Effect of doping on the morphology and multiferroic properties of BiFeO₃ nanorods

Abstract

In this study we report the synthesis of BiFeO₃ nanorods using a sonochemical technique. The nanorods had a diameter of 20–50 nm, a length of 100–500 nm and exhibit aspect ratios in the range of 5–10. However, after doping, the TEM images of Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ and Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃ samples show that the aspect ratios of both the double doped samples have reduced considerably, while retaining the crystallinity of the particles. BiFeO₃ nanorods show a weak ferromagnetic order at room temperature, which is quite different from the linear M–H relationship reported for bulk BiFeO₃. The saturation magnetization of these BiFeO₃ nanostructures has been found to increase on doping with various metal ions (Ba²⁺, Ca²⁺, Mn²⁺, Cr³⁺), reaching a maximum value of 1.35 emu g⁻¹ for the Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ nanostructures. However, saturation of electric polarization was observed only in case of the Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃ nanostructures.

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Introduction

Multiferroic materials yield simultaneous effects of ferroelectricity, ferromagnetism or ferroelasticity in the same material due to their unique and strong coupling of electric, magnetic, and structural order parameters.^1,2 Hence, they offer a wide opportunity for potential applications in information storage, magnetic recording media, spintronic devices and sensors.^3–5 Among all the multiferroic materials studied so far, BiFeO₃ (BFO) which exhibits the coexistence of ferroelectric and antiferromagnetic (AFM) orders has received great attention as it has the potential to be one of the prime candidates for room-temperature magneto electric applications due to its high ferroelectric Curie point (T_C ∼ 1103 K) and the antiferromagnetic (AFM) Néel point (T_N ∼ 647 K). However, both its spontaneous polarization and saturation magnetization are disappointingly low when compared to many standard ferroelectrics and ferromagnets. This is due to the superimposition of a spiral spin structure on BFO's antiferromagnetic order. In this spiral spin structure, the antiferromagnetic axis rotates through the crystal with an incommensurate long-wavelength period of 62 nm, which cancels the macroscopic magnetization and also inhibits the observation of the linear magnetoelectric effect in bulk BFO.^6,7 Hence, for novel electronics applications of BFO, its magnetic and electric properties must be enhanced.

Several studies aimed at upgrading the magnetic and ferroelectric properties of BiFeO₃ have ensued in the last five years. Improvement in magnetic properties at room temperature has been observed in single crystalline bismuth ferrite nanoparticles which show strong size-dependent magnetic properties.^8–10 Also the ferroelectric saturation polarization P_s and remnant polarization P_r of BiFeO₃ nanoparticles have been reported to be higher than that of the bulk material.¹¹ Introducing suitable dopant ions in BiFeO₃ films has also been reported to be a potential method for enhancing its magnetic, electric and magnetoelectric properties.^5,12–17 Hence it was of interest to synthesize doped BiFeO₃ nanoparticles and study the effect of the dopant ions on its magnetic and electric properties at room temperature.

It has always been a challenge to synthesize pure BiFeO₃ as the product is mostly contaminated with secondary phases such as Bi₂O₃ and Bi₂Fe₄O₉.¹⁸ In the present work, we have synthesized phase pure undoped BiFeO₃ and co-doped Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ and Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃ nanostructures through a sonochemical route. Sonochemical synthesis is based on acoustic cavitation resulting from the continuous formation, growth and implosive collapse of the bubbles in a liquid.^19,20 Undoped BiFeO₃ nanoparticles synthesized using the sonochemical technique has been reported earlier but the products showed the presence of some unidentified peaks in the powder X-ray diffraction pattern.²¹ The choice of the dopant ions was based on the fact that replacing Fe³⁺ ions in BiFeO₃ thin films with other transition metal ions such as Cr³⁺ and Mn³⁺ that have better electronic stability is expected to increase the resistance by reducing valence fluctuations in Fe³⁺.^22–25 Also, weak ferromagnetism has been observed at room temperature for single doped BiFeO₃ nanoparticles where divalent cations, viz. Ba²⁺ and Ca²⁺, substitute trivalent cations of Bi³⁺.²⁶ We have also evaluated the magnetic and ferroelectric properties of the sonochemically synthesized Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ and Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃ nanoparticles and the details of this work are discussed herein.

