Enhanced electrical, optical and magnetic properties of BiFeO 3 perovskite nanoparticles co-doped with Y and Cu

The present work includes the fabrication of pure BiFeO 3 and Bi 0.9 Y 0.1 Fe 1 − x Cu x O 3 (x = 0.05, 0.10, 0.15) nanoparticles by the usual sol-gel route. The characterization of structural, morphological, optical, electrical, and magnetic properties of the synthesized samples was achieved through x-ray diffraction spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, an impedance analyzer, and a vibrating sample magnetometer. The Fourier transform infrared study confirmed the presence of Bi–O and Fe–O bonds. The x-ray diffraction patterns confirmed the rhombohedral (R3c) structure as a major phase with the orthorhombic (Pnma) structure as a secondary phase in all samples, and few amount of impurity phase of Bi 25 FeO 40 was also determined. The surface morphology analysis showed the variation of particle size from 63 to 174 nm. The Raman spectra of 13 optical phonon modes, including 4A 1 and 9E symmetric phonons, are observed for all samples, and the positions of these modes are almost similar, except the intensities of A 1 modes. The optical measurements show a reduction in bandgap from 2.17 to 2.03 eV with co-doping, revealing itself as a more promising applicant in photo-voltaics. An enhanced value of the dielectric constant is observed for different doped samples at different frequency regions from 100 Hz to 10 MHz. Magnetic investigation demonstrates the improved ferromagnetism with co-doping of Y and Cu due to distortion in the FeO 6 octahedron. A maximum saturation magnetization of 0.26 emu/gm and a coercivity of 2915 Oe were found for Y concentration of 10% and Cu concentration of 15%.


I. INTRODUCTION
2][3] The potential to create new types of multifunctional devices by influencing charges with applied magnetic fields and spins with applied voltages is what motivates the quest for these materials. 4,5However, such types of substances are quite uncommon because, in the case of ABO 3 perovskite, ferroelectricity requires transition metal ions with d 0 electron arrangement.On the other hand, a partially filled d orbital of transition metal ions is needed for ferromagnetism.The requirements as mentioned above are mutually exclusive for the emergence of ferromagnetic and ferroelectric order in a single phase. 6Among a few known single-phase multiferroic materials at room temperature, BiFeO 3 (BFO) is the most prominent from the perspective of practical applications.Its high G-type antiferromagnetic Néel temperature (T N ∼ 643 K) and high ferroelectric Curie temperature (Tc ∼ 1103 K) as well as unique electrical and optical properties bring out the fascination of several researchers. 3,7Other ARTICLE pubs.aip.org/aip/advforms of bismuth ferrite also serve as vital productive materials.For instance, Bi 2 Fe 4 O 9 is a potential semiconductor gas sensor and an effective catalyst for oxidizing ammonia to NO. 8 The rhombohedral distorted structure with the space group R3c for perovskite ABO 3 (A = Bi; B = Fe) nanoparticles have been reported. 9In bulk BFO, magnetic ordering is antiferromagnetic with a spatially modulated spiral spin.The spin canting due to the exchange interaction along with spin-orbit coupling prevents complete ferromagnetic ordering with the moment rotating with a wavelength of 62 nm, creating a helical magnetic arrangement along with a zero net macroscopic magnetization. 10However, recent studies show weak ferromagnetism for BFO nanowires, nanoparticles, and thin films due to the suppression of spiral spin structure.In addition, BFO has a large leakage current induced by defects caused by impurities and oxygen vacancies, which has made it difficult to use in multiferroic devices. 113][14] Moreover, it is shown that BFO has appealing prospects as photovoltaics 15,16 and photocatalysts. 17arrier exciton in BFO is possible with the available femtosecond laser pulses because of its small bandgap (∼2.2 eV), thereby aiding in the development of ferroelectric ultrafast optoelectronic devices. 18till now, the synthesis of crystalline BFO in a single phase is an intimidating task.0][21] To improve its properties and phase purity, numerous studies with different synthesis processes and different types of doping introduced in BFO have previously been reported by researchers.Doping with Li 3+ , Eu 3+ , and Gd 3+ at the A site of Bi 3+ ions has enhanced the ferroelectric and magnetic properties, [22][23][24] whereas doping with Sc 3+ and Co 3+ at the B site of Fe 3+ has shown enhancement in ferromagnetic properties. 25,26Recently, some investigations have been done with simultaneous substitution of Bi and Fe in BFO by Y and Mn or La and Co, showing the increased electrical, magnetic, and optical properties as well. 27,28Luo et al. reported Y 3+ replacement at the Bi site of BiFeO 3 NPs synthesized by the sol-gel route and showed improved ferromagnetism and effectively suppressed leakage current without the presence of impurity phases. 29Besides that, Agrawal et al. demonstrated an enhanced magnetization and an increased bandgap by Cu doping at the Fe site of BiFeO 3 . 30Therefore, co-doping of Y 3+ and Cu 3+ ions in place of Bi 3+ and Fe 3+ ions, respectively, may lead to simultaneous improvement in magnetic, optical, and electric properties.From that ground, we synthesized the novel multiferroic BiFeO 3 NPs with Y and Cu co-doping and reported the effects on structural, optical, electrical, and magnetic properties of the prepared NPs.

