Effect of Cobalt Substitution on the Structural and Magnetic Properties of Bismuth Ferrite Powders

Abstract

BiFeO₃ (BFO) is a multiferroic material with excellent ferroelectric properties but with poor magnetic behavior. Therefore, we focused principally on the enhancement of the magnetic order of BFO. These multiferroic properties make BFO an excellent candidate for magnetoelectric devices at room temperature. Pure and Co-BiFeO₃ powders were successfully synthesized via the sol–gel method at 700 °C. The effect of Co substitution on the corresponding structural and magnetic properties of BFO was studied. X-ray diffraction and Fourier Transform Infrared Spectroscopy measurements confirmed the rhombohedral perovskite structure in all samples. A secondary phase of CoFe₂O₄ (CFO) was detected for 9, 10, and 15% of Co doping. The scanning electron microscopy images of the Co-BFO particles showed a reduction in the particle size compared to the pure BFO powders. Vibrating sample magnetometry measurements evidenced the ferromagnetic hysteresis loop for the Co-BFO powders with values of saturation magnetization of 4.1 emu/g and a coercivity of 1083 Oe for 15% of Co doping. In this work, we report impurity free samples with notable magnetic properties at the same time, which is a difficult challenge in bismuth ferrite synthesis. This is the first step for later applications in future technology.

1. Introduction

Multiferroic materials have attracted interest from researchers due to their expected spontaneous and switchable ferroelectric, ferromagnetic, and ferroelastic ordering in a single phase. Furthermore, tuning of the magnetic and electric properties can be attained using a magnetic and electric field, respectively, and through electromagnetic coupling. In other words, an external electric field can induce a magnetic response and an external magnetic field can modify the electric polarization in the same material [1]. Among multiferroics, Bismuth Ferrite (BFO) is one of the most important materials exhibiting multifunctional properties at room temperature. BFO is ferroelectric with a Curie temperature of T_C ~ 1103 K and a G-type antiferromagnetic ordering with a Neel temperature of T_N ~ 643 K [2]. These features make BFO a good candidate for applications in electronic devices, data storage, sensors, and spintronics [3,4,5,6,7,8]. Moreover, BFO shows a weak magnetism due its spiral spin structure, which makes doping a promising route to enhance its magnetic properties. This incorporation of dopant species can induce a distortion in the BFO unit cell due to the mismatch between the ionic radius of the host and guest atoms that will change the Fe-O-Fe bond angles and, according to the Goodenough–Kanamori rule, switch the magnetic ordering [9]. Costa et al. [10] improved the magnetic properties by doping BiFeO₃ with Ca in Bi sites. Other dopants, such as Mn, Co, and Ni, have also been considered in an attempt to enhance the corresponding magnetic properties [11,12,13]. In particular, the synthesis of Co-BiFeO₃ nanocrystalline powders involved the presence of Cobalt Ferrite (CFO) as a secondary phase [14]. CFO is a ferromagnetic material that opens the possibility to form BiFeO₃/CoFe₂O₄ nanocomposites, which would combine ferroelectric and ferromagnetic features.

On a general basis, several approaches have been used to synthesize BFO powders, e.g., hydrothermal [15], sol–gel [16], solid state [17], and co-precipitation [18], among others. Sol–gel is a rapid and simple synthesis method that also allows a good control of crystallite size and composition. The preparation of impurity-free BFO is a current challenge, since the scientific literature reports the coexistence of Bi₂₅FeO₃₉ [19] and Bi₂Fe₄O₉ [10]. Selective dissolution, so-called leaching, is an alternative to remove these secondary phases [20], at the expense of prolonging the processing time and generating unwanted liquid waste.

In the present work, we investigated the effect of Co substitution in impurity-free BFO on the structural and magnetic properties of the powders synthesized via a modified sol-gel approach. Depending on the Co doping level, CFO was detected in coexistence with pure BFO; it explained the observed enhancement in the ferromagnetic behavior of the as-synthesized nanocrystalline powders.

