Magnetic and Dielectric Properties of La and Ni Co-substituted BiFeO3 Nanoceramics

The increasing plead for the realization of ultra-fast, miniaturized, compact, and ultra-low power consumption in electronic as well as spintronic devices has propelled the quest for novel multiferroic materials that efficiently enable voltage control of magnetism. The present work reports the phase stability, magnetic and dielectric responses of polycrystalline Bi1−xLaxFe1−yNiyO3 (0 ≤ x ≥ 0.2 and 0 ≤ y ≥ 0.2) multiferroic ceramics synthesized through a simplistic sol–gel approach. The maneuver substitutions of La at A− site of BiFeO3 multiferroic eliminate the secondary phases formed owing to impurities. Rietveld refined XRD analysis reveals the structural transformation of the orthorhombic (Pbnm) phase as La substitution increases. However, an additional lattice distortion is induced as a result of the substitutions of Ni atoms at B− site. A substantial enhancement in magnetic and dielectric responses has been found in the co-doped (Ni and La) sample at both A and B− sites as a result of the size confinement of nano-crystallites, the exchange interaction between Fe3+ and Ni2+ ions, and corresponding variation in Fe–O–Fe bond angles. The dielectric constant has increased substantially in the low-frequency region with simultaneous substitutions of La and Ni at the sites of Bi and Fe, respectively. A careful observation of temperature-dependent magnetization curves (FC and ZFC) indicates a spin glass response with entangled ferromagnetic components. The experimental findings infer that the co-substitutions of La and Ni at their respective sites in Bi1−xLaxFe1−yNiyO3 (0 ≤ x ≥ 0.2 and 0 ≤ y ≥ 0.2) may significantly improve the ferromagnetic and dielectric responses of the studied nanoceramics.


INTRODUCTION
Multiferroics are among the special class of materials having ferroelectric and ferromagnetic phases simultaneously and being studied extensively in the recent past as they offer some of the most promising applications of technological importance [1][2][3][4][5]. Among various multiferroic materials, BiFeO 3 (BFO) has emerged as a potential material offering a wide range of viable applications in next-generation actuators, sensors, non-volatile memory devices (FeRAMs), and photovoltaics as it has many intriguing properties such as robust ferroelectricity, magnetism, and photovoltaic effects [6][7][8][9][10]. Furthermore, BFO allows the tuning of magnetic properties by applying an electric field, i.e., EM coupling (converse ME coupling), and leads to controlled permeability with minimum power consumption. Energy harvesting applications of multiferroics through photovoltaic has been a pioneering area of study by its own and the combination of other ferroic properties leads it to a new track [11][12][13][14]. BFO shows multiferroic responses at room temperature, with Curie temperature (1,103 K) and Neel temperature (647 K) [15]. BFO crystal exhibits rhombohedral distorted perovskite structure with R3c space group (a r = 3.96 Å and α r = 0.6 • ) [16]. Nevertheless, pure and bulk BFO has shown poor performance in various technological applications because of low saturation magnetization, high current leakage, and poor magnetoelectric response at room temperature [17]. Therefore, there has been a continuous effort to improve the multiferroic properties of BFO through controlling their size (in the realm of nanoscale) and thus results in the origin of sizedependent magnetic and electric properties [18,19]. On the other hand, the substitution of other atoms on either of A/B or A and B sites has proved to be another successful approach to alter the aforementioned properties. Despite tremendous advanced researches in the area of BFO, it is still difficult to develop a low-temperature synthesis protocol, which does not involve the formation of intermediate impurity phases [20,21]. These impurity phases have a great influence on magnetic, electrical, and other properties of the multiferroic materials. At the same time, these impurity phases have also been responsible for the large leakage current discussed earlier, owing to the hopping of electrons, weak magnetoelectric coupling, and low resistivity. In recent demonstrations, it was found that chemical substitution or doping has been an effective way to remove the impurities and thus enhancing its multiferroic properties. Aliovalent and isovalent substitutions at Bi and Fe sites have been attempted to overcome this drawback and to enhance the magnetic, electric, and magnetoelectric properties [20][21][22]. Some earlier studies report that doping of isovalent elements at Bi − sites eliminates or at least minimizes the formation of secondary phases [23]. It is also reported that it leads to a structural change as a result of imbalanced 6s 2 lone pair and further affects the intrinsic properties like ferroelectric and magnetic responses. However, isovalent substitution at Fe − site affects its magnetic properties without (or negligibly small) altering its ferroelectric behavior [24][25][26][27][28]. On the other hand, aliovalent doping at Bi or Fe site results to either fill or create oxygen vacancies [17,[29][30][31][32] and finally enhances the multiferroic properties of BFO. Doping of rare earth elements in BFO has been a subject of utmost importance as it leads to structural phase stability through the internal chemical pressure [33]. The small difference in atomic radii of rare earth dopant and Bi causes strong chemical pressure and thus affects the phase stability of the rhombohedral phase of doped BFO. In this context, La doping at Bi − site has been of particular interest because of (1) the large difference in ionic radii of Bi 3+ and La 3+ and (2) the increase in dielectric constant and decrease in leakage current [34]. Similarly, the aliovalent doping of various divalent at the Fe − site significantly improves the ferroelectric behavior of BFO [34]. Ni has been an effective candidate among various B − site substituents in BFO with better multiferroic response [35][36][37][38][39].
Herein, keeping the aforementioned considerations, we have successfully synthesized La and Ni co-substituted BFO multiferroic ceramics, at A and B Fe site lattices, respectively, through a facile sol-gel approach. These substitutions lead to a structural transformation from rhombohedral to orthorhombic phase. The evolution of the new phase opens a new avenue of manipulating the ferromagnetic and ferroelectric properties of BFO. Therefore, a systematic study of the magnetic and ferroelectric properties of La and Ni co-substituted BFO vis-a-vis the structural modifications has been conducted.

