Controlling magnetic, magnetotransport and optical properties of Al codoped Zn-Co-O thin films

Thin films of 5{\%} Co doped ZnO were grown on C- and R-cut sapphire substrates by pulsed-laser deposition, with and without Al codoping ($x$=0--1{\%} Al). Al-doped films retain significant magnetization while exhibiting degenerate semiconductor behaviour. Magnetoresistance of these novel Co-doped ZnO semiconductor films is found to be highly dependent on Al doping and is vanishingly small at ($x>$0.2 {\%}) Large ($\sim $20 {\%} at 2 K) in-plane anisotropy of the magnetoresistance is observed, resembling an AMR effect, which is attributed to Fermi surface anisotropy and most of it has no ``ferromagnetic'' origin. The field dependence of the magnetoresistance can be explained in terms of two-band model and ionised impurity scattering. Hall-effect data indicates completely degenerate electron gas at ($x \quad >$ 0.5 {\%}). High resolution x-ray scattering and magnetisation data on samples with ($x$=0) reviles the presence of Co metal clusters ($\sim $8nm in size) that account for much or all the ferromagnetic magnetisation and exhibit temperature activated decrease of the coercive field. A huge band gap shift is observed with Al doping as a result of \textit{Burstein-Moss} effect. In view of the vanishing magnetoresistive effects at room temperature, it is clear that these Co doped ZnO samples are not dilute magnetic semiconductors.