A. Synthesis
Y, Cu co-doped BiFeO 3 samples with the general formula BiFeO 3 and Bi 0.9 Y 0.1 Fe 1−x CuxO 3 (x = 0.05, 0.10, 0.15) were prepared by the sol-gel route.The precursor solution of 0.1M BiFeO 3 was prepared by incorporating an appropriate amount of Bi(NO 3 ) 3 ⋅ 5H 2 O and Fe(NO 3 ) 3 ⋅ 9H 2 O with the molar ratio of Bi:Fe = 1:1 in 50 ml distilled water under continuous stirring at 80 ○ C to obtain a clear solution.To prevent the precipitation of bismuth hydroxide, a few drops of concentrated HNO 3 were added.In addition, 25 ml of ethylene glycol was added to serve as a binding agent; after being continuously stirred at 80 ○ C, the transparent solution was entirely transformed into a brownish gel.Afterward, the gel was dried in an oven at 120 ○ C for 14 h and ground into a powder.Later, the powder was calcined in air ambient at 650 ○ C for 4 h and ground once again.The Y and Cu co-doped samples with different concentrations were prepared by substituting the stoichiometric amount of Y(NO 3 ) 3 ⋅ 6H 2 O and Cu(NO 3 ) 2 ⋅ 3H 2 O keeping all other experimental conditions as before.For convenience, the nominal compositions of BiFeO

B. Characterization
The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were accomplished with Pyris Series-STA-8000 by heating the BFO powder sample in the temperature range 30-700 ○ C at a rate of 20 ○ C min −1 under air ambient to determine its phase transition temperature.The chemical bonds in the calcined NPs were studied through Fourier transform infrared (FTIR) spectra using a PerkinElmer 100 spectrometer.The crystalline phases and structural characterization of the powder samples were investigated by x-ray diffraction (XRD) (Thermo Scientific ARL EQUINOX 1000) study with Cu-Kα (λ = 1.5406Å) radiation.The surface morphology of the particles was studied using a JSM-7610 field emission scanning electron microscope (FESEM).The samples were analyzed through a MacroRAM Raman spectrometer.The optical bandgap of the samples was measured using a UV-visible spectrophotometer (Shimadzu UV-2600).An impedance analyzer (Agilent 4294-A) was used to evaluate the dielectric behavior of the synthesized NPs.Before dielectric measurement, pellets of 6 mm diameter and 1 mm thickness were fabricated by compressing powders in a uniaxial press.Finally, the magnetic properties were explored using a Lakeshore 8604 model vibrating sample magnetometer (VSM) with variation of the magnetic field at room temperature ranging from 0 to ±1.5 T.

