Control of physical properties in BiFeO3 nanoparticles via Sm3+ and Co2+ ion doping

Highly crystalline BiFeO3 (BFO), Bi0.97Sm0.03FeO3 (Sm-BFO) and BiFe0.97Co0.03O3 (Co-BFO) nanoparticles (NPs) were utilized as potential magnetic hyperthermia agents at two different frequencies in the radiofrequency (RF) range, and the effect of Sm3+ and Co2+ ion doping on the physical properties of the material was examined. The thermal behaviour of the as-prepared powders disclosed that the crystallization temperature of the powders is affected by the incorporation of the dopants into the BFO lattice and the Curie transition temperature is decreased upon doping. Vibrational analysis confirmed the formation of the R3c phase in all compounds through the characteristic FT-IR absorbance bands assigned to O–Fe–O bending vibration and Fe–O stretching of the octahedral FeO6 group in the perovskite, as well as through Raman spectroscopy. The shift of the Raman-active phonon modes in Sm-BFO and Co-BFO NPs indicated structural distortion of the BFO lattice, which resulted in increased local polarization and enhanced visible light absorption. The aqueous dispersion of Co-BFO NPs showed the highest magnetic hyperthermia performance at 30 mT/765 kHz, entering the therapeutic temperature window for cancer treatment, whereas the heating efficiency of all samples was increased with increasing frequency from 375 to 765 kHz, making our doped nanoparticles to be suitable candidates for potential biomedical applications.


Introduction
Multiferroic materials are characterized by the coexistence of (anti)ferromagnetism, ferroelectricity or ferroelasticity and the coupled magnetic, electric and structural order parameters [1].Among all the multiferroics, BiFeO 3 (BFO) is of great interest, as it exhibits both magnetic and ferroelectric transition Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.temperatures well above room temperature.It has a ferroelectric Curie temperature of 1103 K and an antiferromagnetic Néel temperature of 643 K [2] making BFO a promising candidate for a wide range of potential applications in data storage, sensors, spintronic devices, etc [3].Another noticeable application of BFO is its photocatalytic activity that can be used for photocatalytic water splitting [4] and organic waste degradation [5] by virtue of its small band gap (E g = 2.2-2.3 eV), low cost and good chemical stability [6].Due to the narrow band gap, visible light irradiation is able to generate photoinduced charge carriers in BFO, thus making it to be a potential visible light-driven photocatalyst.An even narrower band gap would entail a stronger visible light absorption, thereby enhancing the photocatalytic performance of the material.Additionally, BFO could also be utilized for biomedical applications due to its low toxicity.In fact, biocompatible BFO NPs have already been examined as radiothermotherapeutic agents to intensify the theranostic efficiency [7].
The ferroelectricity of BFO arises from the relative displacement of cations, resulting from the stereochemical activity of 6s2 lone electron pair of Bi 3+ cations, whereas the G-type antiferromagnetic ordering of BFO is superimposed with an incommensurate spin cycloidal structure, leading to an almost vanished net magnetization [2].Metal ion doping in the ABO 3 lattice at the A-sites or B-sites or both A and B sites concurrently is regarded as an efficient method to enhance the optical, ferroelectric and ferromagnetic properties of BFO.The doping-induced structural distortion of BFO can modulate its electronic band structure and dipole-dipole interaction, thus improving the properties of the material [8].Moreover, by decreasing the particle size of BFO below the periodicity of the helical ordering (62 nm), suppression of the modulated spin structure can give rise to a remanent magnetization in nanoscale particles [9].The enhancement of the magnetic properties of BFO NPs could be highly beneficial for magnetic hyperthermia treatment of cancer, where the heat (41 °C-45 °C) generated by magnetic NPs (MNPs) exposed to an alternating (AC) magnetic field at 100-1000 kHz is exploited to shock or kill cancer cells [10].This motivated us to synthesize highly crystalline BFO, Sm-BFO and Co-BFO NPs via a simple chemical co-precipitation method, after annealing at 600 °C for 2 h in air, as described in our previous study [11].It was shown that the combined effect of cation substitution and size confinement resulted in the significantly enhanced magnetization values of Sm-BFO and Co-BFO NPs at room temperature, which was also reflected by their improved magnetically induced heating capabilities under an AC magnetic field of 50 mT/375 kHz, at a nanoparticle concentration of 4 mg ml −1 .Dubey et al [12] reported the evolution of the heating efficiency of Nd 3+ , Gd 3+ and Dy 3+ -doped BiFe 0.95 Mn 0.05 O 3 (BFM) NPs under 60 mT/310 kHz and 80 mT/310 kHz, where 3 mg ml −1 concentration of Dy-doped BFM NPs could heat up to 39 °C.
Therefore, a more detailed investigation into the magnetically induced heating capability of BFO-based aqueous dispersions is lacking, triggering us to study the evolution of the magnetic hyperthermia performance of BFO, Sm-BFO and Co-BFO aqueous dispersions at two different RF frequencies.Here, we report on the effect of rare earth (Sm 3+ ) and transition metal (Co 2+ ) ion doping on the structure, piezoresponse, optical band gap and magnetic hyperthermia efficiency under 30 mT/375 kHz and 30 mT/ 765 kHz.We have observed that the substitution-induced structural changes impose an optical band gap reduction, as well as a notably higher piezoresponse, compared with pure BFO NPs.The heating efficiency of all aqueous dispersions under study increases remarkably with increasing frequency, whereas the aqueous dispersions of both Sm-BFO and Co-BFO could reach the therapeutic window for cancer treatment indicating their potential utilization as magnetic hyperthermia or other biomedical agents.

