Enhanced magnetoelectric coupling in La-modified Bi₅Co_0.5Fe_0.5Ti₃O₁₅ multiferroic ceramics

Abstract

Multiferroic properties of La-modified four-layered perovskite Bi_5−x La _x Fe_0.5Co_0.5Ti₃O₁₅ (0 ≤ x ≤ 1) ceramics were investigated, by analyzing the magnetodielectric effect, magneto-polarization response and magnetoelectric conversion. X-ray diffraction indicated the formation of pure Aurivillius ceramics, and Raman spectroscopy revealed the Bi ions displacement and the crystal structure variation. The enhancement of ferromagnetic and ferroelectric properties was observed in Bi_5−x La _x Fe_0.5Co_0.5Ti₃O₁₅ after La modification. The evidence for enhanced ME coupling was determined by magnetic field-induced marked variations in the dielectric constant and polarization. A maximum ME coefficient of 1.15 mV/cm·Oe was achieved in Bi_4.25La_0.75Fe_0.5Co_0.5Ti₃O₁₅ ceramic, which provides the possible promise for novel magnetoelectric device application.

Introduction

Multiferroic magnetoelectrics, coupling between electric and magnetic orderings, offer a wide scope of potential applications and an excellent abundance with fundamental physics [1,2,3]. The interplay between ferroelectricity and magnetism, namely magnetoelectric (ME) effect, allows an additional degree of freedom in multifunction device designing, which make devices more attractive than the use of ferroelectricity or magnetism only [4,5,6]. Much of the pioneering works on multiferroics have been devoted to introduce these two order parameters in a single-phase material. For example, the observations of electric field-induced spin flop in BiFeO₃ [7], magnetic controlled ferroelectric polarization in La₃Ni₂NbO₉ [8] and spin-assisted ferroelectricity in CaBaCo₄O₇ [9] were attractive from the fundamental and technical perspectives. However, the single-phase multiferroics above room temperature (RT) are rare in nature and exhibit weak ME coupling, originating from the mutual exclusion of ferroelectricity and ferromagnetism in d ₀ electronic structure for the B-site elements [2,3,4]. So continuing effort to search for single phase with high ME amplitude and sensitivity above RT should be undertaken. Several approaches of searching for desirable single-phase multiferroics with ferroelectrically and magnetically combined compound at atomic scale have been addressed, which include superlattices and layer-structured materials, e.g., the bismuth-based layered compound.

Layered bismuth titanates with Aurivillius structure, which can be viewed as inserting the unit of BiFeO₃ into the three-layered [Bi₂Ti₃O₁₀]²⁻ perovskite slabs of the parental Bi₄Ti₃O₁₂ ferroelectrics (nBiFeO₃·Bi₄Ti₃O₁₂), is interesting compounds with the possible ME coupling at RT [10,11,12,13]. By incorporating a magnetic unit into a ferroelectric Bi₄Ti₃O₁₂ block, magnetic properties are enhanced by the coupling between magnetic ions, such as Fe³⁺, Ni²⁺ and Co³⁺ [13,14,15]. More importantly, partially replacing Bi ions by rare earth ions with d-block electronic configuration ions, such as La³⁺, and with f-block ions, such as Nd³⁺ and Sm³⁺, further remarkably improve the ferroelectric polarization, ferromagnetic property and magnetodielectric (MD) effect [16,17,18]. Such rare earth ions substitution approach in Aurivillius structure was mainly demonstrated in four-layer Bi₅FeTi₃O₁₅ ceramics, composed by inserting one mole BiFeO₃ into one mole Bi₄Ti₃O₁₂ [15,16,17,18,19]. Yang et al. found that the Nd-modified Bi₅Fe_0.7Co_0.3Ti₃O₁₅ ceramic presents a large ferromagnetism with 2M _r = 330 memu/g and an enhanced MD response of 0.12% at RT [18, 19]. Recently, Mao et al. [15] realized the enhancement of ferromagnetism and MD effect in Bi₅Fe_0.5Co_0.3Ti₃O₁₅ ceramic by the La substitution, and the observed MD value of Bi_4.25La_0.75Fe_0.5Co_0.5Ti₃O₁₅ reached up to 8.1% under the magnetic field of 1 T. Although several studies have reported on the ferroelectric, ferromagnetic and MD properties in doped four-layer Aurivillius, little attention is focused on the magnetic field dependence of the ferroelectric polarization and ME output. It is expected that such studies will help in further elucidating the ME response in four-layer Aurivillius family in fact. To our knowledge, the ions doping level is crucial for improving ME properties and designing a new class of multiferroics. In this work, we prepared La-doped four-layer Aurivillius Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) system with different doping content, and then systemically investigated the structural, ferroelectric, magnetic properties and ME coupling. The ME coupling is characterized by the MD response, magneto-polarization property and ME voltage sensitivity. Excitingly, the La-substituted ceramics exhibits a significant enhancement in the ME coupling at RT, especially for the Bi_4.25La_0.75Fe_0.5Co_0.5Ti₃O₁₅ ceramic.