A. Thermal stability
The thermal stability of the synthesized parent sample (BiFeO 3 ) has been confirmed by thermogravimetric analysis in the temperature range of 30-700 ○ C. Figure 1 shows the TGA and DTA curves of the synthesized BiFeO 3 powder sample, and a decomposition of the ingredients is observed due to the thermally activated chemical reaction.However, this decomposition follows several steps with increasing temperature, and the first step of 8.7% of weight loss is observed between 30 and 125 ○ C, which is attributed to the loss of physisorbed water.
Upon further heating from 125 to 340 ○ C, the evaporation of the remaining solvent and the crystallization process are ascribed to a considerable weight loss of 10.8% as a second step due to the loss of surface hydroxyl groups and nitrate species and decarbonization. 8eyond this step, a plateau region with a final step of 2.5% weight loss is observed, which indicates that the sample has presumably entered an ordered state at 700 ○ C.This plateau is slightly inclined toward weight loss, but the sample remained significantly unchanged and 78% of residue remains after thermal analysis in the overall range of temperature.In the DTA curve, there was an endothermic peak at 85 ○ C, and then, the plateau was reached at ∼490 ○ C.However, the endothermic peak at 621 ○ C indicates the phase formation temperature.Therefore, the calcination of all synthesized BFO and BYFCO samples has been performed at 650 ○ C expecting the formation of crystalline structure.

B. FTIR analysis
Fourier-transform infrared spectroscopy (FTIR) analysis is a useful spectroscopic technique to know the structure formation as well as information about the redistribution of cations between the perovskite structure of BiFeO 3 and its products.Figure 2 shows the FTIR spectra ranging from 250 to 4000 cm −1 recorded for the synthesized compositions of BiFeO 3 and Bi 0.9 Y 0.1 Fe 1−x CuxO 3 (x = 0.05, 0.10, 0.15), and an almost pure phase of the composition is confirmed due to calcination at 650 ○ C.However, the broadband at ∼3460 cm −1 corresponds to the stretching bond of the H 2 O and OH − groups originating from ethylene glycol or condensation products. 8,31In addition, the band near 1630 cm −1 corresponds to the H 2 O bending vibration.The band at ∼1381 cm −1 is attributed to the stretching vibration bands of NO 3 ions, which are prominent for the higher substitution of Y and Cu in the parent BiFeO 3 sample. 32The absorption band at ∼1195 cm −1 corresponds to the stretching of C-O bonding. 33The peak at ∼755 cm −1 is originated due to the presence of C-H bonding.The strong absorption band between 700 and 400 cm −1 was primarily responsible for the formation of metal oxides.It is attributed to the stretching and bending vibration modes of the Fe-O bond, which are the features of the octahedral FeO 6 groups in the perovskite substances. 8Moreover, the Bi-O absorption band remains within this range.The presence of a metal oxygen band serves as a confirmation that the perovskite structure has formed. 8In our case, the strong peaks around 565 cm −1 in all the crystallites are assigned to the mode of stretching vibrations along the Fe-O axis.7][38] The band at around 1110 cm −1 is also attributed to the vibration of the Bi-O bond. 39Another absorption peak is found at ∼2068 cm −1 for all samples, which is due to the stretching vibrations from the C≡C bond of alkynes. 40

