Introduction

Materials possessing two or more ferroic orders such as ferroelectric, ferromagnetic or ferroelastic simultaneously in the same phase, and also allows the coupling between these ferroic orderings is known as multiferroic materials [1–3]. These materials have attracted much attention during the past decade. The search of these materials is driven by the prospect of controlling charges by applying magnetic field and spins by electric field. The new effect, such as magnetoelectric (ME) effect could be produced by the coupling between different order parameters. For observation of ME effect, the co-existence of electric and magnetic dipoles is the basic requirement. This is the most fascinating characteristic of the multiferroic materials, as interesting physics is associated with it. Due to the co-existence of magnetism and ferroelectricity at room temperature, these materials can be used in multiple state memories, sensors, transducers and data storage devices [4, 5]. The coupling between two order parameters exists, if electric polarization is caused by applying either an electric or a magnetic field. Due to coupling between two order parameters makes it possible to write data bit with an electric field and read it with magnetic field, and vice versa. This provides an extra degree of freedom in device designing [6, 7]. Much of the early work on multiferroic was directed towards bringing these two order parameters in one material [8]. However, in actual, there is shortage of the materials exhibiting magnetoelectric behaviour at room temperature possibly due to the fact that these two order parameters make mutually exclusive groups [9, 10]. Since Ferroelectricty requires empty d-orbitals, while magnetism needs partially or half-filled d-orbitals, and the presently known single-phase multiferroic compounds shows a weak magnetoelectric (ME) coupling at room temperature. From the technological application point of view, we require those types of materials having large ME coupling responses at room temperature [11]. Although the gigantic ME effect in the composite structure has been well demonstrated, the single-phase compounds are still greatly concerned, partially because the multiferroic compounds allow us to tune the ME effect in the quantum level. Hence the search for new single-phase multiferroic materials having large magnetoelectric effect at or above room temperature still continues.

Many multiferroic compounds such as YMnO3, Cr2O3, TbMnO5 have been studied during the past decades [12–14]. The ME coupling responses of most of such known materials at room temperature are too small to be used for technological purposes. The demand of the materials with multifunctional properties is increasing with development towards device miniaturization. Initially the term multiferroic was only referred to single-phase materials and was later on expanded to include any material which exhibit two or more type of long-range orderings. The single-phase multiferroics exhibit the co-existing order parameters only at low temperature, and they additionally have weak magnetic response at room temperature. BiFeO3 (BF) and its solid solutions are some of the widely studied ME systems in recent years. World over, the aim of the researcher has been to improve the ferroelectric and magnetic properties of BF along with improved coupling. Although both ferroelectric and magnetic properties have been demonstrated in several of these BF-based solid solutions. The ME effect in them is usually too small to be used in applications. Recently, a few studies on the Fe- and Ni-doped PbTiO3 have shown a convincing magnetoelectric effect at room temperature [15, 16]. The lead (Pb)-based compounds are highly toxic and non-eco friendly, which restrict their uses from application point of view. As an alternate to Pb-based compounds, bismuth titanate Bi4Ti3O12 (BIT) has gained a lot of attention [17]. Layered structure BIT belong to the Aurivillius family of compounds having general formula [Bi2O2]2+ [A n−1B n O3n+1]2− with n = 3 has low processing temperature than other bismuth layer-structured ferroelectric and a strong anisotropy of the spontaneous polarization (P s) along a-axis and c-axis [18, 19]. Bismuth layer-structured materials are of interest, since they have high Curie temperature, and exhibits physical properties suitable for non-volatile memory devices. However, the high conductivity of these materials at elevated temperatures often prevents the application of higher electric fields during higher temperature poling. The order of conductivity can be reduced in these materials by proper choice of substitution with rare-earth ions at the bismuth site. To our knowledge, there are few reports on multiferroic behaviour of BIT, although Lu et al. have suggested that the substitution of Ti by Fe in BIT will lead to ferromagnetism at room temperature [20]. Recently Chen et al. reported the multiferroic nature of BIT by substituting Fe at the Ti sites, but the system encountered a problem of high loss due to conduction [21, 22]. The volatile nature of Bi3+ ions creates the Bi vacancies accompanied by oxygen vacancies, which reduces the remnant polarization with high dielectric losses of BIT. It has been reported that substitution at the Bi-site by either La3+ or Nd3+ ions having smaller ionic radius than Bi3+ ion led to improved ferroelectric properties [23, 24]. Sm3+ion substitution has been also reported to improve the ferroelectric and magnetic properties of such layered structure compounds by suppressing the concentration of oxygen vacancies and canting of spins in the respective sub-lattices [25, 26]. Hence, in the present study, we have chosen rare-earth Sm3+ ion for substitution at the Bi-site in BIT, which have ionic radius smaller than Bi3+ ion. At present, single-phase multiferroic materials with large magnetoelectric effect are under active experimental investigations. In order to search for the materials exhibiting both the orders at room temperature with low value of dielectric loss, we have partially substituted magnetic cobalt (Co3+) ions at the Ti4+ sites to introduce magnetic properties in the purely ferroelectric compound, and at the same time rare-earth (RE) samarium (Sm3+) ions at the Bi3+-sites to control the high dielectric losses in addition to an improvement in the ferroelectric properties of BIT.

