Preparation and characterization of single-crystal multiferroic nanofiber composites

Journal of Alloys and Compounds 552 (2013) 518–523

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Preparation and characterization of single-crystal multiferroic nanofiber composites Zhaohui Ren, Zhen Xiao, Simin Yin, Jiangquan Mai, Zhenya Liu, Gang Xu, Xiang Li, Ge Shen, Gaorong Han ⇑ State Key Lab of Silicon Materials, Department of Material Science and Engineering, Cyrus Tang Center for Sensor Materials and Applications, Zhejiang University, Hangzhou 310027, PR China

a r t i c l e

i n f o

Article history: Received 14 August 2012 Received in revised form 13 November 2012 Accepted 14 November 2012 Available online 23 November 2012 Keywords: Composite materials Heterojunction Solid state reaction Magnetic measurements AFM

a b s t r a c t One-dimensional single-crystal multiferroic composites consisting of PbTiO3 (PTO) nanofiber-CoFe2O4 (CFO) nanodot were prepared using an in situ solid state sintering method, where pre-perovskite PTO nanofibers and CFO nanodots were used as precursors. Structural analyses by using transmission electron microscopy, scanning electron microscopy and X-ray diffraction determined a epitaxial growth relation between the PTO nanofiber and the CFO nanodot. Ferromagnetism and ferroelectricity of the nanofiber composites were investigated by using vibarting sample magnetometer (VSM) and piezoresponse force microscopy (PFM) Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In the past decade, great efforts have been devoted to multiferroic materials because of their fascinating properties, such as ferroelectricity, ferromagnetism and the coupling between them [1,2]. The coexistence of these properties in a single phase material is desirable for both of fundamental research and applications based on the mutual controls of magnetic and electric fields [3–5]. In particular, a reversal of magnetization was controlled by a ferroelectric polarization field, which can lead to low-energy consumption memory devices [6,7]. However, single phase multiferroics are still rare with weak coupling at room temperature [8]. Alternative approach such as two-phase heterostructures was developed to obtain multiferroic materials. Flexible component and geometry design of the heterostructures allow optimizing magnetoelectric coupling [9,10]. Especially, large magnetoelectric coupling than that of single phase materials has been realized in self-assembled nanopillar composite at room temperature, which is called 1-3 type nanostrucures, such as BaTiO3–CoFe2O4, PbTiO3–CoFe2O4 and BiFeO3–CoFe2O4 [11–13]. It has been demonstrated that simple strain field distribution and reduced substrate clamping are critical for achieving good magnetoelectric coupling in multiferroic nanocomposites [11,14–18]. Being free of the mechanical constraint of film substrate, multiferroic nanofiber composites were investigated theoretically to demonstrate higher magnetoelectric responses than orders of magnitude of multiferroic composite thin films of similar compositions [17]. Therefore, it is ⇑ Corresponding author. Tel.: +86 571 87951649; fax: +86 571 87952341. E-mail address: [email protected] (G. Han). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.11.087

highly interesting to explore multiferroic nanofiber composites with promising properties. Different approaches, including porous anodic aluminum oxide (AAO) templates [19], polycarbonate membrane templates [20] and electrospinning [21], were empolyed to prepare one-dimensional (1D) multiferroic nanocomposites. These nanocomposites, however, usually consisting of polycrystalline particles, which make them rather complex for desirable properties. Investigation on the 1D multiferroic composites with ideal crystallinity has not been reported up to now. In this article, we report a facile preparation of single-crystal PbTiO3 (PTO) nanofiber-CoFe2O4 (CFO) nanodot composites by using an in situ solid state sintering. The nanofiber composites demonstrated ferroelectricity and ferromagnetism at room temperature. Measured at 100 Oe, the magnetism of the nanocomposites is almost temperature-independent below a blocking temperature (TB) of 712 K. This has been determined to arise from a strong magnetoelectric coupling of the heterostructures because of the absence of substrate clamping, as well as simple epitaxial interfaces between the PTO nanofiber and the CFO nanodot. 2. Experimental section The scenario for the preparing these heterostructures can be simply described by Fig. 1. A commercial CFO nanodot powder (Fig. 2b) was used. Single-crystal pre-perovskite (PP) PTO nanofibers [22–24] (Fig. 2a) were synthesized via a polymer-assisted hydrothermal method. Then the powders of CFO nanodots and the PP-PTO nanofibers with different weight ratio were mixed in ethylalcohol, in which solution of sodium dodecyl benzene sulfonate (SDBS) and polyethylene glycol (PEG) were added as additives. After stirring and ultrasonic vibrating, the mixture was sintered at 650 °C for 30 min in air, resulting in brown powders. During the sintering process, the PP-PTO nanofibers transformed into tetragonal perovskite (TP) PTO nanofibers. The final products were characterized by using X-ray diffraction (XRD,

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PFM (AFM, Korea XE-100E) was employed to investigate the topography and ferroelectric property. NSC15 Ti-Pt cantilevers with a force constant of 5.5 Nm1 and resonance frequency of 160 kHz were used.

