1 Introduction

In recent years, multiferroic materials, exhibiting simultaneously the ferroelectric and magnetic order, have been widely investigated due to their potential applications in various advanced devices, such as data storage, actuators, sensors and ultra-high speed telecommunication devices. BiFeO3 (BFO) is one of the multiferroic materials that exhibit the coexistence of ferroelectric and magnetic properties in perovskite structures at room temperature in the same phase. BFO with a rhombohedral-distorted ABO3-type perovskite structure (R3c) exhibits ferroelectricity with high Curie temperature (TC ~ 1103 K). The G-type antiferromagnetic properties (TN ~ 643 K) have recently attracted considerable attention [1, 2]. It possesses a large intrinsic polarization as high as ~ 90 μC/cm2, which is ascribed to the distortion of FeO6 octahedron caused by the presence of 6s2 lone pairs of electrons at the Bi site [3, 4]. However, various constraints restrict the applicability of BFO for practical applications, such as difficult to synthesize pure phase, low ferroelectric property, and high leakage current, etc. [5, 6]. To overcome these problems, many researchers mainly concentrate on the following methods to improve the performance of BFO, including modification of preparation methods (such as the fast liquid phase method [7], microwave method, sol–gel, etc.), the substitution of other ions at A or B sites [8, 9], and formation of solid solution with other ABO3-type perovskite structure (such as BaTiO3 (BTO) and SrTiO3, etc.) to effectively inhibit the generation of impurity and improve the electrical insulativity, especially the solid solution formed with BaTiO3 [10].

The (1-x)BiFeO3-xBaTiO3 (BFO-xBTO) (0.3 ≤ x ≤ 0.4) sample with a morphotropic phase boundary (MPB) with rhombohedral and cubic (or tetragonal) phases can obtain excellent ferroelectric and piezoelectric properties [11,12,13]. The BFO-xBTO (0.10 ≤ x ≤ 0.25) sample with a rhombohedral phase is obtained [12,13,14,15]. The chemical substitution (such as Cr, Co, Nd, Eu, etc.) at the Bi site in BFO-BTO has been investigated, showing the enhancement of magnetoelectric effect, ferroelectric, piezoelectric and magnetic properties due to structural distortion and partial destruction of spiral spin structure [16,17,18,19,20]. The rare-earth element substitution favors the formation of paraelectric orthorhombic phase [21]. The Sm substitution with a smaller ionic radius than Bi at the Bi sites in BFO leads to the improvement of the ferroelectric, piezoelectric and magnetic properties [22, 23]. The Sm-doped BFO undergoes structural transformation from rhombohedral to orthorhombic phase [22, 24, 25]. Therefore, in the present work, a solid solution of 0.79Bi1-xSmxFeO3–0.21BaTiO3 was fabricated by a traditional ceramic technique in air with the rhombohedral and orthorhombic phases. The effects of Sm doping on the microstructure, ferroelectric, ferromagnetic and magneto-capacitance properties of the ceramics were studied.

2 Materials and methods

In this paper, the 0.79Bi1-xSmxFeO3-0.21BaTiO3 (BFO-BTO-Sm-x) ceramics (x = 0.00, 0.01, 0.03, 0.04, 0.06) were successfully prepared by the traditional solid-state reaction method. Raw materials of Bi2O3 (99.9%), BaCO3 (AR), Fe2O3 (99.9%), TiO2 (99.8%) and Sm2O3 (99.99%) were used. The powders were stoichiometrically mixed according to the chemical formula and then ball-milled for 24 h at 450 rpm using ZrO2 balls in a polymer jar with ethanol as a milling media. The mixed precursors were dried in an oven at 80 °C and calcined for 4 h at 800 °C. An additional ball-milling step was conducted for 24 h at 450 rpm following calcination to achieve better homogeneity of the final powders. The synthesized powders were mixed with a PVA binder solution, then pressed into disk samples at 10 MPa and dried at 650 °C for 3 h. Finally, the compacts were sintered at 980 °C for 2 h in air. Silver electrodes were treated on the top and bottom surfaces of the samples at 220 °C for 30 min.

