ZnO based Diluted Magnetic Semiconductors and CoFe2O4 nanoparticles

Diluted magnetic semiconductors are believed to be an ideal candidate for spintronics, since they can be easily integrated into microelectronic technology. To use DMS materials for practical applications it is necessary that its Curie temperature (Tc) must be at or above room temperature. Among the various II–VI semiconductors zinc oxide (ZnO) is one of the most important material, with wide band gap (3.3 eV). Undoped and transition metal (Fe, Cu and Mn) doped ZnO nanorods have been synthesized of average length ~ 1 µm and diameter ~ 50 nm. Room temperature ferromagnetism (RTFM) has been observed in both undoped and doped ZnO NRs. The origin of ferromagnetism in undoped ZnO NRs was attributed to localized electron spin moments resulting from surface defects / vacancies. Significant changes in the M - H loop were observed as doping concentration of transition metal ions changes. The origin of ferromagnetism in doped ZnO NRs was explained by F - centre exchange mechanism. 
ZnO usually tends to grow in the wurtzite structure in the bulk or nanocrystalline form. On the basis of density functional theory it was predicted that the binding energy of Mn is higher on (001) plane of zinc-blende structure than that in the wurtzite structure. Therefore ZnO in zinc - blende structure has been obtained by growing it as a shell on the ZnS nanoparticles (core). Further, the ZnO shell in zinc-blende structure was doped with different concentrations of Mn and its optical and magnetic properties were studied.
Another important aspect of nanotechnology is related to magnetic materials. Cobalt ferrite (CoFe2O4) is a most studied magnetic material due to its very good physical and chemical properties. Oleic acid capped CoFe2O4 nanoparticles of ~ 20 nm have been synthesized using chemical route. The effect of capping molecule concentration and annealing temperature on the magnetic properties has been investigated. Ultra high coercivity (~ 9.47 kOe) has been observed in oleic acid capped CoFe2O4. The high coercivity value was found to be due a cumulative effect of the strain, spin disorder and surface anisotropy developed at the surface due to oleic acid.
To utilize magnetic nanoparticles for various applications it is necessary to coat these nanoparticles with a suitable material that is nontoxic, biocompatible, water soluble and does not hinder the magnetic properties of core material. Silica coating is one of the methods. High coercivity CoFe2O4 nanoparticles were coated with silica. On the basis of various physico – chemical characterizations it was concluded that silica coating does not intervene the coercivity of oleic acid capped CoFe2O4 nanoparticles.