Results and discussion

Undoped BiFeO₃, single doped BiFeO₃ (Bi_0.9Ba_0.1FeO₃, BiFe_0.9Mn_0.1O₃, Bi_0.9Ca_0.1FeO₃, BiFe_0.9Cr_0.1O₃) and double doped BiFeO₃ (Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃, Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃) particles were synthesized using the sonochemical technique. The XRD patterns of the undoped BiFeO₃ as well as that of Bi_0.9Ba_0.1FeO₃, BiFe_0.9Mn_0.1O₃ and Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ are shown in Fig. 1, while those of Bi_0.9Ca_0.1FeO₃, BiFe_0.9Cr_0.1O₃ and Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃ are shown in Fig. 2, respectively. The peaks in all the XRD pattern corresponded to the rhombohedral structure of BiFeO₃ with R_3c space group (JCPDS no: 71-2494). The observed broadening of the peaks in the XRD patterns of all the above samples, compared to that normally seen for bulk BiFeO₃ obtained via solid state synthesis is typical for nanoparticles. No additional peaks were observed in any of the XRD patterns, confirming the formation of phase pure BiFeO₃. Earlier reports on alkaline earth metal doped BiFeO₃ nanoparticles synthesized using a sol–gel technique showed the presence of some unidentified impurity phases.²⁶ The lattice parameters of all the samples are given in Table 1. The lattice parameters of undoped BiFeO₃ is relatively close to the reported literature values (a = 5.587 Å, c = 13.860 Å). However, the lattice parameter values change on doping BiFeO₃. The results suggest that the rhombohedral BiFeO₃ structure undergoes distortion on doping, which involves Fe or Bi displacements relative to the cubic perovskite parent structure. The (104) and (110) peaks are overlapping in case of BiFe_0.9Mn_0.1O₃ and Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃, but can be distinguished in pure BiFeO₃ and for all the other doped samples.

Fig. 1 XRD patterns of (a) BiFeO₃, (b) Bi_0.9Ba_0.1FeO₃, (c) BiFe_0.9Mn_0.1O₃ and (d) Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃.

Fig. 2 XRD patterns of (a) Bi_0.9Ca_0.1FeO₃, (b) BiFe_0.9Cr_0.1O₃, and (c) Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃.

Table 1 Structural parameters of undoped and doped BiFeO₃ samples

Sample	a/Å	c/Å
BiFeO3	5.5811(1)	13.8589(2)
Bi0.9Ba0.1FeO3	5.5799(2)	13.8575(1)
BiFe0.9Mn0.1O3	5.5817(4)	13.8598(1)
Bi0.9Ba0.1Fe0.9Mn0.1O3	5.5778(5)	13.8264(2)
Bi0.9Ca0.1FeO3	5.5673(5)	13.7712(3)
BiFe0.9Cr0.1O3	5.5713(3)	13.7932(5)
Bi0.9Ca0.1Fe0.9Cr0.1O3	5.5768(5)	13.8231(4)

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To further investigate the microstructure and topography, we used transmission electron microscope to image the undoped and doped BiFeO₃ samples on carbon coated copper TEM grids. Fig. 3A shows the TEM image of undoped BiFeO₃ sample obtained via sonochemical synthesis followed by heat treatment at 400 °C for 1 h. It can be seen that the obtained nanocrystalline BiFeO₃ are rod-like with a diameter of 20–50 nm and a length of 100–500 nm. These pure BiFeO₃ nanorods exhibit aspect ratio in the range of 5–10. There are reports on faceted BiFeO₃ nanoparticles synthesized using a sol–gel technique, nanospindles synthesized using a hydrothermal route and also nanowires and nanotubes synthesized using template assisted synthesis.^8,27–29 However, to the best of our knowledge, this is the first report on the synthesis of BiFeO₃ nanorods. The energy dispersive spectrum of the BiFeO₃ nanorods shown in Fig. 4 confirms the presence of Bi, Fe and O in our prepared sample. The atomic ratio of Bi to Fe is approximately 1 : 1. Spectra taken at a number of selected positions of the sample show the presence of the same constituents. The Cu and C signals arise from the TEM grid. Fig. 3B shows the selected area electron diffraction (SAED) pattern taken from the BiFeO₃ nanorod, which exhibits its highly crystalline structure. The indexing of the concentric rings corresponds to the rhombohedral BiFeO₃ structure. The TEM images of Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ and Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃ samples are shown in Fig. 3C and 3D, respectively. It is clear from the images that the aspect ratios of both the double doped samples have reduced considerably, while retaining the crystallinity of the particles. In case of Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃, very few particles with a nanorod shape are seen, as a majority of them have a faceted morphology (Fig. 3C). Compared to Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃, the Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃ samples show the presence of more nanorods (Fig. 3D). This clearly indicates that the nanorod shape of the BiFeO₃ particles is destabilized under the influence of various dopant ions. Such an effect has also been observed in cobalt doped ZnO nanostructures.³⁰

Fig. 3 (A) TEM image of BiFeO₃, (B) SAED pattern of BiFeO₃, (C) TEM image of Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ and (D) TEM image of Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃.

