STRUCTURAL, MAGNETIC AND ELECTRICAL PROPERTIES OF Ho AND Ni CO-DOPED BiFeO 3 MATERIALS

2019 Abstract. Pure BiFeO 3 (BFO) and Bi 1- x Ho x Fe 0.97 Ni 0.03 O 3 ( x = 0, 0.025, 0.05, 0.075, and 0.1) materials were synthesized by a sol-gel method. The influence of (Ho, Ni) co-doping on structural, magnetic and electrical properties of BFO materials were investigated by different techniques as X-ray diffraction (XRD), energy dispersion X-ray (EDX), Raman scattering, magnetic hysteresis ( M-H ) loops, and complex impedance spectra measurement. XRD results showed that all samples were crystallized in the rhombohedral structure with R 3C space group. Crystal lattice parameters ( a , c ) and average crystal size L XRD were ( a = 5.584 Å, c = 13.867 Å, L XRD = 60 nm) for pure BFO, ( a = 5.589 Å, c = 13.875 Å, L XRD = 60 nm) for BiFe 0.97 Ni 0.03 O 3 sample, and then decreased with increasing of Ho content in (Ho, Ni) co-doped samples. Similarly, Raman scattering spectra showed the left shift of active modes Fe-O bonds when doping Ni and right shift when co-doping Ho. These observations confirmed the successful substitution of Ho 3+ and Ni 2+ ions into the host BFO crystal lattice. Magnetic hysteresis loops measurement indicated that all samples exhibited weak ferromagnetic behavior with saturation magnetization M s and remnant magnetization M r of ( M s ~ 0.047 emu/g, M r ~ 0.008 emu/g) for pure BFO which increased gradually for (Ho, Ni) co-doped samples, reached to ( M s ~ 0.702 emu/g, M r ~ 0.169 emu/g) for x = 0 sample. Origin of the enhancement of ferromagnetization in (Ho, Ni) co-doped samples have been

Multiferroic materials, possessing simultaneously ferromagnetic (antiferromagnetic), ferroelectric and ferroelasticity orders, and magnetoelectric (ME) effect in the same structure phase, which have been reported by many previous studies [1][2][3][4]. Multiferroic can be used in electronic devices such as information storage, memory, sensor, and ultrasonic broadcast. Due to competition between ferromagnetic and ferroelectric orders, multiferroic are very rare in nature. BiFeO 3 (BFO) is one of multiferroic materials coexisting antiferromagnetic order ( temperature T N = 643 K) and ferroelectric order (with Curie temperature T C = 1100 K) [3,5]. However, BFO has small saturation magnetization M s and polarization P s which limits its applications. This problem can be solved by modification of magnetic and electrical properties of BFO. Studies have shown that ferromagnetism and ferroelectricity of BFO can be improved by substitution of rare earth ions (Sm 3+ , Nd 3+ , Gd 3+ , Ho 3+ , etc.) into Bi-sites [6][7][8] or transition metal ions (Ni 2+ , Co 2+ , Mn 2+ , etc.) into Fe-sites [9][10][11] or co-doping rare earth and transition metal [12][13][14]. Chakrabarti et al. [15] and Zhang et al. [16] indicated that magnetization of (Eu, Co) or (La, Co) co-doped BFO enhanced several times compared to that of BFO. Ye et al. [17] showed that both ferromagnetic and ferroelectric properties of (Ho, Mn) co-doped BFO were improved compared to that of BFO.
In this work, holmium (Ho) and nickel (Ni) will be co-doped into BFO. Since the appropriate content of 2 ÷ 3 molar% of Ni 2+ dopant was demonstrated in several studies for largest enhancing of multiferroics properties [13,18,19], Ni 2+ concentration in co-doped samples will be kept at a constant of 3 molar%. The content of Ho 3+ ions changes in the range of 0 ÷ 10 molar% to study the effect of (Ho, Ni) co-doping on structural and physical properties of BFO.

