Effect of BaTiO3 addition on structural, multiferroic and magneto-dielectric properties of 0.3CoFe2O40.7BiFeO3 ceramics

Recent interest in SrTiO3 stems from its wide applicability in microwave devices based on the tunable characteristics of dielectric constant in the microwave frequency range. It is obvious that for any such application, SrTiO3 should have a ferroelectric Curie temperature (TC) close to room temperature or higher. By inducing strains by chemical substitutions, it was possible to obtain TC as high as 200C in SrTiO3 modified with Fe. Hysteresis loops obtained confirms the presence of ferroelectric domains. Two apparent transitions, one at ~200 C and another ~300 C were seen in ε′, which are replicated as sharp drops in resistivity curves. These temperatures far exceed the TCs reported in the literature till now and could open new avenues for innumerable other applications for SrTiO3. The magnetic properties of Fe doped SrTiO3 are also investigated. Low doping of Fe exhibits simple antiferromagnetic behaviour. 1.Introduction SrTiO3, a quantum paraelectric or an incipient ferroelectric [1], whose phase transition to the ferroelectric state (TC) is quenched by the quantum fluctuations, allows non-linear properties only at low temperatures[2]. SrTiO3 has long been labelled as a material for high frequency tunable devices, in particular in the microwave frequency range and any possible application needs a high temperature or at least a room temperature dielectric transition. Concerted efforts are in progress to induce ferroelectricity and increase the TC close to room temperature either by chemical substitutions or by induced strains. SrTiO3 doped with Mn and Pr showed polar behaviour with enhanced TC. In the case of Pr-doped SrTiO3, room temperature ferroelecticity was seen [3], with spontaneous polarization decreasing as a function of increased Pr doping. In the Mn-substituted SrTiO3 however, a broad dielectric transition with relaxor-like behaviour was seen with a transition maximum around 50 K [4]. In these two cases, instabilities and there by large amounts of strain created by the addition of the aliovalent with ionic radii close to that of the host ion has been exploited in realizing ferroelectricity. In a recent report Haeni et al [1], have shown that large strains similarly can be induced by other means, for example by depositing epitaxial layers of SrTiO3 on single crystals of DyScO3, enhancing TC by several hundreds of degrees. Following the approach of inducing strains in the lattice by means of chemical substitutions to induce ferroelectricity, we were able to bring up the TC above room temperature. Significantly enhanced TC (up to 200 C) was seen in a polycrystalline film of SrTiO3 doped with 10 mole% of Fe, with strains modifying the crystal structure from a simple cubic (SrTiO3; space group Pm3-m,) to tetragonal (Fedoped SrTiO3). Oxygen non-stoichiometry forces some of the Fe into Fe and in addition, presence of oxygen vacancies could induce considerable strain in the lattice, making the structural transformation possible. It is known that a distorted perovskite structure with tolerance factors <1 could induce spontaneous polarization. SrFeO3-δ, which is a cubic perovskite when δ=0, with the structure transforming into a Brownmillerite orthorhombic phase when δ→ 0.5 [6]. The loss of oxygen, compounded by the disproportionation of Fe and Fe in the lattice together produces lowered crystal symmetry. In the orthorhombic state, in SrFeO2.5 International Conference on Magnetism (ICM 2009) IOP Publishing Journal of Physics: Conference Series 200 (2010) 092010 doi:10.1088/1742-6596/200/9/092010 c © 2010 IOP Publishing Ltd 1 oxygen vacancies were found to have been ordered with an oxygen missing in each alternate octahedra. Similarly, it is also possible to reduce the cubic symmetry in SrTiO3 by means of reducing the stoichiometry of oxygen. This can be achieved by reducing SrTiO3 or by the incorporation of aliovalent ion into the Ti site. We believe that the lowered symmetry achieved by means of doping an aliovalent ion, affected by the symmetry can induce diploe moment in SrTiO3. If the oxygen vacancy ordering also occurs in