Preparation of BFO-based nanoparticles
Nanocrystalline BFO, Sm-BFO and Co-BFO powders were prepared by a simple chemical co-precipitation method, as described in our previous studies [11,13].For Sm-BFO and Co-BFO samples, appropriate amounts of Sm(NO 3 ) 3 •6H 2 O (Acros Organics, 99.9%) and Co(NO 3 ) 2 •6H 2 O (VWR Chemicals, 99.0%), respectively, were used for the synthesis of the doped nanoparticles under study.The synthesis of the initial powders was followed by calcination at 600 °C for 2 h in air.

Characterization
The investigation of the thermal behaviour of the assynthesized non-calcined BFO, Sm-BFO and Co-BFO powders was conducted by thermogravimetric analysis, TGA, (SETSYS 16/18 TG-DTA, Setaram) from room temperature to 1100 °C in air (50 ml min −1 ) at a heating rate of 20 °C min −1 .A sample of 3.5 ± 0.5 mg was placed in an alumina crucible, whereas the same experimental conditions were applied to the same empty crucible before the measurement to eliminate the buoyancy effect.Fourier transform infrared spectroscopy, FT-IR (Cary 670, Agilent Technologies), analysis was performed with a diamond attenuated total reflectance accessory, ATR (GladiATR, PIKE Technologies), at room temperature, to study the metaloxygen bonds of the pure BFO and doped BFO samples within the range of 4000-400 cm −1 with 32 scans and a resolution of 4 cm −1 .Raman spectra were collected at room temperature using a Renishaw Centrus 3C5P20 detector, with a 532 nm excitation source and an exposure time of 1 s.To probe the ferroelectric properties of the NPs, piezoresponse force microscopy (PFM) measurements were performed using a scanning probe microscope MFP-3D (Asylum Research).The NPs were drop-casted onto a conductive carbon tape for the PFM measurements at room temperature.Pt/Cr coated cantilevers Multi 75E-G (Budget Sensors) with a spring constant of 3 N m −1 were utilized.The PFM measurements were conducted at probing voltage amplitudes ranging from U ac = 1.5 to 5 V and a frequency f of around 70 kHz.The Gwyddion 2.47 (Gwyddion, Brno, Czech Republic) [14] software was utilized for the analysis of the PFM images.UV-vis measurements were carried out at room temperature using a PerkinElmer UV-vis spectrophotometer (Lambda 18) equipped with an integrating sphere in reflectance mode over the wavelength range of 200-850 nm and a resolution of 1 nm.The optical band gaps were determined from the corresponding Tauc plots after applying the Kubelka-Munk function on the collected diffuse reflectance spectra (DRS) of the samples.For magnetic hyperthermia, the measurements were conducted using a 4.2 kW Ambrell Easyheat Li3542 system, operating at a frequency of 375 kHz, as well as a SPG-10 Ultrahigh Frequency Induction Heating Machine (Shuangping Corporation), operating at a frequency of 765 kHz.The amplitude of the magnetic field was kept fixed at 30 mT for both frequencies, whereas temperature recording was performed by a GaAs-based fiber optic probe every 0.4 s.

