Cobalt-Substituted Seven-Layer Aurivillius Bi₈Fe₄Ti₃O₂₄ Ceramics: Enhanced Ferromagnetism and Ferroelectricity

Abstract

In this work, Aurivillius-phase Bi₈Fe_4-xCo_xTi₃O₂₄ (7-BFCT, 0 ≤ x ≤ 0.4) powders and ceramics were successfully prepared by the combination of citrate combustion and hot-press methods. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) indicate a successful synthesis of the pure phase of the 7-BFCT ceramics, and all the samples showed a good seven-layer structure of the Aurivillius phase. Partial Fe substituted by Co was found to be effective to enhance both ferromagnetic and ferroelectric properties at room temperature, and the largest remnant magnetization (2M_r) of ~0.69 emu/g was revealed at the composition of x = 0.4. Zero field cooling and field cooling (ZFC-FC) magnetization measurement confirmed its magnetic transition occurring at a high temperature of ~750 K. Correspondingly, the enhanced ferroelectric properties of such Co-substituted ceramics were also investigated.

1. Introduction

The family of Bi-containing Aurivillius oxides are known potentially as single-phase multiferroics and have attracted much attention due to their excellent abundance with fundamental physics (unusual dielectric, magnetoelectric (ME) and high ferroelectric (FE) phase transition temperatures) and great application potential as sensors, and in multi-state memory, energy harvesting, etc. [1,2,3,4,5]. Such compounds have the general formula of Bi₄Bi_n−3Fe_n−3Ti₃O_3n+3 (BFTO), which can be formed by inserting one or more perovskite layers of multiferroic BiFeO₃ (BFO) into the three-layered (Bi₂Ti₃O₁₀)²⁻ perovskite slabs of the parental Bi₄Ti₃O₁₂ (BTO) ferroelectrics, and here n denotes the number of intermediate perovskite-like layers within a full unit cell [6]. The FE and ferromagnetism (FM) of the resulting BFTO can be driven from Ti ions with unoccupied d orbitals and Fe ions with partially filled d orbitals, respectively. Furthermore, (Bi₂O₂)²⁺ layers in Bi₄Ti₃O₁₂ can play the key role as both space-charge compensation and insulation, with an expectation to be able to further reduce the leakage and improve fatigue properties as well [7].

Researchers have already revealed the coupled FE and the unwanted antiferromagnetic (AFM) behaviors in such compounds with n = 4 (4-BFTO) [8,9,10], 5 (5-BFTO) [11,12,13], and 6 (6-BFTO) [14,15]. In 2009, with Co partially substituted for Fe, a new Bi₅Fe_0.5Co_0.5Ti₃O₁₅ (4-BFCT) ceramic was investigated for the first time, which was found able to present a significant coexistence of FE and FM well above room temperature. Its remanent magnetization (2M_r) was improved to 7.8 memu/g, due to a possible existence of superexchange among oxygen-connected Fe and Co ions and the realigned spin structures caused by the substitution [16]. Following this work of using the Co substitution strategy, most recent investigations on Bi₆FeCrTi₃O₁₈ [13], Bi₆Fe_2−xCo_xTi₃O₁₈ [17] (n = 5) and Bi₇Fe_3−xNi_xTi₃O₂₁ [18] (n = 6) also revealed an enhanced FM performance at or above room temperature. Remarkably, Wang et al. reported a plausible intrinsic magnetoelectric (ME) coupling in a five-layered Aurivillius ceramic, SrBi₅Fe_0.5Co_0.5Ti₄O₁₈, even at a high temperature of 373 K, surpassing almost all single-phase multiferroic materials currently under investigation [19]. Furthermore, by a nanoscale structural modulation, such ME coupling may be further enhanced due to the appearance of a new analogous morphotropic transformation effect [20]. In addition, Chen et al. revealed the formation mechanism of Bi₅Fe_0.9Co_0.1Ti₃O₁₅ nanosheets [21]. Apparently in the layered structure, FM properties can also be affected by the structure of the fundamental building blocks (here mainly referring to the layer number) [22]. Thus, it is worth considering what will happen if similarly using this substitution strategy in such layer-structured oxides with even higher n numbers. In fact, B-site substitution in a seven-layer Bi₈Fe₄Ti₃O₂₄ (7-BFTO) has never been done before, and no room-temperature FM was found in this material. Only a few previous works can be found with focuses on synthesis, crystalline structure and magnetic property studies of pure 7-BFTO, and most of them suggested the material’s AFM behavior at room temperature [23,24,25].