2. Materials and Methods

2.1. Materials

All reagents were of analytical grade and used without any further purification. Bismuth nitrate [$\mathrm{B}\mathrm{i}{\left({\mathrm{N}\mathrm{O}}_3\right)}_3·5{\mathrm{H}}_2\mathrm{O}$, 99.9% Alfa Aesar, Massachusetts, MA, USA], iron nitrate [$\mathrm{F}\mathrm{e}{\left({\mathrm{N}\mathrm{O}}_3\right)}_3·9{\mathrm{H}}_2\mathrm{O}$, 99.9% Alfa Aesar], and cobalt (II) acetate tetrahydrate [$\mathrm{C}\mathrm{o}{\left({\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\right)}_2·4{\mathrm{H}}_2\mathrm{O}$, 98%, Alfa Aesar] were used as the precursor salts, whereas ethylene glycol [$\mathrm{E}\mathrm{G},{ \mathrm{H}\mathrm{O}\mathrm{C}\mathrm{H}}_2{\mathrm{C}\mathrm{H}}_2\mathrm{O}\mathrm{H}$, 99% Alfa Aesar] was used as the solvent.

2.2. Synthesis of Pure and Co-BFO Powders

Well-crystallized ${\mathrm{B}\mathrm{i}\mathrm{F}\mathrm{e}\mathrm{o}}_3$ nanocrystalline powders were synthesized via a modified sol–gel and thermal treatment method. Suitable amounts of Bi, Fe, and Co salts were used to achieve the desired atomic fractions of ${\mathrm{B}\mathrm{i}}_{\left(1-\mathrm{x}\right)}{\mathrm{C}\mathrm{o}}_{\mathrm{x}}{\mathrm{F}\mathrm{e}\mathrm{o}}_3$, with x = 0, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, and 0.15. Precursor salts were dissolved in ethylene glycol at 150 °C, followed by drying at 350 °C and milling. The obtained solid precursor was then thermally annealed in air for 30 min at 700 °C to develop the desired crystalline structure in agreement with the temperature conditions used in a previous work [12]. The heating rate was 5 °C/min for all these experiments.

2.3. Materials Characterization

The crystalline structure of the as-synthesized powders was studied by X-ray diffraction (XRD) using a SIEMENS D 500 diffractometer with a Cu-Kα radiation. The morphology and size of the particles were studied in a JEOL scanning electron microscope. Fourier Transform Infrared Spectroscopy (FTIR) was used to identify structural features and investigate the chemical structure of produced samples. The corresponding magnetic properties were measured at room temperature in a Lake Shore 7410 vibrating sample magnetometer (VSM).

3. Results and Discussion

3.1. XRD Analysis

Figure 1 shows the XRD patterns for ${\mathrm{B}\mathrm{i}}_{\left(1-\mathrm{x}\right)}{\mathrm{C}\mathrm{o}}_{\mathrm{x}}{\mathrm{F}\mathrm{e}\mathrm{o}}_3$ (x = 0, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, and 0.15) powders thermally treated at 700 °C for 30 min. Match software verified that all main peaks corresponded to the rhombohedral perovskite structure of BFO. For 0 and 5 at.% of Co doping, the main impurity phases of Bi₂Fe₄O₉ (mullite) or Bi₂₅FeO₃₉ (sillenite) were detected in the 25°–30° diffraction angle range. The presence of these intermediate oxides can be attributed to the thermal decomposition of the initially generated BFO or incomplete transition of precursors into BFO [20]. No peaks coming from any impurities were detected for 8, 9, 10, and 15 at.% of Co doping. Instead, a secondary phase of cobalt ferrite (CFO) was identified as evidenced by the appearance of a peak at ~35.4° that belonged to the cubic spinel structure of CFO [21]. The Co (II) species would have reacted with the available Fe (III) species to form CFO, which also inhibited the formation of impurities: the Fe species were no longer available for the formation of Bi-Fe impurity phases. The presence of cobalt ferrite in Co-doped ${\mathrm{B}\mathrm{i}\mathrm{F}\mathrm{e}\mathrm{o}}_3$ powders has been reported by other researchers but coexisting with a large amount of impurities [14].

