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Structural and magnetic properties of electrospun yttrium iron garnet (YIG) nanofibers
Ceramics International xx (xxxx) xxxx–xxxx
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Structural and magnetic properties of electrospun yttrium iron garnet (YIG) nanofibers Lining Pana, Xinlei Zhanga, Jianbo Wanga,b, Qingfang Liua,
⁎
a
Key Laboratory for Magnetic and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, People's Republic of China Key Laboratory of Special Function Materials and Structure Design of the Ministry of Education, Lanzhou University, Lanzhou 730000, People's Republic of China b
A R T I C L E I N F O
A BS T RAC T
Keywords: Yttrium iron garnet (YIG) Electrospinning Magnetic properties Electron spin resonance
The present work describes the fabrication of yttrium iron garnet (YIG) nanofibers prepared via electrospinning method. The YIG nanofibers with diameters of around 80 nm were formed when calcinations temperature exceeded 750 °C. The morphology and magnetic parameters including saturation magnetization (Ms) and coercivity (Hc) of these samples dramatically change with variation of the annealing temperature. In particular, the Ms displays a steady increase from 3.1 emu/g at 700–21.5 emu/g at 1000 °C, and the Hc shows a different tendency that first increases to a maximum value of 140 Oe at 750 °C, and then reduce distinctly. Finally, the electron spin resonance spectra display strong asymmetry and broad linewidth that are highly affected by shape anisotropy, defects and the presence of trace amount second phase orthorhombic YFeO3 (o-YFeO3) in the YIG nanofibers.
1. Introduction Garnet ferrites have been widely applied in many microwave and magneto-optical devices, including optical isolators, oscillators, circulators, and phase shifters, due to their superior properties such as low damping, high electrical resistivity and controllable saturation magnetization [1–5]. As one of typical garnet ferrites, especially, yttrium iron garnet Y3Fe5O12 (YIG) has attracted widespread attention. Moreover, with the rapidly development of nanotechnology in recent years, the nanostructural YIG materials have also been investigated extensively because of their unique size dependent properties and great potential to act as key performers in the near future [6–8]. Nevertheless, most of researches about YIG in the nanostructural state are still limited to powders [9,10] and films [11,12], and one-dimensional (1D) nanostructures of YIG are seldom reported. Therefore, preparation and investigation of 1D YIG nanomaterials should be carried out. During the past decades, there has been a steady growth of interest in 1D nanostructural magnetic materials not only because of their distinctive properties compared with bulk or particle counterparts, but their potential applications in many fields such as magnetic sensors, ultrahigh-density data storage, electromagnetic devices and magnetooptical switches [13–16]. Up to now, numerous approaches have been demonstrated for preparing 1D magnetic nanomaterials, including precursor thermal decomposition [17], anodized aluminum oxide
⁎
(AAO) template [18,19], and electrospinning [20,21], etc. Among these methods, electrospinning as an extremely simple and efficient method for fabricating ultrafine and continuous nanofibers has successfully received widespread attention of researchers. So far, there are huge quantity reports about 1D magnetic nanofibers synthesized by electrospinning, for instance, Fe, Co, Ni, CoFe2O4, NiFe2O4, SrFe12O19, etc [20,22,23]. Herein, YIG nanofibers have been successfully obtained through electrospinning followed by two-steps calcinations approach. Simultaneously, a series of analytical methods were introduced to reveal their phase content, morphology, magnetic properties and microwave properties in detail. Furthermore, we elucidated the influence of the morphology of YIG nanofibers on its magnetic and ferromagnetic resonance properties. 2. Experimental All chemical regents, which contain ethanol (≧99.7%), ferric nitrate nonahydrate (Fe(NO3)3·9H2O, ≧98.5%), yttrium nitrate hexahydrate (Y(NO3)3·6H2O, 99.9%) and poly(vinyl pyrrolidone) (PVP, average Mw ≈1,300,000 g mol−1, purchased from sigma-aldrich), used in the experiment are of chemical grade. Entire preparation procedures of YIG nanofibers consist of precursor solution preparation, electrospinning and calcinations. The
Corresponding author. E-mail address: [email protected] (Q. Liu).
http://dx.doi.org/10.1016/j.ceramint.2016.10.070 Received 31 May 2016; Received in revised form 10 October 2016; Accepted 11 October 2016 Available online xxxx 0272-8842/ © 2016 Published by Elsevier Ltd.
Please cite this article as: Pan, L., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.10.070
Ceramics International xx (xxxx) xxxx–xxxx
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6H2O and Fe(NO3)3·9H2O mixed solution was added slowly into the PVP/ethanol solution and further continuous magnetically stirred for another 3 h at the room temperature to form a precursor solution for electrospinning. The viscous precursor solution was loaded into a plastic syringe with a stainless steel needle (d =0.8 mm, where d stands for outer diameter of the stainless steel needle) to obtain the precursor YIG/PVP composite nanofibers. The applied voltage and the distance between syringe needle tip and the ground collector were 14.0 kV and 15 cm, respectively. The solution was driven by a syringe pump of a rate of 0.35 mL h−1 during the electrospinning process. At last, the aselectrospun YIG/PVP composite nanofibers were annealed by twosteps calcinations to obtain the resultant nanofibers, being presintering at 100 °C for 2 h and calcined at different temperature T (T =700 °C, 750 °C, 900 °C and 1000 °C) for 2 h in ambient atmosphere, and the heating rate guaranteed 2 °C/min during all calcinations process. X-ray diffraction (XRD) patterns was characterized by a PANalytical diffractometer using Cu Kα radiation (λ=0.15418 nm). The morphology and microstructure of the YIG nanofibers were carried out by field emission scanning electron microscopy (FESEM, Hitachi S4800), transmission electron microscopy (TEM, Tecnai™ G2 F30, FEI), and selected area electron diffraction (SAED). Vibrating sample magnetometer (VSM, LakeShore 7304, USA) was utilized to measure the static magnetic properties of the prepared samples at room temperature. Electron spin resonance (ESR, JES-FA300) was performed using an X-band spectrometer with a magnetic field ranging from 0 to 500 mT at a frequency of 9 GHz.
