DEVELOPMENT OF ROOM TEMPERATURE FERROMAGNETISM IN TRANSITION METAL DOPED ZnO NANOPARTICLES

Abstract

The primary aim of this work was to synthesize and develop intrinsic ferromagnetism at or above room temperature in transition metals doped ZnO nanoparticles and to correlate the structural and magnetic interaction with a view to understanding the origin of ferromagnetic ordering in these nanoparticles. The synthesis process was optimized for preparing nanomaterials to be studied and then crystallized via annealing process at elevated temperature in various atmospheres viz. ambient, forming gas (gaseous mixture of argon (95%) and hydrogen (5%) and oxygen. The effect of annealing on different environments of the samples has been explored. The materials studied include; Zn1-xCoxO (0.00≤x≤0.10), Zn1-x-yCuyCoxO (x = 4%, 0.5% and 0.00≤ y<0.01), Zn0.96-xCo0.04MnxO (0.00≤x≤0.02) and Zn0.96- xCoxMn0.04O (0.00≤x≤0.02). The structural studies via X-ray diffraction show single phase character of Zn1-xCoxO (0.00≤x≤0.10) nanoparticles which are seen to be paramagnetic when annealed in air and become ferromagnetic on annealing in a reducing atmosphere. Electronic characterizations (via XPS) revealed Co ions are in +2 valance state and replaced with Zn ion on the ZnO lattice. The energy band gap of these compositions has been studied via diffuse reflectance spectroscopy (DRS) and found to decrease with Co addition. Optical studies also confirm the presence of Co +2 in substitutional sites. Magnetic studies, correlated with the XPS, clearly suggest that ferromagnetism occurs in the Co dopes samples with the introduction of oxygen vacancies generated by annealing in a reducing atmosphere. Resistivity studies shows a corresponding trend viz. reducing atmosphere increases the conductivity very significantly in the ferromagnetic compositions. The ferromagnetic samples exhibit an apparent change from insulator to metal with increasing temperatures for T>380K and this change along with the magnetic and electrical transport properties were observed to be reversible. The effect of a non-magnetic co-dopant such as Cu ions (concentration <1%) was studied in detail and in these low (Cu) dopant compositions the moment displays a non-monotonic variation with Cu concentration. The XPS of Cu 2p core spectra revealed that ionic state of Cu begins to include the +1 state in addition to the dominant +2 state with increasing Cu concentration. This change correlates with what we interpret as a strong decrease in the concentration of oxygen vacancies and the decrease of the ferromagnetic moment Therefore the effect of Cu at lowconcentrations appears to be indirect; via increasing or decreasing the oxygen vacancies whose role appears to be critical in this context. Furthermore, the effect on the ferromagnetism of this system (ZnO) has been investigated with Mn and Co ions as magnetic co-dopants in an n-type environment. We find that while a purely Mn doped sample exhibits weak ferromagnetism at room temperature, the general effect of Mn as a co-dopant with Co, in an n-type environment, is to decrease the moment strongly. All of our results can be systematically explained within the context of defect mediated ferromagnetism in these wide band gap semiconductors where the coincidence of the spin split impurity (defect) band states and the 3-d states leads to the development of a net moment alongside the formation of spin polarons. The nanoparticle nature of the materials may serve to enhance the density of states and leading to a fulfillment of the Stoner criterion fro ferromagnetism. Thus central to the ferromagnetism in these doped semiconductor nanoparticles is the role of the oxygen vacancies as n-type defects and the states they create within the band gap, with the transition metal ions serving to provide the electrons that fix the position of the Fermi level.