Nanoparticles synthesised through chemical methods are demanded for their monodispersity, dispersability and inexpensiveness. Also, these nanoparticles could be used for a variety of applications such as biomedical applications and in nanocoating. Hence the interest in the shape and size controlled magnetic materials through chemical methods are important.Although oxides can be easily synthesis through chemical methods, the synthesis of metals and alloys require special techniques. Hence polyol process is used in the synthesis of metals and alloys. Although polyol process could be used for the synthesis of metals and alloys, not all the metals and alloys could be synthesised. Hence the research work is undertaken in two directions, one in the synthesis of metals and alloys and second in the understanding of the mechanism in polyol process.
Synthesis of Fe
The synthesis of Fe through chemical methods are difficult to achieve due to the oxidation of Fe. However, proper control of the reaction parameters enables to synthesis pure Fe. The reduction of Fe in polyol is achieved by the use of NaOH as additive along with FeCl2 in ethylene glycol thereby enhancing the reaction kinetics of EG. The enhanced reaction kinetics has thus paved way to reduce Fe which was found to be difficult to reduce so far.
Reaction process in the synthesis of Co
Co nanoparticles could be reduced in polyols easily compared to Fe. However, the choice of suitable precursors is necessary for the reduction else, the reduction may require several hours to occur. In view of these, the basic reactions undergoing in the process from the introduction of a precursor to the final reduction of Co is studied. The following process takes place in polyols Co(Ac)2--> CoEG2Ac2 --> CoEG-->Co2+-->Co
Ferrites
Size controlled ferrite nanoparticles are required for a variety of application, Mn-Zn ferrite nanoparticles of various sizes in the range 20-80 nm were synthesised using a modified oxidation method. The critical size for superparamagnetism is found as 25 nm. The Curie temperature is constant and does not vary with size which supports the claim that Curie temperature depends only on the cation distribution and not the size. The anisotropy constant is determined to be 7.79 kJ/m3 which is higher than the bulk (1 kJ/m3).
Garnets
Garnets are important class of ferrimagnetic materials used for high frequency applications. Although the magnetic properties of ferrites by mechanical milling has been studied extensively, there has not been a single study on the change in magnetic properties on mechanical milling. In view of these, bulk garnets were mechanically milled to synthesis the nanocrystalline form of garnets. But the garnets are found to undergo decomposition into orthoferrite and Fe2O3. As the byproducts are nonmagnetic it was able to study the magnetic properties of the garnets. On milling the saturation magnetisation of gadolinium iron garnet increases whereas that of yttrium garnet decreases. The increase in the magnetisation is due to the shift in the compensation temperature on milling. The defects or oxygen vacancies produced by milling are responsible for canted structure resulting in unsaturated magnetisation.
Permanent Magnetic Materials
Exchange coupled Nd2Fe14B/alpha-Fe nanocomposite permanent magnetic materials are found to exhibit a higher theoretical energy product than single phase Nd2Fe14B. The higher energy product is due to the exchange coupling between the alpha-Fe and Nd2Fe14B phases which combines the higher saturation magnetisation of Fe and higher anisotropy of Nd2Fe14B. However practically not even the energy product of single phase magnets is achieved. Various factors are found to influence the exchange coupling phenomena such as grain size, homogeneity, etc. It was also found that the exchange coupling is affected by the grain boundary anisotropy which is verified by Henkel plot studies.
Sm-Co permanent magnetic materials are suitable for high temperature applications due to their high Curie temperature and anisotropy. However the anisotropy constant of the material depends on the phases of the Sm-Co material which exists as SmCo7, Sm2Co17, SmCo5 etc. We have studied the magnetic properties of the Sm-Co permanent magnetic material with the TbCu7 structure. On milling, the coercivity increases upto the single domain size of 709 nm.