In this paper, we will investigate the multiferroic behaviour of Bi4−x Sm x Ti3−x Co x O12−δ (x = 0, 0.02, 0.05, 0.07) ceramics. The major aim is to introduce the magnetic properties with controlled dielectric loss in purely ferroelectric BIT and correlate it with the ME effect. Therefore, we utilized the solid-state reaction method to prepare the bulk ceramic samples for the structural, dielectric, electrical, ferroelectric, magnetic and magnetoelectric measurements.

Experimental details

Polycrystalline samples of Bi4−x Sm x Ti3−x Co x O12−δ (x = 0, 0.02, 0.05, 0.07, 0.1) ceramics were synthesized using standard solid-state reaction technique. Raw materials of Bi2O3, TiO2, Sm2O3 and Co3O4 with purity ≥99.95 % (Sigma Aldrich) were weighed in stoichiometric amounts. The resultant powder of each composition was ball milled for 5 h in the tungsten carbide jars for better mixing with a 3 wt % excess Bi2O3 to compensate for the bismuth (Bi) loss during post annealing. The milled powders were calcined at 700 °C for 6 h. The post calcined powders were then pressed into pellets of diameter 12 mm and thickness of 1–2 mm using hydraulic press applying pressure of 120 MPa. The pellets were finally sintered at optimized temperature of 850 °C for 4 h.

The structural analysis of the sintered samples was carried out with the help of powder X-ray diffractometer (X’Pert PRO PANalytical) using CuKα radiation (λ = 1.54 Å) over a range of angles (20° ≤ 2θ ≤ 60°) at a scanning rate of 2°/min at room temperature. In order to analyse the surface morphology of the sintered pellets, field emission scanning electron microscopy (FESEM) images were taken with the help of FEI Quanta FEG-450 electron microscope operating at an accelerating voltage of 20 kV. The elemental composition of each sample was carried out using energy dispersion X-ray spectroscope (EDX) attached (Bruker-Nano-X-flash detector 5030), respectively. Crystal structure distortions were studied using Raman spectroscopy (Reinshaw invia Raman microscope) equipped with Argon laser (λ = 514.5 nm) operated at 20 mW. Dielectric measurements as a function of frequency and temperature were carried out using LCR meter (Wayne Kerr 6500B) with high temperature furnace attached. The room temperature dc resistivity was measured by two probe methods (Keithley Electrometer 6221). Ferroelectric hysteresis loop measurements were performed using an automatic P-E loop tracer (Radiant Technologies). Room temperature magnetic measurements of powder samples were performed using a vibrating sample magnetometer (Lake Shore Model No. 662). Magnetoelectric effect was recorded using the laboratory assembled dynamic magnetoelectric coupling set up.