CoFe2O4

3. Results and discussion

Heating

PP-PTO nanofiber

TP-PTO nanofiber

Fig. 1. A proposed scheme for fabrication of the multiferroic nanofiber composites consisting of TP-PTO nanofiber and CFO nanodot.

RIGAKUD-MAX-C with CuKa radiation k = 1.54056 Å), scanning electron microscope (SEM, Hitachi field emission SEM MODEL S-4800 equipped with an EDS detector) and high-resolution transmission electron microscope (HRTEM, FEI F20 transmission electron microscope with acceleration voltage of 200 kV). Element composition at the interface of the two phases was examined by energy dispersive X-ray spectroscopy (EDS). In addition, magnetic property and its temperaturedependence of the sample were tested by using magnetic property measurement system (MPMS) (Quantum Design) and vibrating sample magnetometer (VSM).

SEM images of the precursors and the nanofiber composites with 0, 1/4, 1/10 and 1/20 weight ratio of CFO/PTO were prepared (Fig. 2c–f). As the weight ratio of CFO/PTO is decreased from 1/4 to 1/10 and 1/20, the nanodot aggregation significantly decreases and individual nanodots can be observed on surface of the nanofibers in the sample (1/20) (Fig. 2c–f). To illustrate the intrinsic magnetoelectric property of the heterostructures, the nanofiber composite (1/20) was chosen for detailed investigations considering its simple and clear interface. The 1D morphology of the nanocomposites was basically kept after calcinations. The nanofibers have a diameter of 80–300 nm with a length of a few to tens of micrometers. Distribution of the nanodots on the surface of the nanofibers is relatively uniform although the aggregation could not be avoided. On the other hand, a diameter of CFO nanodots is mainly 50–80 nm, approaching the critical diameter (Ds) of 70 nm, corresponding to a maximum coercive magnetic field (Hc) at about 2 kOe [25]. As shown in Fig. 3, all diffraction peaks can be indexed into perovskite TP-PTO (JPCDS 06-0452) and spinel cubic CFO (JPCDS 03-0864). No impurity peaks have been detected.

Fig. 2. SEM images of (a) PP-PTO nanofibers, (b) CFO nanoparticles, (c) TP-PTO nanofibers, (d) the nanofiber composite with a 1/4 weight ratio of CFO/PTO, (e) the nanofiber composite with a 1/10 weight ratio of CFO/PTO and (f) the nanofiber composite with a 1/20 weight ratio of CFO/PTO.

20

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Intensity (a.u)

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Fig. 3. XRD patterns of pure CFO sample and the nanofiber composites.

Fig. 4 shows TG-DTA curves of the single-crystal TP-PTO nanofibers and the nanofiber composites, corresponding to the samples of Fig. 2c and f, respectively. An endothermic peak situated at 758 K in Fig. 4a corresponds to a ferroelectric tetragonal to paraelectric cubic phase transformation of the single-crystal TP-PTO nanofibers. The phase transformation temperature matches well with that (760 K) of bulk crystal TP-PTO [26]. In contrast, the ferroelectric phase transformation temperature of the nanofiber composite is 748 K. The exothermic peak at 553 K could be attributed to the burning of sodium dodecyl benzene sulfonate (SDBS) and polyethylene glycol (PEG) that remain on the surface

Weight(%)

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Fig. 4. TG-DTA curves of the single-crystal TP-PTO nanofibers (a) and the PTO–CFO nanofiber composites (b).