Incorporation of Co species in Bi sites should give place to a shrinking in the unit cell due to the mismatch in ionic radii between ${\mathrm{C}\mathrm{o}}^{2+}$ (0.78 Å) and ${\mathrm{B}\mathrm{i}}^{3+}$ (1.20 Å); consequently, the diffraction peaks of Co-doped BFO were expected to be shifted toward higher diffraction angles with respect to the peaks for pure BFO powders. However, as Figure 2 suggests for the (012) peak, a shift toward lower diffraction angles with doping (particularly in the Co doping range of 5 to 7 at.%), was observed. This trend may suggest an expansion of the unit cell due to the incorporation of ${\mathrm{C}\mathrm{o}}^{2+}$ (0.78 Å) in the ${\mathrm{F}\mathrm{e}}^{3+}$ (0.64 Å) sites, instead. For 8 and 9 at.% of Co, the shift was not so evident because the formation of CFO should have been the dominant mechanism rather than substitution of Fe species by Co sites. At higher compositions of Co (10 and 15%), both mechanisms would have become evident, i.e., the initial substitution of Fe by Co species and the chemical reaction between Fe and Co to form the CFO phase.

The volume of the unit cell at each doping level was determined as an attempt to corroborate the mechanisms proposed in the incorporation of Co and the formation of CFO (Figure 3). From 5 to 7 at.% of Co, an increase in the volume of the unit cell was observed (incorporation of ${\mathrm{C}\mathrm{o}}^{2+}$ in ${\mathrm{F}\mathrm{e}}^{3+}$ sites); for 8 and 9 at.% of Co, the volume remained almost constant (prevailed formation of CFO rather than the incorporation of ${\mathrm{C}\mathrm{o}}^{2+}$). Finally, for 10 and 15 at.% of Co, an expansion of the volume of unit cell was observed. This expansion in the unit cell took place with the simultaneous detection of the CFO phase by XRD analysis.

3.2. FTIR Analysis

Figure 4 shows the room temperature FTIR spectra of the Co-doped BFO samples synthesized at different levels of doping. The band at 813 ${\mathrm{cm}}^{-1}$ corresponded to the Fe-O stretching vibration, which only was detected for pure and 5% Co-doped BFO powders. Other bands were detected at 539 ${\mathrm{cm}}^{-1}$ (pure BFO) and 550 ${\mathrm{cm}}^{-1}$ (15% Co-doped BFO) and can be attributed to the Fe-O bending vibration in the BFO host lattice. The Fe-O band is attributed to the perovskite structure [22,23]. The shift in this band with the doping level reveals a distortion in the octahedral ${\mathrm{F}\mathrm{e}\mathrm{O}}_6$ groups.

3.3. Scanning Electron Microscopy (SEM) Analysis

Figure 5 shows the SEM images corresponding to 0% and 15% Co-doped BFO powders. Pure BFO powders consist of highly agglomerated particles with an irregular shape. The average particle size was estimated by ImageJ software at ~1.20 µm. In turn, the SEM image for the Co(15%)-BFO powders clearly evidenced less agglomeration, a smaller particle size (average size estimated at 240 nm), and a high monodispersity. The decrease in the particle size was attributed to the incorporation of the Co species in the host BFO. Therefore, this can be explained by an increase in the nucleation rate due to lattice distortions induced by the difference in the ionic radius between ${\mathrm{B}\mathrm{i}}^{3+}$ and the dopant [24].