The structural properties were studied using X-ray diffraction (XRD). The XRD patterns were recorded at room temperature using an X-ray powder diffractometer with Cu Kα radiation. Room-temperature dielectric properties were measured by an impedance analyzer (Agilent 4284A). In a wide frequency range of 100 Hz to 1 MHz, the a.c. electrical conductivity and ferroelectric measurements were carried out as a function of the electric field using the ferroelectric test system TRI- Multiferroic 4 kV at room temperature. The magnetic measurements were performed by using a physical property measurement system (PPMS) at room temperature. X-ray absorption spectroscopy (XAS) of Fe L2,3 and O K-edge were measured at the photoelectron station of Beijing Synchrotron Radiation Facility (BSRF), Beijing, China.

3 Results and discussion

The XRD patterns of BFO-BTO-Sm-x (x = 0, 0.01, 0.03, 0.04, 0.06) ceramics are shown in Fig. 1. This figure shows that all ceramics exhibit a pure perovskite structure. A small impurity phase was detected around 30°, suggesting that Ba3+, Ti4+ and Sm3+ have diffused into the BFO lattices to form a homogenous solid solution. Similar to BFO, the sample of x = 0 exhibits a rhombohedral symmetry (R3c, JCPDS card 71–2494). As shown in Fig. 1b, it can be seen that the diffraction peaks at 2θ ~ 39° gradually change as x increases to 0.06. It suggests that a phase transition occurs after substitutions of Sm3+ for Bi3+. The diffraction peak splitting at 2θ ~ 39° becomes more noticeable for x = 0.03 and 0.04, and then indistinct as x up to 0.06, indicating the coexistence of rhombohedral and orthorhombic (Amm2, JCPDS card 75–1608) phases for the samples with x = 0.01 ~ 0.04. Apart from the structural transition, the peak shifts to a higher angle under Sm substitution due to the smaller ion radius of Sm3+ (0.0995 nm) than that of Bi3+ (0.103 nm) at A sites. To characterize the phase compositions in the samples, the peaks at 38°–40.5° were fitted by the Gaussian function as shown in Fig. 2. The peaks between 38° and 40.5° were fitted to (006)R/(202)R and (102)O/(120)O, where R and O denote rhombohedral and orthorhombic phases, respectively. The ceramic with x = 0 shows a rhombohedral phase with the (006)R/(202)R diffraction peaks as shown in Fig. 2a. As x increases, it shows the transformation from (006)R/(202)R peaks to (102)O/(120)O peaks. The ceramic with x = 0.06 shows an orthorhombic phase with (006)R/(202)R diffraction peaks as shown in Fig. 2e. It indicates that the MPB of rhombohedral and orthorhombic phases is formed for the samples of x = 0.01–0.04.

Fig. 1
figure 1

XRD patterns of the BFO-BTO-Sm-x (x = 0, 0.01, 0.03, 0.04, 0.06) ceramics

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Fig. 2
figure 2

XRD fitting patterns of the BFO-BTO-Sm-x (x = 0, 0.01, 0.03, 0.04, 0.06) ceramics in the 2θ ranges of 38°–40.5°

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Figure 3 shows the SEM images of BFO-BTO-Sm-x (x = 0, 0.01, 0.03, 0.04, 0.06) ceramics. These images were taken from freshly fractured surfaces. The samples show inhomogeneous grain sizes as shown in Fig. 3. The grain size distributions for the samples calculated by nano-measurer software are shown in Fig. 4(a–e). The grain size distributions become more scattered. The average grain size and percentage of porosity are shown in Fig. 4f. The grain size decreases and the percentage of porosity increases under Sm substitution. It means that the electrical insulativity will change as the number of porosity varies in samples.

