Structural and magnetic properties of Sb3+ ions doped Ni–Ba–Co ferrite prepared by sol–gel method

In the present work, nanocrystalline Sb3+ ions doped Ni0.2Ba0.1Co0.7Fe2−xSbxO4 (0 ≤ x ≤ 0.1, step by 0.025) ferrites were prepared via sol–gel method. The spinel-phase structure of samples can be confirmed by X-ray diffraction (XRD) patterns. The composition and structure were further studied by Fourier transform infrared spectroscopy (FTIR). There were two typical characteristic bands ν1 and ν2 in FTIR spectra, which related to the stretching vibrations in spinel ferrite. Energy-dispersive spectrometer (EDS) analyzed the elements of samples. It indicated that the elements of Ni, Ba, Co, Fe, O, and Sb existed in the samples. Vibrating sample magnetometer was used to characterize magnetic properties. The saturation magnetization decreased from 57.65 to 44.50 emu/g with the increasing Sb3+ ions content, which is attributed to Fe3+ ions replaced by the Sb3+ ions. Remanent magnetization and coercivity first decreased and then increased slightly.


Introduction
In the past few decades, spinel ferrites have always attracted much attention due to largely considerable properties they appeared, such as spin glass behavior, magnetic compensation behavior, critical behavior, etc. [1][2][3]. They have a wide range of applications covering industry, environment, and bio-medicine, such as microwave communications [4], microwave absorbing material [5,6], adsorbents [7], catalysts [8], wastewater treatment [9], magnetic resonance imaging, MRI [10], drug delivery, and release [11]. The magnetic, optical and electronic properties of spinel ferrite nanoparticles are affected by many factors. The synthesis conditions and methods are critical factors. Some authors synthesize ferrite nanoparticles by various methods including co-precipitation [12], sonochemical approach [13], electrospinning method [14], sol-gel method [15], microemulsion method [16], thermal decomposition [17], etc. The conditions of pH and temperature also have effects on the morphology and magnetic properties of spinel ferrite [18][19][20][21]. Among a lot of prepared methods, sol-gel method has benefits of gathering safe, less economic consumption, and short experiment period. The heat required in the process is provided by the reaction itself and does not require external supply, which is a key feature of sol-gel method [22]. Among the many spinel ferrites, cobalt ferrite (CoFe 2 O 4 ) has great coercivity and high magnetocrystalline and is suited to magnetic recording applications [23]. Barium ferrite (BaFe 2 O 4 ) is rhombohedral and nonmagnetic material [24]. NiFe 2 O 4 is a sort of soft ferrite regarded as a collinear ferrimagnet [25]. Ni-Ba-Co ferrite is one of the permanent magnetic material. Recently, modification of ferrite by chemical doping is a hot topic of research by scholars. Ashiq et al. [26] prepared NiFe 2-x Sb x O 4 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1) ferrite with the reverse microemulsion method and the crystallite size of samples were in the range of 8-38 nm. Anjum et al. [27] prepared CdSb x Fe 2-x O 4 (x = 0.1, 0.2, 0.3, 0.4, 0.5) with the ceramic route and found that saturation magnetization decreased and coercivity increased with the increasing content of Sb 3? ions. Anjum et al. [28] also prepared CoSb 0.3-Fe 1.7 O 4 thin films via electron beam deposition technique. They found that the post annealing temperature is related to the growth of crystal structure. Lakshmi et al. [29]. prepared Ni-Zn-Sb ferrite by hydrothermal method and found that the lattice parameter decreased with increasing content of antimony. There are no discovered literature on the properties of Sb 3? ions doped Ni-Ba-Co ferrite so far. It is necessary to explore the occupation of Sb 3? ions in the Ni-Ba-Co spinel ferrite lattice. By doping antimony trioxide (Sb 2 O 3 ), we want to explore the effect of nonmagnetic Sb 3? ions on the magnetic properties of Ni-Ba-Co spinel ferrite.
In this work, the Ni 0.2 Ba 0.1 Co 0.7 Fe 2-x Sb x O 4 (0 B x B 0.1, step by 0.025) ferrite were prepared with solgel method and citric acid was used as complexing agent. This work used sol-gel method to prepare ferrite. The advantages of samples prepared this method are high purity and chemical homogeneity on an atomic scale. In addition to this method also saves time. The complexing agents mainly play two roles in the experiment. One is to act as a complexing agent to form a uniform and stable sol in the reaction process. Another is to act as a fuel for sol-gel autocombustion. The effects of Sb 3? ions doped Ni-Ba-Co ferrite were explored.

