Room temperature ferroelectricity and ferromagnetism in Ni-substituted Bi5Ti3FeO15

Aurivillius-type bismuth layer-structured ferroelectric (BLSF) Bi5Ti3FeO15 (BTF) has recently attracted considerable attention as a typical multiferroic material because ferroelectric and magnetic orders coexist, but bulk BTF exhibits antiferromagnetic (AFM) orders and negligible intrinsic magnetoelectric (ME) coupling effects. In this study, nickel-substituted Bi5Ti3FeO15 (Bi5Ti3Fe0.5Ni0.5O15, abbreviated as BTF-Ni) was synthesized using a solid-state reaction method to explore and enhance both the magnetic and ferroelectric properties of BTF. Polarization-electric field P-E loops indicate that the BTF-Ni exhibits considerable maximum polarization P m of 11.9 μC/cm2 and remnant polarization P r of 5.8 μC/cm2, but still keeps a very high ferroelectric Curie temperature (FE T c) of 1029 K, which are much superior to those of pure BTF. Moreover, magnetization-magnetic field M-H loops indicate that BTF-Ni exhibits significant ferromagnetic properties with a large saturation magnetization M s of 60 memu/g, low coercive field H c of 31 Oe at room temperature, and a high ferromagnetic Curie temperature (FM T c) of 698 K, whereas pure BTF has an antiferromagnetic Néel temperature (T N) of 80 K. Our work suggests that nickel-substituted BTF is a potential room-temperature magnetoelectric multiferroic material.


Introduction
Multiferroic materials with ferroelectric and ferromagnetic orders have attracted considerable attention in recent years because of their unique physical properties and potential applications in multifunctional devices, such as sensors, transducers, multi-state memory devices, and energy harvesters [1][2][3][4][5][6][7][8].In particular, Bi 5 Ti 3 FeO 15 (BTF), a typical Aurivillius-type bismuth layer-structured ferroelectric (BLSF) compound, has been extensively investigated because of the coexistence of ferroelectric and antiferromagnetic orders [9][10][11].The structure of the BTF is described as a four-layered intergrown ABO 3 perovskite unit of (Bi 3 Ti 3 FeO 13 ) 2- sandwiched by a (Bi 2 O 2 ) 2+ unit along c axis [10][11][12][13], where Bi 3+ ions in the ABO 3 perovskite unit occupy the A-sites of the ABO 3 perovskite unit, and Ti 4+ and Fe 3+ ions are located at the B-sites with probabilities of 75% and 25%, respectively.The (Bi 2 O 2 ) 2+ layers play key roles in both space-charge compensation and insulation, which are expected to reduce the leakage current.The BTF is considered to be a combination of ferroelectric Bi 4 Ti 3 O 12 and multiferroic BiFeO 3 , herein the Bi 4 Ti 3 O 12 is a typical ferroelectric compound and well-known fatigue-free ferroelectric material by lanthanide substitution at the A-site [14][15][16][17], whereas BiFeO 3 is the only well-studied single-phase multiferroic material that exhibits both ferroelectric and antiferromagnetic orders [18][19][20][21][22][23][24].However, the intrinsic character of the antiferromagnetic state of the BTF along with a low Néel temperature (T N ) limits its application prospects [25][26][27], therefore; it is highly expected that both ferroelectric and ferromagnetic orders can coexist in the BTF.
It has been reported that the magnetic behavior of BTF can be regulated from an antiferromagnetic state to a ferromagnetic state by substituting magnetic ions.For example, room-temperature ferromagnetic properties have been reported in the Co [27], Cr [28][29][30], Mn [31], and Nd [32] single-element modified BTF, Ho/Mn [25], W/Ni [33], and Nd/Co [34][35][36] dual-elements modified BTF.Large magnetic response was observed in the Co/ Nd co-substituted BTF [36].A three-orders-of-magnitude increase in the remnant magnetization M r has been reported in Co-modified BTF (M r ∼ 3.9 memu/g) [27], but the possible presence of a small amount of CoFe 2 O 4 cannot be excluded which might also contribute to the tremendous improvement in the magnetic properties of the Co-modified BTF [35].Ferromagnetism has also been found in Ni-modified BTF, but the mechanism of coexisting ferroelectric and ferromagnetic orders is not well understood [37][38][39].
In this study, Aurivillius-type Bi 5 Ti 3 Fe 0.5 Ni 0.5 O 15 (BTF-Ni) was synthesized using a conventional solid-state reaction method, and its structural, ferroelectric, dielectric, and magnetic properties were investigated to understand the mechanism of coexisting ferroelectric and ferromagnetic orders of BTF-Ni.The results indicate that the BTF-Ni exhibits considerable maximum polarization P m of 11.9 μC/cm 2 and remnant polarization P r of 5.8 μC/cm 2 at room temperature, which are much superior to that of pure BTF.Moreover, BTF-Ni exhibits significant ferromagnetic properties with a large saturation magnetization M s of 60 memu/g and a much lower coercive field H c of 31 Oe at room temperature.Temperature-dependent dielectric and magnetic properties measurements indicate that BTF-Ni has a high ferroelectric Curie temperature (FE T c ) of 1029 K and a high ferromagnetic Curie temperature (FM T c ) of 698 K.These results indicate that BTF-Ni is a room-temperature magnetoelectric multiferroic material.

