Augmentation of optical and magnetic characteristics at ambient temperature in bismuth ferrite-titanium (BiFe1–xTixO3) multiferroic compounds

A facile sol–gel approach was utilized to synthesis BiFe1-xTixO3 multiferroic ferrites, denoted as BFO-xTi (with x from 0.00 to 0.10). XRD analysis revealed that the undoped BFO ferrite exhibited a rhombohedral structure, and a transition from the rhombohedral phase to the pseudo-cubic phase occurred as the Ti content increased. The optical band gap increased from 2.00 eV to 2.23 eV with a Ti substitution at 0.10 mol. The magnetic properties exhibited a transition from antiferromagnetic to ferromagnetic phase due to the structural phase transition and super-exchange interaction between the 3d orbitals of Ti4+ and Fe3+/Fe2+ at the B-sites through oxygen vacacies. The sample with a 0.10 mol Ti substitution demonstrated the highest M s and M r values of 3.68 emu g−1 and 7.21 × 10–3 emu g−1, respectively. Overall, the results suggest that the BiFe1-xTixO3 multiferroic ferrites with optimal composition hold promise for use in magneto-optical devices.


Introduction
Bismuth ferrite (BiFeO 3 , BFO) is a renowned multiferroics with significant antiferromagnetic and ferroelectric properties, characterized by a Néel temperature (T N ) of 370 K and a Curie temperature (T C ) of 830 K, respectively.Concomitantly, BFO exhibits considerable saturation polarization and magnetic moment, the former at 90 μC cm −3 and the latter ranging between 8 and 9 emu cm −3 [1].The ferroelectric behavior of BFO is attributed to the stereochemical activity of the Bi 3+ ion's 6s 2 lone pair, while its G-type antiferromagnetism is due to the superexchange interactions among Fe 3+ ions with half-filled 'd' orbitals [2,3].Additionally, BFO displays an optical band gap proximate to 2.0 eV, which is conducive to applications in magneto-optical devices that can manipulate magnetic fields using light for purposes such as data storage, communication, and sensing [4,5].
However, intrinsic limitations of bulk BFO, including secondary phase formation, high leakage current, and negligible magnetoelectric (ME) coupling at room temperature, as well as an absence of macroscopic remnant magnetization (M r ), restrict its multifunctional application potential [6][7][8].To overcome these challenges and enhance its dielectric, ferroelectric, and magnetic properties, recent researches have focused on the doping of the Bi-site and the Fe-site to improve BFO's multiferroic characteristics [9][10][11].Substitutions at the A-site (with Nd, Y, Gd) and the B-site (with Mn, Zr) have been found to diminish the particle size below 62 nm, consequently altering or inhibiting the material's magnetic spiral structure [12][13][14][15][16].
In this study, the sol-gel method has been employed to synthesis single-phase BiFe 1-x Ti x O 3 (BFO-xTi), with varying levels of titanium (Ti) substitution (x = 0.00, 0.02, 0.04, 0.06, 0.08, and 0.10).This particular substitution was chosen because the ionic radius of Ti 4+ (0.645 Å) closely approximates that of Fe 3+ (0.645 Å), which favors a distorted rhombohedral symetry with an R3c space group and facilitates significant polarization along the [001] hexagonal or [111] pseudocubic direction [17].Furthermore, it is anticipated that substituting Fe 3+ with Ti 4+ would decrease the leakage current density and enhance the ferroelectric properties of BFO [18] Ti 4+ acts as a donor within the oxygen octahedra framework, mitigating oxygen vacancy formation, and preventing the valence fluctuation from Fe 3+ to Fe 2+ [19].Prior investigations into Ti 4+ substitution for Fe 3+ have demonstrated that lattice strains and defects, resultant from the disparity in ionic radii, inhibit grain growth as the level of substitution increases.Comprehensive analyses of the structure, optical, and magnetic properties have unveiled novel behaviors in the single-phase Ti-doped BiFeO 3 , underpinning the improved multiferroic characteristics of the bismuth ferrite samples.