2. Results and Discussion

2.1. X-ray Diffraction

Figure 1 shows the X-ray diffraction spectra of such as-prepared 7-BFCT ceramics. Due to the lack of standard Joint Committee on Powder Diffraction standards (JCPDS) cards of 7-BFCT, the XRD patterns in Figure 1 are compared with standard PDF cards of four-layer, five-layer and six-layer Aurivillius compounds, i.e., Bi₅FeTi₃O₁₈—PDF#38-1257, Bi₆Fe₂Ti₃O₁₈—PDF#21-0101, and Bi₇Fe₃Ti₃O₂₁—PDF#54-1044, all of which show similar XRD patterns. All the XRD patterns in Figure 1 can be indexed by an orthorhombic lattice with the space group of F2mm, and it was found that all the samples are pure Aurivillius phase with no apparent impurity phase appearances. The peak located at about 30.75° slightly shifts to a larger angle (Figure 1b) as the Co concentration increases, indicating the small structural distortion arising from the Co substitution.

2.2. High-Resolution Transmission Electron Microscopy

To verify the successful synthesis of 7-BFCT ceramics, Figure 2 shows the high-resolution transmission electron microscopy images of the 7-BFCT ceramics with various amounts of substituted Co. As shown in Figure 2, all samples had lattices consistent with several perovskite layers sandwiched by two (Bi₂O₂)²⁺ layers. All samples showed a regular Aurivillius structure with c/2 around 3.2 nm, indicating the seven-layer structure [22] (Figure 2a–d). In addition, the lattice parameter c showed a slight decrease with the increase of x, suggesting the successful substitution of Co for Fe inside the lattices. When x = 0.4, although the sample was still a seven-layered structure, some stacking faults started to appear (Figure 2d), suggesting the maximum Co doping concentration in such materials before the secondary phase’s appearance or the structural collapse caused by the too-large Co substitution.

2.3. Ferromagnetic Properties

Magnetic hysteresis loops of the 7-BFCT ceramics at room temperature and the corresponding zero field cooling and field cooling (ZFC-FC) magnetization measurements (the temperature dependence of M under a magnetic field of 500 Oe was recorded) were performed to visualize the ferromagnetic properties of the 7-BFCT, as presented in Figure 3. In Figure 3a, for the sample with x = 0 (the inset at the upper left of Figure 3a), the linear field dependence of the magnetization (M) indicates its AFM nature, similar to the results reported by Srinivas et al. [23]. The ZFC-FC magnetization measurements show that the undoped sample’s Neel temperature (T_N), i.e., the antiferromagnetism-to-paramagnetism transition temperature, was ~247 K (Figure 3b), which further verifies the AFM nature of the pure 7-BFTO at room temperature. Noticeably, with the partial substitution of Co ions, magnetic hysteresis loops of the doped 7-BFCT ceramics (x > 0) all revealed the presence of the ferromagnetic moment (Figure 3a). Obviously, the FM properties of such seven-layered BFCT ceramics were significantly enhanced by the Co partial substitution, and 2Mr reached 0.04 emu/g, 0.30 emu/g, 0.35 emu/g and 0.69 emu/g for x = 0.1, 0.2, 0.3 and 0.4 samples, respectively. This enhancement could be explained by the possible superexchange among oxygen-connected Fe and Co ions, and the structure-modulated spin canting (the canting angle and the length between coupled ions) from the titling of adjacent Fe–O and Co–O octahedrals [16,17,18]. Furthermore, Figure 3c reveals the ZFC-FC measurements of samples with x = 0.1, 0.2 0.3 and 0.4, and the curves indicate the magnetic Curie temperatures (here also simplified as T_N) of such ceramics, the ferromagnetism-to-paramagnetism transition temperatures (marked as the peak of dM/dT curves), were about 776 K, 764 K, 762 K and 750 K, respectively, which are much higher than RT. It also shows a decreasing trend with the increase of the Co substitution concentration, and this can be attributed to the decreased orderliness of B-site ions and the relative stability of the magnetic structure by the increased Co substitution concentration [26]. Also, the ZFC and FC curves show that the spin glass transition temperatures of the 7-BFCT samples with x = 0 to 0.4 were about 247 K, 641 K, 633 K, 625 K and 623 K, respectively. This suggests that the substitution of Co for Fe may introduce some new magnetic exchange interactions which are worth investigating in the future. The detail values of 2M_r and T_N with different Co substitution concentrations are listed in

