Study of Structural and Magnetic Properties of Silica and Polyethylene Glycol (PEG-4000)-Encapsulated Magnesium Nickel Ferrite (Mg0.5Ni0.5Fe2O4) Nanoparticles

Mg0.5Ni0.5Fe2O4 has been successfully synthesized by using co-precipitation method. Two series of Mg0.5Ni0.5Fe2O4 silica encapsulated have been prepared by varying the concentration of silica and variation of PEG-4000 concentration. Analysis of X-Ray Diffraction (XRD) pattern showed that nanoparticles contained Mg0.5Ni0.5Fe2O4 spinel phase and γ-Fe2O3 phase with a particle size of 5.1 nm. The various of silica encapsulation give rise to produce a new phase of SiO2 and increase the particle size to 16.1 nm. PEG-4000 encapsulation affected to create a new phase of γ-FeO(OH), and reduce the particle size down to 4.5 nm. Fourier Transform Infra Red (FTIR) for Mg0.5Ni0.5Fe2O4 showed absorption peaks around 300-600 cm-1 which are M-O bond vibration. After silica encapsulation, there was new bond vibration typical of silica such as Si-O-Si (1049.28 cm-1), Si-OH (779.24 cm-1), and Si-O-Fe (570.93 cm-1). The PEG-4000 encapsulation creates a new vibration for typical of PEG-like of C-O (1103.28 cm-1) and C-H (925.83, 1481.33, and 2924.09 cm-1). Both of encapsulations series have M-O bond vibration indicating the presence of Mg0.5Ni0.5Fe2O4. After silica encapsulation, the coercivity of Mg0.5Ni0.5Fe2O4 decreased from 47 Oe to 10 Oe due to the decrease of particle size. Even though, the discrepancy of particle size as the effect of PEG-4000 encapsulation, the coercivity just slightly reduced to 46 Oe. The saturation magnetization of Mg0.5Ni0.5Fe2O4 decreased from 4.7 emu/g to 1 emu/g after silica encapsulation because diamagnetic SiO2. Otherwise, the saturation magnetization increased to 7.7 emu/g after PEG-4000 encapsulation because of domination of Mg0.5Ni0.5Fe2O4 phase ratio.


Introduction
Recently, research on magnetic nanoparticles (MNPs) has been extensively done. The magnetic nanoparticles have quantum size effect (the transition region between atoms and bulk), and the large surface area dramatically changes some of the magnetic properties and exhibits superparamagnetic phenomena [1]. Spinel ferrite is one of the most important magnetic nanoparticles because their structure, chemical, electrical, and magnetic properties make them ideal for many applications. Spinel ferrite has formula MFe 2 O 4 , where M is a divalent cation.
MgFe 2 O 4 exhibits soft magnetic, high resistivity, non-cytotoxic, and magnetic anisotropy which are lower than of other spinel ferrites because Mg 2+ is carrying no magnetic moment, so the magnetic coupling is purely originated from the magnetic moment of Fe cations [2,3]. On the other hand, 2

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The 4th International Conference on Advanced Materials Science and Technology IOP Publishing IOP Conf. Series: Materials Science and Engineering 202 (2017) 012047 doi: 10.1088/1757-899X/202/1/012047 NiFe 2 O 4 exhibits soft magnetic, high resistivity, cytotoxic [2], and magnetic anisotropy which are higher than MgFe 2 O 4 because Ni 2+ has a magnetic moment. When Mg 2+ ions are combined with Ni 2+ ions with the same composition, Mg 0.5 Ni 0.5 Fe 2 O 4 will be found.
According to properties of ferrites used for biomedical application, ferrite should be biocompatible and non-toxic. Magnetic nanoparticles tend to agglomerate because of their large surface area. Therefore ferrite has encapsulated with organic materials (like chitosan, Polyvinyl alcohol (PVA), Polyethylene glycol (PEG), etc.) or inorganic materials (like gold, silica, etc.) [4]. Silica has silanol groups which can easily react with coupling agents providing strong attachment of surface ligands on magnetic nanoparticles; encapsulated silica increases the stabilization of magnetic nanoparticles in liquid, and it also can prevent agglomeration by increasing the surface charges hence the electrostatic repulses among particles [4]. The silica can produce by using precursor like tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate (TMOS), and sodium silicate (Na 2 SiO 3 ). The precursors like TEOS and TMOS are environmentally unfriendly, toxic, and expensive. Na 2 SiO 3 precursors can be used as a source of silica because they are environmentally friendly, non-toxic, and inexpensive [5,6]. On the other hand, PEG is biocompatible, low-cost, non-toxic, and hydrophilic [7]. PEG with molecular weight under 1000 g/mol is expensive, so PEG-4000 can be used as encapsulation material because it is inexpensive.
Synthesized of MgNiFe 2 O 4 by coprecipitation has been investigated [8,9]. In the recent years, the research about silica encapsulated nanocomposites as NiFe 2 O 4 [10], KFeO 2 [11], etc. has gained much attention. PEG also has much attention as encapsulation materials such as for CaFe 2 O 4 [12], Mn (1-x) Zn x Fe 2 O 4 [13], etc. However, the study of the influence of silica and PEG encapsulated Mg 0.5 Ni 0.5 Fe 2 O 4 on the structural and magnetic properties has not been paid much attention. In this work, Mg 0.5 Ni 0.5 Fe 2 O 4 will be synthesized by using coprecipitation method and encapsulated by varying the concentration of silica and PEG-4000. The influence of silica and PEG-4000 on crystal structure and magnetic properties of Mg 0.5 Ni 0.5 Fe 2 O 4 will also be investigated.