MATERIALS AND METHOD
Pure BiFeO 3 3 .5H 2 O, and Ni(NO 3 ) 2 .6H 2 O were dissolved in 35 ml citric acid solution 1 M. The solution was then mixed by magnetic stirring at temperature of 60 °C for 45 minutes to obtain a reddish-brown transparent solution. Then, temperature of the sol was increased up to 100 °C for 3 hours to evaporate and obtain wet gel. Next, the gel was dried at 130 °C for 4 hours. Finally, dry gel was annealed at temperature 800 °C for 7 hours to obtain powder materials.
For the measurements of complex impedance, powder materials were compressed by a pressure of 20 MPa into round tablets of 6 mm in diameter and 1 mm thick. The round tablets were sintered at temperature of 800 °C for 5 hours to obtain ceramics. Then, the ceramic tablets were polished after measuring accurately their thickness. Finally, the samples were evenly covered with Pt glue as electrode and sintered at temperature of 500 °C for 3 hours.

RESULTS AND DISCUSSION
The chemicals composition of pure BFO and Bi 1-x Ho x Fe 0.97 Ni 0.03 O 3 (x = 0, 0.05 and 0.1) powders investigated by EDX spectra measurement are shown in Fig. 1. As seen in Fig. 1, EDX spectra of all samples exhibit characteristic peaks for Bi, Fe, and O elements. EDX spectrum of BFNO presents one more peak at 7.42 eV corresponding to Ni element while that of (Ho, Ni) codoped samples show two more characteristic peaks for Ho element at 1.35 eV and 6.64 eV. The intensity of Ho characteristic peaks increases as increasing Ho content.    BFO peaks shift obviously toward the lower 2θ angle when doping Ni 2+ ions and then shift slightly to the higher 2θ angle when co-doping Ho 3+ ions. Lattice parameters (a and c) and average crystal size (L XRD ) have been determined by using UnitCell software and Debye Scherrer's formula ( Table 1). As seen in Table 1, the lattice parameters (a = 5.584 Å, c = 13.867 Å) for BFO sample increase to (a = 5.589 Å, c = 13.875 Å) for BFNO sample and then decrease gradually with increasing Ho content. This change can be assigned to difference of ionic radius between Ni 2+ (0.69 Å) and Fe 3+ (0.65 Å) ions as well as the difference of ionic radius between Ho 3+ (1.02 Å) ions and Bi 3+ (1.17 Å) ions. Small ionic radius of Ho 3+ is not enough to fill 12-sided cavity created by BO 6 octahedron, resulting to the rotation of the BO 6 octahedron (B is Fe or Ni) and the reduction of the volume of 12-sided cavity. For (Ho, Ni) co-doped samples with high Ho content, the rotation of BO 6 octahedron is large enough to reduce both a and c parameters as observed in Table 1. Part et al. [18] also suggested that the change in lattice parameters originated from the fact that ionic radius of Ho 3+ is smaller than that of Bi 3+ and the ionic radius of Ni 2+ is larger than that of Fe 3+ .    (Fig. 3a) and clear boundary. When doping Ni 2+ into Fe 3+ site, the particle size decreases obviously and becomes more inhomogeneous (Fig. 3b). The particle size decreases further to less than 1 micrometer when co-doping Ho 3+ into Bi 3+ site (Fig.  3c). Furthermore, particles size and morphology are poorly homogeneous. In particular, the grain boundary becomes less clear. This reveals that Ni 2+ doping and (Ni 2+ , Ho 3+ ) co-doping affect strongly on the particle size and morphology of BiFeO 3 which also can be attributed to the differences of doping ions (Ni 2+ , Ho 3+ ) and host lattice ions (Fe 3+ , Bi 3+ ). The substitution of these doping ions in BFO lattice leads to a certain disorder in crystal structure, resulting to restrict the crystalline growth. Raman scattering spectra of BFO and Bi 1-x Ho x Fe 0.97 Ni 0.03 O 3 are shown in Fig. 4a. According to the group theory, 13 Raman active modes could be desirable for the rhombohedral BFO structure with R 3C space group (Γ = 4A 1 + 9E) [20,21]. However, not all modes could be clearly observed at room temperature [22]. Raman peaks of all samples are fitted by Gaussian function, as are shown in Fig. 4b and Table 2.
