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Effect of Barium Substitution on Ferroelectric and Magnetic Properties of Bismuth Ferrite Rajasree Das and Kalyan Mandal S. N. Bose National Centre for Basic Sciences, Kolkata 700 098, India We successfully synthesized Bi Ba FeO replacing Bi by Ba using a chemical synthesis route. The phase and crystal structure of the samples were studied by X-ray diffraction (XRD). Peaks in the XRD shifted toward lower value with the increase in Ba concentration due to the increase in the unit cell size. Detail thermal behavior was performed by differential scanning calorimetry and differential thermal analysis (DTA) to investigate the change in magnetic and ferroelectric transition temperature, respectively, due to Ba substitution. Magnetic, dielectric, and magnetoelectric properties of the samples were also studied in detail. Antiferromagnetic bismuth ferrite was converted to ferromagnetic materials at room temperature on incorporating Ba in the crystal structure. Polarization versus electric field (P-E loop) and dielectric constant were also found to change significantly in the presence of a magnetic field in the aforementioned samples. Appreciable magnetodielectric effect also indicated effective magnetoelectric coupling within the materials. Index Terms—Fatigue resistance, ferroelectric transition, magnetoelectric coupling, multiferroics. I. INTRODUCTION B ISMUTH ferrite (BFO) is one of the most interesting members [1] of multiferroic family, which shows large magnetoelectric coupling in single phase at room temperature (RT). It has been studied by many groups worldwide due to its importance in fundamental research, as well as in commercial applications. BFO has a rhombohedrally distorted perovskite structure that belongs to the R3c space group [2]. Compared to other multiferroics, BFO exhibits a higher ferroelectric Curie temperature ( C) and a high G-type antiferromagnetic (AFM) ordering ( C) temperature [3]. At RT, pure bismuth ferrite shows an AFM nature because of its complicated cycloidal spin structure with a wavelength of about 62 nm along the axis at RT. Though BFO is considered as a promising candidate for magnetic storage or in the applications of spintronics devices [4], leakage current due to oxygen vacancies or impurities is the major problem in BFO. It has been observed that doping [5]–[7] with ions at A-sites and B-sites reduce the leakage current in BFO and enhance the multiferroic properties. Though Ba-doped BFO shows promising results, few reports are available on this composition [8]–[10]. In this study, Ba-doped BFO are prepared using a chemical route different from the paper of Wang et al. [10]. The temperature dependence of magnetic and dielectric behaviour, fatigue resistance, and magnetoelectric properties are investigated in details, which show some interesting results. II. EXPERIMENTAL DETAILS Bi Ba FeO ceramics ( and 0.25) are prepared by chemical synthesis route in ambient atmosphere. Starting chemicals, such as Ba(N0 ) , Bi(N0 ) , and Fe(N0 ) , are weighed according to the stoichiometric compositions, all supplied by Sigma-Aldrich, St. Louis, MO, with a purity of 99.9%. Solutions of these chemicals are mixed and dried up at 80 C. Homogeneous powders are calcined in two steps: 1) at 500 C for 4 hr and 2) at appropriate temperatures between 865 C and 885 C for 2 hr to get single-phase ceramics. For the measurement of electrical properties, pellets with diameter of 5 mm and thickness of 1 mm are prepared and sintered at 850 C. Both sides of the pellet are pasted with silver paste to connect electric wires for the measurements of dielectric and magnetoelectric properties. Phase and structure of the samples are determined by X-ray diffraction (XRD) using CuK radiation (PAnalytical). The magnetic properties of BFO are measured using a vibrating sample magnetometer (VSM; Lake Shore Cryotronics, Westerville, OH) up to a maximum field of 16 kOe at RT. Impedance analyzer (Agilent 4294 A) is used to measure the change in dielectric constant of the samples within the frequency range 40 Hz to 1 MHz. Ferroelectric hysteresis loops are studied by a polarization versus electric field (P-E) loop tracer (Marine India). III. RESULTS AND DISCUSSIONS A. Structural Analysis Fig. 1 shows the XRD pattern of Bi Ba FeO . Except the first