Fig. 3
figure 3

SEM micrographs of the BFO-BTO-Sm-x ceramics sintered: a x = 0, b x = 0.01, c x = 0.03, d x = 0.04 and e x = 0.06

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Fig. 4
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(a–e) Grain size distribution of the BFO-BTO-Sm-x (x = 0, 0.01, 0.03, 0.04, 0.06) ceramics. f The average grain size and percentage of porosity of the BFO-BTO-Sm-x ceramics

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The P-E hysteresis loop is one of the most important characteristics of a ferroelectric material and gives information on its dynamic polarizability. Figure 5 shows the P-E loops for BFO-BTO-Sm-x ceramics at room temperature. It can be seen that all the samples show an obvious electrical hysteresis. The remnant polarization (Pr) as shown in Fig. 5f increases when x ≤ 0.04, then decreases for higher Sm content at x = 0.06. The maximum ferroelectric characterization is obtained with Pr of 3.83 μC/cm2 and Ec of 4.01 kV/mm at x = 0.04. The enhancement of ferroelectric properties is due to the formation of the MPB of rhombohedral and orthorhombic phases. The rhombohedral and orthorhombic phase possesses different spontaneous polarization directions [26], which may give more total polarizations when the two phases coexist.

Fig. 5
figure 5

(a–e) Ferroelectric hysteresis loops for different x in BFO-BTO-Sm-x ceramics at x = 0.0–0.06. f The variation of polarization (Pr) and electric field (EC) for BFO-BTO-Sm-x ceramics

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Figure 6 shows the leakage current density J of the BFO-BTO-Sm-x ceramics. The leakage current can directly reflect the insulativity of samples. The larger the leakage current is, the smaller the resistance is for the samples as shown in Fig. 6(a, b). It plays an important role in multiferroic properties and piezoelectric properties. It can be seen that all ceramics exhibit lower J values of 1.77 × 10–7-1.16 × 10–5 (A/cm2) under an electric field of 40 kV/cm than that of pure BFO ceramic (8.18 × 10–4 (A/cm2)), suggesting that all ceramics exhibit better electrical insulativity [5]. The high leakage current in BFO is attributed to the presence of charged oxygen vacancies for charge compensation and the reduction of Fe3+ to Fe2+ during sintering [5]. The lower leakage current of BFO-BTO than that of BFO is due to the suppression of the reduction of Fe ions from Fe3+ to Fe2+ leading to the decrease of oxygen vacancies [27]. With the increase of x, the leakage current density becomes larger than that of pure BFO-BTO, which indicates that the insulativity of the BFO-BTO system can be inhibited under Sm substitution. It suggests that both the reduction of Fe ions from Fe3+ to Fe2+ and oxygen vacancies increase under Sm substitution in BFO-BTO. To further understand the conduction mechanism, the logarithmic plots of J-E and its fitting curves are presented in Fig. 6c. After fitting, the logJ-logE curves show a linear relationship. The slope of the fitting curves at low voltage is close to 1.0, indicating that the conduction mechanism follows Ohm’s law. The slight drift at high voltage is attributed to the space-charge-limited conduction (SCLC) [25].

Fig. 6
figure 6

a Leakage current density J of the BFO-BTO-Sm-x (x = 0, 0.01, 0.03, 0.04, 0.06) ceramics; b resistivity vs. electric field of the BF-BT-Sm-x ceramics; c double logarithmic scale log J vs logE fitted of the BF-BT-Sm-x ceramics

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The magnetic hysteresis loops of BFO–BTO–Sm-x ceramics are presented in Fig. 7. The undoped BFO-BTO sample exhibits a partially saturated and weak hysteresis with Ms of 0.504 emu/g and coercive field (HC = 1826 Oe). With the increase of x, the hysteresis loop gradually becomes more saturated, and the saturation magnetization Ms increases rapidly and reaches the maximum at x = 0.03 with the value of 3.43 emu/g; when x > 0.03, the saturation magnetization Ms gradually decreases to 2.45 emu/g at x = 0.06. This change of magnetic properties may be associated with the structural distortion. It is well known that BFO possesses a distorted rhombohedral structure and a G-type antiferromagnetic spin spiral structure [4]. This structure also suppresses the macroscopic magnetization, resulting in the inability to observe the large magnetoelectric effect. The spiral structure might be further distorted, leading to the release of more suppressed magnetization. The sample of x = 0.03 possesses the highest MS because it is in the coexistence state of rhombohedral and orthorhombic phases. In BFO-SrTiO3 and BFO-BTO materials, a similar correlation between crystal symmetry and ferromagnetism has also been reported [28, 29]. The enhanced magnetization of the Sm-doped samples will be interesting for magnetic field sensor applications.