Synthesis
A series of Sb 3? ions doped Ni-Ba-Co ferrites have been prepared using sol-gel method having the general formula Ni 0.2 Ba 0.1 Co 0.7 Fe 2-x Sb x O 4 (0 B x B 0.1, step by 0.025). The preparation of the sample includes some processes which are shown in Fig. 1. Nitrates weighed in stoichiometric ratio and citric acid at the molar ratio of 1:1.2 mixed in 100 mL distilled water. After the solute is completely dissolved, ammonium hydroxide is used to adjust the pH 7 under the condition of uniform stirring. The precursor solution was heated at 80°C for 3 h in magnetic heating and stirring agitator. Then, the solution turned to a wet sol. The wet sol was put in dry blast oven at 120°C for 2 h to form a dry gel. The dry gel was heated with an alcohol lamp. The obtained floccule was ground about 1 h until it forms black powder. The powders were annealed in muffle furnace at 1100°C for 2 h. The obtained powders were ground about 20 min to obtain final samples.

Characterization techniques
The XRD patterns were collected by Germany Bruch diffractometer with a goniometer using Cu Ka radiation (k = 0.15406 nm). The diffracted intensities were recorded in the angular range 20°-80°. The infrared absorption spectra of samples were recorded by Fourier transform infrared spectroscopy (China WQF-510). The morphology and shape of samples were observed by scanning electron microscopy (Japan JSM-6700F). The magnetic measurements were obtained by vibrating sample magnetometer (USA Lakeshore 7304).

Structural properties
The lattice structure of spinel ferrite belongs to facecentered cubic (space group Oh7-F3dm) and is den- where (h k l) are Miller index, 'd' is inter planar spacing. The average crystallite size (D) can be calculated by the Scherrer's formula. The formula is as follows [31]: where 0.9 is shape factor, 'b' is the full width at half maximum of the diffraction angle. The values of lattice constant (a) and average crystallite sizes (D) are shown in Table 1. It is clear from Fig. 5

that lattice
Heat with an alcohol lamp Grind the floccule for 1h to get powder Distilled water Ammonium hydroxide pH=7 Heat the solution at 80 for 3h

Ba(NO3)2·6H2O
Anneal the powder at 1100 for 2h  The reason for the decrease of lattice constant is that Sb 3? ion (0.76 Å ) may be oxidized to Sb 5? ion (0.60 Å ) when samples were annealed at 1100-1200°C [32]. The Sb 5? ions with smaller ionic radius are accommodated on the lattice by replacing Fe 3? ion (0.67 Å ). The increased phenomenon of lattice constant can be related to cation redistribution and bigger ionic radius of Sb 3? ion (0.76 Å ) when the content of Sb 3? ions further increased. The average crystallite sizes of prepared samples decreased from 29.4 to 21.1 nm. The decrease in average crystallite sizes may be due to energy consumption when Sb 3? ions enter into the lattice [33]. The dislocation linear density (d) can be calculated by the formula: The X-ray density (q x ) can be calculated by the formula [34]: where 'M' represents the molecular mass; 'a 30 is volume of unit cell; 'N A ' is the Avogadro's constant, and the value is 6.02214076 9 10 23 mol -1 . The values of dislocation linear density and X-ray density of samples are shown in Table 1. The X-ray density of samples increased with the increasing content of Sb 3? ions. This is because Fe 3? (55.85 g/mol) was replaced by Sb 3? with a bigger molecular mass (121.76 g/mol).  Table 2. The appearance of two bands proves that the spinel structure of the prepared samples. With the increasing content of Sb 3? ions, the m 1 moves to lower frequency from 613 to 603 cm -1 . It indicates that Sb 3? ions replace Fe 3? ions into lattice; thereby, causing the change of bond length. Some small absorption bands are observed in Fig. 6. The band near 1641 cm -1 may be attributed to the stretching vibration of -OH in adsorbed molecular water [36]. The band around 1388 cm -1 is seen as the vibration of antisymmetric NO 3 bond, the decrease or disappearance of the band at 1388 cm -1 can indicate that NO 3 participated in the process of reaction [37]. The band located at 1091 cm -1 may be related to the stretching vibration of residual C-O bond.