Experimental details
Nanoparticles of BFO-xTi where x = 0.00, 0.03, 0.06, 0.10 were synthesized utilizing a sol-gel process.The precursors employed were bismuth nitrate pentahydrate (Bi(NO 3 ) 3 •5H 2 O, 98% purity), iron nitrate nonahydrate (Fe(NO 3 ) 3 •9H 2 O, 99.9%), and tetraisopropoxytitanium (IV) (C 12 H 28 O 4 Ti).A 1:1 volumetric mixture of acetic acid (CH 3 COOH) and acetylacetone (CH 3 COCH 2 COCH 3 ) was used as the solvent system.Initially, Bi(NO 3 ) 3 •5H 2 O and Fe(NO 3 ) 3 •9H 2 O were solvated in a combination of acetic acid and distilled water until a clear solution formed.Subsequently, the acetylacetone was blended into the pre-prepared solution, followed by the addition of tetraisopropoxytitanium (IV).The resultant mixture was agitated continuously for about 6 h at ambient temperature, forming a transparent sol.This sol underwent thermal treatment at 100 °C, resulting in a dehydrated gel.The resultant gel was pulverized and subjected to calcination at 400 °C for 2 h, followed by crystallization at 800 °C for 3 h.Characterization of the nanocrystalline phase within the BFO-xTi was conducted using powder x-ray diffraction (XRD) on a D8-Advance Bruker AXS diffractometer equipped with Cu-Kα radiation (λ = 1.54048Å).To evaluate the morphology of the nanoscale samples, scanning electron microscopy (SEM) was employed.Absorption spectral analysis was performed using an ultraviolet-visible (UV-vis) system (Agilent8453, Palo Alto, CA).The bandgap energy, indicative of direct electronic transitions within the samples, was calculated applying Tauc's relation.The magnetic response, under the influence of an applied field, was assessed by vibrating sample magnetometry (VSM) using a Lake Shore 7407 instrument, at room temperature with a maximal field of 1 Tesla.structural morphology of BFO.The SEM analysis reveals that undoped BFO exhibits microscale grains displaying a hexagonal basal plane configuration with a distribution that is not uniform.Introduction of Ti is observed to result in a diminutive grain size, and the smaller hexagons exhibit increased porosity along with decreased density relative to pure BFO.Correlated with the structural findings, there is an observed trend of escalating internal microstrain concomitant with the rise in Ti substitution level.This phenomemon is accompanied by an elevation in internal energy and stress, a condition that could be attributed to either an expansion in the grain boundary area or a reduction in grain size [8].110) of the BFO-xTi samples progressively merge into a singular peak near 2θ of 31.6°, which is characteristic of the pseudo-cubic perovskite structure [3].Additionally, these peaks undergo a minor shift to the right toward a larger diffraction angle.This might be caused by the disparity between the radius of Ti ions and Fe ions, implying that the replacement of Fe ions by Ti ions could distort the crystal structure.When x further increases to 0.10, the diffraction peaks at 2θ of 31.6°transitioninto a single peak.This suggests a potential change from a rhombohedral structure to a pseudocubic structure.The change in structural phase can be attributed to the disparity in the ionic radius of the substituted ion and the host ion.The radii of cations used in this experiment located at the six-fold coordination site (octahedron site) are given as r(Fe 3+ ) (CN: 6) = 0.645 Å, r(Ti 4+ ) (CN: 6) = 0.605 Å [20,21].The difference in ion radii, calculated as r(Ti 4+ )−r(Fe 3+ ) = −0.040Å, implies that the substitution of Fe 3+ with Ti 4+ would lessen the unit cell volume, resulting in a shift of XRD peaks to the larger angles.This experimental outcome aligns with the research reported by Zalesskii and his co-workers [6].The use of MAUD software for Rietveld analysis on the XRD patterns of the samples verifies the development of a rhombohedral structure characterized by the R3c space group, as shown in figure S1 (in the Supporting Information document).The refined lattice parameters are detailed in tables S1.

Results and discussion
Furthermore, as the concentration of Ti dopant rises, there is a noticeable reduction in intensity along with a widening of the diffraction peaks around the 2θ of 31.6°.This indicates a decrease in the grain size of BFO-xTi resulting from the substitution with Ti.The Scherrer equation is utilized to calculate the average crystalline sizes of BFO-xTi using the (104)/(110) preferred orientations from the XRD data.The formula is represented as: where the constants D, K, λ, β, and θ represent the grain size, Scherrer constant (theoretical value of K is 0.9), wavelength, full width at half maximum (FWHM), and Bragg angle, respectively.As presented in table 1, the D value noticeably reduces from 68.4 nm to 48.4 nm with the increase in the Ti content.This observation completely matches the results obtained from the SEM images in figure 1.