C. Structural analysis
The X-Ray Diffraction (XRD) patterns for BiFeO 3 and Bi 0.9 Y 0.1 Fe 1−x CuxO 3 (x = 0.05, 0.10, 0.15) have been recorded at room temperature (RT-25 ○ C) for 20 ○ ≤ 2θ ≤ 70 ○ .Figure 3 shows the XRD peaks of all the studied samples, and the data have been analyzed by Rietveld refinement 41 using the FullProf software, version 2.05.The regular arrangement of the peaks of (012), (104), (110), (006), (202), (024), ( 116), (122), (018), (214), (208), and (220) confirms the formation of crystalline BFO and BYFCO with the rhombohedral structure (JCPDS card number 71-2494), which corresponds to R3c space group. 42n addition, the refinement confirms the presence of another secondary crystalline phase of BFO and BYFCO that matches the orthorhombic structure of the Pnma space group. 43This secondary orthorhombic phase is dominating in the samples BYFCO-2 and BYFCO-3 as the intensities of the peaks at 2θ ≈ 25 ○ are increased (Fig. 3) due to Y and Cu doping in BiFeO 3 .Such a phase is also observed in the experimental XRD patterns reported by Medina et al. 44 for different Y-concentrations in BiFeO 3 composition.However, Fig. 3 includes some extra peaks to the desired spectra of XRD patterns for the synthesized samples, and the peaks are associated with an impurity phase of Bi 25 FeO 39 that is isostructural to γ-Bi 2 O 3 . 45,46An additional peak of another impurity phase of Bi 2 Fe 4 O 9 (Fe-rich compounds) has been identified at 2θ = 48 ○ for BYFCO-2 and BYFCO-3 samples. 47The low-temperature stability of impurity phases and the metastable and off-stoichiometric nature of BFO are thought to be the prime causes for the formation of these secondary phases.
The unit cell structure of the synthesized BFO, BYFCO-1, BYFCO-2, and BYFCO-3 samples have been extracted for the major phase (R3c) utilizing the Rietveld refined data in a 3D visualization program of the VESTA software, version 3.5.8. Figure 4(a) represents the unit cell of BFO and BYFCO samples with a sixfold coordination of Fe atom by O atom in each composition, and the structure contains 101 atoms, 144 bonds, and 24 polyhedra.From a close observation of the magnified XRD peak at 2θ ≈ 31.6 ○ in Fig. 4(b), it is observed that the peaks are shifting toward the higher values of 2θ, which is an indication of the structural alternation between the R3c and Pnma space groups. 13,48However, the phase percentage (%) has been calculated using the Rietveld analysis reported earlier 49,50 as the phase fraction Wp is expressed by the following equation: 51 where S, Z, M, and V express the Rietveld scale factor, the number of formula units per unit cell, the mass of the formula unit, and the unit-cell volume, respectively.All of the refined data are given in Table I. ).Furthermore, the substitution of Y 3+ cation at the site of Bi 3+ and Cu 3+ cation at the site of Fe 3+ makes the shift of (104) and (110) plane toward the higher angle, which causes the decrease in lattice constant.This is because the size of Y 3+ (1.04 Å) and Cu 3+ (0.54 Å) is smaller than Bi 3+ (1.17 Å) and Fe 3+ (0.69 Å), respectively. 52he crystallite size (ζ) has been determined from the full width at half maximum (FWHM), β, and the Bragg position (θ) as given by the Scherrer formula, 46 where k is the dimensionless constant with a typical value of ∼0.9 and λ is the wavelength of Cu Kα radiation with the value of 1.5418 Å.The value of FWHM (β) was evaluated by the Gaussian fitting of the peaks (104) as it is prominent and almost common in all the samples.The variation of lattice parameters (a and c) and crystallite size (ζ) is illustrated in Fig. 5, and it is observed that all the values gradually decrease due to doping Cu content in the BFO sample.In addition, the lattice strain (εL) was calculated using the following relation:  where c is the lattice constant (c = λ/sin θ) and c 0 is the lattice constant of the ideal strain-free sample, which is 5.20 Å.The dislocation density, δ, of all the samples was estimated using the following relation: The values of εL and δ are reported in Table I, where both the values are higher due to the substitution of Y 3+ at the site of Bi 3+ and Cu 3+ at the site of Fe 3+ .

D. Microstructural analysis
The grain morphology of BFO with co-doping was investigated by FESEM micrographs, and particle size variation is represented by histogram, which are depicted in Figs. 6 and 7, respectively.The undoped BFO shows an average particle size of 63 nm, which is close to the crystalline size calculated by the Debye-Scherrer formula.
For the doped samples, the particle size became 104, 161, and 174 nm for BYFCO-1, BYFCO-2, and BYFCO-3, respectively.That is, the particle size has increased with an increase in doping content, while Table I shows the gradual decrease in crystallite size (ζ) for all samples compared to undoped samples.The particle size would decrease with an increase in doping content for the replacement of Bi and Fe with ions of smaller ionic radius, 52 which proposes the presence of particle agglomeration. 53Irregular grain morphologies have been observed, which could be a sign of the existence of impurity phases.

