fe3O4 nanoparticles and (d) Fe3O4/C self-assemblies.
Fe3O4. In Fig. 2(c), sample with glucose exhibits rod-like arrays (about 200 nm in length) self-assembled by the Fe3O4 nanoparticles (about 8 nm in diameter). In Fig. 2(d), the HRTEM image of Fe3O4/C nanoparticles clearly shows the core/shell structure, with a crystalline core and an amorphous shell. In the core, the d-spacing of 0.253 nm corresponds to the lattice fringe {311} of Fe3O4. The shell can be determined to be carbon from the energydispersive spectrometry result. The size of Fe3O4/C nanoparticles is smaller than that of Fe3O4 nanoparticles, indicating that the carbon layer from the glucose may restrain the growth of Fe3O4 nanoparticles. The formation pocesses of Fe3O4 nanoparticles and Fe3O4/C self-assemblies were as follows: during the hydrothermal reaction process, Fe nanoparticles are obtained through redox at first, and then Fe nanoparticles are oxided to be Fe3O4 nanoparticles, the amorphous C from glucose carbonization is absorbed on the surface of Fe3O4 nanoparticles to form Fe3O4/C self-assemblies. The magnetic hysteresis loops of Fe3O4/C self-assemblies and Fe3O4 nanoparticles are shown in Fig. 3. The shape of loops and the existence of coercive field indicate ferromagnetic behavior of two samples. It is obvious that the saturation magnetization (MS) of Fe3O4/C self-assemblies (41.7 emu/g) is lower than that of Fe3O4 nanoparticles (48.7 emu/g), while the coercivity of the Fe3O4/C self-assemblies is higher than that of Fe3O4 nanoparticles as shown in the inset of Fig. 3. The phenomenon is ascribed to the fact that saturation magnetization drops monotonically with the size of nanoparticles. According to the Stoner–Wolhfarth theory, the coercivity (HC) of the nanosized materials with single-domain particles with uniaxial anisotropy depends on both the anisotropy constant (K) and the saturation magnetization (MS) Hc ¼ 2K=μ0 Ms, μ0 is the permeability constant of the vacuum [10]. In addition, the existence of nonmagnetic carbon layer also affects HC [11]. As shown in Fig. 4(a), the real part (ε0 )/the imaginary part (ε″) of permittivity of Fe3O4/C self-assemblies is lower/higher than that of Fe3O4 nanoparticles, respectively. According to the free electron theory ε″ 1=2πε0 ρf (ρ is the resistivity) [7,8], so the resistivity of Fe3O4/C self-assemblies is lower than that of Fe3O4 nanoparticles. Based on dielectric loss angle equationtg ε ¼ ε″=ε0 , the dielectric loss of Fe3O4/C self-assemblies is higher than that of Fe3O4 nanoparticles, which is helpful to the microwave absorption. In Fig. 4(b), the real part (μ0 ) of permeability of Fe3O4/C selfassemblies is higher than that of Fe3O4 nanoparticles at 2–16 GHz, because of the existence of carbon layer on Fe3O4. Generally, the imaginary part (μ″) of permeability represents magnetic energy dissipation, and a resonance peak of μ″ appearance means a high magnetic loss at a certain frequency. Compared to the Fe3O4 nanoparticles, the peaks of μ″ of Fe3O4/C self-assemblies shift to the high frequency, ascribed to the small size of Fe3O4/C
Fig. 3. Hysteresis loop for the Fe3O4 nanoparticles and Fe3O4/C self-assemblies at the room temperature. The inset shows the enlarged part at a low field range.
F. Xiao et al. / Materials Letters 122 (2014) 103–105
105
Fig. 4. (a) Complex permittivity and (b) complex permeability of Fe3O4/C self-assemblies and Fe3O4 nanoparticles.
nanoparticles. In addition, two overlapped peaks in Fe3O4/C selfassemblies are also observed in Fig. 4(b), as reported elsewhere, one resonance peak of μ″ at 7 GHz is mainly attributed to the natural resonance originated from traditional static magnetic energy, and the other resonance peak at 12 GHz is attributed to energy exchange effect of magnetic nanoparticles from small size effect [12,13]. 4. Conclusions Fe3O4/C self-assemblies were synthesized by a simple hydrothermal method. The average diameter and length of Fe3O4/C selfassemblies is 10 and 200 nm, respectively. The MS of Fe3O4/C selfassemblies is lower than that of Fe3O4 nanoparticles. The dielectric loss of Fe3O4/C self-assemblies is higher than that of Fe3O4 nanoparticles. Due to the existence of carbon layer on Fe3O4, μ0 of Fe3O4/C self-assemblies is higher than that of Fe3O4 nanoparticles at 2–16 GHz. Two overlapped peaks of μ″ of Fe3O4/C selfassemblies are from natural and exchange resonance. Acknowledgments This study has been supported by the National Natural Science Foundation of China (Grant nos. 21071003 and 51201002).
References [1] Liu XG, Ou ZQ, Geng DY, Han Z, Jiang JJ, Liu W, et al. Carbon 2010;48(3):891–7. [2] Liu XG, Feng C, Or SW, Jin CG, Xiao F, Xia AL, et al. Mater Res Bull 2013;48 (10):3887–91. [3] Chen YJ, Xiao G, Wang TS, Ouyang QY, Qi LH, Ma Y, et al. J Phys Chem C 2011;115(28):13603–8. [4] Zhu CL, Zhang ML, Qiao YJ, Xiao G, Zhang F, Chen YJ, et al. J Phys Chem C 2010;114(39):16229–35. [5] Deng Y, Qi D, Deng C, Zheng X, Zhao DY. J Am Chem Soc 2008;130(1):28–9. [6] Chen YJ, Gao P, Wang RX, Zhu CL, Wang LJ, Cao MS, et al. J Phys Chem C 2009;113(23):10061–4. [7] Liu XG, Or SW, Ho SL, Cheung CC, Leung CM, Han Z, et al. J Alloys Compd 2011;509(6):2929–32. [8] Liu XG, Geng D, Shang P, Meng H, Yang F, Li B, et al. J Phys D: appl Phys 2008;41(17):175006 (175006-7). [9] Yan AG, Liu Y, Liu Y, Li X, Lei Z, Liu P. Mater Lett 2012;68:402–5. [10] Caruntu D, Caruntu G, O’Connor CJ. J Phys D: Appl Phys 2007;40(19):5801–9. [11] Liu XG, Geng DY, Meng H, Shang PJ, Zhang ZD. Appl Phys Lett 2008;92 (17):173117 (173117-3). [12] Ma J, Li J, Ni X, Zhang X, Huang J. Appl Phys Lett 2009;95(10):102505 (102505-3). [13] Pan H, Cheng X, Zhang C, Gong C, Yu L, Zhang J, et al. Appl Phys Lett 2013;102 (1):012410 (012410-4).