As seen in Table 2  and E-9 modes shift toward lower frequency in comparison with those of BFO. For the (Ho, Ni) co-doped samples, positions of A 1 -1, A 1 -3, A 1 -4, E-3, E-6, and E-7 modes tend to shift slightly toward higher frequency, positions of E-2, E-4, and E-5 modes change a little, while positions of E-8 and E-9 modes tend to shift strongly toward higher frequency in comparison with those of the x = 0 sample. Previous studies have also showed that the A 1 modes and E modes at low frequency characterize for Bi-O covalent bonds, other E modes at high frequency characterize for Fe-O bonds [23]. The A 1 -1, A 1 -3, A 1 -4, E-3, E-6, and E-7 modes characterize for Bi-O covalent bonds [14,24], while the E-8, E-9 modes characterize for Fe-O bonds [11,25]. So the changes of E-8, E-9 modes confirmed Ni 2+ ions substituted into Fe-sites, and the changes of A 1 -1, A 1 -2, A 1 -3 , A 1 -4, E-3, E-6 and E-7 confirmed Ho 3+ ions substituted into Bi-sites. These results are conformable to the XRD results, which confirmed that Ho 3+ and Ni 2+ ions substituted into Bi-sites and Fe-sites, respectively.  Figure 5a shows magnetic hysteresis loops of BFO and Bi 1-x Ho x Fe 0.97 Ni 0.03 O 3 materials at room temperature. As seen in Fig. 5a, all samples present weak ferromagnetic behavior. BFO sample has saturation magnetization of M s = 0.047 emu/g and remanent magnetization of M r = 0.008 emu/g. When doping with Ni 2+ ions in Fe 3+ site, M s and M r values increase to 0.702 emu/g and 0.169 emu/g, respectively. However, when co-doping Ho 3+ in Bi 3+ site they decrease slightly as observed in Fig. 5b. This reveals that Ni 2+ doping enhances ferromagnetization in BFO that can be attributed to some following reasons: (i) the appearance of ferrimagnetic order Ni 2+ -O 2--Fe 3+ besides antiparallel indirect interaction between Fe 3+ and neighbor Fe 3+ [26,27]; (ii) Ni 2+ replacement in Fe 3+ site causes the charge shortage, in order to neutralize the charge, some Fe 3+ ions transform to Fe 4+ , resulting the ferromagnetic double exchange interaction Fe 3+ -O 2--Fe 4+ ; (iii) the enhancement of magnetization due to the appearance of oxygen vacancies and lattice defects when doping Ni 2+ in BFO lattice [28]. The slight decrease of saturation magnetization and remanent magnetization in (Ho, Ni) co-doped samples in Fig. 5b could be explained by the reduction of oxygen vacancies due to Bi 3+ volatilization [29,30]. However, the M s and M r values in (Ho, Ni) co-doped samples are still larger 12 and 15 times in comparison with those of BFO. Figure 6 shows complex impedance spectra of BiFeO 3 and Bi 1-x Ho x Fe 0.97 Ni 0.03 O 3 (x = 0, 0.05, and 0.1) which can indicate the contribution of grain, grain boundaries, and electrode interface into impedance of materials. As can be seen, simulated curves fit well with the experimental curves in all samples. Fig. 6a presents that the impedance spectrum of BFO consists of one semicircle at high frequency and one semicircle at intermediate frequency, which indicate the contribution of grains and grain boundaries, respectively. The resistance of grain boundaries is much larger than that of grains. Figs. 6b, c, d exhibit only one semicircle that corresponds to the contribution of grain. As observed in XRD and SEM results, particle size was large for BFO and then decreased obviously when doping Ni 2+ and co-doping Ho 3+ into BFO. Furthermore, the grain boundary became less clear after doping Ni 2+ and co-doping Ho 3+ . This may be the reason for the contribution of grains and grain boundaries into impedance of samples. Since the limitation of frequency range, the contribution of electrode interface to impedance can't be detected in all samples.

CONCLUSION
In summary, BiFeO 3 and Bi 1-x Ho x Fe 0.97 Ni 0.03 O 3 (x = 0, 0.025, 0.05, 0.075, and 0.1) materials have been successfully fabricated by sol-gel method. All samples crystallize in rhombohedral structure of BFO materials. Crystal lattice parameters and average crystallite size increased with doping Ni 2+ and decreased gradually with co-doping Ho 3+ . X-ray diffraction patterns and Raman scattering spectra confirmed the successful substitution of Ho 3+ and Ni 2+ ions into Bi-sites and Fe-sites, respectively, which affected obviously to magnetic and electrical properties of BFO. Ni 2+ replacement kept an important role in enhancing ferromagnetization of BFO. Complex impedance spectra showed the main contribution of grains and grain boundaries into impedance of samples.