Table 1. The detail values of 2Mr, 2Pr, T_N and T_C with different Co substitution concentrations.

x	2Mr (emu/g)	2Pr (μC/cm2)	TN (K)	Tc (K)
0	--	0.59	247	1052
0.1	0.04	0.88	776	1054
0.2	0.30	2.90	764	1060
0.3	0.35	4.15	762	1062
0.4	0.69	3.05	750	1065

. In order to recognize the ferromagnetic contribution from the magnetic impurities (e.g., CoFe₂O₄) or the BFCT host in Aurivillius ceramics [19,20], a derivative thermos-magneto-gravimetry (DTMG) measurement was performed in the nitrogen atmosphere at a heating rate of 20 K/min with a 200 Oe applied magnetic field on the sample Bi₈Fe_3.6Co_0.4Ti₃O₂₄ (7-BFCT-0.4). As shown in Figure 3d, there was a peak at 753.4 K in the derivative weight curve, corresponding to the T_N (750 K) measured using the ZFC-FC method, far higher than the 720 K of magnetic impurity CoFe₂O₄ [20]. Therefore, our 7-BFCT ceramics are ferromagnetic.

2.4. Ferroelectric Properties

The room-temperature P-E hysteresis loops of the 7-BFCT samples under a driving field of 150 kV/cm are presented in Figure 4. As shown in Figure 4, the 2P_r of such 7-BFCT showed a great dependency on the Co substitution concentration. The 2P_r first increased with x and reached a maximum value of 4.15 μC/cm² at x = 0.3, and then decreased as the concentration further increased. The increased polarization can be explained by the distortion of the crystalline structure by the Co substitution, as mentioned in the XRD analysis. The decreased of 2P_r at x = 0.4 could be attributed to the existing stacking faults, which may induce the destruction of the insulation of the (Bi₂O₂)²⁺ layer. To rule out the possibility of artificial polarization from the leakage, a pulsed polarization positive-up-negative-down (PUND) measurement was performed and the results are shown in the inset of Figure 4. The measured switched polarization value (ΔP) was similar to the 2P_r obtained from the above hysteresis measurement, indicating the intrinsic ferroelectricity of the 7-BFCT samples.

