Structural and multiferroic properties of Fe-doped Ba0.5Sr0.5TiO3 solids

f ei Chin , fe ) so con do e sa ed site ech y pero e ferro ue to licatio een re , whic echniq ders ectrici use an constant, low dielectric loss, and good thermal reliability. Barium BST igate ceramics exhibit excellent, almost single-phase structures, which Contents lists available at SciVerse ScienceDirect .el Journal of Magnetism an Journal of Magnetism and Magnetic Materials 325 (2013) 24–28 lattice defects and vacancies, which are favorable for single-phaseE-mail address: [email protected] (Z. Guo). strontium titanate, abbreviated as BaxSr1�xTiO3 (BST), is the solid indicate that the preparation of tetragonal BST ceramics with large doping amounts is possible through a non-stoichiometric method. Fe mainly acts as an acceptor to replace Ti in the B-site, leading to the further ionization of Fe ions and the appearance of 0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.08.023 n Corresponding author. Tel.: þ86 010 62332587. BaTiO3 and SrTiO3 are two typical perovskite-based and broad- spectrum electronic functional ceramics with high dielectric the structural and multiferroic properties of Fe-doped Ba0.5Sr0.5- TiO3 (BSTF) ceramics with different doping concentrations. BSTF multiferroic materials possess several distinctive electronic and structural properties such as orbital and charge ordering, local moment formation, and Jahn–Teller distortions [12–14]. namely, the B-site (Ti site) [35]. The preparation and physical properties of bulk Fe-doped ceramics have been rarely reported. In this study, we invest as Ti4þ in BaTiO3) at a B-site should be present for B-site cation off-centering, which will break up the existing ferromagnetic order [8]. However, the coupling between ferroelectric and ferromagnetic orders is highly useful for the new type of multi- ferroic devices by taking advantage of both magnetic and electric polarization, such as electrically read-write magnetic memory devices and other magnetoelectric devices [9–11]. Moreover, cerning Fe-doped BaxSr1�xTiO3 samples, the overwhelming majority focus on BSTF thin films [34–38]. One typical example is the fact that Fe doping of BST films is remarkably effective in suppressing decreasing dielectric constant, and BST films show a minimum dielectric loss with varied Fe doping concentration [34]. Additionally, the electron standing wave method identifies the site where the doped Fe ions are substituted in the lattice, Multiferroic materials, especiall roic materials where more than on been widely investigated [1–3] d fundamental physics and device app materials, numerous studies have b thin films in the past decade [4–7] different substrates using different t ferromagnetic and ferroelectric or conventional mechanism of ferroel materials with ABO3 structure, beca vskite-based multifer- ic order coexists, have their significance for ns. Among multiferroic ported on multiferroic h have been grown on ues. The coexistence of is a challenge to the ty in perovskite-based empty d orbital (such solution of BaTiO3 and SrTiO3. Previous studies mainly focused on BST thin films. Many film growth techniques have been employed to prepare BST thin films, such as metalorganic vapor phase epitaxy [15], sputtering [16], molecular beam epitaxy [17,18], pulsed laser deposition [19], and soft solution processing [20]. BST has been mainly used for dielectric devices such as capacitors for dynamic random access memory because of its high dielectric constant and low dissipation factor [21–25]. BST thin films have also been widely investigated for microwave applications such as phase shifters, tunable filters, tunable resonators, and switches [26–30]. Metal ion-doped BST samples, including thin films and ceramics, have also been investigated [31–38]. Especially, con- 1. Introduction Structural and multiferroic properties o Zhengang Guo a,n, Liqing Pan a,b, Chong Bi a, Hongm M. Yasir Rafique a a Department of Physics, University of Science and Technology Beijing, Beijing 100083, b College of Science, China Three Gorges University, Yichang, Hubei 443002, China a r t i c l e i n f o Article history: Received 7 March 2012 Received in revised form 9 August 2012 Available online 25 August 2012 Keywords: Ceramic Crystal structure Dielectric Multiferroic material a b s t r a c t The structural, dielectric (Ba0.5Sr0.5Ti1�xFexO3; BSTF diffraction (XRD) patterns XRD data revealed that the ferromagnetic orders of th experimental and simulat systems, namely, nearest- pling. The two coupling m behavior. journal homepage: www Fe-doped Ba0.5Sr0.5TiO3 solids Qiu a, Xuedan Zhao a, Lihong Yang a, a rroelectric, and magnetic properties of bulk Fe-doped Ba0.5Sr0.5TiO3 lids prepared by standard solid-state reaction were investigated. X-ray firmed the tetragonal structure of BSTF samples. Rietveld refinements of ping ions lead to unit cell expansion in three directions. Ferroelectric and mples were observed simultaneously at room temperature. According to results, two possible exchange coupling mechanisms exist in the BSTF antiferromagnetic coupling and next-nearest-site ferromagnetic cou- anisms coexist in BSTF systems, leading to extraordinary ferromagnetic & 2012 Elsevier B.V. All rights reserved. sevier.com/locate/jmmm d Magnetic Materials structure formation. The dielectric properties of the samples will remain good, and ferroelectricity and ferromagnetism of BSTF are simultaneously observed. In particular, the magnetism of BSTF samples is somewhat abnormal and possibly involves two differ- ent coupling mechanisms. 