Electrical Control of Magnetoresistance in Highly Insulating Co-Doped ZnO

A Zn0:96Co0:04O film with low electron concentration (about 1:5 _ 1017 cm_3 at 21 K) on a highly conducting Zn0:99Al0:01O layer has been deposited on a-plane sapphire substrate by pulsed laser deposition. To study the magnetoresistance (MR) of depleted, highly insulating Co-doped ZnO an Au ohmic contact and Pd Schottky contact were deposited on the Zn0:99Al0:01O and Zn0:96Co0:04O layer, respectively. Positive MR of 30% with a current of 10_6 A was observed at 5 K. The positive MR decreases drastically at 5 K and changes to negative MR at 50 K with increasing current, which is considered to be due to the bias voltage control of the electron concentration in the Zn0:96Co0:04O layer. Our work demonstrates the electrically controllable magnetotransport behavior in insulating ZnO-based diluted magnetic semiconductors.