the reduced SrTiO3 as seen in Brownmillerite SrFeO2.5, it can also cooperatively incorporate ordering in anion-cation pairs thus bringing in ferroelectricity. As for the magnetic properties are concerned the other end of the series i.e., SrFeO3 (with the complete replacement of Ti with Fe) is a helical antiferromagnet (TN = 134 K) and oxygen stochiometry plays an important role in establishing the magnetic and transport properties in this system [5, 7]. SrFeO3δ (δ ~ 0) is metallic at room temperature and at δ ~ 0.15 metal-insulator (MI) transition takes place with a large change in magneto resistance. As δ → 0.19, a charge ordered (CO) state appears around TN . In an earlier paper we have identified compositions that lie on the boundary between MI and CO insulating states [7]. It is possible to dilute the helical structures in oxides by use of pressure, magnetic field and also by doping non magnetic ions. We have shown that by doping with a non magnetic ion such as Ti leads to a simple antiferromagnetic structure at lower Ti doping levels [7]. In this work the magnetic properties of the complete series of SrFexTi(1-x)O3-δ with x=0.2 to 0.9 have been carried out and it is shown that the ordering temperature decreases with decrease in Fe content. 2. Experimental 2.1 synthesize and characterization In order to achieve the objectives of lowering its cubic symmetry and induce ferroelectricity in SrTiO3-δ, 10 mole% of Fe was doped at the Ti site (hereafter called STF10). Polycrystalline films of 20 μm thick were prepared using screen printing with STF10 sandwiched between two layers of screen printed Pt-electrodes on a sintered alumina substrate. Prior to the screen printing, STF10 powders were prepared by solid state reactions starting with stoichiometric powders of SrCO3, TiO2, and Fe2O3. The ball milled powders were heated at 1400 C, before they were mixed with suitable inks for screen printing. The screen printed film was characterized using X-ray diffraction and has been identified as phase pure without any impurities. Dielectric measurements were done on a Solartron impedance analyzer (1260) in the temperature between 30 and 500 C at frequency 1 kHz, 100 kHz, and 1 MHz at bias fields of 0, 10 and 20 V. Pyroelectric current was measured ramping the temperature at 10 C/m in the temperature range 30-500 C using a Keithley digital multimeter. Ferroelectric hysteresis loops were traced using the Radiant technology’s RT66 test system in the Sawyer-Tower mode at various temperatures. The magnetic properties of different compositions of SrFexTi1-xO3-δ were investigated using a Physical Property measurement system. X-ray diffraction carried out on STF10 thick film screen printed over sintered alumina showed the polycrystalline nature of the film with no traces of impurity. The spectrum included peaks not only those of STF10 but also of Pt, which is used as an electrode, and alumina. This made analysis difficult and to circumvent this problem, instead X-ray diffraction patterns of the STF10 powders were obtained. These screen printed powders are expected not to show any drastic change either in structure or on composition as they were fired at low temperatures to evaporate the inks in the pastes. ‘Figure 1’ shows the X-ray diffraction spectrum of powders of sintered STF10 powders which were screen printed on sintered alumina and fired at temperatures low enough to bring in the required adhesion without modifying the sample phase. The pattern shows a clean SrTiO3 phase without impurities, with slight increase in the volume of the unit cell. Significantly, there are a number of peaks which show clear splits, indicating the transformation of the cubic perovskite structure of SrTiO3 into a lower symmetry. This lowered symmetry, due to the addition of Feat the Tisite could induce non-centrosymmetry, which is responsible for the realization of ferroelectricity in a number of perovskite oxide materials. Two reasons that could result in the lowered symmetry are (1) the addition of Fe, which induces strains in the International Conference on Magnetism (ICM 2009) IOP Publishing Journal of Physics: Conference Series 200 (2010) 092010 doi:10.1088/1742-6596/200/9/092010