Influence of pH Variation on Structural and Magnetic Properties of Ni-Zn Ferrite Nanoparticles Synthesized by Auto Combustion Method

Nickel zinc ferrite (Ni0.5Zn0.5Fe2O4) nanoparticles were synthesized via solution auto ignition combustion method. The ferrite samples were synthesized with different pH values 3, 5, 7 and 8. The as synthesized samples were calcined at 800oC for 4 hours. These synthesized nanopowder have been characterized by XRD, SEM and VSM and effect of pH value on the properties of the ferrite samples were studied. XRD pattern confirmed that samples were crystalline in nature. Crystallite size of ferrite powder increased with increase in basic nature of ferrite nanopaticles. Crystallite size of sample was found 24.86nm, 28.10nm, 33.51nm and 39.61nm at pH values 3, 5, 7 and 8 respectively. The magnetization of samples prepared at different pH values 3, 5, 7 and 8 were found 55.490e.m.u./g, 61.420e.m.u./g, 65.541e.m.u./g and 66.512e.m.u./g respectively.


INTRODUCTION
Ferrite materials draw more attention due to their unique properties 1 .Soft ferrite powder have attracted the researchers and investigators of different fields due to their numerous applications like memory system, medical science instrument etc [2][3][4][5][6][7] .Ferrites are important materials due to their various practical applications, such as magnetic storage devices [8][9] .Parameters like composition, method of synthesis, substitution of different cations, annealing time and temperature, sintered density, porosity and crystallite size alter the properties of material [10][11] .Ferrites materials are comparatively economical for preparation of various types of sensors to probe, such as magnetic field 12 , current 13 , gas concentrations [14][15] , heat treatment 16 and mechanical stresses 17 , and.Ferrite temperature sensors used in biochemical applications [18][19][20][21] .Main reasons of being ferrite samples are attracting researchers due to it's their high resistivity, applicability at higher frequency and higher corrosion resistance that makes them highly important for high frequency applications.Different methods for synthesis of ferrite powder are co-precipitation 23 , sol-gel method 22 , micro emulsion method 26 , hydrothermal 24,25 , reverse micelle 28 , precursor 27 , etc. Complex synthesis process and lower production rate are the main problems of these wet-chemical methods 29 .Sol-gel auto-combustion or auto-ignition preparation method where the chemical sol-gel and ignition process is combined.This powder preparation method shown great possibility in the synthesization of spinel type ferrite nanomaterials.This method of synthesis can be considered as solution combustion technique 30 .It has been applicable for the preparation of different spinel ferrite compounds NFe 2 O 4 , where N could be Zn, Ni, Co, Cu, Mg, Mn ion or its combination [31][32][33][34][35][36][37][38][39][40][41][42] .Nickel-Zinc ferrites are considered as one of the most versatile soft ferrites because of its high resistance and low eddy current losses 43 .In the present course of work we have prepared the Ni-Zn ferrite (NZF) by solution auto combustion method.Normally, in ferrites nickel has a more tendency to occupy the octahedral sites whereas zinc occupies the tetrahedral site.Thus, nickel ferrite has an inverse spinel structure whereas zinc ferrite has a normal spinel structure.Hence, NZF have a mixed spinel ferrite structure.The tetrahedral sites are occupied by Zn 2+ and Fe 3+ whereas the octahedral sites are occupied by Ni 2+ and Fe 3+ .The interactions between the ions in tetrahedral and octahedral sites can alter magnetic and electrical parameters of prepared nano-ferrites.The nickel zinc ferrite properties are also dependent on the methodology adopted for their synthesis, preparative conditions like pH of solution, sintering temperature and time, chemical composition, grain size.Novelty of work is that we have prepared the nickel zinc ferrite powder by economical method.The main objective is to study the effect of pH variation on the structural and magnetic parameters of solution auto ignition synthesized NZF nanoparticles.

