Nowadays, studies are orienting towards the development of new multifunctional materials with properties such as ferroelectricity, ferromagnetism, and multiferrosity. Bismuth ferrite (BiFeO₃) is a good example of multifunctional materials,since it has high Curie and Néel temperatures (T_C = 1083 K, T_N = 673 K) [1,2]. Besides, it exhibits ferromagnetic (FM), ferroelectric (FE), and ferroelasticity properties based on magnetization, electric polarization, and elastic effort, respectively [3]. These properties make the BiFeO₃ a material of great interest in the scientific field. Although some materials (BiMnO₃, TbMnO₃, TbMn₂O₅, YMnO₃, LuFeO_4, and Ni₃B₇O₁₃) show similar properties, only bismuth ferrite presents ferroelectricity and antiferromagnetism at room temperature [4,5].

Bismuth ferrite exhibits a G-type antiferromagnetic (G-AFM) ordering with a long-wavelength period ~62 nm, weak ferromagnetism and linear magnetoelectric effect [6]. In addition, it has a bad circuit ferroelectric, and polarization remnants due to charge defects linked to oxygen vacancies [7].

Recent studies have focused on improving the magnetic properties of bismuth ferrites through smaller particle sizes and avoiding the secondary phases. The bismuth substitution for rare earth elements such as samarium (Sm) [8], gadolinium (Gd) [9], neodymium (Nd) [10,11], holmium (Ho) [12] dysprosium (Dy) [13], praseodymium (Pr) [14] and lanthanum (La) [15], which allowed high densification and the elimination of not multiferroic phases [16], as well as their application in spintronics devices like field-effect transistors, electrical switching, nanoelectronics, magnetoelectric random access memories (MERAMs) and sensors [17].

In this paper Bi_1-xLu_xFeO₃ system (x = 0.00, 0.02, and 0.04) was produced by the solid-state reaction method and characterized, studying the effect of lutetium doping on its morphological, structural, and magnetic properties.

The Bi_1-xLu_xFeO₃ (x=0.00, 0.02 and 0.04) samples were synthesized by solid-state reaction method. Stoichiometric amounts of the oxides Bi₂O₃, Fe₂O_3, and Lu₂O₃ with 99.9% purity were dried, weighed, calcined at 750 °C for 9 h and sintered at 800 °C for 9 h, with intermediate milling and pellet pressing.

The solids obtained were characterized by X-ray diffraction (XRD), using the DRX equipment PANalytical X'Pert PRO-MPD equipped with an Ultra-fast X'Celerator detector in Bragg-Brentano arrangement, using Cu Kα radiation (λ = 1.54060 Å) from 20° to 80° 2 Theta. The results were refined with the GSAS and PCW software. The morphological properties were evaluated by scanning electron microscopy (SEM) with FEI Quanta 200-r equipment. Finally, the magnetic characterization was carried out in a VersaLab-type magnetometer of vibrating sample from Quantum Design company, the measurements were made in the temperature range from 50 - 320 K and magnetic fields from -30 to 30 kOe. The zero-field cooled (ZFC) method was used to measure the magnetization as a function of temperature (at 1000 Oe).

The diffractograms of the Bi_1-xLu_xFeO₃ system are in Fig. 1. BFO, BFO2 and BFO4 are x = 0.00, 0.02 and 0.04, respectively. XRD signals revealed high crystallinity of the solids obtained. The patterns analysis evidenced the majority formation of bismuth ferrite (BiFeO₃) with file code JCPDS-01-086-1518, rhombohedral structure, and space-group R3c (161), with preferential orientation on the (1 1 0) plane and the secondary phase Bi₂Fe₄O₉ with file code JCPDS- 00-025-0090, orthorhombic structure, space-group Pbam (55) with a preferential orientation on the (1 2 1) plane [17]. The formation of the secondary phase was can be attributed to the reduced range of thermal stability that BiFeO₃ presents. This decomposition is associated with high bismuth volatility, considering the synthesis temperature [18]. The XRD analysis shows that the secondary phase decreases when the percentage of lutetium doping increases, which stabilizes the main crystalline phase [19,20]. These results are in accordance with the reported by other authors [21].

Figure 1 Source: The Authors.