Fig. 1. XRD patterns of the YIG nanofibers annealed at 700, 750, 900 and 1000 °C for 2 h.
Table 1 Average crystalline sizes D of YIG nanofibers annealed at different temperature for 2 h. Temperature (°C)
700
750
900
1000
Average crystalline size D (nm)
–
30
39
47
3. Results and discussion
preparation processes of precursor solution are as follows: firstly, the appropriate amount of Y(NO3)3·6H2O and Fe(NO3)3·9H2O with a molar ratio of 3:5 were dissolved in 5 mL distilled water. At the same time, PVP with the concentration of 9 wt% was added into 1 mL ethanol solution, followed by continuous magnetic stirring for 2 h to ensure the complete dissolution of PVP. Subsequently, 1 mL Y(NO3)3·
Fig. 1 shows the XRD patterns of the prepared YIG nanofibers annealed at different annealing temperatures from 700 °C to 1000 °C. It can be obviously observed that the crystallization process of the sample begins at 700 °C. When the sample annealed at this temperature, almost all characteristic diffraction peaks can be assigned to the
Fig. 2. Typical SEM images of the YIG nanofibers annealed at (a) 700 °C, (b) 750 °C, (c) 900 °C, (d) 1000 °C, respectively.
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Fig. 3. Typical TEM images and HRTEM image of the YIG nanofibers annealed at 750 °C (a–b), 900 °C (c–d) and 1000 °C (e–f). Inset shows the corresponding SAED images of YIG nanofibers annealed at 750 °C.
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Fig. 5. Room-temperature ESR spectra of the YIG nanofibers with various calcination temperatures recorded at 9 GHz.
X-ray, and β is the full-width at half-maximum (FWHM). As shown in Table. 1, the crystallite size of YIG nanofibers increases monotonically from 30 nm to 47 nm with the increase of the annealing temperature. Fig. 2 shows the SEM images of YIG nanofibers obtained at different annealing temperatures. The nanofibers annealed at 700 °C have continuous structure with a relatively rough surface and their average diameter is approximately 65 nm in Fig. 2(a). As calcinations temperature rises to 750 °C (Fig. 2(b)), the nanofibers still remain as continuous structures, and their average diameter and roughness are both nearly invariable comparing to the nanofibers annealed at 700 °C. Subsequently, the morphology of the samples is dramatically changed with increasing the calcination temperature further. When annealing temperature increases to 900 °C, the surface of resultant nanofibers becomes smoother due to the growth and coalescence of the particles in the nanofibers under the calcinations process. Almost all of nanofibers become discontinuous and agglomerate for the sample sintered at 1000 °C shown in Fig. 2(d). So, it can be deduced that the morphology of the samples is significantly influenced by the annealing temperature. As a representative, the TEM images of YIG nanofibers are presented in Fig. 3, which provides a further insight into their microstructure. When the nanofibers annealed at 750 °C in Fig. 3(a– b), they show a continuously linear structure, which is in accord with above SEM observation (Fig. 2(b)). It is clear seen that individual YIG nanofiber consists of a multitude of single nanoparticles stacked along the nanofiber axis. The inset shows the SAED images of the YIG nanofibers annealed at 750 °C, which displays the typical single crystalline diffraction spots. According to the TEM and XRD results, the individual YIG nanofiber annealed at 750 °C contains many single crystals. The bright spots are indexed to the (116), (208), (422), (532) and (640) of YIG. A high-resolution TEM image is shown in Fig. 3(b), the fringe spacings are measured to be 0.44, 0.31 and 0.25 nm, which corresponds to the (440), (400) and (422) crystallographic plane of YIG. As shown in Fig. 3(c), the nanofibers still keep the linear structure when the annealing temperature increases to 900 °C, which is in agree
Fig. 4. (a) Room temperature magnetic hysteresis loops of the samples annealed at various temperatures, the inset shows the enlarged sections of the magnetic hysteresis loops at low field, (b) the variation of saturation magnetization (Ms) and coercivity (Hc) with annealing temperature.