Results and discussion

Structural and morphological studies

Figure 1 shows the X-ray diffraction patterns of Bi4−x Sm x Ti3−x Co x O12−δ (x = 0, 0.02, 0.05, 0.07, 0.1) ceramics at room temperature. For x ≤ 0.07 samples all the diffraction peaks correspond to the pure phase of Bi4Ti3O12 having orthorhombic structure (JCPDS card No. 89-7500). At x > 0.07, an impurity peak at around 2θ = 27.89° (indicated by * in the Fig. 1) can be attributed to the formation of secondary phase and is identified as bismuth cobalt oxide (Bi25CoO40) matched with (JCPDS card No. 39-0871). The XRD pattern shows two closely spaced (020) and (200) peaks around 2θ = 33° in the orthorhombic phase of BIT, and on increasing the Sm3+ and Co3+ ions content in BIT, the relative 2θ difference between the peaks decreases with a slight shift towards higher angle side. These observations correspond to the decrease in the lattice parameters, as well as orthorhombicity [27]. It is also confirmed from the calculated values of lattice parameter, unit cell volume and orthorhombicity (δ = 2(a − b)/(a + b)) for all compositions listed in the Table 1. It can be clearly noticed that orthorhombicity (δ) decreases from 2.39 × 10−3 to 0.55 × 10−3 with the increasing content of Sm3+ and Co3+ ions from x = 0 to 0.07 in BIT, implying a relaxation of the structural distortions in the system. The observed relaxation in structural distortions may be attributed to different ionic radii of substituting Sm3+ (0.96 Å) and Co3+ (0.65 Å) ions compared to Bi3+ (1.03 Å) and Ti4+ (0.61 Å) ions of host sites. [28]. Further, the observed slight decrease in c-parameter with increasing Sm3+ and Co3+ content can be attributed to the rotation of TiO6 octahedron [29].

Fig. 1
figure 1

X-ray diffraction patterns of sintered Bi4−x Sm x Ti3−x Fe x O12−δ samples with x = 0, 0.02, 0.05, 0.07 and 0.1

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Table 1 Structural, electrical, ferroelectric, magnetic and magnetoelectric parameters of Bi4−x Sm x Ti3−x Co x O12−δ (x = 0.0, 0.02, 0.05, 0.07)
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Raman scattering study provides valuable information about local structures in the materials. The Raman spectra of Bi4−x Sm x Ti3−x Co x O12−δ (x = 0, 0.02, 0.05, 0.07) ceramic samples at room temperature are shown in the Fig. 2. Theoretically, there are 24 Raman active modes for orthorhombic BIT [30, 31]. In the present system, the Raman modes are observed at around 116, 144, 224, 268, 328, 357, 535, 564, 610 and 850 cm−1. The modes above 200 cm−1 have been assigned as the internal modes of TiO6 octahedra of BIT [30]. In addition, the formation of BIT with orthorhombic structure is identified by splitting modes at 193 and 224 cm−1, 535 and 564 cm−1. Although the mode at 224 cm−1 is Raman inactive according to the Oh symmetry of TiO6, it is often observed because of the distortion of octahedron. The suppression of the mode at 224 cm−1 with increasing substitution can be attributed to the decrease in distortion of TiO6 octahedra and hence the decrease of orthorhombicity [32]. The two bands in the frequency range between 500 cm−1 and 600 cm−1 and the other two bands at 328 cm−1 and 361 cm−1 tend to merge into one another with increase in Sm3+ and Co3+ content, which correspond to the vibrations of the O–Ti–O bending and Ti–O torsional modes [33]. The phonon modes at 116 and 144 cm−1 reflect the vibration of A-site Bi3+ ions in layer-structured pervoskite. The appearance of Raman mode at about 716 cm−1 becomes more prominent with increasing Co content and its frequency is in the high frequency phonon band of the octahedron with A 1g symmetry [34]. Hence, it is reasonable to believe that the new Raman mode resulting from the vibration of Co–O due to the substitution of Ti by Co in TiO6 octahedron. Moreover, the gradual increase in the intensity of the new Raman mode also confirms the cobalt substitution at the B-site in Bi4−x Sm x Ti3−x Co x O12−δ ceramic sample. Hence, the results obtained from Raman spectra correlate well with the XRD patterns.