Thermal behaviour
The thermal behaviour of the as-prepared, non-calcined, BFO, Sm-BFO and Co-BFO powders is investigated by differential thermal analysis (DTA) in the temperature range of 350 °C-850 °C, as presented in figure 1.The exothermic peaks located at 460 °C-490 °C are correspondent to the crystallization of both pure and doped BFO powders [13,15,16].Indicatively, the crystallization temperature of BFO was found to be at 467 °C, whereas the respective temperature of Sm-BFO was slightly increased up to 482 °C.This is likely due to the incorporation of the Sm 3+ cations into the BFO lattice.The substitution of Bi by Sm ions may decrease the diffusion rate of the Bi ions, leading to the slight increase in the crystallization temperature.This observation is consistent with the observations of Sangian et al [17], where Bi ions were substituted by Y ions.Moreover, the endothermic peak observed in the temperature range of 790 °C-800 °C in all samples is correlated with the ferroelectric to paraelectric phase transition, which corresponds to the Curie temperature (T C ) of the material [16,18].The DTA curve of Co-BFO reveals the presence of a small endothermic peak at around 760 °C, which could be attributed to the reduction of the rhombohedral phase, with the simultaneous evolution of an orthorhombic interim phase.Both phases could coexist up to the Curie temperature of 791 °C, in the case of Co-BFO, at which the material becomes paraelectric, owning a non-polar orthorhombic structure [18,19].At this point, it is worth mentioning that R3c is the dominant phase in both pure and doped BFO NPs at room temperature, as reported in our previous study [11].It is also observed that T C is decreased upon doping, following the particle size trend.Indicatively, the BFO NPs own a T C value of 796 °C, as shown in figure 1, which drops to 791 °C for Co-BFO and Sm-BFO NPs, suggesting that T C is slightly decreased with decreasing particle size, as reported in [11].The decreasing trend of T C with the particle size was also reported by Reddy et al [20].
TGA of the samples under study is shown in figure S1 in the supplementary material.In every case, and up to ca. 305 °C, a mass loss step of ca.7% is observed (figure S1(a)), which is attributed to the loss of absorbed water due to humidity, with two maxima of mass loss rate at 99 °C and 151 °C (figure S1(b)), and the decomposition of hydrates and nitrates, with a mass loss rate maximum at ca. 230 °C-280 °C (figure S1(b)) [13,15,21].The second mass loss of ca.1.5% at around 390 °C-460 °C, with a mass loss rate maximum at ca. 425 °C, could be related to the decomposition of remnant nitrates [21].This step is followed by the crystallization of the powders, as mentioned in the DTA analysis, whereas no further weight loss is detected at the Curie temperature.
3.2.Vibration spectra analysis 3.2.1.ATR-FTIR.Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy was carried out to investigate the fundamental absorbance bands of BFO, Sm-BFO and Co-BFO NPs at room temperature.At this point, it is worth mentioning that transmission electron microscopy (TEM) of the corresponding samples revealed an average particle size of 76.8 ± 19.0 nm, 72.8 ± 12.0 nm and 66.5 ± 23.0 nm, for BFO, Co-BFO and Sm-BFO NPs, respectively [11]. Figure 2 demonstrates the respective spectra in the wavenumber range of 2000-400 cm −1 .The absorbance peaks at around 440 cm −1 and 530 cm −1 , for all the samples, are assigned to the O-Fe-O bending vibration and Fe-O stretching of the octahedral FeO 6 group in the perovskite lattice, respectively, indicating the formation of the perovskite with the R3c space group symmetry [13].Another Fe-O peak at around 815 cm −1 denotes the formation of highly crystalline BFO phase [20].Additionally, the stretching vibrations of -NO 3 ions at around 1390 cm −1 and 845 cm −1 , mostly observed in the spectra of BFO and Sm-BFO NPs, disclose the presence of trapped nitrates [13].Finally, the band observed at around 1630 cm −1 is attributed to -OH vibrations [15,22].
Figure S2 in the supplementary material presents the magnified spectra of all the samples in the wavenumber range of 600-400 cm −1 .The observed shift of the Fe-O peak position in the doped samples is attributed to changes in the Fe/Bi-O bond lengths and O-Fe-O bond angles due to the substitution-induced structural distortions [23].It is worth mentioning that the broad nature of the corresponding vibrational bands indicates the co-occurrence of absorbance peaks, which are associated with both Fe-O and Bi-O bonds [13].
3.2.2.Raman spectroscopy.Raman scattering is a powerful technique, highly sensitive to atomic vibrations, that was utilized to identify the phase structure of the BFO, Sm-BFO and Co-BFO NPs of the present work.It is well known that BFO belongs to the R3c space group and the C3v point group.According to the group theory, the irreducible representation is demonstrated as follows: where both A 1 and E phonon modes are transverse optical (TO) vibrational modes, being Raman and IR active, whereas the A 2 modes are inactive in both Raman and IR [24].Among these 13 (4A 1 + 9E) Ramanactive modes at room temperature, Bi atoms are associated with the low frequency modes (up to 167 cm −1 ), Fe atoms are correlated with the modes between 152 and 262 cm −1 , whereas the modes above 262 cm −1 denote oxygen atoms motion [25].
Figure 3 exhibits the room temperature Raman scattering spectra of BFO, Sm-BFO and Co-BFO NPs.The peak positions of the Raman-active modes of all the samples are presented in table 1.In the undoped sample, three prominent peaks observed at 137, 171 and 217 cm −1 can be assigned to A 1 -1, A 1 -2 and A 1 -3 modes, respectively, whereas the A 1 -4 mode, owning a weak scattering intensity, is observed at 469 cm −1 .The other peaks, observed at 111, 262, 276, 329, 362, 429, 523, 546 and 599 cm −1 , can be assigned to E modes.The peaks are well matched with other reports [3,26].
In our previous work [11], we reported substitutioninduced structural distortion in the BFO lattice after doping with Sm 3+ and Co 2+ ions, for Sm-BFO and Co-BFO, respectively, which is also reflected by the corresponding Raman spectra presented in this work.Although all Raman spectra look similar, there are still some noticeable differences among them.Due to the increased lattice distortion, the vibrational mode E−2 (at 262 cm −1 ) in Sm-BFO, as well as the vibrational modes E−6 and E−8 (at 429 cm −1 and 546 cm −1 , respectively) in Co-BFO, could not be activated, as shown in figure 3 and summarized in table 1.In addition, the intensity ratio of the most prominent Raman peaks A 1 −1 and A 1 −2 decreases upon the Sm 3+ -doping and increases upon   doping with Co 2+ ions in the cases of Sm-BFO and Co-BFO, respectively, as compared to the corresponding A 1 −1/A 1 −2 intensity ratio of the pure BFO NPs.This observation signals the BFO crystal lattice distortion after the successful substitution of the host elements by the dopants.As mentioned in our previous study [11], the ionic radii difference between Sm 3+ and Bi 3+ ions, as well as between Co 2+ and Fe 3+ ions, leads to the increased lattice distortion observed in Sm-BFO and Co-BFO NPs, respectively.According to the same report [11], the Rietveld refinement of the corresponding diffractograms disclosed a variation of the lattice parameters (a and c), as well as a unit cell volume contraction upon doping, owing to the ionic radii mismatch between the host elements and the dopants.Additionally, the possibility of structural transformation in Sm-BFO and Co-BFO NPs or the coexistence of other symmetries along with the R3c symmetry was also examined through the quantitative phase analysis conducted by the Rietveld refinement of the corresponding XRD patterns.However, all samples manifested appropriate fit to the rhombohedral R3c space group with reasonable refinement parameters, as reported in [11].
Moreover, the Raman-active modes of Sm-BFO are slightly shifted towards higher wavenumbers, compared with BFO.The frequency of Raman modes is strongly related to the atomic masses and force constant [27][28][29][30].With the substitution of Bi 3+ by Sm 3+ ions in BFO, the average mass at the A-sites of ABO 3 perovskite slightly decreases, since the atomic weight of Sm is smaller than that of Bi resulting in the aforementioned wavenumber shift.Accordingly, a slight shift towards lower wavenumbers, compared with BFO, is observed in Co-BFO, as Co is heavier than Fe.Subsequently, the shifting of the Raman modes in the doped samples further confirms the successful incorporation of the dopants into the BFO lattice, which is accompanied by an increment of structural distortion.These doping-induced structural changes may have a significant impact on both optical and multiferroic properties of the samples under study.