hexagonal YFeO3 phase (h-YFeO3) (JCPDS card # 48–0529), and slight YIG phase (JCPDS card # 43–0507) and orthorhombic YFeO3 (oYFeO3) phase (JCPDS card # 39–1489) are also formed. While the annealing temperature increases to 750 °C, the YIG phase is formed with the slight second phase o-YFeO3. This is due to the h-YFeO3 exists as a metastable form at lower annealing temperature, and the o-YFeO3 is thermodynamically stable [24]. As the sintering temperature increases further from 750 °C to 1000 °C, the samples tend to form the purer YIG phase with the decrease of o-YFeO3 phase, and their corresponding peaks also become sharper and narrower, which indicates the enhancement of crystallinity and crystalline size. The average crystalline sizes D of YIG nanofibers were calculated from the several intense peaks in accordance with the Debye―Scherrer formula:
D =Kλ / βcosθ Where, K is a shape factor with value of 0.89, λ is the wavelength of the
Table 2 The comparison of the properties between the YIG with other structures prepared by other methods and this work. Method
Ms (emu/g)
Hc (Oe)
Structure
Annealing temperature
Reference
Microemulsion co-precipitation method Chemical Solution Deposition Sol–gel process Sol–gel process solid state reaction Sol–gel process Electrospinning
13.98 28.13 0.122 emu/mm3 27 17.98 > 25.7 ~25 19.3
10.6 703 13 25 38.10 < 10 ~20 140
Nanoparticles Nanoparticles Flim Nanoparticles Nanoparticles Nanoparticles Nanoparticles Nanofibers
1000 °C 1200 °C 750 °C 900 °C 1000 °C 1200 °C 800 °C 750 °C
[33] [4] [34] [35] [36] [37] [32] This work
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4. Conclusion
with the SEM observation. And the fringe spacing is measured to be 0.31 nm from RHTEM image, corresponding to the (400) crystallographic plane of YIG. However, the discontinuous and agglomerate nanofibers are obtained from SEM and TEM images at 1000 °C. At the same time, the fringe spacing is also measured, 0.28 nm is corresponding to (420) plane of YIG. As we mentioned, the annealing temperature is an important factor to the microstructure of nanofibers. Fig. 4(a) shows the room temperature magnetic hysteresis loops of all samples. It is found that all magnetic hysteresis loops are typical of a soft magnetic material. The Ms and Hc of the samples estimated from the magnetic hysteresis loops are both shown in Fig. 4(b). Especially, the magnetic characteristic of the sample annealed at 700 °C derives from the h-YFeO3, slight YIG phase and o-YFeO3 phase. According previous report, the h-YFeO3 shows a linear increase with field, resembling a paramagnet [25]. However, the nanofiners show a little different hysteresis loop in this work, owing to the ferromagnetic YIG and o-YFeO3 phases in this sample [25]. As the annealing temperature increases further from 700 °C to 1000 °C, the values of Ms and Hc vary markedly. In particular, the Ms displays a steady increase from 3.1 emu/g at 700–21.5 emu/g at 1000 °C, which are highly affected by the gradual increase of the crystallinity and crystalline size of the YIG phase [26,27]. The value of 21.5 emu/g is slightly lower than that for the bulk YIG (26.8 emu/g) [28]. Besides, the Hc shows a different tendency that first increases to a maximum value of 140 Oe at 750 °C, and then reduce distinctly. The initial increase of Hc with the annealing temperature increases from 700 to 750 °C can be attributed to the existence of second phase o-YFeO3 in YIG nanofibers. This is due to the o-YFeO3 crystallizes in perovskite-type structure, and shows unique magnetic properties such as a large Hc [29]. When the annealing temperature increases further, the Hc decreases gradually with the decreasing of second phase o-YFeO3. Simultaneously, the variation of coercivity of the YIG nanofibers is closely connected with magnetic domain structure and morphology (shape anisotropy and roughness) [30,31]. However, it was reported that the single-domain critical size of YIG is 190 nm [32], which is considerably larger than the crystalline sizes displayed in Table. 1. Thus, it can be inferred that another reason of dramatically decrease of Hc is the morphology of the samples. The magnetic properties of YIG fabricated by other methods as a comparison are shown in Table 2. The typical room temperature ESR spectra for the randomly oriented YIG nanofibers are measured at 9 GHz. As presented in Fig. 5, the ESR spectrum of the YIG nanofibers annealed at 750 °C possesses obviously asymmetry and broad linewidth. Whereas, with an increase of the annealing temperature, the ESR spectrum become narrower and their line shape is more symmetrical compared to the sample calcined at 750 °C. Combining the results of XRD and SEM analysis, it can be deduced that the broadening and asymmetry of resonance peaks are mainly originated perhaps the shape anisotropy, defects and the presence of trace amount second phase. Originally, for a single faultless nanowire, it can be definitely confirmed that its resonance field is highly affected by the orientation of the nanowire with the applied magnetic field direction. Actually, due to nanofibers are randomly oriented in the sample, so it could be assumed that those broad ESR spectra are a superposition of multiple resonance signals which derived from ESR in differently oriented YIG nanofibers [38,39]. Moreover, the real samples we prepared contain a mass of defects, which lead to the existence of a distribution of the orientation of the magnetic moment with the nanofiber axis (theoretically, the magnetic moment are strict parallel to the nanowire axis due to its great shape anisotropy) and also make the magnetic moment in the nanofiber difficult to rotate. Besides, the presence of the second phase o-YFeO3 also modifies the spectral line shape because of nonhomogeneous local magnetic field [40].