E. Raman analysis
The lattice dynamics and structural transition due to cationic disorders have been evaluated from Raman spectroscopy for BiFeO 3 and Bi 0.9 Y 0.1 Fe 1−x CuxO 3 (x = 0.05, 0.10, 0.15) samples.Figure 8 shows the room temperature Raman spectra for the studied samples, and the measurement range was 80-800 cm −1 .In the Raman spectra, the positions of the Raman peaks were fitted by using a Lorentzian profile, and the positions of each Raman mode are given in the top right corner of the image.The Raman mode positions are in good agreement with the reported literature data. 54The peak at ∼144 cm −1 can be assigned to the first normal A 1 mode for the rhombohedral BiFeO 3 system.Group theory analysis predicts that rhombohedral R3c BFO has 13 optical phonon modes, including 4A 1 and 9E symmetric phonons, which are Raman active modes. 55In the present investigation, all studied samples correspond to all of these phonon modes as indicated in Fig. 8.The

F. Optical properties
The (αhv) 2 vs photon energy for pure and doped BFO samples is displayed in Fig. 9, which was created by applying the Kubelka-Munk function 56 to diffused reflectance data.The corresponding energy gap for all samples was calculated from Tauc's relation: 57 αhυ = A(hυ − Eg) n , where α is the absorption coefficient, hν is the absorption energy, A is a constant, which does not depend on photon energy, Eg is the optical energy gap, and n is 1/2 for direct bandgap semiconductors.The linear extrapolation of the (αhν) 2 on the energy axis is implemented to determine the optical bandgap of the prepared samples, which is the difference in energy across the top of the valence band and the bottom of the conduction band. 58he optical bandgap of undoped BFO was 2.17 eV, whereas for both BYFCO-1 and BYFCO-2, the bandgap was found to be 2.11 eV; thereafter, with a further increase in Cu, i.e., for the sample BYFCO-3, the bandgap decreased to 2.03 eV.This decreased bandgap in the doped BFO samples could be due to several factors, including structural deformation due to the size mismatch of the dopants.In particular, the substitution of a smaller A-site cation than Bi causes the change in the Fe-O bond length, and the Fe-O-Fe bond angle causes modifications in the one-electron bandwidth and leads to a reduction in the bandgap of BFO. 59In Table I structure and also a decrease in the Fe-O bond length.Therefore, the decrease in the bandgap of the BYFCO-3 sample is attributed to the increase in the Fe-O-Fe bond angle (ϕ) and the decrease in the Fe-O bond length.Therefore, for all the synthesized samples, the bandgap remains within the visible light region, and the reduced bandgap for BYFCO-3 indicates its potential for application in photovoltaics and optoelectronic devices.