Experimental

Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics were synthesized by a modified sol–gel method. The process involves two steps. Firstly, the parental powders of Bi_4−x La _x Ti₃O₁₂ (0 ≤ x ≤ 1) were synthesized by the citrate combustion method. Analytical reagent grade Ti[OCH(CH₃)₂]₄, Bi(NO₃)₃·5H₂O (5 wt% excess to compensate the volatilization of Bi) and La(NO₃)₃·6H₂O were dissolved into the citric acid solution with glycol in stoichiometry. Ammonia was added into the mixed solutions to regulate PH to 7. Then the precursor solution was evaporated at 90 °C and dried at 120 °C to obtain the xerogel. After that, the xerogel was pre-sintered at 450 °C for 4 h and sintered at 750 °C for 4 h to obtain Bi_4−x La _x Ti₃O₁₂ powders. Secondly, the as-obtained Bi_4−x La _x Ti₃O₁₂ powders were soaked in BiFe_0.5Co_0.5O₃ precursor solution with mole ratio of 1:1, which was taken in stoichiometric amounts using Bi(NO₃)₃·5H₂O, Fe(NO₃)₃·9H₂O, Co(NO₃)₂·6H₂O and citric acid solution. This powder-in-sol precursor hybrid processing allows for a high purification and densification, a uniform dispersion and low-temperature sintering feature. The resultant solution was then subjected to heat at 90 °C with continuous stirring for 4–5 h and dried at 120 °C to obtain the xerogel. Afterward, the xerogel was pre-sintered at 450 °C for 4 h to burn out the organic species. The pre-sintered powders were ground, pelleted and finally sintered at 840 °C for 4 h in air. The sintered pellets were electroded using silver paint on both the surfaces to carry out subsequently electrical and ME measurement.

The phase identification and micromorphology of sintered pellets were characterized using X-ray diffraction (XRD, Philips X-pert PRO) and scanning electron microscope (SEM, TESCAN VEGA 3), respectively. The Raman mode was recorded with Raman spectrometer system (BWS435, B & WTEK). The grain size of the ceramic samples was determined by the mean linear intercept, and the bulk density of pellets was measured using Archimedes’ method. Magnetic hysteresis loops were studied by a vibrating samples magnetometer (VSM, Lakeshore 7404). Dielectric characteristics performed by combining a HP4284 LCR meter. Bulk density of pellets was performed using Archimedes’ method. DC resistivity was measured using precision LCR meter (TH2829C). The effect of magnetic field on the ferroelectric properties was evaluated by a ferroelectric tester based on Sawyer–Tower circuit. The magnetic field source for MD and ferroelectric properties measurement is used a custom designed magnet, and magnetic field direction is parallel to the surface of the pellet. The ME measurement were taken using the lock-in technique. A small alternating magnetic field H (f = 1 kHz, H _ac = 1.0 Oe) was generated using a solenoid that was superimposed onto a magnetic bias up to 3.5 kOe.

Result and discussion

Figure 1 displays the XRD patterns of Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics. Inset shows the magnified XRD patterns in the vicinity of 29.5°–31°. All the XRD peaks agree with the four-layered perovskite Aurivillius structure, in accordance with the standard XRD spectrum (JCPD no. 38-1257). No impurity phases are detected, indicating the monophasic nature of these ceramics. From the inset of Fig. 1, the diffraction peak (119) shifts toward low angle with increasing La content. This shifting is related to the lattice structure distortion driven by the difference of introduced ionic radius. Detailed variations of lattice parameters (listed in Table 1) are evaluated from the Rietveld refinements, and the sample is indexed with the orthorhombic lattice of the space group F2mm. With increasing x, the lattice constant a decreases while the lattice constant b, the lattice constant c and the cell volume increase. We also estimate the orthorhombicity [δ = 2(a − b)/(a + b)] for all samples. It can be clearly noticed that the orthorhombicity (δ) decreases continuously with increasing x. The variation of δ implies the relaxation of orthorhombic structure distortion [20, 21], which may affect the electric and magnetic properties.