In addition, to further realize the ferroelectricity of such 7-BFCT ceramics, the temperature dependence of the dielectric constant (ε) and the dielectric loss (Tanδ) at 1 MHz were taken. Figure 5a shows the temperature dependence of the dielectric constant (ε) of the samples with x = 0, 0.2 and 0.4. Their ε increases with the increase of the temperature and shows a dielectric peak at 1052 K, 1060 K and 1065 K for samples with x = 0, 0.2 and 0.4, respectively. These temperatures are very close to the reported T_c, the ferro-paraelectric phase transition temperature, at ~1049 K [23]. The increased T_c can be explained by the substitution of smaller-radius ions (Co) for larger-radius ions (Fe), which would bring out a smaller tolerant factor and lead to the higher T_c. Figure 5b shows that the dielectric loss (Tanδ) increases rapidly with increasing the temperature. Compared to the sample with x = 0, the Tanδ values of the rest of the samples show a decrease and this may suggest a decreased defect concentration with the Co substitution [27]. The x = 0.4 sample shows a larger Tanδ than that of the sample with x = 0.2, and this could be explained by the increased defect concentration caused by the existing stacking faults, as suggested in both the HRTEM and ferroelectricity analyses before. Table 1 also lists the details of 2P_r and T_c of the samples with different x values.

3. Materials and Methods

3.1. Sample Synthesis

Synthesis of the seven-layer Bi₈Fe_4−xCo_xTi₃O₂₄ (0 ≤ x ≤ 0.4, 7-BFCT) powders and ceramics follows below procedures: stoichiometric amounts of Bi(NO₃)₃·5H₂O (99%), Fe(NO₃)₃·9H₂O (98.5%), Co(NO₃)₃·6H₂O (99%) and Ti(C₁₀H₃₆O) (tetrabutyl titanate, 98%) were firstly dissolved in the dilute nitric acid, then EDTA (ethylenediamine tetraacetic acid) and citric acid were added to form a stable colloidal solution. After dried, burned and heat-treated at 750 °C for 2 h, Bi₈Fe_4--xCo_xTi₃O₂₄ powders were obtained. The corresponding ceramics were prepared by the hot-press method: firstly, the pre-sintered powders were pressed into pellets under a pressure of 80 MPa, then the pellet was placed in an alumina abrasive, and zirconium oxide powders were filled around the pellet as the isolation layer. Finally, the pellets were sintered at 930 °C (x = 0, 0.1), 910 °C(x = 0.2, 0.3) and 900 °C (x = 0.4) for 3 h under a pressure of 10 MPa in the Ar/O₂ mixed atmosphere.

3.2. Characterization

Crystalline structures of the as-fabricated samples were investigated by X-ray diffraction (XRD, D/Max-gA, Toyko, Japan) with a Cu-Ka radiation at λ = 1.5405 Å. Lattice images were taken by a high resolution transmission electron microscopy (HRTEM, JEM-2010, Japanese electronics co., LTD, Tokyo, Japan). Ferromagnetic properties were characterized using a vibrating samples magnetometer (VSM, EV-7, ADE Co., New York, NY, USA), and then the ferroelectric hysteresis loop was conducted using a Precision LC ferroelectric analyzer (Radiant Technology product, New York, NY, USA). Materials’ dielectric properties were measured by an Agilent 4294A precision impedance analyzer. Derivative thermo-magneto-gravimetric (DTMG) measurements were realized by thermos-gravimetric analysis and application of a magnetic field of 200 Oe (TGA Q5000IR, New Castle, DE, USA).

4. Conclusions

In summary, Aurivillius Bi₈Fe_4–xCo_xTi₃O₂₄ (7-BFCT, 0 ≤ x ≤ 0.4) powders and ceramics were successfully prepared by the combination of citrate combustion and hot-press methods. The effect of Co substitution on their ferromagnetic and ferroelectric properties was also investigated. All samples showed a good seven-layered structure with the largest remnant magnetization (2Mr~0.69 emu/g) obtained at x = 0.4, and the ferroelectric and magnetic transitions occurred at ~1065 K and ~750 K, respectively. The sample with x = 0.3 revealed the largest 2Pr~4.15 μC/cm² and T_C~1062 K. The superexchange between Fe-O-Co and the distorted crystal structure is responsible for the enhanced ferromagnetism. Changing of the ferroelectricity can be mainly attributed to the distorted crystalline structure and the existing stacking faults.