In summary, the YIG nanofibers have been successfully synthesized via the sol-gel assisted electrospinning technique in combination with subsequent two-steps calcination process. The XRD patterns show that the YIG phase with the slight additional yttrium orthoferrite (o-YFeO3) phase has been formed when calcination temperature increased to 750 °C. The morphology and magnetic properties of the samples was highly affected by the calcination temperature. The results of ESR spectra at various calcination temperatures demonstrated that the modification of lineshape and linewidth are influenced by anisotropy, defects and the presence of trace amount second phase in the YIG nanofibers. And, it can be foreseen from the results that YIG nanofibers would be a very promising microwave absorbing materials. Acknowledgment This work is supported by the National Science Fund of China (11574121, 51371092). References [1] J. Fujita, M. Levy, R.M. Osgood, L. Wilkens, H. Dötsch, Waveguide optical isolator based on Mach–Zehnder interferometer, Appl. Phys. Lett. 76 (16) (2000) 2158. [2] M. Pardavi-Horvath, Microwave applications of soft ferrites, J. Magn. Magn. Mater. 215–216 (2000) 171–183. [3] M.Y.G.R.M. Grechishki, S.E. Ilyashenko, N.S. Neustroev High-resolution sensitive magneto-optic ferrite-garnet films with planar anisotropy, J. Magn. Magn. Mater. 157/158 (1996) 305–306. [4] M.M. Rashad, M.M. Hessien, A. El-Midany, I.A. Ibrahim, Effect of synthesis conditions on the preparation of YIG powders via co-precipitation method, J. Magn. Magn. Mater. 321 (22) (2009) 3752–3757. [5] C. Hauser, T. Richter, N. Homonnay, C. Eisenschmidt, M. Qaid, H. Deniz, D. Hesse, M. Sawicki, S.G. Ebbinghaus, G. Schmidt, Yttrium iron garnet thin films with very low damping obtained by recrystallization of amorphous material, Sci. Rep. 6 (2016) 20827. [6] A.A. Serga, A.V. Chumak, B. Hillebrands, YIG magnonics, J. Phys. D-Appl. Phys. 43 (26) (2010) 264002. [7] J. Bourhill, N. Kostylev, M. Goryachev, D.L. Creedon, M.E. Tobar, Ultrahigh cooperativity interactions between magnons and resonant photons in a YIG sphere, Phys. Rev. B 93 (14) (2016). [8] M. Aldosary, J. Li, C. Tang, Y. Xu, J.-G. Zheng, K.N. Bozhilov, J. Shi, Platinum/ yttrium iron garnet inverted structures for spin current transport, Appl. Phys. Lett. 108 (24) (2016) 242401. [9] A. Mergen, A. Qureshi, Characterization of YIG nanopowders by mechanochemical synthesis, J. Alloy. Compd. 478 (1–2) (2009) 741–744. [10] S.R. Murthy, Interaction of ultrasonic waves with domain walls on nanocrystalline YIG, Ultrasonics 54 (2) (2014) 479–485. [11] Y. Sun, Y.-Y. Song, H. Chang, M. Kabatek, M. Jantz, W. Schneider, M. Wu, H. Schultheiss, A. Hoffmann, Growth and ferromagnetic resonance properties of nanometer-thick yttrium iron garnet films, Appl. Phys. Lett. 101 (15) (2012) 152405. [12] T. Liu, H. Chang, V. Vlaminck, Y. Sun, M. Kabatek, A. Hoffmann, L. Deng, M. Wu, Ferromagnetic resonance of sputtered yttrium iron garnet nanometer films, J. Appl. Phys. 115 (17) (2014) 17A501. [13] Jiecai Fu. Junli Zhang, Fashen Li, Erqing Xie, Desheng Xue, Nigel J. Mellors, Yong Peng, BaFe12O19 single-particle-chain nanofibers-preparation, characterization, formation principle, ACS Nano 6 (2012) 2273–2280. [14] T. Thurn-Albrecht, J. Schotter, G.A. Kastle, N. Emley, T. Shibauchi, L. KrusinElbaum, K. Guarini, C.T. Black, M.T. Tuominen, T.P. Russell, Ultrahigh-density nanowire arrays grown in self-assembled diblock copolymer templates, Science 290 (5499) (2000) 2126–2129. [15] J. Xiang, Y. Chu, X. Shen, G. Zhou, Y. Guo, Electrospinning preparation, characterization and magnetic properties of cobalt-nickel ferrite Co1−xNixFe2O4 nanofibers, J. Colloid Interface Sci 376 (1) (2012) 57–61. [16] M.J. Hu, B. Lin, S.H. Yu, Magnetic field-induced solvothermal synthesis of onedimensional assemblies of Ni-Co alloy microstructures, Nano Res. 1 (4) (2010) 303–313. [17] Z. Jia, D. Ren, R. Zhu, Synthesis, characterization and magnetic properties of CoFe2O4 nanorods, Mater. Lett. 66 (1) (2012) 128–131. [18] J.H. Min, B.H. An, J.U. Cho, H.M. Ji, S.J. Noh, Y.K. Kim, H.L. Liu, J.H. Wu, Y.D. Ko, J.-S. Chung, Effects of Cu doping on the microstructure and magnetic properties of CoPt nanowires, J. Appl. Phys. 101 (9) (2007) 09K513. [19] L. Cattaneo, S. Franz, F. Albertini, P. Ranzieri, A. Vicenzo, M. Bestetti, P.L. Cavallotti, Electrodeposition of hexagonal Co nanowires with large magnetocrystalline anisotropy, Electrochim. Acta 85 (2012) 57–65. [20] L. Pan, D. Cao, P. Jing, J. Wang, Q. Liu, A novel method to fabricate CoFe2O4/ SrFe12O19 composite ferrite nanofibers with enhanced exchange coupling effect, Nanoscale Res. Lett. 10 (1) (2015).