Fig. 4 EDX spectrum of BiFeO₃ nanorods showing Bi, Fe and O peaks. Note: The Cu and C peaks are due to the carbon coated copper grids used for dispersing the sample.

To investigate the magnetic order at room temperature of our undoped bismuth ferrite nanorods, magnetic measurements were done using vibrating sample magnetometer (VSM). For all the samples we can observe sizable hysteresis with a finite value of the coercive field, remanent magnetization and saturation magnetization that are tabulated in Table 2. A DC magnetization loop of the BiFeO₃ nanorods, recorded at 300 K, is shown in Fig. 5. The data represent the average of all random orientations of the BiFeO₃ nanorods used for the measurements. BiFeO₃ nanorods show a weak ferromagnetic order at room temperature, which is quite different from the linear M–H relationship reported for bulk BiFeO₃.³¹ The weak ferromagnetic order was also observed in BiFeO₃ films, nanoparticles, nanowires and nanotubes.^{26,28,29,32–34} The weak ferromagnetic order generally observed in BFO films and in nanoparticles has been attributed to the size effect. It is well known that the incommensurate spiral spin structure of bulk BFO, with a period of 62 nm, cancels the macroscopic magnetization. The BFO nanostructures with typical dimensions below 62 nm can possess favorable magnetic properties due to their grain size confinement, an effect that has been found to partially destroy the long-range spiral spin structure of bulk BFO. The diameters of our BiFeO₃ nanorods are in the range of 20–50 nm, which is less than the wavelength of the incommensurate spiral spin structure of the bulk material. This would lead to partial destruction of the spiral spin structure in the BiFeO₃ nanorods and hence the incomplete spin compensation becomes measurable, resulting in weak FM behaviors. The maximum magnetization, M_S, measured at the maximum applied field of H_appl = 8 kOe corresponds to M_S ∼ 0.277 emu g⁻¹ (0.016 μ_B/Fe) for the BiFeO₃ nanorods. This is less than that observed in case of BiFeO₃ nanowires (M_S ∼ 0.534 emu g⁻¹) but more than that reported for BiFeO₃ nanotubes (M_S ∼ 0.125 emu g⁻¹).^28,29 This may be attributed to the different shape anisotropy, magnetocrystalline anisotropy and different extents of defects present in the various nanoforms. The coercive field of the nanorods is quite small (H_c ∼ 382 Oe). A shift in the hysteresis loops is also observed in the M–H curves of BiFeO₃ nanorods (shown as inset in Fig. 4). This can be ascribed to the presence of exchange coupling between the ferromagnetic surfaces and the antiferromagnetic cores. The hysteresis loops of undoped BiFeO₃ nanorods exhibit very small remnant magnetization and a lack of proper saturation. This can be attributed to the presence of exchange and dipolar interparticle interactions in our system.

Table 2 Magnetic parameters at room temperature

Sample	Coercive field/Oe	Remanant magnetization/emu g^−1	Maximum magnetization (at 8 kOe)/emu g^−1
BiFeO3	382	0.01	0.28
Bi0.9Ba0.1FeO3	1918	0.04	0.31
BiFe0.9Mn0.1O3	574	0.02	0.32
Bi0.9Ba0.1Fe0.9Mn0.1O3	1942	0.25	1.35
Bi0.9Ca0.1FeO3	350	0.11	0.68
BiFe0.9Cr0.1O3	350	0.05	0.47
Bi0.9Ca0.1Fe0.9Cr0.1O3	1142	0.13	1.06

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Fig. 5 Field variation of magnetization over ±8 kOe at room temperature for the undoped BiFeO₃ nanorods. Inset (right) enlarges M(H) curve showing the presence of hysteresis.