Experimental Method
Two series of silica and PEG-4000 encapsulated magnetic nanoparticle Mg 0.5 Ni 0.5 Fe 2 O 4 were prepared by two steps for each encapsulation materials. Firstly, Mg 0.5 Ni 0.5 Fe 2 O 4 was prepared by using the coprecipitation method. In the second step, silica encapsulated Mg 0.5 Ni 0.5 Fe 2 O 4 was prepared by varying silica concentration along with the using of sodium silicate (SS) precursor. The PEG-4000 encapsulation also prepared by varying PEG concentration. XRD patterns were recorded on Shimadzu XD by using CuK α radiation. FTIR transmission spectra were taken by using FTIR spectrometer Shimadzu Prestige-21. Magnetic properties of samples were studied by using Vibrating Sample Magnetometer (VSM) Riken Denshi.

Preparation of magnetic nanoparticle
The magnetic nanoparticle of Mg 0.5 Ni 0.5 Fe 2 O 4 was prepared by coprecipitation method, by using aqueous solutions of magnesium, nickel, and iron salts. The stoichiometric amount of 2.703 g FeCl 3 .6H 2 O (Merck, Germany) were dissolved in 25 mL of distilled water, 0.508 g MgCl 2 .6H 2 O (Merck, Germany), and 0.594 g NiCl 2 .6H 2 O (Merck, Germany) were also dissolved in 25 mL of distilled water and then 3.37 mL HCl was added. The solution was then added in dropwise to 10 M NaOH (Merck, Germany) solution. The reaction was kept under constant stirring at 90˚C for 1 hour. Precipitated ferrite nanopowder was washed with distilled water for six times. Finally, the sample was dried in a furnace at 95 ˚C for 5 hours.     The FTIR spectrum of Mg 0.5 Ni 0.5 Fe 2 O 4 at Figure. 2.a shows the bands at 3417.86 cm -1 and 1627.92 cm -1 are assigned to the O-H stretching and bending vibration, respectively [11,13,14]. The bands at 362.62 cm -1 correspond to M-O stretching mode of the octahedral group, and 609.51 cm -1 corresponds to M-O stretching mode of the tetrahedral group [8,11,12]. In the FTIR spectrum of silica ( Figure. [11,15,16], at 794.67 cm -1 is Si-OH stretching [11], and at 1087.85 cm -1 is Si-O-Si stretching asymmetric [6,15]. Furthermore, in the FTIR spectrum of silica encapsulated Mg 0.5 Ni 0.5 Fe 2 O 4 nanoparticles ( Figure. 2 [6,17], Si-OH stretching at 779.24 cm -1 [11], and Si-O-Si stretching asymmetric at 1049.28 cm -1 [6,15] clearly show the presence of silica on Mg 0.5 Ni 0.5 Fe 2 O 4 . The bands at 300.9 cm -1 and 347.19 cm -1 correspond to M-O stretching mode of the octahedral group, and the band at 686.66 cm -1 corresponds to M-O stretching mode tetrahedral group [8,11,13].  Table 4 shown the decrease of coercivity from 47 Oe to 10 Oe affected by increase of silica encapsulation onto Mg 0.5 Ni 0.5 Fe 2 O 4 . The coercivity dependence of particle size, if the coercivity is proportional to 1/t , then the nanoparticles are multidomain. If the coercivity is proportional to t , then the nanoparticles are single-domain. This behavior is illustrated in Figure. 3. Therefore, all samples before and after silica encapsulation are multi-domain due to 6

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The 4th International Conference on Advanced Materials Science and Technology IOP Publishing IOP Conf. Series: Materials Science and Engineering 202 (2017) 012047 doi:10.1088/1757-899X/202/1/012047 the increase of particles size after silica encapsulation. The diamagnetism of SiO 2 phase [1] on nanoparticles also caused coercivity value decreasing as increasing silica concentration. The saturation magnetization of Mg 0.5 Ni 0.5 Fe 2 O 4 was 4.7 emu/g. The saturation magnetization decreased as increasing silica concentration, due to diamagnetism properties of SiO 2 at nanoparticle. SiO 2 caused the magnetic response of Mg 0.5 Ni 0.5 Fe 2 O 4 weaker, so, by increasing silica concentration, the magnetic anisotropy became weaker than before encapsulation.  Figure 4a shows that it is indexed as (220) Table 5 shows the particles size of Mg 0.5 Ni 0.5 Fe 2 O 4 is 5.1 nm and PEG-4000 encapsulation became 4.5 nm. PEG-4000 has larger molecular weight and a long chain of monomer, so there are a lot of particles trapped by polymer chain and then the growth of crystal decreases and the particle size becomes smaller [18]. Also, Table 5 shows that X-ray density has no significant difference after PEG-4000 encapsulation. Otherwise, the strain became higher than before encapsulation due to the increase of the average lattice spacing because of the substituted O atoms to nanoparticles.
The FTIR spectrum of PEG-4000 ( Figure 5c) has bands at 3448.72 cm -1 and 1635.64 cm -1 which correspond to the O-H stretching and bending vibrations. The bands at 956.69 cm -1 is C-H bending (out of plane), at 1465.9 cm -1 is C-H in plane bending (scissoring), at 2885 cm -1 is C-H stretching (symmetric), and at 1064.71 cm -1 and also 1111 cm -1 are stretching ether C-O from -CH 2 -OH group in PEG-4000 [12,19].