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Structural and magnetic properties of electrospun yttrium iron garnet (YIG) nanofibers Lining Pana, Xinlei Zhanga, Jianbo Wanga,b, Qingfang Liua,
⁎
a
Key Laboratory for Magnetic and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, People's Republic of China Key Laboratory of Special Function Materials and Structure Design of the Ministry of Education, Lanzhou University, Lanzhou 730000, People's Republic of China b
A R T I C L E I N F O
A BS T RAC T
Keywords: Yttrium iron garnet (YIG) Electrospinning Magnetic properties Electron spin resonance
The present work describes the fabrication of yttrium iron garnet (YIG) nanofibers prepared via electrospinning method. The YIG nanofibers with diameters of around 80 nm were formed when calcinations temperature exceeded 750 °C. The morphology and magnetic parameters including saturation magnetization (Ms) and coercivity (Hc) of these samples dramatically change with variation of the annealing temperature. In particular, the Ms displays a steady increase from 3.1 emu/g at 700–21.5 emu/g at 1000 °C, and the Hc shows a different tendency that first increases to a maximum value of 140 Oe at 750 °C, and then reduce distinctly. Finally, the electron spin resonance spectra display strong asymmetry and broad linewidth that are highly affected by shape anisotropy, defects and the presence of trace amount second phase orthorhombic YFeO3 (o-YFeO3) in the YIG nanofibers.
1. Introduction Garnet ferrites have been widely applied in many microwave and magneto-optical devices, including optical isolators, oscillators, circulators, and phase shifters, due to their superior properties such as low damping, high electrical resistivity and controllable saturation magnetization [1–5]. As one of typical garnet ferrites, especially, yttrium iron garnet Y3Fe5O12 (YIG) has attracted widespread attention. Moreover, with the rapidly development of nanotechnology in recent years, the nanostructural YIG materials have also been investigated extensively because of their unique size dependent properties and great potential to act as key performers in the near future [6–8]. Nevertheless, most of researches about YIG in the nanostructural state are still limited to powders [9,10] and films [11,12], and one-dimensional (1D) nanostructures of YIG are seldom reported. Therefore, preparation and investigation of 1D YIG nanomaterials should be carried out. During the past decades, there has been a steady growth of interest in 1D nanostructural magnetic materials not only because of their distinctive properties compared with bulk or particle counterparts, but their potential applications in many fields such as magnetic sensors, ultrahigh-density data storage, electromagnetic devices and magnetooptical switches [13–16]. Up to now, numerous approaches have been demonstrated for preparing 1D magnetic nanomaterials, including precursor thermal decomposition [17], anodized aluminum oxide
⁎
(AAO) template [18,19], and electrospinning [20,21], etc. Among these methods, electrospinning as an extremely simple and efficient method for fabricating ultrafine and continuous nanofibers has successfully received widespread attention of researchers. So far, there are huge quantity reports about 1D magnetic nanofibers synthesized by electrospinning, for instance, Fe, Co, Ni, CoFe2O4, NiFe2O4, SrFe12O19, etc [20,22,23]. Herein, YIG nanofibers have been successfully obtained through electrospinning followed by two-steps calcinations approach. Simultaneously, a series of analytical methods were introduced to reveal their phase content, morphology, magnetic properties and microwave properties in detail. Furthermore, we elucidated the influence of the morphology of YIG nanofibers on its magnetic and ferromagnetic resonance properties. 2. Experimental All chemical regents, which contain ethanol (≧99.7%), ferric nitrate nonahydrate (Fe(NO3)3·9H2O, ≧98.5%), yttrium nitrate hexahydrate (Y(NO3)3·6H2O, 99.9%) and poly(vinyl pyrrolidone) (PVP, average Mw ≈1,300,000 g mol−1, purchased from sigma-aldrich), used in the experiment are of chemical grade. Entire preparation procedures of YIG nanofibers consist of precursor solution preparation, electrospinning and calcinations. The
Corresponding author. E-mail address: [email protected] (Q. Liu).
http://dx.doi.org/10.1016/j.ceramint.2016.10.070 Received 31 May 2016; Received in revised form 10 October 2016; Accepted 11 October 2016 Available online xxxx 0272-8842/ © 2016 Published by Elsevier Ltd.
Please cite this article as: Pan, L., Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.10.070
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6H2O and Fe(NO3)3·9H2O mixed solution was added slowly into the PVP/ethanol solution and further continuous magnetically stirred for another 3 h at the room temperature to form a precursor solution for electrospinning. The viscous precursor solution was loaded into a plastic syringe with a stainless steel needle (d =0.8 mm, where d stands for outer diameter of the stainless steel needle) to obtain the precursor YIG/PVP composite nanofibers. The applied voltage and the distance between syringe needle tip and the ground collector were 14.0 kV and 15 cm, respectively. The solution was driven by a syringe pump of a rate of 0.35 mL h−1 during the electrospinning process. At last, the aselectrospun YIG/PVP composite nanofibers were annealed by twosteps calcinations to obtain the resultant nanofibers, being presintering at 100 °C for 2 h and calcined at different temperature T (T =700 °C, 750 °C, 900 °C and 1000 °C) for 2 h in ambient atmosphere, and the heating rate guaranteed 2 °C/min during all calcinations process. X-ray diffraction (XRD) patterns was characterized by a PANalytical diffractometer using Cu Kα radiation (λ=0.15418 nm). The morphology and microstructure of the YIG nanofibers were carried out by field emission scanning electron microscopy (FESEM, Hitachi S4800), transmission electron microscopy (TEM, Tecnai™ G2 F30, FEI), and selected area electron diffraction (SAED). Vibrating sample magnetometer (VSM, LakeShore 7304, USA) was utilized to measure the static magnetic properties of the prepared samples at room temperature. Electron spin resonance (ESR, JES-FA300) was performed using an X-band spectrometer with a magnetic field ranging from 0 to 500 mT at a frequency of 9 GHz.