Fig. 7
figure 7

(a–e) M–H loops of BFO-BTO-Sm-x ceramics measured at room temperature; (f) variations of saturated magnetization Ms and coercive field Hc of the BFO-BT-Sm-x ceramics

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Figure 8a shows the frequency variations of the dielectric constant for BFO-BTO-Sm-x ceramics from 100 Hz to 1 MHz frequency. It can be seen that the values of the dielectric constant decrease with increasing frequency. The observation has been explained by the phenomenon of dipole relaxation wherein at low frequencies the dipoles are able to follow the frequency of the applied field [27]. It can be seen that the dielectric constant of pure BFO-BTO is higher than that of the ceramics at x = 0.01 and 0.06, and the dielectric constant of the ceramics at x = 0.03 and 0.04 is higher than that of pure BFO-BTO. The highest dielectric constant is at x = 0.04, which is consistent with the P-E loops. Figure 8b shows the magneto-capacitance of BFO-BTO-Sm-x ceramics with different magnetic fields. The magnetoelectric effect is a known property in most of multiferroics induced by the coupling between electrical and magnetic dipoles. A magnetoelectric material gets strained after the application of a magnetic field. This strain induces stress in the material. This stress generates an electric field on the ferroelectric domains, leading to a modification of the dielectric constant [30]. The change rate of dielectric constant under different magnetic field intensities can indirectly characterize the magnetoelectric coupling effect between electrical and magnetic dipoles. Magneto-capacitance (MC) is defined as the change in dielectric constant (εr) value with an external magnetic field. It is also a sign of the magnetoelectric properties of a multiferroic material. Its mathematical expression is,

Fig. 8
figure 8

a Dielectric constant of BFO-BTO-Sm-x (x = 0–0.06) ceramics at room temperature. b The magneto-capacitance of BFO-BTO-Sm-x ceramic with the different magnetic fields at 5 kHz

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$$MC\left(\mathrm{\%}\right)=\frac{{\varepsilon }_{r}\left(H\right)-{\varepsilon }_{r}(0)}{{\varepsilon }_{r}(0)}\times 100\%$$

where εr(H) and εr(0) denote dielectric constants at applied magnetic field H and zero fields, respectively [30, 31]. The values of MC for Sm-substituted BFO-BTO ceramics are higher than that of BFO-BTO under the magnetic field of 10 kOe. The value of MC increases to 1.3% at x = 0.03 and then decreases to 0.16% at x = 0.04. It is noted that the sample for x = 0.03 owns a best-saturated magnetization Ms and better residual polarization Pr, showing stronger magnetoelectric coupling. As x up to 0.06, the value of MC increases to 0.99%, where the crystal structure just deviates from the MPB. The high resistivity and low leakage current lead to the large strain and energy transfer requisite for an enhanced magnetoelectric coupling [32,33,34]. Due to the structural transformation from rhombohedral to orthorhombic, the destruction of spiral spin structure leads to the enhancement of magnetic properties [22, 35, 36]. Thus, the magnetoelectric coupling of x = 0.06 sample with lower leakage current and without R3c phase is stronger than that of x = 0.04 sample. It indicates that the magnetoelectric coupling enhances under Sm substitution.