Ferroelectric and piezoelectric properties
To probe the ferroelectricity in the BFO, Sm-BFO and Co-BFO NPs, PFM was applied, which is a trustworthy method for studying ferroelectric and piezoelectric properties at the nanoscale, even within individual NPs [31,32].The PFM results of the samples under investigation are shown in figure 4. Figures 4(a), (d) and (g) manifest the topography of BFO, Sm-BFO and Co-BFO NPs, respectively.It is observed that the NPs are mostly agglomerated.The series of topography images confirms the particle size reduction upon doping, which was reported in our previous study [11].The intensity of the LPFM signal depends on the in-plane component of the polarization [12].As can be seen in the LPFM phase images, a large number of particles are found to be in a single-domain state.However, there are also some particles that appear to be multidomain, as seen in figure S3 in the supplementary material for Co-BFO NPs, indicatively.In the LPFM amplitude images, the brighter zones correspond to NPs that exhibit larger piezoresponse [12].
A comparison between the investigated samples is shown in figure 5, with respect to the normalized LPFM amplitude signal.The broader distribution observed for Co-BFO indicates a larger contribution of NPs with higher piezoresponse.Assuming that the distribution of the crystallographic directions of the individual NPs is the same for all studied samples, one can conclude that Co-BFO has the highest shear piezoresponse, followed by Sm-BFO, whereas BFO presents the smallest shear piezoresponse among the studied samples.It is observed that even a moderate doping of nominally 3 mol% with Sm 3+ and Co 2+ ions in the cases of Sm-BFO and Co-BFO, respectively, increases the piezoresponse of the BFO NPs.
Since piezoresponse is roughly proportional to the local polarization value, it can be concluded that A-site doping of BFO NPs with trivalent (Sm 3+ ) cations, as well as B-site doping with divalent (Co 2+ ) cations, provides an increase in polarization in comparison to that in pure BFO.The increased polarization in Sm-BFO and Co-BFO correlates with the doping-induced modification in the crystal structure of BFO, which is already discussed in the Raman spectroscopy section of the present work, as well as in the structural analysis of our previous study [11].The incorporation of dopants with different ionic radii, compared with the host elements, into the BFO lattice, may induce structural changes that strongly affect the off-center displacement of Bi in the rhombohedral (R3c) structure of BFO, which is responsible for the intrinsic polarization of the material.