G. Dielectric measurements
The plots of the real part (ε ′ ) and imaginary part (ε ′′ ) of the dielectric constant (permittivity) with frequency from 100 Hz to 10 MHz at ambient temperature are shown in Figs.10(a) and 10(b), respectively.In addition, the variation of dielectric loss tangent for the same frequency range is shown in Fig. 11.
The dielectric constant, ε ′ , was calculated from the frequencydependent capacitance values by using the following equation: , where C is the measured capacitance, A is the pellet crosssectional area, d is the pellet thickness, and εo is the free space permittivity.As shown in Fig. 10, different doped samples predominate at various frequency ranges for both the real and imaginary parts of the dielectric constant.The values of the real part have increased for doped samples compared to pure BFO except for Y and Cu concentrations of 10% (BYFCO-2), which shows a maximum value up to 5.5 kHz.It shows a continuously decreasing trait until 100 kHz and afterward shows nearly constant values.This low-frequency dispersion can be described from the context of space charge polarization discussed by Maxwell and Wagner (M-W).The space charges at low frequencies can follow the field frequency.However, at higher frequencies, they undergo relaxation. 60In the case of BYFCO-1 and BYFCO-3 samples, the values of the real part of permittivity are increased compared to undoped   Frequency (Hz) BFO BYFCO-1 BYFCO-2 BYFCO-3 BFO.This behavior indicates a higher dielectric response, which might be ascribed to the presence of oxygen vacancies and displacement of Fe 3+ ions. 61 II.Dielectric loss tangent shows the energy dissipation in a dielectric system.Figure 11 depicts the dielectric loss for all the synthesized samples, which is the ratio of the imaginary part to the real part of the dielectric constant.From Fig. 11, it is observed that dielectric loss tangent for all the samples have very low values except BYFCO-2, which shows a broad and intense peak at ∼100 kHz.This kind of relaxation is induced by point defects, and therefore, its peak intensity is directly proportional to the defect density and can be modified by annealing in oxidizing atmospheres. 62
All samples exhibit saturated hysteresis loops with finite values of saturation magnetization (Ms) because of the weak ferromagnetic nature of the samples.Previous reports have shown antiferromagnetic order of bulk BFO, 63 whereas we found ferromagnetic ordering in BFO nanoparticles.This ferromagnetic ordering could be due to the structural dissimilarities of the nanoparticles and the inherent spin arrangement. 3,20,64The M-H curve reveals that the undoped BFO NPs are somehow ferromagnetic, while the Y and Cu substitutions made the rest of the samples more ferromagnetic.The magnetic properties of ferrites are strongly influenced by the cationic distribution in the perovskite structure.The A-A, A-B, and B-B superexchange interactions are responsible for magnetization in spinel ferrites. 65The enhanced magnetism could be attributed to the increase in the superexchange interactions.This is due to the modifications in the bond length and bond angle of Fe-O and Fe-O-Fe by Y and Cu co-doping in BFO, which may affect the FeO 6 octahedron tilting angle consequently suppressing modulated spin structure. 66Table III shows that a maximum saturation magnetization, Ms, of 0.26 emu/gm was found for BYFCO-3.Afterward, Ms slightly decreased to 0.24 emu/gm for BYFCO-2, and then for BYFCO-1, it became 0.20 emu/gm.These results are concomitant to the microstructure and particle size (Figs.6 and  7) as the average particle size is largest for BYFCO-3 compared to other studied samples.Since the larger particle size implies a smaller surface-to-volume ratio, the compensation of magnetic moments at the surface of BYFCO-1 and BFO is much higher than that of BYFCO-2 and BYFCO-3.Therefore, the net magnetization has increased in BYFCO-2 and BYFCO-3.However, the lower magnetic moment of Cu 3+ (3.87 μ B ) compared to that of Fe 3+ (5.92 μ B ) contributes to weakening the magnetic saturation (Ms) and remnant magnetization (Mr), resulting in a gradual variation of Ms and Mr. BiFeO 3 has a small retentivity, Mr, and coercivity, Hc, of 0.011 emu/gm and 838 Oe, respectively.Both retentivity and coercivity are maximum for BYFCO-3 (0.069 emu/gm, 2915 Oe), and the other two samples BYFCO-1 and BYFCO-2 show intermediate values.The variation of these properties may be due to shape anisotropy, magnetocrystalline anisotropy, and magnetoelastic anisotropy. 6he M-H loops display an asymmetric shift of the hysteresis loop along the applied magnetic field axis, which is known as the exchange bias (EB) effect.It indicates that ferromagnetic and antiferromagnetic domains may coexist in this material.The EB effect is due to the coupling of multiple magnetic domains at the interface between the ferromagnetic and antiferromagnetic domains. 67The EB field, H EB , was calculated from the hysteresis loop asymmetry along the applied field axis by using the following relation: 67,68 where the left and right side coercivities are H c1 and H c2 , respectively.The EB effect usually appears when a system is cooled  below the Néel temperature (T N ) in the presence of an external field. 69The H EB values in Table III are obtained in the absence of a cooling magnetic field at ambient temperature, and the biasing effect is not notably significant.Thus, from the perspective of practical applications, these samples open the door to prospective applications.The magnetic moment, μ, which is related to the saturation magnetization per formula unit in Bohr magneton, μ B can be calculated by the following formula: μ = M×M s 5585 , where M is the molecular weight and Ms is the saturation magnetization.Deduced values of magnetic moments are listed in Table III.The variation of μ agrees well with Ms.The magnetocrystalline anisotropy constant, K, was calculated with the Stoner-Wohlfarth 70 K = 1 2 MsHc, where Ms is the saturation magnetization and Hc is the coercivity, shown in Table III.The calculated values of the anisotropy constant varied between 79 and 379 erg/gm.Magnetic susceptibility indicates the degree of magnetization of a material in response to an applied magnetic field.The magnetic susceptibility, χ, is calculated using the following formula: χ = M H , where M is the magnetization and H is the applied magnetic field strength.The variation of the magnetic susceptibility, χ, with an applied magnetic field, H, for undoped BFO and Y, Cu co-doped BFO samples obtained from magnetization data is shown in Fig. 13, indicating the low susceptibility of all the samples.It is observed that the susceptibility is maximum for BYFCO-3, while it is minimum for pure BFO.