Chemicals
All the chemicals used in the present work were of analytical reagent grade and used as received without any further purification.

Synthesis
In a typical synthesis, a facile sol-gel method has been used to prepare Bi 1−x La x Fe 1−y Ni y O 3 (0 ≤ x ≥ 0.2 and 0 ≤ y ≥ 0.2) (BLFNO) powders. Briefly, stoichiometric amounts of the starting materials were dispersed in deionized water (100 ml), followed by a constant magnetic stirring at 200 rpm and 80 • C. An additional quantity of bismuth nitrate (∼7.5 wt.%) was added to indemnify Bi owing to its volatile nature. A few drops of concentrated HNO 3 were also added to avoid the precipitation of bismuth hydroxide. Furthermore, ethylene glycol (50 ml) was added in the solution which forms an organic 3D tangled network and resulted in a transparent blackish-red solution. The asobtained solution was further kept in an oven held at 150 • C, and a xerogel powder was thus obtained. The xerogel powder was ground gently and calcined at 300 • C for 4 h. The asobtained powder was pelletized in the rectangular form through a hydraulic press at a pressure of 3.5 tons per square inch. The pellets were sintered at 550 • C for 2.5 h and allowed to quench normally in the air.

Characterizations
The structural features and crystalline properties of the assynthesized powder samples were accessed through a powder Xray diffraction carried out by PANalytical X-ray diffractometer using CuK α radiation (λ = 1.5406 Å at 40 kV and 30 mA), with scanning in the range 2θ of 15 • to 80 • . The as-obtained XRD data were further analyzed by Rietveld refinement using FullProf Programme. The microstructures and morphologies of the as-synthesized materials were studied by transmission   [29,[40][41][42]. The formation of these impurity oxides and thereby substantial suppression with La doping at A site may be described by the fact that Bi 3+ and Fe 3+ ions have not been very stable in aqueous solution and get hydrolyzed in water rapidly. Oxide formation occurs fast   The XRD patterns of the studied Bi 1−x La x Fe 1−y Ni y O 3 samples also reveal that the peak splitting behavior decreases gradually with increasing x contents. Furthermore, the diffraction peaks (104), (110) and (006), (202) get merged to the respective single peaks, suggesting the distortion of the rhombohedral structure by La substitutions [40]. It is known that with the decrease in the average A − site ionic radius, rhombohedral to orthorhombic phase transition occurs in BiFeO 3 system because of the chemical pressure exerted by a relatively small rare earth (RE) substitution at Bi site [40]. Moreover, the co-substitutions of La and Ni at their respective sites lead to a continuous shift in 2θ values toward a higher angle as a result of a significant decrease in the unit cell parameters and the volume as evident from Table 1 also. Figure 2 depicts the Rietveld refined XRD patterns of the selective samples. The pseudo-Voigt function was used to fit the peak profile of pure BFO (space group R3c) and for La and Ni co-substituted samples (space group Pbnm) [40]. The observed, calculated, and difference for XRD patterns of Bi 1−x La x Fe 1−y Ni y O 3 have been found in fairly well-agreement. The tolerance factor τ (τ = (r A + r O )/ √ 2(r B + r O ), where r A , r B , and r O represent the ionic radii of the A, B, and O sites, respectively) has been calculated and summarized in Table 1. Ni substitution leads to a decrease in tolerance factor and thereby the driving force for octahedral rotation increases.

