Engineering of magnetic properties in doped bismuth ferrite materials

The engineering of magnetic behaviour of Li/Zn doped BiFeO3 had been done by synthesized a polycrystalline of BiFeO3, Bi0.96Li0.02FeO3, and Bi0.95Zn0.05FeO3. Investigation of crystallite structure and magnetic properties of the sampel had been done by X-ray diffraction and VSM analysis. At room temperature, the lithium and zinc doped bismuth ferrite has conducted a different magnetic behaviour. Within the ferromagnetic region, an increases of magnetic saturation or enlarger magnetic coercivity were identified. Doping lithium resulted in increasing magnetic saturation (Ms) and magnetic remanent (Mr), significantly. Meanwhile, doping zinc resulted in enlarger of magnetic coercivity coincide with the reveal of Bi20FeO40 as the second phase. 1 Intoduction Bismuth ferrite or BiFeO3 (BFO) become the most studied materials due its wide range of physical properties which promises for many applications [1]. Ferroelectric (TC=830 °C) and antiferromagnetic (TN=370 °C) ordering above room temperature, were devoted many researchers to understand and tailor these physical properties to fulfil the requirement of specific technology applications, i.e. multiferroic. One of the vital engineerings on the multiferroic application is the enhancement of magnetic properties. Many works have been done to enlighten the role of doping towards the magnetic properties of BFO. Different types of doping can be rising the magnetic moment of BFO, while the reduction size of BFO was also increased by the magnetic moment [2]–[4]. Du et.al. [5] found that the La-doped BFO in Bi-site caused the effect of lattice parameter increases which resulted in increasing of magnetic moment. In common, Khomchenko et. al.[6] have reported that Sm-doped BFO in Bi-site was significantly enhanced for the spontaneous magnetization, while the phase transition from a rhombohedral to an orthorhombic was conducted by the composition. On the other hand, Zn or Li-doped BFO into Fe-site can significantly be enhanced the magnetic saturation of BFO by the effect of crystallinity improved or particle-size reduction, have been reported [7], [8]. Recently, Baqiah et. al. [9] have reported that BiTiO3 phase content incorporation with the BFO matrix tends to increase the magnetic saturation [10]. a Corresponding author : naufal@ui.ac.id © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). MATEC Web of Conferences 218, 04027 (2018) https://doi.org/10.1051/matecconf/201821804027 ICIEE 2018


Intoduction
Bismuth ferrite or BiFeO 3 (BFO) become the most studied materials due its wide range of physical properties which promises for many applications [1]. Ferroelectric (T C =830 °C) and antiferromagnetic (T N =370 °C) ordering above room temperature, were devoted many researchers to understand and tailor these physical properties to fulfil the requirement of specific technology applications, i.e. multiferroic.
One of the vital engineerings on the multiferroic application is the enhancement of magnetic properties. Many works have been done to enlighten the role of doping towards the magnetic properties of BFO. Different types of doping can be rising the magnetic moment of BFO, while the reduction size of BFO was also increased by the magnetic moment [2]- [4]. Du et.al. [5] found that the La-doped BFO in Bi-site caused the effect of lattice parameter increases which resulted in increasing of magnetic moment. In common, Khomchenko et. al. [6] have reported that Sm-doped BFO in Bi-site was significantly enhanced for the spontaneous magnetization, while the phase transition from a rhombohedral to an orthorhombic was conducted by the composition. On the other hand, Zn or Li-doped BFO into Fe-site can significantly be enhanced the magnetic saturation of BFO by the effect of crystallinity improved or particle-size reduction, have been reported [7], [8]. Recently, Baqiah et. al. [9] have reported that BiTiO 3 phase content incorporation with the BFO matrix tends to increase the magnetic saturation [10].
Hence, it is interesting to engineering the magnetization of doping BFO. Different doping-site and different type of materials affect the complexity of magnetic behavior in BFO [11]. The increases in magnetic coercivity or magnetic saturation have different benefits and applications [12], [13]. In order to synthesize doped BFO, there are several preparations technique such as hydrothermal, solid-state reaction, co-precipitation, and solgel [5], [8], [14], [15]. Of all these processes, a sol-gel technique has been widely used because of its easy, straight-forward and its final product is highly pure [7], [16]. Moreover, this technique produces an ultrafine porous powder uniformly which can be accommodating industrial-scale production.
This paper aimed to perform engineering of magnetic behavior in low content Li/Zn doped BFO. To further understand by XRD and VSM analyze, the polycrystalline sample of BiFeO 3 , Bi 0.96 Li 0.02 FeO 3 , and Bi 0.95 Zn 0.05 FeO 3 were synthesized through a sol-gel route. The phase present and magnetic behavior of the sample is assessed. were weighed and dissolved in 200 ml of de-ionized water in a glass beaker. Then, citric acid (3:1 citric acid and metal ratio) was added as a chelating agent. These solutions were mixed and heated at 80 °C with constant stirring until it becomes a very thick gel, then dried in an oven at 120 °C for 24 h to be xerogel. During the drying process, keep it from the impurity or uncontrolled combustion. After that the xerogel was collected, ground and heated at 600-750 °C for 5 h. Finally, all the samples were ground.

