Ferromagnetism, variable range hopping, and the anomalous Hall effect in epitaxial Co:ZnO thin film

A series of high quality single crystalline epitaxial Zn _0.95Co _0.05O thin films is prepared by molecular beam epitaxy. Superparamagnetism and ferromagnetism are observed when the donor density is manipulated in a range of 10 ¹⁸ cm ^-3-10 ²⁰ cm ^-3 by changing the oxygen partial pressure during film growth. The conduction shows variable range hopping at low temperature and thermal activation conduction at high temperature. The ferromagnetism can be maintained up to room temperature. However, the anomalous Hall effect is observed only at low temperature and disappears above 160 K. This phenomenon can be attributed to the local ferromagnetism and the decreased optimal hopping distance at high temperatures.