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Synthesis and Magnetic Properties of Mn- Zn Ferrite Obtained by Decomposition of Precursor by Sunlight

Magnetic nano-materials are extensively studied because of their wide range of applications. The Mn-Zn ferrite is amongst the most important magnetic materials and has attracted a great interest in technology because of its use in memory or data storage devices, magnetic recording heads, switches and other electromagnetic gadgets. Low temperature single phase nano crystalline MnxZn (1-x) Fe2O4 (x = 0.4, 0.5 and 0.6) were prepared by decomposition of precursor by sunlight. Stoichiometric amounts of manganese oxide, zinc oxide and ferric oxide are taken as starting materials. Mixture of these oxides were homogenized by using ball-mill and then treated with a ligand to produce a precursor. The samples obtained after the decomposition of precursor were characterized by infrared spectral analysis (IR), X-ray diffraction (XRD) and atomic absorption spectroscopy (AAS). The lattice parameters increase with increase in Mn content (8.4386-8.4552 A0). The particle size, determined by Scherer formula, is found to be in nano range between 17nm to 26 nm. The saturation magnetization values are in the range 22.47emu /g 41.63emu /g and hysteresis loss is found to be low for all the samples. The Curie temperature (Tc) is in the range 473K – 558K.


Introduction
Magnetic iron oxide nanoparticles are of high interest due to their unique properties. Among them Mn-Zn ferrites are one of the most widely used magnetic materials due to their low hysteresis loss and high saturation magnetization The applications of these versatile magnetic material ranges from actuators, electromagnetic gadgets, switches, sensors, transformers etc. [1] to biomedical applications [2]- [3]. Even though, it has many applications in various fields as bulk material [4] as well as their new applications as nanoparticles [5], an increase in the demand for the development of new synthesis routes for Mn-Zn ferrite nano particles, still exists and are explored by different group of researchers. Therefore, the improvement and understanding of various new synthesis routes that allow the achievement of highly crystalline materials is a constant need. As reported in the literature by different researchers [6]- [7]. It is very important to have the accurate control of chemical composition and particle size distribution for the preparation of high quality Mn-Zn ferrite powders. The most common method of producing Mn-Zn ferrite powders in the large amount in industry is via the traditional ceramic technology, involving solid phase reactions at high temperatures, of mixture of oxides [8]- [9], which is often faced with difficulties of providing high quality powders required for the high performance devices because of the poor phase and compositional control, in homogeneities in compound, as well as of larger and wider particle size distributions and impurities. Many novel and innovative wet-chemical methods are tried successfully in recent years to prepare high quality ultrafine powders or nanopowders, including the co-precipitation [10]- [11], hydrothermal process [12]- [13], sol-gel [14]- [15] and sol-gel auto-combustion [16]- [17]. In this study a novel method was tried in which solid oxides of manganese, zinc and iron were ball milled to obtain a homogenous mixture which was then treated with a ligand hydrazinium acetate to obtain a precursor which was exposed to sunlight and on drying undergoes auto combustion-self decomposition-to give ultrafine Mn-Zn ferrite.

Experimental
The stoichiometric amounts of pure 99.9% manganese dioxide (Sigma-Aldrich make), zinc oxide (Thomas Baker make) and ferric oxide (Thomas Baker make) was taken as starting materials. The mixture was then ball-milled with ball to material ratio of 10 at 80 rpm speed for 10 hrs in Acmas Technocracy Ball Mill (model Acm -8203) to obtain a powdered mixture. This mixture was then treated with calculated amount of aqueous hydrazinium acetate and homogenized to a thick paste. The resulting paste containing mixed metal oxides was exposed to sunlight as shown in the figure 1 it dried slowly by the sun rays. It was observed that, on drying the paste swells and undergoes auto combustionself decomposition-to finely divided solid powder which was found to be magnetic in nature. The procedure was repeated for other samples and, the final product, in each case, was used for characterization as well as to study the structural, magnetic and electrical properties. The IR spectra of Mn-Zn ferrite samples were recorded by using FTIR Shimadzu Model IR prestige 21 series spectrophotometer. In a typical experiment, the solid ferrite sample was finely ground along with the pure and dry KBr, in the ratio 1:10. Fine grinding is required for the sample to be uniformally mixed with KBr. The mixture was then put in a sample holder and placed in the sample chamber of the IR spectrophotometer. The absorption spectrum for the sample was recorded in the wavelength range 1000 cm -1 to 400 cm -1 .
Atomic absorption spectroscopy is a method for elemental analysis. It is also useful in detecting, both qualitatively and quantitatively, the trace metals and it is independent of the molecular form of the metal in the sample. The method is highly sensitive and can detect different metals in concentration of less than one ppm.
The structural characterization of the prepared Mn-Zn ferrite nanopowders was carried out using Rigaku, X-ray advance Power diffractometer using Cu Kα radiation (λ = 1⋅54183 Å). The step size employed was 0⋅02°, in the range of 20 0 -80 0 .The average particle size T was calculated using most intense peak (311) employing the Scherer formula. The particle size and morphology was carried out using Scanning Electron Microscope Model JEOL 5800LV.  Infrared spectra of various ferrites have been studied by Waldron [18], reporting two bands in the region 1000 cm -1 to 200 cm -1 . He attributed the high frequency (υ 1 ) band to tetrahedral metal-oxygen bond and second frequency (υ 2 ) band to octahedral metal-oxygen bond corresponding to:

