Ferrites, both soft and hard, are used in many technological applications because of their excellent magnetic and electrical properties. Spinel ferrite (soft ferrite) with the general formula MFe₂O₄, where M = Mn, Co, Ni, Cu or Zn or a mixture of these ions, posses unique electronic or physical structure and chemical stability [1,2]. It is an ideal material for high-frequency passive components because of its high permeability, resistivity and permitivity. These materials have been used technologically in magnetic recording media and magnetic fluids for the storage and/or retrieval of information, magnetic resonance imaging (MRI) enhancement, catalyses, magnetically guided drug deliveries, sensors and pigments [3,4]. The magnetic properties of these ferrites can be changed by the substitutions of various kinds of M²⁺ among divalent cations (Zn²⁺, Mn²⁺, Ni²⁺, Mg²⁺, Co²⁺, Cu²⁺, etc.) or by introducing a relatively small amount of rare-earth ions [5]. However, the properties of these materials are determined by their chemical composition, microstructure and process mechanism. So, the interesting features are that their properties can be enhanced by controlling the chemical composition and the preparation methodology.

Spinel ferrites accommodate cations among two possible available interblaner sublattices namely tetrahedral (A) sites and octahedral (B) sites. Spinel ferrites can be either normal spinel (M²⁺)_A[Fe³⁺ Fe³⁺]_BO₄ or inverse spinel with half of the trivalent ions in the A-sites and the other half together with divalent ions in the B-sites [6-8]. Ferrites of similar composition differ in their physical and chemical properties depending on the distribution of cations, having different oxidation states, among the available tetrahedral and octahedral sites. Moreover, substitution of magnetic and non-magnetic cations in ferrite materials changes their magnetic and electrical properties [9,10]. The change in physical and chemical properties of ion substituted spinel ferrites arises from the ability of these compounds to distribute the cations amongst the available tetrahedral and octahedral sites [11].

Nanocrystalline Ni-Mn ferrite system is one of the most substituted ferrites due to its excellent properties which is mainly applied in electrical devices and in catalyses. The Ni-Mn ferrite (Mn_xNi_1−xFe₂O₄) crystallizes in an inverse spinel structure and can be represented by (Mn_xFe_1−x)_A (Ni_1−xFe_1+x)_BO₄ where x is defined as the inversion parameter and depends upon preparation conditions and heat treatment [1,11-18]. Spinel nanocrystalline ferrite displays enhanced magnetic and electronic properties by virtue of their unique electronic or physical structure which may be harnessed for technological applications [2,4,19,20]. Moreover, some phenomena like modified saturation magnetization, superparamagnetism, metastable cation distributions, etc., have been observed in nanoparticles of various ferrites [18,21,22].

The conventional way to synthesize ferrites, involves the use of high-temperature solid-state reaction between the oxides/carbonates and then annealing at high temperatures (>1200˚C), has some inherent disadvantages. These include chemical inhomogeneity, coarser particle size and introduction of impurities during ball milling. Various soft chemical methods [23-29] like mechanical milling, inert gas condensation, hydrothermal reaction, oxidative precipitation, sol-gel synthesis and reverse micelle technique are employed for the preparation of nanoferrites. Most of these methods are economically unfeasible for large scale production. The oxalate precursor technique was used for synthesis of ferrites where a single phase of nanocrystalline ferrite powders with homogeneous microstructure, narrow size distribution and uniform shape at low processing temperature can be produced. The oxalate precursor technique uses the carboxylic groups of oxalic acid to chelate the required cations in a proper solution conditions. Chelating metal ions in solution using oxalic acid allows the formation, after drying, of a solid oxalate precursor with uniformly distributed cations in the required ratio. Relatively low annealing temperatures were required to form the corresponding oxides since all cations are mixed at molecular scale.

In the present investigation, we synthesized a pure Mn_0.2Ni_0.8Fe₂O₄ ferrite using the oxalate precursor method. X-ray diffraction (XRD), scanning electron microscope (SEM) and a vibrating sample magnetometer (VSM) were utilized in this study. The effect of annealing temperature (400˚C - 1100˚C) on the formation, microstructure and magnetic properties of MnFe₂O₄ ferrites was also investigated.

The oxalate precursor method was applied for the preparation of Mn-Ni ferrite (Mn_0.2Ni_0.8Fe₂O₄). Chemically grade ferric chloride (FeCl₃∙6H₂O), manganese chloride (MnCl₂∙2H₂O), nickel chloride (NiCl₂∙H₂O) and oxalic acid anhydrous (C₂H₂O₄) as source of organic were used as starting materials. Pre-calculated stoichiometric ratios of ferric chloride, manganese chloride and nickel chloride solutions and containing equivalent amounts of oxalic acid were prepared. The mixture of ferric chloride, manganese chloride and nickel chloride solution was firstly prepared and then stirred for 15 minutes on hot plate magnetic stirrer, followed with addition of an aqueous solution of oxalic acid to the mixture with stirring. The solution was evaporated at 80˚C with constant stirring rate until dryness and then completely dried in a dryer at 100˚C overnight. The dried powder then obtained as precursors. The block flow sheet for synthesis of Mn_0.2Ni_0.8Fe₂O₄ ferrite materials by oxalic acid method is shown in Figure 1.

Thermal analysis of uncalcined precursor was carried out. The rate of heating was kept at 10˚C/min between room temperature and 1200˚C. The measurements were carried out in a current of nitrogen atmosphere. For the formation of the Mn-Ni ferrites phase, the dry precursors were calcined at a rate of 10˚C/min in static air atmosphere up to required annealing temperatures (400˚C - 1100˚C) and maintained at the temperature for annealing time 2 h.

The crystalline phases present in the different annealed samples were identified by X-Ray diffraction (XRD) on a Brucker axis D8 diffractometer using Cu-Kα (λ = 1.5406) radiation and secondary monochromator in the range 2θ from 10˚ to 80˚. The ferrites particles morphologies were observed by scanning electron microscope (SEM, JSM-5400).

The magnetic properties of the ferrites were measured at room temperature using a vibrating sample magnetometer (VSM; 9600-1 LDJ, USA) in a maximum applied field of 10 kOe. From the obtained hysteresis loops, the saturation magnetization (Ms) was determined

Nanocrystalline Mn_0.2Ni_0.8Fe₂O₄ has been synthesized via oxalate precursor route. The effect of annealing temperature on the formation, crystalline size, morphology and magnetic properties were systematically studied. The results showed that:

1) The oxalate precursor of Mn-Ni-Fe mixture decomposed thermally in multistep weight loss up to about 680˚C.

2) Mn_0.2Ni_0.8Fe₂O₄ was started to form at much lower annealing temperature (≤400˚C).

3) The lattice parameters were increased with increasing annealing temperature up to 1000˚C. The average crystalline size increased by increasing the annealing temperature.

4) Single well crystalline ferrite was obtained at 800˚C with crystallite size about 109 nm.

5) The saturation magnetization of the ferrite powders continuously increased with the increase in annealing temperature.

6) The formed Mn_0.2Ni_0.8Fe₂O₄ ferrite powders had good soft magnetic properties with maximum saturation magnetization (48.2 emu/g) was achieved at annealing temperature 1100˚C.