Experimental procedure
Aurivillius-type BLSF Bi 5 Ti 3 FeO 15 (BTF) and Ni-substituted BTF (BTF-Ni) were prepared using a conventional solid-state reaction method.Analytical grade Fe 2 O 3 (99.99%),Bi 2 O 3 (99.8%),TiO 2 (99.8%), and Ni 2 O 3 (99.3%)were selected as the raw materials and weighed according to their stoichiometric proportions.The compositions synthesized in this study were Bi 5 Ti 3 FeO 15 and Bi 5 Ti 3 Fe 0.5 Ni 0.5 O 15 (BTF-Ni).The oxide powders were batched and mixed via ball-milling using ZrO 2 balls in polyethylene bottles for 12 h with ethanol as the medium.The mixture was dried and calcined at 1043 K for 3 h.Then it was milled again under the same conditions to make the powders uniform, dried, ground, and granulated with 5 wt% polyvinyl alcohol (PVA) binder.The granulated powders were subsequently pressed into pellets (12.5 mm in diameter and 1.5 mm in thickness) under a pressure of 120 MPa.To prevent the evaporation of bismuth (Bi) ions and maintain the desired stoichiometry, these pellets were placed in sealed Al 2 O 3 crucibles and fully embedded in matching compositions.Finally, the BTF and BTF-Ni ceramics were sintered at 1253 K for 3 h using the ordinary firing method and then freely cooled to room temperature.
The crystalline phase structure of the powder from the sintered disks was analyzed using x-ray diffraction (XRD) with Cu K α1 radiation (D8 Advance, Bruker AXS GMBH).The surface microstructure of the sintered ceramics was characterized using scanning electron microscopy (SEM, S-4800, Hitachi, Tokyo, Japan).The magnetic properties as functions of the magnetic field and temperature were measured using a superconducting quantum interference device (SQUID, Quantum Design MPMS, USA).For ferroelectric and dielectric measurements, the sintered samples were first polished parallelly to 0.2 mm and 0.5 mm, respectively, and then silver electrodes were screen-printed on both surfaces of the polished samples and fired at 843 K for 20 min.The ferroelectric hysteresis loops (P-E, J-E loops) were measured using a ferroelectric analyzer (TF 2000E, aixACCT Systems Co., Aachen, Germany) with a high-voltage amplifier (TREK 610E, 10 kV, TREK, Medina, NY, USA).During the measurements, an electric field of 150 kV cm −1 was applied at a frequency of 1 Hz at various temperatures ranging from room temperature to 353 K.The dielectric properties were measured as a function of temperature at a frequency of 1 MHz using a computer-controlled precision impedance analyzer (E4990A, Agilent Technologies Inc., Santa Clara, CA, USA).