Figure 1

XRD patterns of Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics. Inset shows the magnified XRD patterns in the vicinity of 29.5°–31°

Table 1 Rietveld refined lattice parameters of Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics

In order to understand the structural modification by substitution, the Raman spectra of Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics are recorded and shown in Fig. 2. The observation of Raman modes at around 61, 89, 121, 154, 241, 264, 316, 469, 554, and 855 cm⁻¹ confirmed the pure parental Bi₄Ti₃O₁₂ with orthorhombic structure [21, 22]. Based on the mass consideration of vibrational frequencies, the low-frequency Raman mode around 61 cm⁻¹ is dominated by the displacement of Bi³⁺ ions in Bi₂O₂ layers and the triplet bands at 89, 121 and 154 cm⁻¹ modes are assigned to the motion of Bi³⁺ ions (A-site) in the pseudo-perovskite slabs [23]. As shown in inset of Fig. 2, the mode at 61 cm⁻¹ decreases slightly until x = 0.75 and then sharply drops with further increasing La content. The slight variation of this mode indicates the absence of La³⁺ ions participating the Bi₂O₂ layers in the substitution process with x below 0.75. Over this doping content, however, the sharp decrease implies that the introduced La³⁺ ions may partially occupy the Bi³⁺ in the Bi₂O₂ layers. For the triplet bands, the mode at 89 cm⁻¹ softens continuously, while the modes at 121 and 154 cm⁻¹ show a hardening tendency with increasing x. The evolution of the triplet bands mainly related to the A-site Bi³⁺ ions replaced by La³⁺ ions in the perovskite slab. Similar behaviors have been observed previously in La-modified Bi₄Ti₃O₁₂ [21] and Nd-modified Bi₄Ti₃O₁₂ [24] bismuth layer perovskites. The high-frequency internal modes above 200 cm⁻¹ with A _1g character are related to the TiO₆ octahedron. The main modes at 264 and 855 cm⁻¹ show the peak broadening and reduction in the intensity with increasing x, indicating the tilting of TiO₆ octahedron. This observation confirms the increase in c-parameter with increasing La content [25]. Since the appearance of splitting modes at 241 and 554 cm⁻¹ in Bi₄Ti₃O₁₂ is Raman inactive according to the O _h symmetry of TiO₆, it often reflects the distortion of orthorhombic structure [20]. The suppression of these two modes with increasing La substitution in Raman spectra indicates the relaxation in distortion of TiO₆ octahedron and the decrease in orthorhombicity, which is in accordance with the result of XRD patterns.

Figure 2

Raman spectra of Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics in wavenumber ranging from 30 to 1000 cm⁻¹ at RT. Inset shows the dependence of the low-frequency mode observed around 61 cm⁻¹ on La doping content x

The SEM images of the cross sections of Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics are shown in Fig. 3. All ceramics have the plate-like grains, which are typical microstructural characteristic for Aurivillius bismuth layered compounds. The average size of plate-like grains decreases gradually from 3 to 1 μm with increasing La content. The low diffusion of La segregates at the grain boundaries and impedes the motion of grain boundaries during sintering, thus suppressing the grain growth [26]. On the other hand, the substitution of La ions for Bi also reduces the volatilization of Bi³⁺ and suppresses the formation of oxygen vacancies. The decrease in oxygen vacancies prevents the migration of ions between plate-like grains and then inhibits the grain growth. In addition, a few scattered micropores are observed, according to the relative density of ~ 90% (shown in Table 2) in the ceramics. The observed micropores were mainly located on the grain boundaries, relating to the random orientated and anisotropic plate-like grain growth.