Characterization
In order to characterize the phase present and the structure of that phase in the sample, powder XRD data were collected on the PANanalytical Diffractometer (Model: X'Pert Pro) in the diffraction range of 20 o -70 o . Magnetic hysteresis measurements in the room temperature (27 o C -300 K) and fields range of 0 -1.4 T were performed using a vibrating sample magnetometer (VSM) to identify the magnetic behavior established in each sample.

Structure of the sample
The summary of our XRD results is shown in Figure 1. The amounts of BFO phase present in all the samples and their corresponding lattice parameters were analyzed and calculated by Rietveld method using X'Pert HighScore plus software with ICSD 98-018-1983 as a reference database (see Table 1). The XRD patterns confirm the presence of strong (012), (104), (110), (202), (024), (214) peaks in all samples. These peak pattern were consistent with the BFO phase structure (rhombohedral structure system with space group R3c) from the previous investigations [3], [4]. The XRD obtained for the BFO and BLFO compound were indexed with lattice parameters a = b = 5.580 Å and c = 13.874 Å , as mentioned in Table 1. However, doping Zn into Bi-site in BFO-Z were causes the lattice parameter declined because the atomic radius of Zn (142 pm) is smaller than that Bi atom (143pm). This content had resulted in the decrease of lattice parameters with a = b = 5.576 Å and c = 13.863 Å, comparing to BiFeO 3 sample (see Table 1). These occasion coincided with the reveal of Bi 20 FeO 40 (reference: ICSD 98-004-1937) which is cubic structure and space groups I23, as the second phase accompanying the BFO phase present in BFO-Z sample (Figure 1).  In our circumstance, the refinement goodness of fit (GOF) parameters of all sample were <1.6 with R-factors not more than 7 (<10). As mentioned in Table 1, a broad peaks of the powder XRD sample is due to the crystallite size of BFO phase. Scherrer formula was used to calculate the crystallite size (D) of the BFO nanoparticles, where λ is the wavelength of Cu-Kα radiation, and B is them full-width at half-maximum (FWHM). Nevertheles, the B-unit should be converted into radian. We found that the crystallite size of our sample is varied (193.87 nm, 73.840 nm, and 136.930 nm) which may be caused by particles coagulation.
Three dimensional of BiFeO 3 and Bi 25 FeO 40 phase structure were illustrated from refinement data results using VESTA software as shown in Figure 2. From the picture, we can be seen the oxygen atoms were forming a polyhedral shape with the Fe-atom as the center. On the BiFeO 3 phase structure, we can see that doping Li can replace some of Bisite precisely. Meanwhile, we can see in the Bi 25 FeO 40 phase structure that Bi-atom arranged polyhedral shape much more than the Fe-atom.   Table 2.

Magnetic properties
The BFO and BFO-Z sample shows a low saturated hysteresis loop M s (0.08 emu/g and 0.14 emu/g, respectively) which is M r of both samples is identical value (0.02 emu/g). In comparison, the M s of BFO-L is the larger (4.89 emu/g) more than BFO and BFO-Z with the M r is become 1.42 emu/g, as compensate for the Li present in the structure. Refer to Table 1

Conclusion
The BFO, BFO-L, and BFO-Z samples were synthesized by the sol-gel process for engineering the magnetic behavior of that sample. All the samples were crystalized in rhombohedral with space group R3c, precisely. The lattice parameters of BiFeO 3 in BFO and BFO-L samples exhibit identical value, while the lattice parameters of BiFeO 3 phase decreased with doping of Zn content. The magnetic saturation and magnetic remanent increased with doping Li. Further, magnetic coercivity broadens coincide with the reveal of Bi 25 FeO 40 phase structure after doping Zn in the sample. With this results, the behavior of magnetic properties in bismuth ferrite had been successfully engineered.