Fig. 2
figure 2

Raman spectra of Bi4−x Sm x Ti3−x Co x O12−δ (x = 0, 0.02, 0.05, 0.07) at room temperature

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The FESEM micrographs of Bi4−x Sm x Ti3−x Co x O12−δ (0 ≤ x ≤ 0.7) are shown in Fig. 3. From the surface morphology, it is clearly observed that randomly oriented plate-like grains appear in the samples, and the grain size increases gradually with increasing Sm and Co content in BIT. Figure 4 shows the elemental composition of all the considered samples, which indicates that Sm3+ and Co3+ ions are well incorporated into the system. The weight and atomic percentage of each sample are listed in Table 2.

Fig. 3
figure 3

FESEM images of Bi4−x Sm x Ti3−x Co x O12−δ samples for a x = 0, b x = 0.02, c x = 0.05 and d x = 0.07

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Fig. 4
figure 4

EDX of Bi4−x Sm x Ti3−x Co x O12−δ samples for a x = 0, b x = 0.02, c x = 0.05 and d x = 0.07

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Table 2 EDX analysis of Bi4−x Sm x Ti3−x Co x O12−δ (x = 0.0, 0.02, 0.05, 0.07)
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Dielectric studies

Figure 5a shows the variation of dielectric constant (ε r) at room temperature as a function of frequency over the range 1 kHz–1 MHz for all the considered Bi4−x Sm x Ti3−x Co x O12−δ (0 ≤ x ≤ 0.7) ceramic samples. It is observed that dielectric constant of BIT without Sm3+ and Co3+ content decreases rapidly with frequency (up to 105 Hz) and remains constant at higher frequencies, indicating strong dielectric dispersion. The observed decrease in dispersion in dielectric constant with increase in both the frequency and Sm3+ and Co3+ content may be attributed to the possible reduction in the space charge effect. Due to volatile nature of Bi3+ ions, the defects related to oxygen vacancies are created during heat treatment of the material. Addition of Sm3+ improves dielectric properties of BIT as it suppresses the volatility of Bi3+ ions and reduces the oxygen vacancies in the system. The Bi and oxygen vacancies get trapped at sites like grain boundaries with the creation of space charges. No doubt, the oxygen vacancies should arise naturally to ensure charge neutrality when the Co3+ ions are substituted at the Ti4+ sites of the sample. The decrease in dielectric constant and loss tangent with composition and frequency indicates that the process of reduction in oxygen vacancies due to substitution of Sm3+ ions dominates over the process of their creation by the Co3+ ions in the system leading to overall decrease in the oxygen vacancies and hence the space charge effect. It is worth mentioning here that when the pure specimen is substituted with Sm3+ and Co3+ ions, a significant reduction in dispersion of dielectric constant can be clearly noticed at low frequencies (see Fig. 5a), such type of behaviour has been reported in Fe-doped PbTiO3 system by Verma et al. [35]. Hence substitution of Sm3+ ions plays the major role to reduce the defects and dielectric losses in the BIT system. Similar inference can be drawn from the frequency dependence loss tangent (tanδ) curves for these samples as shown in Fig. 5b, where the pure sample appears to be more conducting compared to the samples with Sm and Co content. The decrease in the interfacial or space charge polarization has been also supported by the results obtained from the FESEM (see Fig. 3), which reveals that the size of the grain increases with increasing content of Sm and Co in BIT. Moreover, the growth in the size of the grain reduces the volume fraction of the grain boundaries, leading to decrease in the space charge polarization. The dc resistivity (ρ) of the considered ceramic samples measured at room temperature (RT) also increases upon increasing the concentration of Sm3+ and Co3+ ions (see Table 1). All these results show that the dielectric properties of BIT system improve with Sm3+ and Co3+ ions substitution.