Leveraging the sensitivity of Raman spectroscopy to the symmetry of crystal structures, this technique allows us to probe structural changes and phase transitions in BFO due to Ti doping.As illustrated in figure S3 (in the Supporting Information document), the Raman spectra for BFO-xTi (where x = 0.00, 0.04, 0.08) were recorded at room temperature, showcasing the Raman active modes.Based on group theory, the modified BiFeO 3 rhombohedral structure encompasses 18 optical vibration modes (4A1+5A2+9E), with A1 (TO) and E (LO) modes being Raman and infrared active, respectively, while the 5A2 modes are Raman inactive.Consequently, the deformed rhombohedral perovskite structure, categorized under the space group R3c, manifests 13 Raman active phonon modes (4A1+9E) that travel in both longitudinal and transverse directions [22]; specifically, four A1 modes along the c-axis (A1-1, A1-2, A1-3, and A1-4) and nine E modes in the x-y plane.Among these, the A1-1 and A1-2 modes demonstrate strong scattering intensity, in contrast to the weaker intensity of A1-3 and A1-4 modes, with the nine E modes exhibiting moderate intensity levels [23].However, resonance frequency and line shape vary by sample characteristics.Polarized phonon modes of A1-1, A1-2, and E1, as identified in prior studies, are linked to spin-phonon coupling, magnetic anisotropy, and electromagnetic coupling, respectively [23][24][25].The present analysis reveals subtle shifts in the intensity for modes A1 (corresponding to Bi-O bonds) and E modes (in the higher frequency region corresponding to Fe-O vibrations) with Ti doping.Notably, figure S3 highlights two pronounced peaks at approximately 139 and 172 cm −1 for the A1-1 and A1-2 phonon modes, with a diminished peak at 215 cm −1 for A1-3.Raman spectral analysis indicates a significant decrease in A1-1 mode intensity coupled with a noticeable increase for A1-2 and A1-3 modes as Ti doping rises to 0.04, suggesting alterations in spin-phonon coupling.The observed increase in the intensity of the A1-2 Raman mode with higher levels of Ti incorporation into BFO suggests that magnetic anisotropy is predominantly influenced by spin-dependent scattering and spin-phonon scattering mechanisms.With higher Ti substitution, we note a reduction in the E4 mode's intensity at 276 cm −1 , which also shifts to a lower frequency, eventually converging with the E3 phonon mode at 258 cm −1 .Concurrently, there's a gradual decline in the E8 mode's intensity at 532 cm −1 , whereas the E9 mode at 621 cm −1 experiences a significant rise in intensity.The changing intensity patterns in modes A1-2 and E4, alongside the fusion of the E4 mode with the E3 phonon mode at a reduced wavenumber and the contrasting trends between the E8 and E9 modes, hint at a structural shift towards a pseudo-cubic structure, echoed by our XRD findings.This transition is further supported by the influence of the smaller Ti 4+ ion radius compared to Fe 3+ ions, applying a compressive force on the BFO lattice's B site and leading to lattice distortion.These observations delineate a transition from a rhombohedral to a more symmetric pseudo-cubic structure (P4mm), aligning with the structural transformations detected through XRD analysis.
Figure 3 delineates the (a) ultraviolet-visible (UV-vis) absorption spectra and (b) Tauc plots of (αhv) 2 against the photon energy (hv) for BiFe 1−x Ti x O 3 samples with x ranging from 0.00 to 0.08.The absorbance profile reveals a prominent absorption peak near 450 nm, which is attributed to the electronic transitions between Fe and O ions in the lattice [26].This peak affirms that the synthesized BFO-xTi samples exhibit strong photoresponsive behavior within the visible spectrum, as discerned in figure 3(a).The determination of the  direct bandgap energy has been conducted by extrapolating the linear segment of the Tauc plots, as indicated in figure 3(b).Here, the fundamental bandgap energy of pure BFO (E g = 2.00 eV) involves the electronic transition from the valence band primarily consisting of O 2p states to the conduction band composed of Fe 3d states.This bandgap undergoes a noteworthy incremental shift to 2.23 eV upon the introduction of 0.10 Ti.