one, all other samples show single-phase perovskite structure, which implies that the doping does not lower the stabilization of BiFeO . BiFeO shows additional low intensity peaks of Bi O and Bi Fe O at , and 30.52 . As Bi is more volatile than Fe, iron-rich phases, such as Bi Fe O , are usually formed while preparing BFO. But those impurities disappear in Ba-doped BFO as Ba(NO ) in the solid solution could accelerate the formation of BiFeO perovskite structure. Peak positions shift toward left with the increase of doping concentration, indicating increment in the lattice parameter. Thus, Ba substitution leads to an increase in the unit cell volume since the ionic radius of Ba is larger than that of Bi . Separation Fig. 3. (a) Magnetization M as a function of applied field. (b) Magnetization Ba FeO . versus temperature plot of Bi with increase in cell volume. In BaTiO , the various transition temperatures shift down on compression [12]. Fig. 1. XRD patterns for Bi Ba FeO samples. The symbols “*" and “ " indicate the presence of the impurity phase in the pure BFO sample. Fig. 2. (a) DSC heating curve indicating magnetic transition temperature . (b) DTA curve indicating ferroelectric transition temperature for Ba FeO . Bi between (0 1 2) and (1 1 0) diffraction peaks (see the inset of Fig. 1) is reduced with Ba substitution, which implies that the rhombohedral structure may be distorted to a monoclinic or a tetragonal structure. This change might be important for the ferroelectric properties of the compounds. However, an Mn-doped BFO thin film showed similar kind of changes [11]. B. DSC and DTA Analysis Thermal analysis of the samples has been carried out with DSC (30 C to 500 C) and DTA (200 C to 900 C) to study the magnetic and ferroelectric transition temperatures, respectively. DSC results for Bi Ba FeO are shown in Fig. 2(a). As can be seen in the figure, the heat flow data show a kink-like anomaly at 369.2 C for (heating curve). Magnetic transition shifts toward higher temperature on Ba doping, which is 386.2 C for . DTA results are shown in Fig. 2(b). For the parent compound, a peak obtained at 831 C is attributed to the ferroelectric transition in this compound. The peak is shifted toward higher temperature in the Ba-substituted compounds and reaches 868 C for exhibiting ferroelectric characteristics over a wide temperature range. This increase in can be due to decrease of pressure C. Magnetic Analysis The magnetic hysteresis loops, as shown in Fig. 3(a), illustrate the linear-field dependence of magnetization M for pure BFO indicating its AFM nature. However, Ba-doped samples are ferromagnetic at RT and the characteristics of the loop depend on doping concentrations. Remanent magnetization and coercivity are maximum in the case of . Although the reason behind the occurrence of FM in Ba-doped BFO samples is not clearly understood, this can be attributed to the change in interaction between Fe and Fe breaking down the balance between the antiparallel sublattice magnetization [13]. Change in canting angle [14] or spiral spin modulation [15] may also be responsible for this phenomenon. Fig. 3(b) shows the temperature dependence (250 C to 700 C) M of Bi Ba FeO in the presence of a magnetic field, Oe. For all Ba-doped samples, an AFM transition is observed, with a ferro/ferrimagnetic nature above C . However, above C also, these samples have another magnetic transition. In the case of pure BFO, a sharp increase in M takes place at C . On attaining a maximum at 500 C, M starts decreasing and reaches the paramagnetic state at C . Other samples also show the similar results. M is the maximum for beyond which it decreases. To understand the magnetic behavior, hysteresis loops are measured at temperatures below , above and within them, and they are shown in Fig. 4(a)–(c) for the pure BFO. In between and , pure BFO shows a hysteresis loop with very small coercivity Oe. This magnetic behavior above the Neel temperature can be due to the existence of the impurity phase of Bi Fe O because Bi Fe O is paramagnetic at RT [16]. Linear-field dependence of M is observed at temperatures higher than . To confirm the magnetic state of the pure BFO at different temperature, Arrott’s plots ( versus H) have been plotted in Fig. 4(d). It shows a positive intercept on the y-axis indicating spontaneous