Room temperature ferromagnetism in finite sized ZnO nanoparticles

We report systematic investigations on the evolution of nanostructured ZnO, room temperature ferromagnetism, and tunable optical properties of ZnO nanoparticles prepared by a mechanical alloying process. It was observed that both un-milled and as-milled powders exhibited a wurtzite structure, but average crystallite size decreased and the effective strain increased for the initial periods of milling. Paramagnetic nature observed in un-milled ZnO gradually unveils room temperature ferromagnetic ordering with modest moment and coercivity. A maximum moment of 0.013µB/f.u. at 12 kOe applied field and a coercivity of 172 Oe were obtained for 40 h milled ZnO powder. Thermo-magnetization data reveal a clear magnetic phase transition from ferromagnetic to paramagnetic state around 500 °C, which shifts slightly towards higher temperature with an increasing milling period up to 20 h. Annealing of as-milled ZnO powder and UV-vis studies display a drastic reduction in room temperature magnetic moment and blue-shifting of excitonic absorption peak. The observed ferromagnetic properties are intrinsic and discussed on the basis of finite size effect, defect density due to oxygen vacancy. These nanoparticles with tunable magnetic and optical properties are promising to find applications in multifunctional spintronic and photonic devices.