Results and Discussion
stretching vibration 450-385 cm -1 here O is oxygen, Me O is metal in the octahedral site and Me T in the tetrahedral site. The metal-oxygen absorption bands (a) and (b) are pronounced for all spinel structures and essentially for ferrites, which are also seen in these samples. IR spectral data of all the ferrite samples prepared by these methods are found to show two peaks in the range 588-569 cm -1 and 462-447 cm -1 which are in agreement with the reported value [19]- [20].
Mn-Zn ferrite powder samples of different compositions of Mn and Zn were accurately weighed and dissolved in minimum quantity of AR grade concentrated hydrochloric acid and the solution was diluted to ppm level by taking care that each powder sample has concentration of Fe ions in the range of 2 to 9 ppm, Mn ions in the range of 1 to 3 . 6 ppm and Zn ions in the range of 0 . 4 to 1 . 5 ppm, as per the requirement of the AAS spectrophotometer of GBC Model 932AA.  The values of lattice constants "a" calculated by using Scherer formula from the X-ray diffraction data for samples prepared by these were found to be in good agreement with the reported values [21]- [22].     properties are due to the influence of cationic stoichiometry and their occupancy in the specific sites, in addition to the formation of dead layers on the surfaces, existence of random canting of particle surface spins, non-saturation effects due to a random distribution of particle size and deviations from the normal cation distribution [25]. Moreover, it is known that Mn is unlikely at octahedral sites even at very fine crystalline sizes for ferrites with x ≥ 0.6. Consequently, no Fe 3+ -O-Fe 3+ magnetic interaction is possible in the Mn-Zn ferrites. On the other hand, a part of total Mn 2+ may occupy octahedral sites with x < 0.6 [26]- [27], therefore, it is not surprising that Mn-rich ferrites yield high values of saturation magnetization. All the samples show low values of coercivity indicating soft nature of the ferrites. The variation of the Curie temperature with increasing Mn content in Mn-Zn ferrite prepared by mechano-chemical method is given in Table 4, for prepared samples. It can be seen that, there was a systematic increase in the value of the Curie temperature (T c ) from 473K for Mn 0.4 Zn 0.6 Fe 2 O 4 to 558K for Mn 0.6 Zn 0.4 Fe 2 O 4 as the manganese amount increases in these ferrites [20]. This is due to the substitution of non-magnetic Zn 2+ ions by the magnetic Mn 2+ ions in the samples. The value of T c is found to be higher in the case of the ultrafine powder than that in bulk ferrites. This is due to the deviation of cation distribution in a nano-sized particle as in comparison with its bulk counterparts [28]. In general, magnetic properties are controlled by exchange interaction of the metallic ions on the two interactive sub-lattices A and B. The change in T C may be positive or negative depending on the boundaries, the geometry, and the interaction. It is also possible that T c decreases due to some unknown surface effect. For small particles a significant fraction of atoms are on the surface, and it is reasonable to expect their magnetic interactions to be different. As such, a different average Curie temperature [29] is observed.

Conclusion
The samples Mn x Zn 1-x Fe 2 O 4 with x=0.4, 0.5 and 0.6 prepared using mechano-chemical method with photocatalytic auto combustion-self decomposition, were found to have spinel structure. This was confirmed using XRD and IR spectroscopy. The chemical composition was confirmed with AAS.
The particle sizes were estimated using Scherer formula and were found to be in nano range. Nano-materials produced by this method show high values of saturation magnetization. Tc increases with Mn content and was found to be higher than for the bulk material.
The simple mechano chemical method using metallic oxides, as starting material, adopting photo catalytic auto combustion-self decomposition for the synthesis of the final material was found to produce high performance nanoparticles of Mn-Zn mixed ferrite material.