Preparation of NZF nanoparticle
Nickel zinc ferr ite (Ni 0.5 Zn 0.5 Fe 2 O 4 ) powder samples were prepared by solution autocombustion technique.Metal nitrates Zn(NO 3 ) 2 •6H 2 O, Ni(NO 3 ) 2 •6H 2 O, Fe(NO 3 ) 3 .9H 2 O were used as starting materials.These nitrates were added separately in double distilled water and stirred for 20 minutes at 70 o C 30 .Solution of citric acid in double distilled water poured into stirred metal nitrate solution, and then ammonia solution was added to continuous stirred solution for adjustment of pH value.The pH of solution was adjusted to 3, 5, 7 and 8 using ammonia solution.The mixed solution was kept onto a hot plate with continuous stirring at 100 o C.After continuous approximate 7 hours of stirring the viscous brown black gel formed.After the formation of a highly dense sticky black brown gel, the temperature was raised to 120 °C.The temperature was then increased rapidly upto 200°C, huge amounts of gases like H 2 O, CO 2 and N 2 were produced and when heated at 250-300 o C, the dried gel burnt out completely to form a loose powder.The loose powder was ground in pestle motor for 30 minutes to form fine powder.Finally, the as prepared fine powder calcined at 800°C for 4 h (heating rate of 10°C/min).

Characterization Techniques XRD
The structural parameters of synthesized NZF were investigated by X-ray diffraction (XRD; The Bruker D8 X-ray diffractometers a).The XRD pattern of samples were taken in the angle range 10-80 o with 2 o /min.

SEM
The morphological nautre of samples was scanned using scanning electron microscopy equipped with an energy dispersive X-ray spectrometer (SEM, JEOL).

VSM
Magnetic nature of prepared nanoparticles were studied by a vibrating sampler magnetometer (VSM PAR-155 at IIC, IIT, Roorkee) Range: 0.00001 to 10000 e.m.u., Magnetic field: 10 to +10 kOe , Temperature range : Room Temperature).where k is the structure factor, b is the full width at half maxima, l is the X-Ray wavelength, q is the Brag Diffraction angle.Due to the fast combustion rate and high ignition temperature with increasing pH, higher pH produced the powder with larger crystallite size and good crystallinity.Crystallite size of sample NZ1, NZ2, NZ3 and NZ4 were  The SEM images of heat-treated NZF are presented in Fig. 3a, 3b, 3c and 3d.The SEM images demonstrates that the NZF particles have a strong tendency to hold together, forming agglomerates because of the magnetic interactions between ferrite particles during preparation process [46][47] .Grain size of NZ1, NZ2, NZ3 and NZ4 were found 24.86nm, 28.10nm, 33.51nm and 39.61nm respectively.These voids are also present in the as synthesized sample.This may be attributed to the interaction between the magnetic nanoparticles, in turn these particles grow to some extent to form larger particles at high pH of the solution 48 .

Magnetic properties
The magnetic parameters of ferrites nanoparticles were altered by different parameters  4 and Fig. 5.The magnetic parameters like saturation magnetization, coercivity were measured from M-H loops listed in Table 2.
M-H curve of the samples show hysteresis, indicating that the samples exhibit good ferromagnetic behavior.Magnitude of magnetization of Ni-Zn ferrite nanoparticles depend on occurrence of Fe ions at A or B site.Zn 2+ ion is non magnetic in nature merely occupies the A site.The concentration of Zn ion increases with increase of pH value from 3 to 8. Therefore when the content of Zinc is excessive,  Zn 2+ occupies most of the A site.Accordingly, the number of magnetic ion Fe is sharply reduced in the A site.More the migration of Fe 3+ ions from A to B site results in enhancement in magnetization, due to more ions at B site as compared to A site.Hence total magnetization i.e.Ms=MB −MA increased.Nilar et.al have also reported the similar type of results 44,49,50 .Fig. 6 demonstrates the graph between magnetization and crystallite size with different pH values 3, 5, 7 and 8.

CONCLUSIONS
The NZF samples were successfully prepared with different pH values by solution auto ignition method.The impact of pH on the magnetic and structural parameters of NZF has been investigated.The XRD results clearly shows that the crystallinity and average crystallite size of powder samples increases with an increase in pH value.Crystallite size of sample was found 24.86nm, 28.10nm, 33.51nm and 39.61nm at pH values 3, 5, 7 and 8 respectively.All the prepared samples NZ1, NZ2, NZ3 nad NZ4 are single-phase ferrites with cubic spinel structures.Auto ignition or auto combustion process resulted in the formation of nano-sized (24.86-39.61nm), highly reactive and crystalline nickel zinc ferrite.The magnetization of samples NZ1, NZ2, NZ3 and NZ4 were measured 55.490e.m.u./g, 61.420e.m.u./g, 65.541e.m.u./g and 66.512e.m.u./g respectively.

Fig. 6 :
Fig. 6: Variation of crystallite size with M s for different pH values of ferrite sample