Comparable phenomena, such as an increased optical bandgap resultant from Mn doping in BFO, have been previously reported by Zhou et al which may be a manifestation of the Burstein-Moss effect [7,27].This effect is witnessed in semiconductors where doping raises the carrier concentration, filling the lower energy states and impulsing the Fermi level into the conduction band, thus enlarging the bandgap.In the context of BFO, Ti substitution for Fe injects additional electrons into the system, consequently elevating the Fermi level and augmenting the bandgap.It should also be considered that Ti addition might alter the local atomic environment, lattice structure, and electronic configuration, collectively contributing to the bandgap widening.An analogous escalation in the optical bandgap has been documented with Co doping in BFO due to a reduction of the state density in the valence band [8].It is remarked that the amplification in bandgap energy is not a consequence of quantum confinement, which is supported by the SEM imagery.
Therefore, it can be ascribed to: (1) the Burstein-Moss effect where states in proximity to the conduction band become populated, causing a bandgap shift towards shorter wavelengths (blue shift) with increasing Ti concentration, acting effectively as a donor [7,27]; (2) the cumulative effect of Ti substitution on BFO's band structure, which may be due to the modification of Fe-O bond lengths and Fe-O-Fe bond angles resulting in a decreased state density in the valence band [28].These alterations contribute to the shift in the fundamental absorption edge towards higher photon energies.The observed increase in optical bandgap with the incrementation of Ti doping signifies the potential application of these materials for UV and blue-greenactivated photocatalysts or in optoelectronic devices.
To elucidate the improved magnetization, we conducted a detailed examination of the XPS spectra for both pure BFO and BFO-0.08Tisamples, as depicted in figure S2 (in the Supporting Information document).This XPS analysis aimed to assess the oxidation states of the cations within these samples, thus revealing how changes in the valence state of the Fe ion influence the magnetic characteristics.The spectra in figure S2 present data for the different regions: (a) Bi 4f, (b) Fe 2p, (c) O 1s, and (d) Ti 2p for the BFO-0.08Tisample, and (e) Bi 4f, (f) Fe 2p, and (g) O 1s for the pure BFO sample.In both samples, the Bi 4f peaks, appearing at 4f 7/2 and 4f 5/2 with a gap of approximately 6.0 eV, affirm the Bi-O bond's presence and the Bi 3+ valence state [29].For the BFO-0.08Tiand pure BFO samples respectively, the Fe 2p spectrum is deconvoluted into two peaks representative of Fe 2p 3/2 and Fe 2p 1/2 at 709.63 eV and 723.14 eV, and 712.24 eV and 724.90 eV, signifying the coexistence of Fe 3+ and Fe 2+ ions [30].Notably, the ratio of Fe 3+ /Fe 2+ ions has increased from 0.56 in the BFO to 0.83 in BFO-0.08Ti,indicating a higher Fe 3+ ion concentration, which subsequently enhances the super-exchange interaction of Fe-O-Fe and, in turn, the magnetic properties [31].The spectra for O 1s in BFO-0.08Ti and BFO, shown in figures S2(c) and (g), indicate a redistribution of oxygen within the lattice and oxygen vacancies, with the ratio of vacancies to lattice oxygen notably decreasing from 0.79 to 0.38 [32].The Ti 2p spectrum in figure S2(d) showcases peaks at 458.22 eV and 465.81 eV, validating the presence of Ti 4+ ions due to the peaks' alignment with TiO 2 characteristics [33].Ti4+ replacements at Fe sites in BFO help to balance charge concentrations and regulate Fe 2+ /Fe 3+ valence fluctuations, effectively curtailing the formation of oxygen vacancies in the BFO framework [32].Thus, these results suggest that the increased concentration of Fe 3+ ions coupled with a reduction in oxygen vacancies are key contributors to the enhancement of BFO's structural intactness and its magnetic attributes.