Optical band gap analysis
The optical properties of the NPs are determined using diffuse reflectance spectroscopy (DRS).Figure 6 displays the room temperature UV-vis absorbance spectra of BFO, Sm-BFO and Co-BFO NPs derived from the respective diffuse reflectance (R) spectra using the Kubelka-Munk function which is plotted as a function of photon energy (eV).At first glance, all the samples exhibit strong light absorption in the visible light region, indicating that both pristine and doped BFO NPs, under study, can respond to visible light for a potential photocatalytic reaction.In each spectrum, two transitions are observed, corresponding to the two anomalies within the energy range of 1.5-3.5 eV.From the absorbance spectra of the samples under investigation, the peaks centered at around 1.91 eV are attributed to the 6 A 1g → 4 T 2g excitation, arising from the d-d crystal field excitations of Fe 3+ ions [34].Such excitations are formally forbidden, because they change the total spin of Fe 3+ from / = S 5 2 to / = S 3 2.However, spin-orbit coupling relaxes the spin selection rule, giving rise to these transitions [35].Above 2 eV, absorbance increases remarkably for each sample, with a small shoulder centered at around 2.5 eV.This absorbance feature is assigned to the charge transfer (C-T) excitations and correlates with the Fe 1 3d-Fe 2 3d intersite electron transfer [13].Although all the absorbance spectra look similar, there appears to be a slight redshift in the d-d and C-T transition bands of the doped samples, revealing a substitution-induced increase in the internal chemical pressure, because of the changes in the FeO 6 octahedral coordination [36].The band gap of BFO is mainly formed by the strong hybridization of Fe 3d and O 2p orbitals [37].The optical band gap energy of the NPs is determined by the Tauc equation [38][39][40][41]: where α, h, v, E g , A and n represent the absorption coefficient, which can be replaced by the Kubelka-Munk function, F(R), for the reflectance spectra, Planck's constant, frequency of the illumination, energy of the optical band gap, a proportional constant and the power index, depending on the nature of the electronic transition, respectively.For direct band gap materials, such as BFO, n takes the value of 1.The inset of each absorbance spectrum shows the respective Tauc plot of ( ) ahv 2 as a function of energy.In all the cases, the E g values are obtained by extrapolating the linear part of the plots to ( ) = ahv 0 2 axis.At this point, it is worth mentioning that the abrupt UV absorption presented in figure 6(c), as well as the clockwise-like rotation of the corresponding Co-BFO Tauc plot, are attributed to the almost black color of the Co-BFO nanopowder, which exhibits intense absorbance within the measured UV-vis energy range.Eventually, the band gap values of BFO, Sm-BFO and Co-BFO NPs were found to be 2.11 ± 0.08 eV, 2.06 ± 0.05 eV and 1.97 ± 0.03 eV, respectively, suggesting that the doped samples show enhanced absorbance in the visible light region, compared with pure BFO.This would be beneficial for both photocatalytic and solar cell applications.The obtained energy band gap values are in good agreement with previous reports [3,13,37].The decrease in the optical band gap of Sm-BFO and Co-BFO NPs can be ascribed to the rearrangement of the molecular orbital and the dopinginduced distortion in the FeO 6 octahedra [36].Moreover, the enhanced presence of oxygen vacancies in Sm-BFO and Co-BFO samples, as reported in [11], may also lead to the apparent energy band gap reduction, as oxygen vacancies always result in sub-band gap defect states [42].