Fig. 1 (b) shows the zoom-in of the 31^o-33^o 2 Theta range. A shifting toward smaller angles was observed, which indicated the structural distortion generated by the ionic radius difference of substituent cation, which shows the correct lutetium doping in bismuth ferrite [22,23].

Rietveld refinement allowed determining the lattice parameters (Table 1). Fig. 2 shows the refined X-ray diffractogram of BFO2, a high correlation between the experimental and theoretical data was observed, with preferential orientation in the plane (1 1 0) located at 32o 2 Theta and Fig. 2 (b) shows the cell unit obtained with PCW software, which is characteristic of these ferrites [24].

Table 1

Source: The Authors.

Figure 2 Source: The Authors.

Fig. 3 shows the micrographs obtained for each material, particle size distribution was determined using the image J. software. In the micrographs can be observed the presence of interconnected particles with well-defined edges, irregular shapes and sizes. Particle sizes were between 2.25 and 4.50 μm, the values are attributable to lutetium insertion at the A site of the ferrite. These results were correlated with the data obtained by Rietveld refinement (Fig. 4). Lu³⁺ cation has an ionic radius of 25% less than Bi³⁺ cation, causing the unit-cell volume reduction and the decrease in particle size [25]. Additionally, doping can suppress oxygen vacancy concentration, which leads to smaller particle sizes. This behavior is common in the substitution of rare earth elements into the bismuth ferrite structure [26].

Figure 3 Source: The Authors.

Figure 4 Source: The Authors.

Magnetic hysteresis loops, obtained at 50 and 200 K (Fig. 5), exhibits typical magnetism of bismuth ferrites systems. These magnetic properties are associated with unpaired electrons on the Fe³⁺ d-orbital, located at the B site. Remnant magnetization values increase with the higher x values (Table 2), which is due to the ionic radium of Lu³⁺ is lower compared to the one for Bi³⁺. Therefore, doping affects the bonds between Bi-O and Fe-O [27-28].

Figure 5 Source: The Authors.

Table 2

Source: The Authors.

The results show a linear increase in magnetization in regards to the applied field without magnetic saturation. This is attributed to antiferromagnetic ordering and spin structure in the material [29]. The zoom-in section of hysteresis loops showed a slight ferromagnetic behavior for each material, the small hysteresis loops of BiFeO₃ is due to parasitic ferromagnetism caused by the magnetic moments and spin canting effect in the antiferromagnetic network [30,31].

The magnetization results are in accordance with the phases found by XRD. Ferromagnetic ordering is associated with the presence of a secondary phase (Bi₂Fe₄O₉) and the antiferromagnetic ordering is caused by the main phase [32].

Magnetization as a function of temperature is shown in Fig. 6. Some transitions occur at two temperatures: at 250 K is presented the typical transition PM-AFM characteristic of bismuth ferrite systems and at 120 K was presented a slight curvature attributed to the lutetium insertion. It should be noted that Lu³⁺ ion is non-magnetic, and Bi³⁺ ion has a low energy multiplet, which can contribute to the Van Vleck-type magnetic susceptibility. Under 120 K, the negative values are due to the susceptibility of the antiferromagnetic phase [33].

Figure 6 Source: The Authors.

Some researches attribute the increase of magnetization to size particle reduction, which is because of enhancing the uncompensated antiferromagnetic spins in the material surface. In contrast, the magnetization increase was not associated with a direct contribution of lutetium, since it is not a magnetic ion. Nevertheless, it was attributed to the high structural stability of the new material and the decrease of the lattice parameters. [34,35].

Lutetium doped Bismuth ferrite in low percentages (2% and 4%) was synthesized by solid-state reaction using lower temperatures than those reported in the literature. The materials showed a majority phase of rhombohedral structure and space-group of R3c (161) with the presence of the secondary phase (Bi₂Fe₄O₉). These results were confirmed by Rietveld refinement, which showed that the insertion of lutetium cation improves structural stability and decreases the lattice parameters. SEM analyzes showed the formation of particles with smaller sizes by increasing the x value, this is due to the small Lu³⁺ ionic radius. The increase of magnetization, allowed to demonstrate that lutetium substitution was performed efficiently and favored the secondary phase diminution and size particle reduction, generating better magnetic properties.