Growth temperature dependent evolutions of microstructural, optical and magnetic properties of Zn_0.75Co_0.25O films

Zn_0.75Co_0.25O films are fabricated via reactive electron beam evaporation. The influence of growth temperature on the microstructural, optical and magnetic properties of Zn_0.75Co_0.25O films is investigated by using x-ray diffraction, selected area electron diffraction, field emission scanning electron microscope, high resolution transmitting electron microscope, photoluminescence (PL), field dependent and temperature dependent DC magnetization, and x-ray photoelectron spectroscopy (XPS). It is shown that Zn_0.75Co_0.25OO films grown at low temperatures (250--350℃) are of single-phase wurtzite structure. Films synthesized at 300 or 350℃ reveal room temperature (RT) ferromagnetism (FM), while superparamagnetism for 250℃ fabricated films is found above 56K. PL and XPS investigations show favour towards the perspective that the O-vacancy induced spin-split impurity band mechanism is responsible for the formation of RT FM of Zn_0.75Co_0.25O film, while the superparamagnetism of 250℃ fabricated film is attributed to the small size effect of nanoparticles in Zn_0.75Co_0.25O film.