Figure 3

SEM morphology of the Bi_1−x Nd _x FeO₃ ceramics a x = 0, b x = 0.25, c x = 0.75 and d x = 1

Table 2 Summary of the physical, ferroelectric, magnetic and ME parameters of the Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics

Magnetic hysteresis loops of the Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics measured at RT are shown in Fig. 4. The well-saturated magnetic hysteresis loops indicate the typical ferromagnetic ordering. For the La free sample, the remnant magnetization M _r and coercive field H _c is determined to be 35 memu/g and 291 Oe, respectively. With increasing La doping content, the M _r increases continuously and maximizes at x = 1.0 with the value of 131 memu/g, while the H _c decreases sharply with the x ≤ 0.5 and then increases slightly, shown in inset of Fig. 4. The improved ferromagnetism in the Co-substituted Aurivillius phase has been previously reported, mainly originating from the Dzyaloshinskii–Moriya (DM) interaction between Fe³⁺ and Co³⁺ through oxygen ions [12]. For the La-doped Aurivillius oxides, the further enhanced ferromagnetism is mainly attributed to the enhancement of Fe–O–Co clusters coupling. The 4d unoccupied orbits in La³⁺ ions partly accept the lone pairs of oxygen, leading to an intensively canted spin structure. This canted spin structure strengthens the coupling between the Fe-based and Co-based sublattices via the antisymmetric DM interaction. Moreover, the relaxed lattice distortion by the La³⁺ replacement for Bi³⁺ ions may release the locked latent magnetization and then contribute to the ferromagnetism interaction [27].

Figure 4

Magnetic hysteresis loops of Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics with the inset of the magnified loops in low field

Figure 5 exhibits the variation of dielectric constant ε′ and loss tangent tanδ with frequency in Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics. In the whole frequency range, the ε′ decreases as increasing x from 0 to 0.75. In the layered bismuth titanates systems, it is inevitable that the volatilization of Bi existed in the structure during the sintering process. Many studies confirm this fact [12,13,14,15]. Due to the decrease in Bi in Bi–O structure, charge defects of oxygen vacancies are produced in the Bi₅Ti₃Co_0.5Fe_0.5O₁₅ ceramic. These defects along with 6s ² lone pair of Bi make a contribution to space-charge polarization under an external electric field. This is supported by the result of relatively strong dielectric dispersion in undoped sample, which shows a rapid decrease at low frequency (< 1 kHz) and remains constant at high frequency. The addition of La³⁺ reduces the formation of oxygen vacancies and restrains the space-charge polarization, leading to the decrease in ε′. Similar variation for the frequency dependence of tanδ is observed in inset of Fig. 5, where the tanδ of doped samples decreases significantly at low frequency as compared to the undoped Bi₅Ti₃Fe_0.5Co_0.5O₁₅ sample. However, in the high doping level of x = 1, a slight increase in ε’ can be found in Bi₄LaTi₃Fe_0.5Co_0.5O₁₅ ceramic. This is caused by the substitution of La³⁺ for Bi³⁺ at the (Bi₂O₂)²⁺ layers. Bi₂O₂ layers act as both a reservoir for oxygen ions and an insulating layer to obstruct space charges hopping [28]. According to the analysis of Raman, the sharply decrease in the mode around 61 cm⁻¹ at x = 1 indicates that the introduced La³⁺ ions partially occupy the Bi³⁺ in the Bi₂O₂ layers. The incorporation of La³⁺ ions into Bi₂O₂ layers destroys the original effects of the insulating layers as well as the space-charge compensation, which release oxygen vacancies and produce space charges. The increased space charges at grain boundaries contribute to interfacial polarization and lead to the increase in ε′.

Figure 5

Variation of dielectric constant ε′ and dielectric loss tanδ with frequency for the Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics

Investigation over the magnetic-field-dependent dielectric properties, magnetodielectric (MD) effect, may give an understanding on ME coupling between the electric and the magnetic origins. MD effect is defined as MD = 100% × [ε′(H) − ε′(0)]/ε′(0), where ε′(H) and ε′(0) are the ε′ at magnetic field and zero field, respectively. Figure 6a shows the frequency dependence of MD with the magnetic field of 0.8 T for Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics. The MD of all ceramics experience continuous decrease with increasing frequency and the curves become flat at high frequency. The negative value of MD implies the decrease in ε′ at the applied magnetic field. The magnitude of MD increases with increasing magnetic field up to 0.8 T at 1 and 100 kHz, as shown in Fig. 6b. The maximum MD values reach up to 2.54% at 1 kHz and 1.18% at 100 kHz in x = 0.75. In layered Aurivillius structure, two major factors influence the MD response: the extrinsic effect, i.e., Maxwell–Wagner effect, and the intrinsic effect [29, 30], i.e., spin reorientation and structure distortion [31,32,33]. To see the influence of Maxwell–Wagner effect on the MD behavior, the dependence of tanδ on the magnetic field is measured and shown in inset of Fig. 6a, which is defined as Δtanδ = 100% × [tanδ(H) − tanδ(0)]/tanδ(0); where tanδ(H) and tanδ(0) are the dielectric loss at magnetic field and zero field, respectively. Similarly, the Δtanδ presents negative value, suggesting the decreased tanδ in the presence of applied magnetic field. The decrease in the ε′ and tanδ at the applied magnetic field indicates that the dominant mechanism of the MD effect is not the extrinsic effect, i.e., Maxwell–Wagner effect. This phenomenon can be explained by the Maxwell–Wagner capacitor model, which shows the observed MD and Δtanδ present an opposite sign as function of the applied magnetic field [30]. In our case, the observed MD and Δtanδ exhibit the same sign (negative value) under the magnetic field. Therefore, the MD behavior is reasonably ascribed to the intrinsic effect: the incurred lattice distortion driven by the La³⁺ substitution and the polarization response of Fe²⁺ and Fe³⁺ local dipoles induced by the spin reorientation under the magnetic field. In view of improved magnetic and MD properties in the high La modifying content for Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅, the observations of the notable magneto-polarization and ME response are deserved.

Figure 6

a Dependence of MD effect on frequency at the magnetic field of 0.8 T in Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ at RT. Inset shows the variation of tanδ on the magnetic field. b Variation of MD effect as a function of magnetic field at the frequency of 1 and 100 kHz (in inset)

Figure 7a shows the ferroelectric hysteresis loops of the Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) measured at 1.5 kHz. The loops reflect clear ferroelectricity. Also, the loops exhibit less unsaturation under the electrical field of 70 kV/cm, and this result is different from the saturated loops measured at the high electrical field of 230 kV/cm in Bi₅Ti₃Fe_0.5Co_0.5O₁₅ system [11]. To exclude the effect of leakage-related contribution, the remanent polarization P _r dependence of frequency from 400 Hz to 2 kHz in undoped Bi₅Ti₃Fe_0.5Co_0.5O₁₅ ceramic was determined and is shown in inset Fig. 7a. The remnant polarization decreases rapidly from 400 Hz to 1.2 kHz, but keeps less change in the high-frequency region beyond 1.2 kHz. This indicates that the ferroelectricity is intrinsic nature (domain switching) and the leakage current is almost independent on the ferroelectric behavior in high-frequency region [27]. From Fig. 7b, the 2P _r increases sharply with increasing x, reaches the maximum at x = 0.75, and then decreases with further substitution. The largest 2P _r (4.45 μC/cm²) is about 3 times larger than that of the undoped sample with 2P _r = 1.5 μC/cm². This significant enhancement of 2P _r on La doping may be attributed to non-centrosymmetic structure distortion and reduction in the oxygen vacancy concentration. By the occupation of La³⁺ ions at Bi³⁺ ions site in the perovskite slab, a lone s ² pair of electrons of Bi³⁺ ion hybridization with an empty p orbital of Bi or O ion results in the non-centrosymmetic distortion of perovskite-like block and gives rise to the ferroelectric order [34]. On the other hand, the reduction oxygen vacancies tend to trap and weaken the segregation of vacancy at domain walls and speed up the polarization switching [35]. Over the doping content 0.75, the deterioration of 2P _r may arise from the induced leakage in ceramics. As discussed earlier, the incorporation of La³⁺ ions into Bi₂O₂ layers destroys the original effects of the insulating layers as well as the space-charge compensation and deteriorates the P _r. This can be further confirmed by the measured resistivity R _dc as shown in Fig. 7b. The R _dc presents a similar variation tendency to that found in 2P _r.