of the sample after heating treatment. Another possible contribution to this exothermal peak could be OH desorption from the surface of the nanofibers [27]. Fig. 5a presents a TEM image of a single nanofiber composite attached with nanodots. As shown in Fig. 5b, a single heterostructure is composed of nanofiber (100 nm in diameter) and nanodot (43 nm). EDS spectra in Fig. 5c indicates element distribution along the purple line in Fig. 5b, confirming that the nanofiber and the nanodot are PTO and CFO, respectively. From Fig. 5c, there is a clear transition area for element distribution from PTO nanofiber to CFO nanodot, corresponding to an interface between two phases. HRTEM characterization (Fig. 5d) shows a clear and simple interface with a slight distortion of lattice image. Due to the different space situation for the nanofiber and the nanodot, atomic lattice images of the nanofiber and nanodot near the interface are obtained one by one along certain axis. As shown in Fig. 5e and f, respectively, the nanodot spacings of d(220) = d(202) = 2.94 Å are very close to those (2.97 Å) of spinel CFO (JPCDS 03-0864), and the nanofiber lattice spacings of d(100) = d(010) = 3.90 Å are in good agreement with those (3.90 Å) of TP-PTO (JPCDS 06-0452). HRTEM images and highly regular fast Fourier transformation (FFT) patterns (inset of Fig. 5e and f) indicate that the nanodot and the nanofiber are well single-crystal in nature, and the nanofiber has a longitudinal axis along [1 0 0] of TP-PTO. Fast Fourier transformation (FFT) pattern of the interface regions (inset in Fig. 5d) suggests an epitaxial growth. According to Fig. 5d–f, the angle between (2 2 0)CFO and (1 0 0)PTO is nearly 45o and thus (2 2 0)CFO is basically parallel to (1 1 0)PTO (2.76 Å), corresponding to about 6.5% lattice mismatch. This is favorable for heterostructure formation overcoming a low energy barrier [15]. As discussed above, intimate contact and simple interface between the PTO and the CFO have been achieved in the single-crystal nanofiber composite without substrate clamping, with which a good magnetoelectric (ME) property is expected. Room-temperature ferromagnetism of the nanofiber composites is also identified by magnetic field (M–H) hysteresis loops, as depicted in Fig. 6b. The coercive field (Hc) value for the pure CFO nanocrystals is about 2.3 kOe nearing that of a critical diameter (Ds) (Fig. 6a), below which the nanodots become single domain [25]. Considering the diameter distribution of the CFO nanodots, single-domain nanodots could therefore make dominated contribution to the magnetism of the samples. In contrast to that pure CFO nanodot, the nanofiber composite has a low Hc of 710 Oe (Fig. 6b), which was mainly contributed to thermal agitation [28,29]. Combining with large magnetostriction and the low coercive field of CFO may lead to an enhancement of ME coupling effect. Measured at 100 Oe, zero field cooled (ZFC) magnetization (M)-temperature (T) curve of the pure CFO nanodots is shown in Fig. 6c, exhibiting typical single-domain particle behavior. The blocking temperature (TB) of the pure CFO nanodots is about 732 K, corresponding to the thermal transition from the blocked ferrimagnetic state to the superparamagnetic one. In principle, magnetic field effect on the ferroelectric phase transition temperature or ferroelectric phase transition induced magnetism change of the ferromagnetic phase are used to investigate an ME coupling effect [11,15]. Due to a higher Curie temperature of the PTO nanofiber than TB of the CFO nanodot, it is impossible to observe an abrupt change in magnetism of CFO nanodot near Curie temperature. Fig. 6d depicts ZFC M–T curve of the nanofiber composite, measured at 100 Oe. However, the nanofiber composite demonstrates almost temperature-independent magnetism from room temperature to 650 K with TB lowering from 732 K to 712 K (Fig. 6d). This is significantly different from that of pure CFO nanodot in Fig. 6b. Such magnetic property has never been observed previously in single-domain CFO

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Fig. 5. TEM and HRTEM results of the nanofiber composites: (a) a typical TEM image of the PTO nanofiber with dispersed CFO nanodots on its surface, (b) HRETM image of a heterostructure composed of a single CFO nanodot and TP-PTO nanofiber, (c) EDS element analysis of the heterostructure scanning along the purple line in Fig. 5b, (d) HRETM lattice image of the heterostructure interface, with the inset showing fast Fourier transform (FFT) pattern of the interface area; (e) and (f) HRETM lattice images of the CFO nanodot and TP-PTO nanofiber of Fig. 5d. The insets of Fig. 5e and f are corresponding FFT patterns. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

nanoparticles [28,29], as well as multiferroic nanostructures. On the other hand, the applied magnetic field (100 Oe) for M–T measurement is too low to tailor the magnetism of the nanofiber composite. Therefore, the observed result in Fig. 6d should be attributed to a ME coupling via effective strain transferring through simple phase boundary interfaces. On the basis of the result in Fig. 5d, a and b lattice parameter expansion and thus the (1 1 0) spacing expansion of the PTO nanofiber would be coupled to the (2 2 0) of the CFO nanodot, resulting in a decrease of magnetization and TB. The magnetostrictive strain can be estimated as e ¼ 32 ks ½ðM=Ms Þ2  13; where ks is the saturation magnetostriction coefficient and M and Ms are magnetization and saturated magnetization, respectively [25]. Because the applied magnetic field is very weak (100 Oe) during M–T curve measurement, its induced magnetostrictive strain and a change of magnetization are almost negligible. However, as temperature increased the in-plane lattice parameter of PTO increases and could be coupled to CFO