Fig. 1. XRD patterns of the YIG nanofibers annealed at 700, 750, 900 and 1000 °C for 2 h.
Table 1 Average crystalline sizes D of YIG nanofibers annealed at different temperature for 2 h. Temperature (°C)
700
750
900
1000
Average crystalline size D (nm)
–
30
39
47
3. Results and discussion
preparation processes of precursor solution are as follows: firstly, the appropriate amount of Y(NO3)3·6H2O and Fe(NO3)3·9H2O with a molar ratio of 3:5 were dissolved in 5 mL distilled water. At the same time, PVP with the concentration of 9 wt% was added into 1 mL ethanol solution, followed by continuous magnetic stirring for 2 h to ensure the complete dissolution of PVP. Subsequently, 1 mL Y(NO3)3·
Fig. 1 shows the XRD patterns of the prepared YIG nanofibers annealed at different annealing temperatures from 700 °C to 1000 °C. It can be obviously observed that the crystallization process of the sample begins at 700 °C. When the sample annealed at this temperature, almost all characteristic diffraction peaks can be assigned to the
Fig. 2. Typical SEM images of the YIG nanofibers annealed at (a) 700 °C, (b) 750 °C, (c) 900 °C, (d) 1000 °C, respectively.
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Fig. 3. Typical TEM images and HRTEM image of the YIG nanofibers annealed at 750 °C (a–b), 900 °C (c–d) and 1000 °C (e–f). Inset shows the corresponding SAED images of YIG nanofibers annealed at 750 °C.
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Fig. 5. Room-temperature ESR spectra of the YIG nanofibers with various calcination temperatures recorded at 9 GHz.
X-ray, and β is the full-width at half-maximum (FWHM). As shown in Table. 1, the crystallite size of YIG nanofibers increases monotonically from 30 nm to 47 nm with the increase of the annealing temperature. Fig. 2 shows the SEM images of YIG nanofibers obtained at different annealing temperatures. The nanofibers annealed at 700 °C have continuous structure with a relatively rough surface and their average diameter is approximately 65 nm in Fig. 2(a). As calcinations temperature rises to 750 °C (Fig. 2(b)), the nanofibers still remain as continuous structures, and their average diameter and roughness are both nearly invariable comparing to the nanofibers annealed at 700 °C. Subsequently, the morphology of the samples is dramatically changed with increasing the calcination temperature further. When annealing temperature increases to 900 °C, the surface of resultant nanofibers becomes smoother due to the growth and coalescence of the particles in the nanofibers under the calcinations process. Almost all of nanofibers become discontinuous and agglomerate for the sample sintered at 1000 °C shown in Fig. 2(d). So, it can be deduced that the morphology of the samples is significantly influenced by the annealing temperature. As a representative, the TEM images of YIG nanofibers are presented in Fig. 3, which provides a further insight into their microstructure. When the nanofibers annealed at 750 °C in Fig. 3(a– b), they show a continuously linear structure, which is in accord with above SEM observation (Fig. 2(b)). It is clear seen that individual YIG nanofiber consists of a multitude of single nanoparticles stacked along the nanofiber axis. The inset shows the SAED images of the YIG nanofibers annealed at 750 °C, which displays the typical single crystalline diffraction spots. According to the TEM and XRD results, the individual YIG nanofiber annealed at 750 °C contains many single crystals. The bright spots are indexed to the (116), (208), (422), (532) and (640) of YIG. A high-resolution TEM image is shown in Fig. 3(b), the fringe spacings are measured to be 0.44, 0.31 and 0.25 nm, which corresponds to the (440), (400) and (422) crystallographic plane of YIG. As shown in Fig. 3(c), the nanofibers still keep the linear structure when the annealing temperature increases to 900 °C, which is in agree
Fig. 4. (a) Room temperature magnetic hysteresis loops of the samples annealed at various temperatures, the inset shows the enlarged sections of the magnetic hysteresis loops at low field, (b) the variation of saturation magnetization (Ms) and coercivity (Hc) with annealing temperature.