Results and discussion
Figure 1 shows the room-temperature X-ray diffraction (XRD) patterns of the BTF and BTF-Ni ceramic powders, which reveals the presence of only a bismuth layer-structured ferroelectric compound with m = 4 within the compositions.It can be seen that the strongest intensity of diffraction peak is the (119) diffraction peak in the patterns, which coincides with the fact that the most intense reflection of BLSFs is (112 m+1) [40][41][42], herein m = 4, i.e. (119) diffraction peak.The XRD patterns of BTF and BTF-Ni are almost the same and the peak positions shift slightly, therefore, the theoretical density of BTF-Ni does not change significantly compared with that of BTF.The results reveal that both BTF and BTF-Ni adopt the polar orthorhombic space group A2 1 am and both are single-phase with no detectable secondary phases, which is consistent with earlier reports [43].Figures 2(a Figure 3 (a) shows the polarization-electric field (P-E) hysteresis loops of the BTF-Ni ceramics measured at 1 Hz under an external applied electric field of 150 kV cm −1 at different temperatures.The P-E hysteresis loops are unsaturated owing to relatively small external driven electric field; however, a higher external driven electric field can cause sample breakdown.The maximum ferroelectric polarization P m was obtained experimentally under a maximum electric field of 150 kV cm −1 .It is clear that P m and remnant polarization P r both increase,  but the coercive field E c decreases with increasing temperature.Consequently, the BTF-Ni presents sizable polarization values with P m of 12.0 μC/cm 2 and P r of 5.8 μC/cm 2 (herein E c is 70.0 kV cm −1 ) under the electric field of 150 kV cm −1 at room temperature.Furthermore, when the temperature increases to 353 K, enhanced polarization values with P m of 23.0 μC/cm 2 and P r of 7.5 μC/cm 2 (herein E c is 54.4 kV cm −1 ) are achieved for the BTF-Ni.The significantly improved P m and P r , as well as the reduced E c , can be ascribed to the increased mumber of electrically active domains as the temperature increases.Usually, relatively low-quality P-E loops at high electric fields arise from the relatively high leakage current generated by high conduction, which is common in multiferroic materials [45].As shown in figure 3(b), the leakage current density is very small, approximately 10 −2 mA/cm 2 under an electric field of 150 kV cm −1 .Therefore, the lower leakage current density of BTF-Ni results in high-quality P-E loops.
To explore the ferroelectric phase transition temperature and dielectric properties of BTF-Ni, the temperature dependence of the relative dielectric constant ε and dielectric loss tanδ measured at 1 MHz as a function of temperature are presented in figure 4. As shown in figure 4 (a), the dielectric constant peak corresponding to the FE T c appears at 1029 K, which is almost the same as the FE T c (∼1032 K) of the BTF [46], revealing that nickel substitution did not significantly change the FE T c .Furthermore, figure 4 (b) shows that, below 600 K, the dielectric loss tanδ is relatively low.However, the dielectric loss tanδ increases sharply when the temperature is higher than 600 K because of the higher electric conduction at high temperatures [43].This abnormal dielectric phenomenon has also been observed in other BLSFs, which are derived from the oxygen vacancies due to the volatilization of bismuth during the sintering process in Aurivillius-type oxides [47].The good dielectric stability of BTF-Ni makes it a promising candidate for high-temperature dielectric applications.
We now discuss the magnetic properties of BTF-Ni.The magnetic hysteresis (M-H) loops measured at 5 K and room temperature are shown in figure 5(a).The magnetization saturation and coercive field of the loops indicate that the BTF-Ni is ferromagnetic at room temperature.The magnetization of BTF-Ni at 5 K exhibits a hysteresis loop with a coercive field H c of 38 Oe, a remnant magnetization M r of 5.2 memu/g, and a saturation magnetization M s of 100 memu/g.Meanwhile, the M-H loop at room temperature presents a smaller value with an H c of 31 Oe, a M r of 4.3 memu/g, and M s of 60 memu/g (0.051 μB/f.u.).The room temperature remnant polarization M r (4.3 memu/g) of BTF-Ni is higher than that of cobalt-modified BTF (Bi 5 Ti 3 Fe 0.5 Co 0.5 O 15 , M r = 3.9 memu/g) [27], and the M s (60 memu/g) of BTF-Ni is about eight times larger than that of Bi 5 Ti 3 Fe 0.5 Co 0.5 O 15 (M s ∼7.6 memu/g).Importantly, the nickel-modified BTF exhibits a significantly decreased coercive field H c (31 Oe), which is one order of magnitude smaller than that of the cobalt-modified BTF (205 Oe).In contrast, the M-H plot (not shown here) of the BTF exhibits linear behavior, implying the characteristics of antiferromagnetic materials [48][49][50].This means that partial substitution of Ni ions for Fe ions can significantly enhance the ferromagnetism of the BTF.Now we turn to discuss the origin of the observed ferromagnetism in BTF-Ni.The BTF is known to be antiferromagnetic with a Néel temperature of 80 K, in which the magnetic moments of the Fe atoms are aligned in opposite directions with equal magnitude [51].The antiferromagnetic order at a low Néel temperature is due to the indirect exchange interaction of Fe-O-Fe.While Ni atoms uniformly substitute Fe atoms at random, the ideal occupancy of Fe 3+ and Ni 3+ ions in BTF-Ni is 1:1.Owing to the differences between the magnetic moment of Fe 3+ and Ni 3+ ions, according to the Goodenough-Kanamori rules, the net magnetic moment will be ±2μ B per Fe 3+ -O 2− -Ni 3+ owing to the antiferromagnetic order.Therefore, in the ideal case of uniform and random doping, antiferromagnetism, rather than ferromagnetism is expected.However, antiferromagnetism is not observed in BTF-Ni.In fact, we observed ferromagnetism with a magnetic moment of 0.051μ B /f.u. at room temperature, which is much smaller than the isolated ionic magnetic moments of Fe 3+ (5μ B ) and Ni 3+ (3μ B ).This implies that ferromagnetism does not originate from the Fe or Ni metal clusters.First, the substitution of Ni 3+ for Fe 3+ slightly distorted the sublattices.Considering the canted spin structures due to the distorted crystal structure, the observed ferromagnetism may partially originate from spin canting based on the octahedral tilting between the adjacent Ni-O and Fe-O octahedra and the Fe-O and Ni-O sublattices, tending to the existence of a weak ferromagnetic (FM) state caused by the Dzyaloshinsky-Moriya (DM) interaction in Aurivillius-type BLSF BTF-Ni [52].On the other hand, oxygen vacancies inevitably exist, accompanied by A change in the valence of Fe or Ni in the Ni-substituted BTF.This magnetism may be partially attributed to the interaction of magnetic ions (Fe 3+ , Fe 2+ , and Ni 3+ ions) via oxygen vacancies as a medium.Oxygen vacancies combined with magnetic ions tend to form bound magnetic polarons (BMPs), which cause ferromagnetic interactions [52][53][54].