Fig. 6 shows the room temperature magnetic hysteresis curves for the single doped Bi_0.9Ba_0.1FeO₃, BiFe_0.9Mn_0.1O₃ and double doped Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ samples. There is a small increase in the maximum magnetization (M_S) of single doped BiFeO₃ nanostructures with Mn and Ba doping (M_S ∼ 0.31 emu g⁻¹) compared to that observed in BiFeO₃ nanorods. These effects may originate from the variable valence and dissimilar transition metal ions. The highest value of M_S (1.35 emu g⁻¹), M_R (0.25 emu g⁻¹) and coercive field H_c (1.94 kOe) occurs in the double doped Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ nanostructures. The high coercive field value obtained in case of single doped Bi_0.9Ba_0.1FeO₃ nanostructures was comparable to that reported for Bi_0.95Ba_0.05FeO₃ nanoparticles.²⁶ However, Bi_0.9Ba_0.1FeO₃ and Bi_0.9Ba_0.1Fe_0.8Mn_0.2O₃ samples synthesized via pyrolysis of xerogel precursors exhibited higher M_S and coercive field values compared to our nanostructures. This may be due to the presence of P4mm phase in their samples, which is absent in our case, since in systems like BiFeO₃, different synthesis methods often lead to different competing structures. The M–H loops for Bi_0.9Ca_0.1FeO₃, BiFe_0.9Cr_0.1O₃ and Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃ are shown in Fig. 7. In this case also, though the highest value of M_S (1.06 emu g⁻¹), M_R (0.13 emu g⁻¹) and coercive field H_c (1.14 kOe) occurs in the double doped Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃, it is less than that observed for Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ nanostructures. Bi_0.9Ca_0.1FeO₃ show the highest M_S (0.68 emu g⁻¹) value among all the single doped samples though it is less than that reported for Bi_0.9Ca_0.1FeO₃ nanoparticles synthesized using a sol–gel route.²⁶

Fig. 6 Field variation of magnetization over ±8 kOe at room temperature for the Bi_0.9Ba_0.1FeO₃, BiFe_0.9Mn_0.1O₃ and Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ samples.

Fig. 7 Field variation of magnetization over ±8 kOe at room temperature for the Bi_0.9Ca_0.1FeO₃, BiFe_0.9Cr_0.1O₃ and Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃ samples.

Fig. 8 shows the ferroelectric properties of the undoped BiFeO₃ nanorods and double doped Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ and Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃ nanostructures, investigated by the P–E loop measurements. At a maximum applied electric field of ±500 V cm⁻¹, the remanent polarization (P_r) is 0.21 μC cm⁻² and the coercive field (E_c) is 155 V cm⁻¹ for the BiFeO₃ nanorods. The P_r value is much less than that reported in case of BiFeO₃ nanoparticles and thin films, but higher than that observed in case of bulk BiFeO₃.^11,35,36 Saturation of polarization is not observed in case of the BiFeO₃ nanorods as well as Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ nanostructures at a maximum applied electric field of ±500 V cm⁻¹ and ±2000 V cm⁻¹, respectively. However, a high saturation polarization (P_s) of 10.5 μC cm⁻², P_r of 7 μC cm⁻² and E_c of 1957 V cm⁻¹ is observed in case of the Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃ nanostructures. The P–E hysteresis loops of all samples indicate their ferroelectric nature but with lossy features. This leaky feature is the least in case of Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃.

Fig. 8 FE hysteresis loop for BiFeO₃ nanorods, Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ and Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃ nanostructures.

To understand this behavior, we have to first note that in BiFeO₃, small amounts of Fe²⁺ ions and oxygen vacancies exist.³⁷ Incidentally, BiFeO₃ shows p-type conductivity,³⁸ which can be understood by considering the substitution of a small amount Fe²⁺ ions in Fe³⁺ positions (acceptor doping of Fe³⁺ by Fe²⁺). When Ca²⁺/Ba²⁺ and Cr³⁺/Mn²⁺ is added to BiFeO₃, Ca²⁺/Ba²⁺ is supposed to substitute Bi³⁺ because of the close ionic radii of Ca²⁺/Ba²⁺ and Bi³⁺. Such acceptor doping of Bi³⁺ by Ca²⁺/Ba²⁺ is expected to generate oxygen vacancies without the liberation of electrons.³⁹ Normally, the oxygen partial pressure in the ambience is sufficient to incorporate oxygen into the structure to nullify the oxygen vacancies and show p-type conductivity. The hole generated can be consumed by Fe²⁺ in Fe³⁺ position resulting in lower acceptor doping of Fe³⁺ by Fe²⁺ in BiFeO₃ with consequent decrease in conductivity. When Cr³⁺ substitutes Fe³⁺, the acceptor doping of Fe³⁺ by Fe²⁺ is further reduced since Cr³⁺ is very stable electronically and this further causes a decrease in the conductivity. Hence, Ca²⁺ and Cr³⁺ co-doped BiFeO₃ show comparatively non-lossy ferroelectric hysteresis loops compared to undoped BiFeO₃ and Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ nanostructures.