Magnetic hyperthermia
Magnetic particle hyperthermia was conducted to assess the magnetically induced heating capabilities of BFO, Sm-BFO and Co-BFO aqueous dispersions.The concentration of the NPs was kept constant at 4 mg ml −1 in all cases.The magnetic hyperthermia measurements were carried out under an applied AC magnetic field of 30 mT at 375 kHz and 765 kHz.The duration of the exposure to the magnetic field was 600 s at both frequencies.
Figures 7(a) and (b) show the magnetic hyperthermia curves of the BFO, Sm-BFO and Co-BFO dispersions in water at 375 and 765 kHz, respectively, including water as a reference.As can be observed in figure 7(a), pure BFO reached a maximum temperature of 35.4 °C, whereas Sm-BFO and Co-BFO reached a maximum temperature of 38.5 °C and 41.6 °C, respectively, suggesting that the doped BFO samples exhibit enhanced heating efficiency under the applied AC magnetic field of 30 mT/375 kHz, compared with pure BFO dispersion in water.Accordingly, as seen in figure 7(b), the maximal temperatures attained for the exposure duration of 600 s were found to be 39.6, 42.0 and 45.0 °C for the aqueous dispersions of BFO, Sm-BFO and Co-BFO, respectively, under the applied AC magnetic field of 30 mT/ 765 kHz.Magnetic heating efficiency can be quantified by the specific loss power (SLP), using the following equation [10]: where C is the volumetric specific heat capacity of the sample, m f is the mass of the dispersion (ferrofluid), m NPs is the NPs mass in the colloidal dispersion and ΔT/Δt is the average value of the initial slope at the start of the heating process.The temperature differences (ΔT) within the heating stage of 600 s for all aqueous dispersions, as well as their  respective SLP values obtained by the initial slopes, are presented in table 2.
The augmented magnetic features of Sm-BFO and Co-BFO NPs at room temperature, as reported in [11], are reflected by the increased SLP values at both frequencies, compared with that of pure BFO.In fact, the room temperature magnetization values of BFO, Sm-BFO and Co-BFO NPs at 9 T were found to be 0.6073 ± 0.0304, 0.8480 ± 0.0424 and 1.3656 ± 0.0683 Am 2 kg −1 , respectively [11].Dubey et al [12] reported that the magnetic hyperthermia efficiency depends upon both the magnetic and the microstructural properties of the NPs.Subsequently, the higher magnetization value of Co-BFO at room temperature resulted in the enhanced heating efficiency of the Co-BFO aqueous dispersion at both frequencies, followed by the aqueous dispersions of the Sm-BFO and BFO NPs.The moderate doping of 3 mol% in the single doped Sm-BFO and Co-BFO NPs has not caused any remarkable changes in the morphology of the NPs, as reported in our previous study [11], however the doping-induced particle size reduction in the cases of Sm-BFO and Co-BFO may affect the heating capabilities of the corresponding NPs, leading to their increased hyperthermia performance, compared with that of pure BFO.It is also disclosed in table 2 that the SLP value of each sample increases with increasing frequency suggesting that the applied frequency is strongly related to the magnetically induced heating performance of the aqueous dispersions.This behaviour is consistent with other reports, where it was shown that the heating efficiency scales linearly with the frequency [10,43].Observing the maximum attained temperatures, as presented in table 2, it can be concluded that the aqueous dispersion of Co-BFO NPs enters the therapeutic window of 41 °C-45 °C [44] within the heating stage of 600 s, under the applied AC magnetic field of 30 mT/ 375 kHz, whereas both Sm-BFO and Co-BFO dispersions reached and almost surpassed the aforementioned temperature window at the frequency of 765 kHz.These results are auspicious for the potential utilization of these promising materials in hyperthermia tests for cancer treatment and other biomedical applications.