RESULTS AND DISCUSSION
The insight of the morphological aspects of Bi 1−x La x Fe 1−y Ni y O 3 (x = 0.0, 0.2 with y = 0.0 and 0.2) nanoceramic samples has been studied by employing scanning electron microscopy. Figure 3 represents the typical SEM micrograph of as-synthesized samples. It depicts the agglomeration of nanosized particles and, in turn, reveals that the progressive doping of La and Ni contents affects the morphologies of the sample. SEM micrograph of pristine BFO indicates that the grains have been rather non-uniform and the grain size varies in the range ∼100-150 nm. Moreover, the SEM micrographs of La and Ni co-substituted BiFeO 3 point that the particle morphology slightly changes with reduced grain size. It infers the effect on the crystallization habit of BFO particles by doping contents.
TEM studies have been carried out to further investigate the morphological aspects of   The temperature and field dependence magnetization of the as-synthesized samples have been investigated for assessing the magnetic behavior of the nanoceramics. Figures 5A,B traces the room temperature magnetization hysteresis (M-H) loop of the samples. The magnetization increases linearly with an increasing magnetic field, suggesting the existence of weak ferromagnetism arising as a result of canting of antiferromagnetic sublattices, for pristine BFO. The magnetic moment gets enhanced substantially with La doping at A − site in BiFeO 3 . Such enhancement has already been reported elsewhere [41,42]. Figures 5A,B indicates that magnetization changes with Ni − doping in Bi 1−x La x Fe 1−y Ni y O 3 (x = 0.2 and y = 0.05, 0.07, 0.10, 0.15, and 0.20) even in the low field range, i.e., up to 1 T. It can also be observed that the magnetization increases to a maximum for y = 0.10 and further decreases with a higher content of Ni. It is reported that the magnetization has been induced in BiFeO 3 by regulating the particle size smaller to the enduring spin periodicity of 62 nm as well as by chemical substitution [29,40,43]. In the present study, the particle sizes have been found close to the spin periodicity so the reduction in particle size plays a vital role in the enhanced magnetization. The enhancement in the magnetization of Bi 1−x La x Fe 1−y Ni y O 3 can also be attributed to the substitution of a small amount of Ni 2+ doping at B − site which perturbs the Fe-O-Fe bond angles ( Table 1). As the superexchange interaction has been sensitive to bond angles, the spiral structure may get destroyed completely by co-substitution of La 3+ and Ni 2+ , and thereby leads to the enhancement in magnetization [39].
The ZFC and FC curves of Bi 1−x La x Fe 1−y Ni y O 3 (x = 0.2 and y = 0.0, 0.05, 0.07, 0.10, 0.15, and 0.20) nanoceramics under applied magnetic field of 500 Oe reveal a non-monotonic temperature dependence and irreversibility. It also shows a distinct separation of up to 300 K between FC and ZFC curves for pristine BFO owing to the relaxation process [44]. Figure 5C shows that the separation increases with lowering the temperature, indicating the existence of some glass-like component, and has been consistent with the assumption that the magnetic properties of BFO ceramic get readily affected by the nanostructured nature [45].
The dielectric constant and dielectric loss of the as-synthesized Bi 1−x La x Fe 1−y Ni y O 3 nanoceramics have been investigated in the frequency range 100 Hz to 1 MHz at room temperature, for evaluating the dielectric performance. The dielectric properties of the samples in this frequency range are rarely available because of the persistence of leakage current. Figure 6A demonstrates the decrease in the values of dielectric constant with increasing frequency. This variation of dielectric constant points out the dispersion that occurred led by Maxwell interfacial polarization and has been in accord with the Koops phenomenological theory [46]. The dispersion of dielectric constant may be caused by the hopping of electrons between Fe 2+ and Fe 3+ ions. The large values of the dielectric constant in Bi 1−x La x Fe 1−y Ni y O 3 can be understood as the replacement of Fe 3+ by Ni 2+ that has been expected to introduce a higher density of vacancies and increase the probability of hopping conduction mechanism. The initial slow decrease in the dielectric constant has been consistent with the Koops model [46]. Moreover, the polycrystalline samples have high grain boundary density and so the nature of the grains and grain boundaries affects the electrical properties [47]. The inhomogeneity occurred because of grains and grain boundaries that lead to high dielectric value.
The room temperature frequency dependence of dielectric loss for Bi 1−x La x Fe 1−y Ni y O 3 nanoceramics is presented in Figure 6B. It shows that the dielectric loss decreases smoothly with the increase in the frequency. The dielectric loss increases for all the samples with decreasing frequency without having any loss peak. It signifies that the synthesized material is homogeneous and stoichiometric with uniform distribution of the grain/particles.

CONCLUSION
Summarily, Bi 1−x La x Fe 1−y Ni y O 3 nanoceramics have successfully been synthesized by ethylene glycol-based solgel method. The XRD pattern of the pristine BiFeO 3 reveals the rhombohedral distorted perovskite structure having space group R3c and substantial decrement in impurity phases has also been observed with La substitution at A − site in BiFeO 3 . Rietveld refinement suggests the induced lattice distortion with La and Ni co-substitution in Bi 1−x La x FeO 3 nanoceramics. Enhancement in the magnetic behavior has been observed with Ni contents in the sample as a result of the suppression of cycloidal spin structure arising from the size confinement effect, exchange interaction between Fe 3+ and Ni 2+ ions, and distortion in Fe-O-Fe bond angle. Notable enhancement in dielectric ordering and reduction in loss tangent have also been observed in the samples by the co-substitution of La and Ni in BiFeO 3 nanoceramics.

DATA AVAILABILITY STATEMENT
The datasets generated for this study are available on request to the corresponding author.

AUTHOR CONTRIBUTIONS
ASr performed the sample preparation, collected the experimental data, and completed the writing of the manuscript. ASi collected the structural data like XRD and analyzed it. OS, KM, and JS helped in the manuscript writing. HT was involved in data analysis and editing the draft. All authors contributed to the article and approved the submitted version.