2. Experiment BSTF ceramics were prepared by the standard solid-state reaction technique. The powders of high-purity SrCO3, TiO2, Fe3O4, and BaTiO3 served as the raw materials. The nominal Fe-doped atomic fractions x for the prepared BSTF samples were 0.05, 0.10, 0.15, 0.20, and 0.40, respectively. The ground and mixed SrCO3 and TiO2 powders were sintered to synthesize SrTiO3. The obtained SrTiO3, Fe3O4, and BaTiO3 were then used to synthesize BSTF samples. The sintering process was carried out by preheating at 950 1C for 12 h, followed by holding the temperature at 1300 1C for 24 h and finally, cooling to room temperature with a rate of 5 K/min. The sample structures were characterized by X-ray diffraction (XRD) with Cu Ka radiation and Rietveld refinement. General Structure Analysis System (GSAS) package was employed to refine the crystal structure of samples from the XRD data [39]. Micro- structures of the BSTF solids were studied by a field emission scanning electron microscope (FE-SEM) (Zeiss SUPRA55) with an operating voltage of 10 kV. The samples were polished for ferro- electric and dielectric property measurements. The ferroelectric property of the samples was measured using a RT6000VHS ferroelectric analyzer. The dielectric measurements were carried out with an impedance analyzer (Agilent, 4294 A), spanning a frequency range of 40 Hz–1 MHz. The magnetic property of the sample was characterized by a vibrating sample magnetometer system (Versalab, Quantum Design Co.). 3. Results and discussion XRD patterns and the refinement spectra of BSTF samples are shown in Fig. 1. All the diffraction peaks are indexed, and the phase of tetragonal structure is identified. Even at a high doping level (x¼0.40), the XRD pattern of the prepared sample is in good agreement with the tetragonal BST phase (JCPDS No. 89-0274, vertical bars). Full-pattern matching refinements of the XRD spectra were performed using the GSAS program package to obtain more detailed information on the crystallographic struc- ture of BSTF solids. The fitted powder patterns are in good agreement with the respective experimental data, denoted by Rp and Rwp factors listed in Table 1. No other observable impurity phase exists in the system. According to the refinements, the Fe ions are embedded in the BST crystal lattice and substitute the Ti men Table 1 X-ray diffraction refinement results of BSTF solids. Fe-doped level (nominal x) Lattice parameters Occupancy (Ti:Fe) R factors a (A˚) c (A˚) V (A˚3) Rwp (%) Rp (%) 0.05 3.957598 3.952448 61.906 0.9463:0.0457 4.79 3.23 0.10 3.959624 3.957481 62.048 0.8755:0.0805 6.63 4.91 0.15 3.961049 3.958633 62.111 0.8486:0.1393 5.17 3.65 0.20 3.961966 3.958967 62.145 0.8058:0.1831 5.44 3.86 0.40 3.972238 3.970080 62.643 0.6243:0.4021 7.41 4.74 Z. Guo et al. / Journal of Magnetism and Magnetic Materials 325 (2013) 24–28 25 Fig. 1. Observed (circle) and calculated (solid line) XRD profiles for the GSAS refine calculated profiles: (a) x¼0.05, (b) x¼0.10, (c) x¼0.20, and (d) x¼0.40. The vertical ba t of BSTF samples along with the plots of the difference between the observed and rs indicate the peak position of the reflections on JCPDS. ions because no obvious secondary phase is observed in the diffraction patterns. The lattice parameters of the BSTF phase are carefully deter- mined by GSAS refinement, as listed in Table 1. The lattice parameters of the BSTF samples exhibit a slight variation, but the crystal structures remain almost the same. The values of lattice parameter a and c increase with the Fe doping level, indicating a lattice expansion of BSTF systems. For instance, the unit cell volume increases from 61.906 A˚3 to 62.643 A˚3, which can be attributed to the different sizes between Fe3þ and Ti4þ ions and the possible ion vacancies presented in the lattices. According to the refined results, Fe and Ti ion occupancy in the samples match well with the experimental design. However, several lattice vacancies appear in Ba sites and O sites because of the lack of cations. These vacancies may be favorable for stabilizing the tetragonal structure of BSTF systems. Microstructures and morphologies of samples were observed by SEM. The representative SEM