Figure 9 shows the O K-edge XAS spectra corresponding to the transition from O 1 s to 2p transition hybridized with the unoccupied Fe 3d, Bi 6 s/6p, and Sm 4f orbitals [37, 38]. The O K-edge absorption consists of two regions at ~ 530–536 eV (A and B) and ~ 536–545 eV (C and D). For BiFeO3, the Fe ions generally have two states 3d5(Fe3+) and 3d6(Fe2+). The O 2p hybridized with the unoccupied Fe 3d has mostly 3d characters and splits into t2g and eg bands by ligand field [39, 40]. The peak A corresponds to the hybridization of O 2p with unoccupied Fe 3d. The peak B can be attributed to the hybridization of O 2p with Bi 6 s/6p orbitals [41, 42]. The intensity of peak B increases with the increase of Sm content, indicating the increase of hybridization strength of O 2p–Bi 6 s/6p orbitals as the system approaches the MPB. The Bi 6p-O 2p hybridization is the main origin of FE polarization due to the off-center displacement of the Bi ions in BiFeO3 [43]. The improvement of Bi 6 s/6p-O 2p hybridization leads to the enhancement of local FE polarization. The peak C centered at ~ 542 eV corresponds to the hybridizations of O 2p with the unoccupied Fe 4 s/4p, Sm 5d, and Bi 6d orbitals [38, 44,45,46]. The peak C of Sm-substituted BFO-BTO ceramics is broader than that of pure BFO-BTO. The broadness of XAS lines is recognized as an intrinsic property of oxygen vacancies [47, 48]. It indicates that oxygen vacancies increase in amount under Sm substitution,

Fig. 9
figure 9

O K-edge synchrotron XAS spectra of BFO-BTO-Sm-x (x = 0–0.06) ceramics

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Figure 10 displays the normalized Fe L2,3-edge XAS spectra, due to the transitions from the Fe 2p3/2 to the unoccupied Fe 3d orbitals [49]. In the FeO6 octahedron, the coupling between the Fe 3d and the O 2p orbitals causes a splitting of the fivefold degeneracy into triple-degenerate t2g orbitals (lower photon energy) and double-degenerate eg orbitals (higher photon energy) [50]. The energy gap between t2g and eg bands is ~ 1.4 eV. To determine the transformation of Fe3+ to Fe2+ for the BFO-BTO sample, the Fe L2,3-edge XAS spectra of Fe3+ and Fe2+ obtained by calculation using CTM4XAS are presented in Fig. 10a. It can be seen that Fe ions in the BFO-BTO sample coexist in the form of bi- and tri-valence. As shown in Fig. 10b, the intensity of t2g increases with the increase of Sm content, indicating that the ratio of the amount of Fe3+ to Fe2+ decreases. The increase of t2g suggests the increase of the unoccupied state of Fe 3d, resulting in the reduction of the charge transfer between Fe 3d and O 2p. It suggests that the hybridization between Fe 3d and O 2p decreases. The enhancement of magnetic property can be caused by the existence of the double-exchange interaction of Fe3+-O-Fe2+ due to the creation of Fe2+ [51, 52]. Thus, the decrease of hybridization of O 2p with unoccupied Fe 3d results in the improvement of magnetization.

Fig. 10
figure 10

Fe-L3,2 edge synchrotron XAS spectra of (a) BFO and BFO-BTO, and (b) BFO-BTO-Sm-x (x = 0–0.06) ceramics

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4 Conclusions

In this paper, the 0.71Bi1-xSmxFeO3-0.29BaTiO3 (x = 0, 0.01, 0.03, 0.04, 0.06) solid solution ceramic samples were successfully prepared. Through the analysis of the XRD structure, it was found that the method of introducing BaTiO3 and BiFeO3 to form a solid solution could greatly inhibit the impurity phase in the process of BiFeO3 preparation. XRD patterns show that the coexistence of rhombohedral and orthorhombic phases was formed at x = 0.01–0.04, called MPB structure. The better residual polarization Pr = 3.83 μC/cm2 (x = 0.04) and the maximum saturation magnetization 3.43 emu/g (x = 0.03) are obtained near the MPB, so that we can observe the apparent magneto-dielectric coupling effect in the sample of x = 0.03, and the magneto-capacitance is as high as ~ 1.3%, the ferroelectric properties of the system can be effectively improved, especially the ferromagnetic properties have been greatly improved under Sm substitution. When the structure is close to MPB, the hybridization strength between Bi 6sp and O 2p increases, thus the ferroelectric properties of the system are improved. From Fe L2,3- edge XAS spectra, the intensity of t2g increases as Sm content increases, indicating that the hybridization strength of O 2p with Fe 3d decreases. The reduced hybridization results in the improvement of magnetization. It indicates that the system has potential application in the field of information storage for new devices.