Figure 4 depicts the magnetic hysteresis (M-H) loops of BFO-xTi (x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10) at room temperature, with the applied field's scope varying between: (a) −5000 Oe to +5000 Oe and (b) −65 Oe to +65 Oe.These curves allow extraction of essential magnetic characteristics such as magnetization at the highest field (M h ), coercive field (H c ), and remanent magnetization (M r ), which are then noted in table 2. Using the law of approach to saturation (LAS), we can determine the anisotropy constants by fitting the magnetization curves in high magnetic field regions (H ?H c ) based on the given relation (see figure 5) [28,34,35]: In this case, M(H) represents the magnetization at the applied field H.The parameters M s , a, b, and χ (representing saturation magnetization, inhomogeneity parameter, factor proportional to K 1 anisotropy constant, and susceptibility) are determined via the fitting method and documented in table 2 as well.BFO demonstrates magnetic hysteresis (M-H) loops characterized by poor ferromagnetic properties at ambient temperature, a phenomenon stemming from the antagonistic interplay between antiferromagnetic and ferromagnetic ordering.The antiferromagnetic characteristics of BFO are principally attributed to the direct exchange interactions facilitated by the B-O-B bonds, wherein B represents Fe 3+ and/or Ti 4+ ions [36].Empirical data for BFO, reflected in figure 4, reveals a nearly linear augmentation alongside minimal values of M s and M r on the magnetization curves, corroborating the material's antiferromagnetic nature.Notably, the magnetism of the BFO sample undergoes a transition towards weak ferromagnetism concomitant with the increased substitution of Ti.The addition of Ti has been found to significantly enhance the ferromagnetic properties of BFO at ambient temperature.With the introduction of Ti into BFO, there's a notable transition from antiferromagnetism to ferromagnetism.This substitution results in a marked improvement in both the saturation magnetization (M s ) and the remanent magnetization (M r ) of BFO-xTi, as detailed in table 2. Notably, the saturation magnetization for BFO-0.10Tiskyrockets to a value of 3.7 emu g −1 , which is more than a fivefold increase compared to the 0.7 emu g −1 measured for pure BFO.Despite this, the Mr values remain quite low, only increasing from 2.4 × 10 −3 emu g −1 for BFO to 7.2 × 10 −3 emu g −1 for BFO-0.10Ti.The coercive field in BFO-xTi samples decreases strongly form 40.5 Oe to 4.8 Oe with an increase of Ti-doped concentration.These figures compellingly demonstrate that BFO-xTi materials exhibit ferromagnetic characteristics at room temperature.The observed enhancement in magnetization is posited to originate from the super-exchange interaction involving Fe 3+ cations mediated through oxygen vacancies (Fe 3+ −V O ̈−Fe 3+ ).This interaction is believed to be an outcome of the stochastic dispersion of Ti 4+ cations throughout the native lattice structure of BFO.Consequently, the observed magnetization may be explained by both surface phenomena and intrinsic selfdefects, as referenced in the literature [37].Coey and colleagues have reported a substantial impact of the effective radius of hybrid orbitals on the interaction strength between magnetic ions through oxygen vacancies [38,39].Consequently, the ferromagnetic interactions within the BFO-xTi system are enhanced, facilitated by Ti 4+ cations which modulate the interaction and spacing of magnetic ions via oxygen vacancies.