Conclusions
In summary, doping BFO NPs with Sm 3+ and Co 2+ ions affected the crystallization temperature of the initial powders, whereas the Curie transition temperature decreased upon doping.A series of topography images revealed a particle size reduction in the doped NPs obtained in the present work.In all the investigated samples, the absorbance bands correlated with the O-Fe-O bending vibration and the Fe-O stretching of the octahedral FeO 6 group in the perovskite compound indicate the formation of the R3c phase, which was further verified by Raman spectroscopy in all compounds.However, shifting of the Raman-active phonon modes in Sm-BFO and Co-BFO NPs disclosed structural distortion of the BFO lattice, confirming the successful incorporation of the dopants into it.The ferroelectric properties of the doped NPs were significantly improved, compared with pure BFO, as revealed by the increasing trend of the in-plane polarization component upon doping.The Co-BFO NPs exhibited the largest piezoresponse owing to the increased structural distortion, as compared with Sm-BFO and BFO NPs.Furthermore, the  doping-induced increase in the internal chemical pressure caused a slight redshift in the d-d and C-T transition bands of the doped NPs, because of the changes in the FeO 6 octahedra.The corresponding Tauc plots manifested an optical band gap reduction upon doping indicating the stronger visible light absorption of Sm-BFO and Co-BFO NPs, thus making them promising candidates for potential photocatalytic and solar cell applications.The aqueous dispersions of BFO, Sm-BFO and Co-BFO NPs under the applied AC magnetic fields of 30 mT/375 kHz and 30 mT/765 kHz, with a nanoparticle concentration of 4 mg ml −1 , were also tested to study their magnetically induced heating efficiency.In all cases, sufficient heating (ΔT >13 °C) was achieved reaching the therapeutic window.The aqueous dispersion of Co-BFO NPs exhibited the highest heating efficiency at 30 mT/765 kHz owning a SLP value of 127.2 ± 12.0 W g −1 .Additionally, it was also shown that the SLP values of all aqueous dispersions increase with increasing frequency, while keeping both the magnetic field amplitude and the nanoparticle concentration constant at 30 mT and 4 mg ml −1 , respectively.The aqueous dispersions of Sm-BFO and Co-BFO NPs entered the therapeutic window of 41 °C-45 °C under the applied AC field of 30 mT/765 kHz, rendering them as promising magnetic hyperthermia agents.