image for the sample (x¼0.40) is shown in Fig. 2, which indicates the dense nanoparticle surfaces and the well-distributed crystallites. The inset of Fig. 2 shows that the grain size of the sample is uniform and ranges from 200 nm to 600 nm. The dielectric and ferroelectric properties of BSTF solids with a thickness of 0.5 mm to 1.0 mm were investigated. Fig. 3 shows the frequency dependence of the dielectric constant ranging from 40 Hz to 1 MHz and the loss of samples. As the Fe doping level increases, dielectric constant decreases significantly. The electric dipole response in the samples decreases at higher frequencies because the electric dipoles need some time for realignment and cannot follow electric-field changes. The dielectric loss remains in the same range, which is always less than 0.15, but the losses increase slightly with increasing Fe doping level. Fig. 4 shows the typical ferroelectric hysteresis loops measured at a frequency of 100 Hz for the capacitors with a diameter of 5 mm at room temperature, indicating the ferroelectric behavior of BSTF sam- ples. Although the slightly distorted hysteresis loops include an effect of leakage current, the saturation polarization of BSTF samples (x¼0.10) reaches 12 mC/cm2 at an applied electric field of 100 kV/cm, with remnant polarization of 4.7 mC/cm2 and coercive field (Ec) of 31 kV/cm. For the high-doping level sample (x¼0.40), the saturation polarization decreases rapidly by only half as much as that of the sample (x¼0.10), which is in agreement with the changing pattern of dielectric property. The rapidly decreasing saturation polarization is caused by two conditions. These conditions are the rapidly increasing leakage current due to increasing Fe concentration and the doped Fe ions that weaken the ferroelectricity of BSTF samples because of the strong magnetic coupling between the doped Fe ions. The ferromagnetism of the samples was measured at room temperature and the results are shown in Fig. 5. All the BSTF samples exhibit ferromagnetism at room temperature with clear hysteresis loops. The saturation magnetization has a remarkable characteristic, with the maximum appearing in Fe doping level x¼0.15 in this study. The inset shows the dependence of saturation magnetization of the samples on Fe doping level. The residual and saturation magnetization of samples increase with the Fe doping level, which reach a peak of 0.37 and 2.03 emu/g, respectively, when Z. Guo et al. / Journal of Magnetism and Magnetic Materials 325 (2013) 24–2826 Fig. 2. SEM images with different magnifications for the BSTF samples of x¼0.40. Fig. 3. Frequency dependence of dielectric constant and dielectric loss of BS Fig. 4. Ferroelectric hysteresis loops of BSTF samples. TF samples with different doping levels ranging from 40 Hz to 1 MHz. Z. Guo et al. / Journal of Magnetism and Magnetic Materials 325 (2013) 24–28 27 Fig. 5. Magnetic hysteresis loops of BSTF samples. Inset: saturation magnetization versus Fe doping level. Fe doping level is lower. However, when Fe doping level con- tinuously increases, the residual and saturation magnetization of samples significantly decrease. As shown in the inset of Fig. 5, the saturation magnetization of the samples decreases rapidly when the Fe doping level is higher than 15%, which is even less than the minimum doping amount (x¼0.05). The ferromagnetic behaviors in BSTF systems possibly involve different exchange coupling mechanisms. The coupling in such systems occurs between the doped Fe ions due to two possible reasons, namely, nearest-site antiferromagnetic coupling and next-nearest-site ferromagnetic coupling. According to this assumption, ferromagnetic coupling is predominant between the doping Fe ions at a lower doping amount because the Fe ions in the systems tend to disperse and become independent. However, when Fe doping amount increases to a certain degree, the proportion of the nearest Fe ions increases sharply and antiferromagnetic coupling is exhib- ited. As a result, the saturation magnetization of the samples decreases at high doping levels. A simple simulation was performed as a comparison, in which the Fe ion spins were assumed to be in an antiparallel arrange- ment in the nearest-site lattices and in a parallel arrangement in the next-nearest-site lattices. In the simulations, a three- dimensional lattice model consisting of one thousand lattices were adopted to calculate the magnetic moments generated by all the Fe ions distributed randomly in the lattice model. The simulation result and a sketch of the lattice model are illustrated in Fig. 6. The maximum magnetic moments appear in Fe doping level x¼0.13, which is close to the experiment results. The direction of change exhibited by the variation trend of the simulation curve is consistent with that of the experimental results. 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