phase, which would induce a decrease of magnetization due to its negative magnetostriction coefficient (ks), as reported in BaTiO3 (BTO)-CFO nanocomposite films [15]. This, combined with intrinsic increase of magnetization of the CFO nanodot, leads to almost temperature-independent magnetism in a range from room temperature to 650 K in Fig. 6d. As discussed above, this nanofiber composite could be desirable objects to explore electric field controlling of magnetization, even a reversal of magnetization [30]. Ferroelectricity and piezoelectric response of the nanofiber TPPTO has been confirmed by the investigation of PFM (Fig. 7). The topography image of the nanofiber is acquired by atomic force microscopy (Fig. 7a and b), showing that the diameter of the nanofiber is approximately 140 nm. A typical piezoelectric hysteresis loop and a ‘‘square’’ PFM hysteresis phase loop can be observed in Fig. 7c and d, respectively, which confirm the existence of piezoelectricity and ferroelectric polarization in the single-crystal perovskite PbTiO3 nanofiber with a diameter of 140 nm.

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Fig. 6. Magnetic properties of the nanofiber composites: (a) M–H curve of the pure CFO sample at room temperature, (b) M–H curve of the nanofiber composites at room temperature, (c) and (d) Zero field cooled (ZFC) M–T curves of the pure CFO nanodots and the nanofiber composites measured at 100 Oe.

Fig. 7. Atomic force microscope (AFM) characterization results of the nanofiber. (a) Topography of a single TP-PTO nanofiber, (b) height profile of the nanofiber with a diameter of 140 nm, (c) piezoresponse curve of the nanofiber detected by DC-EFM; and (d) phase curve of the nanofiber corresponding to (c).

4. Conclusions One-dimensional multiferroic composites, consisting of singlecrystal PTO nanofibers and CFO nanodots, have been prepared by using a simple and rapid in situ solid state sintering. These nano-

structures were investigated to demonstrate a simple epitaxial growth relation between the PTO nanofiber and CFO nanodot. The present findings clearly indicate that this simple preparation method could be extended to other PTO or Pb(Zr, Ti)O3 (PZT)-based multiferroic nanostructures. In particular, without a substrate

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clamping, the nanofiber composites may offer an opportunity to understand intrinsic property of a single multiferroic heterostructure for optimized materials. Acknowledgments This work was financially supported by the National Science Foundation of China (Grant No. 51232006, 51102212), and the Fundamental Research Funds for the Central Universities. References [1] S. Roy, S.B. Majumder, J. Alloys Compd. 538 (2012) 153. [2] Y.D. Xu, G. Wu, H.L. Su, M. Shi, G.Y. Yu, L. Wang, J. Alloys Compd. 509 (2011) 3811. [3] W. Eerenstein, N.D. Mathur, J.F. Scott, Nature 442 (2006) 759. [4] R. Ramesh, N.A. Spaldin, Nat. Mater. 6 (2007) 21. [5] L. Zhu, Y.L. Dong, X.H. Zhang, Y.Y. Yao, W.J. Weng, G.R. Han, N. Ma, P.Y. Du, J. Alloys Compd. 503 (2010) 426. [6] M. Liu, J. Lou, S.D. Li, N.X. Sun, Adv. Funct. Mater. 21 (2011) 2593. [7] J.T. Heron, M. Trassin, K. Ashraf, M. Gajek, Q. He, S.Y. Yang, D.E. Nikonov, Y.-H. Chu, S. Salahuddin, R. Ramesh, Phys. Rev. Lett. 107 (2011) 217202. [8] N.A. Hill, J. Phys. Chem. B 104 (2000) 6694. [9] J. Ma, J.M. Hu, Z. Li, C.-W. Nan, Adv. Mater. 23 (2011) 1062. [10] L. Li, X.M. Chen, H.Y. Zhu, J. Alloys Compd. 526 (2012) 116. [11] H. Zheng, J. Wang, S.E. Lofland, Z. Ma, L. Mohaddes-Ardabili, T. Zhao, L. Salamanca-Riba, S.R. Shinde, S.B. Ogale, F. Bai, et al., Science 303 (2004) 661. [12] J.H. Li, I. Levin, J. Slutsker, V. Provenzano, P.K. Schenck, R. Ramesh, J. Ouyang, A.L. Roytburd, Appl. Phys. Lett. 87 (2005) 072909.

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