Figure 1 .
Figure 1.DTA curves of the initially amorphous, as-prepared, BFO, Co-BFO and Sm-BFO powders.Crystallization temperature at around 467 °C and Curie temperature at ca. 796 °C are indicated by arrows.

Figure 2 .
Figure 2. ATR-FTIR spectra of BFO, Sm-BFO and Co-BFO NPs at room temperature.The arrows indicate stretching and bending vibrations of various bonds.

Figure 3 .
Figure 3. Raman scattering spectra of BFO, Sm-BFO and Co-BFO NPs at room temperature.Raman active A 1 and E modes are indicated within the measured frequency range.

Figures 4 (
b), (e) and (h) display the corresponding lateral PFM (LPFM) amplitude images, whereas the respective LPFM phase images are shown in figures 4(c), (f) and (i).

Figure 4 .
Figure 4. PFM images of BFO, Sm-BFO and Co-BFO NPs at room temperature.The first column on the left (a), (d), (g) represents the topography of BFO, Sm-BFO and Co-BFO, respectively.The column in the middle (b), (e), (h) displays the lateral PFM amplitude images of BFO, Sm-BFO and Co-BFO, respectively, whereas the last column on the right (c), (f), (i) contains the lateral PFM phase images of the corresponding samples.

Figure 5 .
Figure 5.Comparison of the lateral PFM amplitude among BFO, Sm-BFO and Co-BFO NPs at room temperature.Number of events, presented by the vertical axis, are taken from all over the image of the corresponding LPFM amplitude of each sample.

Figure 6 .
Figure 6.UV-vis DRS spectra of BFO (a), Sm-BFO (b) and Co-BFO (c) NPs obtained at room temperature.Insets display the corresponding Tauc plots fitting the energy band gap of the samples.

Figure 7 .
Figure 7. Magnetic hyperthermia curves of BFO, Sm-BFO and Co-BFO aqueous dispersions with a nanoparticle concentration of 4 mg ml −1 at 375 kHz (a) and 765 kHz (b).Pure water is also included at both frequencies as reference.

Table 1 .
Raman-active modes for BFO, Sm-BFO and Co-BFO NPs at room temperature.

Table 2 .
Magnetic hyperthermia results of all aqueous dispersions under an alternating magnetic field of 30 mT at 375 kHz and 765 kHz.