Fig. 5
figure 5

Frequency dependence of a dielectric constant (εr) and b loss tangent (tanδ) of Bi4−x Sm x Ti3−x Co x O12−δ (x = 0, 0.02, 0.05, 0.07) at room temperature

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Figure 6a and b shows the variation of dielectric constant and loss tangent as a function of temperature measured at 100 kHz. It is observed that the ferroelectric phase transition temperature (T c) decreases with increase in Sm3+ and Co3+ ions content (see Fig. 6a; Table 1), which may be attributed to the reduction of orthorhombic distortions in the present system with the substitution. [36].The temperature dependence of loss curves illustrates the decreasing trend in the loss tangent with increasing content of Sm and Co.

Fig. 6
figure 6

Temperature dependence of a dielectric constant (εr) and b loss tangent (tanδ) of Bi4−x Sm x Ti3−x Co x O12−δ (x = 0, 0.02, 0.05, 0.07) at 100 kHz

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Ferroelectric studies

Figure 7 shows the room temperature polarization–electric field (P–E) hysteresis loops of Bi4−x Sm x Ti3−x Co x O12−δ (0 ≤ x ≤ 0.7) ceramic samples observed at a fixed frequency of 50 Hz. It can be noticed that the shape of ferroelectric loop improves on increasing content of Sm3+ and Co3+ ions, indicating the reduction in losses related to conduction. This observation also compliments the results of our dc electrical resistivity measurements. On increasing the substitution, a significant enhancement in the remnant polarization (2P r) and coercive field (2E c) has been also observed (see Table 1). This clearly shows that the ferroelectric properties of BIT are improved significantly with the addition of Sm and Co content. It is evident, since Sm3+ and Co3+ ions substitution in BIT leads to a decrease in the orthorhombicity, which in turn weaken the segregation of vacancy related defects at domain walls and speed up the movement of the domain walls [37].

Fig. 7
figure 7

Ferroelectric hysteresis loops of Bi4−x Sm x Ti3−x Co x O12−δ (x = 0, 0.02, 0.05, 0.07) at room temperature

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Moreover, the grain size of Bi4−x Sm x Ti3−x Co x O12−δ ceramics increases gradually with increasing content of Sm and Co in BIT which leads to increase the domain variants [38]. Consequently, the volume fraction of grain boundaries reduces, thereby decreasing the probability of trapping space charge at the boundaries. Due to this, the pinning of neighbouring domains gets reduced, which makes domain reorientation easier [39]. This then leads to an increase in the domain alignment, and hence the remnant polarization.

Magnetization studies

Figure 8 shows isothermal magnetization (M) versus applied magnetic field (H) curves for all the samples measured at room temperature (300 K). An anti S-type curve has been obtained for pure (x = 0) sample, demonstrating the diamagnetic nature of BIT. Upon increasing the content of Sm and Co, the M-H curve gradually changes into symmetric S-type hysteresis loop (although non-saturating), indicating the origin of weak ferromagnetism. It might be speculated that the present magnetism originated from the Sm3+ and Co3+ ions that have successfully entered into the pseudo-pervoskite unit cell. The remnant magnetization (2M r) of the considered samples increases with increasing content of magnetic ions (see Table 1), which can be attributed to the decrease in interionic distances as revealed from XRD analysis and the enhancement in the strength of magnetic interaction. The origin of room temperature ferromagnetism in sample with x = 0.07 might be explained on the basis of following mechanisms; (i) The oxygen vacancies arise naturally to ensure charge neutrality, when Co3+ is substituted at the Ti4+-site, leading to Co3+–□–Co3+ network in the structure, where (–□– denotes oxygen vacancy). In these vacancies, an electron gets trapped having a down spin, which form the F-centre [22, 40]. Since Co3+, 3d6 also have unoccupied minority spin orbitals, so it might be speculated that F-centre exchange mechanism is also suitable for Co3+–□–Co3+ network [41]. The F-centre exchange mechanism was employed earlier to explain the ferromagnetism in Fe-doped PbTiO3 and Fe-doped BIT [22, 42]. The orbital of the trapped electron will overlap the d-shells of both the Co3+ ions having spin-up. The exchange interaction between the two neighbouring Co3+ ions via F-centre gives rise to direct ferromagnetic coupling. (ii) In another mechanism Co3+–O–Co3+ coupling might occur, which would favour the formation of ferromagnetic state. (iii) There is a possibility that cobalt may exist in the +2 valence state. This may lead to the emergence of Co3+–O–Co2+ group in the structure. The double exchange interaction existing in this group would also lead to ferromagnetic state [41]. To confirm the exact valence state of Co in the system further studies are required. The presence of Sm3+ ions in the system also contributes to the magnetism, because in pervoskite layered structure of bismuth titanate, the spins of Sm3+ ions in the sub-lattices are canted instead of being parallel. This leads to the weak canted antiferromagnetic behaviour. The extent of canting of spin structure increases with the substitution of Sm3+ ions [26]. Hence simultaneous substitution of Sm3+ and Co3+ ions leads to the enhancement in magnetization of the Bi4−x Sm x Ti3−x Co x O12−δ ceramic samples.