Experimental

All the reactions were carried out at room temperature under ambient conditions. High purity bismuth nitrate [Bi(NO₃)₃·5H₂O], ferric nitrate [Fe(NO₃)₃·9H₂O], manganese acetate [Mn(OOCCH₃)₂·4H₂O], chromium nitrate [Cr(NO₃)₃·9H₂O], barium nitrate [Ba(NO₃)₂] and calcium chloride [CaCl₂] were obtained from commercial sources (Aldrich). In a typical procedure, an aqueous solution of Bi(NO₃)₃·6H₂O and Fe(NO₃)₃·9H₂O was sonicated for 20 min. To the mixture, 5 ml of tetraethylene glycol (TEG) was added and sonicated for further 10 min. The pH of the solution was then adjusted to ∼8 by adding ammonia solution and the resultant mixture was irradiated with a high intensity (100 W cm⁻²) ultrasonic radiation operating at 20 kHz, under air at room temperature for 100 min. This was done by the direct immersion of a titanium horn (13 mm diameter) to a depth of 6 cm in the solution. The subsequently formed white precipitate was centrifuged, washed with distilled water and finally with acetone. The product was heated under air in furnace at 400 °C for 1 h.

For the doped BiFeO₃ nanostructures, the method is similar to that reported for undoped BiFeO₃ but here a stoichiometric amount of barium nitrate/manganese acetate or calcium chloride/chromium nitrate was also added to the reaction mixture. We have prepared Bi_0.9Ba_0.1FeO₃, BiFe_0.9Mn_0.1O₃, Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃, Bi_0.9Ca_0.1FeO₃, BiFe_0.9Cr_0.1O₃ and Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃ nanostructures using the sonochemical technique. The products obtained were subjected to thermal treatment, and resultant residues were characterized by XRD.

Characterization

X-Ray diffraction (XRD) measurements were carried out on a Philips Instrument, operating with Cu-Kα radiation (λ = 1.5406 Å) and employing a scan rate of 0.02° s⁻¹ in the scattering angular range (2θ) of 15° to 80°. Silicon was used as an external standard for correction due to instrumental broadening. The average crystallite size was calculated from the diffraction line width based on Scherrer's relation: d = 0.9λ/Bcosθ, where λ denotes the wavelength of X-rays and B is the corrected full width at half maxima (FWHM). EDS analyses was were carried out using an Inca Energy 250 instrument coupled to Vega MV2300t/40 scanning electron microscope. Conventional TEM micrographs were recorded on JEOL 2000FX. The particulates obtained, were dispersed in methanol solution and then deposited on the carbon coated copper grids for TEM/SAED studies. Magnetization of powder samples was measured using an EG&G P.A.R. vibrating sample magnetometer (model 4500). The magnetic hysteresis loops (Mvs.H) was measured at 300 K with H = ±8 kOe. Electric field (P–E) hysteresis loops of the samples (5 mm diameter pellet with sputtered gold as top electrode and silver paste as bottom electrode) were measured by the modified Sawyer–Tower circuit at a 1 kHz ac frequency.

Conclusion

Phase pure BiFeO₃ nanorods have been successfully synthesized using a facile sonochemical technique. These nanorods show a weak ferromagnetic order at room temperature, which is quite different from the linear M–H relationship reported for bulk BiFeO₃. On doping, the nanorod shape of the BiFeO₃ particles is destabilized and we get nanostructures with a reduced aspect ratio. Addition of various dopants in BiFeO₃ nanorods, alters their magnetic as well as ferroelectric properties to different extents. The highest value of M_S (1.35 emu g⁻¹), M_R (0.25 emu g⁻¹) and coercive field H_c (1.94 kOe) occurs in the double doped Bi_0.9Ba_0.1Fe_0.9Mn_0.1O₃ nanostructures. However, saturation of electric polarization was observed only in case of Bi_0.9Ca_0.1Fe_0.9Cr_0.1O₃ which exhibited a P_s of 10.5 μC cm⁻², P_r of 7 μC cm⁻² and E_c of 1957 V cm⁻¹. This material also showed enhanced ferromagnetic properties compared to the bulk BiFeO₃. Thus, fine tuning of the concentration of these dopants in BiFeO₃ nanostructures can lead to better materials for different potential applications.

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