SEM and chemical elements analysis
The morphological micrographs of the prepared Ni 0.2 Ba 0.1 Co 0.7 Fe 2-x Sb x O 4 (0 B x B 0.1, step by 0.025) ferrite annealed at 1100°C are displayed in Fig. 7. Figure 7 show the varying degrees of agglomeration with the increasing content of Sb 3? ions. The agglomeration appears among particles due to the magnetic dipole-dipole interactions along with Van der Waals force [38]. It indicates that every particle is composed of a lot of small grains. Energy-dispersive spectrometer is used to analyze elements and ingredients of prepared samples. The EDS diagrams are displayed in Fig. 8. It can be confirmed that there are five elements in pure sample, which are Ni, Ba, Co, Fe, and O. It can be observed that the existence of Ni, Ba, Co, Fe, O, and Sb element in doped samples.

Magnetic properties
The magnetic characterization of Ni 0.2 Ba 0.1 Co 0.7-Fe 2-x Sb x O 4 (0 B x B 0.1, step by 0.025) is recorded at room temperature. The hysteresis loop of the prepared samples is shown in Fig. 9. Values of coercivity (H c ), remanent magnetization (M r ), saturation magnetization (M s ) are shown in Table 3. It shows from   [39]. Due to the partial replacement of magnetic ions (Fe 3? ) with Sb 3? (nonmagnetic ions), the magnetization of B site decreased, which weakens superexchange A-B. Accordingly, saturation magnetization decreased. For spinel ferrite, the Bohr magnetic moment can be calculated by the formula [40]: where 'M W ' is the molecular weight. The value of  Table 4. The demagnetization curve located in the second quadrant of the hysteresis loop is an important basis for examining the hard magnetic materials. The coercivity is the strength of the reverse magnetic field that needs to be applied to reduce the magnetization of the magnetized magnet to 0. The coercivity decreased from 952.31 to 800.68 Oe when the content of Sb 3? ions increased up to x = 0.075, then the coercivity slightly increased from 800.68 to 844.35 Oe. A similar phenomenon occurs in the reference [27], the coercivity first decreased and then increased with the increasing content of Sb 3? ions in CdSb x Fe 2-x O 4 series ferrites. The coercivity depends highly on some factors such as particle size, anisotropy constant and lattice imperfection. The different Sb 3? ions content entered into Ni-Ba-Co ferrite lattice consuming energy, which leaded to different degrees of agglomeration. The particle distribution is uneven, the domain wall needs more energy to move, which leaded to a larger coercivity. Anisotropy constant (K) can be estimated by the following formula [41]: where 'l 0 ' is magnetic permeability in vacuum (equal to 1 in Gauss unit system). The value of anisotropy constant is displayed in Table 4. From Fig. 10b, it can be observed that 'K' decreased with the increasing content of Sb 3? ions. The value of squareness S (M r / M s ) and coercivity squareness (S*) can be seen in Table 3     Magnetic field (kOe)  Fig. 11. As far as an ideal single domain particle is concerned, the value of dM/dH at H ? 0 is 0 [42,43]

Conclusions
The sol-gel method was used to prepare Ni 0.

Declarations
Conflict of interest The authors declare no competing financial interests.