Prevailing literature indicates that both Fe 3+ and Fe 2+ ions coexist within BFO matrices [19,29].The Fe 2+ ion is known to contribute to magnetic anisotropy, whilst the Fe 3+ ion is anticipated to exhibit isotropic behavior due to its electronic configuration.The observed reduction in coercivity within BFO-xTi samples upon Ti integration is attributable to an increased Fe 3+ /Fe 2+ ionic ratio coupled with a decline in the first-order magnetocrystalline anisotropy constant (K 1 ), as delineated in table 2, where K 1 is proportional to 'b'.Furthermore, studies by Sun et al. have demonstrated that the Fe 3+ /Fe 2+ concentration ratio escalates with progressing Ti substitution [19].And a bolstered Fe 3+ presence serves to augment the super-exchange interaction (Fe 3+ −V O ̈−Fe 3+ ), consequently ameliorating the magnetic properties [31].Lin et al observed the presence of oxygen vacancies within BFO matrices, positing that the substitution of Fe sites by Ti 4+ ions could offset charge concentrations and regulate Fe 2+ /Fe 3+ valence fluctuations [32].Here, it is suggested that an  increased concentration of Fe 3+ ions alongside prevalent oxygen vacancies plays a crucial role in defining the BFO framework and modifying its magnetic characteristics.Additionally, a phase mixture embodying rhombohedral and pseudo-cubic structures might also play a role in enhancing the magnetism of the BFO-xTi composite.x-ray diffraction analysis has provided evidence of a potential phase transition from a rhombohedral to a pseudo-cubic structure within the BFO-xTi system.Incorporation of Ti, with its comparatively smaller ionic radius, suppresses the intrinsic helical cycloid spin structure endemic to BFO, thereby aiding in unlocking its latent magnetic potential [40].This structural transformation, initiated by Ti substitutions, modifies the Fe-O bond angles and lengths, as well as introducing slight distortions in FeO 6 octahedra proximate to a cationic vacancy.Such distortions involving Fe 3+ ions within FeO 6 octahedra effectively disrupt the long-range cycloidal spin structure, acting as sources for enhanced magnetic performance [32].Hence, Ti substitution prompts alterations in Fe-O bond lengths and distortions within FeO 6 octahedra which ultimately suppress the extended cycloidal spin configuration and bolster the intrinsic magnetization potential [40].

Conclusion
Using the sol-gel technique, BiFe 1-x Ti x O 3 multiferroic ferrites, identified as BFO-xTi with x values from 0.00 to 0.10, have been successfully synthesized.XRD data was evident that BFO-xTi transitioned from a rhombohedral structure to a pseudo-cubic phase when Ti 4+ replaced Fe 3+ .An increment in Ti content led to smaller crystalline sizes, attributed to the raised compressive stress within the BFO crystal lattice.An incremental Ti doping saw a slight rise in the bandgap energy, from 2.00 eV to 2.23 eV.Pure BFO exhibited modest ferromagnetic properties, with saturation magnetization (M s ) at 0.7 emu g −1 and remanent magnetization (M r ) at 2.4 × 10 −3 emu g −1 , but these values surged to 3.7 emu g −1 for M s and 7.2 × 10 −3 emu g −1 for M r with 0.10 Ti substitution.The enhancement in M r and M s associated with Ti doping may be derived from an amplified Fe 3+ −V O ̈−Fe 3+ superexchange interaction and/or the blend of rhombohedral and pseudo-cubic phases interrupts the spiral modulation of spin.These outcomes highlight the promise of BiFe 1-x Ti x O 3 multiferroic ferrites for applications in technologies reliant on visible-light photocatalysis and magneto-optical features.

Figure 1
Figure 1 presents SEM micrographs of BiFe 1-x Ti x O 3 nanoparticles with varying Ti concentrations (x = 0.00, 0.02, 0.06, 0.08) at a magnification scale of 20 μm.These images elucidate the influence of Ti doping on the

Figure 2 (
a) depicts the XRD patterns for the BFO-xTi powder samples where x ranges from 0.00 to 0.10, whereas figure 2(b) highlights the enlarged XRD patterns at 2θ from 30°-33°.XRD patterns measured with the step size of 0.0202 deg./step and the scan rate of 0.04 deg./second.The illustrations suggest that the samples are pure single-phase.Pure BFO powder samples as revealed by the XRD patterns possess a rhombohedral perovskite structure (R3c) identifiable by the double peaks (104)/(110) and (006)/(202) at 2θ angles of 31.6°and39.0°, respectively.This finding agrees with the data from the International Centre for Diffraction (JCPDS File no.01-086-1518) [2].

Figure 4 .
Figure 4. (a) Room temperature magnetic hysteresis loops (M-H) for BFO-xTi under a magnetic field from −5 kOe to 5 kOe, (b) Magnetization of samples focused in low magnetic fields from −65 Oe to 65 Oe, (c) The comparison of the M s and M r values in this study to those in previous research.

Table 1 .
The bandgap energy (E g ) and grain size (D) of the samples.

Table 2 .
The values of coercivity (H c ), remanent magnetization (M r ), saturation magnetization (M s ) and the fitting parameters of LAS curve for BFO-xTi.