hexagonal YFeO3 phase (h-YFeO3) (JCPDS card # 48–0529), and slight YIG phase (JCPDS card # 43–0507) and orthorhombic YFeO3 (oYFeO3) phase (JCPDS card # 39–1489) are also formed. While the annealing temperature increases to 750 °C, the YIG phase is formed with the slight second phase o-YFeO3. This is due to the h-YFeO3 exists as a metastable form at lower annealing temperature, and the o-YFeO3 is thermodynamically stable [24]. As the sintering temperature increases further from 750 °C to 1000 °C, the samples tend to form the purer YIG phase with the decrease of o-YFeO3 phase, and their corresponding peaks also become sharper and narrower, which indicates the enhancement of crystallinity and crystalline size. The average crystalline sizes D of YIG nanofibers were calculated from the several intense peaks in accordance with the Debye―Scherrer formula:
D =Kλ / βcosθ Where, K is a shape factor with value of 0.89, λ is the wavelength of the
Table 2 The comparison of the properties between the YIG with other structures prepared by other methods and this work. Method
Ms (emu/g)
Hc (Oe)
Structure
Annealing temperature
Reference
Microemulsion co-precipitation method Chemical Solution Deposition Sol–gel process Sol–gel process solid state reaction Sol–gel process Electrospinning
13.98 28.13 0.122 emu/mm3 27 17.98 > 25.7 ~25 19.3
10.6 703 13 25 38.10 < 10 ~20 140
Nanoparticles Nanoparticles Flim Nanoparticles Nanoparticles Nanoparticles Nanoparticles Nanofibers
1000 °C 1200 °C 750 °C 900 °C 1000 °C 1200 °C 800 °C 750 °C
[33] [4] [34] [35] [36] [37] [32] This work
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4. Conclusion
with the SEM observation. And the fringe spacing is measured to be 0.31 nm from RHTEM image, corresponding to the (400) crystallographic plane of YIG. However, the discontinuous and agglomerate nanofibers are obtained from SEM and TEM images at 1000 °C. At the same time, the fringe spacing is also measured, 0.28 nm is corresponding to (420) plane of YIG. As we mentioned, the annealing temperature is an important factor to the microstructure of nanofibers. Fig. 4(a) shows the room temperature magnetic hysteresis loops of all samples. It is found that all magnetic hysteresis loops are typical of a soft magnetic material. The Ms and Hc of the samples estimated from the magnetic hysteresis loops are both shown in Fig. 4(b). Especially, the magnetic characteristic of the sample annealed at 700 °C derives from the h-YFeO3, slight YIG phase and o-YFeO3 phase. According previous report, the h-YFeO3 shows a linear increase with field, resembling a paramagnet [25]. However, the nanofiners show a little different hysteresis loop in this work, owing to the ferromagnetic YIG and o-YFeO3 phases in this sample [25]. As the annealing temperature increases further from 700 °C to 1000 °C, the values of Ms and Hc vary markedly. In particular, the Ms displays a steady increase from 3.1 emu/g at 700–21.5 emu/g at 1000 °C, which are highly affected by the gradual increase of the crystallinity and crystalline size of the YIG phase [26,27]. The value of 21.5 emu/g is slightly lower than that for the bulk YIG (26.8 emu/g) [28]. Besides, the Hc shows a different tendency that first increases to a maximum value of 140 Oe at 750 °C, and then reduce distinctly. The initial increase of Hc with the annealing temperature increases from 700 to 750 °C can be attributed to the existence of second phase o-YFeO3 in YIG nanofibers. This is due to the o-YFeO3 crystallizes in perovskite-type structure, and shows unique magnetic properties such as a large Hc [29]. When the annealing temperature increases further, the Hc decreases gradually with the decreasing of second phase o-YFeO3. Simultaneously, the variation of coercivity of the YIG nanofibers is closely connected with magnetic domain structure and morphology (shape anisotropy and roughness) [30,31]. However, it was reported that the single-domain critical size of YIG is 190 nm [32], which is considerably larger than the crystalline sizes displayed in Table. 1. Thus, it can be inferred that another reason of dramatically decrease of Hc is the morphology of the samples. The magnetic properties of YIG fabricated by other methods as a comparison are shown in Table 2. The typical room temperature ESR spectra for the randomly oriented YIG nanofibers are measured at 9 GHz. As presented in Fig. 5, the ESR spectrum of the YIG nanofibers annealed at 750 °C possesses obviously asymmetry and broad linewidth. Whereas, with an increase of the annealing temperature, the ESR spectrum become narrower and their line shape is more symmetrical compared to the sample calcined at 750 °C. Combining the results of XRD and SEM analysis, it can be deduced that the broadening and asymmetry of resonance peaks are mainly originated perhaps the shape anisotropy, defects and the presence of trace amount second phase. Originally, for a single faultless nanowire, it can be definitely confirmed that its resonance field is highly affected by the orientation of the nanowire with the applied magnetic field direction. Actually, due to nanofibers are randomly oriented in the sample, so it could be assumed that those broad ESR spectra are a superposition of multiple resonance signals which derived from ESR in differently oriented YIG nanofibers [38,39]. Moreover, the real samples we prepared contain a mass of defects, which lead to the existence of a distribution of the orientation of the magnetic moment with the nanofiber axis (theoretically, the magnetic moment are strict parallel to the nanowire axis due to its great shape anisotropy) and also make the magnetic moment in the nanofiber difficult to rotate. Besides, the presence of the second phase o-YFeO3 also modifies the spectral line shape because of nonhomogeneous local magnetic field [40].