IV. CONCLUSION
The rhombohedral structure of the R3c space group was determined in both pure and doped BiFeO 3 (BFO) NPs as a major phase, while the orthorhombic structure of the Pnma space group resided as a secondary phase.Due to Y and Cu co-doping in BFO, this secondary phase amount (%) has been increased.In addition, a slight amount (1.1%-3.7%) of Bi 25 FeO 40 was also observed in all samples as an impurity phase depending on the doping.However, the crystallite size gradually decreased due to Y and Cu doping in BiFeO 3 samples, while the grain sizes increased from 63 to 174 nm, confirming the agglomeration of the NPs.A substantial decrease in the optical bandgap (2.03 eV) was determined for the B 0.9 Y 0.1 Fe 0.85 Cu 0.15 O 3 sample, which reveals itself as a potential candidate for use in photovoltaic and optoelectronic devices.The composition B 0.9 Y 0.1 Fe 0.9 Cu 0.1 O 3 showed a relaxation peak in the dielectric loss tangent (tan δ) induced by point defects, and all other samples displayed low values of tan δ.All the synthesized nanoparticles exhibited ferromagnetic ordering with higher ferromagnetic properties for Y and Cu co-doping in BFO.The maximum values of saturation magnetization (0.26 emu/gm), retentivity (0.069 emu/gm), coercivity (2915 Oe), magnetic moment (0.014 μ B ), and anisotropy constant (379 erg/gm) were detected for the composition B 0.9 Y 0.1 Fe 0.85 Cu 0.15 O 3 , for which the magnetic susceptibility is also higher.Finally, the composition B 0.9 Y 0.1 Fe 0.85 Cu 0.15 O 3 is the most suitable for magnetic storage applications.

FIG. 3 .
FIG. 3. Rietveld refinement of x-ray diffraction patterns recorded for BiFeO 3 and Bi 0.9 Y 0.1 Fe 1−x CuxO 3 (x = 0.05, 0.10, 0.15) NPs, where red circles represent the observed data (I Obs ), black lines illustrate the calculated pattern, and blue lines show their difference (I Obs − I Cal ).

FIG. 4 .
FIG. 4. (a) Representation of unit cell structure of BFO and Bi 0.9 Y 0.1 Fe 0.9 Cu 0.1 O 3 samples obtained by the Rietveld refinement of XRD data and (b) magnified XRD patterns of (104) and (110) diffraction peaks along with Gaussian fitting of (104) to obtain the crystallite size (ζ) and lattice strain (ε L ).
Figure 10(b) shows that imaginary parts are sharply decreased at low-frequency regions for BFO, BYFCO-1, and BYFCO-3 samples, whereas BYFCO-2 shows an increase in the imaginary part with a broad peak at ∼15.6 kHz.The values of dielectric constants (real part and imaginary part) at different frequency regions are provided in Table

TABLE III .
Different magnetic parameters calculated from the magnetic hysteresis curve.