Fig. 8
figure 8

Isothermal magnetization hysteresis (M-H) of Bi4−x Sm x Ti3−x Co x O12−δ (x = 0, 0.02, 0.05, 0.07) samples at room temperature

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Magnetoelectric studies

The ferroelectric and magnetization studies reveal that both ferroic orders exist in Bi4−x Sm x Ti3−x Co x O12−δ ceramic for x = 0.02, 0.05 and 0.07 respectively. To observe the extent of coupling between these two ferroic order in the single-phase material, ME voltage across each sample under applied dc magnetic field was measured using laboratory assembled magnetoelectric coupling setup. All the measurements were recorded under a constant ac magnetic field (H ac = 3Oe) and at frequency f = 993 Hz. The ME coupling co-efficient (α = dE/dH), is calculated from the output voltage [43]. Figure 9 shows the variation of α as a function of applied dc magnetic field (H dc) at a fixed ac magnetic frequency 993 Hz for Bi4−x Sm x Ti3−x Co x O12−δ (x = 0.02, 0.05 & 0.07), measured at room temperature. The coupling co-efficient has shown a non-linear behaviour with the applied dc magnetic field for all the samples and is found to increase with Sm and Co substitution (see Table 1). The maximum value of coupling co-efficient α = 0.65 mV cm−1 Oe−1 has been recorded for x = 0.07. These results clearly demonstrate the existence of a coupling between the two order parameters. The optimized dc magnetic field at which maximum coupling occurs between the two ferroic order shifts towards higher field with increase in Sm and Co content, which is a critical parameter for the operation of multiferroic-based devices to maximize the energy conversion between electric and magnetic fields. In single-phase multiferroic materials, the ME coupling arises mainly from the interaction between electric and magnetic sub-lattices through the stress or strain transmitting from one sub-lattice to another [44]. In the presence of magnetic field, the strain-induced magnetic sub-lattice induces a stress on the electrical sub-lattice which is realized as ME output.

Fig. 9
figure 9

Magnetoelectric coupling co-efficient (α) as a function of dc magnetic field under H ac  = 3Oe at 993 Hz

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Conclusion

In summary, we have synthesized the Lead free, single-phase polycrystalline samples of Bi4−x Sm x Ti3−x Co x O12−δ (x = 0, 0.02, 0.05, 0.07) ceramics by standard solid-state reaction technique. The introduction of Sm3+ and Co3+ ions leads to the relaxation of the orthorhombic distortion in the system. The dielectric constant and loss tangent are found to decrease with substitution, which has been explained in terms of reduction of space charge effect. Substitution has significantly reduced the dispersion both in dielectric constant and loss tangent. There is a compositional decrease in ferroelectric phase transition temperature (T c), which may be due to reduction of orthorhombic distortions. It is observed that dc electrical resistivity, remnant polarization (2P r) and magnetization (2M r) increases with increasing Sm3+ and Co3+ contents. Magnetoelectric coupling co-efficient (α) value of 0.65 mV cm−1 Oe−1 has been found in Bi3.93Sm0.07Ti2.93Co0.07O12−δ ceramic sample. The present study clearly demonstrates the multiferroic behaviour of the Sm3+ and Co3+ ion-substituted BIT, which may find useful application in designing cost-effective electromagnetic devices such as multiple state memories and data storage devices.