In summary, the YIG nanofibers have been successfully synthesized via the sol-gel assisted electrospinning technique in combination with subsequent two-steps calcination process. The XRD patterns show that the YIG phase with the slight additional yttrium orthoferrite (o-YFeO3) phase has been formed when calcination temperature increased to 750 °C. The morphology and magnetic properties of the samples was highly affected by the calcination temperature. The results of ESR spectra at various calcination temperatures demonstrated that the modification of lineshape and linewidth are influenced by anisotropy, defects and the presence of trace amount second phase in the YIG nanofibers. And, it can be foreseen from the results that YIG nanofibers would be a very promising microwave absorbing materials. Acknowledgment This work is supported by the National Science Fund of China (11574121, 51371092). References [1] J. Fujita, M. Levy, R.M. Osgood, L. Wilkens, H. Dötsch, Waveguide optical isolator based on Mach–Zehnder interferometer, Appl. Phys. Lett. 76 (16) (2000) 2158. [2] M. Pardavi-Horvath, Microwave applications of soft ferrites, J. Magn. Magn. Mater. 215–216 (2000) 171–183. [3] M.Y.G.R.M. Grechishki, S.E. Ilyashenko, N.S. Neustroev High-resolution sensitive magneto-optic ferrite-garnet films with planar anisotropy, J. Magn. Magn. Mater. 157/158 (1996) 305–306. [4] M.M. Rashad, M.M. Hessien, A. El-Midany, I.A. Ibrahim, Effect of synthesis conditions on the preparation of YIG powders via co-precipitation method, J. Magn. Magn. Mater. 321 (22) (2009) 3752–3757. [5] C. Hauser, T. Richter, N. Homonnay, C. Eisenschmidt, M. Qaid, H. Deniz, D. Hesse, M. Sawicki, S.G. Ebbinghaus, G. Schmidt, Yttrium iron garnet thin films with very low damping obtained by recrystallization of amorphous material, Sci. Rep. 6 (2016) 20827. [6] A.A. Serga, A.V. Chumak, B. Hillebrands, YIG magnonics, J. Phys. D-Appl. Phys. 43 (26) (2010) 264002. [7] J. Bourhill, N. Kostylev, M. Goryachev, D.L. Creedon, M.E. Tobar, Ultrahigh cooperativity interactions between magnons and resonant photons in a YIG sphere, Phys. Rev. B 93 (14) (2016). [8] M. Aldosary, J. Li, C. Tang, Y. Xu, J.-G. Zheng, K.N. Bozhilov, J. Shi, Platinum/ yttrium iron garnet inverted structures for spin current transport, Appl. Phys. Lett. 108 (24) (2016) 242401. [9] A. Mergen, A. Qureshi, Characterization of YIG nanopowders by mechanochemical synthesis, J. Alloy. Compd. 478 (1–2) (2009) 741–744. [10] S.R. Murthy, Interaction of ultrasonic waves with domain walls on nanocrystalline YIG, Ultrasonics 54 (2) (2014) 479–485. [11] Y. Sun, Y.-Y. Song, H. Chang, M. Kabatek, M. Jantz, W. Schneider, M. Wu, H. Schultheiss, A. Hoffmann, Growth and ferromagnetic resonance properties of nanometer-thick yttrium iron garnet films, Appl. Phys. Lett. 101 (15) (2012) 152405. [12] T. Liu, H. Chang, V. Vlaminck, Y. Sun, M. Kabatek, A. Hoffmann, L. Deng, M. Wu, Ferromagnetic resonance of sputtered yttrium iron garnet nanometer films, J. Appl. Phys. 115 (17) (2014) 17A501. [13] Jiecai Fu. Junli Zhang, Fashen Li, Erqing Xie, Desheng Xue, Nigel J. Mellors, Yong Peng, BaFe12O19 single-particle-chain nanofibers-preparation, characterization, formation principle, ACS Nano 6 (2012) 2273–2280. [14] T. Thurn-Albrecht, J. Schotter, G.A. Kastle, N. Emley, T. Shibauchi, L. KrusinElbaum, K. Guarini, C.T. Black, M.T. Tuominen, T.P. Russell, Ultrahigh-density nanowire arrays grown in self-assembled diblock copolymer templates, Science 290 (5499) (2000) 2126–2129. [15] J. Xiang, Y. Chu, X. Shen, G. Zhou, Y. Guo, Electrospinning preparation, characterization and magnetic properties of cobalt-nickel ferrite Co1−xNixFe2O4 nanofibers, J. Colloid Interface Sci 376 (1) (2012) 57–61. [16] M.J. Hu, B. Lin, S.H. Yu, Magnetic field-induced solvothermal synthesis of onedimensional assemblies of Ni-Co alloy microstructures, Nano Res. 1 (4) (2010) 303–313. [17] Z. Jia, D. Ren, R. Zhu, Synthesis, characterization and magnetic properties of CoFe2O4 nanorods, Mater. Lett. 66 (1) (2012) 128–131. [18] J.H. Min, B.H. An, J.U. Cho, H.M. Ji, S.J. Noh, Y.K. Kim, H.L. Liu, J.H. Wu, Y.D. Ko, J.-S. Chung, Effects of Cu doping on the microstructure and magnetic properties of CoPt nanowires, J. Appl. Phys. 101 (9) (2007) 09K513. [19] L. Cattaneo, S. Franz, F. Albertini, P. Ranzieri, A. Vicenzo, M. Bestetti, P.L. Cavallotti, Electrodeposition of hexagonal Co nanowires with large magnetocrystalline anisotropy, Electrochim. Acta 85 (2012) 57–65. [20] L. Pan, D. Cao, P. Jing, J. Wang, Q. Liu, A novel method to fabricate CoFe2O4/ SrFe12O19 composite ferrite nanofibers with enhanced exchange coupling effect, Nanoscale Res. Lett. 10 (1) (2015).
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