ABSTRACT
The importance of fabrication and the understanding of the magnetic properties of oxides of metals attracted scientist and engineers from various fi elds. The lithium ferrite is one of the most used versatile magnetic materials. They are important components for microwave device and memory core due to their high values of Curie temperature, saturation magnetization, and so on [1, 2]. A number of lithium ferrite compositions comprising of the oxide of lithium and iron, and if desired the oxides of other metals such as Mn, Ni, and Zn are known to be useful in applications requiring soft ferrimagnetic properties. The lithium ferrites are usually synthesized and have been prepared by mixing and milling together powder mixture of lithium carbonate, iron oxide, and oxides of other suitable metals. After prolonged milling the powder mixture is calcined at an elevated temperature of about 1,000oC to co-react the individual oxides into the spinel lithium ferrites which is then processed into the desired components [3, 4]. The solid state reaction has some inherent disadvantages like chemical inhomogeneity, introduction of impurities during ball milling, and so on. The high temperature induced lower magnetization due to formation of Fe3O4 and α-Fe2O3. However, most of the factors degrading the properties are overcome in chemical synthesis method. The art of preparing lithium ferrite by conventional method is not suitable for preparing chemically uniform lithium ferrites of extremely small grain size. The quest for synthesis of small grain size demands the synthesis of ultrafi ne ferrite materials [5]. Consequently, it led to the discovery of various synthesis methods. Several patents disclose various wet chemical techniques namely sol-gel, auto combustion, coprecipitation, Pechini method, and so on [6-9]. In the chemical coprecipitation process an aqueous solution of suitable salts of iron, lithium, and other desired, suitable materials is mixed with a precipitating agent that will cause the precipitation of a fatty acid salt of lithium with the hydroxides of the other metals present in the solution. The precipitate represents a substantially uniform mixture of precursor compounds of the ferrite metals. The precipitate is fi ltered from the mother liquor and dried. The dry precipitate is heated in air at an elevated temperature, preferably about 200oC to dehydrate the precipitate and to burn out carbonaceous matter leaving a residue of the oxides of the respective metals. The particles are then heated at a temperature of 400-500oC to co-react the lithium oxide, ferric oxide, and other metal oxides present to form a spinel lithium ferrite. At this stage, the ferrite particles which are extremely
small in size about 100 Angstrom are formed. It is then sintered at about 1,100oC. The sintering operation results in some grain growth, although such growth can be advantageously inhibited by the presence of a small amount of bismuth. It is to be noted that the grain size is signifi cantly smaller than the grain size obtained in conventionally prepared lithium ferrites. However, the coprecipitation process takes a very long duration to obtain the required materials. This is to overcome in the sol-gel auto combustion process also known as the citrate precursor method. In this method, the respective nitrates and citric acid are heated making a spontaneous combustion process to occur giving the required ferrite sample in just a few second. A brief discussion of what is spinel structure ferrite, different preparation methods, basic magnetic, and Mössbauer background and some characteristic properties of Li-Zn-Ni ferrites prepared by citrate precursor method is given.