magnetization in between and only [17]. D. Ferroelectric Properties It is very difficult to get good ferroelectric properties in BFO ceramics due to the presence of oxygen vacancies. How- Fig. 4. (a)–(c) Magnetic hysteresis loops at different temperatures. (d) versus H plots of BiFeO . Fig. 6. Dielectric constant versus frequency (50 Hz to 1 kHz) plots of Ba FeO ceramics at RT. Bi Fig. 5. Ferroelectric hysteresis loop of Bi kV cm and Hz. Ba FeO ceramics at Fig. 7. Dielectric constant versus frequency ( Bi Ba FeO sample at different applied fields. ever, with Ba substitution, better ferroelectric properties are observed. This can be explained by the distortion of oxygen octahedral by the relative displacement of the equatorial and apical oxygens in Ba-doped samples as observed in A-site ion-substituted Bi Ti O [18]. Enhanced ferroelectricity observed in Bi La Ti O is due to rotation of TiO octahedra in the – plane accompanied with a shift of the octahedron along the axis. RT P-E loops of Bi Ba FeO samples are shown in Fig. 5 at the frequency of 50 Hz and a maximum field of kV cm . Saturation can be obtained with the application of higher electric field or by poling the samples with high voltage. The Ba BFO sample shows the maximum ( C/cm ) among the Ba-doped BFO ceramics. Electrical switching causes the trapping of electronic charge at the domain wall. Fatigue endurance of the samples is also an important factor for practical applications. Fatigue occurs in BFO mainly because of the change in the positions of oxygen vacancies. Doping of Ba reduces movable charge density due to which fatigue resistance changes with an appropriate doping concentration. Fatigue characteristics of all the samples are carried out at an applied field kV cm and frequency 50 Hz. Hz–2 kHz) plots of the Inset a of Fig. 5 shows that the value remain almost the same up to read and/or write switching cycles for sample . TheP-E loop shows the same nature before and after the fatigue measurement (see inset b of Fig. 5). Though Ba BFO shows a higher value initially than the other Ba-doped samples, the fatigue resistance of Ba BFO is very much less than the sample. The value of Ba BFO decreases continuously from C/cm to a lower value with the switching cycles. Compared to others, the sample with the concentration shows the maximum fatigue resistance with value about C/cm throughout the measurement cycles. E. Dielectric Measurements Fig. 6 shows the variation of dielectric constant of the samples measured from 50 Hz to 1 kHz at RT. changes noticeably with the Ba concentration. Ba BFO shows the maximum at RT. The trend observed here is closely similar to that of ferroelectric measurements. The magnetoelectric coupling of the sample Ba BFO (which shows the maximum fatigue resistance) is clearly shown in Fig. 7. The dielectric constant of the sample increases ACKNOWLEDGMENT The work of R. Das was supported by Council of Scientific and Industrial Research, India, for a fellowship. This work was supported by S. N. Bose National Center for Basic Sciences under the Project SNB/KM/10-11/57. REFERENCES Fig. 8. MD effect versus field plots of the Bi frequency. Ba FeO sample at a different with the applied magnetic field. Temperature dependence of of the sample Ba BFO from RT to 460 C is shown in the inset of Fig. 7. Further increase of temperature was not possible due to the limitation of sample holder. It is observed that of the sample increases sharply above 360 C. This sharp increase can be considered due to the magnetic phase transition around this temperature. Fig. 8 shows a field dependence of the magnetodielectric (MD) effect, expressed by at a different frequency . A large change in the MD value is observed while increasing from 400 to 500 Hz, afterward it shows almost the same MD value. Obstruction of charge carriers at the grain boundaries enhances the MD effect in our polycrystalline samples [19]. IV. CONCLUSION Magnetic and ferroelectric properties of BFO are found to change significantly with Ba substitution in place of Bi. Above the Neel temperature of the pure BFO, a large enhancement in magnetization is observed in Ba-doped samples. Transition temperatures (magnetic and ferroelectric) are shifted toward higher temperature with the increase in the Ba concentration. 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