3.4. M-H Measurements

Figure 6 shows the magnetic hysteresis loop measured at room temperature for the Co-doped ${\mathrm{B}\mathrm{i}\mathrm{F}\mathrm{e}\mathrm{O}}_3$ powders. All Co-doped BFO samples exhibited a well-saturated ferromagnetic hysteresis loop, contrasting with the pure BFO that displayed a M-H dependence typical of a material with an antiferromagnetic ordering. As suggested before, the incorporation of ${\mathrm{C}\mathrm{o}}^{2+}$ ions in the ${\mathrm{F}\mathrm{e}}^{3+}$ sites could be considered as the predominant mechanisms for 5–7% at.% of Co doping that would have induced changes in the Fe-O-Fe bond angles and, hence, negatively affected the antiferromagnetic ordering. In turn, the enhancement in the magnetic properties for the Co-doped BFO was attributed to the formation of the ferromagnetic cobalt ferrite at higher doping levels. The changes in Fe-O-Fe bond angles, induced by substitution of Fe by Co species, should have also been involved with the enhancement on magnetization in those Co-doped BFO samples.

Figure 7 summarizes the variation of the remnant and saturation magnetization with the doping level. As said before, this trend was attributed to distortions in the Fe-O-Fe bond angles and the ferromagnetic ordering provided by the co-existing cobalt ferrite. The reduction in the particle size in the doped samples (as evidenced by SEM images) would modify the spiral spin structure of the non-doped BFO that could also induce ferromagnetic ordering. [23,25,26,27]. In our case, the 15% Co-doped BFO powders exhibited a maximum saturation magnetization (4.1 emu/g) that was 43 times higher than that of non-doped BFO (

Table 1. Magnetic parameters for the pure and 15% Co-doped BFO powders thermally treated at 700 °C for 30 min.

Samples	Coercivity (Oe)	Saturation Magnetization (emu/g)
pure BFO	14.1	0.1
15% Co-doped BFO	1083	4.1

). Furthermore, the coercivity was drastically enhanced from 14.1 Oe up to 1083 Oe, for the pure and Co-BFO phases. These result trends in the magnetic properties can be attributed to the presence of Cobalt ferrite, in good agreement with those observations from Tang et al. [14]. However, Tang reported the synthesis of samples with a high impurity level and the presence of cobalt ferrite only for 20 and 25% of Co doping. In the present work, the presence of impurity phases was completely inhibited, and the formation of cobalt ferrite was detected for Co doping levels as low as 8% Co.

4. Conclusions

In summary, pure and Co-doped ${\mathrm{B}\mathrm{i}\mathrm{F}\mathrm{e}\mathrm{O}}_3$ powders were well synthesized with the sol–gel method. XRD and FTIR analysis confirmed the formation of Co-doped ${\mathrm{B}\mathrm{i}\mathrm{F}\mathrm{e}\mathrm{O}}_3$ powders with rhombohedral perovskite structure. The formation of cobalt ferrite in Co-doped ${\mathrm{B}\mathrm{i}\mathrm{F}\mathrm{e}\mathrm{O}}_3$ powders inhibited the formation of impurities; Fe species would have been consumed to form the cobalt ferrite phase rather than become available to stabilize the Bi-Fe impurities. SEM images showed a particle size of 1.20 µm for the pure BFO and a decrease to 250 nm for the 15% Co-doped BFO. This reduction in particle size was attributed to the mismatch in the ionic radius between ${\mathrm{B}\mathrm{i}}^{3+}$ and ${\mathrm{C}\mathrm{o}}^{2+}$. All the doped samples exhibited ferromagnetic behavior. The optimal doping condition was obtained for 15% Co-doped BFO with high values of saturation magnetization and a coercivity of 4.1 emu/g and 1083 Oe, respectively. The magnetization values were increased as the Co-content increased probably due to the distortion of the Fe-O-Fe bond angles and the presence of ferromagnetic cobalt ferrite coexisting with the ${\mathrm{B}\mathrm{i}\mathrm{F}\mathrm{e}\mathrm{O}}_3$ host lattice. Otherwise, a balance between the electrical and magnetic properties of the Co-doped ${\mathrm{B}\mathrm{i}\mathrm{F}\mathrm{e}\mathrm{O}}_3$ should be investigated for future applications in spintronics and memory devices.