Conclusion
In summary, Aurivillius-type bismuth layer-structured ferroelectric (BLSF) Bi 5 Ti 3 FeO 15 (BTF) with Ni substitutions was successfully synthesized by a conventional solid-state reaction method.The microstructure and the ferroelectric, dielectric, and magnetic properties were investigated in detail.The ferroelectric properties measurements show that the BTF-Ni has a maximum polarization P m of 12.0 μC/cm 2 , a remanent polarization P r of 5.8 μC/cm 2 , and a coercive field E c of 70.0 kV/cm under the electric field of 150 kV cm −1 .The magnetic properties measurements show that the BTF possesses a large saturation magnetization M s of 60 memu/g, a low remanent magnetization M r of 4.3 memu/g, and a coercive field H c of 31 Oe at room temperature.The results also reveal that the FE T c and FM T c of BTF-Ni are 1029 K and 698 K, respectively.The results confirm that BTF-Ni is a room temperature ferroelectric and magnetic material, and also indicate that BTF-Ni possesses superior ferroelectric and ferromagnetic properties, demonstrating the possibility of Ni-substituted BTF as a potential room-temperature magnetoelectric multiferroic material.
Figure1shows the room-temperature X-ray diffraction (XRD) patterns of the BTF and BTF-Ni ceramic powders, which reveals the presence of only a bismuth layer-structured ferroelectric compound with m = 4 within the compositions.It can be seen that the strongest intensity of diffraction peak is the (119) diffraction peak in the patterns, which coincides with the fact that the most intense reflection of BLSFs is (112 m+1)[40][41][42], herein m = 4, i.e. (119) diffraction peak.The XRD patterns of BTF and BTF-Ni are almost the same and the peak positions shift slightly, therefore, the theoretical density of BTF-Ni does not change significantly compared with that of BTF.The results reveal that both BTF and BTF-Ni adopt the polar orthorhombic space group A2 1 am and both are single-phase with no detectable secondary phases, which is consistent with earlier reports[43].Figures2(a) and (b) show the SEM images of the fresh surfaces of BTF and BTF-Ni, respectively.The grains of BTF and BTF-Ni are preferentially formed in the shape of thin platelets, which is a typical

Figure 2 .
Figure 2. SEM micrographs of fresh surfaces of (a) BTF and (b) BTF-Ni, and cross-sectional SEM images of (c) BTF and (d) BTF-Ni.

Figure 3 .
Figure 3. Ferroelectric polarization-electric field (P-E) loops and corresponding current density-electric field (J-E) loops of the BTF-Ni at a frequency of 1 Hz at various temperatures from room temperature (RT) to 353 K.

Figure 5
(b) shows the M-T curve from room temperature to 780 K for BTF-Ni at a magnetic field of 100 Oe.Clearly, BTF-Ni shows a spontaneous magnetic moment, indicating its ferromagnetic nature.The sample undergoes a ferromagnetic to paramagnetic (FM-to-PM) transition at 698 K (defined as the temperature corresponding to the peak of dM/dT shown in the inset of figure 5(b)), which is the ferromagnetic Curie temperature (FM T c ) of BTF-Ni.

Figure 4 .
Figure 4. Temperature dependence of (a) the dielectric constant ε and (b) dielectric loss tanδ of BTF-Ni measured at a frequency of 1 MHz.

Figure 5 .
Figure 5. (a) The magnetic hysteresis (M-H) loops of BTF-Ni at room temperature and 5 K.The inset enlarges the magnetic hysteresis loop of the applied magnetic field from −200 to 200 Oe.(b) Magnetization of BTF-Ni as a function of temperature measured at H = 100 Oe.The inset is the temperature dependence plot of dM/dT.