Figure 7

a Ferroelectric hysteresis loops of Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics. Inset shows the dependence of the remanent polarization P _r in Bi₅Ti₃Fe_0.5Co_0.5O₁₅ on frequency from 400 Hz to 2 kHz. b Variation of 2P _r and resistivity R _dc as a function of La content

In order to demonstrate the ME coupling aroused by La doping contribution, ferroelectric hysteresis loops of the Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics was performed under the magnetic field. Figure 8a shows the variation of remanent polarization ΔP _r and coercive field ΔE _c with x in Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics, which are defined as ΔP _r = P _r(H) − P _r(0) and ΔE _c = E _c(H) − E _c(0), where P _r(H), E _c(H) and P _r(0), E _c(0) are the polarization and coercive field measured with and without magnetic field at 0.8 T, respectively. The ΔP _r increases with increasing x and reaches a maximum value of 0.6 μC/cm² at x = 0.75. This increase trend is indicative of coupling between the two ordered ferroic parameters. In layered Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ structure, the weak magnetic moment by the measurement of magnetization indicates a canted antiferromagnetic ordering (spin structure). Such canted antiferromagnetic structure induces an electric polarization based on the DM interaction, which will be added to the ferroelectric polarization dominantly driven by Bi³⁺ ion displacement. The canted antiferromagnetism contributed to the polarization can be flipped by external magnetic field [13, 36]. By substituting La ions for Bi ions, the intensified canted spin structure and the enlarged ion displacement in Bi–O sublattice facilitate the ferroelectric polarization induced by inverse DM interaction under the magnetic field. Moreover, the enhanced polarization under the applied magnetic field partly arises from the strain coupling between the magnetic and ferroelectric domains. This can be confirmed by the changed coercive field ΔE _c shown in Fig. 8a. Due to the coupling between the magnetic and ferroelectric domains, the strain would induce a stress and then generate an additional electric field. This field orients the ferroelectric domains, causing a high pinning of domains [37, 38]. Consequently, a large coercive field is required to remove such additional aligning strength and switch more domains. At present, it is difficult to identify how the magnetic field will affect the polarization by current measurements, i.e., strain-mediated or DM interaction, or even both. Further research is required to elucidate this point. Figure 8b displays the variation of ME voltage coefficients α _ME with applied magnetic field H at 1 kHz for Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics. The coupling coefficient of all ceramics exhibits a non-monotonically ME behavior with increasing applied magnetic field and attain a peak value in the range of 200–400 Oe. The peak value of 1.15 mV/cm·Oe at H = 300 Oe is achieved for x = 0.75 sample, which is higher than that in the layered Aurivillius Bi₅FeTi₃O₁₅ (0.63 mV/cm·Oe at 300 Oe) ceramic [39], Bi_4−x Sm _x Ti_3−x Ni _x O_12+δ ceramic (0.47 mV/cm·Oe at 300 Oe) [20] and Bi_4−x Sm _x Ti_3−x Co _x O_12-δ ceramic (0.62 mV/cm·Oe at 300 Oe) [40]. Enhanced ME coupling was driven by La modification in four-layer Aurivillius ceramics, which will broaden the number of RT multiferroic candidates for ME multifunctional device application.

Figure 8

a Dependence of the changed remnant polarization ΔP _r on x without and with magnetic field of 0.8 T for the Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics at RT. Inset shows the ferroelectric hysteresis loops without and with magnetic field of 0.8 T for x = 0.75 sample. b Variation of ME voltage coefficients α _ME with magnetic field for the ceramics

Conclusions

La-modified Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (0 ≤ x ≤ 1) ceramics with enhanced ME coupling have been prepared by a modified sol–gel process. The XRD patterns and Rietveld refinement confirmed the formation of a pure Aurivillius ceramics, and Raman spectroscopy revealed the relaxation of orthorhombic distortion and the Bi displacements in the Bi₂O₂ layers. The La substitution was found to significantly improve the ferroelectric and magnetic properties of the Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ ceramics at RT. The enhanced ME coupling of the La-doped Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ ceramics was demonstrated by the large modulation of MD response, magneto-polarization behavior and direct ME output. The largest ME response was obtained in Bi_5−x La _x Ti₃Fe_0.5Co_0.5O₁₅ (x = 0.75) with a MD value of 2.54%, magneto-polarization variation of 0.6 μC/cm² and ME conversion coefficient of 1.15 mV/cm·Oe, which may further encourage the potential use for four-layer Aurivillius ceramics in multifunctional ME device applications.